Surface roughness Ra and Rz

Ra averages the profile deviations over the evaluation length, Rz rates the maximum profile height within a sampling length. Rz therefore reacts much more sensitively to single scratches and peaks and is often more meaningful for function-critical surfaces.

Ra and Rz describe different properties of the same profile. The ratio of Rz to Ra depends on the profile shape and thus on the manufacturing process: it tends to be lower for turned surfaces and higher for ground ones. The common empirical range is Rz ≈ 4 to 7 · Ra; an exact formula does not exist.

ISO 21920 replaces ISO 4287 (parameter definitions), ISO 4288 (specification operators) and ISO 1302 (drawing indication). It combines parameters, measurement conditions and drawing indication in one series of standards, redefines default measurement conditions and refines, among other things, how Rz is determined over the sampling lengths. Two changes affect acceptance directly: the default is now the maximum tolerance acceptance rule instead of the 16 % rule, and surface imperfections count as part of the specified surface under ISO 21920-3, so they have to be taken into account when locating the measurement trace. The same drawing indication therefore states a stricter requirement than before.

The N grades originate from the withdrawn ISO 1302 and are no longer intended on new drawings, where Ra or Rz are stated directly with a numerical value. On legacy drawings and in shop-floor practice the N grades are still widespread; N7 corresponds to Ra 1.6 µm, for example.

Rz is written in the surface texture symbol with its value in µm, for example Rz 6.3. Per ISO 21920-1 the value is by default an upper limit governed by the maximum tolerance acceptance rule: no measured value may exceed the limit. The symbol Tmax may be added for clarity. If the 16 % rule is intended instead, it has to be requested explicitly with T16 %. Under the superseded ISO 1302 it was the other way round: the 16 % rule was the default and the maximum rule was indicated by the suffix max (Rz max 6.3).

Rough turning is typically at Ra 6.3 to 25, finish turning and milling at Ra 0.8 to 6.3, drilling at Ra 1.6 to 12.5, broaching at Ra 0.4 to 3.2, grinding at Ra 0.1 to 1.6 and lapping or polishing below that.