Process control for electrical discharge machining apparatus

In electrical discharge machining apparatus for machining a workpiece with an electrode by causing intermittent electrical discharge across a gap between said electrode and said workpiece, said apparatus comprising drive means operable to advance and retract the electrode with respect to the workpiece, a power supply for supplying the electrical discharge across said gap, a microprocessor control system for controlling both the electrical discharge from said power supply and the relative positioning of said electrode by said drive means with respect to said workpiece, and means sensing the voltage and current across said gap, the improvement which comprises an amplifier stage (78) supplying a control current for control of the drive means, an input circuit (44) associated with said amplifier stage (78), said input circuit (44) comprising a resistor network having an effective value controlled by a digital output (60) of the microprocessor representing a desired feed rate for the electrode, a second amplifier stage (68), said second amplifier stage (68) comprising means (70, 72, 74) receiving one analog input signal representative of actual gap voltage and an input circuit (42) comprising a further resistor network having an effective resistance controlled by a digital output (54) of the microprocessor representing a desired gap voltage, the second amplifier stage having an output (76) connected as another input to the first amplifier stage (78).