Material Characterization for Metal-Cutting Force Modeling

Widely applicable machining simulation programs require reliable cutting force estimates, which currently can be obtained only from process-dependent machinability databases. The greatest obstacle to developing a more basic, efficient approach is a lack of understanding of material yield and frictional behavior under the unique deformation and frictional conditions of cutting. This paper describes a systematic method of specifying yield stress and friction properties needed as inputs to process-independent cutting force models. Statistically designed end turning tests are used to generate cutting force and chip thickness data for a mild steel and an aluminum alloy over a wide range of cutting conditions. Empirical models are fit for the cutting force and model-independent material parameters such as the tool-chip friction coefficient and shear stress on the shear plane. Common material yield behavior assumptions are examined in light of correlations between these parameters. Results show no physically meaningful correlation between geometric shear stress and strain measures, a weak correlation between geometric stress and strain rate measures, and a strong correlation between material properties and input variables such as cutting speed and rake angle. An upper bound model is used to fit four- and five-parameter polynomial strain-rate sensitive constitutive equations to the data. Drilling torques calculated using this model and an empirical turning force model agree reasonably well with measured values for the same material combination, indicating that end turning test results can be used to estimate mean loads in a more complicated process.