Ductile machining of single-crystal germanium for freeform surfaces diamond turning based on a long-stroke fast tool servo

Abstract

Freeform surfaces of single-crystal germanium are required increasingly in advanced infrared optics, but there are difficulties in processing due to the high hardness, brittleness and low fracture toughness of germanium itself. In the present study, the machining mechanisms and parameters of single-crystal germanium were investigated via ultra-precision diamond turning based on a long-stroke fast tool servo. To realize ductile regime machining, a ductile machining model in ultra-precision turning of germanium considering both feed rate, cutting depth and surface slope of freeform surface was proposed and discussed. A novel taper cutting test was adopted to determine the critical ductile–brittle transition depth. Subsequently, XYPFS were fabricated under different cutting parameters to demonstrate the effectiveness of proposed model. The morphologies of machined surfaces and cutting chips were identified and compared. Finally, a smaller feed rate and cutting depth should be used to reduce its maximum undeformed chip thickness and realize the ductile machining for a freeform surface having a large surface slope. The freeform surface of single-crystal germanium with a form error of ~1258 nm PV and surface roughness of ~1.92 nm Ra was successfully fabricated. In summary, this research provides a comprehensive understanding of ductile machining mechanisms of diamond turning based on a LFTS as well as is meaningful in the manufacturing of freeform surfaces on germanium or other brittle materials with optically qualified surfaces and high-precision.