Cutting characteristics of implant materials in milling

The demand of implant parts has recently been increasing with aging of the population progresses. In terms of biocompatibility and mechanical strength, stainless steels and cobalt chromium molybdenum (CoCrMo) alloy have been applied to implant parts. Laser sintering parts processed in additive processes have also been manufactured to adapt for each human body. However, the machinability of these materials is much lower than the conventional ones such as carbon steels. Residual stresses are also associated with the cutting forces for long lives of implant parts. This paper discusses the cutting processes of CoCrMo alloy, sintered stainless steel (SUS420J2), and wrought stainless (SUS304) in ball end milling with the cutter axis inclination. The cutting characteristics are compared in the cutting force, the surface finish and the residual stress. The cutting force of the sintered stainless steel includes large dynamic components due to non-uniformity of material with voids. Z component in the cutting force becomes large in milling of CoCrMo alloy. The surface finishes, then, are discussed in terms of the cutting force and the vibration. The surface finish also depends on the tool vibration associated with the dynamic response of tool and adhesion. The residual stresses measured in the feed direction of the tool and the width direction of the groove for some cutting parameters and the testing materials. In order to discuss the mechanical effect on the residual stress, the residual stresses are associated with the resultant cutting force in the feed and the width directions. The sensitivity of the residual stress to the cutting force is discussed for the testing materials. The residual stress in the width direction has large dependency on the cutting force, which may be controlled by the cutting parameters and the tool geometry. The residual stress of the feed direction depends on the material behavior controlled by the material properties.