Why Surface Finish Matters
Surface finish (or surface roughness) describes the texture of a machined surface. It affects sealing, wear, fatigue life, coating adhesion, and appearance. If you’re designing parts that mate, seal, or slide against each other, you need to specify surface finish on your drawing.
Common Surface Finish Values
| Ra (µin) | Ra (µm) | Process | Typical Application |
|---|---|---|---|
| 500 | 12.5 | Rough turning, sawing | Non-critical surfaces, rough stock |
| 250 | 6.3 | Turning, milling | General machined surfaces |
| 125 | 3.2 | Standard milling/turning | Most machined mating surfaces |
| 63 | 1.6 | Fine turning, grinding | Bearing surfaces, hydraulic cylinders |
| 32 | 0.8 | Grinding, honing | O-ring grooves, seal surfaces, precision fits |
| 16 | 0.4 | Fine grinding, lapping | Precision bearings, optical components |
| 8 | 0.2 | Superfinishing, lapping | Gauge blocks, high-precision seals |
Quick Rules of Thumb
- If it seals — 32 µin or better (o-ring grooves, gasket surfaces)
- If it slides — 16–32 µin (bearing journals, piston bores)
- If it’s press-fit — 63 µin or better
- If it doesn’t matter — don’t specify it (saves money)
How to Call Out Surface Finish on a Drawing
Use the surface finish symbol (✓ with a number) and specify Ra in microinches (µin) or micrometers (µm). Place it on the surface or on a leader pointing to the surface.
If you don’t specify a surface finish, the shop will machine it to whatever their standard process produces — which is usually fine for non-critical surfaces but can be a problem for seals and fits.
Relationship to Manufacturing Cost
Better surface finish = more machining time = more money. Going from 125 µin to 32 µin might double the cost of that feature. Only call out tight finishes where they actually matter.
See also: O-Ring Groove Design (requires 32 µin Ra for groove surfaces)