Dimensional Factors

Surface Finish/Waviness. Figure 5 indicates the surface finish (expressed as the roughness average, Ra) that can be achieved with a variety of manufacturing processes. Traditionally, the requirements for finishing processes have been compromises between achieving better surface finishes while at the same time reducing the total finishing time. Recent developments in microgrinding with fixed abrasives have enabled this process to compete with the slower lapping operation using loose abrasives (Ref 14).

Fig. 5 Ranges of average surface roughness produced by various processing methods. Source: Machinery's Handbook, 24th ed., Industrial Press, 1992, p 672

Beyond their well-known importance for tribology (Ref 15, 16) and aesthetics, surface finish and waviness can have a significant impact on the surface integrity of a part, because wear behavior of surfaces is influenced by the surface roughness. Huang (Ref 17) has studied the fatigue life of turned and ground superalloys and has shown that the surface roughness, especially at high temperature, and residual stresses of the samples had more effect on fatigue resistance than surface hardening does.

Stress relaxation can be a finishing operation per se. Reference 18 describes a variety of thermal and/or mechanical operations can be applied for stress relieving. For example, 90 s in a salt bath at 345 °C (650 °F) relieved 80% of the residual stress of ground steel parts (Ref 19). Even if no stress relaxation technique is used, fatigue-related relaxation sometimes occurs due to microplastic deformation. The rate of relaxation is greatest when the finishing process produces the steepest residual stress gradient normal to the surface (Ref 20). Thus, sand-blasted or rolled parts could be expected to relieve their residual stress much faster than milled ones, for metallic components.

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