On Critical Conditions for Chip Transformation from Continuous to Serrated in High-Speed Machining

Abstract

For most of materials, a chip transformation from continuous to serrated takes place at a relative high cutting speed which is called the critical cutting speed (CCS). Serrated chips at CCS have different characteristics from those at higher cutting speeds. In this paper, the chip transformation is analytically investigated. The deformation in the primary shear zone (PSZ) during the transformation is analyzed. The critical shear strain at CCS for chip transformation is proposed. Cutting Experiments are carried out with four metals, and metallographical and morphological investigations on the chip transformation are conducted. The results show that serrated chips can be produced if the shear localization along a shear plane occurs before the shear plane reaches to the middle of PSZ. At CCS, the flow stress of the shear plane passing through the PSZ reaches maximum at the middle of PSZ and then decreases with further straining. The high thickness of localized shear bands makes the serrated chip at CCS look as a wave. At CCS, the shear strain of chip segments is approximately equal to the critical shear strain for chip transformation. Influences of material hardess (brittleness) on chip transformation are also discussed.