Although 45HRC hardness is the starting point for hard turning, hard turning is often performed on workpieces with hardness above 60HRC. Hard turning materials usually include tool steel, bearing steel, carburized steel, and special materials such as Inconel, corrosion-resistant and heat-resistant nickel-based alloys, and tungsten-chromium-cobalt alloys. According to metallurgy, materials with small hardness deviations (less than 2 HRC) over the depth of cut show the best process predictability. The parts most suitable for hard turning have a small aspect ratio (L / D). Generally speaking, the L / D ratio of the unsupported workpiece is not greater than 4: 1, and the L / D ratio of the supported workpiece is not greater than 8: 1. Although the slender part is supported by the tailstock, it may still cause cutter vibration due to excessive cutting pressure. In order to maximize the rigidity of the system for hard turning, the overhang should be minimized. The extended length of the tool must not be greater than 1.5 times the height of the tool bar.