ELECTRICAL MACHINING METHODS

• Document Type: CREST
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• Document Number (FOIA) /ESDN (CREST): CIA-RDP81-01043R002300210012-6
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 127
• Document Creation Date: December 23, 2016
• Document Release Date: September 20, 2013
• Sequence Number: 12
• Content Type: REPORT

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Table of Contents
Electrical Machining Methods, by Professor K.V.Olsen,
G.E., Institute of Machine Tool Research, D.T H
Electrolytic Machining
Electric Spark Machining - The Development in Russia
The Development in the U.S A
The Development in England
Development in Germany.
Electric Arc Machining
Ultrasonic Machining
Other Electrical Machining Methods
Bibliography
Patent Literature
12
22
37
60
75
192
100
112
120
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ELECTRICAL MACHINING METHODS
by
Professor K.V.Olsen,
C.E., Institute of Machine Tool Research, D.T.H.
621.791.7.054
In the course of the last half score of years one has occasionally heard of and
seen descriptions in technical journals of methods for machining electrically conduc-
tive materials - primarily iron and metals, especially the hard types - in which,
along with mechanical machining in various forma, use is made of electric energy.
All of this technology has been of rapid growth, and although young in years - at
least, as it is practiced now - it has been divided up into a series of different
methods of machining, which differ primarily in electric principles; each of these,
individually, can apparently be adapted to definite lines of work. Within these
lines, the methods can be profitably applied to various tasks which are otherwise
quite difficult.
At present, extensive research is being done in many places, and the results
achieved have partly led to directives for applying the methods to practical use, and
partly to the development of a series of machines which have already been placed on
the market. In the Danish literature, as far as is known, only one short mention
(Bib1.41) is made of this new technological field; consequently, a survey will be
given below of the various electric machining methods, their basic principles, fields
of application, advantages, drawbacks, and limitations. It should be mentioned at
the same time that the discussion includes only actual machining, whereas other meth-
ods of treatment for which electrical energy is also employed are disregarded, for
example surface treatment in electrolytic baths, electrolytic polishing, and others
which have been known for a fairly long time.
The most modern electric machining methods which have appeared in recent years-
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:- more exactly those with which more systematic work has been done since about 1945.4
- all have their roots in the technological development in other fields. Exactly is
this period of time there has been a great advance in the automobile and aircraft ia.
duatries; jet engines, gas turbines etc. are in the process of rapid development,
where the limitations, in many cases, imam to lie in the procurement of suitable mml
- higher temperatures.
IA. .
The treatment of such materials, new and better ones of which are constantly' :
often difficult to work. This is speoifically true in view of the fact that the very
. parts made of the most difficult materials often require the most complicated machin-
ing. More indirectly, the more difficult materials result in a aborter life for the
i_J
--, cutting tools, i.e., they must be ground more frequently and thus make increased de-
mands an the tool-grinding mill.
Since increasing use is being made of tools clad with hard metal, both in goner.
grinding process; it seams that the demands are actually becoming too high for the
available types suitable abrasives i.e., primarily diamonds and diamond dust, which
are used either directly in the grinding of hard metal or in precision grinding and
lapping of these materials.
The Diamond Shortamo
In the U.S.A., and in England, and Russia as well, a sever shortage Of diamonds
' for grinding has prevailed during and sines the war; at the same time, the demand his
risen considerably, leading to a sharp increase in price. In the U.S.A. and Russia,
efforts have therefore been made to save diamonds and to develop grinding methods
which would eliminate or at least reduce the consumption materially; one of the moans
toward this end was the use of the above-mentioned electric machining methods.
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Insofar as Russia is concerned, no data seems to be available to throw light on
this situation, but in the U.S.A. developments are being followed closely by a speci-
al commission which evaluates the current situation. In its reports, the shortage of
diamonds is established as a fact, and it is made clear that this constitutes a long-
term phenomenon which can only be
expected to be further aggravated.
From a long-range viewpoint, there-
fore, it is necessary both to econ-
omize and to search for other ab-
rasives and methods.
A statistical evaluation of
2
Toisl OW*/
/Production
-41^7c
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1
rvSkiroptel
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01134 $14 MI $40 NO VI4 I 1946 MI MO $42
Fir.1 - Graph of the World Production of
Industrial Diamonds in the Years 1934 -
1952. The broken line indicates the U.S.A.
import for the same time.
The reason that,
in 1941 - 1945, this exceeded the total
production be several million carats is due
to the fact that during this time a stock-
pile of about 20 million carats was used
up. (U.S. Bureau of Mines)
possibilities for an exact definition of how
attributed to each purpose, but it is
this situation is given in a re-
port (Bib1.36) which also empha-
sizes that, practically speaking,
all industrial diamonds come from
the Belgian Congo, from which they
are imported either in the form of
whole diamonds for use in the manu-
facture of cutting tools (diamond
tools) or as diamond bort for use
in diamond disks for grinding of
tools. At present, there are no
much of the total consumption must be
stated, according to the best opinion, that as
far as the U.S.A. is concerned the requirements for diamond dust constitute about
80% of the total diamond import.
Figure 1 shows the world production of industrial diamonds in the years 1934 to
1952 and also the quantity imported into the U.S.A. The curves show that, for all
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practical purposes,
le rising sharply.
creased production
change was made to
all industrial diamonds go to the U.S.A. and that the consumption
The sudden jump upward after 1940 is due both to the greatly in-
on account of the war and to the fact that just at this time a
an increased use of hard-metal tools.
That the import of the U.S.A.
alone in the years 1941 - 1945 was
able to exceed the world production
by several million carats is due to
the fact that a stockpile of about
20 million carats was worked up be-
forehand, of which 11 million was
in England and the rest in Africa
and other places. This surplus had
been exhausted at the close of the
war, after which the consumption
dropped somewhat, only to climb
once more from 1949 to the present
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11111111111111111111111111111111111// 111111101KII
1111111111.111111111111111,1111111,11 Imp no
mum ..s..--mzionaii11111411 till
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11111111111111111111111INIMP/1111: MINIM
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1011111111MOMIIIIII=1=111111111
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.11
Fig.2 - Import Curve from Fig.1, and Solid
Curve Showing the Hard Metal Production of
the U.S.A., with Certain Applications
Omitted
heitts.
Figure 2 shows the same import curve, but related to the production of hard met-
al in the U.S.A.; it will be noticed that the hard-metal curve for the last few years
deviates from the diamond curve.
Before the appearance of hard-metal tools, the demand for diamonds was limited.
Since hard metal has come into general use, and since it has become standard practice
to grind them with diamond wheels, the consumption has risen sharply and will rise
further with further development in hard-metal tools. As an indication of this grow-
th, the commission mentions that the sale of hard-metal tools in a certain period of
1952 was 67-", higher than the sale in the corresponding period of the year before.
To arrive at practical conclusions it is necessary to calculate how much of the
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imported diamond dust is consumed in the various operations for which diamond wheels
are used. This is done in Table 1, which shows that not less than 80% of the total
import is used for diamond wheels for grinding of hard metal.
Table 1
How crushing bort is used.
Application
Total crushing
bort imports
pct.
Diamond wheels for grinding cemented carbides
Diamond saws of all kinds
Glass, quartz, grinding and optical curve generators
Diamond dressing tools, drills, dental tools, optical laps
Diamond and carbide wire dies, and other dies
Diamond compounds for lapping
Misdellaneous small uses
4
154.
80.4
6.0
3.0
4.3
4.3
1.5
0.5
100.0
Percentagewise Distribution in the U.S.A. of Diamond Bort over the various
Fields of Application (From the Iron Age)
A further analysis of the distribution of diamond consumption is given in Table
2. The last three groups are still so new that they do not take up appreciable quan-
tities of diamond bort, but it is expected that in the future they will increase
greatly in importance and require increasing amounts of diamond bort. The first five
groups comprise actual tool grinding and (aside from group h) account for about 70%
of the total consumption of diamond bort in the U.S.A., only 50% of it for single-
point tools, i.e. turning steel, milling steel etc., and an apparently so minor an
item as grinding of chip breakers takes up no less than 20% of the consumption.
The Table gives also a comparison of the four electric machining methods, elec-
trolytic grinding, electrosparking, electroarcing and ultrasonic as well as grinding
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by means of diamond wheels. The letter A denotes that the method in question is al-
ready in use under production conditions; the letter B denotes that the method seems
applicable and that it is promising, but that for the present it is limited because
Table 2
Job Item or Operation
Percent
use of
Diamond
Bort
Applicability of Methods
Diamond-
wheel
Grinding
Electro-
lytic
Electro-Electro-
sparking arcing
Ultra-
sonic
1. Sharpening of single-
point tools
30
A
2. Grinding single-point-
tool chip breakers
20
A
3. Sharpening milling cut-
ters and broaches
15
A
13
r ? lrinding projectile cores
5
A
5. Shaping and finishing
dies
4
A
13
A
A
6. Forming of turbine-
bucket attachments
7. Rifling of gun barrels
8. Machining and grinding of
compressor disks
The Table shows percentage distribution of the total diamond consumption in the
U.S.A. percentagewise among eight important fields. The first five groups account
for about 75%, while all of the grinding with diamonds takes up about 85% of the
total consumption. The remaining portion, or about 15 is used for all other
purposes. (From American Machinist)
of incomplete knowledge of its behavior; the letter C denotes that the method may be
applicable, that it does not seem immediately promising, but that it can eventually
be developel so that it will become useful; finally, 0 denotes limited applicability
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or none at the present stage of development. The three asterisks next to the !Ps in-
dicate that the method is not suited for hand grinding but is applicable to machine
grinding.
Accordin7 to the Table, the demand for diamonds is mainly for chip-removing
tools, while group 5, for the time being, accounts for 4,t% It should be remembered
that the development of the cutting and punching techniques is not included. As the
application of this technique becomes more widespread, there is a steadily increasing
tendency to manufacture most tools of. hard. metal in order to get the benefit of the
very much longer life of these materials.
These tools, depending on circumstances, can be manufactured of a single piece
of hard metal or assembled of many, but both types have in common that the basis for
the often complex form of the tools requires extensive designing. The situation is
very difficult in cases where the tools contain fairly small irregular holes, which
must be milled directly in the hard metal. Although the production of hard metal is
a technology which has been brought to great perfection and permits sintering of pre-
cise molds with correctly placed cut-outs, holes, and even finished threads etc., all
tools assembled from such ready-sintered parts must be further treated to give the
necessary precision and sharpness.
Survey
On the basis of this information on the American situation (which certainly is
also valid for other places) and in view of the known fact that the consumption for
the period after 1951 has surpassed the import, together with the assumption that
this will not appreciably increase in the future - intensive work has been done on
bringing down the consumption by economy and by development of new grinding and ma-
chining methods, of which the most important methods at present seem the electric
types.
In addition, it can be more generally stated that the electric machining meth-
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ods may be valuable in cases where, in general, it is a matter of machining the many
hard materials appearing from time to time on the market and which are hard to ma-
chine with conventional methods. Examples of such materials which are difficult to
machine, in addition to hard metal, are titanium, vitallium, vanadium, Alnico, chrome-
nickel steel, tantung, manganese steel, stellite and others, and of course also ordi-
nary tempered steel.
Although the above discussion has only mentioned the application of the electric
methods in combination with grinding, there is nothing to hinder their use in turn-
ing, boring, milling, etc. The more detailed account to follow will show examples
of this. The methods seem to be in vigorous development, and the larger industrial
countries are working intensively on improving the technique and advancing a produc-
tive scale.
Leading in that respect are apparently the U.S.A., Russia, and England, with
Germany following closely in recent years. All these countries today have machines
and equipment on the market for one or more of the electric machining methods.
American statements indicate that a large number of experimental machines are used
in the U.S.A. for the individual machining methods and that constant work is being
done on developing and improving the technique and in perfecting the machines and
equipment. It is also stated that, as early as 1950, there were more than 4000
"Electro-Spark" machines in use in Russia and Poland. Corresponding figures for oth-
er countries are lacking.
All in all, it seems that the electric machining methods, in broad outline, have
been developed in the period since around 1940. Certain of the fundamental princi-
ples however have been known much longer - since around 1925 - but did not, at that
time, acquire great significance and therefore have apparently remained neglected
until a short while ago.
The Americans maintain in their journals that they are the originators of this
development which, according to their interpretation, had its beginning around 1940
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and resulted in Electro-Sparking machines put on the market in 1943. They loaned the
Russian Purchasing Commission in 1944 prospectus material on the equipment and in
1945 furnished Russia with Elex plants for electro-sparking. They maintain now that
the Russians imitated this equipment with the result mentioned above, that a very
significant number of machines are in use today in Russia and Poland.
Without taking a position on this assertion, it must be conceded that the Ameri-
cans since 1940 and later have done intensive work in this field; this is attested
both by the journals and the patent literature. But at the same time it should be
pointed out that the electric machining methods consist of four processes which are
different in principle, and that there is not always a sufficiently sharp distinction
possible between them. This also brings up the possibility that one country can be
first in the one field while others are first in other fields. Finally it should be
pointed out as a simple fact that one of the electric methods is based on a German
discovery of 1924 (Bib1.48) while another rests on an observation which a Russian,
Vladimir Gusev, had protected by patent No.335 003 in England in 1929. In addition,
another fundamental work was carried out by a Russian - Lazarenko - described in the
Russian technical journals after 1946, and patented in England - No.637 793 of 1946.
An Englishman, Rudorff, also contributed to the development with his patent No.637
872 of 1947, and finally may be mentioned the p4ent of an American, Teubner, No.2
650 979 of 1950
When subjecting this new tehnological field to a closer examination, it will
be natural to subdivide it according to the electric machining principles, which are
quite divergent since the methods have different spheres of application and require
different apparatus. Unfortunately, the terminology is very ambiguous (there is as
yet none in Danish) so that Danish denotations must be taken as a choice of words
which include the concepts but which must not be conceived as the absolutely cortect
or on1:: terminology which can be used. Also some of the foreign terms are mislead-
ing; therefore the following classification gives the usage most often applied in the
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foreign literature.
The field can accordingly be subdivided into the following five groups; an at-
tempt will be made below to give a complete account of the technology, possibilities,
equipment etc. for each of the mentioned electric machining methods.
1. Electrolytic Grinding:
Electrolytic process,
Elektrolytische Abtragung,
Elektrolytisches Schleifen,
Elektrolytisk bearbejdning,
Elektrolytisk met ode.
2. Spark Machining:
Electro-sparking process, occasionally, but incorrectly
arc-machining,
Electro-erosion machining,
Atom expulsion, now most frequently electrical
discharge machining,
Funkenerosion,
Funkenentladung,
Elektrisches Bohrverfahren,
Elektro-Funken-Methode,
Elektrisk gnistbearbejdning,
Gnistmetode.
3. Electro-Arcing Process:
Arc Machining,
Disintegration,
Electrical erosion,
Electro-drilling,
Lichtbogeneros ion,
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Cesteuerter Lichtbogen,
Elektrisk buebearbejdning,
Duemetode
4. Ultrasonic Process:
.Ultrasonic drilling,
Electro -abrading,
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Ultraschallverfahren,
Ultralydbearbejdning,
? Ultralydmetode.
5. Combination Processes:
Andre metoder
Kombinationsmetoder.
It should be noticed that the division into groups is not always sharp and that
certain characteristics can produce a cumulative effect of several processes. Group
5 contains the processes which cannot be classified in other places and processes
used in combination with one or more of the other methods.
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ELECTROLYTIC MACHINING
The idea of replacing the conventional mechanical machining with a method for
removal of a layer of material by chemical agents is very old. The principle has al-
so found extensive application, for example, in mordanting, where dross etc. are re-
moved and in chemical pickling baths where metal surfaces are freed of impurities.
Various other attempts were made to machine the materials chemically, including
the suggestion by de Fehse in 1931 (D.R.P. 564 081) to change the dimensions or the
hard metal while maintaining the geometric form, by exposing the natal to a hot
stream of an oxidizing gas or by direct heating to 700 - 800?C in atmospheric air
with subsequent mechanical removal of the oxide layer.
Somewhat later - in 1943 and occasioned by the shortage of suitable abrasives
during the war - Ballhausen (D.R.P. 847 390) attempted to sharpen the edge of hard-
metal tools by heating them to about 800?C and then brushing off the oxide layer
formed. Attempts to dissolve the hard metal by chemical means are well known and
constitute essentially a modification of the technique applied in the sharpening of
steel files. None of these methods, however, led to the desired results, and they
have never come into extensive practical use.
Electrolytic machining is based on anodic dissolution of the workpiece in an
aqueous electrolyte. This machining method is applied preferably in combination with
grinding - and then, as a rule, with lathe tools and the like - or for shortening
hard materials such as cutting hard metal plates from longer pieces, etc.
In principle, the method consists in connecting the workpiece to the positive
pole o: a direct-current source and the negative pole to a rotary disk, while the
object to be machined - just as in conventional grinding - is passed quite close but
without actual contact taking place. The workpiece is meanwhile immersed in a hiAly
conductive electrolyte, or the electrolyte itself is applied to the disk in the form
like the antifreeze solution in grinding. When the positive workpiece
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oVosc),
being machined passes the negative disk, an anodic disintegration of the part takes
place; the amount of material removed will be directly proportional to the current
strength and the time. In addition, the amount of material removed from the place
of work will be much greater, the more the distance between anode and cathode is
narrowed.
The method which thus is a purely electrochemical process, apparently was first
suggested in 1924 by the Germans Pirani and Schroeter (Bib1.48), and in 1929 the Rus-
sian Gusev took out an English patent (335 003) on a similar procedure.
Practical Data
The direct voltage applied is as a rule quite low, normally less than 25 volts
and most frequently only between 15 and 20 volts. The amperage on the other hand is
large; American sources indicate that 25 amp/cm2 are necessary for operations that
are more than mere experiments. Other information mentions up to 65 amp/cm2 in grind-
ing hard metal and up to 240 amp/m2 for work on steel.
The feature which makes electrolytic machining a new process and differentiates
it from the well-known electrolytic polishing - which is based on the same principle
but uses a current intensity of only a few amperes per square centimeter - reported-
ly is the much higher current density. The great speed with which the process takes
place, and the fact that it can be kept under complete control, are especially
emphasized.
Since the space between the work and the wheel can be made very narrow - of the
order of magnitude of 0.01 - 0.02 mm - the current density, as mentioned, becomes
high. roreover, there is some indication that this clearance is significant for the
precision which can be obtained in the machining and for the resultant quality of
surface finish. It is emphasizeq that the local heating is very slight, so that the
surface is not damaged by scratches, crack formations, etc. and ,that practically no
wear of the wheel takes place.
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,s cathode, preferably ordinary soft carbon steel of a thickness of 0.5 to 2.0
nm is used. The lo:rer of these values is used in work on the periphery or shorten-
ing, while the higher value is best applied in grinding on the side of the wheel.
The most common thickness used seems to be about 1 mm and it is also stated that the
speed of working increases with thinner wheels. Other wheel materials, which are
occasions11,r used, are copper-plated carbon steel and cast iron. The diameter of the
wheel seems to vary between about 100 and 300 mm but is most often near 150 mm. The
peripheral speed is stated by the various sources, with considerable uniformity, to
lie between 8 and 12 misec and cannot be confused with the about twice as high peri-
pheral speeds which are used in some of the other machining methods like, for example,
those in groups 3 and 5.
As electrolyte, an aqueous solution of a metal salt or a weak acid is used.
Different sources mention more or less identical aqueous solutions of concentrated
sodium silicate, concentrated hydrochloric acid, sodium salt of weak acids, sodium
chloride, caustic soda, etc., and there seems to be agreement that waterglass gives
the best effect and the fastest working. The specific gravity of the electrolyte is
given as between 1.28 and 1.34, and it appears that increased specific gravity pro-
duces more rapid grinding.
It is pointed out as a defect of the method that the current is not limited to a
fairly small localized portion of the work, but extends over the whole of the surface
covered by the electrolyte. Since the relative current intensity, as mentioned be-
fore, is greater the less space there is between the anode (workpiece) and the cath-
ode (wheel), different current intensities will pass through different portions of
the worl:, resulting in an uneven "grinding". As a consequence, no excessive accura-
cy can be expected.
The requirements normally made in r;rinding a tool with hard metal are that the
tool, when magnified about 200 times, will not show scratches and will not have, a
surface rourimess above 50,1 in. r. m. s. The first requirement is easily met in
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electrolytic grinding, and the second can be met: although this is not always so,
some specimens showed surface finishes of the order of 5 - 154 in r. m. s.
More or less successful attempts have been made to limit the electrolytic effect
to a smaller area by using uncoated electrodes, but this complicates the conditions
considerably. However, it seems that, at proper selection of all factors concerned
not the least of these being the choice of a suitable electrolyte - surfaces with a
finish acceptable for general purposes can be obtained.
Results Obtained
This electric machining method apparently is highly suitable for the above-
mentioned tool grinding, especially where material is to be removed from large sur-
faces. On the other hand, the method is not suitable for machining of small surfaces
or for boring of small and deep holes. This is apparently due to the above-mentioned
dissimilar current passage which also has a tendency to remove corners and edges.
This is without doubt a condition to be taken into consideration in tool grinding,
where a sharp edge is desired. The edge grinding is moderate, but can be tolerated,
especially since honing of the sharp edge phase grinding is usually performed.
Another important question in this connection is the life of an electrolytically
ground tool compared to that of the same tool ground in a conventional fashion. The
available experimental data are insufficient to give a general answer to this ques-
tion and the pertaining statements are extremely divergent. American information
tends to indicate that the electrolytically ground tool has essentially the same life
as one ground by a ceramic-diamond wheel, while Russian information indicates that
electrolytically ground tools have a service life 2 to 3 times as long as normally
ground tools. An independent consideration would indicate that the correct answer
to the question lies somewhere between these two positions, since it is quite natur-
al to count on a longer life for the electrolytically ground tool, inasmuch as it is
free of the cracks and scratches which even the best, normally ground, tool cannot
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escape.
Concerning the working speed with pure electrolytic machining methods as descri
ed here there is no sufficient information available, but a comparison with other re/
lated electrical methods perhaps permits the conclusion that the grinding time is
more or less the same, possibly a few per cent less than with ordinary grinding.
The electrolytic method is not used extensively in practice; among other things
for the additional reason that it has a tendency to change into other methods. For
example, if contact takes place between the work and the wheel, which can easily- oc-
cur with the small space involved, and electric arc is formed which changes the ef-
fect to that described under electric-arc machining in group 3. Another natural var-.
iation consists in replacing the metal wheel with a diamond wheel; this results in a
purely mechanical effect, and the process changes character and must be placed in
group 5, where it will be described in more detail.
Setup Assemblies
In practice, the machining can be carried out by machines designed especially
for the purpose or by conventional grinding machines rebuilt so as to include a tank
for the electrolyte, a metal wheel, insulation between the wheel and the work, and
finally a suitable direct-current source. If absolutely necessary, built-up experi-
mental equipment can be used. It is the two last forms which up to now have the
greatest dissemination, while the first type is only seldom found described in the
literature or met in practice. If the pure electrolytic machining is abandoned, for
example if, as just mentioned, the process changes in character, then this modified
technique - which will be described in more detail in the other group - will have
available several machines of the two first types.
As a typical example, Fig.3 shows a Russian electrolytic tool grinding machine
for hard metal. The table dimensions are 450 x 160 mm with a 250 mm displacement.
The transverse movement is effected by the spindle dock and amounts to 85 mm. The
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chuck height is 180 mm and tools up to 30 x 45 x 315 mm can be ground.
As cathode, a wheel of steel or cast iron is used, with a diameter of 85 mm and
a rotational speed of 1280, 1600, and 2000 rpm corresponding to peripheral speeds of
piece are immersed in a suitable electrolyte
during grinding, while the machine is opera,.
ted by pushbuttons from the panel. The cur-
rent is supplied by a direct-current unit
built into the base. The machine works as de-
wheels, eliminated subsequent patching, and
avoided the, risk of grinding cracks and lo-
cal deformation of the hard metal. Reported-
iy although up to now only under lab-
age of the electrolyte.
oratory conditions. This was done at the
N.P.L. in Teddington, where some very small specimens are said to have been used
which cannot be produced by ordinary methods, since the structure and the surface
layer are destroyed and the measurements are misleading. In addition, the method
satisfied requirements for tolerances of ? 0.001 mm and high surface finish. The ma-
chining therefore had to be done by electrolytic means, using the arrangements shown
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A suitable electrolyte was fed from a retort along a rubber tube to a metal
cathode in a ,:lass chamber and discharged from a nozzle in this chamber onto the
workpiece. As cathode, first a copper tube was used which was later replaced - to
prevent attack by the electrolyte - by a platinum wire in the glass chamber. The
workpiece represented the anode and was simultaneously turned and passed back and
forth past the jet, in which the current density can attain 200 amp/cm2.
Fig.4 - Diagrammatic Sketch for Electrolytic Turning.
The electrolyte, size of the jet, and electrical data
must be matched to the different materials.
The movement of the workpiece must be adjusted in such a way that the anode dis-
solves the Metal evenly on all portions of the surface. The cam wheel must therefore
be especially adapted with this in view and, in this case, is heart-shaped, a design
that has given good results.
As starting material, bars 16 mm long and about WO in diameter were used.
These are screwed into a brass cartridge and locked tight. The spindle runs in two
ball bearings and a tube and is driven at a speed of 2000 rpm by a belt drive. Thus
the "cutting speed" is at maximum about 20 m/min or around 0.33 m/sec, thus being
significantly lower than in electrolytic grinding. The spindle dock, i.e., the tube,
is moved back and forth by the heart-shaped cam wheel in connection with a spring,
and is rTuided in this movement by five ball bearings.
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As electrolyte, a combination of ammonium chloride and hydrochloric acid was
used in this experiment, and to avoid corrosion the workpiece, before clamping, was
rinsed with water from another flask. This setup was combined with an optical in-
strument from which the diameter of the workpiece can be read off, with an accuracy
of 0.0001" *.
In another experiment with the same workpieces, a voltage of 70 v, and an elec-
trolyte consisting of a mixture of copper ammonium chloride and hydrochloric acid, '
the workpieces were "turned dawn" from 3.2 mm to 1.6 mm in 15 min. To achieve a tol-
erance of ? 0.0025 mm at this speed of machining, the "turning lathe" had to be stop-
ped within an accuracy of 1.5 sec. It is very probable that this can be obtained
more easily and quickly if the machining is halted by cutting off the current.
The turning can, in addition, be varied in many ways. The rough planing can be
done for example on a watchmaker's bench and the polishing as indicated here. The
speed can be changed almost continuously by changing the composition of the electro-
lyte, and the precision can be correspondingly increased by lowering the speed.
Attempts have also been made to use two flasks, each with its own electrolyte,
one for rough planing and one for slaw polishing to great surface smoothness.
If the workpiece is turned around slowly without the forward and back motion, a
necking is obtained instead of a longitudinal turning; if the process is continued
long enough, this will finally shear off. The width of the necking is about twice
as great as the nozzle.
For metallurgical purposes electrolytic turning, seems to offer advantages in
* It is not inconceivable that this electrolytic turning is based on a modification
of the principles which underlie the Danish "electrolysis pipette". This apparatus
was used for electro-polishing and has been known everywhere - even abroad - since
about 19h3. The principle in the two installation is entirely the same, only the
workpiece is moved in the "turning", while in the electro-polishing it is stationary.
(Information given the author by Prof. ')r. Tech. Knuth-Winterfeldt).
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1 several areas. Among others, this is true in the production of test specimens where
the structure and surface layer must not be ruined by mechanical machining and where
Fig. 5 - Electrolytic Turning Lathe, Schematic Representation.
Simultaneously with the rotation, the workpiece is moved back
and forth when the cylinder which carries the spindle is dis-
placed by the cam arrangement shown. The insert shows, magni-
fied 20 times: On the left, a notch turned in a workpiece of
pure iron with a diameter of lit"; in the center, the starting
material; and on the right, an electrolytically turned specimen.
there is need for great precision and mirror-finished surfaces. The industrial pos-
sibilities cannot be estimated at present, since they have not yet been tested. It
is however not out of the question that the method can be of value in some areas.
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The fact that experiments have shown that hard materials can be machined by this mot
of speaks for this assumption as does also the rapidity with which the machining can
take place, as well as the control of surface smoothness through regulation of the
speed.
Despite the applicability of the electrolytic machining method within certain
'fields, it still is the least used of the electric machining methods and has resulted
in the least production equipment on the market. If this is so, despite the fact
that the method definitely has the advantage over the other methods, because of the
relatively simple electric equipment, of being considerably cheaper, the reason pre-
sumably lies in the circumstances
mentioned under grinding, namely that it is diffi,. '
cult to obtain exact planes and surfaces of high quality with this method.
EXperiments by the N.P.L. seem to contradict this statement since there high ma-
chining precision and smooth surfaces were obtained. The comment on this is that a
different technique was used, that the experiments are of a more recent date, and
finally that the method may be better suited for turning than for grinding.
If this is the case, it will be interesting to follow future developments, and
to keep track where and how the method can be adapted to industrial purposes.
Literature: Bib1.5, 7, 8, 12, 14, 15, 16, 20, 32, 36, 42, 45d, 48, 59, and 60.
Patents: Swedish No. 76 026, German Nos. 564 081 and 847 390, English No. 335 003.
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;.0
FLECTRIC SPARK MACHINING
THE DEVELOPMENT IN RUSSIA
It is apparently in this machining method that the greatest development has tak-
en place. Young as the technique is, a comprehensive periodical literature exists
on the subject, and the method is also rather jlly treated in the patent literature.
Apparently, the development is carried on - at any rate in the western part of the
country - by individual industrial concerns; it is also typical that this has result-
ed in there being put on the market several machines ready for production which are
at the disposal of the industry for making use of the advantages offered by this
method.
Since Russia, the U.S.A., England, and Germany have individually made important
contributions to the clarification of the subject, not only by theoretical consider-
ations but also by descriptions of the machines and equipment produced, it is hardly
possible or useful - also because of the great number of sources - to give a general-
ly valid cross-sectional view over this large volume of data where divergent concepts
still exist. It will be more useful to summarize the development in the individual
countries and then draw conclusions on this basis. This will be done in the follow-
ing Section where, for practical reasons, we begin with the Russian development, be-
cause the available literature contains chiefly theoretical considerations, while
the sources from the other countries have, in many cases, placed great weight on the
practical problems.
The Development in Russia
As far as can be seen, the point of departure here is apparently the discovery
by ki.Ousev, which is described for example in the Swedish patent No. 76 026 of 1929.
By means of the device described there, round or arbitrarily shaped holes are pro-
duced in the metal by forcing an insulating tube of the desired shape against the
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workpiece. The tube is surrounded by a gasket abutting the workpiece; inside the in-
sulated tube a shorter hollow electrode
Fig.6 -
W.Gusev's Swedish
Patent,
No.76 026 of 1929, Showing the
Head of the Tool, with the Insu-
lating Tube (1) pressed against
the Workpiece. Between (1) and
the cathode (3) an electrolyte is
circulated which is prevented from
leaking by the gasket (2). The
current between the anode and the
only included here
cathode produces a hole with a bawl-
shaped base which corresponds to
the 'outer dimensions of (1).
is installed. Through the resultant space
an electrolyte, e.g. a 3 - V, hydrochloric
acid solution is fed; if the workpiece is
connected to the positive pole of a current
source while the hollow electrode is connec-
ted to the negative pole, a current will be ?
produced, resulting in an arc which will dis-
integrate the metal; the loose particles are
flushed away by the electrolyte.
The hole produced will exactly corre-
spond to the outer dimensions of the insulat-
ing tube. There seems to be a rather great
distance between the end of the hollow elec-
trode and the workpiece. The "frilling
speed" is determined by the current deAsity
and the speed of circulation of the electro-
lyte. At a voltage of 110 v; a current den-
sity of 150,- 300, amp/cm2, and a speed of
circulation of 300 - 600 m/min, the patent
claims state that a satisfactory drilling
speed is attained, but without indicating
what is meant by this.
This is not a spark method, but belongs
in group 1, or possibly in group 3, and is
in order to give the log-
ical development, the next step of which is
the work by Natalia and Paris Lazarenko, de-
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scribed in the Russian technical journals in the period after 1946 and patented in
England, France, and Switzerland.
Fig.7 - Basic Principle of the Electric Spark Method. The workpiece (1)
and the tool (2) are connected to the positive and negative poles of a
direct-current source, while the current is regulated by the resistance R
and is read off the ammeter A1'
Parallel to this, a variable capacitor C
is connected, and the discharge current is read off A2. Above the schemat-
ic, a.diagram, with the discharge current as abscissa and the capacitance
as ordinate, shows the limiting curve C between the area A where spark dis-
charge is produced, and the area C where arc discharge is produced.
(Lazarenko: Swiss patent No. 257 460
' The basic idea of Lazarenkots discovery - and thus of all forms known up to now
for electric spark machining - is given in Figs.7 and 8, which are taken from the
Swiss patent No. 257 468. As shown in Fig.7, at the bottom, an electrode (2), suit-
ably shaped as the tool, is connected with the negative pole of a direct-current
source whose positive pole is connected across the variable resistance R to the work-
piece (1), while at the same time the current intensity can be read off the ammeter
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A1.
the
-^
Parallel to the workpiece and tool a variable capacitance C is connected, and
discharge current in the
circuit can be read off the instrument A2.
According to the patent specifications,
changes in this circuit will produce electric
discharges between the workpiece and tool,
during which particles are torn from the
workpiece or tool, so that a metal transfer
is initiated. If the discharge takes place
through an electric arc the direction of
transfer is from cathode to anode, while if
the circuit is made to discharge with elec-
tric sparks, the metal transfer proceeds in
the opposite direction, i.e., from anode to
cathode;
It has been found by experiment that,
for a definite value of the electric field
intensity between the electrodes, the direc-
tion of the transfer will change and that
this value is dependent (among other things)
on the chemical properties of the electrodes.
It is further indicated that, by changing
Fig.8 - "Drilling Machine" using the
Spark Method. The workpiece (1) is
immersed in a vessel (8) with fluid
and connected to the plus pole. (9)
is an insulating plate. The tool (2)
is set in a holder (4) and can be vi-
brated by the solenoid (5), opening
and closing the circuit.
(Lazarenko, Swiss patent No.257 468)
the constants of the circuits, it is possible to determine whether spark or arc dis-
charge is produced and thus to change the direction of metal transfer.
In the main, there are three factors which are decisive for the conditions,
namely the type and composition of the electrodes, the medium through which the dis-
charge takes place as well as its condition, and finally the magnitude of the elec-
tric parameters. It is further indicated that the optimum effect is obtained at the
instant of transition between spark and arc discharge.
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Briefly stated, the electric spark machining method consists in accumulating a
high electric energy in the circuit and then discharging this energy in the form of
a spark which, for a brief instant, touches the place on the workpiece which it is
desired to machine. The high density of energy applied to this spot causes a break-
down of the surface, so that its particles are torn loose and carried away.
Figure 7 at the top shows the relationship between the discharge current A and
the capacitance, with the curve C indicating the limiting line between the area A
where spark discharge takes place, and the area B where there is an arc discharge.
The working should therefore be done in the area A, since the heat generation
is less with spark formation than with arc formation, so that the risk of ruining the
workpiece by heat, or structural changes, etc. is less here. In addition, the spark
is easier to direct against a single exact point than the arc, which increases the
precision of the work; finally, the spark gives a more concentrated energy discharge
than the arc and thus produces an instantaneous and powerful effect.
In practice, this machining method is carried out by inserting the workpiece and
the tool as electrodes in an electric oscillatory circuit which gives a spark dis-
charge with a suitable predetermined frequency. When the circuit is closed, parti-
cles are dislodged from the positive workpiece and transferred toward the negative
tool. If the operation is stopped here, the particles will continue their travel
and precipitate on the tool, changing its form and size. The particle transfer must
therefore be stopped. This is most easily done by filling the spark gap with a fluid
which, according to the patent specifications, can be stationary or flawing and con-
sist either of a dielectric material or a passive electrolyte.
Machines for Electric Spark Machining
Mechanically, the method is carried out by machines which greatly resemble small
bench drilling machines. However, there are no rotating parts, since the only neces-
sary movement is a suitable perpendicular potential which can maintain a constant
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spark gap under the workpiece.
Such a 'drilling machine" is shown, in principle, in Fig.8. The workpiece is
mounted in a vessel (8) filled with liquid and connected with the positive pole of a
direct-current source. The vessel is insulated by a plate (9) from the tension
plate (3), which is adjustable in all three directions. The tool (2) (here intended
for cutting) fits in the holder (4) and is connected to the electric circuit as
shown, while the resistance R is omitted here, so that the capacitor charges and dis:-
charges directly. This is accomplished by a vibratory motion which is imparted to
the holder by means of the solenoid (5), which causes a rapid opening and closing of
the contact (6).
Lazarenko also mentioned application of the method with rotating tools. He used
wheels of 0.5 to 1 mm thickness, approached to the workpiece until formation of a
spark, while water or a dielectric fluid was run over the point of cutting.
In a paper (Bibi. 2), Lazarenko describes complete drilling machines and a grind-
ing machine, operating on these principles. In addition to the above description, it
should be mentioned that the vibration unit consisted of a cam wheel which, driven
over belts by a motor, raised the spindle 1 mm after which it was returned by a
spring in the hollow spindle. The tool, with which arbitrarily shaped holes could be
"drilled", was made of brass. The partial voltage was created by a special variable
motor through a double worm gear, with a change-over ratio of 1:360. Besides this,
the voltage could be regulated through a special gear with the three transmission ra-
tios of 3:1, 1:1, and 1:3. Finplly, the machine was also equipped with manual
tension.
The electric system was built into a separate panel and consisted of a rheostat
which permitted a variation in current intensity from 1.3 to 30 amp, and of a capac-
itor C which could be adjusted between 2 and 500 u f.
As liquid, oil or kerosene was used, to a'depth of at least 100 mm over the
workpiece.
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After the resistance and capacitance lifers adjusted to the job to be done, the
only point that needed watching was the voltage, since a constant spark gap had to
Fig.9 - Grinding Machine for Turning Steel, etc. The grinding is
done with a rotating metal wheel. Speed 500 rpm. Good results
were obtained without defects in the tool. Time required unknown.
(According to Lazarenko)
be maintained. This control is exerted over the ammeter in the oscillatory circuit.
If the voltage is reduced, the current drops from the determined value to zero, while
an increase in the voltage causes the current intensity to rise to the maximum. The
control can therefore be effected automatically by installing a relay- into the volt-
age circuit and adjusting it according to the desired current intensity. Then, on
any variation in the circuit, the relay will cut the voltage motor in or out. The
power is 2.5 kw, including 0.25 kw for the vibrator and 0.25 kw for the partial
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voltages.
Figure 9 shows a grinding machine tool for turning steel etc. The machine has
three translational and three rotational movements for adjustment so that right and
left-handed tools can be ground with any angles desired. Tools up to 20 x 30 mm
*shaft cross section can be fixed in a suitable holder.
N'..N.\\\\*NN ? ????
Fig.10 - General Setup of Experimental Apparatus for "Grinding" of Tools
by:the Electric Spark Method. B is motor-alternator to supply the unit
with direct current. C denotes the tool that is to be "ground" and D the
"grinding wheel". This is driven by an AC motor E, while the pump F
circulates the "coolant". (According to V.A.Krivoukhova)
The "grinding" was done with a metal wheel rotating at 500 rpm or, in some cases,
with variable resistances and capacitances were used so that current intensities be-
tween 0.1 and 30 amp and capacitors between 2 and 400 utif.
It is-asserted that good results were obtained, with flat surfaces without
scratches or other defects.
Results of the Experiments
As background for this theoretical work and the first attempts to construct ma-
chines for the practical application of the method, the Russian professor V.A.Kri-
voukhova investigated grinding methods and precision grinding with the help of the
spark method. The purpose was to find out the requirements for speedy and effective
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grinding of hard metal, resulting in excellent surface finish while preserving the
characteristics of the hard metal during the grinding process.
In Krivoukhovats experimental setup, the work was done with rotating wheels whose
peripheral speed could assume the values of 5, 10, and 15 m/sec (980, 1970, and 2950
ft/min). Lazarenkots electric circuit was used, with the voltage being regulated be-
Fig.11 - The Modified Spindle in the EXperimental Machine (a Standard
Grinding Machine). The current is supplied to the holder A and the
ring B and is fed from there to the "grinding wheel" D. E and F are
insulating wheels. (From V.A.Krivoukhova)
tween 20 and 220 volts, the current changed between 0.1 and 150 amp, and the capaci-
tance in the oscillatory circuit varied between 1 and 400 upf.
As wheel materials, cast iron, steel, copper graphite, or aluminum were used.
Experiments were made on hard metals of types T 15 K (79% NC, 16% TiC, and 6% Co)
and DK8 (92:'J `.IC and 8% Co). As medium in the spark gap various oil emulsions and air
were used.
The setup for the experiment is shown in Fig.10, where a suitable rebuilt grind-
ing machine of otherwise conventional design is used. The electrode wheels are
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mounted on the modified spindle, and the entire arrangement is shown in Fig.].].. A
motor-alternator B (Fig.10) furnishes the required direct current. The tool is de-
noted by C and the "grinding wheel" by D. This is driven by an AC motor E. The
parts are hooked into an oscillatory circuit, as described before. The pump F cir-
culates the stream of liquid.
Fig.12 - The Quantity of Material P
grImin
which is "ground away" placed
in Relation to the Current Intensity
at various Peripheral Speeds of the
Wheel. Materials, hard metal, grade
T 15 K. Tension 20 volts.
(From V.A.Krivoukhova)
The details of the spindle are shown in
Fig.11. The current is conducted from a
brush holder A to the ring B and from there
through C to the grinding wheel D. B and D
are insulated from the spindle by textolite
rings E and F. This change of the spindle
represents the greater part of the modifica-
tion to permit use of a standard grinding ma-
chine for electric spark machining.
With this arrangement, a series of ex-
periments was made to determine the influence
of the individual factors. For example,
Fig.12 shows the amount of material removed P,
measured in gram/min, as a function of the
current intensity J amp, at a constant voltage of 20 v disregarding the capacitance.
The hard-metal quality used is T 15 K.
The greatest efficiency is obtained at a speed of 10 m/sec (1970 ft/min), and
practical experience has shown that an optimum value is reached at a wheel speed of
12 - 15 mi/sec. If softer materials than hard metal are ground and a law current in-
tensity of about 20 amp is applied, peripheral speeds of 25 - 30 m/sec (4900 - 5900
ft/min) are recommended. These latter speeds must be considered as the absolute max-
imum in fine-grinding.
Throughout the experiments, an attempt was made to determine the optimum elec-
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tric data; from more than 2600 individual experiments it was found for example that,
.for the hard metal BK 8, the voltage should be 20 - 25 v. Figure 13 this the rela?
tionship between P (gm/min) and I (amp), at different values of the capacitance C,
Fig.13 - At a voltage of 20 v and a
Peripheral Speed of 10 m/sec the Re-
lation between the Material Removed
P in gm/min and the Current Inten-
sity is Plotted at various Values of
the Capacitance G, for the Hard
Metal BK 8
(According to Krivoukhova)
es are plotted for
-
for which which many curves are included. The whe-
el speed is constant, 10 m/sec, the voltage
20 v, and the hard metal used, BK 8.
In Fig.14 the relation between P (gra/min,
and the capacitance C (1.11) is plotted for
BK 8 at the two wheel speeds of Sand 10
m/sec (980 and 1970 ft/min). The voltage is
220 v. These two odd curves indicate that
the effect increases with increasing capaci-
tance, and also that, at constant capacitance,
the effect increases with increasing peri-
pheral speed.
The influence of the capacitance on the
surface finish was also investigated and is
plotted in Fig.15. The height of the rough-
nesses was measured (unit unknown) and corre-
lated with the capacitance ( p.f). Four curv-
various values of the voltage V and the current intensity A. In
these experiments a cast iron wheel was used, and a fluid known as "Aviation Oil
Mark M.S.".
It is assumed that the best surfaces are produced with a current intensity from
3 to 5 amp and capacitances from 1 to 5 if, at voltages of about 20 v and wheel
speeds of 5000 - 6000 ft/min (25 - 30 m,/sec).
The ground surface of the hard metal was examined metallographically and showed
no scratches or structural changes. This means there is no danger of damaging the.
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material, thus making the technique suitable for production purposes.
A better criterion for the finial: can be obtained in experiments on the life ofi
tools ground by the spark method and conventional grinding nethods. Several materi-
als, including steel and cast iron, were machined under the following conditions:
rough planing t x s 1.5 x 0.47 mm; polishing t x
s 1.0 x 0.21 mm; and fine poi,. :
Fig.14 - Relation between the Amount
of Material Removed P (gm/min) and
the Capacitance C (u f), Plotted for
the Hard Metal BK 8 at Peripheral
Speeds of 5 and 10 m/sec (980 and
1970 ft/min) The voltage is con-
stant at 220 volts
(According to V.A.Krivoukhova)
Fig.15 - The Four Curves show the
Variation in Surface Smoothness with
the Capacitance C. The curves are
marked with the voltage and current
intensities used. The symbol H is
the maximum height of the roughness,
in a unit not indicated
(According to V.A.Krivoukhova)
ishing t x s = 0.50 x 0.16 mm; t denotes the depth of cut and s the voltage.
Figure 16 shows an example of such a life curve, where X denotes a tool ground
by the spark method and Y a normally ground tool. The spark method here resulted in
- 10'7) longer life, and it is claimed that industrial experiments on a larger scale
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gave even more pronounced differences, since an up to 50% longer life is mentioned
for spark-ground tools.
Finally, Fig.17 shows the relation between the surface smoothness H of the work-
piece and the cutting speed in IA/min, for a tool X ground by the spark method and an-
other Y ground normally. Here too the spark method is superior.
1 1
t II ; ? ?
I ? ? ? ? ? ? 1. ? 1
. f.-
: .
rt., r",
Fig.16 - Life Curve for a Tool X
Ground by the Spark Method and a
Tool Y Ground by Conventional Meth-
ods. X has a 5 - l0 longer life
than Y
(According to V.A.Krivoukhova)
Fig.17 - Relation Between the Surface
Smoothness H of a Workpiece and the
Cutting Speed in m/min, for a Tool X
Ground by the Spark Method and a Tool
Y Ground Normally
(According to V.A.Krivoukhova)
Since the hard metal is brittle, the strong pressure against a grinding wheel
may cause crack formation, especially if the wheel pitches somewhat. In addition,
harmful heat effects may occur. In the electric spark' method, tool and wheel do not
touch each other at all, and these risks are entirely avoided. The method is there-
fore especially suitable for brittle materials.
General Pointers
In industrial application of the method, it can be recommended that the follow-
ing general lines be followed:
As "coolant" it is advisable to use a well-filtered "Aviation Oil" or an emul-
* of "Phulsol" boiled with 30 - 501 water. Deposits of dirt, dust,
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and metal particles or other impurities should be removed from time to time. The
flow of liquid to the cutting point should be ample, but fully controlled.
The rotating grinding wheel is preferably made of tempered cast iron; if pro-
files are to be shaped it is best for it to be made of steel or copper. The grinding
surface should be kept even and smooth and in normal grinding the tool should be mov-
ed back and forth so that the grinding is not done in the same place all the time.
The direction of rotation should always be such that "grinding" is done toward ?
the edge; checking that the wheel is under negative voltage is done by testing wheth-
er there is contact before the work is begun. When the side of the wheel is used,
the spark gap should be kept below 0.01 mm, while with "grinding" toward the periph-
ery this can be increased to 0.03 mm. The wheel should run freely and without notice-
able vibrations.
The tool should have good electric contact with the positive direct-current
source, and all contact surfaces must therefore be absolutely clean.
The total time for grinding a hard metal tool (turning steel) with a shaft cross
section 20 x 30 mm by the spark method is given as 6 min, of which the machining time
takes up 3.5 min. For comparison, the total time for standard grinding of the same
tool is given as 4.5 to 5 min. In coarse grinding of the various surfaces, a voltage
of 20 - 30 v is recommended at a current intensity of 120 - 250 amp and a capacitance
of 300 - 500 f. In fine-grinding, values of 15 - 20 v, 3 - 7 amp, and 1 - 20 uf
can suitable be chosen. The speed of the wheel in coarse-grinding is between 12 and
14 Ir/sec and in fine-grinding, between 25 and 30 m/sec.
It is stated that the cost of conventional grinding is 70 - 100% greater than
in the spark method, and the new technique is regarded as especially suitable for
"grinding" of forming steel, thread steel, millers, and cogwheel tools, or wherever
a great surface smoothness is desired.
Literature: Bib1.2, 25, 42, 49, 50, 59, AO, and 61.
35
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!Patents:
Sweden No. 76 026, England Nos.11335 003 and 637 793, FranCO No. 937 762,
, \_and Switzerland No. 257 468.
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THE DEVELOPMENT IN THE U.S.A.
The American development of the spark method seem to have started around 1920
,and originated with forme of machining in which electric energy was used for produc-
i .ing higher temperatures. Holz (U.S.A. 1 333 311; 1918) used a friction saw without
'teeth for connecting the workpiece and the wheel, each with its pole connected to an
'4 alternating-current source; this resulted in quicker sawing since the heat of trio-
tion was increased by the heat developed by the electric current at the point of
"4 contact.
Similarly, Clawson discovered (U.S.A. 1 620 519; 1922) that an extra grinding
effect would be produced between two conducting surfaces which moved over each other,
when each of them was connected with its pole to a source of current.
Grumpelt (U.S.A. 1 556 325; 1924 and 1 701 919; 1925) suggested a friction saw
with toothed edge, which was shaped like a cam wheel in that the many small teeth
were supplemented by one large tooth. The workpiece and the wheel were attached to
a cxusrent source and the actual friction effect was increased by the heat from the
electric arc formed between the workpiece and the wheel, whose length increased con-
stantly towards the large tooth of the wheel. Strobel/s patent (U.S.A. Re 20 035;
1922) is in line with this.
None of the investigations mentioned concern spark machining, but they are in-
cluded here to give the general background. They seem, on the other hand, to be the
direct point of departure for arc machining, and several of them have essential
points in common with it, without however being identical. In addition, the arc
method can actually be regarded as a step on the road to spark machining.
The literature search had meanwhile advanced to the middle of the Thirties with-
out finding traces of the spark method, and-the first study which the author succeed-
ed, in finding which even faintly suggests spark machining is Clark's patent No. 2
308 860 of 1940. This concerns a tool for drilling rock, concrete, etc., and is
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Fig.18 - Tool for Drilling Bedrock, Concrete, etc. Across the
capacitor (13) a voltage is built up which, at a certain strength
determined by the "timer" (10), is discharged through the mercury-
vapor tube (16) and the mining drill (20), so that an electric spark
jumps between the tungsten electrode (26) and the rock (21). This
is heated locally, and the resultant thermal tensions split the sur-
face layer. First American experiment found in spark machining.
(r.S.Clark, U.S.A. patent No. 2 308 860)
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shown in Fig.18. In this arrangement, a power source between (5) and (6) is connect-
ed with a transformer (7) and, at the same time, provides a 'timer" (10) with energy
across (8) and (9). Across the secondary of the transformer a capacitor (13) is con-
nected. One side of the capacitor is connected with the ground, while the other side
leads across (15) to a mercury-vapor tube (16) and across (14) to the "timer". The
mercury-vapor tube is connected over (19) with the hollow mining drill (20). The
"timer" is connected with three grids in the tube (16) across (22), (23), and (24). '
The mining drill consists of a metal tube whose lower end carries a tungsten elec-
trode (26), whose distance from the ground is determined by the shoe (27), mounted
to an insulated plug (28) with the air passages (29) and (30).
During operation, the capacitor is charged, and when the voltage has reached a
certain point, determined by the adjustment of the "timer", the mercury vapor is
ionized and the circuit from ground across the capacitor, mining drill, and back to
ground is closed; this releases a spark discharge between the electrode (26) and the
bedrock (21). The rapid discharges and the relatively high energy which is discharg-
ed will cause considerable heating of the rock surface, and since bedrock has only
a law level of conductivity such high thermal tensions will form rapidly that the
layer breaks up.
After this time, the development seems more rapid. Starting with the beginning
of the Forties, a series of patent specifications appears - Harding, Holfelder, rc-
Kechnie - which, from the technique of the Twenties and Thirties via the arc method,
finally arrive (around 1950) at the pure spark method. Primarily, the development
seems to have been carried forward by the Elox Corporation, richigan, and sometime
later by the Hethod X Company, Philadelphia, the daughter company of Firth Sterling
Steel and Carbide Corporation. Others also have taken part in the work, such as the
National Pureau of Standards which solved a single concrete problem, and staff mem-
bers of the Carnegie Institute of Technology, who have collaborated with ethod X;
finally, also other firms have become interested in the matter in more recent times.
39
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-
0 . -
'
:11 iSnark,Zrillinir of Diamonds
,)
The drilling of small holes in diamonds for use as draw dies, etc. is the field
worked most intensively during recent yowls. In the course of thist_application-of_
electric methods
-4
011 -1 ?
_
?
was introduced, particularly spark machining, and it appears that
the technique gradually developed in this
field has promoted spark machining in
general.
V.O.Allen and R.A.Macintosh suggest in
U.S.A. patent No. 2 300 855 of 1941 to drill
holes with a diameter of 0.05 ami and less
if
diamonds, with the help of electric sparks.
Their method is a further development of the
idea in German patent No. 672 832 (Bergmann
et al, 1937; see also "Development in Ger-
many"), and the setup is shown in Fig.19.
The diamond is mounted to a holding
plate of a conductive material and immersed
into an electrolyte. The electrode A, which
ile4w/410' oasts
Fig.19 - Setup for Spark Drilling
Of Diamond Nozzles. Hole diameters
of 0.05 mm and less. The electric
equipment consists of a vacuum-tube
generator which furnishes high-
frequency currents (6000 - 30,000
kc) to the spark gap. Working vol-
tage up to 5000 v
(From U.S.A. pat. No. 2 300 855)
preferably should be of tungsten,
so that its diameter is less than
red hole and it rests against the
is tapered
the desi-
surface of
the diamond with a light pressure. The set-
up operates on high-frequency alternating
current which is provided by the Hartley generator shown. The frequencies are norm-
ally between 6000 and 30,000 kc, and voltages up to 5000 v are used.
The surface of the diamond is coated with a thin film of electrolyte through
which the spark strikes. This film can be replaced by a stream of humid air blowing
through the spark gap (see Fig.20). It is suggested that gases be used which are
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__readily ionized and as a container heavy metal ions, since this promotes the machin-
It is maintained that the use of high frequencies and high voltage gives high
_
drilling speeds. It is said that, under the conditions mentioned, holes can be dril
led in a diamond of 1.30 in thickness in 2 - 3 hours, while this would take days unde
the conditions of the German patent mentioned.
Fig.20 - Increase in Machining Speed, as Claimed, by Replacing Immersion
of the Diamond in an Electrolyte by Blowing a Stream of Readily Ionizable
Moist Air or Gas through the Spark Gap
(From U.S.A. patent No. 2 300 855)
In U.S.A. patent No. 2 377 159 of 190, J.Kurtz at al begin with the same funda-
mental idea. Diamond nozzles are normally bored from two sides, and it is now pro-
posed to eliminate difficulties by making the two holes coincide exactly. This is
done by first plane-grinding the diamond in the conventional way and then centering
it on one side. With this preliminary drilling as point of departure, the specimen
is spark-drilled to a depth of a little over half the thickness of the diamond, so
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that a conical hole results. The mechanical arrangement is, on the whole, analogous
to that shown in Fig.19, but the tungsten electrode is made to rotate. As electro-
lyte, nitric acid or sulfuric acid is mentioned. High-frequency currents of 25 kc or
more are also used here. The design is for about 100 watts, and machining is done
at voltages of 350 - 1500 v.
After the one side has been drilled, the
other side is centered carefully (in the reg-
ular manner), and a hole is drilled in the
same position as the first, but not so deep
that the holes come together. The diamond is
Fig.21 - Device for Spark Drilling
of Small Holes in Diamond Draw
Plates. The diamond rests on a
brass block and the needle elec-
trode, representing the "drill", is
held with a light pressure against
the surface of the diamond. The
machining is done at high voltages
(General Electric Co.)
rangement similar to the one used is
drilling are given in Fig.22.
then dried carefully and placed between two
tungsten electrodes resting on the conical
walls of the holes. After this the diamond
is drilled through, with a cylindrical hole
joining the two conical drillings by letting
a high-frequency current with a voltage of
10,000 to 30,000 v pass between the elec-
trodes in gas.
It is said that this method permits dril-
ling holes as small as 0.006 mm.
The National Bureau of Standards devel-
oped a method for producing quite small holes
- under 0.35 mm - in draw plates of diamonds
for drawing hard thread (chromium-nickel,
carbon steel, etc.) where great roundness
and small tolerance are required. An ar-
shown in Fig.21, and ten different steps in the
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The first step in the drilling is making a hole of 0.35 - 0.50 ram depth. An
electric circuit similar to Lazarenkofs is used, only the capacitor is charged to a
high voltage before it is allowed to discharge
1104 SS No
?1
sperfoul AND marl IMI1.TAA1
sAALLNI4
NIS* VoLtA?t
INICNANICAL CONIN?
FINAL NISH VIILFASI 1011.1.11141
?
14
CLICTNOLYTiC NICK SIAILLINS
%MN MaCI
'MAL SOMAS
011.LL11141 MTN
PIACI
(LICINOL114C INIiLLIII? WON
siNes ?
IA
jvl
FINISONNS ANIS P?USNINS
through the point of a needle-shaped
electrode, which is held in con-
tact with the surface of the draw
plate. The spark jumps from the
point of the needle over the stir-
face of the diamond to a brass
block on which it rests. The
drilling speed rises with the
electric energy to an upper Jimit
where the diamond will become dam-
aged; however, there is warning
beforehand when the needle whose
diameter is about 0.5 mm, becomes
red hot.
The secondary cone is drilled
by law voltage sparks which are
discharged into an electrolytic
solution. A platinum-iridium
needle is used, which is lowered
till it touches the botton of the
primary cone and rests against it
with a light pressure (about 0.2
gm). Another electrode is immer-
sed in the electrolyte, at a lit-
tle distance. If low voltage -
spark jumps from the point of the
Fig.22 - Drilling of Draw Plates Proceeds in
the Ten Steps Shown and is Done by a Combina-
tion of Spark Drilling, Mechanical Drilling,
and Electrolytic Drilling. This method is
said to have resulted in savings of over 100
hours per draw plate
(National Bureau of Standards)
about 90 v - is applied between the electrodes, a
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0_
?1,needle, and a conical hole is formed direcitly under the point. The hole io even and
_
!-
!smooth and its form and conicity can be controlled through proper choice of-the_elec.
!trolyte; the size depends on the pressure :of the needle against the bottom of the
hole.
10_1
- 1tric operations,
12-1
_Jcates that this method,
_.;
The entire operation is done in ten stepe which, besides the two mentioned elec
includes pure mechanical drilling (see Fig.2).
in comparison to the
The report Jodi-
ones ordinarily used, saves about 100
machining hours, and that draw plates drilled in this way, compared with others by
photoelastic methods, are demonstratably free of stressed surface layer, which other-
wise reduces the life and leads to destruction.
.$
Elox Machines
The appearance of equipment for spark machining is due not only on the above-
mentioned research but undoubtedly also to the observations which were made in elec-
tric etching of metal surfaces, where the wear was observed which occurred each time
a spark jumped at the point of contact.
The first machines built - placed on the market in 1943 - also used the same
electric alternating-current circuit as the etching equipment. Such a piece of equip-
ment was developed about 1942 by Waidler and Holfelder at Curtiss-Wright Corp. and
is described in "Wings" of July 1942. The machining voltage was 110 v, and. as an
electrode a copper tube was used, through which air was passed to entrain the parti-
cles. The first equipment of this construction is shown on the left in Fig.23,
while the right side shows a diagrammatic representation.
The equipment contains a transformer producing a secondary voltage of 3 - 15 v,
whose one side is attached to the workpiece, while the other pole is connected to a
cooling tube, coiled around the magnetic electrode holder. During the machining,
the electrode will therefore vibrate and thus open and close the circuit, thus pro-
ducing the spark.
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These first types were difficult to control and worked slowly; in addition, the
electrodes were worn out in About the same time as the hole was bored. They were
?
C034.ANT OUT.-
Fig.23 ? The First Spark Machining Equipment with Electric Alternating
Current Circuit of the Type Used with Electric Etching. Put on the
Market in 1943. The equipment worked slaray, had a very large electrode
consumption, and was difficult to control. To the right is seen one of
the earliest applied principles for a device for removing broken frag?
ments. It works with 3 - 15 v alternating current from a grid transfor?
mer. One side of the secondary winding is attached to the wwkpiece
and the other side to the tube for the coolant and, through it, to the
electrode. The cooling tube is coiled around the electrode holder,
which is nagnetic. This imparts a vibrating motion to the electrode,
which closes and opens the circuit thus effecting the spark formation.
originally built to salvage parts in which taps, rivals, drills, etc. had become
stuck and which were too expensive to discard for this reason. From this came the
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name udieintegrator".
The machinery was further improved when MeKechnie abandoned the alternating cur-
rent and caused a direct current to discharge across a spark gap of a certain length,
filled with liquid, electrolyte, or dielectric material (see Fig.24). The left-hand
portion shows an Elox machine, model 11-10, put on the market in 1950 and working with
electrodes of 36-inch length, which, aside from up and down displacement, can also
rotate in both directions. The machine is calculated for disintegration and has a
servo-controlled voltage which maintains a definite spark length.
The top right of the figure shows rectification of the current, without the prin-
ciple in Fig.23 being otherwise altered. The bottom of the diagram, at the left
shows the transfer of the principle to "grinding" or "sawing" and, at the right, a
servo control which, by the changes in the mean voltage of the spark, moves the elec-
trode and the workpiece closer together or farther apart, so that the spark gap is
kept constant.
According to this system, 'klrillingu can be done considerably faster than with
the earlier one, and since the electrode is now negative, excessive wear is avoided.
The model H-10 "drills" holes down to about 1 mm, and at that size a hardened steel
plate of 1 inch thickness can be drilled through in 4 - 5 min. Similarly, it is sta-
5
ted that a hole is "drilled" in about 10 min.
16"
The holes are about 0.15 FM larger than the electrode and the precision, for
the rest, depends on the speed. The possibilities of variation are great, and as a
curoisity is can be mentioned that a special machine has been built for deep-hole
drilling which operates with a head on each end for drilling from two sides. Holes
are "drilled" by this from 2 mm and less to a depth of more than 700 mm.
The development described here as well as the pertaining machinery, originated
with the Elox Corp. and can hardl: be considered a.pure spark method, since the ma-
chines lack the oscillatory circuit which otherwise characterizes all spark machin-
ing. Actually, the method is intermediate between spark and arc machining and is
4.6
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TO RECTIrIER
CIRCUIT AS
IN DIAGRAM
&ROVE
Fig.24 - On the Left an =lax Machine, rodel M-10, Put on the Market in 1950 and ,
Intended for Etractin-; Broken raps, 3rilLin: of Holes, etc. It operated with
36" long tube-shaped electrodes and has a servo-controlled tension mechanism to
maintain a constant spark gap. The electrode holder can rotate both ways as in
a drill press, whose standard parts are incorporated in the machine. The right
top shows the electric wiring diagram of the machine; note that now it is oper-
ated b:; arect current. The left bottom shows the principle uscl in sawing,
etc., and to the right the servo circuit, which records the
avera,:e tension of the spark and causes variations in it. 'to affect the pressure,
so that a constant spark gap is maintained.
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characteristically also known as ?intermittent are.
Accordingly, spark machining is a method for removing metal from the workpiece
without physical contact between it and the tool. The principle consists in causing
carefully controlled electric sparks to jump between an electrode serving as the
tool and the spot on the workpiece to be machined. The method can be applied to all
types of a machining such as drilling, thread-cutting, turning, etc., but for the
present it is used mostly for the first of these types.
Up to now, the methods have been considered to be slower than normal and only
in the most recent years - presumably from around 1952 - has the technique been so
developed that this situation changed.
In practice, the spark method has been used in extracting cracked taps, rivals,
etc., drilling irregular holes, contour machining, grinding tools, and it should be
mentioned that the economical area of application will increase greatly in the fut-
ure. The materials machined are therefore those of the hard type which can be ma-
chined only with difficulty or not at all in the desired form by other methods. As
typical examples may be mentioned tempered tool steel, hard metal, alnico, chrome-
nickel alloys, manganese steel, stellite, hardened cast iron, tantung, etc., but al-
.so softer materials like copper, copper alloys, aluminum alloys, lead, zinc, etc.
can be machined by this method.
The spark method is thus based on the realization that it is possible, by caus-
ing a spark to jump between tool and workpiece, to remove from the latter particles
of a predetermined size and in the desired quantity, while the exact dimensions of
the workpiece are accurately maintained, resulting in great smoothness of the sur-
face. The control of the voltage and discharge current, the spark time, and the
time intervals between the different sparks, the frequency and other factors deter-
mine the machining speed, the precision, and the surface smoothness.
Since these electric data can be varied inside wide limits, the particles torn
from the workpiece can be from atom size up to a pinhead. The bombarding materj:al
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is not affected, since the spark effect of the circulating liquid is strictly limit-
ed to heating up only a very small localized area of the workpiece. This is heated
to very high temperatures, which are in fact indicated up to 10000P to 150000 C.
Normally very little material is removed from the tool electrode in relation to
that which is machined off the workpiece, when only direct current is used, with the
workpiece connected to the positive pole. The abraded material is continuously
rinsed away from the spark zone by the fluid flow as well as by the electric forces.
There is no physical contact between the workpiece, and the tool and since the spark
gap can be made quite small, machining can be done with voltages as low as 10 - 15v.
Theory
There have been many theories on the mode of operation of spark machining.
The following theory - as well as the preceding discussion - was originated by
the Elox Corp. It is assumed that the negative tool electrode normally has a number
of free electrons which arrive at and depart from the surface. As soon as the volt-
age between the electrodes has reached a certain magnitude, a stream of electrons
will jump from the tool to the workpiece; these electrons will strike the surface of
the workpiece and cause the atoms in this area to increase in activity, so that the
temperature of the solid surface increases until the material is first fluidized and
then vaporizes, with great increase in volume.
When this increase in volume occurs suddenly, such high voltages are generated
that particles are torn from the surface. The torn-off particles have a positive
charge and are ionized, and will concentrate at the negatively charged tool. There-
fore, a stream of liquid is fed through the spark gap, which is to flush away the
particles and prevent them from settling on the tool. Nevertheless, some particles
will reach the tool and gradually erode its surface. The ihdividual steps in this
process are illustrated in Fig.25.
The tool electrode does not need to rotate, but if this is possible, it should
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be mentioned that, in such a case, high precision is achieved, making it difficult
for the particles to adhere to the surface and short-circuit the spark gap.
Viewed theoretically, there is no limit to the speed at which the machining can
be done, since the amount of energy that can be discharged through the spark may be
tremendous. Therefore, practical considerations set a limit here. The size of the
particles can be determined from the voltage, the current intensity, and time, i.e.,
the energy; there are great possibilities of variation. The speed of machining, the
size of the particles, and the wear on the tool vary with these three factors, even
when their product is kept constant.
General Pointers
As for the speed of machining, the following values are given (1952) for cubic
centimeter which can be removed per minute, provided that there is a surface rough-
ness which, measured with a profilometer, does not exceed about 60 ? inches
(L 1.50 ?): zinc alloys 3.25, steel and steel alloys 0.4, nickel and nickel alloys
0.8, tungsten carbides 0.065, and titanium carbides 0.33, entirely disregarding the
hardness of the material. Lower speeds give better surfaces, while higher speeds
decrease it. The figures given (which, in addition, seem rather high) are not al-
ways a measure for the machining speed. Thus, in drilling, a hollow electrode is
generally used; with this, only a ring-shaped opening is drilled so that the piece
in the center drops out entirely.
The machining can also be divided into a rapid rough grinding and a slower pol-
ishing, where the precision, with care, can be made nearly as great as in lapping,
i.e., within about 0.005 mm.
The electrode material can be chosen according to the end use. Often, technic-
ally pure copper is used for tap-drawing, drilling of less exact holes, etc. If
great precision is needed, eround molybdenum electrodes can be used. With proper
control of the process, however, it is impossible to make full use of the higher
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COOltiot
Cooled port/On
OJ? ot 1140 pop
Mowing o PonOa
Caw', 5,01,41,
to o eriOpn
Cfotir
Fig.25 ? Assumed Operating Principle of the Spark Method During a Single Dis?
charge, which must be Regarded as an Independent Operation. At point A, the
voltage between the electrodes increases, and the negatively charged elec?
trons move toward the surface of the tool. At point B, the voltage becomes
so high that the electrons strike the surface and move toward the tool at
high velocity. At point t, the same process takes place, but now a liquid
flows through the hollow tool electrode and nut through the spark gap. At D,
the electrons strike the surface of the workpiece and activate its atoms.
This portion of the surface is heated to such a high temperature that it va?
porizes. At the same time, some of the electrons strike the atoms in the sur?
face and knock the electrons out of their paths. so that a positively charged
cloud is produced above the surface. At E, the negative tool attracts the
positively charged and overheated particles in the forth of vapor, which Moves
toward the tool. The particles are cooled off rapidly and are washed away by
the stream of liquid. The spark ceases at this instant, so that only the re?
moved metal has been heated.
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melting point of molybdenum, and for many purposes the price is too high. Brass and
similar alloys are both mechanically and electrically well suited to many jobs.
/ .1 ; ! 1 .' 1
Fig.26 --Profilometer Diagram of Fine Grinding of the Hard Metal Carboloy 55-B, Mag-
nified 25 times. 1 mm = 10 11 in. The surface roughness is 19 11 r.m.s.
In machining a suitable ',cooling liquid" is used. Water and soluble oil m.x-
tures can be used for rapid and coarser jobs, as for example tap-drawing. In more
precise jobs, hydrocarbon or dielectric fluids are recommended. The fluid is fed
through the spark gap under a pressure which, depending on circumstances, may vary
between 3 and 30 kg/ cia2, at an average value of about 8 - 9 kg/cm2. The liquid can
be supplied in three ways, either from above through the hollow electrode, or from
below through the electrode or, as the case may be, through a hole bored in the work-
piece, and finally it can be supplied from the side.
The number of sparks per second is said to lie between 20,000 and one million,
while other sources indicate much lower values.
Electric Spark Grinding
Grinding can be done by rebuilding an ordinary grinding machine and providing
it with electric equipment. In this machine, a brass wheel is used rotating at a
speed of 50 - 500 rpm. The speed is not critical, but it must be below the normal
speed of a grinding wheel. Figure 26 shows a profilometer diagram of fine grinding
of the hard metal Carbaloy 55-B, magnified 25 times.
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Tool grinding by this method has recently been taken up by the Pratt & Whitney
Aircraft Division, which has installed a rebuilt Browne & Sharpe No.10-N tool-grind-
ing machine as well as an Elox machine in the grindery, which every year grinds a-
round two million tools. The equipment is
used especially for figure tools. The ta-
ble of the machine carries a fiberglass
vessel, with transformer oil as the dielec-
tric where the profiled brass is immersed
halfway into the liquid. The travel of the
table is varied by installing a special
gear, to obtain a variation between 4.25"
and 10" per minute. The spark gap is kept
constant from 0.003 to 0.005 mm, with the
Fig.37 - Machine, Modified for Spark
, help of the previously discussed servocon-
Grinding (4 Hard Metal Tools. This is
trol. In addition, see Fig.27.
clamped in a vise on th4 'platen which
With a normal finish of about 60i-J. in
carries a vessel with transformer oil.
r.m.s., 0.055 cm3 of hard metal and 0.06
The tools are kept submerged under the
cm3 of hard steel could be removed per ran-
workpiece.
ute,'consideralay less than the previously
mentioned amounts.
The wheel shows very little wear and is cheap in comparison with diamond and
grinding wheels; roundings can be produced with a precision within a few hundredths,
without formation of ridges or heat as I the case in normal grinding. An addition-
al advantage lies in the fact that all materials can be ground with the same wheel,
which therefore can be permanently installed.
Investigations have shown that hard-metal tools, ground in this way, make a
better cut than diamond wheels without lapping. The surface roughness is kept sta-
tionary between 20 and 25 IL in r.m.s., but if desired this can be reduced to 2
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Experiments have also shown that the spindle must be extremely rigid in
intain the very small spark gap and that the dielectric liquid must be
and predict considerable future expansion of the method.
The other American firm, Method X Co., which has worked with the spark method,
maintains that this is a
dyne-Controlled Electrode Voltage. Capacity ?kw.
It is expressly stated
that the "cooling liquid" must be a dielectric, and most often a hydrocarbon is se-
I.ormally, kerosene is used and a specially developed fluid "Dielectro X",
which is said to give increased speed of machining, since the spark will free hydro-
;en which, in turn, will ionize the path of the spark. The Method X Co., alp() gives
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another theory which is more in line with the one developed by the Elox Corp.
Figure 28 shows a Method X machine, rebuilt as a drill press. The electrode
voltage is automat-
ic and is con-
trolled by an amp-
I lidyne amplifier.
The capacity is
7 kw, and the time
of discharge is
Fig.29 - Operating Principle of the Method X Equipment for
Thread-Cutting. The thread electrode is brought for-
ward by the tbread control 32. At the lower left
thread cutting is shown, perhaps in a bottom cavity
with simultaneous drilling of the center. Notice
that here too Lazarenkols circuit is used.
(E.E.Teubner, U.S. Patent No. 2 650 979).
Figure 29
shows a diagram of
a Method X machine
for thread cutting.
The thread elec-
trode is carried to
the control 32.
Also shown is the
cutting of threads
in ground holes and
removal of the core
by boring. If an
electrode with in-
ternal thread is
used, an outside
thread can of
course be produced
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A 6.3 mm hole can be drilled through a hard metal plate of 5.3 mm thickness in 35 -
38 sec, and if the inductance in the secondary of the pulse transformer is lowered,
the time can be further reduced to 22 sec. In a hard metal plate of 12.5 nun thick-
ness, a thread can be cut in barely 10 min, while the time was formerly about 90 nin.
It is of interest that the surface smoothness reportedly at this high machining
speed is not inferior to that obtained with older equipment; this is explained by
the fact that passage of the sparks is more regular. The drawback in the oscillator
circuit was the fact that the spark, with all its irregularities, formed a definite
link in the discharge circuit. In the new model 5 developed by Method X a series of
steeply rising and falling current-producing pulses is superimposed on the discharge
point. With the rapid rise and steep drop, the path of the spark becomes ionized '
more rapidly, so that machining can be done at a higher frequency. The pulses are
produced by a capacitor, charging across a transformer and discharging across the
rotating spark gap.
This high energy also necessitates a more rapid circulation of the dielectric,
since most of the energy is transformed into heat. While this was formerly adequate
at a circulating speed of about 1 ltr/min, about 5 ltr/min are required in the new
model. This amount is also necessary in order to remove the large quantity of par-
ticles which must be carefully filtered. It must be borne in mind that more than
2 mm3/min machined-off material is inVelved here, to which must be added the materi-
al abraded from the tool electrode.
Spark Grinding of Chip Breakers
At the Watertown Arsenal, a Method X machine has been in use for grinding chip
breakers in hard metal plates. Originally, brass electrodes were used, which resul-
ted however in considerqble wear; later, change-over to copper electrodes with an
0.003 mm chromium layer was made. This resulted in a reduction in wear on the elec-
trodes to a fourth, about equal to half the amount of hard metal.
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In the grinding process, a special holding tool is used, where 40 pieces of 1"
plates are set up in four rows of 10 inches (in some cases, 80.5" plates). Four
plate electrodes, one for each row, "grind" the chip breakers during the machining.
As a coolant, kerosene was used, and the electrodes had to be adjusted from time to
time with a file. With this arrangement, it was possible to grind 6000 chip breakers
4
per month in one shift, where formerly it took the work of three shifts to achieve
the same result in grinding with diamond wheels.
The next step in the development was to test hard metal as electrode material.
It was shown that the wear of the electrode amounted to about one half the wear of
the workpiece. Therefore, if a hard-metal plate is used as electrode it is provided
with a chip breaker at the same time as it itself produces chip breakers in two other
hard-metal plates.
A holding tool was therefore constructed for ten plates in one row, just as the
electrode holder could also take care of ten plates, so that the one plate, during
the machining process, grinds chip breakers in the next one. The chip breakers were
about 0.40 mm deep. First, about 0.25 mm was removed from the lower plates by a
quick rough planing (approx. 12 min), followed by two polishings which removed the
rest (time, about 5 min each) and finally by a fine polishing in which practically no
material was removed (time, 3 - 5 min).
After each such cycle the lower plates were shifted outward, while the cop
plates were exchanged after every second cycle. Two complete operations thus give
30 completed plates and take 50 - 60 min. The surface smoothness achieved by this
machining is 25 - 301L inch r.m.s. 0.7010.
As a final example, Fig.30 shows a production spark drilling machine which can
drill eight radiating elliptic holes in a ring-shaped housing of stainless steel,
with an inner and outer wall. The drilling takes place in four steps, each in its
head, and the hole tolerance is 0.11 mm. Floor to floor time is 110 min. The mach-
'ne was *built by the Cincinnati Milling Machine Company.
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Bibliography references: 3, 4, 5, 7, 10, 23, 24, 31, 32, 33, 34L 35, 38, 39, 42,
44, 59, and 60.
Patents: British Nos. 578 933 and 580 4114 U.S. Nos. 2 300 855; 2 308 860,
2 377 159; 2 438 941: 2 476 965; and 2 650 979.
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THE DEVELOPMENT IN ENGUND
The first British machine for spark machining appeared about 1952. The develop-
ment apparently is based on U.S. and Russian inventions as well as on D.W.Rudorfffs
British Patent No. 637 872 of 1947. The idea in it has been used and developed by
"Sparcatron Ltd., Tuffley Crescent, Gloucester, which now, under the name of ilaparca-
troels furnishes spark-machining equipment in the prevalent form as small drill
presses.
Parallel with this, but no doubt not much earlier, machining was taken up by .
another British firm, Wickman Ltd., Coventry, which developed a corresponding mach,-
ine sold under the name "Erodomatic" which, as far as is known, was given great pub-
licity during the International Machine Tool Exposition in Brussels in 1953*.
Electric Spark Turning
Rudorffes patent also made use of Lazarenkots oscillator circuit, but, in prin-
ciple, it gave a more constructive elaboration not only of the machines used but al-
so of the electric circuit used. Rudorff also showed more diversified types of ma-
chines, such as a band saw, a circular saw, and a lathe.
*In the March and June 1955 issues of "Machines Francaises", G.Cany in the ar-
ticle "Intermittent Arc Machining" with "Steel" as source, mentions that the first
patents on which spark machining is based can presumably be traced back about 75
years. It is also mentioned here that even Priestley (1733 - 1804)0 while causing a
spark to jump between the tip of a needle and a plate noticed a dark central stain,
surrounded by colored rings. "The surface looked as though it had been ground or
corroded by violent explosions." This is due to the fact that very small pits and
melted metal particles are present in the center plate. (Solution advanced after
printing of the manuscript.)
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Figure 31 thus shows a greatly schematized lathe, with the corresponding elec-
trical circuit. Rudorff expressly presupposes spark formation and wishes to prevent
/45
Fig.31 - Schematized Spark Lathe. The work is carried out as with conventional
lathes. The electric circuit is as indicated by Lazarenko, but for higher
voltages is tapped from the auxiliary battery 102. The coil (87) with the
iron core (86) and the springs (84), (88) cause the "Turning steel" (69a)
to vibrate with an amplitude of several millimeter, so that the "turned-
off" surface is even and smooth. (D.W.hudorff, from Swiss Patent Lo.
273 469)
arc formation by feeding a coolant through the spark gap. He claims that the type
of liquid is less essential, and the patent specifications include electrolytes, di-
electric materials, mixtures with at least one of each type, and gas mixtures con-
sisting of at least two gases.
The claims also include control of the tool electrode so as to maintain a spark
gap of constant size;Jsimultaneously, this prevents the hydrodynamic boundary layer
from being penetrated.
In the lathe shown in Fir.?1, the workpiece (50 is clamped between pinions and
driven by the motor (61). The "turning steel" consists of an iron rod (69), which
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is surrounded by an insulating layer (70) and mounted in the steel holder (73) so
that only the point (69a) is free. The electrode is connected with the negative
pole. The steel holder can be shifted freely back and forth in a lead parallel to
the turning line and is fastened over the springs (84) and (88) to a pair of stand-
ards on the upper carriage. The slide system of the lathe is otherwise normal of
the conventional type. In front of the spring (88) an iron core (86) belonging to
the solenoid (87) is inserted. The elasticity is so adjusted that the steel holder
is drawn to the left when the coil has no current and is made to vibrate when an al-
ternating current is sent through the circuit. The amplitude of these vibrations is
of the order of magnitude of very few millimeter and is controlled during the "turn-
ing" process in order to achieve a smooth and even surface.
The motor (61) and the coolant pump (90) are started by the switches (91) and
(92). In the main circuit, the current is controlled by the variable resistance
(TO; by means of the switch (95) the capacitance can be adjusted to two values.
The positive pole (100) is connected to the spindle through the brush and tow-ring
(100 and (105). In starting, it was found advisable to connect the auxiliary bat-
tery (102), which produces a higher voltage than the main current source.
Theory
hudorff himself thias that, ihe effect in spark machining results from the
thermal effect (46). The temperatures in a spark are much higher than in an arc, .so
that the spark strikes the surface very suddenly in a quite small area - rather at a
point - and will cause a vaporization. The high temperatures in the electric spark
are explained by the high current density which prevails in the spark cross section.
For a spark current strength in air of 250 amp, Meek (Bib1.54) has calculated a cur-
rent density of 29,000 amp/cm2 after a time of 0.5 IL sec from the beginning of the
discharge. Since the cross section of the spark increases rapidly during the dis-
charge, this current density decreases, after an additional 0.3 IL sec, to 3700
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amp/cm2. For an arc of 250 amp, Meek computes, by way of comparison, a current den-
sity of only 500 up/cm2; this would explain the higher temperature of the spark.
The same author also calculated the instantaneous effect in the spark and indi-
cates, for a spark of 500 amp, an effect of 1000 kw/cm after one microsecond from
the beginning of the discharge. The temperature of the spark, accordingly, can be
determined by spectroscopic means: in thus manner, temperatures up to 12,0000 ; have
been established.
The atomizing effect of the electric spark has been known for a long time and
presumably applied for the first time by Th.Svedberg more than 50 years ago for the
production of metal colloids; in this connection, it should also be mentioned that
OrvaAer
kad,er
Calcitmer
/WA
0140ag
? 1.
Fig.:12 --Diagram of Sparcatronts Spark Generator, Fed by an Ordinary Mains, with
Holder Transformer, Rectifier, Control Circuits, and Row of Capacitors
spark discharge across a capacitor circuit was used as early as 1919 by-V.hohlschut-
ter for the same purpose [4. Elehtrochem. Vol.25 (1919) p.:109]. For machining, the
electric spark has only recently been applied (cf. the previously discussed develop-
ment). Aside from the sources mentioned, the field has also been explored by Jones
(Bib1.55,56,57) who assumes that the vaporized amount of material, outside of the
energy of the spark, is also dependent on the melting point of the metal in question.
In all spark machining, the tool is given negative and the workpiece positive
voltage, since this will cause wore material to be removed from the workpiece than
from the electrode. Jones explains this with the following example: A spark jumps
between two nickel electrodes with a current density of 1,000,000 amp/cm2. fhe cur-
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'
Z-71
9.
!rent density in the anode can be calculated as 100,000 amp/cm.2aiiirthi-iiithisdillOT1
0,
,as 20 v. The cathode will have an energy density of 2000 kw/cm?, while the energy j
density in the anode will be 20,000kw/cm2. Jones shows that, under these conditions
the cathode will, within a time of 2.5 x 104 sec, heat to the ieltini point Of niC1-
kelp while the anode reaches this temperature.in 2.5 x 3.078 sec. Since the current 1
strength is high and since the cloud of electrons reaches the anode before .the posi-,
tive stream of ions reaches the cathode, the surface of the anode is machined more !
I l?
rapidly than the surface of the cathode. In the case of spark discharges, which
vary from 10-4 to 10-5 sec, the time difference for the heating of the anode and of
the cathode will nevertheless be without significance.
The results cited by Jones were obtained with discharge in air at atmospheric
pressure and, in a single case, at 3 atm pressure in a spark chamber'. In spark map. :
chining, the spark jumps nearly always (at least in practice) through a liquid,
which is preferably a dielectric. This is, however, of no significance for the
above explanation since the spark will vaporize the surrounding fluid so that the
path of the spark goes through a vapor atmosphere. The material removed from the
workpiece will be entrained as metal vapor by the cooling fluid and condense in it,
and the flushed-away particles will mainly have a spherical shape.
It is possible that, aside from the vaporizing of the metal, mechanical forces
are involved, such as oscillations which arise on pulsations in the vapor layer of
the spark path (Hib1.58).
The quantity of removed metal is proportional to the discharged energy which, .
in turn, is proportional to the capacitance and the square of the voltage. However,
there are limits to the increase in these amounts; obviously, the energy will in-
crease, but an increase in capacitance will necessarily lead to a decrease in the
time constant, so that lower discharge frequencies result. The curve giving the
ratio of quantity of material removed to the capacitance will therefore rise to a
certain point and then level off (cf. Fig.11i).
ob.
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Figure 32 shows the principle of the electric system used by Sparcatron, built
up as a special panel. The voltage and capacitance of the oscillator circuit can be
Fig.33 - Sparcatron Spark Drill Press with Electronic Servocontrol to Keep the Spark
varied and adapted to different jobs, whether it is a question of rapid rough plan-
ing or slower fine machining with greater need for precision and surface finish.
Figure 33 shows Sparcatronts construction of a drill press with servocontrol of
the spark gap. The tool electrode is mounted in a cartridge which is held in place
by the servo system. The clamping of the workpiece and the mechanical parts of the
machine are standard and have been described before. The fluid is a dielectric fed
downward through the hollow electrode; the reservoir is combined with an effective
plied; this voltage is compared with a reference
system is in balance when the spark gap has the proper size. If the position of the
electrode is wrong, a pulse is generated which leads to a positive or negative error
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signal, depending on whether the spark gap is too small or too large. This signal
is fed to an electronic amplifier and, after amplification, is supplied to a DC motor
which, across various gear transmissions
with cogwheel and racks, builds up the vol-
tage at the electrode. By means of this
control over the DC motor, the spark gap
continuously tends to stabilize itself at
the preset value.
By reason of the system's inertia, the
adjustment can be delayed slightly, just as
the spark gap can be inclined to execute
oscillations about its proper position.
This is counteracted by introducing a sta-
Fig.34 -
Surface Layer of a
Spark-Mach,-
ined Surface, Magnified 100
Times. No changes or only
mimor changes can be seen in
the outermost lapr.
bilizing feedback. Therefore, a DC genera-
tor is coupled to the DC motor, and the vol-
tage furnished by it - which is proportional to the rate of discharge - is fed back
to the amplifier. In the newest models, the spark gap, at a working voltage of 110v
can be kept constant within 1/100 - 1/200 mm. The spark frequency is given as be-
tween 300 and 1800 per second, or significantly lower than indicated by the Ameri-
cans.
Despite such excessive temperatures in spark discharge, the material is not af-
fected by this; investigations have shown that the surface layer, affected by the
temperature rise, is less than 0.002 - 0.003 mm (see Fig.34)-
Figures 35 and 36 show a fairly simple Sparcatron bench drill press, with the
corresponding cabinet for the electric control equipment. The machine itself can be
more complicated, as is the case in the equipment with greater table movement. The
dock" contains the elements which, through the electronic servocontrol,
keep the spark gap constant. The machine also can be furnished with rotating elec-
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trode holders and in a model in which the electrode rotates additionally about a
given axis.
Figure 37, finally, shows a setup for spark machining of one half of a well for
turbine blades. The well, made of tool steel, is tempered. An extra brass elec-
trode is placed on top of the well..
Fig.35 - Simple
Spa rcatron
Spark Drill
Press with Table Displaced in
Two Directions. The "drilling
head" with the electrode hold-
er has a servo-controlled dis-
charge. In other models, the
holder can rotate and tUrn
about a prescribed axis.
General Observations
Sparcatron states in some sources that
in general, paraffin can be used as a cool-
ant, while other sources recommend kerosene
or special fluids. It is said that, for
the electrodes, almost any soft material
which is a good conductor can be used.
Graphite has been used, but generally brass
is preferred since it is easily machined to
the desired shape. A drilled hole acquires
the same shape as the electrode but is a
little larger. For round holes, the excess
is about 0.03 mm and with properly performed
machining, the excess may be twice'the
spark gap. Under equal conditions, the
ex-
cess will be constant; it is possible to
make allowance for this by making the elec-
trode correspondingly smaller. Such cor-
rections or allowances can be set up in Ta-
bles or curves. It is also worth noticing that tapered or stepped holes of many
diameters can be produced.
For simple drilling, the electrodes are fairly cheap to produce. however, this
method is often used in actual shaping work. This applies to the production of
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forms, sinks, pressing tools, etc. In this case, the electrod??t?iij-liar- 1
more expensive. With this as a model, the workpiece is prepared and can be used as i
an electrode to fabricate a new electrode exactly like the original.
? A much used and simple method for re-
Fig.36 - Cabinet
for
ducing the diameter of an electrode, in or-,
r-
der to produce the desired free access to a
given hole size, consists in immersing it ,
in an acid bath for a suitable time.
In the production of more complicated
electrodes, it has been the practice in
some plants to execute these in phosphor
bronze and "Cronite", casting them so that '
all refinishing is reduced to an absolute ?
minimum.
The speed of machining is said to be
less than with conventional machining, but
it is thought nevertheless that it can be
increased considerably in the future. As a
concrete example, it is mentioned that a
plate of 1/8" thickness was drilled through with a 1/4" graphite
the Electric Equip-
ment of the Sparcatron Machine
glass-hard steel
electrode in 3 min. Hard metal was drilled through with a 7/32" graphite electrode
to a depth of 9/16" in the space of an hour. These speeds seem considerably less
than those given by the Americans, which is possibly due to the fact that the Ameri-
can equipment apparently has a somewhat greater power. Besides this, such data on
speed can hardly be compared unless all conditions are alike, not the least of these
being the surface smoothness achieved, which, generally speaking, is inversely pro-
portional to the speed of machining.
Electric Spark Grinding
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_
The method does not seem immediately suited to free-hand grinding, but in spite
of this, Sparcatron has developed a small hand grinding machine. In all essentials,
it resembles a conventional grinder with diamond wheels.
Sparcatron has also developed the spark
method with round grinding. An ordinary
round grinding machine is rebuilt so the
spindle dock is insulated from the machine
and the voltage is applied across a brush
and draw-ring. The machining is done as in,
conventional round grinding, except that the
workpiece and electrode do not touch. The
electrode which is a wheel can, in some
Fig.37 - Spark Machining of One Half of cases, be stationary, but it is preferred to
let it rotate. Such an arrangement is shown
a Tempered Well for Turbine
in Fig.38, while Fig.39 gives a spark-ground
Blades. On top, an extra brass
workpiece with four different surface smooth-
electrode is shown
nesses.
The change from one smoothness to another is performed at the same voltage with-
out change in the machine itself, merely by regulating the electric constants in the
circuits. By this process, surface smoethnesses down to about 5 4 inch can be a-
chieved; at lower capacities, it can be reduced still further. In addition, it
should be mentioned that all electric spark machining produces surfaces which, in
comparison with those produced in standard machining, are absolutely free of longi-
tudinal grooves of machining. The surface pattern is completely smooth and without
scratches. This also explains that microcracks disappear, which leads to higher fa-
tigue strength and less risl, of breaking. The smooth surface pattern has the spec-
ial property, like bearing surfaces, to hold the oil; therefore, such spark machin-
ing is suitable for mating surfaces.
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No data are given on the peripheral speed of the wheel or on the optimum table
speed and rotation of the workpiece. Here the previously mentioned figures by Pratt
&Whitney may be instructive. Conversely, it was reported that, from a workpiece of
a diameter of 3/4" and a length of 4 3/4" 0.04 mm per minute (measured on the diame-
ter) can be removed by grinding.
Fig.38 - The Spark Method Applied to Round Grinding and Carried out by a Suitably
Rebuilt Machine
in rough planing, a voltage of about 22 v and a current intensity of about 15
amp are suggested. In polishing, the current intensity must be reduced to about 3
Erodomatic Equipment
The other British firm which builds spark-machining equipment - Wickman Ltd. -
gives more or less the same explanation of the process as the previously mentioned
However, while many sources quote electrolyte, dielectric material, or
one. some-
times air as coolant, Wickman sLates expressly that the liquid must not be a conduc-
tor or a dielectric; this is regarded as a criterion for whether a given method is
to be considered span l machiniiv orallothf.rform of electric machining. The function
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of the fluid there is to deionize the spark gap after each discharge, which is stated
as having a duration of some few millionths of a second, and to flush away the re-
moved particles.
Fig.39 - A Spark-Ground Workpiece with Four Different Surface Smoothnesses.
It is said that spark machining, done at high speed, cannot stand comparison
with conventional machining processes. However, it is believed that this situation
will improve in the future. The application of greater energies increases the speed,
but with the limitations mentioned above.
Wickman maintains also that the wear of the electrode is excessive and increas-
es with increasing machining speeds. In certain jobs, where only simple and cheap
electrodes are required, it can nevertheless often be worthwhile to force the speed,
particularly ih rough planing. In addition, it is claimed that the load capacity of
a given material is also a contributing factor for the speed with which it can be
spark-machined.
The spark gap here is also chosen up to about 0.013 mm and, according to avail-
able information, machining speeds between 0.008 and 0.013 nm are reached. The sur-
face smoothness, as a rule, is between 20 and 8 4 inch, but can easily be as much as
for example it 4 inch; in this respect, it is emphasized that the surface smoothness
for a given job can be selected in advance and adjusted by regulating the machine
constants - something which can hardly be done to the same extent in traditional ma-
chining methods.
Wickman also remarks that spark-machined surfaces do not have a glossy or shiny
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appearance but rather that of a fine sand-blasted surface. The advantage in the
smooth surface without directional traces is emphasized, and the significance of the
fact that the surface does not (as is usual)
have cracks, scratches etc., is easy to
comprehend. The many small depressions of
which the surface .is composed reportedly
have a depth-diameter ratio of around 0.3.
In this connection it is of interest
that it is claimed that a spark-ground tool
.
has a 5 - 10% longer life that one ground
by conventional methods and that the method
is cheaper in addition.
Wickmants spark drilling machine "Ero-
domatic" is for 6.6 kv-amp. In one opera-
tion, the "drill dock" is mounted to two
cylindrical columns along which it moves.
The significance of the precision of such a
machine is strongly emphasized, and in con-
sequence of this the two slides of the ta-
Drill Press Erodomatic
ble are moved by precision lead screws with
Fig.40 - Mickman's 6.6 1w-amp Spark
accurately graduated scales.
The machine is equipped with an effective filter for the coolant and a pump for
circulating it. The flow of liquid can be changed in direction so that it either
can run down through the hollow electrode or be sucked up through it, depending on
whether the inside or the outside shape is to be transferred to the workpiece.
Erodomatic also has servo-controlled voltage feed to keep the spark gap con-
stant. The various possible feeds are: 1. manual voltage; 2. automatic voltage
with a) nonrotating electrode, b) rotating electrode, c) rotating and oscillating
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electrode, d) oscillating but nonrotating electrode; 3. automatic servo-controlled
rotation, in combination with a lead screw unit for producing thread and spiral
tracks.
Wickman has also produced an electric
free-hand spark-machining unit, available
under the name Erodosharp. With this,
hard-metal tools are "ground" by electric
means in the same manner as with carborun-
dum or diamond wheels. The machine (see
Fig.41) is constructed like a conventional
grinding machine for turning steel and is
of the regular revolving design so that
grinding can be done at the desired angles.
Instead of the standard grinding wheel, a
Fig.41 - Wickman's Free-Hand Spark Grind,-
cast-iron wheel is used here, which consti-
ing Machine Erodosharp Constructed like
an Ordinary Grinding Machine for Steel.
Inset shows the "grinding wheel", which
is provided with radially slanted
grooves which feed oil from a central
chamber over the periphery of the wheel.
tutes the negative tool electrode.
The forward-turning end face of the
wheel has an annular oil chamber, with sev-
eral slanted grooves cut in the surface,
forcing the oil from the chamber by centri-
fugal force. The oil will then coat the
end face of the wheel in a thin film and act as a dielectric during the grinding,
while simultaneously maintaining a spark gap between the turning steel and the
grinding wheel. It is emphasized that it is quite easy in practice to do grinding
in this way, but the steel must not be forced against the wheel to the extent that
the oil film would break.
With regulation of the electric constants of the circuit rough grinding, fine
grinding, and polishing are performed, where the latter replaces the usual lapping
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with diamond wheels. The wheel speed is around 1400 rpm.
Despite the fact that the method can be adapted to practically all forms of
machining, Wickman considers the spark method as the best suited for machining& in
which simple cutting tools are not normally used, since then advantage can be taken
of the fact that the electrode works on a larger area, so that the wear is reduced.
As stated above, the spark method is considered slower than normal machining, but in
an evaluation of this fact it must be taken into consideration that the spark method
is often applicable where ordinary machining fails. It is estimated that the sole
purpose of drilling out cracked taps, rivals etc. will justify the purchase of such
equipment for an ordinary business.
Wickman claims that the staff working on the development of this process and on
the production of the necessary machinery are of the opinion that electrospark mach-
ining is a method which offers great possibilities for the future. Theoretical con-
siderations have shown that, in coming years, it will be possible to increase the
present operating speeds so greatly that per unit time, it will be possible to re-
move just as much or even more material than with conventional methods.
The spark machining, therefore, justifies not only the investigations on new
machining methods and the work on developing suitable machine tools, but it is also
expected that this new technique will make its mark in allowing a freer and less
traditional design and elaboration of many machine elements and tools and make pos-
sible the use of harder and less easily worked materials, whose application is, for
the moment, somewhat limited.
Bibliography references: 9, 113, 16, 17, 18, 22, 26, 30, 31, 42, 46, and 59.
Patents: USA ho. 2 526 423; Switzerland ho. 273 469.
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DEVELOPM:T IN GERMANY
Spark machining seems to have appeared later in Germany than in the other coun-
tries mentioned, and the German development apparently is also based on their tech-
nique. However, it must not be forgotten
that fairly early and especially in the oth-
er electric machining methods (especially
Groups 1 and 3), a valuable independent con-i
tribution was made (cf. the earlier cited
sources).
The first contribution to actual spark
machining which the author succeeded in
finding is the German patent 1o.672 832 of
1937 granted to A. Bergmann, W.Dawihl, and
Fig.42 - Procedure for Drilling Holes in
Diamonds. The diamond (3) is set in a
lead plate (2) and is coated with the e-
lectrolyte (4). The needle-type elec-
trode (6) touches the workpiece with a
light pressure. If the voltage between
the electrodes (5) and (6) exceeds a
certain value, there will be a spark
%
0.Fritsch. It concerns a method for pro-
clueing depressions and holes in diamonds and
other naterials difficult to machine and is
shown in Fig./12.
Here (1) is a glass container with a
holding plate (2) in the bottom. This is
made of lead or similar material, and the
worlipiece (3) is set into it. Over the
whole is poured a' conductive liquid (4)
discharge, which hollows out the diamond.
such as diluted sulfuric acid, sodalye, or
(From German Patent ho.672 852)
a salt solution. An electrode (5), which
is not attacked by the fluid is submerged in this without touching the diamond or
the holdinr plate. Another needle-shaped electrode (6) is inserted in a bushing (7)
and is held against the workpiece with a light pressure by a weight (not shown) or by
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a spring arrangement.
If voltage is applied across (5) and (6), a current will flow between the elec-
trodes. If the voltage is increased above a certain amount, e.g., 40 volts, then,
according to the patent specifications, the current will drop sharply, accompanied
by a spark discharge between the point of the electrode (6) and the diamond; this
produces a depression in the diamond. If the voltage remains below the given amount
only an electrolytic discharge will take place, which does not affect the diamond.
At a sufficiently high voltage, the electrolysis is replaced by the spark formation
mentioned.
- Above, Schematized, the Charg-
ing and Discharging of the Capacitor;
Below, the Voltage Course as Shown in a
Cathode-Ray Oscillograph. Only the
charging can be seen. The discharge oc-
curs so rapidly that the light inten-
sity is too low to blacken a film.
(Reference frequency 5 kc) a) Capaci-
tor voltage; b) Time t
The form of the depression or the hole
is governed by the magnitude of the voltage
If this is high, i.e., above about 80 v, a
cylindrical drilling will result, while
voltages of about 40 - 60 volts give coni-
cal holes.
The machining speed is dependent on
the workpiece, the electrolyte, and the vol-
tage, but data are lacking. The method can
be used with direct as well as with alter-
nating current and is said to be purely
electromechanical.
The process seems basically similar to
that developed by the National Bureau of
Standards for drilling holes in small dia-
mond draw stones (see Figs.21 and 22) and
may possibly have been the point of depar-
ture for it.
The investigations in Germany of actual spark machining were apparently started
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around 1951; at present, there are two firms in each field which have developed the
machinery to a point where it could be placed on the market. These are Friedrich
Krupp, Essen, and Deutsche Edelstahlwerke (D.E.W.) Krefeld, and as far as can be de-
termined from available information, both also have obtained .a license from the
above-mentioned American firm Method X Company.
SI
' ? ? -)N:,-.4`' ? ;
? .
? n
Fig.44 - Surface of a 4-mm Drilling in a 2.5-mm Thick H2 Plate, Spark-Drilled with
220 Voltage. On the left: C = 24f ; J = 0.8 amp; working time = 12 min.
On the right: C = 31til ; J = 2.2 amp; working time = 6 min.
Both German equipment is designed along the lines of the previously mentioned
equipment and also make use of Lazarenkols circuit. The first of the mentioned
firms explains that the line voltage U0 across the resistance R charges the capaci-
tor whose voltage Uc will increase exponentially with the time constant 11 = RC. If
the spark gap is so small that it can be bridged, the capacitor will discharge
through the spark with an essentially smaller time constant. The brief spark will
be maintained until the capacitor voltage drops below the breakdown of the spark;
due to the (very small) inductance of the circuit, high-frequency oscillations may
be generated. This again changes the capacitor. Determining factors for the fre-
quency of the sawtooth oscillations produced in this way are not only the line volt-
age Uo, but also the time constant ti= RC and the size of the spark gap.
The upper portion of Fig.43 shows schematically the flow of the voltage past
the capacitor; the bottom gives an oscillogram in which, by a suitable choice of the
resistance It and the capacitance C, a suitably slow series of sparks is produced.
The increase in the voltage is clearly defined, while the discharge took place so
17
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rapidly that the light intensity of the cathode-ray oscillograph was too low for
blackening the film.
Results Obtained
The machining speed is regulated by varying R and C. However, this cannot be
done entirely independently, since in the case of a high capacitance, the resistance
Table 3
Favorable Electric Conditions in Drilling Through a 5 mm Thick Plate. Line Voltage
220 v. Working time, quantity of material removed, and surface roughness are also
given.
Electrode
diam.
Ppm'
Volume remereii Roushness
Electr?4e Current Cap?- Time*
corgss?Sect?Zr
tength !Jane, per min
fmnii) 1A1 imu F) town) ongu.inanj I (?1
3,0
10,0
10,0
O u5
0.25
0.1
0,3
4,0
1,0
4,0
3,0 34
4,3 10
3.0 32
15,0 10
13,0 50
31,0 7,4
15,0 131
53,0 ic
0,01
0,4
0,3
2,0
.13,0
3,0
15,0
* For a plate thickness of 5 mm.
must not be too low, which would result in an arc rather than in a spark. The most
favorable values for a given job are best found by experiment. Table 3 shows the
values found by this firm under various machining conditions.
Column 1 gives the diameter of the electrode, while column 2 indicates the mea-
sured size of the drilled. hole, followed by the cross s-ction of the electrode in
mm2, the current strength in amp, and the capacitance in g. It does not appear di-
rectly from the source what material is under discussion, but from the context it
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can be estimated that it must be hard metal with a thickness of 5 mm. The next col-
umns give the working time in minutes for drilling through plates and the quantity of
material in mm3 removed per minute, while the last column indicates the profile depth
of the machines surface in II The working voltage, in all cases is 220 v.
It will be noticed that two sets of
electric conditions are given for each elec-
trode diameter. One of these is so selected
that there will be a short working time and
resulting lack of surface smoothness, the
other so that a good surface is obtained, at
a still acceptable working rate.
? Finer surface finishes are easily ob-
? tained, but at the cost of working time.
The roughnesses given in the Table are said
Fig.45 - Surface in a Spark-Drilled Hole.
to be the maximum occurring profile differ-
Material: chromium-vanadium steel. Ca-
pacitance C = 50a ; current strength
tam n waviness. As also indicated by the
2 amp. The fusion beads are clearly
photomicrographs (Fig.44) of the surface,
discernible
the microgeometry is significantly better
than would be assumed from the figures in the Table; if these figures had been given
for example in have or h,5, the resultant values would have been of the same general
r
magnitudes as previously given by the British and Americans.
Roughly speaking, the Table indicates that the least roughnesses (at the limita-
tions cited above) obtainable with still acceptable speeds is about 25 - 30 i, while
a greater working speed will result in destruction of the surface to a depth of about
1001L. Figure 44 gives an idea of the obtained finish, showing the surface of a 4-mm
drilling in an H2 plate of 2.5 mm thickness and a working voltage of 220 volts. For
the picture on the left we have C = 2 Ri , I = 0.8 amp, drilling time 12 min. On the
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ws-:74,
LA---
s,? '''''-'w. ?
? D'O>,--, -... ,: e 1 4 ... ....
' ? ' 7rilifits 'd 7.7 -51.7 '4,-1 i?--- 2:2 iiiiiViiiai, Iiiiiiiiiilor 6 alii7-4- 4 's-11'' ' ' ^ 71.' ''' K
..?_ ..?. :-.,... _ . . ? . , ,
. . ,
,
_.' _It_is stated that, in the luminpus ar6 #s.3..f#Aamperaturos as high.e .._ ___?? _.. asor evn _ ... 41.... ,.? el..
4 1
.?1?04
?- ..:above 50,000ft., The temperature of the electrode in an arc or a
u
..:?and, does not exceed 40009K. Thus, spark;machininf? is based on
_'very high temperatures which arise in extremailocalised areas.
nal stresses which lead to the detachment of individual trystels
_crystal bond, as shown in Fig.44 on the let.
If a !park-machined surface is'ixaMined
under a microscope, signs of fusion will be
observed even with hard metal. Those fusedl
particles are not exclusively due to the oo-1
belt but apparently also fusion of the tung-
sten carbide in machining. If the particles
i
are filtered from the fluid and the powder
residue is examined under the microscope, it
is possible to demonstrate ditungsten car-
bide. A smaller portion of this powder con-
sists of the normal hexagonal carbide .W2C
with a lattice constant of a = 2.986
while themmain part consists of a cubit face
)
centered crystalline phase which probably
also has the formula W2C. It is known from investigations by K.Becker [Z.F.Elektro-
chemie 34 (1928) pp.640-42] and J.J.Lander and L.H.Gerner (Am. Inst. Min. and-Met.
Eng.; Tech. Publication No.2259, 1947) that W2C has a high-temperature modification
for which the latter authors found a lattice constant of a = 4.16 while the con-
stant for particles from spark machining is an insignificant 4.228 A.
It is interesting that the same cubic phase also appears when pure tungsten car-
bide without a binder is spark-machined with a pure tungsten carbide electrode. The
Fig.46 - Grinding of a Surface in the
Same Material as in the Preceding Pho-
tograph. Working conditions: voltage
220 v; current strength 2 amp; capaci-
tance 56 ; thickness of the material
6 mm, electrode diameter 6 mm; drilling
time 15 min.
spark, onsthe other
the very brief' and
This produces ther
0.
or separation.of,the
?
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?
same phase also appears in drilling of metallic tungsten with a tungsten metal
trode.
The fusion appears more pronounced when working fusible metals such as iron or
sine. Thus Figs.45 and 46 show the surface of a spark-machined tempered chromium-
vanadiUm steel (Ent 8272) with a composition of 0.4% Cr, 0.8% Ni and 0.1% V. In
Fig.45, where C has a value of 50 pf and the current strength is 2 amp, fusion beadi
.are clearly seen.
The high temperatures cause formation of a special surface, in which copper :
particles appear; this means that these particles must have traveled from the cath-
ode to the workpiece and deposited there or were welded to its surface. In drilling
electrolyte iron with a microhardness of 170 kg/mm2 (20 gm load), the surface layer,
produced has a microhardness of 790 kg/mm2. This layer contains small amounts of
cementite and individual needles of martensite; the main part cannot be eroded metal-
lographically and, to judge from X-ray examinations, consists of austenite. Since,
among other things, this phase is stabilized by a high carbon content, it is possi-
ble that this is due to a carburization. The particles produced in the spark mach-
ining consist of cementite and tetragonal martensite. This surface hardening might
have significance in production of sinkers and similar tools of steel.
In summary, it is stated that this machining is of a complex nature and that it
proceeds somewhat differently, dOpon4pg)on the characteristics of the machined ma-
terial. Apparently the removal of material is due to several causes, in that spall-
ing caused by thermal stress, vaporization, and cathode atomization each plays a
role, and a more exact analysis of the whole process must be reserved for later
thorough investigations.
It is further indicated that a spark-machined surface for most applications
must be retouched, to remove the affected surface layer. Although this is only thin
(according to Table 3, between about 40 and 150 g) it nevertheless indicates de-
struction of the surface, making it preferable to remove it by subsequent polishing 0
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or lapping; this, however, is not considered an obstacle in practical spark method.
Experience by D.E.W.
The other German firm interested in spark machining, D.E.L, has named its met-
hod Elbo-process (from "electric boring process"). The fundamental principles are as
previously described. This equipment works with voltages between 10 and 110 v and
can give between 1000 and 10,000 discharges per second. Kerosene is used as a coolant,
and it is stated that the size of the spark gap depends on the penetrating power of
the dielectric fluid and the applied voltage. During flashover of the spark local
fusion and vaporization takes place in most metals accompanied in hard metals by de-
tachment of small particles. Beads of fusion and accumulated small particles have a
diameter between 10 and 40 smaller machining voltages give smaller particles and
better surface finish.
The surface smoothness achieved is stated by this source as about 20 - 40 11;
only on application of very high voltages will more or less deep grooves and occas-
ionally small scratches appear.
In different hard metal types, the roughness and the tendency to scratch forma-
tion increases in the same order as the sensitivity in grinding, other conditions be-
ing equal. The qualities Fl and H2 should always be machined with low voltage; the
tougher qualities such as u, to G6 are far less sensitive. Sintered parts of drill
G2
carbide cannot, from what is said, be spark-machined, since particles of millimeter
size are torn out.
It is therefore asserted by this firm that retouching is necessary, and a mach-
ing allowance of 50 - 100 u is given as suitable in most cases. Workpieces which can
be mounted in round or plane grinding machines are easily retouched with diamond
wheels. Irregular workpieces are lapped with crushing bort, or drilling carbide
paste, smeared on cast-iron forms of the desired shape is used for speedy removal of
the rough surface layer. This applies especially where the work can proceed auto-
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matically, as in cases where the workpiece can be mounted in a filing machine and
forced by a weight against the up-and-down moving lapping strip.
For the electrode, copper or brass is recommended and for smaller holes, tung-
Fig.47 - Friedrich Krupp's Electric Spark-Machining Equipment, Consisting of a Drill
Press with Electronically Controlled Voltage Feed and Electric Control Pan-
el. On the right, the drill press is shown in more detail.
sten. The spark gap is given as 50, 100, and 150 4 with 20, 60, and 110 volts mach-
ing voltage, i.e., somewhat higher than that indicated by other sources.
Elbo mentions, in contrast to other sources - apart from the first Elox. machine
- a considerable electrode wear. In machining of hard metal, the wear of the elec-
trode reportedly is three times the amount..of material removed, i.e., a 30-mm piece
of electrode is used to drill a hole of 10 mm depth. In addition, the electrode is
worn along the periphery so that a lancet-shaped hole is formed or, in drilling
through, a conical opening. If the hole is to be cylindrical, a long electrode is
needed, at least 4 times the drilling depth. For bottom holes, several sharp-edged
or hollow electrodes must be used. Extremely tapered drillings require electrodes,
whose conical shape is considerably.more tapered than the desired hole. For ecamp1
a tapered drilling with a 90? cone angle without pre-drilling was produced with a
copper electrode whose cone angle was 220.
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In the production of cylindrical holes, it is preferred to have the electrode .
rotate. For very fine holes a mire in used, which moist be accurately running and
sufficiently rigid. If the thinnest practicable electrode is &unwed to consist of ?
a 0.10 mm thick tungsten, mire, then the smallest spark gap - 50 LI - will result in a
minimum hole diameter of 0.2 an. The great-
est drilling depth with this diameter is
2mm. If the hole diameter is increased to
0.3 mm, the drilling depth rises to 5mm.
The electrodes must then, as mentioned be-
fore, be sufficiently long to keep the hole
from, becoming conical.
The limitations in the drilling depth
become obvious when considering that kero-
sene must be able to penetrate to the bottom
of the hole and flush away the loosened par-
Fig.48 - Electric Spark Drilling Machine
ticles. In larger drillings, this presents
by D.E.W. (Elbo process) with Automatic-
no difficulties, since kerosene can be fed
ally Controlled Voltage Feed to Keep the
through the hollow electrode. If the elec-
Spark Gap Constant
trode is rotating, the effect can also be
achieved by providing the electrode with milled-off surfaces, to permit easier pene-
tration of the fluid.
The electrode wear, Elbo states, seems to be far greater than that indicated by
the other sources mentioned. Only for the first completed American Elox machine
(Fig. 23) is a wear approximately comparable with this ever mentioned, where it was
reported that the electrode became worn nearly as fast as the hole was bored. The
wear was sharply reduced, however, on changing to a DC circuit. Wickman also men-
tions a relatively great electrode wear, without its reaching, however, the values
given here.
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With respect to the machining time, it is also stated by the Elbo people that: "
this is longer than with standard types of machining. This is stated to be the rea-
son lehr it is used primarily where ordinary machining is possible only with
difficul-
ty or not at all, as for example with hard metal. The ratio of working speeds in
spark machining and ordinary machining is said to be of the same general order as
that of lapping with drilling carbide or diamond dust to grinding with earborundum or
diamond wheels.
When one considers that, at each spark discharge, a certain amount of the work-
piece, and three times this amount of the ?
electrode, is vaporized and that for elec-
trical reasons the spark frequency cannot be
increased above certain limits, it will be
understood that the working speed and the
energy which can be converted in the spark
gap are limited; this is even more true if,
for a suitable surface finish, the sensitiv-
ity of the workpiece must also be taken into
consideration.
The longer time required is counterbal-
anced by the fact that electric machining
often can proceed entirely automatically.
Fig.0 - Simultaneous spark drilling of
5 0.8 In holes in a 1 mm thick cut plate
of hard metal. Working time in all 20
minutes.
requires no hand work and, being set 4,
As a typical example, it is mentioned that a copper electrode will remove 0.08 -
0.15Egn of hard metal per minute and that, in this process, an electric force of about
1 kw is converted in the spark gap.
Both German statements as to the machining speeds indicate considerably lower
speed than the Russian, British and especially the American data, a fact which is no
doubt due to the newness of the technique and the relative low power of the machines.
In Fig.48 the D.E.W. drill press for spark machining is presented. This mach-
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inc does not differ basically from the other equipment described. The voltage feed
is controlled in this machine by a Leonard device, which maintains the proper size
of the spark gap.
Figure 49
gives an example of the drilling of a series of holes in a 1.0 mm
thick plate of hard metal. The diameter of the holes is 0.8 mm. Five wire elec-
trodes are fixed in the holder, and appli-
cation of a low working voltage all holes
,
can be produced simultaneously in 20 min.
Figure 50 shows a ring shaped miller
o 04
? with inside toothing, which cannot be pro-
duced by conventional methods of machine
?? tooling. The electrodes used are shown in
the lower portion of the photograph, on the
Fig.50 - An Example of a Job not Produc-
left a new one, on the right a used one.
ible with Conventional Machine Tooling
The electrode is attached to a holder so
Methods. With spark machining, the mu-
that it can slide into the opening. The
ler with inside toothing was produced
excessive wear is clearly indicated by the
with the help of the electrode shown,
difference between the new and used elec-
which was fixed in a holder. On the
trode, especially when it is taken into
left a new electrode, on the right a
consideration that the used electrode had
used one which has produced three teeth.
only produced three teeth.
Finally, Figure 51 shows two flash tools, whose thread profiles are produced by
the electrode shown. The steel was first shaped like the one shown on the left. It
will be noticed that is is possible to machine the steel shaft and the hard metal
plate simultaneously.
From the preceding it appears that the results obtained in Germany up to now
do not compare with those of the Russians, British, and especially the Americans.
It is emphasized, however, that this is only the beginning of a development, and
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that the future may show how far the process can be improved and which jobs should
preferably be done by spark machining.
Bibliography references: 23a, 42, 44, 45b, 59, and 60.
Patents: German No.672 832.
Resume
The account given in the preceding with respect to spark machining development
in Russia, the U.S.A., England, and Germany shows that in all these countries in-
tensive work is being done on the related problems and that apparently the method is
considered valuable as ameans of saving ex-
pensive diamond grinding wheels. In addi-
tion, the method is considered of great
significance for machining newer materials,
which are always being produced and which
can be machined only with difficulty when
using conventional methods. This is true
Fig.51 - Flash Tool in which the Thread
whether such materials enter into structur-
Profile (M20) was Produced with Spark
al elements or are used in the production
Machining, with the elec.-ode shown.
of tools.
The hard metal as well as the steel
The same principle used as a basis for
shaft are provided with profiles,
the spark machining applies everywhere. An
electric spark is caused to jump between a negative electrode and the positive work-
piece, while simultaneously a liquid is fed through the spark gap which is to deionize
the spark path, cool, and flush away the removed material. The spark voltage is al-
ways produced by charging a capacitor across a resistance with this capacitor being
connected in parallel, across the spark gap; direct current is used. This oscilla-
tor circuit is characteristic of all spark machining and was described for the first
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time by Lasarenko, wile was also the first to use this circuit in combination with
suitable equipment for applying the process.
Although the type of cooling liquid is given by different sources as widely va-
rying, there is general agreement, at least in practice, in using a dielectric mister-
ial, preferably kerosene, transformer oil, or certain liquids produced for the pur-
pose.
The circuit constants can be varied within wide limits, and the choice has An
influence on the machining speed and the fineness of the surface. High voltage, cur-
rent strength, and capacitance give great speed but little fineness, and vice versa.
Also of influence is the spark frequency and thus the amount of energy discharged.
The statements by various sources are quite different, and since the conditions
used for the available experimental results cannot always be determined with suffic-
ient certainty, it would be difficult to find a common denominator for the *widely di-
vergent information on machining speeds, available from the many publications on the
item.
The American reports seem to be the most optimistic, the German the least, and
the reports from Russia and England occupy a middle position. This obviously is con-
nected with the entire development in which the U.S.A. is, for the moment, far out in
front, while Germany entered the field only a short time ago, and England and Russia
take up the intervening position.
This is also reflected in the machinery which has been put on the market in the
respective countries. In the beginning, these operated with effects of one or two
kilowatt, while the German equipment is now at about 3 kw; the Russian slightly high:-
er, and the British at 6 - 7 kw. Opposed to this are American spark machining equip-
ment with effects which, a year ago had reached 12 kw.
It also seems that the Americans, aside from increasing the working speed by
applying greater effects, have also been able to preserve and improve the process
control by modifying the electric circuit and, by making suitable structural arrange-
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ments, have also been able to control the spark discharge itself so that flame of the
limitations mentioned by others have, to a certain extent, been successfully elimin-
ated.
There seems to be considerable agreement as to the surface finish which can be
achieved by spark machining. For many purposes, where an even smooth surface is de-
sired, it is common to encounter orders of magnitude of 60 4 inch (s, 1.504), and
often values as low as 15 - 20 R inch (0.4 - 0.5 4), or even down to about 5 - 10 4
inch (0.10 - 0.25 ?). The requirement for a sufficiently fine surface is apparently
not an insurmountable technical obstacle but can be achieved only at the cost of
greater machining time.
The possible machining precision is also, generally speaking, a matter of agree-
ment. According to the various accounts, it can reach thousandths of a millimeter,
but here too it is true that the closer tolerances involve increased time.
On the other hand the opinions are somewhat divided as to the surface layer af-
ter spark machining. The Americans say that it is not affected or destroyed, while
the Germans mention a destroyed surface layer of 0.05 - 0.10 mm thickness, which
must be removed by subsequent lapping with crushing bort or drilling carbide. Mid-
way between these two concepts, the British maintain that the surface is affected to
a depth of 0.002 - 0.003 mm, but in general this is not considered to require a re-
finishing.
The working speed depends on the effect used, the electric constants in the cir-
cuit, charging and discharging frequencies, and on the controllability of the spark
discharges themselves; in addition, the material in the workpiece to be machined is
of essential significance, inasmuch as the materials which traditionally are consid-
ered difficult to machine require the longest time also in this process. At the
same time, it is important that the spark pap be maintained constant; in practice
this is quite impossible unless the electrode voltage is servo-controlled. This,
in turn, has the advantage that entire machinings, once started, can run automatic-
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ally without continual inspection. To give general guide lines for the working
speeds, it can be stated with all possible reservation that, at a surface smoothness
of about 60 4 inch (Jig 1.50 ), 0.05 - 0.15 cm3/min or 0.80 - 2.25 g/min of hard met-
al can be removed, while the corresponding figures for hard steel are 0.05 - 0.25
cm3/min or 0.40 - 2.0 gm/min and for tempered steel around 0.1 cm3/min or 0.80 gm/
min. Softer materials can be machined more rapidly, and it should furthermore be
remembered that figures can be obtained which are considerably above or below those
mentioned here.
The electrode material is not critical, and often brass or copper is used. As
to the wear, there seems to be some disagreement. Aside from the first equipment,
the Americans claim that it is extremely minimal. The Germans report excessive wear
leading to distortion of the workpiece. it is emphasized that three times as much
of the electrode is abraded as there is material removed from the workpiece. The
Russians mention nothing as to wear, while the British, with reports of a reasonable
we4r, tAe the in-between standpoint.
Spark machining hA taken place mainly as a type of drilling, but has also of-
ten been applied to t.read-cutting. Not only inside but also outside shapings can
be performed merely by shaping the electrode correspondingly. The method has also
been applied with success in other forms for machining such as round grinding, plane
grinding, freehand grinding, turning, reducing, and sawing.
The spark method must indeed not be hastily compared with the conventional
forms of machining, so long as it is a question of usual materials. This is not the
case when it comes to materials that are difficult to machine, where standard meth-
ods either are very slow or fail entirely. Here the spark method can often compete
or even be the only answer. This must be taken into consideration in evaluating the
circumstances, and it must also be remembered that the process, once started, can
run automatically, so that the same operator can take care of many spark machines at
the same time, or perform other work in the meantime. Under conditions where this
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can be practiced, the question of a slightly longer or shorter working time is there-
been successfully used in various machinings. At the same
of machines for the process has been so steady that equipment
meaning that the method has moved from the preliminary stage
to the shop as a piece of production equipment like the rest
The new technique is only in its infancy, but if it fulfills its promise it
d be interesting to follow its future development, to observe its adaptation to
1 lobs of the shop, and to see what place it will take among the conventional
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ELECTRIC ARC MACHINING
In discussing the electric spark machining in the Section "Development in the
U.S.A." reference was made to the first beginnings of arc machining; at the same
time, an American device by the Elox Corp. was described which must be considered as
closest to an electric arc machining equipment. Figure 52 shows schematically ano-
Fig.52 - Diagram for Arc Machining. The current source Q furnishes a continuous or
pulsating direct current and is connected across the variable resistance R
with the electrode and workpiece. The latter, WS, is immersed in a coolant
DF. The spindle is set in H and carries the electrode WE. At the top,
there is a vibrator head VK which makes the spindle swing, so that an arc
is continually turned on and off between workpiece and electrode. The in-
set shows an oscillogram of the voltage slope. (W.Ullmann)
ther arc machining equipment and the corresponding electric circuit. This consists
merely of a current source which generates a steady or pulsating direct current and
which is connected across a variable resistance with the workpiece and tool. It is
preferable to use direct current, since the electrode wear is otherwise too great.
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The workpiece WS is immersed in a vessel with cooling liquid and the electrode WE is
fixed in the spindle, which is controlled in the bearing H. Above the spindle, the
vibrator head VX is fed by a usual 50-cycle mains. This causes the electrode con-
tinually to rise and drop on the workpiece,
thus continually connecting and disconnect-
ing an electric arc. The inset in the pic-
ture shows an oscillogram of the voltage
Fig. 53 - Schematic Diagram of the Fourth
Stage of the Production Equipment of the
Cincinnati Milling Machine Co., Shown in
slope.
In this method a dielectric can be
used as coolant, such as kerosene, trans-
former oil, or specially prepared liquids
(electrolytes such as waterglass or ordin-
ary water). The two latter liquids are
used especially for jobs which do not place
excessive demands on surface smoothness,
such as reducing and drilling of cracked
Fig. 30. The elliptical holes are drilled
cutting taps, etc.
out with a tube-shaped electrode. In
the three preceding stages the holes are
drilled in the three outer covers.
The result in arc machining are analo-
gous to those given earlier for spark mach-
ining. However, the arc frequency is con-
siderably lower than the spark frequendy so that the machining is done to a greater
degree by fusion of the surface. If the liquid used is an electrolyte, the condi-
tion mentioned is supplemented by a chemical effect. The surface will be less good,
partly as a result of the rougher fusing, partly because the arc has a tendency to
follow the once established and ionized path, regardless of whether, at a given mo-
ment, this represents the shortest way between tool and workpiece.
Arc Machining Equipment Produced
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Are machining can presumably be traced back to around 1920, and the first com-
mercial equipment appeared around 1942. Although equipment of this type has moitly
been used in jobs in which the smoothness of the surface is less essential, this
does not preclude finer machinings
from being carried out. There are
only a few machines of this type
on the market.
Figure 23 shows a diagram for
an Elox machine, and Fig.24 gives
a machine with an effect of 2.5 kv
amp. The spark gap is kept con-
stant with the help of a servo con-
It is stated that, with
Fig.54 - Portable Elox. Machine for racting
this machine, a 1-inch steel plate
Cracked Cutting Taps, etc. The electrode hold-
er is attached in a drill press, and the cabin-
can be drilled through with a
5/16" electrode in 5 min, and cor-
et with the electrical circuit is connected to
respondingly a bu tap can be
the power line. As coolant, ordinary water is
drilled out to a depth of 3/4" in
used. Weight about 25 kg.
5 - 6 min.
Figure 27 shows electric arc grinding with an Elox machine, and the production
equipment shown in Fig. 30, built by thC Cincinnati Milling Machine Co., also works
with an Elox machine. The last stage of the machine is shown in Fig. 53. The outer
diameter of the housing is 38" and the inner 22". The tolerance of the elliptical
holes to be drilled is I 0.13 =I, and the four holes are to be flush within 0.025 mm
while the angle between the axis of the holes and the vertical is to be kept within
a tolerance of 4. 5?. The 32 holes in the housing were drilled in a total time of
110 min. Tube electrodes of brass are used, having the shape of the elliptical
hole; the electrodes are fixed in four carriages, whose voltage feed is servo-con-
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trolled and which, in addition, is provided with an auxiliary feed driven by com-
pressed air. As coolant, a dielectric material is used, kept in a jacket which en-
cases the entire housing.
Since the end faces of the electrode must accurately fit against the surface of
the housing, four electrodes, each on its carriage, must be used because of the dif-
ferent diameters of the four walls. The first electrode drills through the outer
wall, the next electrode projects through the hole formed and drills the next mall,
etc. The drilled-out pieces are held in the cavity of the electrode by means of a
permanent magnet.
Machines especially designed to drill out cracked taps etc. are often made por-
table. They consist of an electrode holder which can be mounted on the spot in an
available drill press, together with a cabinet with the electric equipment. The en-
tire unit need only be connected to the electric circuit, and the coolant can be
taken from an ordinary faucet at the site. Such an outfit is shown in Fig.54; it
works rapidly and cheaply and is quite mobile, since it only weights around 25 kg.
Aside from the Elox machine mentioned, corresponding machines for extracting
taps are made by the French firm Qualitex in Paris and by the Czekoslovaken Re-
search Institute VUMA.
General Pointers
The working time in arc machinifiglas in spark machining, depends on the voltage,
current strength, and arc frequency. The tension is preferably low, usually 20 - 30
v, while the current strength may be as high as a few hundred amperes.
The precision varies with the electrical constants, so that high machining
speed gives little surface smoothness and vice versa. It is possible to reduce the
roughness depth as far as 5 11 inch r.m.s., but values of 20 - 60 P. inch are more us-
ual. The precision can normally be kept within some few hundredths millimeter and,
with care, it is said that it can be reduced to 0.005 mm.
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. . 4 .
6 ............ ..... .......... ... ,....._ .. ..,.... .....- -
?
' The electrode, which is generilly brass or copper - in?rsrer cases with Simi. i
?
work, ground molybdenum rods are used - is subject to some wear which, however, is
relatively small in arc machining. It is stated that, in tool steel, it is possible
to drill a combined hole depth of 419 with a total wear of the copper electrode not
exceeding 1'1.
Arc Grinding of Spheres
As a special application of the arc method, it was used around 1950 by a Russian
firm to refinish ball bearings. This was done in a special lapping machine (see
Fig. 55 - Russian Are Machining Equipment for Lapping Balls for Ball Bearings. The
upper wheel is divided into three mutually insulated segments, each of which
is connected to the secondary of a three-phase transfmrmer. The lower wheel
rotates at a speed of 500 - 600 rpm. The balls are fed in the center of the
upper wheel and move toward the periphery where they drop down.
Fig.55) whose upper stationary wheel consisted of three segments .insulated from each
other as well as from the stand. Each of the segments was connected to the second-
ary of a three-phase transformer which furnished a 20 - 25 volt voltage. The lower
wheel rotated with a speed of 500 - 600 rpm. The balls were fed through a hole in
the center of the upper wheel and gradually worked themselves toward the periphery
where they dropped down. At the points of contact between balls and lapping wheel
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Nee
gi? ,
tt4
?-_
'a series of fine electric arcs was formed, which partly decarbonized the outer layer
and partly removed smaller particles, which were flushed away by a stream of water.
The current strength was of the order of 1200 - 1500 amp.
At a At a voltage of 15 - 20 v,
balls of 13/16" (xi 20 mm) balls
, were reduced in diameter with a
Fig.56 -
proved. If the voltage is lowered
Capacity 30? mm. The spindle dock is set on a
further to 0.5 - 5 v, a surface is
carriage which is operated by compressed air.
obtained which, practically speak-
The wheel is a soft iron plate of 300 mm size
speed of 0.3 - 0.4 mm/min, while
the rounding was kept within
1/100 - 2/100 mm. t was a pre-
!
1'1 requisite that all the balls had
the same original size. The sun-
Arc Machine for
Reducing
face, after machining, shoved
grooves of about 0.05 mm, depth
which were later removed, along
with the decarbonized surface lay-
er, by ordinary lapping.
If the voltage is lowered to
7 - 12 v, the ball diameter is re-
duced at 0.06 - 0.10 mm/mm, and
the surface is considerably im-
ing, has a polished appearance and
and 1 mm thickness, which rotates at 800 rpm.
great regularity of shape. In
The working voltage is 15 - 25 v, current.
this case, the working speed is so
strength up to 40 amp (VUMA)
of the diameter per minute is removed. It is
97
greatly reduced that only 0.03 mm
claimed that the old method could ban-
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die 3000 balls in 8 to 9 days, while the arc method mentioned here rewired only 3
to 4 days, and at the same time resulted in considerable current saving.
Reducing with Electric Arc Machines
A special field for arc machining lies in sawing, reducing, etc. Figure 56
shows a Czech machine, designed like a circular saw. The spindle dock is placed on
a carriage which moves on rollers against a compressed-air cylinder. The amount of
voltage is regulated by the electric working circuit. The spindle dock carries a
0.5 kw Motor which, over V -type belts, drives the working spindle at a speed of 800
rpm. As a wheel, an ordinary soft iron plate of about 1 mm thickness is used. If
this is about 300 mm in size
Pio
the cutting speed will be about 13 m/sec. As a coolant
os
flo
?
Fig. 57 - Dependence of the
on the Current Strength in
ducing hachine (VUMA)
terial
driven
up to 150# mm.
waterglass of 1.30 sp.gr. is used, some-
times with an admixture of Na3PO4 at a ra-
tio 1:20, which is said to reduce the wear
on the wheel.
As a current source, a cooled three-
phase selenium rectifier is used, which can
give around 40 amp, with 15 - 25 v. With a
variable resistance in the circuit, the ma-
chine can work in four steps at varying am-
Cutting Tditalt,-,
perage. The capacity of this machine is as
a Large Re-
high as 30 mm round material.
A larger analogous machine can cut ma-
The wheel is 800 mm in diameter and runs at about 300 rpm,
by an 0.8 kw motor. The wheel width is 1.5 - 2 mm, the voltage as in the
small machine, i.e., up to about 25 v, while the current strength may reach 200 amp.
Figure 57 shows the cutting time for this machine as a function of the current
strength, indicating for example, that a 100 mm round iron piece can be cut in
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10 min at 160 amp.
A similar arc machining equipment, but somewhat smaller, was constructed about
three years ago in the metallurgical laboratory of the Darmstadt Polytechnic Insti-
tute where it has been frequently used with good results (Bib1.40,41). This machine
uses a pulsating direct current which is produced by superimposing a steady direct
current with the alternating current from a transformer connected in the circuit.
The remaining data are in line with what was indicated before.
This machine, as far as can be seen, represents the first Danish experiment in
which use was made of electric Machining methods. But it should be mentioned in
this connection that the Laboratory for Machine Tools, Darmstadt Polytechnic Insti-
tute, as early as around 19h8 - 1950, had an electric arc machining device for test-
ing. The machine was of quite simple construction and exclusively intended for
drilling out cracked cutting taps, etc. The tests were very brief and not very suc-
cessful, since the electric circuit proved to be out of order; therefore, work was
discontinued and the machine returned.
Bibliography references: 4, 5, 7, 10, 16, 22, 23a, 32, 37, 38, 39, 40, 41, 42, L3,
45a, 49, 59, 60, 61.
Patents: Swedish No.76 026; German No.672 832; British No.335 003, No.507 392, and,
No.578 933; French !o.1 024 353; Swiss No.298 974; and U.S.A. Nos.1 333 311,
1 620 519, 1 556 325, 1 701W?; Re..20 035, 2 258 A80, 2 374 348, 2 383 382,
2 415 690, 2 441 954, 2 650 979.
99
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-
ULTRASONIC MACHINING
Including ultrasonic machining in a discussion of electric machining methods is
not entirely correct. The chip removal proceeds here purely mechanically; however,
the method of producing the necessary energy as well as the working procedure in the
entire process, makes it natural to treat ultrasonic machining together with elec-
tric machining methods.
Ultrasonics, as is generally known, is mechanical vibrations of such high fre-
quency that they cannot be perceived by the human ear, i.e., from about 20,000 cy-
cles upward. If a bar is exposed to ultrasonic vibrations in a longitudinal direc-
tion, it can in the presence of resonance and at sufficiently high energy, be set
into mechanical longitudinal vibrations, with amplitudes of several hundredths milli-
meter.
It has been known for some years that it is possible to bore holes in hard met-
al with such a vibrating rod. Since this moves only in a longitudinal direction,
holes can be drilled with arbitrary profiles, corresponding to the cross section of
the rod. This effect can be increased greatly if, during the working process, a
finely divided grinding compound is added since this will permit direct grinding or
lapping. Since the machining is thus purely mechanical, this method is the only one
of those described here, which can 134-G-ed with nonconductive materials.
The process can actually be compared with the effect of a compressed air hammer.
whose chisel vibrates at low frequency, in that the individual grinding grain corre-
sponds to the chisel. The use of the numerous cutting edges and the change to high-
er frequencies denote a great increase in the precision of the work. At the same
vibration energy per surface unit, the acceleration forces increase proportionally
to the frequency. A grain which performs 10,000 vibrations per second exerts, at the
same amplitude, the same force on the workpiece as al0,000times heavier grain which
executes only one vibration per sec. It is therefore possible with fine grains and
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?
*
high frequencies to achieve precise and rapid work.
Normally, frequencies between 20 and 30 kc are used in ultrasonic drilling, as
a rule around 25 kc. Higher frequencies will improve the precision, but at the same
time give lower efficiency, just as the drill holder whose length is determined by
the resonance factors, becomes
shorter with increasing frequen-
cies. Even at 100 kc, the drill
holder may be as short as 3 cm,
which is impractical in many cases.
Fig. 58 - Drill Head in an Ultrasonic Drill Press.
A is a vibrator heat consisting of a stack of
nickel plates, which under the i/iflaenep of high-
frequency vibrations in the coil D will execute
longitudinal vibrations. At the node of the vi-
brations, the vibrator head is attached to C. A
highly permeable yoke E carries polarizing poles
F which are fed with direct current. B is a
Principle of the Ultrasonic Drill-
ing Machine
Ultrasonic machines are con-
structed like small drill presses,
which may be more or less complex.
Examples are known of the simplest
design, in which the drill head
slides in precision ways and in
which the material is moved by
precise. cross slides; in this
type, the machines are actually
similar to jig drill presses.
A vacuum-tube generator is
included in the equipment, which
delivers high-frequency vibrations
to the drill head, where they are
converted to mechanical longitud-
soldered piece of stainless steel with attachment
inal vibrations. The process is
thread for the drill holders G,H, and K (Mullard).
shown in Fig. 58.
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In the sketch, A is the vibrator head which is constructed of a stack of nickel
plates whose magnetostriction properties are utilized. This is demonstrated by the
fact that certain materials (iron, nickel, cobalt), when exposed to an alternating
current, will change dimensions in time. The length of the plates corresponds to a
half a wavelength; at the vibration node, the plates are attached to the cross piece
C.
The upper end of the plates is surrounded by a coil D which is fed with alter-
nating current from the generator, so that the vibrator head A will execute longitu-
dinal vibrations in synchronism with the imposed high-frequency vibrations. Here, E
is a highly permeable yoke, wound with a polarizing pole F which is supplied with
--
NEJETIviliRATOR
Dilly, COIL
1
APIPtI1.10- POwill OUTPUT
POWSIE UNIT NED
TRANSDUCER
POLAAIZING
POLAMUNG
COIL
NO0/1.1. CLAN/
? MATCHING.
STUEI
DRILL HEAD
Fig. 59 - Schematic Setup of an Ultrasonic Drill Press with Electric Circuit (Mullard),,
direct current. The lower end of the vibrator head is soldered to a piece B of
stainless steel, with attachment threavzbr the drill holders G, H, and K.
The length of the drill holder must be so adjusted that its natural frequency
corresponds to that of the vibrator head, in order to obtain maximum amplitude. At
25 kc, this means that the drill holder should be about 12 am long. The vibration
energy is transferred from the end face of the vibrator head to the usually much
smaller end face of the drill holder; if this is to take place without loss, the vi-
bration amplitude must be inversely proportional to the size of the faces. This
presupposes a definitely conical form of the drill holder, which is made in practice
102
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with an exponential producer curve.
The amplitude at the end face of the vibrator head in the equipment described
here (which is for 50 watts) is 13 4 and three drill holders are used which, accord-
ing to shape and size, increase the amplitude in the end faces of the drill holder
2, 3, or 6 times, so that the amplitudes here
est drill holder gives the least oscillation.
The drill holders can thus be regard-
Figure 59 shows schematically the entire arrangement with electric circuit.
e 60 shows a very simple 50-watt ultrasonic drill press by the British firm
Mullard. It can hardly be of simpler de-
sign, since it consists only of the drill
the arm shown, which is counterbalanced.
The material is placed loosely in the dish-
Column, Counterbalanced and Manually
erated with an Arm. The material is
placed in the dish-shaped base plate
frequency of 20 kc which, however, can be
varied slightly in either direction. The
direct current to the polarizing pole is
also tapped from the generator which, for
this purpose, contains a rectifier.
11
This sn 11 machine drills holes from
0.15 to about 13 mm, at a depth up to 13
mm. If an abrasive is used with grain size
120 (American mesh), the dimensions can be
(Nullard).
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kept within 0.05 - 0.06 mm. If finer grains are selected, such as 1000, the preci-
sion can be increased to around 0.01 mm. The depth of roughness can be brought down.
to less than 1 u, depending on the grain size selected.
The working time is dependent on the hardness of the workpiece and the size of
the drill. A square hole with sides of in can
be drilled with silicon carbide in sodium
glass at a speed of 2 mm/min. With tungsten
carbide, when using drilling carbide as an a-
brasive, a speed of about 0.1 mm/min can be
achieved.
In addition it must be remembeeed that,
in this method, it is useless to try to in-
crease the speed by exerting more force on the
drill, since the only result is a reduction in'
vibrations that partially stop the machining
process. The pressure should be quite low at
all times.
Figure 61 shows a 250-watt ultrasonic de-
vice, also by Millard. Here the entire "drill
do61K with the vibrator head, etc., is con-
structed as a unit which contains a soldered
precision way for movement of the head. The
drill head itself is constructed in principle
as in the smaller machine, but instead of be-
ing operated manually, a compressed-air cylin-
der is added which automatically performs the operating motions and periodically
lifts the drill to permit feeding of new abrasive, after which the drill is lowered
again. The air pressure is applied at about 4 kg/cm2, and the air consumption is
Ultrasonic Device. The drill head
is in principle as in the machine
-in Fig.60, but the voltage is ap-
plied by means of a built-in
compressed-air cylinder. The drill
dock contains a soldered way and is
intended for attachment to an avail-
able machine (Nullard).
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_) around 0.2 0/min.
The electric as Well as the pneumatic equipment, together with all control units
and instruments, is assembled on a panel from which all connections lead to the
drill dock via a simple flexible hose.
The capacity of this machine is up to hole diameters of 50 mm and depths to
supervision, since the abrasive can
trasonic drill press Diatron for 500
Fig.62 - 500-Watt Ultrasonic Drill Press of
Precision Design. The vibrator head is
cooled in an oil chamber, which contains a
copper coil through which water flows. The
tool is held with a constantly regulated
pressure against the workpiece (Diatron).
a) Drill pressure; b) Exchangeable drill head
actual precision machine with cross
slides for shifting the material, and
The drill head is mounted in the dock
with a soldered precision way and can
be moved by counterbalance and almost
without friction. During the work,
the tool is held elastically against the workpiece with a constant pressure which
can be read from a dial gage and whose amount can be set between 100 and 1500 gm,
depending on the diameter of the hole.
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The vibrator head is constructed in a way similar to that previously described,
but is based on the greater electric effect applied in an oil chamber, which is kept
cooled by sending a stream of water through a copper coil in the oil bath. This pre-
vents overheating of the vibrator head since the efficiency at best is about 70% and
since 30% of the energy introduced is therefore converted into heat. The amplitude
is near 0.1 mm.
In the U.S., ultrasonic drill presses have been built, which accurately corre-
spond to those mentioned here, e.g., Cavitron and Raytheon. In these, effects as
high as 1.5 kw are used so that the working speed is naturally increased. Figure 63
Fig.63 - American 1.5 kw Ultrasonic Drill Press with Electric Equipment
in the Control Panel (Sheffield Cavitron).
shows a full-scale Cavitron machine.
General Pointers
Concerning the working speed, it is to be expected that this will rise with
increasini7, effects. This is true, however, only within certain limits. If, for ex-
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ample, the 500-watt machine is considered and it is remembered that the end faces of
the vibrator head have an area of about 10 cm2, Lis corresponds to an evenly dis-
tributed vibration energy of 50 watts/cm2. Such a value presupposes a relatively
high fatigue limit of the material of the vibrator head. It has, therefore, been
necessary to develop materials which partly had a high breaking strength and partly
possessed a high magnetostriction effect with small eddy current loss - hysteresis -
and low magnetic loss.
There is an even greater need for good breaking strength and elastic properties
of the drill holder. In the end face of the holder, all the energy is concentrated;
since it is quite thin - probably about 1 cm2 - this means that the energy density
is of an order of 500 watt/cm2. Such an energy concentration cannot be withstood by
even the best types of steel, since stresses and strains are set up which lead to
deformations far above the elastic limits. This result in overheating of the mater-
ial and, after a fairly long time, lead to fatigue cracks. With smaller holes, it ?
is therefore impossible to force the speed by increasing the effect, since the ma-
terials used set an upper limit for the power of the machines. With the use of small
tool dimensions and large effects, the tool will become red-hot in a short time.
The effect is therefore so laid out that the vibration energy is imparted to the
individual grinding grains, which are distributed over the end face of the tool and
make this execute vibrations about. it resting point in the longitudinal direction
-
of the tool. The end face of the tool, meanwhile, is also ground and acquires a
hollow shape. With through-holes, this makes no difference, but with part-way holes
the end face must occasionally be smoothed if a level bottom is desired.
Obviously there will also be grinding grains between the walls of the hole and
the sides of the tool, which leads to a grinding effect here
slightly conical. This is more pronounced at deeper holes'.
which makes the hole
If a hard metal plate
of 5mm thickness is bored through, the hole will be about 0.1 mm larger at the top.
Under certain conditions, e.g., with a matrix, this conicity can be used to advan-
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-ttage while in other cases it is impermissible, so that the hole must be made cylin-
drical by using two or. three successive drillings and making these remove less and
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!less material.
The method can be used in machining hard and brittle materials, such as hard
-zmetal, tempered steel, titanium, glass, ceramics, germanium, diamonds, etc., but it
10
is not adapted to softer and tougher materials. In the latter, the grinding grains
cannot work effectively, since they will settle firmly in the workpiece and just lap:.
the tool. A case-hardened workpiece thus can be drilled rapidly as long as thema-
chining is done in the hardened surface layer, but after this layer is broken
through, the speed quickly reduces.
The drill holder is preferably made of high-quality steel. In eases where the
end face is to be especially small, monel metal can be used with advantage, since
the breaking strength and elasticity limit of this material are very high, so that
the previously described difficulties disappear to a certain extent.
The drilling tool itself is made of a tough but not brittle material such as or-
dinary soft iron, pure carbon steel with 0.2 - 0.5% C, St. 85.11, or a tough and
nonwearing Cr - Ni steel. The choice depends on the requirements made. If the tool
is to be nonwearing and keep its shape, Cr - Ni steel is chosen, i.e., for example
in drilling deep holes. With more complicated shaping, St. 85.11 is often prefera-
ble since it is easier to machine..- eteo? _In all cases, the drill holder as well as
the drilling tool are soft and therefore fairly easy to machine. The tool cannot be
screwed or brazed to the drill holder.
As an abrasive, depending on the circumstances, aluminum oxide, silicon carbide,
drilling carbide, or perhaps diamond dust is selected, with the size of the grains
adjusted to the purpose. The abrasive is preferably triturated in water or oil and
circulated by pumping.
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,ts'At.11.
_
Precision and Working Speed
With careful working, a precision of several thousandths millimeter can be 4,
ichieved. This presupposes the use of many tools in succession which work each with
a decreasing grain size; nevertheless, the first drilling may easily take from 10
:1_ to 100 as long as each of the subsequent drillings, when there is considerably more
material to be removed. If possible, it is profitable to use a tube-shaped drill
and to drill a piece or prebore by another method - with conductive materials, e.g..,
with one of the earlier named electrical methods.
At 280 grains (grain size 60 - 80 ? ), a surface roughness of 1? can be achieved,
and with finer grains even less. As opposed to the electric methods, ultrasonic ma-
chining gives a smooth and shiny surface, which can simply be polished, so that a
completely finished tool can be made with this method.
It should be mentioned that this method is also useful for rapid cutting. In
that case, the tool must have a thread and neither the tool nor the workpiece must
be permitted to rotate with a speed adapted to the pitch of the thread. Another
feature is the fact that it is also possible to drill crooked holes by using a cor-
respondingly bent tool.
The working speed is dependent on a series of factors, such as the hardness of
the morkpiece and its breaking strength. The more brittle the material, the more
rapidly can it usually be machined. The dpyth of the hole also plays a part, in
that the work progresses more slowly the deeper the drill penetrates. This is due
to the difficulty of the abrasive in penetrating through the narrow opening between
the wall of the hole and the tool. The method is nevertheless always slower than
the conventional methods of machining and also slower than the electric machining
methods. Its main advantage is the great precision which can be achieved and which
only in exceptional cases demands reCinishing. There is no destruction of the sur-
face as described for the electric methods.
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Table 4
Amount of Material Removed in mm3/min and Electrode Wear
Material
Material
Removed
mm3/min.
Wear
Tool/Workp. I
in %
200
0.8
Glass
Agate .
54
4
Ruby
30
8
Hard ceramic
26
6
Hard metal H 1
10
50
Hard metal G 5
6
70
Sintered drilling carbide . . . .
4.5
50
Dural
3.8
25
Brass lis 63
3.6
40
Quick steel
2.2
200
Steel
1.8
100
Chrome stee1,12;:
1.6
200
1
From a German source, Table 4 lists working speeds for various materials. The
tool used was a hollow six-sided piece irt. 85.11 with 27 mm2 area (8mm needle
width); the abrasive was drilling carbide, 220 mesh, and the machine effect was
500 watts. In addition, the wear of the tool, measured in percent of the wear of
the workpiece, is given. The figures must be accepted with some caution and can
only be considered indicative.
Fields of Application
The ultrasonic method is widely used for producing tools which are used for
non-chip-mloving machinings. As mentioned, the limitation is mainly jn the size of
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the woDkpiece, but with just such tools usual grindings can also be difficult, es-
pecially if the workpiece cannot be divided.
Figure 6) shows an example of ultrasonic machining in which a chipbreaker is
ground with a piece of flat iron soldered to the
drill holder. As an abrasive, drilling carbide is
used. The work, according to the statements, took
several minutes which, generally speaking, corre-
sponds to the time for ordinary grinding with dia-
mond wheels.
Figure 65 shows an example of a cutting plate,
executed in hard metal and drilled by means of ul-
trasonics.
Bibliography references: 19, 22, 27, 29,
:18, 42, 45, h5c, 60.
Breaker. The tool is a piece
of flat iron. Drilling carbide
was used.
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AIL.*
;
()TOR ELECTRICAL MACHINING METHODS
?
Aside from the electric machining methods mentioned preyiously talerp.4r, others!
Ithich are less known or which find application within more limited areas. These
10__Imethods cannot be quite as readily classified in the groups mentioned before and, in!
12 .any cases, consist of combinations of one or more of the other methods.
1
DrillinK of Very Small Holes
.!6
By spark machining, holes with a diameter as small as about 0.1 mm can be
0__drilled by means of thin tungsten wire; in the watchmaker's technique it is known
2.2 that, with special drills and with extreme care, this can be reduced to hole diame-
ters of 0.05 mm. By a specially developed technique, using precision-ground ordinary
sewing needles it is possible to drill through thin foil plates with holes down
to 0.005 mm, but here the possibilities cease.
It is rather rare that such small or even smaller holes are required, but such
sizes are occasionally required
for diaphragm apertures in preci-
sion optical instruments and, for
example, in electron microscopes.
By means of recent electronic
optics, it is now possible to ob-
tain sharp focusing of corpuscular
Executed in hard metal and set in a steel block.
rays and thus produce an intense
heating of a very small area. If a surface is struck by such an electron ray, whose
voltage may reach over 100,000, it is vaporized and a hole appears, while the mater-
ial is again condensed by the cold parts of the equipment. The.drilling is done in
special devices under vacuum.
Fig.65 - Cutting
Plate
Drilled by
Ultrasonics.
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774.446.....41?6.4 . "?..te",t,?.3,.-. - .7,',,ii. - "..-----"..7-..,,-,,,,,,,-.,?-, 11-,14%;.,:v tL..,,,41
..?- ?-. . --, .
% ? , - -- ?
-1 The drilling time is extraordinarily short; in a few seconds, holes with diame-
,
z
..,:l
..-.--1
...3.ters between 5 and 10 ? can be drilled in 7 - 8 nun thick plates of glass, ruby or
b...i- similar material. This permits drilling of holes with diameters below 1 P.
Coating of Steel with Hard Metal
?
1 ?
Another method, which perhaps has somewhat greater possibilities of practical
4.
...- f application, consists in coating parts subject to excessive wear (as, for example,
1
1
cuttingtools of carbon steel and high-speed steel) with a thin layer of hard metal,
,so that it acquires a considerably increased resistance to wear; as far as tools are
concerned, this will result in a longer service life - judging from reports, up to
3 to 5 times.
In this method a vibrator can be used, designed roughly like an electric re-
cording pen. A hard-metal electrode is used, which is held against the tool at the
spot where it is to be coated; when the electrode vibrates, a series of sparks is
generated so that hard metal passes from the electrode to the workpiece, where it is
deposited as an evenly distributed thin layer. During the process, the workpiece is
connected to the negative pole of the circuit and the electrode to the positive pole.
Instead of the simple setup mentioned, a spark-machining device can be used,
where the process is allowed to take place in air.
It is assumed that the very.fine hard-metal particles, which are transferred to
the workpiece, diffuse into its surface and that the abrasive resistance i8 further
increased as a result of the reaction of the surface with atmospheric nitrogen.
Finally, it is possible that a local thermal reaction takes place as a result of the
rapid heating and cooling during spark flashover.
The applied layer is very resistant, but it is claimed that it is too thin to
serve as an anticorrosion coating. The coating is so hard that it can be worked
only by lapping with drilling carbide.
The characteristics of the surface layer depend on the electrode material, the
11?
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voltage, capacity, and current strength in the electric circuit, the duration of the
process, and also on the roughness of the surface treated. Hard metal which con-
tains ti-i,aniur carbide gives a more wear-resistant layer than pure tungsten carbide.
In parts for measuring instruments and in tools for fine machining, graphite elec-
trodes are often used, which give a very smooth surface finish.
According to the desired fineness of the layer, voltages can be selected be-
tween 50 and 220 v, capacitances between 70 and ?50 af, and current strengths be-
tween 0.25 and - amp. The obtained layer thickness is fror less than 0.01 to
about 0.1 mm. The lower these values, the finer will be the layer and the less the
thickness. The finer the surface to be treated, the better will be the layer; the
workpiece must be absolutely clean and free of grease, rust, etc.
Under given conditions, the quality of the surface layer is determined to a
high degree by the duratior of the process. If this is continued beyond a certain
l!mit/ dark stains appear in the surface layer which indicate that local destruction
has taken place. 1=ormally, the surface is treated until the spark picture changes
'a\
? ?
character. This is'indacated by the sparks losing their brightness and original
fort, after a certain time has passed.
As far as it is possible to say new with respect to 7eneral practice, tools
treated ir this way are preferably used in heavy cutting of hard metals and only to
a lesser degree in jobs with ,_:rat speed and small chip areas, in softer Materials,
or in simple machininr.
When a tool treated in this way is ground, it must be refinished and a new coat-
ing of hard metal must be applied. The coating is applied only at spots of greatest
year, and it is important Lhat the edge is not directly exposed to the effect of the
sparks. The treatment therefore berinn nli htly in bacl, of the edge and moves slow-
ly toward it, but without proveeding so atr forward that the sparks are drawn di-
rectly fro; thc rutting edge.
as fAr ts can he determined, the method was first proposed in 19V) by the
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Russians Ivanov and Titov and has apparently found extensive use in Russia, not only.
for cutting tools but also in machine parts in general. In a large factory in
Czechoslovakia the author had the opportunity to see all spiral drills treated by
this method, where a female worker in the tool grinding shop, in addition to watch-
ing several automatic grinding machines, also coated drills with hard metal.
According to Russian sources, the price for such equipment is between 2000
and 6000 kr. The necessary effect is about 0.8 - 1.2 kw. The time for treating a
surface of our square centimeter varies between 0.5 and 3 min, and the power consump-
tion in treating 1000 tools with 16 x 25 mm shaft cross section is 1.2 - 1.5 kw - hr.
The consumption of hard metal containing titanium is 2 to 5 mg per tool.
Electrolytic Grinding with Diamond Wheels
Electrolytic grinding has already been described. This method, however, can be
varied in different ways so that the process, to some degree, shifts character.
Figure 66 shows three such possibilities. The top portion shows pure electrolytic
grinding with a simple DC circuit and the tool connected to the positive pole. As
liquid, an electrolyte is used, and as a blade an iron plate or cast-iron wheel. If
the circuit is modified so as to correspond to Lazarenkols the center picture is ob-
tained. This results in a type of spark grinding, rising a dielectric material as
fluid. The wheel is preferably of?tegl. or cast iron. Machines of this type have ?
?-?
been built in individual cases, and such a small grinding machine is shown in Fig./1.
The process is actually that of spark grinding. Finally, an alternating-current cir-
cuit can be used, as shown in the bottom figure.
If the conventional wheel in the upper two pictures is replaced by a metal-
bound diamond wheel, a process will result which has proved highly advantageous in
grinding hard-metal tools and other hard materials. This procedure apparently was
developed originally in America, around 1950.
Figure 67 shows an experimental setup. The diamond wheel is mounted to the ma-
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_Ichine and is insulated, and the negative Voltage is fed to the spindle through a ma
-
_icury_bath. ,The workpiece is attached in S holder on a crose_slidesioehiohje_dis=:__
4_i
placed on rollers. The spindle speed is infinite-
ly variable, and th? workpiece is guided back and
16 ?
a)
- Three Methods of Elec-
24...-itrolytic Grinding, Shown Sche-
.:. matically. At the top a simple
DC circuit, next a system, with
Lazarenkols oscillator circuit,
- 'and at the bottom an AC circuit.
If the iron wheel in the two up-
per figures is replaced by a
metal-bound diamond wheel, an
especially effective grinding
is obtained.
a) Circuit diagram; b) Disk;
c) WOrkpieee; d) Working fluid
forth over the wheel by means of a crank mechanism
which is attached to the slide. The workpiece is
forced against the wheel, with a suitable pressure
tapped from a compreised-air cylinder.
In the experiments, the workpiece was made to
move back and forth for 20 min over the wheel, at
rate of 35 times a minute and at a pressure betweeil
the workpiece and the wheel of 3 kg/0=2. The
wheel speed was about 2600 - 3500 rpm which, for
the 611 wheel used, corresponds to grinding speeds
of 20 - 27 m/sec.
The rectifier furnishes a voltage up to 20 v
and 75 amp, and the machining is done at current
densities of 25 - 50 amp/cm2.
The effect in the process is partly an anodic
eolutio.n_as in the pure electrolytic grinding,
- ,
partly a mechanical removal of material by means
of the diamond wheel.
It is asserted, however, that there is no reg-
ular grinding involved. The wheel will circulate
the coolant which otherwise would tend to deposit a film on the workpiece, rapidly
halting the process. It is the function of the diamond grains to assure a suitable
distance between wheel and workpiece and to cut the film which settles on the work-
piece. The cobalt of the hard metal will also be dissolved more rapidly than the
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carbides and these, which are relatively poor conductors, will remain behind; how-
ever, since they are now exposed, it is relatively easy to remove them by the
Fig.67 - Experimental Setup for Electrolytic Grinding with Diamond Meels.
A crank motion leads the workpiece back and forth over the wheel, while it
is held against it with a constant pressure by the compressed-air cylinder.
diamond.
Figure 68 shows some experimental results. The curves A and B give the quanti-
ty of hard metal (Carboloy 78 B) in cubic inches per minute times 1000, which can be
removed. Here, A corresponds to a current density of 45 amp/cm2, B to.24 anp/cm2;
C is the result of a normal grinding with bakelite-bound diamond wheels and D with a
metal-bound diamond wheel just like those in curves A and B. The curves also show
the duration of the experiment, which in many cases was fairly lon7, to get an oppor--
tunity to judge the wear of the diamond wheel.
The surface roughness was measured every half hour with a profilometer and av-
erage values for the grindings h, 8, C, and D of 14, 11, 6, and 64 in rms.
The wear of the diamond wheel was so negligible in the electrolytic -rinding
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that it could only be measured after 40 hours. At that time, there was a wear
of 1.5 it, per cubic inch of hard metal removed. For the wheel according to curve C,
1 4
4
? 3
3
a
ki
9.
a
11
CUOVI CW11110 NNW/ iV CIIINOIMG venlig MUW4
Ao? Poo froI.
A
390
133
Now,
O Ni..
10
Itiskoilhe IMO.N1011.114 Nom,
itisSyht 04044 loom P.
C1141?01.111011114 01(1041100.111 IS.
Om we set.???? 0.1314101:14* Wee
20
TIME. HOURS
30 40
Fig.68 - Ground-Off Hard fletal (Carboloy 78 B) in Cubic Inches per flinute,
Times 1000.
A - Electrolytically ground, current density 45 amp/cm2, B - Ditto, current
density 2h amp/cm2, C - Normal grinding with bakelite-bound diamond wheel,
D - Ditto, with metal-bound diamond wheel. The time axis indicates increased
duration.
the corresponding wear was 20??, i.e., the diamond consumption in the electrolytic
grinding is less than 1.;.; of the consunption in normal grinding with a bakelite-bound
wheel.
The surface roughness can be reduced farther than the figures indicate, if
proper care is exercised; especially rood results are obtained if the grinding is
done for the last few seconds without fluid. The surface is dull and smooth as in
all other electric machining methods. It is also indicated that, in this form of
grinding, there is a certain rounding of the corners which is. greater the longer the
;Tinding lasts. If the -;rinding process is short, the problem is of practically no
rA,:n5ricance and the effect can be reduced or avoided by coating the corners with a
118
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thin layer of shellac.
As liquid, various electrolytes 'ere used here, of which a 5 sodium nitrite
solution proved to be the best suited, all things considered.
With this
simple electriC circuit, care must be taken that no spark or arc for-
mation occurs, since this may cause excessive destruction of the surfaces, which
means that there is an upper limit
for the useful current densities. If
a more complicated electric circuit
is used, a certain spark formation
can be allowed, since this can now be
better controlled. Such an electric
machine has been built by the American
firm Anocut Engineering Co., Chicago,
and is furnished for use in combina-
tion with conventional grirding ma-
chines which rust be modified so as
to have the spindle insulated. Fig-
ure 69 shows this equipment in combi-
Fig.69 - Electronically Controlled Power and
Control Yachinery for Use in Electrolytic
Grinding, in Combination with ebuilt Ya-
chines with Insulated Spindle
nation with a rebuilt plane grinding machine.
The equipment consists of a rectifier which furnishes h - 16 v and /00 amp, and
is thus considerably more powerful thar the one mentioned previously. In addition,
the cabinet contains an electronic control syster with two functions: First, it ad-
justs the voltage and current strength to the right amount for the greatest possible
removal of material and next, the spark formation is adjusted to a suitable value.
On the one hand, this is sufficiently intense to speed up the ;rinding process con-
siderably and, on the other hand, not so intense as to prevent a srlooth and satis-
factory surface or as to lead to arc rorration. This is done by reducing the volt-
age; the ,,ntire control is fully autoratic, without assistance by the operator.
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Electrolytic grinding with diamond wheels apparently is a promising electric ;
machining method, not only due to the fact that the necessary equipment is relative-
ly cheap and readily obtainable but also to the fact that the grinding itself offers
great advantages. These primarily include economy which consists in savings of dia-
mond bort over ordinary grinding; laboratory experiments showed a saving of 99%
while the saving under shop conditions is given as 80 - 907. In addition, on the
average about six times as much material per unit time is removed in electrolytic
grinding as in normal grinding, which means that the grinding costs are reduced to
about :13`5 of the normal. To this must be added the fact that a surface is obtained,
whose roughness is similar to or less than that obtained in regular grinding and
which, in addition, is free of grinding scratches and in the long run will be more
uniform than a normally ground surface. This should mean - is apparently confirmed
by experiments - longer life for the tool or higher rates of cutting.
Bibliography references: 3, 5, 7, 8, 15, 21, 31, 31a, 42, 45d, 45e, 47, 60.
BIBLIOCEAPHY
American Machinist
1. - 8 May 1947, p.162.
2. Lazarenko, B.F. and N.I. - Machininj by Erosion. 18 Dec. 19h7, p.194.
,. 7 Method X "Machines" Carbides by Atom Expulsion. ? Sept. 1951, p.194.
lardirg,A.V., Matulaitis,V.E. - Arc Machining makes Hard-to-Cut Jobs Easy.
? March 1952, p.16
- rew Processes will rachine the "Unnachinable". 17 March 1952, p.130
6. Denroat,(i.H. - Sort Steel and Ultrasonics Machine Carbide. 15 Sept. 1952, p.1/t1
;. Ashburn,nnderson:- "hy and How; rust Conserve Diamond Bortt )0 rov. 1952,
P.'51
8. :"etzr.er,L.H., YeelEric,G.- Electrolytic Crindinc. 10 Nov. 1952, p.154
0. f.;eed,r.q. - Dritair, too :lac 71ectro,parl, epot or (1,). 10 Fo?,.
1:.10
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1952, p.159
10. Harding,H.V., Matulaitis,V.E., Stoke,R.C. - Electrical-Discharge Machining.
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12. - Highlights in Twelve Months Progress Shown at Open Day at the M.P.L.
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13. Seed,M.G. - Britain, too, is Developing Electro-Spark Machining. 28 June 1952,
P.977
14. - Russia is Building High-Output Machine Tools. 17 Jan. 1953, p.109
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19. Wood,R.G. - Ultrasonics Cuts Carbide with Mild Steel Tools. 26 Sept. 1953,
p.1601
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Wheel Wear. 14 rov. 1953, p.1910
22: - Roport on Electro-Erosion Machining. 15 May 1954, p.833
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Machinery, London
24. Poller,J.S. - New Method of "Machining' Hard Metals. 21 Feb. 1952, p.322
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31a. - Toughening Metals by Electro-Spark Treatment. 25 May 1956, p.818
The Tool Engineer
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The Iron Age
- New Method for Drilling Diamond Dies. 10 July 1947, P.64
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Hard Alloys. 26 July 1951, p.65
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p.203
?7. - Pussian Machine for Electromechanical Machining of Bearing Balls. Translated
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(1950), p.10. 1? Parch 1952, p.98
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8. Elwers,G. Eew Machining Technique Evaluated. 20 March 1952, p.10:1
19. Weber,J. - Another Electro-Machining Method Shows Promise. 29 May 1952, p.88
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40. - Method for Setting up Yetallographic Grinding Tests. 30 Sept.1952, p.799
Inpenioren.
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42. Hintiber, EUdinger;0. - Eecent Processes of Metal Machining, Especially Electro-
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45a. Spizig,S. - Drilling with Controlled Arc. May 1955, p.246
45b. Axer,H. - Fine Machining by Electro-Erosion. Nov.1955, p.554
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Uov. 1955, p.557.
45d. Koscholke,G. - Electrolytic Grinding. iov. 1955, p.562
h5e. Spizig,S. - Experiments with Electrolytically Grinding Diamond Wheels.
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Elektrotechnische Zeitschrift
k6. rudorff,D.W. - The Sparcatron jpark Cutting Process. .21 June 1951, p.195
1,-)2
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80ther Sources
47. von Ardenne,M. - The Electron Microscope. Berlin (1940) p.162
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Vol.3 (1953), p.57/10
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Vol.33 (1943), p.218
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graphic Analysis. Trans.J.Soc.Chem.Industry. (1945) p.317
54. lieek?J.M. - The Nature of the Electric Spark. Trans.Liverpool Eng.Soc., Vol.75
(1950) p.122
55. Jones, F.L1. - Eechanism.of the Electric Spark. lature, Vol.165 (1950) p.9o0
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(1950) p.298
57. Jones,F.L1. - Electrode Evaporation and the Electric Spark. Nature,Vol.157
(1942) p.298
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Spark Discharges. Phys.Rev., Vol.88 (1952) p.137.
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Machines Francaises
60. Cany,G. - Intermittent Arc Machining. No.9, March 1955; No.10, June 1955;
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12/i.
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Technische aundschau
61. Ullmann,W. - Electro-Erosion Metal Machining. Nos.21 and 23, (1955)
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Patent Literature
Sweden: No. 76 026 Gussef 1929
Germany: Nos. 564 081 Fehse 1931
672 832 Bergmann, Dawihl and Fritsch 1937
847 390 Ballhausen 1943
England: Eos. 335 003 Gussef 1929
507 392 Bergmann, Dawihl and Fritsch 1937
578 933 Elox Corp. 1944
580 411 Callite Tungsten Corp. 1943
627 713 Lazarenko 1946
637 793 Lazarenko 1946
637 872 Rudorff 1947
France: Nos. 9?7 762 Lazarenko 19h6
1 024 353. Eberle 1950
Switzerland: Nos. 257 468 Lazarenko 1946
273 469 Rudorif 1948
298 974 Arcolux 1949
U.3.A.: Nos. 1 333 311 Holz 1918
1 620 519 Clawson 1922
1 556 325 Grumpelt 1924
1 701 919 Grumpelt 1925
Fe 20 035 Strobel 1936
2 258 480 Bergmann, Dawihl and Fritsch 1937
125
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Nos. 2 308
860
Clark 1940
2
300
855
Allen et al. 1941
2
374
348
H. V. Harding
2
377
159
Kurtz et al. 1943
2
383
382
H. V. Harding
2
393
383
H. V. Harding
2
415
690
Holfelder
2
438
941 Peters et al. 1946
2
441
319
H. V. Harding
2
476
965
Emerson et al 1946
2
501
954
MdKechnie
2
526
423
.Sidorff 1948
2
650
979
Teubner 1950
126
;
47a
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