Micro- and nano-crystalline CVD diamond coated tools in the turning of EDM graphite

Electrical discharge machining is a widely used technique, namely in metal mould shaping. High quality EDM graphite is the foremost choice as electrode material but the random oriented graphite aggregates cause an abrasive action over the tools used in its machining. Wear-resistant diamond tools were then chosen as an adequate option to guarantee tight tolerances and good surface finishing. In this work, microcrystalline and nanocrystalline CVD diamond coated inserts are used in turning operations. Cutting forces were always very low (< 20 N) as also was the tool wear. An important result is the absence of diamond film delamination showing the high adhesion level offered by the silicon nitride ceramic substrates for CVD diamond coating. The smoother nanocrystalline coatings allowed to obtain a better workpiece surface roughness than the microcrystalline ones.