In order to let a wire cut electric discharging machine (hereinafter as wire cut EDM) achieve a high processing speed, a huge current must be provided, and therefore a large amount of energy must be supplied to the gap between the wire electrode of the wire cut EDM and the workpiece within a very short period of time (<3 uS) for processing a workpiece. Therefore, the amount of energy must be controlled during the process of discharge.

A conventional discharge control circuit of a wire cut EDM, as shown in FIG. 1, is a transistor-capacitor circuit generally comprising a current limit resistor 3 connected in series to a power source 1, a transistor 4, a capacitor 2, and a wire electrode 5.Because of the limitation of charging and discharging time of the capacitor and the problem of energy control during the discharging of the capacitor, the wire cut EDM cannot achieve a high processing speed.

The present invention has been accomplished to solve the above said problem. In one aspect the present invention offers a novel method for controlling a wire cut EDM such that the discharge current of the wire cut EDM is controlled within a predetermined range and also the processing speed is improved. The main circuit includes a high voltage power source and a first switching transistor operated by a control unit containing a pulse generator [hereinafter simply as a control unit], and the testing circuit includes a low voltage power source, a second switching transistor, a resistor, and a diode, wherein the second switching transistor is also operated by the same control unit.

The control unit is used to detect the voltage drop between the wire electrode and the workpiece to determine whether the gap between the wire electrode and the workpiece is within a predetermined range suitable for discharging operation. The device of the invention thus eliminates the capacitor and the current limiting resistor used in the conventional wire cut EDM,and the processing speed is improved. Only a small amount of current passes through the resistor employed in the testing circuit, and therefore the resistor employed in the device of this invention does not present any problem.