Physical modelling with experimental validation of high ductility metal cutting chip formation illustrated by copper machining

Abstract

This paper addresses problems of predicting chip formation in high strain machining conditions. A complete physical model of chip formation requires both plasticity and chip/tool friction models. Friction models are commonly partly phenomenological, with friction coefficients measured from the conditions in which the models are applied. This paper’s thesis is that friction emerges from the plastic response of the chip material in contact with the cutting tool. Extremely large strains are generated in the contact region. In the case of machining highly ductile metals large strains also occur in the bulk of the chip. This paper applies a Mechanical Threshold Stress plasticity model extended to high strains (equivalent strains >5) to simulating chip formation in copper machining, without assuming measured values of friction coefficients. In the case of copper machining there is not a unified source of experimental knowledge against which to validate simulations. There is a need to provide such a source. This paper reports extensive results from machining three coppers in general engineering conditions. At all cutting speeds there remains a systematic difference between the simulated and experimental chip thicknesses. In addition, at low cutting speeds an experimental observation is that chip formation cycles between low and high thicknesses. The simulations do not predict this. The experiments show the cycling to occur when the chip thickness rises to 10 or more times the uncut thickness. It is speculated with some evidence that the cycling is associated with plastic failure rather than with strain hardening, as is currently commonly given as the explanation. Modelling large strain plasticity and failure of highly ductile metals, for metal machining simulations, remains incomplete.