On Chip Morphology, Tool Wear and Cutting Mechanics in Finish Hard Turning

Abstract

Topography of surfaces produced in finish hard turning using cubic boron nitride (CBN) tools is affected by a large number of factors including tool wear and the mechanics of the chip formation process. This paper shows first that tool wear rates are affected by interactions between the work material and the binder phase of the CBN tool. For finish hard turning, low CBN content, ceramic binder tools give longer lives and better finish than high CBN content metallic binder tools. For low CBN tools, wear rate is directly related to the microstructure of the work material and to the CBN grain size. SEM studies suggest that chip morphology is independent of work material microstructure, but varies with tool wear. Orthogonal cutting tests show that, above a critical speed, segmented chips are formed by catastrophic localized shear and that chip segmentation spacing may be reflected in a modulation of the machined surface. Segment spacing is a function of depth of cut, rake angle, and surface speed, approaching a limiting value with speed. Specific cutting energies decrease with speed, also approaching an asymptote. A simple mechanical model gives reasonable predictions of segment spacing along the original surface, although a full thermo-plastic model will be required to account for other aspects of the chip formation process.