Feed-deviation preventive numerically controlled EDM method and apparatus

A numerically controlled method of electroerosively machining a workpiece with a tool electrode wherein a succession of electrical voltage pulses are applied between the tool electrode and the workpiece and at least partly result in effective electrical discharges across a machining gap to electroerosively remove material from the workpiece while drive means is furnished with a sequence of incrementally driving command signals from a numerical controller for relatively advancing one of the tool electrode and the workpiece into the other to continue machining of the workpiece along a programmed path, the method comprising the steps of:

• (a) sensing said command signals to produce a first detection signal representative of a commanded relative advance to be effected between the tool electrode and the workpiece along said programmed path for a given time period;
• (b) sensing the effective electrical discharges caused in the machining gap to produce a second detection signal representative of an actual advance effected between the tool electrode and the workpiece along such path in such time period;
• and
• (c) comparing said first and second detection signals to produce an output signal indicative of a deviation of said actual advance from said commanded advance, said first detection signal being a first train of signal pulses whose number for said time period is representative of a distance of said commanded advance for said time period and said second detection signal being a second train of signal pulses whose number for said given time period is representative of a distance of said actual advance for said time period.