The invention relates to a process and apparatus for monitoring the thermal stresses or constraints of an electrode wire being displaced longitudinally between two guide members in an apparatus for cutting an electrode workpiece by erosive electrical discharge, the presence of an electrical discharge being detected at a predetermined sector along the length of the wire in the machining zone between the electrode wire and the workpiece.
In the course of cutting a workpiece in a travelling wire EDM apparatus, the electrode wire is subjected to thermal stresses which limit the level of the permissible current of the electrical discharges, and consequently the speed at which the workpiece is cut, or material removal rate. The thermal stresses are caused by resistance heating of the wire and by the heat conveyed to the wire by the electrical discharges themselves. The latter type of stresses, which may be the cause of many unknown reasons for the rupture of the wire, is dependent to a large degree on the density of the electrical discharges along the active length of the wire. An abnormal concentration of electrical discharges, due to electrical discharges occuring very close to each other, causes a local heating of the wire which could be a cause of wire rupture.
Diverse methods have been proposed in the past for monitoring the distribution of the electrical discharges along the active portion of the wire. For example, published patent DE-OS No. 3.026.508 and Japanese No. 53-64899 disclose circuits which subdivide the wire in sectors and which permit determination of the sector in which an electrical discharge has occurred. A signal is provided when a predetermined number of consecutive electrical discharges have occurred in the same sector. Such known methods, however, are not sufficient to provide an indication of the actual heating of the wire. As a consequence of the high repetition rate of the electrical discharges and of the relatively wide time constant during which the heat is dissipated from a wire sector, a critical temperature may be reached in a given sector even though the electrical discharges occurring in that sector are not consecutive. Consequently, although a known criteria of the number of consecutive electrical discharges is necessary, it is not sufficient in order to warn against abnormal heating of the wire.