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Machinability and Surface Integrity Part 11

Tham khảo tài liệu 'machinability and surface integrity part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Machinability and Surface Integrity 369 Figure 189. The influence of the cutting edge s condition on the resultant machined surface integrity 370 Chapter 7 In Fig. 189c a white-layer i.e. for this ferrous drilled part being a localised untempered martensitic phase of 63 HRc91 exists beneath the recast and redeposited layer in this case produced by a dull drill s cutting lips and margins. Due to the fact that the recast layer i.e. heat-affected zone - HAZ has a similar metallurgy to that of the white-layer with the delineation of these white-layers regions and their accompanying HAZ s are not clearly defined. This latter HAZ is a complex metallurgical condition comprising of some untempered martensite UTM over-tempered martensite OTM while beneath these layers the bulk substrate material remains unaffected. The thickness of these white-layer zones is strongly influenced by both the actual plastic deformation created here and to a lesser degree by the thermal influence of the passage of the tool s edge over the machined surface as heat penetrates into the locality of the component s surface. Probably the worst abusive machining conditions that can exist are when drilling holes in workhardening materials having long length-to-diameter ratios i.e. L D ratios of 12 1 with inadequate coolant supply creating high levels of friction this condition being exacerbated by an inefficiency produced by a dulled drill s cutting lips. Virtually all tooling even the most sharp - the notable exception here being monolithic faceted natural diamond cutting edges have a finite tip radius of 8 gm i.e. see Fig. 184 - high-lighting the tool tip rounding effect this results in increased forces and tool wear which can transform the surface metallurgy by thermo-mechanical generation. The case has already been made concerning the fact that machining processes impart residual stresses into the surface layers as indicated in the schematically-represented milling conditions shown in Fig. 190 and