Tool wear mechanism in continuous cutting of hardened tool steels

Abstract

Precision hard turning provides an alternative to grinding in some finishing applications. Rapid tool wear, however, remains an impediment to the process being economically viable. This experimental study investigates microstructural aspects of cubic boron nitride (CBN) tool wear in finish hard turning. Results indicate the following: (1) carbide sizes of the workpiece have significant effects on tool wear; (2) the flank wear rate can be correlated with mean carbide diameter of the workpiece; (3) the wear resistance increases monotonically with decreasing CBN grain size; (4) in finish turning with low CBN content tools, the governing wear mechanisms is fine scale attrition by microfracture and fatigue. A powder metallurgy M50 bar hard turned using an ultrafine CBN grain tool shows flank wear less than 45 μM VB max after 6.2 km cutting distance; this wear rate equals 3.1 μm/km, over an order of magnitude lower than when machining conventional M50 under identical conditions. The surface finish is better than 80 nm R a.