The Metrics of Surface Roughness

The reference line xx is established by the skid as it slides along supported by the crests of the peaks meanwhile the profile of the surface is generated by the vertical excursions of the stylus, yy, in conjunction with the horizontal movement in the x-direction. To convert these raw data into a statistical average with a symmetrically distributed deviation, the reference line xx must be translated so that the area bounded by the profile curve over the length L is equally distributed on both sides of the line, as in Fig. 12-5. Statistical treatments of surface profile data are discussed at length in Sections 12.3 and 12.4. 

TABLE 12-1. CHARACTERISTIC ROUGHNESS VALUES OF VARIOUS SURFACES 

Our ultimate interest in the topography of surfaces is in relation to contact over an area. Cut sections and stylus probes yield only linear diagrams. Interferograms, where as many as 10 fringes can be observed c-i- Fig. 12-4), yield information on the surface structure of a band approximately 0.25 mm wide. 

The meaning of contact seems obvious from experience, until it becomes necessary to define it accurately. To establish that contact has occurred in a mechanical device, the investigator invokes one of two principles. Either contact is said to be achieved when a critically minimal resisting force is first sensed as two bodies are moved toward each other or else contact is defined as the elimination of space between the two bodies. The latter definition suffers from an intrinsic ambiguity because of the difficulty in establishing the actual location of a real surface in the first place. 

Real contact on a macroscopic scale occurs over an area. As was shown in the discussion of friction, we must distinguish between the apparent and the true area of contact. The structural aspect of real surfaces has put the word asperity into the working vocabulary of surface 

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