Fundamental Investigation of Dry EDM Plasmas

Abstract

Dry electrical discharge machining (DEDM) has been developed as an alternative manufacturing process to the traditional EDM in oil dielectric. The absence of liquid dielectric allows DEDM to be performed by simpler and environmentally friendlier machines. The erosion in DEDM mainly occurs due to the bombardment of the workpiece material by charged particles produced by micro electric discharges. Therefore, understanding of the fundamental properties of micro discharge plasmas is necessary in order to explain the erosion mechanisms in this process. The present work introduces well-established methods from the field of plasma physics as effective tools to analyse DEDM plasmas and their interactions with the electrode material. Advanced interpretation of optical emission spectroscopy supported by a commercial collisional-radiative (CR) model gives a new insight into the complex DEDM micro discharges. Emission spectra simulations indicate that the studied DEDM plasmas must either have an electron temperature profile peaking at the plasma centre or an electron beam formed within the discharge. In addition, the discharges performed with a point-type cathode are similar to hot anode vacuum arcs (HAVA) with plasma properties compatible with the formation of a hot anode spot. The properties of a HAVA, with inert cathode and active anode, can explain the very small tool electrode wear obtained in DEDM. Moreover, DEDM discharges are mainly formed of metal vapour originated from the electrodes. In contrast, discharges in oil are dominated by components of the dielectric with just little metallic contamination. Finally, the anode power deposition in DEDM is calculated in the present work by a thermal plasma model supported by emission spectra simulations and by electrical circuit simulations with a modified Cassie-Mayr model. This investigation shows that most of the discharge power, between 80% and 90%, is deposited onto the anode material.