Numerical modelling and simulation of surface roughness of 3-D hemispherical convex micro-feature generated by reverse micro-EDM

Reverse micro-electrical discharge machining (RMEDM) is a non-contact electro-thermal micro-machining process that is primarily used for generating high aspect ratio extruded 2.5-D features with different cross-sections like square, circular and triangular. The authors have already evolved a method to generate 3-D hemispherical convex micro-feature by using pre-drilled tapered blind hole as a tool in RMEDM. Debris suspension in dielectric as well as debris adhesion to walls of either electrodes in the confined space in this method leads to reduction in inter-electrode gap creating secondary and higher order discharges. These secondary and higher order discharges occurring mostly on the sides of 3-D hemispherical convex micro-feature leads to higher crater size which increases surface roughness on the sides of the micro-feature as compared to other regions on the surface. In this paper, an attempt has been made to numerically model, simulate and experimentally investigate this variation in surface roughness on the 3-D hemispherical convex micro-feature by incorporating a multiphysics simulation approach including different physical processes viz. electrostatics, heat transfer and fluid flow occurring during RMEDM as well as the effect of secondary and higher order discharges which play a key role in determining the variation in surface roughness on the micro-feature. Simulated result matches well with the experimentally determined surface roughness (maximum error of 13%), thereby confirming that the nature of discharges (viz. secondary and higher order) occurring during RMEDM is responsible for variation of surface roughness on the generated 3-D hemispherical convex micro-feature.