Ductile Machining Mechanism for Continuous Freeform Surface by Ultra-precision Raster Milling

Abstract

For diamond machining of brittle materials, micrometer- or even nanometer-scale machining parameters have to be deliberately selected to guarantee the ductile material removal process, which accordingly leads to a significant limitation in the achievable azimuthal height variation of the generated freeform surface, as well as a reduction in efficiency. In this study, a novel ductile machining model based on ultra-precision fly cutting is proposed for diamond machining of brittle materials for freeform surfaces with large azimuthal height variation. Compared with diamond turning and diamond milling, the chip thickness of fly cutting is not only determined by the adopted machining parameters but also is a function of the configured swing distance. Thus, by employing a large enough swing distance, much thinner chips can be generated by fly cutting even when adopting large cutting depths and feed rates, accordingly achieving the ductile machining of freeform surfaces and structures with large azimuthal height variation on brittle materials, as well as improving efficiency. This model was validated through numerical calculation of the chip thickness for fly cutting and experimental demonstration by fly cutting two kinds of freeform surfaces, namely micro-grooves and an F-theta lens. Through evaluating the resulting surface topography, form error, chip morphology, and material phase transformation, the study concludes that freeform surfaces characterized by tens of micrometers of azimuthal height variation can be directly fabricated on single-crystal silicon with nanoscale surface roughness and microscale form error.