Rupture failure and mechanical strength of the electrode wire used in wire EDM

Abstract

In the application of wire EDM, wire rupture is very troublesome and impedes further increase of cutting speed. Instead of the spark characteristics or the temperature distribution, the rupture mechanism and the mechanical strength of the wire are the focus of this investigation. Wire rupture is a mechanical failure in essence, although the process heat has significant influence on the occurrence of the failure. Before the strength analysis, sufficient wire tension is confirmed as a requirement for wire bow tolerance rather than being an arbitrary choice. Greater cutting speed demands greater wire tension to maintain the same small bow error. Tight tolerance and high cutting speed can only be promised at the expense of high mechanical loads on the wire electrode. A plane-stress model is used to describe the stress distribution, and the solutions are given as Airy's functions. The yield strength is then analyzed to reveal the influence of the loads and other parameters on wire yielding. Since wire fracture occurs frequently in practice, it follows that fracture is emphasized in this investigation. The fracture toughness is evaluated in accordance with the stress-intensity factor as well as with the strain-energy release rate. As far as the yield strength and fracture toughness are concerned, the correlation amongst various parameters including loads, materials properties, and geometrical parameters, is determined. Their effects on wire strength are revealed quantitatively. In general, wire tension and spark pressure are the two major causes for wire rupture. The two loads need to be increased with the increase of cutting speed and the reduction of bow tolerance. Only the careful design of the wire material and structure can assure the required wire strength in application especially when high cutting speed is attempted with a tight bow tolerance.