EDM power supply for generating self-adaptive discharge pulses

Abstract

An EDM (electric-discharge machining) power supply system for generating self-adaptive discharge pulses wherein an electrode is spacedly juxtaposed with a workpiece across a discharge gap while a dielectric liquid coolant is passed therethrough. The electrode and the workpiece are relatively displaced during the machining of the latter to maintain the gap spacing generally constant via a servomechanism. According to the invention, there is applied across the electrode and the workpiece a direct-current arc-striking voltage sufficient to initiate discharge across the gap while permitting the voltage to build up thereacross to a level constituting a function of conductivity characteristic of the gap and to decay with a discharge across the gap. An analog signal is derived across the gap and represents the voltage buildup and decay thereacross. Machining current flow through the gap across the electrode and the workpiece is triggered by a digital signal derived when the analog signal exceeds a threshold value and initiation of the discharge is induced by the arc-striking voltage. A second digital condition terminates the machining current flow which is controlled by a semiconductive power switch turned on and off instantaneously in dependence upon the digital conditions. A limited current high-voltage source is connected in a closed loop circuit with the electrode, the workpiece and the gap to provide the voltage buildup across the latter, while the voltage across the gap is detected by a voltage divider or the like and the output of this voltage divider is supplied via an integrating circuit in a squaring or gating-type logic device, e.g., a Schmitt trigger capable of producing the digital output for triggering the semiconductive power switch of the machining-current power supply.

I claim: 
     
       1. A method of machining a conductive workpiece comprising the steps of: a. spacedly juxtaposing an electrode with said workpiece across a discharge gap while passing a dielectric liquid therethrough.   b. relatively displacing said electrode and said workpiece during the machining thereof to maintain the gap spacing generally constant;   c. applying across said electrode and said workpiece a direct current arc-striking voltage sufficient to initiate discharge across said gap while permitting said voltage to build up thereacross to a level constituting a function of conductivity characteristics of said gap and to decay with discharge across said gap;   d. deriving an analog signal representing the voltage build up and decay across said gap; and   e. triggering a machining current flow through said gap across said electrode and said workpiece upon said signal exceeding a first threshold value and upon initiation of a discharge by said voltage, and terminating said machining current flow through said gap upon detection of a value of the analog signal declining to a second threshold value less than said first threshold value.   
     
     
       2. The method defined in claim 1, further comprising the steps of establishing a first digital condition upon said signal exceeding said first threshold value and a logically converse second digital condition upon said signal attaining said second threshold value; and switching a machining current source connected across said electrode and said workpiece on and off instantaneously in response to the establishment of said first and second conditions, respectively. 
     
     
       3. The method defined in claim 2 wherein said arc-striking voltage is applied across said electrode and said workpiece by a limited-current DC source connected across said gap in a closed-loop circuit therewith, said analog signal being derived by detecting the gap voltage and producing a sensed signal proportional thereto, and integrating said sensed signal to form said analog signal. 
     
     
       4. In the electric discharge machining of a conductive workpiece wherein an electric discharge machining electrode is spacedly juxtaposed with the workpiece across a machining gap flooded with a dielectric liquid coolant and a source of electric machining current is connectable across said electrode and said workpiece to pass an impulsive erosive electric discharge across said gap, the improvement which comprises the steps of: a. connecting said electrode, said gap and said workpiece in a closed-loop circuit with a limited-current voltage source;   b. deriving an analog signal from said gap indicative of gap recovery from a preceding discharge and related to gap conductivity;   c. producing with said analog signal at least one digital triggering signal upon said analog signal attaining a predetermined threshold value; and   d. a periodically triggering said source of machining current on and off instantaneously with said triggering signal, the analog signal being compared with at least one threshold value to produce a first digital state and with another threshold value less than the said one threshold value to produce a second digital state, one of said digital states corresponding to said digital triggering signal, said source of machining current being turned on and off in accordance with said digital states.   
     
     
       5. The improvement defined in claim 4 wherein said analog signal is derived in step b, by the steps of deriving a signal representing voltage buildup across said gap, and integrating the voltage-buildup signal by controlling the charge and discharge of a capacitor therewith to produce said analog signal. 
     
     
       6. The improvement defined in claim 5, further comprising the step of clipping said analog signal beyond a predetermined level thereof to prevent an excessive duration of triggering of said source of machining current. 
     
     
       7. A power supply for electrical discharge machining wherein a tool electrode is spacedly juxtaposed with a workpiece constituting a counterelectrode across a machining gap flooded with a dielectric liquid coolant, said power supply comprising: a main source of machining current including an electronically triggerable power switch connected in series with said electrodes and said gap;   an auxiliary source of arc-striking voltage connected in a closed-loop arrangement with said gap for building up a voltage thereacross;   sensing means including a voltage-dividing resistor having a pair of output terminals connected across said gap for producing an output indicative of the breakdown conditions of said gap;   means including an integrating network having a capacitor bridged across said output terminals for forming an analog signal from said output related to the gap conditions;   threshold gating means including a Schmitt trigger responsive to said analog signal for establishing a first digital state and a second digital state, said threshold gating circuit being connected to said power switch for substantially instantaneously rendering same conductive and terminating conductivity of said power switch upon the occurrence of said digital states respectively, said Schmitt trigger having an input transistor with a base connected to said capacitor.   
     
     
       8. The power supply defined in claim 7 wherein said power switch is a bank of power transistors connected in parallel between said main source and said electrodes and having bases triggerable in parallel by said threshold gating means. 
     
     
       9. A power supply for electrical discharge machining wherein a tool electrode is spacedly juxtaposed with a workpiece constituting a counterelectrode across a machining gap flooded with a dielectric liquid coolant, said power supply comprising: a main source of machining current including an electronically triggerable power switch connected in series with said electrodes and said gap;   an auxiliary source of arc-striking voltage connected in a closed-loop arrangement with said gap for building up a voltage thereacross;   sensing means including a voltage-dividing resistor having a pair of output terminals connected across said gap for producing an output indicative of the breakdown conditions of said gap;   means including an integrating network having a capacitor bridged across said output terminals for forming an analog signal from said output related to the gap conditions; and   threshold gating means including a Schmitt trigger responsive to said analog signal for establishing a first digital state and a second digital state, said threshold gating circuit being connected to said power switch for substantially instantaneously rendering same conductive and terminating conductivity of said power switch upon the occurrence of said digital states respectively, said Schmitt trigger having an input transistor with a base connected to said capacitor, and means establishing two threshold values for said Schmitt trigger effective to produce an output signal upon the voltage across said capacitor exceeding one of said threshold values and terminating said output signal upon the voltage across said capacitor attaining a second threshold value less than the first threshold value, the output signal of said Schmitt trigger being applied to said power switch for rendering same conductive.   
     
     
       10. The power supply defined in claim 9, further comprising amplifier means between said Schmitt trigger and said power switch. 
     
     
       11. The power supply defined in claim 9, further comprising an electronic breakdown device connected across said capacitor for clipping the voltage appearing thereacross upon the latter voltage exceeding a predetermined magnitude. 
     
     
       12. The power supply defined in claim 11 wherein said breakdown device is a Zener diode. 
     
     
       13. The power supply defined in claim 9, further comprising means including a source of direct current and a resistor in series with said capacitor and said source of direct current for charging said capacitor, said voltage divider being connected to said capacitor so as to buck the charging voltage supplied thereto through said resistor. 
     
     
       14. The power supply defined in claim 9, further comprising a rectifier diode connected between one of said output terminals and said capacitor. 
     
     
       15. The power supply defined in claim 9 wherein said main source of machining current includes a direct current source and a bank of power transistors in series with said direct current source and said electrode, and wherein said auxiliary source includes said direct current and a current-limiting resistor in series therewith connected across said electrodes. 
     
     
       16. The power supply defined in claim 9 wherein said major source includes a relatively low-voltage high-current DC source connected in series with a bank of power transistors across said electrodes and wherein said auxiliary source includes a relatively high-voltage low current DC source connected across said electrodes. 
     
     
       17. The power supply defined in claim 16, further comprising a rectifier diode connected in series with said low voltage high current DC source to block high voltage from said high-voltage low current DC source. 
     
     
       18. The power supply defined in claim 16, further comprising a current-limiting resistor in series with said high-voltage DC source. 
     
     
       19. The power supply defined in claim 9 further comprising means for adjusting the charging and discharging rate of said capacitor during the electrical discharge machining process. 
     
     
       20. The power supply defined in claim 19 wherein the last-mentioned means includes a plurality of taps of said voltage divider and sequence-switching means successively connecting said capacitor with said taps. 
     
     
       21. The power supply defined in claim 19 wherein said voltage divider is a variable resistor, the last-mentioned means including a wiper adapted to sweep along said variable resistor. 
     
     
       22. A power supply for electrical discharge machining wherein a tool electrode is spacedly juxtaposed with a workpiece constituting a counterelectrode across a machining gap flooded with a dielectric liquid coolant, said power supply comprising: a main source of machining current including an electronically triggerable power switch connected in series with said electrodes and said gap;   an auxiliary source of arc-striking voltage connected in a closed-loop arrangement with said gap for building up a voltage thereacross;   sensing means connected across said gap for producing an output indicative of the breakdown conditions of said gap;   means forming an analog signal from said output related to the gap conditions;   threshold gating means responsive to said analog signal for establishing a first digital state and a second digital state, said threshold gating circuit being connected to said power switch for substantially instantaneously rendering same conductive and terminating conductivity of said power switch upon the occurrence of said digital states respectively, said means forming said analog signal from said output including an integrating network connected between said sensing means and said threshold gating means, said sensing means including a voltage divider connected between said electrodes across said gap and having output terminals connected to said integrating network, said integrating network including at least one capacitor in circuit with said output terminals and chargeable at a rate controlled by a potential appearing across said output terminals, said threshold gating means including a Schmitt trigger circuit having an input connected to said capacitor and means establishing two threshold values for said Schmitt trigger circuit effective to produce an output signal upon the voltage across said capacitor exceeding one of said threshold values and terminating said output signal upon the voltage across said capacitor attaining the second threshold value, the output signal of said Schmitt trigger circuit being applied to said power switch for rendering the same conductive; and   a timer for connecting sequentially successive portions of said voltage divider in circuit with said capacitor for adjusting the charging and discharging rate of said capacitor during the electrical discharge machining process.   
     
     
       23. The power supply defined in claim 22, further comprising a plurality of taps of said voltage divider and sequence-switching means including said timer successively connecting said capacitor with said taps. 
     
     
       24. The power supply defined in claim 22, further comprising amplifier means between said Schmitt trigger circuit and said power switch. 
     
     
       25. The power supply defined in claim 22, further comprising an electronic breakdown device connected across said capacitor for clipping the voltage appearing thereacross upon the latter voltage exceeding a predetermined magnitude. 
     
     
       26. The power supply defined in claim 25 wherein said breakdown device is a Zener diode. 
     
     
       27. The power supply defined in claim 22, further comprising means including a source of direct current and a resistor in series with said capacitor and said source of direct current for charging said capacitor, said voltage divider being connected to said capacitor so as to buck the charging voltage supplied thereto through said resistor. 
     
     
       28. The power supply defined in claim 22, further comprising a rectifier diode connected between one of said output terminals and said capacitor. 
     
     
       29. The power supply defined in claim 22 wherein said main source of machining current includes a direct current source and a bank of power transistors in series with said direct current source and said electrodes, and wherein said auxiliary source includes said direct current source and a current-limiting resistor in series therewith connected across said electrodes. 
     
     
       30. The power supply defined in claim 22 wherein said major source includes a relatively low voltage high-current DC source connected in series with a bank of power transistors across said electrodes and wherein said auxiliary source includes a relatively high-voltage low-current DC source connected across said electrodes. 
     
     
       31. The power supply defined in claim 30, further comprising a rectifier diode connected in series with said low-voltage high-current source to block high voltage from said low-current DC source. 
     
     
       32. The power supply defined in claim 30, further comprising a current-limiting resistor in series with said high-voltage DC source. 
     
     
       33. A power supply for electrical discharge machinining wherein a tool electrode is spacedly juxtaposed with a workpiece constituting a counterelectrode across a machining gap flooded with a dielectric liquid coolant, said power supply comprising: a main source of machining current including an electronically triggerable power switch connected in series with said electrodes and said gap;   an auxiliary source of arc-striking voltage connected in a closed-loop arrangement with said gap for building up a voltage thereacross;   sensing means connected across said gap for producing an output indicative of the breakdown conditions of said gap;   means forming an analog signal from said output related to the gap conditions; and    threshold gating means responsive to said analog signal for establishing a first digital state and a second digital state, said threshold gating circuit being connected to said power switch for substantially instantaneously rendering same conductive and terminating conductivity of said power switch upon the occurrence of said digital states respectively, said means forming said analog signal from said output including an integrating network connected between said sensing means and said threshold gating means, said sensing means including a voltage divider connected between said electrodes across said gap and having output terminals connected to said integrating network, said integrating network including at least one capacitor in circuit with said output terminals and chargeable at a rate controlled by a potential appearing across said output terminals, said threshold gating means including a Schmitt trigger circuit having an input connected to said capacitor and means establishing two threshold values for said Schmitt trigger circuit effective to produce an output signal upon the voltage across said capacitor exceeding one of said threshold values and terminating said output signal upon the voltage across said capacitor attaining the second threshold values, the output signal of said Schmitt trigger circuit being applied to said power switch for rendering same conductive, said Schmitt trigger circuit including a first transistor connected to said capacitor and a second transistor connected to said first transistor and producing said output signal of the Schmitt trigger circuit, said means establishing said threshold values including at least one variable resistor connected in circuit with at least one of said transistors, said capacitor being a variable condenser.   
     
     
       34. The power supply defined in claim 33 wherein said power switch is a bank of power transistors connected in parallel between said main source and said electrodes and having bases triggerable in parallel by said threshold gating means. 
     
     
       35. The power supply defined in claim 33, further ocmprising amplifier means between said Schmitt trigger circuit and said power switch. 
     
     
       36. The power supply defined in claim 33, further comprising an electronic breakdown device connected across said capacitor for clipping the voltage appearing thereacross upon the latter voltage exceeding a predetermined magnitude. 
     
     
       37. The power supply defined in claim 36 wherein said breakdown device is a Zener diode; 
     
     
       38. The power supply defined in claim 33, further comprising means including a source of direct current and a resistor in series with said capacitor and said source of direct current for charging said capacitor, said voltage divider being connected to said capacitor so as to buck the charging voltage supplied thereto through said resistor. 
     
     
       39. The power supply defined in claim 33, further comprising a rectifier diode connected between one of said output terminals and said capacitor. 
     
     
       40. The power supply defined in claim 33 wherein said main source of machining current includes a direct current source and a bank of power transistors in series with said direct current source and said electrodes, and wherein said auxiliary source includes said direct current source and a current-limiting resistor in series therewith connected across said electrodes. 
     
     
       41. The power supply defined in claim 33 wherein said major source includes a relatively low-voltage high-current DC source connected in series with a bank of power transistors across said electrodes and wherein said auxiliary source includes a relatively high-voltage low-current DC source connected across said electrodes. 
     
     
       42. The power supply defined in claim 41, further comprising a rectifier diode connected in series with said low-voltage high-current source to block high voltage from said low current DC source. 
     
     
       43. The power supply defined in claim 41, further comprising a current-limiting resistor in series with said high-voltage DC source. 
     
     
       44. The power supply defined in claim 33, further comprising means for adjusting the charging and discharging rate of said capacitor during the electrical discharge machining process. 
     
     
       45. The power supply defined in claim 44 wherein the last-mentioned means includes a plurality of taps of said voltage divider and sequence-switching means successively connecting said capacitor with said taps. 
     
     
       46. The power supply defined in claim 44 wherein said voltage divider is a variable resistor, the last-mentioned means including a wiper adapted to sweep along said variable resistor. 
     
     
       47. The power supply defined in claim 44 wherein the last-mentioned means includes a timer for connecting sequentially successive portions of said voltage divider in circuit with said capacitor. .Iadd. 48. A process for shaping a workpiece using electric current comprising the steps of: positioning a workpiece adjacent a working electrode such that the electrode and the workpiece are separated by a working gap,   impressing an intermittent pulse voltage across said working gap,   detecting the time period between the instant at which a voltage is impressed across said working gap and the instant at which a substantial current begins to flow within said working gap; and,   controlling the length of time during which said substantial current flows directly in response to the length of said detected time period. .Iaddend..Iadd. 49. A process as in claim 48, wherein said step of controlling includes the steps of:   reducing the length of time during which said substantial current flows in said working gap below a maximum predetermined length of time provided said time period is less than a predetermined interval; and,   permitting said substantial current to flow for said maximum predetermined length of time provided said time period is greater than said predetermined interval. .Iaddend. .Iadd. 50. An apparatus for shaping a workpiece by electric current comprising:   switching means for impressing a voltage across a working gap between an electrode and a workpiece, said voltage remaining at a no-load level until a substantial current flows in said working gap,   timing means for measuring the time period during which said no-load voltage exists across said working gap; and,   control means coupled to said no-load voltage time measuring means for controlling the duration of flow of said substantial current directly in response to the output from said no-load voltage time measuring means. .Iaddend.