Ultra-Precision Turning of Ferrous and Non-Ferrous Material by Sapphire Tool

Ultra-precision machining of ferrous alloys remains challenging because conventional diamond tools suffer severe thermochemical wear, whereas ultrasonic vibration-assisted cutting requires complex and costly equipment. This study investigates single-crystal sapphire as an alternative cutting-tool material for ultra-precision machining of both non-ferrous and ferrous metals. A sapphire tool was fabricated from a polished wafer, laser-shaped into an equilateral triangular insert, vacuum-brazed onto a tungsten carbide carrier, and finished by ultra-fine grinding to yield a well-defined cutting edge. Ultra-precision turning experiments were conducted on copper and 420 stainless steel using a Moore Nanotech 350FG lathe, and the performance of the sapphire tool was benchmarked against conventional diamond (copper) and cubic boron nitride (CBN) tools (stainless steel) under comparable cutting conditions. Surface roughness (Ra) and topography were characterized using an optical surface profiler, while scanning electron microscopy and atomic force microscopy were employed to assess tool wear and cutting-edge geometry. The sapphire tool produced mirror-like surfaces with average surface roughness (Ra) values of 6.4 nm on copper and 39.1 nm on 420 stainless steel, compared with 1.3 nm for diamond on copper and 92.9 nm for CBN on stainless steel. Across both materials, sapphire generated regular, stable tool marks and exhibited minimal wear, with no catastrophic edge degradation or clear evidence of severe chemical interaction with the steel workpiece. These results demonstrate that sapphire is a viable tool material for extending diamond turning-level surface quality to stainless steel without ultrasonic assistance.