Tham khảo tài liệu '21st century manufacturing episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 294 Metal-Products Manufacturing Chap. 7 Figure Regions of heat generation. Controlling the Machining Process 295 Diamond is not the stable form of carbon at atmospheric pressure. Fortunately it does not revert to the graphitic form in the absence of air al temperatures below l 500 C. In contact with iron however graphitization begins just over 730 C and oxygen begins to etch a diamond surface at about 830 C. It is also disappointing that diamond tools are rapidly worn when cutting nickel and aerospace alloys. Generally they have not been recommended for machining high-melting-point metals and alloys where high temperatures are generated at the interface. Ihe family with the highest hot hardness is the alumina-based A12O3 group and these are favored for high-speed facing of cast iron. Cast iron machines with a well-controlled shower of short chips that facilitate high-speed cutting. However the A12O3 -based materials are also very brittle and they have limited use for cutting steels. Empirically it can be shown that tool life decreases with increases in cutting speed as shown in Figure . It turns out that the prolific F. w. Taylor also took great interest in this topic. The optimization of cutting speeds fell in naturally with his interests in the principles of scientific management. By the time the results of his Taylor equation were applied to the Midvale Steelworks a productivity gain of 200 to 300 was achieved on the machine tools which also created a 25 to 100 increase in the wages of the machinists. Taylor found that if the data are replotted on log-log axes a straight line is obtained for most tool-work combinations. This observation led to a wide series of plots of the type shown in Figure . The famous Taylor equation relates the cutting speed V and tool life T to the constants n and c particular to each tool work combination. VT - C logT logV logC Tool life T is also sensitive to feed rate with V and d held constant and to .
