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Tham khảo tài liệu 'metal machining - theory and applications episode 1 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Friction lubrication and wear 73 Table 2.4 Tool surface roughness and contact stress severity data Tool finish Roughness data w Al HSS Cu HSS Brass Carbide Steel Carbide Ra Lum Aq CVD coated 0.2-0.5 3-7 1.2 1.9 2.8 1.8 Ground 0.1-0.25 2-4 1.2 1.9 2.8 1.8 Super-finished 0.03 0.4 1.2 1.9 2.8 1.8 and PE2 in Figure 2.25. PE1 represents theoretical analyses Appendix 3 when the roughness is imagined to be on the tool surface and PE2 when it is imagined to be on the chip. However for large values of s klocal both regions have almost the same upper boundary with c equation 2.26 approximately equal to 1. One would then expect s m --------- 2.28 klocal In those circumstances when m is measured to be 1 this seems to be a reasonable relation. For example in Figure 2.23 for the free machining steels when the rake face temperature is below 600 C m is roughly the same as the ratio of m for the steel to that for the plain carbon steel at the same temperature. However equation 2.28 cannot explain observations of m 1 of the sort recorded in Figure 2.23 b for the non-free machining steel or for the free machining steels above 600 C. Friction coefficients greater than 1.0 The plastic contact mechanics modelling reviewed in Appendix 3 which leads to c 1 for the most part assumes that the asperity does not work harden and that the load on the asperity is constant through its make and break life cycle. In the final section of Appendix 3 there is a brief speculation about departures from these assumptions that could lead to larger values of c and to m 1. All proposals require the shear strength of the junction to be maintained while the normal stress is unloaded. It is certain that for this to occur the strongest levels of adhesion must exist between the asperities and the tool. The freshly formed unoxidized nature of the chip surface created by the parting of the chip from the work typically less than 10-3s before it reaches the end of the contact length and the high temperatures reached
