A study of materials swelling and its characterization on nano-surface generation in ultra-precision machining

Abstract

Ultra-precision machining (UPM) including single-point diamond turning (SPDT) and ultra-precision raster milling (UPRM) makes use of a monocrystal diamond cutting tool which possesses nanometric edge sharpness and wear resistance to generate a super mirror-like surface on precision components which have a surface finish in the nanometer rang and form reproducibility in a submicrometer range. In SPDT and UPRM, not only the relative magnitude of the process factors affect the surface generation, but also the behaviour of the materials at the submicrometer level depth of cut. The nano-surface generation in UPM is very complex in that it involves the combined process of burnishing, elastic recovering, plastic deformation and materials swelling. Materials swelling in generally refer as the elastic-plastic response of the work piece when the cutting tool is removed. Materials swelling would cause a tool mark to deviate from the theoretical tool profile expected to be imprinted on the machined surface. This can neither be explained by the tool feed, machine vibration nor by the tool nose waviness alone. Traditional metrological tools for surface measurement such as surface roughness parameters are inadequate to isolate and study the effect of swelling from other process parameters. Therefore, the development of a new analytical method for the study of material swelling and its characterization is important in ultra-precision machining studies. A quantitative technique based on the multi-power spectrum analysis method (MSAM) and a multiple data dependent system method (MDDSM) was adopted to identify and characterize materials swelling from other process factors in nano-surface generation. Different swelling responses have been successfully identified and delineated by the surface characterization system from various cutting experiments on various kinds of materials. It was found that the effect of materials swelling is significant both in SPDT and UPRM and there is a strong correlation between materials swelling and surface roughness. This research can give us a better understanding of the underlying mechanism of material swelling and its characteristics. Furthermore, it boosts our knowledge of nano-surface generation in UPM. In addition, it can help to define the lower limit of surface roughness that can be achieved for a given cutting strategy. The findings are useful to guide the selection of cutting conditions to generate optics microstructures and optical surfaces which demand a high quality finish down to the nanometric level. In addition, the surface characterization system described in this thesis can quantify the effect of cutting parameters on materials swelling, decrease the time needed for the analysis and display useful information to guide the choice of process parameters.