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Roughness depth Rz
To determine the averaged roughness depth Rz, a measuring segment is first established, and in turn, this is divided into (usually five) individual measuring lengths of equal size. The range of the two extreme values in the roughness profile, for the respective individual sections, is subsequently divided by the number of measuring sections.
Average roughness value Ra
The arithmetic average roughness value Ra describes the arithmetic average of all deviations in the roughness profile from the median line within the measuring length. The surface roughness established by the calculation is thereby less vulnerable to extreme values than the roughness depth.
A conversion of Rz to Ra and the reverse is not possible as 1:1 scale. However, a conversion interval that contains all possible values can be specified. The rule of thumb that repeatedly emerges for converting Rz = Ra*7 does not go far enough, and based on the extensive scattering of values, can lead to bad surprises in practice.
Roughness class N
Additionally, the surface quality is frequently subdivided into 12 roughness classes or roughness grades. Each roughness class from N1 to N12 has a maximum permissible Ra value according to DIN ISO 1302.