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Abstract
Micro milling operation is one of the advanced machining processes to manufacture miniature parts in biomedical, optics and electronics industries. The chip thickness in micro milling is comparable to the tool edge radius, which brings strong size effect to the cutting performance and makes prediction of the cutting forces more challenging. In this thesis, an analysis of the micro milling process has been carried out based on an experimental study to cut different materials using micro flat end mills on a precision CNC machine tool. Influences of cutting parameters on cutting forces and machined surface quality have been evaluated. A cutting force model has been developed by determine the key coefficients from finite element method, with a simulation study and model verification presented.
It has been found that cutting forces occurred at low feed per tooth are relatively high by assessing the averaged peak forces from the experiments. When feed per tooth is relatively close to tool edge radius, the forces were not growing in linearity with the increasing feedrate. This finding indicates the significance of ploughing phenomenon as an effect of tool edge radius in micro milling. Also, machining at high spindle speed may contribute to process instability and significant peak force variation, and have negative effect on work surface quality. Moreover, the workpiece machined by 0.9 mm diameter tool achieved better surface quality than those done by 0.6 mm tool. It is shown that reducing depth of cut can reduce burr formation. In addition, the width of deformed chips has been measured considerably close to the depth of cut value.
To investigate the size effect on cutting performance, the prediction of cutting forces derives from a simplified orthogonal process. A finite element model is developed to simulate two-dimensional cutting forces in orthogonal micro cutting of brass, which has addressed the ploughing and tool edge effects. The FE model is used to evaluate the critical chip thickness and to extract the two force coefficients for modelling cutting forces in micro milling. A generalized analytical force model based on numerical findings is proposed to predict the micro milling force by considering the tool trajectory and tool runout. The cutting force coefficients are modelled as a function of instantaneous uncut chip thickness, which is independent of cutting speed but influenced by tool edge radius. The simulation results of micro milling forces are compared against experimental measurement, where an agreement of force trends is shown along with the increasing feedrate and depth of cut.