Determination of workpiece flow stress and friction at the chip-tool contact for high-speed cutting

This paper presents a methodology to determine simultaneously (a) the flow stress at high deformation rates and temperatures that are encountered in the cutting zone, and (b) the friction at the chip-tool interface. This information is necessary to simulate high-speed machining using FEM based programs. A flow stress model based on process dependent parameters such as strain, strain-rate and temperature was used together with a friction model based on shear flow stress of the workpiece at the chip-tool interface. High-speed cutting experiments and process simulations were utilized to determine the unknown parameters in flow stress and friction models. This technique was applied to obtain flow stress for P20 mold steel at hardness of 30 HRC and friction data when using uncoated carbide tooling at high-speed cutting conditions. The average strain, strain-rates and temperatures were computed both in primary (shear plane) and secondary (chip-tool contact) deformation zones. The friction conditions in sticking and sliding regions at the chip-tool interface are estimated using Zorev's stress distribution model. The shear how stress (k(chip)) was also determined using computed average strain, strain-rate, and temperatures in secondary deformation zone, while the friction coefficient (mu) was estimated by minimizing the difference between predicted and measured thrust forces. By matching the measured values of the cutting forces with the predicted results from FEM simulations, an expression for workpiece flow stress and the unknown friction parameters at the chip-tool contact were determined. (C) 1999 Elsevier Science Ltd. All rights reserved.