US3109086A

US3109086A - Methods and apparatus for machining metal

Info

Publication number: US3109086A
Application number: US124953A
Authority: US
Inventors: Cole Michael
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-08-12
Filing date: 1961-07-18
Publication date: 1963-10-29
Anticipated expiration: 1980-10-29
Also published as: GB908195A

Description

Oct. 29, 1963 M. 'C'OLE 3,109,086

METHODS AND APPARATUS FOR MACHINING METAL Filed July 18, 1961 FIG. 1'

T a -22a INVENTOR:

Michael Cab,

3,ltl9,086 METHUDS AND APPAEATUd lFUiu MACl'llhllNG METAL Michael Cole, 29 Maids Causeway, Qamhridge, England Filed .luly l8, i961, filer. No. iii i353 Claims priority, application Great Eritain Aug. l2, 196d 13 Claims. (til. 219-59) The present invention relates to methods and apparatus for machining metals and is more particularly concerned with the spark machining of metals.

It is well known to effect the cutting or machining of metal by the transfer of material as a result of a rapid series of sparks between the metal and a cutter electrode.

In such a process usually a specimen to be machined is immersed in a bath of an insulating liquid of high dielectric constant and low viscosity, together with a cutter electrode. The specimen and electrode are connected in an electric circuit including a capacitor connected in parallel with the specimen and the cutter electrode and a resistor connected in series with the capacitor to a source of DC. potential, the positive electrode of the capacitor being connected to the specimen and the negative to the cutter electrode. As the cutter electrode is caused to approach the specimen the gap between them reduces until the liquid breaks down under the applied potential. The capacitor discharges and subsequently recharges through the series resistor and the cycle is repeated. If the cutter electrode is caused to approach the specimen still further so that the gap between the specimen and cutter electrode is slightly less than the break-down distance, a rapid series of discharges ensues; if the time constant of the capacitor and resistor are sufficiently large, the formation of a continuous arc between the cutter electrode and the specimen can be avoided and a rapid series of sparks obtained. Each spark effects the transfer or removal of a volume of metal from the specimen and the volume of metal so transferred is approximately proportional to the energy of the spark. The surface of the specimen after such machining, though often cut with sufficient dimensional accuracy, is frequently rough and further mechanical treatment of the specimen is necessary if a flatter and smoother finish is required.

According to one feature of the present invention, a method of machining metals comprises the steps of immersing in a dielectric liquid a metal specimen to be machined and a cutter electrode, bringing an annular portion of the cutter electrode into proximity with the surface of a specimen to form a gap therebetween, connecting the specimen and cutter electrode in an electric circuit, appropriately spacing the surface of the specimen from the cutter electrode and establishing a series of spark discharges between the cutter electrode and the specimen for transferring material from the specimen, rotating the cutter electrode to effect flow of liquid through the gap between the surface of the specimen and the annular portion of the cutter electrode to effect removal of transferred material from the gap, and progressively reducing the energy of the spark discharges by progressively selecting parameters of the electric circuit whereby to produce first a generally flat, rough surface and then a progressively flatter and smoother surface on the specimen.

As the cutter electrode is brought into proximity with the surface of the specimen, the gap will be reduced until the dielectric liquid breaks down under the applied voltage between the specimen and the cutter electrode. Sparking will initially occur between high spots on the surface of the specimen and the cutter electrode. By suitably maintaining the spacing between the cutter elec- It, i hhhdh Patented @ct. 29, 1953 trode and the specimen, the material will be successively removed from the specimen and would normally tend to accumulate in the gap in the form of a suspension and would form a bridge of conductive particles, allowing the spark to travel further through the dielectric liquid. Particles of metal so transferred from the specimen could readily escape from the edges of the gap but would tend to build up in the centre of the gap, with the result that more metal would be removed from the centre region and a concave surface would develop.

A further feature of the invention resides in a cutter electrode whose face is of such conformation, as for example by the presence of radial or more or less radial slots or grooves therein, as to enable it to exercise a pumping action, more particularly a centrifugal pumping action, over the specimen surface. The rotation of the cutter electrode thus can effect flow of liquid dielectric through the gap, and such flow not only can remove particles of metal transferred from the specimen but also can bring a fresh supply of dielectric liquid into the gap. It is found that by successively reducing the energy in the spark discharges, an initially generally flat but rough surface can be rendered very much flatter and much smoother.

By the method of the present invention a substantially strain-free surface of high dimensional accuracy can be obtained. Moreover, by the method of the present invention it is possible to obtain a surface on the specimen which is suitable for metallographic examination after a light mechanical or light electrolytic polish.

The method of the present invention is particularly applicable to the preparation of substantially unstrained metal single crystal specimens.

According to a further feature of the present invention apparatus for the spark machining of metal comprises a bath for receiving a liquid dielectric, means for mounting within the bath a specimen for machining, a rotatable cutter electrode having a plurality of substantially radially extending grooves on its side facing the specimen mounting means, a direct current electric circuit including the specimen mounting means and the cutter electrode and capacitance means connected in parallel therewith, means for effecting relative movement between the cutter electrode and the specimen when mounted in said specimen mounting means to establish between an annular region of the cutter electrode and a surface of the specimen, a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of the metal from the specimen, means for rotating the cutter electrode to create flow of dielectric liquid through said gap to carry away transfer metal, and selector means for varying the parameters of the electric circuit for progressively decreasing the energy of the spark discharges during a machining operation.

By providing the cutter electrode with a plurality of radially extending grooves and by utilising only the outer annular portion thereof in the gap, a continuous flow of dielectric liquid through the gap can be created for carrying away transferred metal and for bringing fresher liquid dielectric into the gap.

The energy in the spark discharges can be reduced over Wide ranges during a single machining operation. It is envisaged that the range can be as low as 10 to l or as high as many thousand to one. For example, in a machining operation lasting about 30 minutes the range of variation of energy can be of the order of 10-301. A coarser range removes the grosser excrescences on the specimen more rapidly.

The electric circuit preferably comprises a capacitor connected in parallel with the specimen and the cutter electrode, and a resistor connected in series with the caaroaoae a pacitor to a source of DC. potential. Variation in spark energy can be obtained by varying any of the parameters of the electric circuit, namely the resistance of the resistor, the capacitance of the capacitor, the voltage of the source of DC. potential; in practice it is, however, convenient to allow the voltage to remain substantially unchanged. Whilst a variable capacitor and/or a variable resistor could be used, it is preferable to provide a series of capacitors and switching means for bringing one, two or more selectively into circuit, then likewise a series of resistors and switching means for bringing one, two or more selectively into circuit.

The cutter electrode may conveniently be in the form of a disc having on one surface a plurality, for example six, substantially radially extending grooves and may conveniently be provided with a central well. The disc may conveniently be of brass or tungsten, though in many cases it may be preferable for the disc to be of the same material as the specimen to be machined.

The invention also envisages the provision of control means for sensing any variations in the average voltage existing between the cutter electrode and specimen when the apparatus is in operation and controlling drive means for moving the face of the cutter electrode towards or away from the surface of the specimen in accordance with the magnitudeand direction of such sensed variations.

The invention will be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic side elevation of one embodiment of the invention,

FIG. 2 is an inverted plan view of a cutter electrode, and

FIG. 3 is a detail illustrating a modification to the circuit of FIG. 1.

Disposed in a bath 819 adapted to receive a liquid dielectric indicated generally at Ill, is means 12 for mounting a specimen 13 to be machined. A cutter electrode 14 is carried on the end of a shaft 15 which is rotatably mounted in bearings 16, 1'7 and carries a driving pulley 1% connected by a belt 19 to a pulley driven by an electric motor 20. The shaft 15 can be raised and lowered by a mechanism 17 to bring the underface of the cutter electrode '14 nearer or further away from the upper surface 21 of a specimen 13. The specimen l3 and the cutter electrode 14 are connected by

leads

28 and 27 in an electric circuit involving a capacitor 22. connected in parallel with them and connected in series with a resistor 23 to a source of direct current supply. The capacitor 22. conveniently comprises a series of capacitors 2 2a as shown in FIG. 3, switching means 34 being provided for selectively bringing into circuit one, two or more capacitors connected in parallel with one another. Likewise, whilst resistor 23 may be a variable resistor as indicated in FIG. 1, it preferably comprises a plurality of fixed resistors 23a as shown in FIG. 3, switching means 31 and 32 being provided to enable one or more resistors or combinations of resistors to be brought into use singly or severally in series or in parallel with one another.

The leads 27a. and 28a of the modified circuit of FIG. 3 will be connected respectively to the elect-rode l4 and the specimen l3 illustratedin PEG. 1 in place of the

leads

27 and 28.

The under face of the cutter electrode 14 is indicated more clearly in FIG. 1 and is provided with a central well 24 and a plurality of radially extending grooves 25.

In operation a specimen to be machined is mounted eccentrically to the axis of the cutter electrode 14- preferably so that only an outer annular portion of the cutter electrode lies over the surface to be machined. The specimen and the cutter electrode are disposed below the level of dielectric liquid 11 in the bath lid. The cutter electrode 14 is set in rotation by connecting the electric motor to an electricity supply, conveniently AG. by leads 26.

A source of DC. potential is connected to the electric circuit embodying the resistor 23, the capacitor 22, the cutter electrode 14- and the specimen 113. It will be noted that the cutter electrode 14 is connected by

lead

27 or 27a to the negative side of the capacitor 22 and the spec-imen 13 by

lead

23 or 28a to a positive side of the capacitor 22. If the mechanism 17 is operated to advance the cutter electrode 14 towards the surface 21 of the specimen 13, a point will be reached at which the liquid dielectric in the gap between the specimen and the cutter electrode breaks down under the applied potential. The capacitor 22 discharges as a spark between the cutter electrode and the specimen and subsequently recharges through the resistor 23 to repeat the process. If the cutter electrode is'advanced slightly nearer to the surface of the specimen an almost continuous succession of spark discharges can be obtained. In practice, it is found that a gap between the face of the specimen and the cutter electrode, equal to approximately of that at which breakdown initially occurs, gives satisfactory operation.

On each spark discharge metal is transferred from the specimen l3 and any metal particles which would otherwise collect in the liquid dielectric in the gap between the cutter electrode and the specimen, are dispersed by the how of liquid dielectric created by the grooves 25 when the cutter electrode '14 is rotated. The parameters of the electric circuit are adjusted as the machining proceeds so that the energy in the spark discharges is progressively reduced. During a machining operation the range of energy reduction may be as low as :1 or as great as several thousand to one. Such reduction in the energy of the spark discharges is most conveniently effected by reducing the capacitance of the capacitor 22 and increasing the resistance of the resistor 23 whilst maintaining an approximately constant potential of the D.C. source but it will be understood that variations in the potential of the DC. source may also be utilised within a range of from about 30 volts to about 330 volts.

By Way of example it has been found that satisfactory results can be obtained in machining a circular specimen of approximately /2" in diameter with a cutter electrode approximately 5 in diameter provided with six substanstantially equi-angularly spaced radial grooves and rotated at a speed of approximately 300 r.p.m., if the parameters of the electric circuit are such that the capacitor 22 initially has a capacitance of approximately miorofarads and the resistor 23 has a resistance of 12 ohms and are progressively reduced over a period of approximately 30 minutes to a capacitance of .01 microfarad and a resistance of 10 kilohms, the direct current supply potential remaining substantially unchanged at 200 volts.

It will be understood that the process may be extended for longer periods of time and that the cutter electrode may be rotated at higher or lower speeds.

It is preferable that the gap between the cutter electrode 14 and the surface of the specimen 13 should, during operation of the apparatus, be maintained substantially at an optimum value. In operation it is found that such optimum value is approximately two-thirds of the breakdown gap for a given dielectric and DC. source potential. For many purposes kerosene is a suitable dielectric and it has been found to have a breakdown voltage of approximately 100 volts per one thousandth of an inch. Other suitable dielectrics are light transformer oil and distilled water. Moreover, it has been found that the breakdown voltage across the gap bears an approximately linear relation to the size of the gap over a small range around the breakdown voltage, and thus variations in the gap can effectively be sensed by a voltage control device 29 and used to control a servo-motor 30 in driving connection with the mechanism 17 to lower or raise the cutter electrode 13.4 in dependence upon the direction and magnitude of sensed variations in working voltage between the cutter electrode and the specimen.

By the method of the present invention it is possible to arouses produce sufiiciently dimensionally accurate substantially strain-free shapes in many metals, either for single crystal fabrication or for metallographic preparation.

If, however, chemical contamination of the specimen is undesirable, such as might occur from deposition of material from the cutter electrode, or from decomposition of the liquid dielectric, it can be reduced by a suitable choice of material of the cutter electrode and dielectric fluid. For many purposes brass or tungsten are suitable materials for the cutter electrode, though for minimising contamination it is preferable that the cutter electrode should be of the same metal as the specimen. Contamination is believed :to be confined to a thin surface layer and can be removed by a light etch.

As an indication of the degree of flatness and smoothness of the surface that can be obtained by the method of the present invention, reflection micrographs of typical surfaces produced by machining aluminum in a mediumlight range have indicated the presence of craters approximately to microns across and about 2 microns deep. Talysurf measurement of the same specimens gave a centre line average surface texture figure of 10 microinches.

I claim:

1. A method of machining metals comprising in combination the steps of immersing in a dielectric liquid a metal specimen to be machined and a cutter electrode, bringing an annular portion of the cutter electrode into proximity with the surface of the specimen to form a gap therebetween, connecting the specimen and cutter electrode in an electric circuit, appropriately spacing the surface of the specimen from the cutter electrode and establishing a series of spark discharges between the cutter electrode and the specimen for transferring material from the specimen, rotating the cutter electrode to effect flow of liquid through the gap between the surface of the specimen and the annular portion of the cutter electrode to effect removal of transferred material from the gap, and progressively reducing the energy of the spark discharges by progressively selecting parameters of the electric circuit.

2. A method of machining metals by spark discharges comprising the steps of immersing in a dielectric liquid a metal specimen to be machined and a cutter electrode, bringing an annular portion of the cutter electrode into proximity with the surface of the specimen to form a gap therebetween, connecting the specimen and cutter elec trode in an electric circuit, appropriately spacing the surface of the specimen from the cutter electrode, establishing a series of spark discharges between the cutter electrode and the specimen to transfer material from the specimen, and simultaneously rotating the cutter electrode to elfect flow of liquid through the gap between the surface of the specimen and the annular portion of the cutter electrode to effect removal of transferred material from the gap, and progressively reducing the energy of the spark discharges by progressively selecting parameters of the electric circuit whereby to produce first a generally flat, rough surface and then a progressively flatter and smoother surface on the specimen.

3. A method of preparing a metal specimen for subsequent metallographic examination including the preparatory stage of machining the specimen by spark discharge, said stage comprising the steps of immersing said specimen and a cutter electrode in a dielectric liquid, bringing an annular portion of the cutter electrode into proximity with the surface of the specimen to form a gap therebetween, connecting the specimen and cutter electrode in an electric circuit, appropriately spacing the surface of the specimen from the cutter electrode, establishing a series of spark discharges between the cutter electrode and the specimen to [transfer material from the specimen, and simultaneously rotating the cutter electrode to effect flow of liquid through the gap between the surface or" the spec imen and the annular portion of the cutter electrode to 8 effect removal of transferred material from the gap, and progressively reducing the energy of the spark discharges by progressively selecting parameters of the electric circuit whereby to produce first a generally flat, rough surface and then a progressively flatter and smoother surface on the specimen.

4. The method according to claim 3 including the step of reducing the energy of the spark discharges over the range of 1080 to l in about 30 minutes.

5. The method according to claim 3 in which the series of spark discharges are established by connecting a capacitor in parallel with the specimen and cutter electrode and charging the capacitor from a source of D.C. potential at a controlled rate and in which the step of progressively reducing the energy of the spark discharges comprises varying the capacitance of the capacitor and varying the controlled rate of charge thereof whilst maintaining the source of DC. potential substantially constant.

6. A method of spark machining metals comprising in combination the steps of immersing a metal specimen to be machined and a cutter electrode in a dielectric liquid, bringing an annular portion of the cutter electrode into proximity with a selected surface of the specimen to form a gap therebetween, connecting a capacitor in parallel "with the specimen and the cutter electrode, charging the capacitor from a source of DC. potential at a controlled rate, appropriately spacing said surface of the specimen from the cutter electrode and establishing a series of spark discharges between the cutter electrode and the specimen and simultaneously rotating the cutter electrode, said discharges transferring metal from the specimen and the rotation of the cutter electrode effecting flow of liquid through the gap between the surface of the specimen and the annular portion of the cutter electrode to effect removal of transferred material from the gap, progressively varying the capacitance of the capacitor and the controlled rate of charge thereof whilst maintaining said source of DC. potential substantially constant to progressively reduce the energy of the spark discharges over the range of 1000 to 1 in about 30 minutes, whereby to produce first a generally flat, rough'sur-face and then a progressively flatter and smoother surface on the specimen.

7. Apparatus for the spark machining of metal comprising a bath for receiving a liquid dielectric, means for mounting within the bath a specimen for machining, a rotatable cutter electrode having a plurality of substantially radially extending grooves on its side facing the specimen mounting means, a direct current electric circuit including the specimen mounting means and the cutter electrode and capacitance means connected in parallel therewith, means for effecting relative movement between the cutter electrode and the specimen when mounted in said specimen mounting means to establish between an annular region of the cutter electrode and a surface of the specimen a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of the metal from the specimen, means for rotating the cutter electrode to create flow of dielectric liquid through said gap to carry away transferred metal, and selector means for varying the parameters of the electric circuit for progressively decreasing the energy of the spark discharges during a machining operation.

8. Apparatus for the spark machining of metal comprising a bath for receiving a liquid dielectric, means for mounting within the bath a specimen for machining, a rotatable cutter electrode having a plurality of substantially radially extending grooves and a central well on its side facing the specimen mounting means, 'a direct current electric circuit including the specimen mounting means and the cutter electrode and capacitance means connected in parallel therewith, means for effecting relative movement between the cutter electrode and the specimen when mounted in said specimen mounting means to establish between an annular region of the cutter elecarcades trode and a surface of the specimen a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of the metal from the specimen, means for rotating the cutter electrode to create fiow of dielectic liquid through said gap to carry away transferred metal, and selector means for varying the parameters of the electric circuit for progressively decreasing the energy of the spark discharges during a machining operation.

9. Apparatus for the spark machining of metal comprising a bath for receiving a liquid dielectric, means for mounting Within the bath a specimen for machining, a rotatable cutter electrode having a plurality of substantially radially extending grooves on its side facing the specimen mounting means, a direct current electric circuit including the specimen mounting means and the cutter electrode and capacitance means connected in parallel therewith, means for effecting relative movement between the cutter electrode and the specimen when mounted in said specimen mounting means to establish between an annular region of the cutter electrode and a surface of the specimen a gap appropriate for the establishment of a series of spark discharges therea-cross for the transfer of the metal from the specimen, means for rotating the cutter electrode to create a flow of dielectric liquid through said gap to carry away transferred metal, selector means for varying the parameters of the electric circuit for progressively decreasing the energy of the spark discharges during a machining operation, sensing means responsive to variations in the average voltage existing between the cutter electrode and specimen when the apparatus is in operation, and control means responsive to said sensing means and controlling said relative movement effecting means to move the face of the cutter electrode selectively towards and away from the surface of the specimen in accordance with the magnitude and direction of such sensed variations.

10. Apparatus for the spark machining of metal comprising a bath for receiving a liquid dielectric, means for mounting Within the bath a specimen for machining, a rotatable cutter electrode having six substantially radially extending grooves and a central well on its side facing the specimen mounting means, a direct current electric circuit including the specimen mounting means and the cutter electrode and capacitance means connected in parallel therewith, means for effecting relative movement between ithe cutter electrode and the specimen when mounted in said specimen mounting means to establish between an annular region of the cutter electrode and a surface of the specimen a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of the metal from the specimen, means for rotating the cutter electrode to create flow of dielectric liquid through said gap to carry away transferred metal, and selector means for varying the parameters of the electric circuit for progressively decreasing the energy of the spark discharges during a machining operation.

11. Apparatus for spark machining of metal comprising a bath, liquid dielectric in said bath, means for mounting within the bath and below the level of said liquid therein a specimen for machining, a rotatable cutter electrode having a plurality of substantially radially extending grooves and a central well on its side facing the specimen mounting means, at least one capacitor, means connecting said at least one capacitor in parallel with said specimen mounting means and said cutter electrode, at least one resistor, means connecting said at least one resistor in series with said at least one capacitor to a source of DC. potential, means for effecting relative movement between said cutter electrode and a specimen when mounted in said specimen mounting means to establish between an annular region of the cutter electrode and a selected surface of the specimen a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of metal from the specimen, means for rotating 8 the cutter electrode to create flow of dielectric liquid through said gap to carry away transferred metal, and selector means for varying the capacitance of said at least one capacitor and the resistance of said at least one resistor for progressively decreasing the energy of the spark discharges during a machining operation.

12. Apparatus for spark machining of metal comprising a bath, liquid dielectric in said bath, means for mounting Within the bath and below the level of said liquid therein a specimen for machining, a rotatable cutter ele trode having a plurality of substantially radially extending grooves and a central well on its side facing the specimen mounting means, capacitor means, means connecting said capacitor means in parallel with said specimen mounting means and said cutter electrode, resistor means, means connecting said resistor means in series with said capacitor means to a source of DC. potential, means for effecting relative movement between said cutter electrode and a specimen when mounted in said specimen mounting means to establish between an annular region of the cutter electrode and a selected surface of the specimen a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of metal from the specimen, means for rotating the cutter electrode to create flow of dielectric liquid through said gap to carry away transferred metal, and selector means for varying the capacitance of said capacitor means and the resistance of said resistor means for progressively decreasing the energy of the spark discharges during a machining operation.

13. Apparatus for the spark machining of metal comprising a bath for receiving a liquid dielectric, means for mounting Within the bath a specimen for machining, a rotatable cutter electrode having a plurality of substantially radially extending grooves on its side facing the specimen mounting means, a plurality of capacitors, a plurality of resistors, first switching means and second switching means, an electric circuit including the specimen mounting means, the cutter electrode, said capacitors, said resistors and said switching means, said first switching means connecting a desired number of said capacitors in parallel with said specimen mounting means and said cutter electrode, and said second switching means connecting selectively a desired number of said resistors in series with the selected number of said capacitors to a source of DC. potential, drive means for efiecting relative movement between said cutter electrode and said specimen mounting means to establish between an annular region of the cutter electrode and a selected surface of a specimen when mounted in said specimen mounting means a gap appropriate for the establishment of a series of spark discharges thereacross for the transfer of metal from the specimen, said gap being established below the normal level of liquid dielectric in said bath, means for rotating said cutter electrode to create flow of dielectric liquid through said gap to carry away transferred metal, said switching means enabling the energy of the spark discharges to be progressively decreased during a machining operation, sensing means responsive to the average voltage existing between said cutter electrode and said specimen when the apparatus is in operation, and control means responsive to said sensing means and controlling said drive means to effect movement of the face of the cutter electrode selectively towards and away from said surface of said specimen in accordance with the magnitude and direction of such sensed variations. 7

References Cited in the file of this patent UNITED STATES PATENTS 2,920,180 Ullmann et a1. Jan. 5, 1960 FOREIGN PATENTS 164,233 Australia July 20, 1955

Claims

1. A METHOD OF MACHINING METALS COMPRISING IN COMBINATION THE STEPS OF IMMERSING IN A DIELECTRIC LIQUID A METAL SPECIMEN TO BE MACHINED AND A CUTTER ELECTRODE, BRINGING AN ANNULAR PORTION OF THE CUTTER ELECTRODE INTO PROXIMITY WITH THE SURFACE OF THE SPECIMEN TO FORM A GAP THEREBETWEEN, CONNECTING THE SPECIMEN AND CUTTER ELECTRODE IN AN ELECTRIC CIRCUIT, APPROPRIATELY SPACING THE SURFACE OF THE SPECIMEN FROM THE CUTTER ELECTRODE AND ESTABLISHING A SERIES OF SPARK DISCHARGES BETWEEN THE CUTTER ELECTRODE AND THE SPECIMEN FOR TRANSFERRING MATERIAL FROM THE SPECIMEN, ROTATING THE CUTTER ELECTRODE TO EFFECT FLOW OF LIQUID THROUGH THE GAP BETWEEN THE SURFACE OF THE SPECIMEN AND THE ANNULAR PORTION OF THE CUTTER ELECTRODE TO EFFECT REMOVAL OF TRANSFERRED MATERIAL FORM THE GAP,