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Metal Machining Episode 4

Tham khảo tài liệu 'metal machining episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Work material characteristics in machining 83 Fig. 3.1 Shear stress levels and work hardening severities of initially unstrained commonly machined aluminium copper iron b.c.c. and f.c.c. nickel and titanium alloys recently these were not machinable. The data come from compression testing at room temperature and at low strain rates of initially unworked metal. The detail is presented in Appendix 4.1. Although machining generates high strain rates and temperatures these data are useful as a first attempt to relate the severity of machining to work material plastic flow behaviour. A more detailed approach taking into account variations of material flow stress with strain rate and temperature is introduced in Chapter 6. Work heating is also considered in Chapter 2. Temperature rises in the primary shear zone and along the tool rake face both depend on fUworktanf kwork. Figure 3.2 a summarizes the conclusions from equation 2.14 and Figures 2.17 a and 2.18 b . In the primary shear zone the dimensionless temperature rise DT pC k depends on fUworktanf kwork and the shear strain g. Next to the rake face the additional temperature rise depends on fUworktanf kwork and the ratio of tool to work thermal conductivity K . Figure 3.2 b summarizes the typical thermal properties of the same groups of work materials whose mechanical properties are given in Figure 3.1. The values recorded are from room temperature to 800 C. Appendix 4.2 gives more details. Figures 3.1 and 3.2 suggest that the six groups of alloys may be reduced to three as far as the mechanical and thermal severity of machining them is concerned. Copper and aluminium alloys although showing high work hardening rates have relatively low shear stresses and high thermal diffusivities. They are likely to create low tool stresses and low temperature rises in machining. At the other extreme austenitic steels nickel and titanium alloys have medium to high shear stresses and work hardening rates and low thermal diffusivities.