Electrical discharge machining servo control circuit

Claim 1

A servo feed control circuit for use in conjunction with an electrical discharge machining apparatus wherein workpiece material is removed by periodic power pulses passed between a tool electrode and a workpiece across a dielectric coolant filled gap including an electronic output switch connected between a power supply and said gap for providing machining power pulses to said gap at preset on off times and frequencies, a sensing network connected to said gap for providing a gap voltage signal representative of gap condition, a reference voltage signal network, an electrically operated servofeed means connected between said networks for controlling relative movement between said electrode and said workpiece responsive to the difference between said signals, wherein the improvement comprises a diode coupled between said gap and said sensing network, a decoupling means connected between said gap and said sensing network, said decoupling means comprising an electronic switch having its principal electrodes connected across said diode in opposite polarity thereto, and a keying means coupled between said output switch and said decoupling means for rendering it operable during machining pulse off-time, said keying means comprising a network connected between the output of said output switch and the control electrode of said last mentioned electronic switch.