Mechanism And Technology Of High Speed Precision Hard Turning With PCBN Tool

The high-speed machining of hardened steel components with Polycrystalline Cubic Boron Nitride (PCBN) inserts has become a very popular machining process in the automotive industry and the aerospace industry in recent years. A significant benefit of the high-speed hard turning as a single point cutting is the capability to produce complex forms of workpiece with the inherent motion capability of modern machine tools. Moreover, because of its economic and environmental advantages in many machining tasks, fine hard turning represents an attractive alternative to grinding. However, there is still much reluctance in adopting hard turning as a finishing process because the high temperature and high levels of specific forces occurr in hard machining operations, leading to very short tool life and poor surface quality if a poor tool material or improper cutting conditions is chosen. In this thesis, the cutting mechanics of high-speed precision hardened steel cutting, tool wear and machined surface quality are studied with experiments of high speed precision hardened steel GCr15 (the hardness of HRC 62~65) with PCBN tools, and combining with finite element method (FEM).Firstly, comparing with analytical model, a two-dimensional(2D) and three-dimensional finite element model with general finite element analysis software Deform 2D&3D for high speed precision hardened steel cutting is developed by considering the chip-tool friction, the workpiece material deformed under high strain, high stain rate and high temperature using the Johnson-cook (JC) equation, local remeshing criterion combining with the Cockroft and Latham criterion used for the material damage. The 2D finite element model can predict the thermo-mechanical variables of machining process, such as saw-tooth chip and tool wear in high speed hard cutting. The 3D finite element model can predict cutting forces and cutting temperature in high speed hard cutting. The feature of cutting forces in high-speed precision hardened steel cutting is differented from conventional cutting because of the materials soften phenomena, especially in precision hard cutting the depth of cut being the same order as the width of chamfered tool, leading to the Dead Metal Zone near chamfer. By applying the minimum energy principle to predict shear angle and ploughing forces studied with a slip line field model, a theoretical model for cutting forces with chamfered cutting tools, is developed to modify the parallel shear zone of Oxley's orthogonal cutting model. A mathematical model based on genetic algorithms is developed to study the effect of the geometries of chamfered cutting tools and cutting parameters on cutting forces and to optimize cutting parameters.PCBN tool wear mechanism in high speed precision hardened steel cutting is different from that in conventional cutting. By applying a scanning electron microscope (SEM) analysis and energy spectrum analysis, the effects of the geometries of chamfered coated cutting tools, coated cutting tools, and cutting speeds on PCBN tool wear are experimentally investigated to analyze PCBN tool wear mechanisms and to identify different tool wear mechanisms under high speed precision hard cutting.Finally, the surface integrity of GCr15 under high-speed hard precision cutting is systematically investigated. The effect of the geometries of chamfered cutting tools and cutting parameters on surface roughness, white layer and microhardness, is experimentally studied with coated PCBN tools and uncoated PCBN tools to design appropriate PCBN tools ( so called"wiper tools") for producing very good surface quality in high-speed precision hard cutting. By applying a scanning electron microscope analysis and energy spectrum analysis, the mechanism of the formation of white layer is studied.