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Turning and Chip-breaking Technology Part 3

Tham khảo tài liệu 'turning and chip-breaking technology part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Turning and Chip-breaking Technology 53 b X 200. 12m min X 100 120m min c X 50 120m min a X 2000 cutting speed 12m min Figure 25. Variations in chip morphological surfaces at different cutting speeds giving an indication of the various shearing mechanisms. Source Watson Murphy 1979 and later by Herbert 1928 . Around this time the cutting speeds were steadily improving with the arrival of new cutting tool materials such as cemented carbide. In 1937 Piispanen introduced his so-called Deck of Cards principle as an explanation of the cutting process see Fig. 24 for Piispanen s idealised model with Fig. 25 depicting sheared chips at a range of cutting speeds . Here Piispanen s model depicts the workpiece material being cut in a somewhat similar manner to that of a pack of cards sliding over one another with the free surface an angle which corresponded to the shear angle ọ . So as the tool s rake face moves relative to that of the workpiece it engages one card at a time causing it to slide over its adjacent neighbour this process then repeats itself ad finitum - during the remainder of the cutting process. Some important 54 Chapter 2 limitations are present with Piispanen s model namely that it exaggerates strain in homogeneity shows tool face friction as elastic rather than plastic in nature considers shearing takes place on a completely flat plane assumes that BUE does not occur takes an subjectively assumed shear angle takes no account of either chip curling or prediction of chip tool length. NB Piispanen s model is easily understood and does contain the major concepts in the chip-forming process - admittedly for simple shear in the main. By way of further information concerning chip morphology the micrographs of chip surfaces illustrated in Fig. 26 show in these cases that the morphology indicates a semi-continuous chip form. These chip forms point towards the fact that noticeable periodic variations have occurred perhaps as the result of the stress becoming unstable