Power source circuit for electric discharge machine

• US 4,395,612 A
• Filed: 02/11/1981
• Issued: 07/26/1983
• Est. Priority Date: 02/16/1980
• Status: Expired due to Term

• Current Assignee: Fujitsu Fanuc Ltd (Fanuc Corporation)
• Original Assignee: Fujitsu Fanuc Ltd (Fanuc Corporation)
• Inventors: Izumiya, Syunzo
• Primary Examiner(s): Shaw, C. C.
• Application Number:
  US06/233,501
• Time in Patent Office: 895 Days
• Field of Search: 219/69 S, 219/69 P, 219/69 C
• US Class Current: 219/69.18
• 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 power source circuit for use with an electric discharge machine which machines a workpiece by a discharge produced between an electrode and the work. When an arc forms in a working gap defined between the electrode and the work, the OFF period of a switching element inserted in the charging path of a capacitor for storing discharge energy is increased so that current flowing across the working gap is cut off for a longer period of time than in the case of a normal operation, thereby quickly restoring the insulating qualities of the gap following arcing.

Claims

1. An improved power source for electric discharge machines of the type wherein a first digital oscillator drives a switching element in the charging circuit of a capacitor which repeatedly produces machining discharges through a gap between an electrode and a workpiece in order to machine the workpiece, the machining discharges occurring at a first voltage which is higher than the breakdown voltage of the gap following a machining discharge, wherein the improvement comprises:

first means for detecting the voltage across the gap;

second means responsive to the first means for turning the switching element OFF at least once, for a duration longer than that determined by the first oscillator, before the gap voltage has risen to a second voltage following a machining discharge, the second voltage being higher than the breakdown voltage of the gap following a machining discharge but lower than the first voltage at which a machining discharge occurs, said second means including a second digital oscillator having a frequency lower than the first digital oscillator, a first gate having a first input responsive to the output of the first oscillator and a second input responsive to the output of the second oscillator, and third means for communicating the output of the first gate to the switching element; and

additional means for keeping the switching element ON, following a machining discharge, as the gap voltage rises from a level above the breakdown voltage of the gap following a machining discharge to the first voltage at which a machining discharge occurs.

2. The power source of claim 1, wherein the first means comprises a voltage divider formed from first and second resistors series-connected at an intermediate connection point, the voltage divider being connected in parallel with the capacitor, and wherein the second means additionally comprises first comparator means for starting the second oscillator following a machining discharge, the first comparator means having a first input responsive to the voltage at the intermediate connection point between the first and second resistors, a second input responsive to a reference voltage which is substantially equal to the second voltage, and an output to which the second oscillator is responsive.

3. The power source of claim 2, wherein the third means comprises a second gate having an output to which the switching element is responsive and a first input responsive to the output of the first gate.

4. The power source of claim 3, wherein the second gate has a second input, and wherein the additional means comprises second comparator means having a first input responsive to the voltage at the intermediate connection point between the first and second resistors, a second input responsive to a third voltage which is higher than the breakdown voltage of the gap following a machining discharge but lower than the first voltage at which a machining discharge occurs, and an output to which the second input of the second gate is responsive.

5. The power source of claim 4, wherein the third voltage is higher than the second voltage and wherein the second oscillator is ON when it is not oscillating as a workpiece is being machined, the oscillations of the second oscillator beginning with an ON pulse each time the second oscillator begins oscillating.

6. The power source of claim 5, wherein the switching element is a transistor having a base connected to the output of the second gate.

7. A power source circuit for an electric discharge machine which receives electricity from a power supply means for supplying electrical power and produces an electrical discharge between a workpiece and an electrode separated from the workpiece by a gap, comprising

a capacitor connected across the gap;

switching element means, responsive to a drive signal, for connecting the power source means to the capacitor to charge the capacitor;

means for generating a gap voltage signal that is proportional to the voltage across the gap; and

pulse generating means responsive to said gap voltage signal for generating said drive signal for said switching element means, said pulse generating means including

first oscillator means for generating a pulse output signal that has predetermined ON and OFF periods,

a first comparator having one input responsive to a first reference voltage and another input responsive to said gap voltage signal,

second oscillator means responsive to the output of said first comparator for generating an output signal that is ON when said gap voltage signal exceeds said first reference voltage and that oscillates when said gap voltage signal is less than said first reference signal, the oscillations being alternate ON and OFF pulses that are respectively ON for longer than said predetermined ON period and OFF for longer than said predetermined OFF period,

a gate having one input responsive to the output signal of said first oscillator means and another input responsive to the output signal of said second oscillator means,

a second comparator having one input responsive to said gap voltage signal and another input responsive to a second reference voltage, and

gate means for OR-ing the outputs of said gate and said second comparator to produce said drive signal for said switching element means.