Suppression of tool damage in ultraprecision diamond machining of stainless steel by applying electron-beam-excited plasma nitriding

Mirror surface machining of stainless steel with single-crystalline diamond tools is proposed in this study by applying a new nitriding method, called electron-beam-excited-plasma (EBEP) nitriding, to workpiece surfaces as pretreatment. It is well known that mirror surface finish of steel workpieces by conventional diamond cutting is unachievable owing to rapid tool wear. Nitriding of steel workpieces has been one of the several attempts to prevent the rapid tool wear of diamond tools. It has been reported that the rapid tool wear is caused by thermochemical interaction between diamond and steel, and that the wear can be greatly reduced by nitriding of steel. However, hard compounds formed on the outmost surfaces of workpieces by the conventional nitriding methods can cause micro-chippings of cutting tools. The authors has recently developed a new nitriding method called EBEP nitriding, in which a high dissociation rate for nitrogen molecules is achieved using the electron-beam-excited-plasma, and iron-compounds-free nitriding has been realized. Therefore, the EBEP nitriding is applied to a typical mold material, modified AISI 420 stainless steel, aiming at suppressing the micro-chippings as well as the thermochemical tool wear during diamond cutting of the stainless steel. The conventional ion nitriding and the gas nitrocarburizing are also applied to the same stainless steel in comparison. Chemical components of the nitrided workpiece surfaces are analyzed by an electron prove micro-analyzer (EPMA) and an X-ray diffraction (XRD) in advance, and turning experiments are conducted with single-crystalline diamond tools. Subsequently, changes in cutting forces and roughness of finished surfaces and tool damages after the turning experiments are evaluated. Finally, mirror surface machining by using the EBEP nitriding is demonstrated, and its advantages and disadvantages in the diamond cutting of stainless steel are summarized in comparison with the conventional nitriding methods.