Variation of surface generation mechanisms in ultra-precision machining due to relative tool sharpness (RTS) and material properties

The need for ultra-fine surface generation by machining in micrometer and sub-micrometer scales is increasing rapidly for fabrication of micro products. However, there are limitations of bringing down the process from macro to micro to ultra-precision level due to the variation of the physical phenomenon responsible for surface generation process which is significantly dependent on tool edge radius. To analyse the tool edge radius effect, the governing process parameter identified as the ratio of undeformed chip thickness (a) to tool edge radius (r), which is known as relative tool sharpness, RTS (a/r). However, there are lacking of mathematical models to quantify the micro-mechanics of the material deformation during the surface generation process. In this study, a mathematical model has been established for the material removal mechanism with relation to edge radius effect. The novelty of this model lies in its ability to quantify the instantaneous material flow angle (cone) for a particular tool-workpiece combination with relative tool sharpness (RTS). The proposed model is able to capture the trend of the change of surface generation Mechanism from shearing to extrusion to ploughing and rubbing. The transition of the deformation behaviour is predicted form the mathematical model and orthogonal cutting experiments were conducted for the validation of the results for two materials (Mg and Cu alloy). Under identical machining conditions, micro-chips and machined surface integrity of Cu and Mg alloy exhibited different characteristics. For the best nanometric finishing, the 'extrusion-like' machining zone is identified where the RTS value for Cu alloy is found smaller than that for Mg alloy. Hence, smaller grain material (Cu alloy,similar to 35 mu m) provides smaller RTS value than larger grain material (Mg alloy,similar to 126 mu m) for achieving superior surface finishing. Therefore, material properties play an important role for the relative tool sharpness (RTS) during the variation of high quality surface generation mechanism.