NEAR-MIRROR-FINISH PHENOMENON IN EDM USING POWDER-MIXED DIELECTRIC

This paper presents a study of the near-mirror-finish phenomenon in el ectrical discharge machining (EDM) when fine powder is introduced into the dielectric fluid as a suspension at the tool-workpiece or inter-e lectrode gap during machining. For this study, the dielectric flushing system of a conventional die-sinking EDM machine was specially modifi ed to inject and distribute the powder into the dielectric fluid, espe cially at the gap between the tool and the workpiece. Machining was pe rformed on various types of steel with different types of powder suspe nsions at a peak current of around 1 A. Particular combinations of pow der-mixed dielectric and workpiece have been found to produce mirror-f inish or glossy machined surfaces. Close scrutiny of the mirror-finish surfaces reveals shallow overlapping re-solidified discs with smooth rims, unlike typical EDMed surfaces, which are typically covered with deep craters, pock marks and globules. The various factors affecting t he generation of the mirror-like surfaces are discussed. The appropria te settings of electrode polarity and pulse parameters and the correct combination of workpiece material and powder characteristics have a s ignificant influences on the mirror-finish condition. The use of negat ive electrode polarity (i.e. with the tool as the negative electrode, which is a condition normally used for finishing EDM) is necessary to achieve the minor-finish condition. Other features of the powder-mixed dielectric EDM are shorter machining time, more uniform dispersion of the electrical discharges, and stable machining. Based on the results of the experimental investigation, the types of material composition, powder properties and machine setting in bringing about near-mirror c onditions are discussed. (C) 1998 Elsevier Science S.A. All rights res erved.