Friction characteristics at tool-chip interface in steel machining

The split-tool method is extensively employed to evaluate friction characteristics at the tool-chip interface in steel machining. In this investigation medium plain carbon structural steels with different hardnesses are turned using TiN cermet, P20 and K20 carbide tools. Based on the measured stress distributions on the rake face, friction characteristics are discussed from the viewpoint of the mechanical and thermal properties of work and tool materials. Hardness and thermal conductivity affect the chip contact length as well as the coefficient of friction at the lightly loaded sliding zone near the chip leaving point. Friction becomes severe as the hardness of the workpiece is low and/or the thermal conductivity of the tool or its affinity with the workpiece is high. Discussion is extended to an investigation into the friction characteristics of high manganese steel and low-carbon free-cutting steel. Friction characteristics of a cut-away tool are found to be the same as those of a natural contact length tool, when an 18%Mn-18%Cr steel is turned using a P20 tool. A high friction condition close to the chip leaving point is also cut away by the restriction of contact length. When a low-carbon leaded resulphurized steel is turned using a P20 tool, both chip contact length and friction coefficient decrease markedly compared with the machining of an equivalent plain carbon steel. This is due to the lubrication effect of the free-cutting additives. The finite-element machining simulation in conjunction with the friction characteristic equation thus determined is applied to the illustration of machinability improvement through the use of free-cutting steel.