Ra (arithmetic mean deviation)
Average absolute height deviation of the roughness profile from its mean line.
In turning, theoretical Ra from tool geometry is about f squared divided by (32 times the nose radius), with feed per revolution f and nose radius in mm; multiply the mm result by 1000 for micrometres. Real surfaces come out rougher through built-up edge, vibration and a worn edge.
General shop guidance in our own words. The drawing and the standard it cites decide; verify on a sample part.
Typical finish by process and the turning feed and nose-radius formula: Surface finish guide.
Related surface roughness parameters
Tell the advisor the feature, the tolerance on the drawing and the material. It answers with machining choices, brand-neutral.
Definition written in our own words and checked against engineersedge.com. Symbol and term names follow ISO 21920-2; we do not reproduce standard text. See the full index.