US3504152A - Method for high-frequency electric-impulse metal finishing and an apparatus for accomplishing the same - Google Patents

Description

March 31 Filed Aug. 8

1970 A. 1.. LIVSHITS ETAL.

ETHOD FOR HIGH-FREQUENG AND AN APPARATUS Y ELECTRIC-IMPULSE MB 'IAL FINISHING FOR ACCOMPLISHING THE SAME 2 Sheets-Sheet l 3,504,152 SE METAL FINISHING Mar c ih 3 1 1970- v ME I A. L. LIVSHITS ET THOD FOR HIGH-FREQUENCY ELECTRIC-1M *Sheet 2 AND Filed Aug. 8,- 1963 United States Patent Oihce 3,504,152 Patented Mar. 31, 1970 3,504,152 METHOD FOR HIGH-FREQUENCY ELECTRIC- IMPULSE METAL FINISHING AND AN APPA- RATUS FOR ACCOMPLISHING THE SAME Abram Lazarevich Livshits, Arkady Timofeevich, Kravets, and Mikhail Iljich Boorda, Moscow, U.S.S.R., assignors to Experimentalny Nauchno-lssledovatelsky Institute Metallorezhushtchikh Stankov Filed Aug. 8, 1963, Ser. No. 300,896

Int. Cl. B23p 1/08 U.S. Cl. 219-69 6 Claims ABSTRACT OF THE DISCLOSURE A method for high-frequency electric-impulse metal finishing by an electrode-tool acting as an anode, between which and the work, acting as a cathode, electric impulse discharges are excited by unipolar voltage pulses with an amplitude ranging from 100 to 600 volts, said electric impulse discharges being characterized by a ratio of a single pulse to average energy over a cycle lying within 1 to 5. The method characterized by the above parameters is implemented by the provision of an apparat-us comprising an oscillator, a power amplifier, an output transformer, two semiconductor valves and a nonlinear resistance.

This invention is related to a novel electric erosion treatment-a method of high frequency electric-impulse working and an apparatus to accomplish said method.

A very important problem in the art of electric erosion treatment nowadays is to improve the purity of the worked surface. This is an important problem for both electric-spark and electric-impulse (electric) erosion treatment.

It has been found that generally, in any kind of electric erosion treatment, the roughness of the finished surface depends upon the energy of discharges which influence said surface (becoming rougher with the increase of impulse energy), and the total metal take-off is determined by the average amount of the energy supplied to the work. Therefore there is a tendency, when it is desired to improve the purity, to conduct finishing by dropping the energy of an individual impulse and increasing the impulse common sequence frequency.

It would be quite natural, in increasing discharge sequence frequency, to maintain an average power at a high level of rough running conditions. It would seem possible to obtain the efficiency of rough running conditions but with a greater surface purity. However, two interconnected factors prevent this in reality.

The first factor is that, with an equal energy introduced into the finishing area, energy consumption of the process is greater, the higher is the frequency and consequently the higher is dispersion of metal, that is with an equal general energy introduced at low and high frequencies in the latter case there will be less metal takeoff.

The second factor is that amount of roughness is approximately proportional to the discharge energy raised to the third power. Therefore, to decrease roughness, for instance, two-fold, that is by one class according to national and foreign standards, the impulse energy should be dropped 2 =8-fold. To raise the surface purity from the first to the eighth class, that is by seven classes, the impulse energy must be dropped at least by 2 that is approximately 2.5 million times, and therefore the upper frequency limit (considering 400 cycles as the impulse frequency in rough running conditions) must be made 10 kilocycles, and this is about a few thousand times greater than has been obtained for the time being experimentally or in industry. At the same time the action of the first factor is manifest to a still greater extent, that is energy consumption increases.

Therefore, in view of technical difiiculties in obtaining heavy current periodic impulses (due to the absence of adequate electronic and semi-conductor devices, the difficulty of frequency supply in the range of megacycles to the tool and the work, low efficiency and the like), generally, together with the increase of frequency within reasonable bounds, impulse energy is also controlled at the expense of dropping its amplitude, that is at the expense of decrease in the total energy introduced into the finishing area. And this still more greatly reduces the difference between fine and rough running conditions.

In connection with the abovementioned factors reduction of the energy consumption of the process at high frequencies is of primary importance.

Energy consumption of the process largely depends on the state of metal removed from the recess-in vapourous or liquid state. Naturally, in the first instance energy consumption of the process is greater as part of the energy is spent to make metal boil and turn it into vaporous state. In turn, as experiments and theoretical considerations show, the state, into which metal removed from the recess is brought, depends on the ratio of the energy of an individual impulse (W to its average value over a repetition cycle (W )l W /W Here, two fundamentally different methods may be distinguished, one of which (high-frequency electric-spark finishing) is widely known and is used in the USSR and abroad, and the other of which (high-frequency elect-ricimpulse finishing) is the object of the present invention.

The known method of high-frequency electric-spark working is characterized by a frequency ranging from a few kilocycles to hundreds of kilocycles.

Here, the value ,U.=W /W ranges approximately within 10 to 100. As a result, the ratio of the current impulse duration to its amplitude (sec/amp) is very small, that is with a short time of impulse application the spark discharge has a very lagre instantaneous power, and this results in a preferably electronic process (that is preferable heat generation due to the braking energy of electrodes on the anode work), high instantaneous temperatures of the anode spot (above 10,000" C.), mostly explosion evaporation of metal (a vaporous stagemetal is removed in the shape of vapours and vapour jets), elevated, due to this fact, energy consumption, optimal, from the point of view of wear and the rate of take-off, direct polarity of electrodes corresponding to the electronic process (the tool serves as a cathode and the worked article as an anode).

As a result of a large ratio of impulse energy to the average energy within a cycle (,u is great) the thermal wear of the tool cathode is increased. Therefore, it is not the practice to utilize high-resistant carbon-graphite electrodes which at these high temperatures of the cathode spot no longer possess sufficient thermal strength and do not restore by themselves due to carbon compounds obtained in pyrolysis of a working fluid and deposited on the carbon-graphite electrode tools. For this reason metal electrode tools (such as: made of copper, brass) are used in this method of working, these being more expensive and more workable than carbon-graphite electrode tools. A outcome of the same factor (high instantaneous temperatures caused by high values of y. is the levelling (alignment) of take-01f rates in Working the materials with very distinctive 'thermophysical properties. It is known, for instance, that the take-off rate for steel comes down to the level of the take-off rate for metal-ceramic solid alloys sharply reducing the efficiency of finishing steel articles by this method.

Stability of the finishing process on a large area also is reduced because metal evacuation through a very narrow long slit in vaporous state is obstructedat high metal dispersion the particles are cooled faster and thus their energy is given up quicker to the surrounding space and the electrode surface.

A characteristic feature of high-frequency electricspark working also consists in utilization of alternating reversed voltage and current impulses, as at such high instantaneous powers the effect of a back wave is by far not the main one" from the point of view of its influence on wear. As a result of this fact, the impuse generators are provided, as a rule, with a capacitance energy accumulator (capacitor) discharging through a thyratron to the primary winding of the impulse transformer, an erosion load being connected to its secondary winding. Generally the circuit of impulse generators for high-frequency electric-spark working resembles th known means of impulse radar modulation.

The proposed novel method of high-frequency electricimpulse working is also characterized by a frequency ranging from a few kilocycles to hundreds of kilocycles.

However, the value of a at the same frequencies and the same mean power is smaller at least by one order (1 p. 5). Hence, the ratio of impulse duration to its amplitude (sec/amp.) is accordingly by one order greater. The result'of this is that the ion process dominates in the proposed method of working, that is heat generates mostly due to ion bombardment on the cathode work and the metal is taken off not in a vaporous state, but mainly in a liquid state, causing smaller energy consumption in the process. In this case reversed polarity of electrodes is optimal with respect to the compared process of known high-frequency electric-spark working, that is the tool serves as an anode and the work as a cathode. This fact necessitates the utilization of a carhon-graphite tool, as together with the temperature drop on the anode spot, the electrode tool is self restored by pyrolysis of the working fluid taking place in the working process.

Thermophysical properties of materials, like in the case of low frequency, considerably affect the rate of metal take-off; the ratio of the steel take-off rate to that of a solid alloy becomes 5 to times greater, which fact determines the serviceability of the method proposed for finishing shaped steel articles. Metal take-off in a liquid state facilitates its evacuation in tooling large areas.

A distinctive feature of the proposed method for highfrequency electric-impulse treatment is also the uttilization of unipolar (instead of alternating reversed impulses used in electric-spark working) voltage and current impulses, which fact, together with the small value of the factor of ,u, determines the principal changes in generator circuits; the capacitance energy accumulator becomes unessential and the output transformer is materially simplified; however, for obtaining unipolar impulses, rectifiers should be provided at the output of the generator. Thus, generators for high-frequency electric-impulse treatment are fundamentally different in their block diagram from the known generators for high-frequency electric-spark working.

Aonther fact should be mentioned. In current use now rough electric-impulse work is carried out preferably at a low frequency (within 1 kilocycle), but motor-generators at a frequency of 400 cycles are actually used. Unipolar arc impulses are used in this working method, the article being connected to the cathode and the electrode to the anode at small values of [L (1 /.L, 55). Existing theoretical concepts on the nature of electric erosion treatment lie in that if absolute impulse duration is reduced approximately to quantities less than 10- sec. electronic process occurs in the main with all the features described above arising from the process of highfrequency electric-spark treatment, including the obligatory connection of the work to the anode. Therefore it was supposed that in the event of a sharp increase of frequency, by several orders, while known electricimpulse low-frequency working, a coincidence of the, physical and technological characteristics of the two kinds of electric erosion treatment-electric-spark and electric-impulse treatments would take plae. This is why all the explorers and inventors in the USSR and abroad have heretofore used ,m values within the range of electricspark treatment (l0 l00) at the increase of frequency, and till now no attempts have been made to create impulse generators for high-frequency electricimpulse treatment. Therefore the effect of transfer of the abovesaid peculiarities characteristic of low-frequency electric-impulse treatment of high frequency was not foreseen, lie the novelty was not in the fact that the absolute impulse duration determined the character of the process (electronic or ionic polarity of electrodes, take-off rate in working steel and solid alloys, the possibility of working large areas and so on), but the ratio of the energy of an individual impulse, W to the average energy over a cycle, W and the consequent ratio of duration of a current impulse to its amplitude (sec./ amp.)

The abovementioned distinctive features of the method we propose to raise the efilciency of finishing shaped steel articles ten-fold as well as reduce several times the wear of the tool and to accomplish finishing of large parts having a finishing area of scores of thousand square millimeters.

To achieve the proposed method of sizing, unipolar voltage impulses of negative polarity (the work as a cathode, the tool as an anode) with an amplitude within 100 to 600 volts are supplied to the machined work from an impulse generator, for instance, of the type described hereinafter; in the erosion process the ratio of the energy of an actuating current impulse should be maintained within the limits from 0.2 to 0.9. The frequency of the impulse sequence is selected in dependence upon the required surface purity and is established within the limits from 2 to 1,000 kilocycles. In this case sizing running conditions are controlled at first by frequency (from lower frequency to higher frequency) and then by the mean current (from heavier current to weaker current) till the predetermined surface purity is obtained. Adjustment from lower frequency to higher frequency is essential because it provides for greater efficiency in preliminary metal take-off at a lower wear of the tool.

As the increase of impulse frequency is accompanied by certain rise in relative tool wear, it is preferable to take-off the minimum allowance at a higher frequency. Further current drop at a maximum frequency does not cause an increase in the wear and ensures higher surface purity, but at a correspondingly lower efliciency. Therefore in the proposed system of adjustment minimum allowances and metal take-off are provided for unfavourable running conditions from the point of view of wear and efficiency, which fact in general provides for obtaining the predetermined surface purity for minimum possible time and at minimum absolute wear of the electrode tool.

In the course of operation, the electrode and the work are brought so closely to each other as to cause a breakdown of the dielectric when voltage impulses are supplied to the electrode. Depending on the electric running conditions, on the dimensions and the shape of the worked surface, in order to stabilize the process, the electrode tool is actuated to perform oscillatory motion relative of the work or the work is actuated to perform oscillatory motion relative of the electrode tool.

The oscillatory motion may be performed in the direction of feeding and on a plane perpendicular to said direction, in particular in the two inter-perpendicular directions. The amplitude of oscillations in the direction of feeding may range from to 0.1 mm., and the amplitude of oscillations on a plane perpendicular to the direction of feeding may reach several millimeters.

For removing erosion by-products during operation, fluid is forced into the space between the electrode tool and the work, or is sucked out of this space.

For realization of the proposed, method a generator is required to obtain exclusively negative voltage impulses and currents with a small u ratio, which are able to cause discharges between the tool and the Work, and a means is essential to impart oscillatory motions to the electrode tool and the work.

Hereinafter we describe one of the preferred modifications of these units with reference to the appended drawings.

FIG. 1 shows a principal electric diagram of a unipolar impulse generator able to accomplish the proposed method of electric impulse high-frequency metal working.

FIG. 2 shows a general view of a mechanical means for imparting oscillations to the electrode tool and the work. a

FIG. 3 shows a diagrammatic arrangement of drive electromagnets which compose said means.

FIGS. 4 and 5 show sectional views along AA and B-B in FIG. 2.

The impulse generator, shown in FIG. 1, comprises an oscillation exciter, a voltage amplifier, a power amplifier, an output transformer and a rectifier block.

The oscillation exciter comprises two tubesa pentode 1L and a triode 2L.

The frequency of oscillations is established within a wide frequency range by means of condensers 1C, 2C and resistors 1R, 2R, 3R and 4R, switched over by switches S S S and S To stabilize the amplitude of impulses a non-linear resistor (a thermistor T 1) is connected to the cathode circuit tube 1L. Resistors SR and GR provide an anode load on the exciter. The grid bias is obtained automatically as a result of the anode current flowing through resistors 7R and SR. The screen grid is fed through resistor 9R; condenser 3C serves for disconnecting the screen grid, condenser 4C is a blocking element, condenser 5C is a duct capacitor and connects the anode of the tube IL with the grid of the tube 2L, resistor 10R serves as a leakage for tube 2L.

Voltage from the exciter, through a duct condenser 6C and by means of potentiometer 11R controlling the signal level from zero to maximum, is delivered directly to the voltage amplifier grid arranged on the triode 3L. This amplifier amplifies primary oscillations up to the rated value.

Resistor 12R acts as anode load of the voltage amplifier (or of the tube 3L). The grid bias is provided at the expense of the voltage drop at the cathode resistor 13R. Condenser 7C is a blocking element, resistor 14R acts as a leakage for tube 3L.

Amplified oscillations are supplied to the power amplifier sub-final stage through the condenser 6C, said amplifier being mounted on two pentodes 4L and SL. The tube grids are separated by resistors 15R and 16R. Condenser 9C is connected in between the screen grids and the cathodes.

The grid bias is supplied from an individual power source manufactured in accordance with common circuits.

The power amplifier sub-final stage serves to amplify the level and the power amplifier terminal or output stage.

Connection with the terminal stage is achieved by an interstage transformer 1T whose core is assembled from laminations up to 0.1 mm. thick.

Voltage in opposite phases is delivered from two secondary windings of the transformer IT to the power amplifier terminal stage, said amplifier being connected to four pentodes 6L, 7L, 8L and 9L, arranged according to a two-way circuit with two tubes in each arm.

The power amplifier terminal stage functions in the grid current duty. To avoid self-excitation the tube grids are separated by resistors 17R, 18R, 19R and 20R.

Resistors 21R, 22R, 23R and 24R are used as tubes leakage.

The screen grids of tubes 6L to 9L are supplied through resistors 25R and 26R. Condensers 10C and 11C are connected in between the grids and the cathodes of the tubes.

Anode circuits of the terminal stage of tubes 6L, 7L, 8L and 9L are supplied from a high-voltage rectifier comprising diodes 1V to 6V in accordance with a six-phase circuit. To reduce pulsations a filter is provided in the high-voltage circuit, said filter comprising a choke D and a condenser 12C.

Alternating voltage is delivered to diodes 1V to 6V from a high-voltage transformer 2T. The anode feed circuit is connected with the tube cathodes through con-' mm. thick of high magnetic permeability.

The laminated magnetic core of the transformer is manufactured with a stepped section to allow winding on cylinder frames, to rise technological features of the construction and to somewhat reduce consumption of active materials.

To reduce magnetic core saturation which influences the shape of an output impulse, the transformer magnetic core is provided with a gap. To avoid breakdown in transient processes the transformer windings are filled with an epoxy resin.

Voltage from the transformer secondary winding is supplied to a rectifier comprising two diodes 7V and 8V, which transforms alternating current to unipolar impulses.

As the generator is intended to operate at high frequencies regular power diodes do not ensure a satisfactory complete cutoff of the second half-wave. Therefore another diode 8V is connected discordantly with the first diode 7V parallel with erosion gaps E and E Since in idle running the entire load occurs on the tube anodes, which may thereby be put out of order, they should be protected.

For this purpose a non-linear resistor, more particularly an incandescent lamp L, is connected in parallel with the secondary winding of transformer 3T, and the anode voltage is selected to be 0.7 to 0.8 of the rated value for the tubes.

Current amplitude is controlled by resistors 27R and 28R connected in series with the erosion gap. FIG. 1 shows an example of operation with two electrodes. The generator also permits operation with one electrode. The switching over is achieved by switches S and S A mechanical means for imparting oscillations to the electrode tool and the work (FIGS. 2 to 5) is manufactured as a convenient compact structure. As is understood from FIG. 2, it comprises a body 1, said body may be easily fixed in a spindle 4 by means of a cone 2 connected with the body by screws 3. The mechanical means may also be easily arranged in the tool head of an electric erosion machine. The cores of electromagnets 7, excited by coils 8, are fixed on the flange of cone 2 by screws 5 and pins 6. The armatures 9 of the electromagnets are bolted to the ring 11 by screws 10, said ring resting upon a projection of the body 1 by means of a thrust ball bearing 12. A central part 18 is coupled with the electromagnet armatures from beneath by screws 13 and pins 14 through an intermediate steel ring 15 and rings 16 and 17 made of insulating material, the electrode tool 19 fixed in a holder fastened to said central part. In the upper part the ring 15 is provided with two V-shaped grooves furnished on both sides of the central opening, and a movable connection of the ring 15 with the ring 22 is achieved by means of said grooves and balls 21. Ring 22 is provided with four V-shaped grooves: two grooves on the lower side, the direction of said grooves coinciding with the direction of the grooves of ring 15 for connection through balls 21 with ring 15, and two grooves on the upper side, said grooves being arranged in the direction perpendicular to the direction of the grooves of the ring 15. Ring 22 is moveably connected with ring 24 by the upper grooves and the balls 23, ring 24 being fixed in the body 1. Thus, the central part 18 freely moves in two perpendicular directions, but cannot rotate.

A cup 25 is fastened on the flange of cone 2, a fiat former 26 being placed inside said cup 25. The former is bolted by screws 27 and pins 28 to a nut 29. A pin 30, passing through cup 25 and nut 29, prevents former 26 from turning. Former 26 can move towards the axis of screw 31 by means of screw 31 rotated by a handle 32, linked with the screw by a pin 33. A pivot 36 with a cone head 37 is mounted in rings 11 and 15 on bearings 34 and 35. The purpose of former 26 and pivot 36 is to limit the travel of central part 18 on a plane perpendicular to the direction of the electrode tool fed towards the work. Voltage is intermittently supplied to the coils of electromagnets 8, so that the armatures 9 of the electromagnets move central part 18. It moves on until head 37 of pivot 36 approaches former 26. The sequence of connecting the electromagnets is selected in such a way that pivot 36 may be enabled to slide on the surface of former 26 gradually feeling its surface. As a result of the rigid connection between pivot 36 and central part 18, the latter moves along the same trajectory. This will be readily seen on FIGS. 4 and 5. Trajectory 38 of the pivot in FIG. 3 is similar to trajectories 39 of any point of tool 19.

The former 26 may be ofany complicated shape and therefore the electrode tool may be supplied any complicated trajectory. In shaping the former similar to the contour of the worked holes, preserving the angles of the given drawings, the electrode tool may intermittently feel the worked surface completely reproducing the surfaces of the required shape with the given acute angles on the workpiece 40. If high precision of acute angles is not required the former is manufactured as a ring.

Due to the movement of the electrode tool on the worked surface, the gap between the electrode tool and the work occasionally varies from a quantity corresponding to the value of the breakdown gap and up to quantities considerably greater than this value (up to several millimeters). Thus, the outlet is occasionally freed to release erosion by-products from the working area.

In the course of working, the dimensions of the worked surface may be increased without the electrode tools being changed by increasing the amplitude of oscillatory motions of the electrode tool by means of handle 32, which moves the former 26.

Dielectric fluid is pumped through a pipe connection 41 and holes 42 in the electrode tool. Work 40 and electrode tool 19 are immersed in a dielectric fluid bath 43. To prevent inside penetration of fluid contaminated by erosion by-products, a flexible packing 44 is provided. The electrode tool is connected to the positive pole of the abovedescribed impulse generator 45, while the workpiece is connected to its negative pole. The entire arrangement is linked with the functional feed mechanism of the electric erosion machine.

An arrangement similar to the above described apparatus may be mounted on the electric erosion machine table to impart oscillations to the worked part.

What we claim is:

1. An impulse generator for high-frequency electric metal finishing by electric impulse discharge excited by unipolar voltage impulses of an amplitude from to 600 volts with a ratio of the energy of a single impulse to an average impulse over a cycle ranging from 1 to 5, comprising an exciter-including a phantastron, means including a voltage amplifier connected directly to the output of said exciter for amplifying primary oscillations up to rated value, a power amplifier having its input connected to said voltage amplifier, a magnetic core output transformer having an air gap in its magnetic core and its primary winding'connected to the output of said power amplifier, said transformer having a secondary winding, and two output terminal circuits connected to said secondary winding, one of said output terminal circuits comprising a semiconductor valve preventing flow of current during one half of a cycle and allowing flow of current ther'ethrough to a metal finishing load circuit during the other half cycle of each operating cycle, the other of said output terminal circuits comprising a second semiconductor' valve connected across a series circuit formed of said secondary winding and said first semiconductor valve. 2. An impulse generator for high-frequency electricimpulse metal finishing according to claim 1, wherein a non-linear resistor is connected in parallel with the secondary winding of an output transformer.

3. An impulse generator for high-frequency electricmetal finishing according to claim 1, wherein a non-linear resistor is connected in parallel with an exciter tube.

4. An impulse generator for high-frequency electricimpulse metal finishing according to claim 1, wherein voltage, feeding amplifier tubes, amounts to 0.7 to 0.8 of the rated value.

5. An impulse generator for high-frequency electricimpulse metal finishing according to claim 1, wherein the windings of the output transformer are filled with epoxy resin which prevents a possible breakdown when transient processes occur.

6. An impulse generator for high-frequency electricimpulse metal finishing according to claim 1, wherein the output transformer is manufactured with an air gap in the magnetic core. i

References Cited UNITED STATES PATENTS 2,794,110 5/1957 Griflith. 2,996,601 8/ 1961 ,Sweetman. 3,072,777 1/ 1963 McKechnie.

' 3,062,985 11/1962 Webb 2l969 X 3,089,059 5/1963 Porterfield et al. 2l969 X 3,114,029 12/1963 Pascoe et al. 2l969 3,135,852 6/1964 Bentley et al. 2l969 3,144,541 8/1964 Hill 2l969 3,292,040 12/1966 Ullmann et al. 2l969 X R. F. STAUBLY, Primary Examiner

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