Transient Workpiece Temperature Analysis in the EDM Processes of High Electric Resistance Materials Considering Joule Heating

Abstract

This paper investigates the removal mechanism of high electric resistance materials in EDM processing by calculation of the distribution of the workpiece temperature taking the Joule heating caused by voltage drop in the workpiece into account. The voltage drop in the workpiece is extensive in the EDM processing of high electric resistance materials such as conductive ceramics. The voltage drop was therefore estimated by a numerical calculation, and its results were used to calculate the distribution of Joule heating inside the workpiece. Then considering both the Joule heating and the heat flux from the arc column, the time dependent workpiece temperature was computed by the finite-difference method. The expansion of arc column with time lapse which affects greatly the result of the above calculation was obtained experimentally and taken into account in the calculation. The calculated results show that the voltage drop mostly occurs in the vicinity of the discharge spot, and the calculated total voltage drop coincides well with the measured one. From the computed temperature distribution it is found that the Joule heating causes a temperature rise nearby the discharge spot which is comparable to what is given by the heat flux from the arc column, which was confirmed by the measured material removal by a single pulse which is greater than the removal of steel. Consequently, sufficient cooling of the discharge spot may assure the stability of machining and increase the material removal rate of conductive ceramics rather than metallic materials.