An Experimental Study on the Geometry of Microholes in Microelectric Discharge Machining

Shape and dimensional geometry of blind microholes produced by microelectric discharge machining (micro-EDM) is investigated experimentally to explore the relational dependency with respect to machining time. For this purpose, microholes are machined with machining times ranging from 1–80 min on plastic mold steel samples using tungsten carbide tool electrode and hydro carbide dielectric liquid composed of mineral and synthetic oils. All other operational machining parameters such as pulse time, gap voltage, average current, and so forth, are kept constant during machining. Depth, diametric expansions, end-tip shape, wall side parallelisms of holes were analyzed both on surface and cross-sections of the samples. Consequently, nine different end-tip shape formations were observed, and these shapes discussed on basis of dielectric liquid circulation and sparking conditions during machining. Enlargement in hole diameter was predicted by defining a band-width of 20 µm hole expansion. Moreover, parallel wall-side length was expressed as the microhole depth where 5% reduction in entrance diameter was measured at the tapered section.