Surface defect machining : a new approach for hard turning

Abstract

Hard turning is emerging as a key technology to substitute conventional grinding processes, mainly on account of lower equipment cost, short setup time, and a reduced number of process steps. This is, however, being impeded by a number of challenges required to be resolved, including attainable surface roughness, surface deteriorations, surface residual stresses and metallurgical transformations on the machined steel surface (white layer). In this thesis, a novel approach named Surface Defect Machining (SDM) is proposed as a viable solution to resolve a large number of these issues and to improve surface finish and surface integrity. SDM is defined as a process of machining, where a workpiece is first subjected to surface defects creation at a depth less than the uncut chip thickness; either through mechanical and/or thermal means; then followed by a normal machining operation so as to reduce the cutting resistance. A comprehensive understanding of SDM is established theoretically using finite element method (FEM). Also, an experimental study has been carried out for extensive understanding of the new technique. A good agreement between theoretical and experimental investigations has been achieved. The results show very interesting salient features of SDM, providing favourable machining outcomes. These include: reduced shear plane angle, reduced machining forces, lower residual stresses on the machined surface, reduced tool-chip interface contact length and increased chip flow velocity, as well as reductions in overall temperature in the cutting zone and changing the mechanism of chip morphology from jagged to discontinuous. However, the most prominent outcome is the improved attainable surface roughness. Furthermore, SDM shows the ability to exceed the critical feed rate and achieve an optical surface finish upto 30 nm. A scientific explanation of the improved surface roughness suggests that during SDM, a combination of both the cutting action and the rough polishing action help to improve the machined surface. Based on these findings, it is anticipated that a component machined using the SDM method should exhibit improved quality of the machined surface, which is expected to provide tremendous commercial advantages in the time to come.