Near-mirror-finish phenomenon in EDM using powder-mixed dielectric

摘要

This paper presents a study of the near-mirror-finish phenomenon in electrical discharge machining(EDM) when fine powder is introduced into the dielectric fluid as a suspension at the tool-workpieceor inter-electrode gap during machining. For this study, the dielectric flushing system of aconventional die-sinking EDM machine was specially modified to inject and distribute the powderinto the dielectric fluid, especially at the gap between the tool and the workpiece. Machining wasperformed on various types of steel with different types of powder suspensions at a peak current ofaround 1 A. Particular combinations of powder-mixed dielectric and workpiece have been found toproduce mirror-finish or glossy machined surfaces. Close scrutiny of the mirror-finish surfacesreveals 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 factorsaffecting the generation of the mirror-like surfaces are discussed. The appropriate settings ofelectrode polarity and pulse parameters and the correct combination of workpiece material andpowder characteristics have a significant influences on the mirror-finish condition. The use ofnegative electrode polarity (i.e. with the tool as the negative electrode, which is a condition normallyused for finishing EDM) is necessary to achieve the mirror-finish condition. Other features of thepowder-mixed dielectric EDM are shorter machining time, more uniform dispersion of the electricaldischarges, and stable machining. Based on the results of the experimental investigation, the types ofmaterial composition, powder properties and machine setting in bringing about near-mirrorconditions are discussed.