Tool life and surface integrity issues in continuous and interrupted finish hard turning with coated carbide and CBN tools

Abstract

In this paper, turning experiments on hardened steel were carried out with three kinds of cylindrical workpiece surfaces – continuous, interrupted, and fully interrupted – using coated carbide and cubic boron nitride (CBN) tools. The objective of this work was to investigate the conditions under which coated carbide tools give optimum results in terms of both tool life and surface integrity of the machined surface in comparison to costly CBN tools. The turning performance was evaluated in terms of maximum flank wear of the inserts and issues related to surface integrity like white layer formation, microhardness variation, residual stress generated, and surface roughness of the machined surface. The results indicated that the longest tool life was achieved with CBN tools, but the accumulated machining time of all the four cutting edges in coated carbide tools was comparable to that of CBN. The surface integrity achieved with carbide tools was also comparable to that achieved with CBN. The performance of carbide tools deteriorated at higher cutting speeds. The surface roughness value with all inserts under all cutting conditions was below 1.6 µm. The thickness of the white layer only varied from 0.4 to 1 µm, and no white layer was observed with interrupted and fully interrupted surfaces. Compressive residual stress was observed on the machined surface with a selected tool life criterion.