Electrical discharge machining of conductive CVD diamond tool blanks

The paper reviews the manufacture, physical/mechanical properties and cutting tool applications of chemical vapour deposited (CVD) diamond and makes comparisons with polycrystalline diamond (PCD). The morphology of CVD diamond is significantly different from that of PCD, the former exhibiting continuous columnar grains extending from nucleation sites, whereas the latter involves an agglomeration of intergrown diamond crystals with a minor secondary phase of cobalt. Experimental work is detailed involving the electrical discharge wire machining (EDWM) and electrical discharge grinding (EDG) of boron doped CVD diamond (CVDITE CDE). Results from Taguchi experiments are presented which were carried out to assess the effect of generator operating parameters on sparked surface/edge quality and productivity. Typically when EDWM (roughing) CVDITE CDE surface roughness values were ∼0.7–2.78 μm Ra while cutting speeds were ∼0.87–2.14 mm/min. Sparked PCD surfaces were generally of lower quality, even when using fine diamond (2 μm) products, however, cutting speeds were substantially increased by as much as 100%. When EDG, CVDITE CDE cutting speeds of ∼0.11–0.26 mm/min were obtained. Visual examination of machined surfaces in general showed that the CVD diamond was cleanly cut, with no obvious surface degradation, however, in a minority of cases which were not operating parameter specific, there was wholesale loss of the columnar diamond crystals.