Study of alternating current flow in micro-EDM through real-time pulse counting

Abstract

Following the general guidance on the influence of the machining polarity in micro-EDM, usually a straight polarity is applied (with the tool electrode being connected to the negative pole of the generator output), since the bombarding of the “light” electrons on the workpiece is assumed to be the primary material removal mechanism. Therefore, a flipping of the polarity in the micro-EDM machining process shall result in a high volumetric relative wear, which has been experimentally verified. The initial objective of this research is to reduce the tool wear by inserting a diode in the discharge circuit to block the reverse current flow. However, in the experiments an even higher volumetric relative wear has been observed with the inclusion of a diode. To investigate in depth the influence of the alternating current flow on the tool-material interaction, a pulse counting method based on the detection of the current peak has been proposed to count the number of effective pulses. This counting of the effective sparks enables the subsequent analysis of the mean material removal per discharge and tool wear per discharge. The analysis of the measurements obtained in both sparking cases, with a diode and without a diode, indicates that the reverse current is contributing positively to the material removal on the workpiece and the volumetric relative wear. Therefore, it is not economically proper to block the reverse the current flow in micro-EDM milling. At lower open voltage, the simple empirical linear model without reverse current flow can still serve as a reference for the in-situ tool wear prediction and the subsequent tool wear compensation. However, a correction to the model has to be made at a setting of higher open voltage.