How to define surface quality?

The mean roughness value Ra is a measurement of the roughness of a technical surface that has been produced, for example, by machining, forming or reshaping processes.

The value is specified in µm. Above this, 12 roughness classes (N) are defined:

0,025 no visible machining traces
0,1 Ultrafine sanding
0,2 Fine sanding
0,4 Presanding
0,8 Rough sanding
1,6 arbitrated
3,2 scrubbed
12,5 gravity die casting

In addition to the roughness depth, the shape of the surface structure also plays a role in the qualitative evaluation. Depending on the ‘topography’, for example, the cleanability or corrosion resistance changes.