Surface Roughness (Ra) Explained: Values, Symbols and Cost
What is the difference between Ra 3.2 and Ra 0.8, which value suits which surface and how is it shown on a drawing? A practical surface finish guide for CNC parts.
Table of contents 5 sections
Surface roughness determines how a part looks, how it feels and how it works with other parts. Choosing the right Ra value guarantees function and avoids unnecessary finishing operations.
In short: A standard CNC machined surface is typically around Ra 3.2 µm and is sufficient for most faces. Specify Ra 1.6 or 0.8 for sliding and sealing surfaces; lower values may require grinding or polishing.
What is Ra?
Ra (arithmetic average roughness) is the average distance of the peaks and valleys of a surface profile from its mean line, expressed in micrometres (µm). The smaller the value, the smoother the surface. Ra alone does not describe every aspect of a surface, but it is the most widely used parameter on technical drawings.
Common Ra values and applications
| Ra (µm) | N grade | Typically achieved by | Where it is used |
|---|---|---|---|
| 6.3 | N9 | Rough machining | Hidden, non-functional surfaces |
| 3.2 | N8 | Standard CNC machining | General machine parts, most surfaces |
| 1.6 | N7 | Fine machining (finishing pass) | Sliding surfaces, gasket faces, visible parts |
| 0.8 | N6 | Very fine machining or grinding | Bearing seats, precision fits |
| 0.4 | N5 | Grinding, honing | Hydraulic cylinders, high-precision surfaces |
| ≤ 0.2 | N4 and below | Lapping, polishing | Gauges, optics and critical sealing surfaces |
How is it shown on a drawing?
Surface roughness is indicated with a symbol placed on the relevant surface (ISO 1302, now superseded by ISO 21920-1). The Ra value is written above the symbol. A closed symbol means material removal is required; a symbol with a circle means no material removal is allowed.
- General surface finish: a general symbol and value (e.g. Ra 3.2) near the title block applies to all surfaces unless stated otherwise.
- Specific surfaces: surfaces needing a better finish are marked individually (e.g. Ra 0.8).
Stating Ra 3.2 as the general value and marking only the critical surfaces keeps the drawing clean and tells the manufacturer exactly which surfaces matter.
How roughness affects cost
A lower Ra value means slower feeds, extra finishing passes, special tools or additional operations such as grinding and polishing. Going from Ra 3.2 to 0.8 can multiply the machining time of that surface. Specify low values only where function requires them.
Watch out: anodizing, painting or bead blasting change surface roughness. If you need a value after finishing, state it clearly; otherwise the Ra value is usually interpreted as after machining.
Checklist
- General surface finish value stated on or near the title block
- Sliding, bearing and sealing surfaces marked individually
- Visible (cosmetic) surfaces identified
- Clear whether the value applies before or after finishing
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