Electrical discharge machining servo control circuit

Claim 1

A servo feed control circuit for use with an electrical discharge machining apparatus wherein workpiece material is removed by a tool electrode from a workpiece across a dielectric coolant filled gap including an electronic output switch connected between the power supply and said gap for providing machining power pulses to said gap at presettable on-off times and frequencies, a keying network connected to an intermediate drive stage for said output switch for providing a keying signal during pulse on-time, sensing means connected to said gap for providing a gap voltage signal representative of gap spacing, and an electrically operated servo feed means operatively connected to said networks for providing relative movement between said electrode and said workpiece responsive to said signals, wherein the improvement comprises: a field effect transistor having its current conducting electrodes operatively connected between said sensing network and one terminal of said electrically operated servo feed means, said keying network operatively connected to the gate electrode of said field effect transistor for rendering it conductive during machining pulse on-time.