Preliminary study of the effects of crystal orientation of a CVD monocrystalline diamond in micromilling of Ti-6Al-4V

Monocrystalline diamond structures (synthetic and naturally occurring), offer the potential to exploit the anisotropic characteristics of this material in cutting tools by enabling the best crystallographic orientation to be selected to suit the application conditions of the tool. This paper investigates the performance of monocrystalline chemical vapour deposition (CVD) diamond tools in micromilling of a titanium based alloy, Ti-6Al-4V. A comparison of single-blade milling cutters using two different crystallographic orientations is made on a high-precision four-axis miniature machine tool. Employing the cutting edges of the monocrystalline diamond with the rake/flank faces of {100}/{110} and {110}/{100} orientations respectively, the progressive wear of the tools has been micrographed, the coincidental workpiece surface quality assessed, and machined chips inspected. Examinations using scanning electron microscopy reveal clear differences in the predominant failure modes of the cutting edges: the {110} rake orientation showing abrasion; the {100} rake orientation showing fracture/cleavage. While the measured roughness (R _a, R_z) of the micromilled workpiece surfaces for the two diamond cutting edge orientations were not significantly different, the surface morphologies show the transition from cutting to tearing and dragging of the workpiece material. The analyses of tool wear and workpiece surface quality proved that monocrystalline CVD diamond cutting edges with preferential crystallographic orientation along rake and clearance faces can be successfully utilized for interrupted cutting operations (i.e. micromilling) of alloys which react with diamond, such as those based on titanium.