Ultra-precision machining process is used to produce optical components that require nanometric surface roughness. A singe crystal diamond (SCD) cutting tool is typically used in this process that can maintain a very sharp and uniform cutting edge with an edge radius (sharpness) of tens of nanometers. Using such an SCD tool, nanometric surface roughness can be achieved when machining several materials such as nonferrous alloys, plastics, and polymers. However, excessive tool wear occurs when a SCD tool is used for cutting ferrous alloys because the carbon in the diamond structure has an affinity towards the iron and diffuses into the workpiece. Thus, even though popularly used in common engineering applications, optical components made out of ferrous alloys are seldom used despite their advantages.
In this study, we look for an alternate single crystal tool to machine ferrous-based optics. Based on hardness, bulk crystal size, and chemical inertness with steel; we choose alumina single crystal (sapphire) as the cutting tool material. A lapping setup and process is developed to fabricate a sharp cutting-edge sapphire tool with zero rake and seven degree clearance angle. An accurate edge reversal technique to characterize the sapphire tool’s cutting-edge radius is developed. A nano indent is made using the sapphire tool on two polished pure copper blocks held together in a precise vise. After the nanoindentation, the copper blocks are separated, and the cross-section profile of the indentation mark, a replica of cutting-edge geometry, is observed in an SEM. The cutting-edge radius on sapphire tool obtained is comparable to the SCD tool edge radius. Further, orthogonal cutting of steel is performed in an ultra-precision machine using the fabricated sapphire tools. The surface finish on the workpiece, cutting forces, and the sapphire tool wear are studied for different depths of cut and cutting speeds.