The Mechanisms of Chip Formation in Machining Hardened Steels

Abstract

The formation of saw-tooth chips is one of the primary characteristics in the machining of hardened steels with geometrically defined cutting tools. Catastrophic failure within the primary shear zone during saw-tooth chip formation is usually attributed to either cyclic crack initiation and propagation or to the occurrence of a thermo-plastic instability. The results presented here show that the primary instability resulting in the formation of saw-tooth chips is initiation of adiabatic shear at the tool tip and propagation partway towards the free surface. Depending on the work material hardness and cutting conditions, catastrophic failure within the upper region of the primary shear zone occurs through either ductile fracture or large strain plastic deformation. Prior to the onset of chip segmentation, which occurs with increases in work material hardness and cutting speed, there is a transition in the morphology of the free surface of continuous chips, from the familiar lamellar structure to what has been termed a “fold-type” structure. This transition is attributed to the operation of thermally softened micro-shear zones, which, it is suggested, are a precursor to adiabatic shear initiation.