A novel method to switch machining mode between Micro-ECM and Micro-EDM using oxide film on surface of tungsten electrode

Abstract

A novel method to switch the machining mode between micro-EDM and micro-ECM in the same electrolyte using the same tool electrode and pulse generator circuit was proposed in this research. This method uses the electrostatic induction feeding method where a pulse voltage is coupled to the working gap through a capacitance. The switch between the EDM and ECM modes was realized by utilizing the oxide film formed on the surface of the tungsten electrode. To switch the ECM mode to the EDM mode, the polarity of the diode attached in parallel to the working gap was changed to control the generation of the oxide film. In the ECM mode, the diode was connected so that it is forwardly biased when the polarity of the tool electrode is positive, thereby the oxide film was not formed on the tungsten electrode. In the EDM mode, since the polarity of the diode was reversed, the oxide film was formed, thereby discharge could be ignited. The switching ability was verified from the waveforms of gap current and voltage which showed that discharge occurred in the EDM mode, whereas there was no discharge waveform observed in the ECM mode. In the ECM mode, the influences of total amplitude and rise/fall time of pulse voltage were investigated. In the EDM mode, the discharge probability with different concentrations of electrolyte was researched first. Then, the influence of the feeding capacitance C1 on the EDM mode was investigated. Finally, through-holes were machined by conducting the EDM and ECM modes in this order to obtain higher material removal rate in rough machining and better surface roughness without residual stress in finish machining.