This paper proposes a thermo-mechanical orthogonal cutting finite element model (FEM) to investigate the variation of chip morphology from continuous chip to small and large saw-tooth chip. The corresponding experiments of hard turning AISI 52100 steel are conducted to validate the proposed FE model. Three one-factor simulation experiments are conducted to determine the evolution of chip morphology along feed rate, rake angle and cutting speed respectively. The chip morphology evolution is described by the variations of dimensional values, saw-tooth degree and chip segmental frequency. The research suggests that chip morphology transit from continuous to sawtooth chip with increasing the feed rate and cutting speed, and changing a positive rake angle to a negative rake angle. There exists a critical cutting speed at which the chip morphology transfers from continuous to saw-tooth chips. The saw-tooth chip segmental frequency decreases as the feed rate and negative rake angle value increase, but increases almost linearly with the cutting speed. The larger negative rake angle, the larger feed rate and high cutting speed dominate the sawtooth chip morphology while positive rake angle, small feed rate and low cutting speed determine continuous chip morphology.

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