Electrochemical Machining of Metals

Abstract

Electrochemical machining (E C M) imposes the need for conditions not encountered in electro-deposition or electropolishing. Current densities (30–1000 A/in² or more) are appreciably higher than limiting values for anodic dissolution in electrolytes violently agitated according to metal finishing practices. For them, the diffusion film at the anode surface has been reported in the literature to be of the order of 0·003 cm in thickness. For E C M the diffusion film is apparently about one-tenth as thick. This thinness can be maintained only by forced convection with turbulent flow of electrolyte under conditions of high Reynolds number. Cavities, as for forging dies, are made in minutes by E C M instead of in hours as by conventional machining and are exactly replicated because the tool (cathode) is not worn. For the steel examples cited, anodic dissolution current efficiency is 97–100% at current densities up to 120 A/in² (4.5 V) at 80–100°F with electrolyte flow velocities of the order of 100 ft/sec for acid electrolyte or up to 200–300 ft /sec for neutral salt electrolyte. Smoothness of the surface by ECM is in the range of 15–40 r m s microinch for stock removal rate of about 0·016 in/min.