| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 105 === The demand of precision products has increased recently, and there is breakthrough in the development of ultra-precision turning technology for producing precision components. Diamond tool has been widely used in ultra-precision turning because of its excellent mechanical and chemical properties. However, we can’t cut ferrous metals or any material that contained ferrous metal by using the diamond tool directly, since the graphitization leads by combination of ferric and carbon in diamond causes the tool serious chemical wear. The main purpose of this research was to study the machining characteristics of ultra-precision turning NAK80 steel by using single crystal diamond tool, and analyze the tool wear in different machining environment. In the end, we presented several methods to suppress the tool wear cause by graphitization. According to the result, cutting temperature, contact time and chip built up on tool edge or workpiece are the most important factor to effect tool life. By using sulfated castor oil solution with BN powder, tool life as well as surface roughness would be improved. Compared with dry cutting, it was noted that tool wear was decreased by 40%, and roughness was decreased by 70%. It was believed that BN powder can serve as a physical barrier, preventing the catalytic reaction between carbon and Ferric. On the other hand, after pre-cooling to prevent incomplete cutting from direct cooling, there was 80% reduction in tool wear and over 60% improvement in surface roughness by using liquid CO2 as coolant in turning, causing by slowing down the reaction rate of graphitization. Finally, the effect of combining cryogenic method with adjusted cutting fluid or other assisted method in turning ferrous metals has been discussed. In summary, the most effective way to improve the tool wear is cryogenic turning with liquid CO2.
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