surface finish ra chart
Surface Finish Ra Chart
Ra is the average roughness of a surface, expressed in micrometres, and it is the only surface callout a drawing should use. Adjectives like smooth or fine cannot be measured, and a supplier who is asked for smooth will deliver something plausible and unexplainable. The chart below is what each value actually requires.
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- As machined
- Ra 1.6–3.2 µm
- Fine turning / milling
- Ra 0.8 µm
- Grinding
- Ra 0.4–0.2 µm
- Polishing
- Ra 0.1 µm and better
Surface finish is the most over-specified callout on a drawing, because “smooth” is a feeling rather than a number until it is converted into Ra and priced.
Specifications
| Standard | ISO 4287 / ASME B46.1 |
|---|---|
| Default | Ra 3.2 µm unless stated on the drawing |
| Functional machined face | Ra 1.6 µm |
| Sealing face | Ra 0.4–0.8 µm, ground |
| Conversion | 1 µm = 39.37 µin · RMS ≈ 1.1 × Ra |
| Cost step | Below Ra 0.4 µm requires grinding |
Frequently asked questions
What does Ra 1.6 mean?
An average surface roughness of 1.6 micrometres, about 63 microinches. It is the usual default for a functional machined face and the finish a sharp tool produces on a normal turning or milling pass.
How do I convert Ra to RMS?
Multiply Ra by about 1.1 for RMS, and multiply micrometres by 39.37 to get microinches. So Ra 1.6 µm is roughly 1.8 RMS, or 63 µin.
What Ra can CNC machining achieve without grinding?
Around Ra 0.8 µm with a finishing pass on a rigid setup. Below that, the surface is produced by grinding, honing or polishing rather than by a cutting tool.
Why does a sealing face need a ground finish rather than a turned one?
Because turning leaves a directional lay — fine grooves that run around the part — and a seal can leak along them even when the Ra number looks acceptable. Grinding leaves a random lay that seals.
Do I need to specify Ra on every surface?
No. A general note covering the whole drawing (Ra 3.2 µm unless stated) plus specific callouts on the functional faces is the clearest way to specify surface finish, and it is also the cheapest.
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