Experimental investigation of changing polarity in Powder mixed electric discharge machining (PM-EDM)

Abstract

It is the common practice in EDM to make tool positive and work piece negative (Normal polarity), but researches shows that reverse of it is also possible in which tool is negative and work piece is positive ( Reverse polarity). Paper discusses the effect of polarity on the machining characteristics with chromium powder suspended dielectric in electric discharge machining. High metal removal rate, low tool wear rate and surface roughness are conflicting goals, which cannot be achieved simultaneously with a particular combination of control settings. To achieve the best machining results, the goal has to betaken separately in different phases of work with different emphasis. A L9 orthogonal array is designed for both polarities. Copper is used as tool material and EN-19 alloy steel is selected as work piece material with normal and reverse polarities. The study reveals that Normal polarity is suitable for higher metal removal rate and lower tool wear but reverse polarity gives minimum surface roughness as compared to normal polarity. Normal polarity gives 10-21 times more MRR and 4 times less tool wear rate as compared to reverse polarity, and reverse polarity gives 1.5-2 times better surface finish as compared to Normal polarity.