Electrical discharge machining pulse control method and apparatus

Abstract

A system for controlling machining pulse on-off time to limit gap power in response to a short or an open circuit machining gap condition in electrical machining systems wherein a multivibrator applies a preset on-off time at an electronic switch between the power supply and the gap. The condition detected at the gap in the form of an electrical signal is delivered via a delay network to a control circuit adapted to shunt one of the impedances of the resistance-capacitance network of the multivibrator.

I claim: 
     
       1. In an apparatus for machining a conductive workpiece by passing electrical-discharge pulses between a tool electrode and said workpiece across a dielectric-coolant filled gap, a machining power circuit for supplying said pulses to said gap, said machining power circuit comprising a power supply; a first electronic switch having a pair of principal electrodes connected in series with said power supply and said gap, and a control electrode; a multivibrator having its output connected to the control electrode of said first switch for operating it with predetermined on-off time, said multivibrator including a pair of second electronic switches biased and coupled for alternate operation while having respective control electrodes and at least one resistance-capacitance network connected to the control electrode of one of said pair of said second switches for controlling said on-off time, said network having a time-constant-determining resistance; sensing means connected to said gap for providing an electrical output signal responsive to gap short circuit condition; control means for altering the resistance of said network to decrease switch on-time responsive to said signal; and delay means coupled between said sensing means and said control means for delaying its operation a predetermined time interval after occurrence of said condition, said control means comprising a third electronic switch having its principal electrodes connected in shunt with said time-constant-determining resistance of said network for varying the effective magnitude of said time constant and a control electrode connected to said gap through said delay means. 
     
     
       2. In an apparatus for machining a conductive workpiece by passing electrical-discharge pulses between a tool electrode and the workpiece across a dielectric-coolant-filled gap, comprising a machining-power circuit for supplying said pulses to said gap comprising a power supply, and a periodically operated electronic switch of preset on-off time having its principal electrodes connected in series with said power supply and said gap for providing said pulses thereto and having a control electrode, the improvement which comprises sensing means connected to said gap for providing an electrical output signal responsive to gap open circuit condition, control means operatively connected to the control electrode of said switch for decreasing its on-time responsive to said signal, and delay means coupled between said sensing means and said control means for delaying its operation a predetermined time interval after occurrence of said condition. 
     
     
       3. The improvement defined in claim 2 wherein: said sensing means includes a voltage-dividing resistor connected across said gap;   said delay means includes a delay line having an inductance with one terminal connected to said voltage-dividing resistor and a multiplicity of capacitors having corresponding terminals connected to said voltage-dividing resistor and tapped to said inductance; and   said control means includes a differentiating network connected across another terminal of said inductance and said corresponding terminals of said capacitors, and at least one amplifying transistor connected between said differentiating network and said control electrode.   
     
     
       4. The improvement defined in claim 3, further comprising an integrating network connected between said differentiating network and said amplifying transistor. 
     
     
       5. The improvement defined in claim 4 wherein said periodically operated electronic switch comprises a plurality of parallel-triggered switching transistors each connected to said gap through a respective group of power transistors. 
     
     
       6. The improvement set forth in claim 2 in which said sensing means comprises a variable inductance for providing an output signal which is a function of relatively high frequency discharges occurring across the gap during normal cutting. 
     
     
       7. In an apparatus for machining a conductive workpiece by passing electrical-discharge pulses between a tool electrode and said workpiece across a dielectric-coolant-filled gap, a machining power circuit for supplying said pulses to said gap, said machining power circuit comprising a power supply, a first electronic switch having a pair of principal electrodes connected between said power supply and said gap and a control electrode, a multivibrator having its output connected to the control electrode of said first switch for operating it with predetermined on-off time, said multivibrator including a pair of second electronic switches biased and coupled for alternate operation, each of said pair of second switches having a respective control electrode connected to a different resistor-capacitor network, sensing means connected to said gap for providing an electrical output signal responsive to gap short circuit condition, a first control means connected between said sensing means and one of said resistor-capacitor networks for changing its time constant and decreasing pulse on-time, a second control means connected between said sensing means and the other of said resistor-capacitor networks for changing its time constant and increasing pulse off-time, and delay means connected between said sensing means and each of said control means for controlling its operation a predetermined time interval after occurrence of said condition. 
     
     
       8. The method of electrical discharge machining comprising the steps of periodically connecting a power supply to a machining gap through an electronic switch to provide machining pulses of a predetermined on-off time and frequency thereto, wherein the improvement comprises sensing for the presence and absence of substantially higher frequency discharges across said gap, providing a control output signal responsive to the absence of said high frequency discharges, such condition being representative of gap open circuit and employing said signal to increase the off-time and decrease the on-time of said machining pulses. 
     
     
       9. In an apparatus for machining a conductive workpiece by passing electrical pulses between a tool electrode and a workpiece across a liquid-filled gap, the improvement which comprises: a power supply;   periodically operable electronic switch means having at least one control electrode and at least one pair of principal electrodes in series between said power supply and said gap for supplying said pulses thereto;   periodically operable multivibrator means having an output connected to said control electrode for triggering said switch means with preset on-off time, said multivibrator means including at least one resistance-capacitance network having a time-constant-determining impedance for controlling on-off time;   sensing means connected across said gap for providing an electrical output signal responsive to a condition of said gap;   control means between said sensing means and said impedance and bridged thereacross for altering the effective impedance value and the time constant of said resistance-capacitance network in accordance with said signal; and   delay means between said sensing means and said control means for delaying operation of said control means for a predetermined time interval after occurrence of said condition.   
     
     
       10. In an apparatus for machining a conductive workpiece by passing electrical pulses between a tool electrode and a workpiece across a liquid-filled gap, the improvement which comprises a power supply: periodically operable electronic switch means including at least one first electronic switch having a control electrode and a pair of principal electrodes and a plurality of parallel-connected second electronic switches each having a control electrode connected in circuit with the principal electrodes of said first switch and a pair of principal electrodes connected in circuit with said power supply and said gap for supplying said pulses thereto;   periodically operable multivibrator means having an output connected to said control electrode of said first switch for triggering said switch means with preset on-off time, said multivibrator including a pair of alternately conductive electronic switches and respective resistance-capacitance networks in circuit with each of said alternately operable electronic switches, each of said resistance-capacitance networks having a respective time-constant-determining impedance for controlling on-off time;   sensing means connected across said gap for providing an electrical output signal responsive to a condition of said gap;   control means between said sensing means and at least one of said impedance and bridged thereacross for shunting the bridged impedance and altering the effective impedance value and the time constant of the corresponding resistance-capacitance network in accordance with said signal; and   delay means between said sensing means and said control means for delaying operation of said control means for a predetermined time interval after occurrence of said condition.   
     
     
       11. The improvement defined in claim 10 wherein: said multivibrator means is a transistor multivibrator;   said alternately operable electronic switches are transistors having bases tied to the respective resistance-capacitance networks; and   the impedance bridged by said control means is a resistor of the corresponding resistance-capacitance network.   
     
     
       12. The improvement defined in claim 11 wherein said power supply includes: an alternating-current source;   a saturable reactor in series with said source;   a rectifier in series with said saturable reactor and said source and connected between said saturable reactor and said periodically operable electronic switch means; and   further control means responsive to the electrical current flow through said gap for controlling said saturable reactor.   
     
     
       13. The improvement defined in claim 11 wherein said control means includes: a pair of switching transistors each having its principal electrodes connected across one of said impedances and respective control electrodes;   an integrating network connected to the control electrodes of said switching transistors; and   a differentiating network connected between said delay means and said integrating network.   
     
     
       14. In a method of electrical discharge machining of a conductive workpiece with a tool electrode across a machining gap wherein a power supply is periodically connected to the machining gap through an electronic switch to provide machining pulses of a predetermined on-off time frequency to said gap and the discharge includes waveform harmonics of a higher frequency than that of said machining pulses and representative of a normal-gap cutting condition, the improvement which comprises the steps of sensing selected relatively high-frequency harmonics representative of the normal-gap cutting condition across said gap; deriving an electrical control signal as a function of the frequency level of said harmonics; and controlling the pulse on-off time with said signal. 
     
     
       15. In an apparatus for electrically machining a conductive workpiece by passage of electrical machining pulses across a coolant-filled gap between a tool electrode and said workpiece, said apparatus comprising a machining power circuit for supplying said pulses to said gap and including a power supply, a periodically operated electronic switch of preset on-off time and principal electrodes connected in series with said power supply and said gap, said electronic switch having at least one of its on-time and off-time parameters controllable, the improvement which comprises sensing means continuously connected to said gap for providing an electrical output signal continuously responsive to gap condition, control means between said sensing means and said switch for varying one of said parameters in response to said signal, and delay means between said sensing means and said control means for delaying its operation a predetermined time interval after occurrence of said abnormal gap condition. 
     
     
       16. The improvement defined in claim 15 wherein said control means includes an integrating circuit network responsive to the continually sensed gap condition for varying one of said parameters. .Iadd. 17. An EDM apparatus of the type comprised in part of a power supply producing periodic output machining pulses for generating electrical discharges across a machining gap formed between an electrically conductive tool and an electrically conductive workpiece, wherein the improvement comprises: a. means connected to the tool and workpiece for detecting the presence and absence of a noise signal during the discharges; and   b. means connected between the detecting means and the power supply for modifying the production of the machining pulses in response to detecting the absence of the noise signal. .Iaddend. .Iadd. 18. An EDM apparatus of the type comprised in part of a power supply producing periodic output machining pulses for generating electrical discharges having a predetermined energy level across a machining gap formed in a dielectric medium between an electrically conductive tool and an electrically conductive workpiece, wherein the improvement comprises:   (a) means electrically connected to the tool and workpiece for detecting the absence of a noise signal during the electrical discharges;   (b) means connected between the detecting means and the power supply for changing the energy level of the electrical discharges in response to detecting the absence of said noise signal. .Iaddend. .Iadd. 19. An EDM process for removing metal having the steps of initiating a spark discharge across a machining gap, melting a volume of metal and terminating the spark discharge after a predetermined period of time commensurate with a predetermined energy level, wherein the improvement comprises:   (a) detecting the absence of a noise signal across the gap during the spark discharge; and   (b) reducing the energy level of succeeding spark discharges for a predetermined period of time. .Iaddend.