ELECTRONIC CONSIDERATIONS IN THE THEORY AND DESIGNOF ELECTRIC SPARK MACHINE TOOLS

Everard M. Williams, Fellow I.R.E., and James B. Woodford, Jr., Member I.R.E.Department of Electrical Engineering

Carnegie Institute of TechnologyPittsburgh 13, Pennsylvania

The electric spark,l or 'sparkover', machin- melting points lower than about 6000C in theing process is one of several processes in which case of the anode or 4000C in the case of thea machining action is produced more or less cathode. For this and other reasons it seemsdirectly by the action of an electric current at probable that the anode surface temperaturethe surface to be machined. This particular does not exceed 6000C. Although numerousprocess is of economic value2 in working other- theories have been advanced to explain the re-wise unmachinable or difficult to machine moval of anode material it seems probable2 thatmaterials such as sintered carbides and high a major factor in the anode erosion is thetemperature alloys, and in the production of un- electric field force produced by a high voltageusual shapes and forms in any conducting material. gradient within the surface of the anode due to

the high current density at and below the anodeIn an electric spark machine a series of surface. This force acts on the positive ions

electric discharges pass in a liquid dielectric', in the crystal lattice and where the force ex-between a tool electrode (cathode) and workpiece ceeds the ultimate tensile strength material is(anode). Both tool and the adjacent portion of detached and drawn into the discharge path.the workpiece are eroded by the action of the Cathode erosion is not as well understood butdischarges; the tool electrode is progressively a major factor in cathode erosion appears to beadvanced toward or along the workpiece until the bombardment eroded by particles of the anodedesired,shape is produced in the workpiece. The material. In a sustained discharge the currentdischarges are initiated by sparkover of the density eventually decreases to a very low valuedielectric and automatic controls are usually of the order of 4000 amperes per square inchemployed to maintain the tool electrode in and predominantly thermal phenomena ensue. Forsufficiently close proximity to the workpiece this reason discharge duration is limited into allow sparkover at a voltage in the range of spark machining to the initial high current-about 50 to 200 volts depending on machining density interval. Following the termination ofconditions. Discharge currents range from a few a discharge the discharge path must be deionizedamperes to currents as high as 50,000 amperes, sufficiently so that subsequent applications ofwith durations from a fraction of a microsecond voltage will cause sparkover through anotherto about 100 microseconds, and repetition rates liquid channel rather than through the originalfrom a few pulses per second to 20,000 or more, channel which may remain in the gaseous statedepending upon machining conditions and upon the for several pulse intervals.design of the electrical system.

Early use of electric spark machining andT'he physical phenomena involvred may be the first electrical system appear to be due

briefly summarized as follows: The formative to the Lazarenkos3. Figure 1 shows theirtime of the discharge is approximately 15 milli- electrical system, which is still used in smnemicroseconds, during which the voltage drops of the simpler devices commercially available.from the initial sparkover voltage to a voltage In this system the condenser C is initiallyof about 20 volts. The discharge path has the charged by the d-c source through resistor R.shape of a tapered cylinder with a somewhat The electrode is advanced toward the workpiecelarger area at the anode than at the cathode. until sparkover takes place. A high-currentThe liquid within this cylinder is substantially damped-oscillatory discharge ensues and theinstantaneously converted to a gas column under condenser is discharged. The discharge-pathextremely high pressure. The initial area of then deionizes and the condenser recharges.the discharge is determined by the density of This system is essentially a relaxation systematoms within the gas which provide the positive with a repetition rate controlled by the RCions to neutralize space-charge. Initial time constant and the electrode spacing. Forcurrent densities are about 10i amperes per heavy machining operations, in which large con-square inch and are independent of the actual denser capacity is required, stable operationdischarge current. If the discharge is sus- can generally only be achieved for repetitiontained beyond the formative time the expansion rates of the order of 50 pulses per second.of the gas column results in progressively Attempts to increase repetition rates by reducinglower positive ion densities and correspondingly the charging resistance result in a transitiondecreased current densities. At the end of 1 to a continuous discharge due to failure of themicrosecond, for instance, the current density sparkover path to deionize. The Lazarenkohas decreased to about 4 x 106 amperes per system is also characterized by a varyingsquare inch. Anode or cathode composition has sparkover distance because successive sparkoversno effect on current densities unless these generally take place over a range Of voltageselectrodes are constructed of materials writh intermediate between zero and the d-c supply

voltage. The voltage required for sparkover is* commonly a hydrocarbon and usually kerosene continually varying because of variations of

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electrode-to-workpiece spacing caused by irregu- greater duration than the limiting values listedlar servomechanism operation and the accumulatiam above. The open circuit voltage of a sparkof machined particles in the discharge gap. machine is designed for values of the order ofThese voltage variations result in variations in about 200 volts or less in order to providemachining tolerances. In view of the limitations accuracy in machined surfaces. As soon as spark-of the relaxation type of circuit efforts have over takes place the maximum discharge currentbeen directed toward electrical systems with Inax is set by the inductance of the machineindependent pulse-timing and fixed pulse-voltage. tool assembly at a value of

In the independent timing-system the dis- Ima = 200/L x tcharge gap is supplied with pulses of presetcurrent duration, repetition rate and open-circuit voltage. This system permits greatly in which L is the inductance of the dischargeincreased machining speeds, more accurate con- circuit and t the elapsed time of the discharge.trol of finishes and more accurate machining The current is always less than this valuetolerances. because of dissipative components and the

impedance of the source. Figure 2 shows anTechniques for generating short, accurately experimental tool head in which discharge circuit

controlled pulses have been highly developed inductance has been minimized by the use of ain spark machine circuitry. Spark machining coaxial assembly. For reasons of access andhas important characteristics which differ from flexibility mechanical heads have been builtthe radar pulse art. Some of the particularly with the somewhat less efficient arrangement ofimportant differences are: Figure 3 in which a basket like network of leads

replaces the solid sheath of the coaxial en-1. The spark machine requires a low- closure. In the machine tool of Fig. 3 the

voltage high-current pulse rather than a very pulse source is placed in the chamber immediate-high voltage low current pulse. ly beneath the work tank.

2. The maximum average power capacity An electrical system which operatesdesired in the spark machine is considerable successfully with the machine tool of Fig. 3higher than that of ordinary radar sets. has the following characteristics:

3. The electric spark machine power supply Average power output approx. 10 KWoperates into an extremely variable load in the Peak power per pulse approx. 450 KWsense that the discharge gap is essentially Pulse duration 17 microsecondsopen-circuited at the beginning of the pulse. Pulse repetition rate 2880 pulses perSparkover may not occur at all, or may occur secondat any time during the pulse (at certain times Open-circuit pulse voltage approx. 100in the machining process the tool may also be voltsshort-circuited to the workpiece). Following Peak discharge current approx. 4500sparkover the current in the discharge gap is ampereslimited primarily by the inductance of thedischarge circuit and this, in turn, places a In this system energy is stored in pulse-premium on design of the machine tool for forming networks at a potential of approximatelyminimization of inductance in the discharge 10 kv. This energy is derived from a three-phasecircuit. bridge rectifier. The pulse forming networks

are switched to discharge into the primary of a4. Time-jitter has no effect on the system 100 to 1 pulse transformer by means of a rotary

and pulse-shape is relatively unimportant. spark gap. Fig. 4 shows the rotary gap usedand Fig. 5 the pulse transformer. The switched5. Equipment is operated by untrained high-voltage output is delivered by coaxial

personnel, essentially machine tool operators, cable to the pulse transformer located directlyand requirements as to simplicity are stringent. beneath the work tank of the machine.Cost is of major importance.

The pulse-forming-network discharge producesExperimental independently time systems a large negative pulse following the positivehave been constructed and operated within the pulse. A negative pulse at the machine tool isfollowing limits. undesirable because it results in machining at

the tool instead of the workpiece. In additionPeak pulse power 5 megawatts maximum the energy stored in the magnetic field of theRepetition rate 20,000 maximum primary system produces a very high voltage atAverage power 15 KW maximum the spark gap switch which inhibits deionization.Pulse current 20,000 - 30,000 This is augmented when the electrode-tool system

amperes maximum fails to sparkover (due to excessive gap spacing)Duration 0.2 - 50 microseconds or is in a short-circuit condition. The majordesign problem in this unit has, therefore, been

The design of the mechanical heads of concerned with providing adequate but inexpensivespark machines is such as to limit peak powers damping (including dissipation of stored energyand pulse durations to less peak power and components ) for thle negative half-pulse at the

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pulse-transformer primary. Since very nearly the tool electrode drivle motor. The systemall the pulse-network energy shows up in this includes a velocity feedback generator on thenegative pulse when the work gap fails to fire electrode drive motor shaft. The servomechanismor is in short-circuit a damping system with a design is critical since the optimum electrode-capacity of the order of 8 kilowatts dissipation to-workpiece spacing is of the order of 0.0005is required. The peak current required for com- inches.plete damping is approximately 600 amperes.Although pulse damping diodes with sufficient As a short calculation of the energycapacity are available corierically the cost of conditions in this machine will show very littleproviding complete damping in the first half of the average power in the electrical systemcycle would increase the cost of the overall unit is dissipated in the electrical discharge itself.by about 30%. The damping system actually This situation is more or less inherent in theemployed consists of a parallel network of four process because of the ratio of open circuit6C21 tubes (diode connected) in series with a voltages (in the range 100 - 200 volts) to dis-resistor bank. The resistors limit the peak charge voltage (20 - 30 volts). Provision forand average current requirements of the diodes dissipation of surplus energy will probably,and, moreover, assume most of the power dissi- therefore, always be a serious problem inpation since the tube drop is very much less apparatus design. It is of some interest tothan the resistor drop. The peak voltage across note, however, that the total energy consumedtubes and resistor combinations reaches about 20 in an electric-spark material removal operationKV. in service but the average anode dissipation compares favorably with the energy consumed inrequired is only 1KW. The rotary gap voltage more conventional grinding operations on thedrops sufficiently rapidly to permit stable same material.interrupting operation. Both tool wear androtary spark gap wear would be improved byheavier damping but the increased cost of more Referenceseffective damping has not seemed economicallyjustified. It is expected that high-current 1. "Theory of Electric Spark Machining",junction diodes will eventually be available for Everard M. Williams, Trans. A.I.E.E., Vol. 71,direct damping in the pulse transfomer secondary 1952.circuit.

2. "Recent Developments in the Theory and DesignThe electrical unit associated with this of Electric Spark Machine Tools", E. M. Williams,

high power spark machine tool includes in R. E. Smith, and J. B. Woodford, Jr., Trans.addition to the pulse power supply a servo- A.I.E.E., Vol. 73, 1954.mechanism. This servomechanism receives itssignal from information derived from the voltage 3. "Procede et Machine Pour Le Travail deacross the machining gap. The information is Metaux", N. Lazarenko, B. Lazarenko, Swiss Patentamplified and fed to an amplidyne which controls 257,468, April 1, 1949.

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IBATHWORKPIECE

Fig. 1Circuit used by the Lazarenkos to supply electrical

power to an electric spark machine.

Fig. 2 Fig. 3Experimental electric spark machine tool Shop model of electric spark machine

head in which discharge circuit inductance tool with coaxial outside conductorhas been minimized by the use of simulated by a multiplicity

a coaxial assembly. of single leads.

9i L X4 <if XC9g@X 1>1t A.

Fig. 4 Fig. 5Rotary spark gap used for pulse switching 100 to 1 pulse transformer used with

of 10 KV average power at 10 KV. 10 KWI unit. Unit is forced-oil-cooled.

n.