Finish machining of hardened steels using CBN cutting tools.

摘要

There are a variety of precision mechanical components such as bearings, fuel injectors and gears, which are made of hardened steels, and have to be machined to a high degree of precision. Because of the stringent performance and functional requirements for these components, surface quality in terms of surface topography, microhardness, microstructure, and residual stresses must be specified and achieved through the appropriate manufacturing process sequence. The availability of hard cubic boron nitride (CBN) cutting tools has opened up the possibility of achieving these requirements by a finish machining process, while requiring little or no grinding. A study has been made of the nature of surfaces of M2 high speed steel produced by finish machining with CBN tools. This has involved measurement of forces, surface finish, residual stresses, form errors and metallurgical changes associated with CBN turning of hard steels. The force measurements have enabled a characterization of the mechanics of the finish machining process in terms of the mechanical and thermal loads. It is found that the specific energy for machining M2 steel is around 7 Joules/mm{dollar}sp3{dollar} which is ten times smaller than that found in grinding. Using forces, an estimate has been made of the friction coefficient at the tool-chip interface and the shear plane temperature. The surface finish and form measurements show that roughness values down to 0.13 {dollar}rmmu m Rsb{lcub}a{rcub},{dollar} and straightness, roundness, cylindricity, and parallelism values of 1 {dollar}mu{dollar}m or smaller can be attained through the proper selection of process parameters. Various aspects of machining residual stresses have been investigated such as the effect of process parameters on the stress state, principal stresses, repeatability of the process in the context of residual stress generation and the evolution of residual stresses from heat treatment through to finish machining. A qualitative correlation has been established between work surface temperature and the residual stress state, just as in the grinding of hardened steels. The effect of process parameters on microhardness, microstructure and chip formation has also been investigated. The implications of this study to the finish machining of precision mechanical components are discussed.