In travelling wire EDM apparatus wherein a workpiece is cut by machining electrical discharges by means of an electrode wire, the cutting speed is limited by the thermal stresses exerted on the wire in the course of machining. The stresses aredue on one hand to the heat produced by each electrical discharge and on the other hand to the ohmic losses of the machining current flowing along the wire. An important portion of the heat produced during machining is evacuated by the machining fluidinjected between the wire and the workpiece in the machining zone; however there is a limit to the amount of heat that the machining fluid can handle. The only source of heat on which it is possible to act in order to increase the machining speed is theheat due to ohmic losses, or Joule effect.

It is conventional to supply the machining current to the electrode wire via two contacts, one located on one side and the other on the other side of the machining zone. The power dissipated as heat depends from the position where an electricaldischarge occurs along the machining zone, the power being minimum when the electrical discharge occurs at one end of the machining zone and maximum when the electrical discharge appears at the middle of the machine zone. However, when an electricaldischarge is produced proximate one end of the machining zone, the portion of machining current flowing from the other contact through the length of electrode wire in the machining zone simply produces a useless heating of the wire.

The present invention has for principal object to decrease the losses due to parasitical heat being generated in a workpiece cutting operation in a travelling wire EDM apparatus. The present invention thus permits to prevent excessive localizedheating of the electrode wire to take place due to concentration of faulty electrical discharges at any given location along the wire. The invention permits to select the end of the electrode wire at which machining current is supplied as a function ofthe position of each electrical discharge, such as to decrease heating of the wire due to the resistance of the wire, more particularly in the central portion of the machining zone where the wire is subjected to substantially ineffective cooling.