Surface Finish
Surface Roughness (Ra) Explained for Buyers
Ra is the average height of the tiny peaks and valleys on a surface. Learn which values a machine can hit, and when polishing is money wasted.
Key takeaways
- Ra is an average roughness value, measured in micrometres or microinches.
- Lower Ra means a smoother surface, and usually a higher price.
- Ra 3.2 is a normal machined finish. Ra 0.8 needs extra work.
- Only call out Ra where the surface function needs it.
What Ra measures
Every machined surface has tiny peaks and valleys. Ra takes a sample length, measures how far the profile deviates from the mean line, and averages those deviations.
It is a single number for roughness. It does not tell you the shape of the profile, only the average height of the texture.
Ra is written in micrometres (µm) or microinches (µin). One micrometre is 0.001 mm.
Ra versus Rz
Ra is an average. Rz is the average of the highest peak and deepest valley in each sample. That makes Rz more sensitive to a single scratch or burr.
For most buying decisions, Ra is enough. If a surface must seal or resist fatigue, Rz is the better conversation.
What the numbers feel like
| Ra (µm) | Feel | Typical process |
|---|---|---|
| 12.5 | Visible coarse tool marks | Rough milling, saw cut |
| 6.3 | Clearly visible marks | Standard milling or turning |
| 3.2 | Fine marks, smooth to touch | Good CNC finish |
| 1.6 | Very fine, almost uniform | Finishing pass, reaming |
| 0.8 | Smooth, slight sheen | Fine turning, light grinding |
| 0.4 | Mirror-like under light | Grinding, lapping |
| 0.1 | Reflective | Polishing, superfinishing |
What each process reaches
| Process | Typical Ra (µm) |
|---|---|
| CNC milling, roughing | 3.2 to 6.3 |
| CNC milling, finishing | 1.6 to 3.2 |
| CNC turning, finishing | 0.8 to 1.6 |
| Reaming | 0.8 to 1.6 |
| Surface grinding | 0.2 to 0.8 |
| Bead blasting | 1.5 to 3.0, uniform matte |
| Anodizing | Slight increase in roughness |
| FDM 3D printing | 8 to 20, layer lines |
| SLA 3D printing | 1.0 to 3.0 |
How to call it out
The check mark symbol on a drawing means a surface texture requirement. The number next to it is the Ra value, usually with the unit in the title block.
- A general note like "Ra 3.2 unless otherwise specified" covers most parts.
- Add a specific callout only on sealing faces, bearing seats and sliding surfaces.
- State the measurement direction if the finish is directional, such as a turned shaft.
Why over-specifying wastes money
Getting from Ra 3.2 to Ra 0.8 usually means a second finishing pass, a smaller stepover or a different tool. Getting to Ra 0.4 often means grinding, which may need a separate supplier and an extra setup.
On a large part, that can add 20% to 40% to the price for a surface nobody touches.
Common questions
What is a good surface finish for CNC machining?
Ra 3.2 µm is a normal as-machined finish. Ra 1.6 looks and feels noticeably better for a modest cost.
Does anodizing change Ra?
It can increase roughness slightly because the oxide layer follows the surface and builds on it. Deep scratches stay visible after anodizing.
Is a lower Ra always better?
No. Some surfaces need texture to hold lubricant or adhesive. A very smooth surface can be worse for bonding.
The bottom line
Ra is a useful shortcut for describing a surface, but it is not a quality score. Specify the finish where it matters, and let the rest of the part be machined normally.