Surface Finish
Glossary · Tolerance & Measurement
At a glance
- Category /
- Tolerance & Measurement
- Also called /
- Ra, roughness, surface roughness
- Related /
- 3 terms
- Source /
- Written in the shop
Surface finish describes the fine texture left on a machined surface, most often stated as Ra in microinches or micrometres.
Also called: Ra · roughness · surface roughness
In practice
Ra is the average roughness — the mean deviation of the surface from its own centreline. A rough turned surface might be 125 Ra; a ground bearing journal might be 16 Ra or better. It is controlled by tooling, feed rate and the final operation, not by tolerance.
Why it matters
Seal and bearing surfaces have finish requirements as strict as their dimensional ones. A shaft that is exactly the right diameter but too rough will shred a seal in service.
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