Surface roughness, not surprisingly, is closely related to uncut chip thickness. RT Roughness
Rt roughness is the SI parameter for maximum surface roughness, the maximum difference between peak height and valley depth within the sampling length. As a first approximation, Rt is independent of depth of cut but is dependent on vw, vs, C ■ r, and de. The relationship between surface roughness and specific grinding energy can also be readily obtained by direct substitution.
Rt is but one of several measures of surface roughness. Two other common roughness standards are Ra roughness and Rz roughness.
2.2.16 Ra Roughness
Ra roughness is the arithmetic average of all profile ordinates from a mean line within a sampling length after filtering out form deviations.
2.2.17 RZ Roughness
Rz roughness is the arithmetic average of maximum peak-to-valley readings over five adjacent individual samplings lengths. Rt and Rz values are much larger than Ra roughness values for measurements from the same surface.
Two other parameters related to surfaces, especially those used for rubbing contact, are defined as follows:
2.2.18 Material or Bearing Ratio
Material or bearing ratio, tp, is the proportion of bearing surface at a depth p below the highest peak.
2.2.19 Peak Count
Peak count, Pc, is the number of local peaks that project through a given band height. tp is less for grinding than for other operations such as honing, although it can be improved to some extent by a two-stage rough-and-finish grind with wheels of very different grit size. Pc can be controlled somewhat by adjusting dress parameters.