Electric discharge machining apparatus

An electrical discharge machining apparatus having a power supply for supplying a succession of voltage pulses to a machining gap formed between the tool electrode and a workpiece electrode for causing electrical discharges therebetween, comprising:

• means for generating a signal representative of the voltage across the machining gap;
• means responsive to said machining gap signal for generating:
• a) a first signal indicative of when a discharge voltage is less than a first predetermined level;
• b) a second signal indicative of when a discharge voltage exceeds a second predetermined level;
• and
• c) a third signal indicative of when a discharge waiting time is less than a predetermined value;
• a first carbide formation detector, responsive to said first signal and said second signal for cutting off said pulses when the discharge voltage is below said first predetermined level for a predetermined number of pulses and then above said second predetermined level for a predetermined number of pulses;
• a second carbide formation detector, responsive to said third signal for cutting off said pulses when the discharge waiting time is less than said predetermined value for a predetermined number of pulses.

A method of controlling an electric discharge machine to prevent concentrated and continuous arc discharges, said machine having a power supply for supplying a succession of voltage pulses to a machining gap formed between a tool electrode and a workpiece electrode to generate discharges therebetween, said method comprising the steps of:

• monitoring the voltage across the machining gap;
• generating a first signal when the gap voltage is less than a first predetermined level during a discharge for a first predetermined number of pulses;
• generating a second signal when the gap voltage is greater than a second predetermined level during a discharge;
• generating a third signal when the discharge waiting time is less than a reference time period;
• cutting off the supply of voltage pulses to the machining gap when the first signal is present and then the second signal is present for a second predetermined number of pulses;
• and
• cutting off the supply of voltage pulses to the machining gap when the third signal is present for more than a third predetermined number of pulses.