PROTECTIVE SYSTEM FOR ELECTRICAL DISCHARGE MACHINING POWER SUPPLY CIRCUIT

• US 3,649,802 A
• Filed: 12/28/1970
• Issued: 03/14/1972
• Est. Priority Date: 12/28/1970
• Status: Expired due to Term

• Current Assignee: Colt Industries Incorporated
• Original Assignee: Kurt H. Sennowitz
• Inventors: Sennowitz, Kurt H.
• Primary Examiner(s): Staubly, R. F.
• Application Number:
  US05/102,009
• Time in Patent Office: 442 Days
• Field of Search: 219/69C
• US Class Current: 219/69.13
• CPC Class Codes: B23H 1/024 Detection of, and response to, abnormal gap conditions, e.g. short circuits preventing short circuits or other abnormal discharges by altering machining parameters using adaptive control B23H7/16

Abstract

A circuit is provided to sense an electrical parameter of the gap to determine whether electrical discharge machining is progressing properly. A gap parameter such as gap voltage or a voltage directly related to such parameter is compared to a variable reference voltage to provide reduction or interruption of machining power pulses. A control is included which conjointly controls frequency of machining pulses and the magnitude of the variable reference voltage.

Claims

1. In an electrical discharge machining apparatus for machining an electrically conductive workpiece by passing electrical discharges across a dielectric coolant filled gap between a tool electrode and the workpiece, an electronic output switch and a power supply operatively connected across said gap, a pulser means operatively connected to said switch for operating it with pulses of selectively variable on-off time and frequency, wherein the improvement comprises a circuit for reducing the power content of said pulses responsive to gap abnormal condition, said circuit comprising a variable reference voltage network, a sensing network operatively connected to said gap for sensing a parameter representative of gap condition and providing a voltage output representative thereof, a cutoff means operatively connected to both said networks and operable to reduce the ontime of said pulser responsive to a predetermined difference between said voltages, and means operatively connected between said pulser means and said reference voltage network for varying the magnitude of said reference voltage in response to the frequency preset on said pulser.

2. In an electrical discharge machining apparatus for machining an electrically conductive workpiece by passing electrical discharges across a dielectric coolant filled gap between a tool electrode and the workpiece, an electronic output switch and a power supply operatively connected across said gap, a pulser means operatively connected to said switch for operating it with pulses of selectively variable on-off time and frequency, wherein the improvement comprises a circuit for reducing the power content of said pulses responsive to abnormal gap voltage condition, said circuit including a variable reference voltage network, a sensing network coupled to one terminal of said gap for sensing gap voltage and providing a voltage output representative thereof, a cutoff means operably connected to the output of both said networks and operative to reduce the on-time of said pulser responsive to the relative values of said voltages, and means operably connected between said pulser and said cutoff means for varying the magnitude of said reference voltage in accordance with the frequency setting of said pulser means.

3. In an electrical discharge machining apparatus for machining an electrically conductive workpiece by passing electrical discharges across a dielectric coolant filled gap between a tool electrode and the workpiece, an electronic output switch and a power operatively connected to said gap, a presettable multivibrator operatively connected to said electronic switch for operating it with pulses of selectively variable and on-off time frequency, wherein the improvement comprises a protective circuit for reducing the on-time of said pulses responsive to gap abnormal condition, said circuit comprising a variable reference voltage network, a sensing network coupled to one terminal of said gap for providing an output responsive to gap voltage condition, a cutoff switch operatively connected to both of said networks and having its output operatively connected to said multivibrator to reduce its on-time, and a control means operatively connected between said multivibrator and said reference voltage network for varying the level of said reference voltage network responsive to the frequency setting of said multivibrator.

4. The combination as set forth in claim 3 wherein said control means is operable to lower the level of said reference voltage at relatively low frequency settings of said multivibrator and to raise the level of said reference voltage at relatively high frequency settings of said multivibrator.

5. The combination as set forth in claim 3 wherein said multivibrator comprises an astable multivibrator including a variable capacitor operatively connected thereto for selectively varying the frequency of said multivibrator and wherein said reference voltage network includes a variable resistor, said control means comprising a ganged operating means operatively connected between said variable capacitor and said resistor.

6. The combination as set forth in claim 5 wherein said multivibrator includes a pair of transistors rendered alternately conductive and nonconductive and wherein said cutoff switch comprises a third transistor having its control electrode and one of its principal electrodes each connected to a different one Of said networks, and its other principal electrode coupled to the control electrode of one of said multivibrator transistors for controlling its conduction.

7. In an electrical discharge machining apparatus for machining an electrically conductive workpiece by passing electrical discharges across a dielectric coolant filled gap between a tool electrode and the workpiece, an electronic output switch and a power supply operatively connected to said gap, a presettable multivibrator operatively connected to said electronic switch for operating it with pulses of selectively variable frequency and on-off time, wherein the improvement comprises the protective circuit for reducing the on-time of said pulses responsive to gap short-circuit condition, said circuit comprising a variable reference voltage network, a sensing network coupled to said gap for providing an output responsive to gap voltage condition, a cutoff means operatively connected to both of said networks and having its output operatively connected to said multivibrator to reduce its on-time, and a control means operatively connected between said multivibrator and said reference voltage network for conjointly varying the level of said reference voltage network in accordance with the frequency setting of said multivibrator.

8. The combination as set forth in claim 7 wherein said control means is operable to lower the level of said reference voltage at relatively low frequency settings of said multivibrator and further operable to raise the level of said reference voltage at relatively high frequency settings of said multivibrator whereby changeover is made between rough and finishing operation.

9. The combination as set forth in claim 8 wherein said multivibrator comprises an astable multivibrator including a variable capacitor operatively connected thereto for selectively varying the frequency of said multivibrator and wherein said reference voltage network includes a variable resistor, said control means comprising a ganged operating means operatively connected between said variable capacitor and said resistor.

10. The combination as set forth in claim 9 wherein said multivibrator includes a pair of transistors rendered alternately conductive and nonconductive and wherein said cutoff switch comprises a third transistor having its control electrode and one of its principal electrodes each connected to different one of said networks and its other principal electrode coupled to the control electrode of one of said multivibrator transistors for controlling its conduction and thereby controlling the on-time of said pulses.