"Ra" appears on most drawings, yet it is often misunderstood. This article explains what it means, what is realistic in CNC machining, how it differs from Rz, and how to call it out so that smoothness is paid for only where it is required.
What Ra is. Ra is the average roughness of a surface, measured in micrometres (µm) as the mean deviation of the surface profile from its centre line. Lower means smoother. It describes texture and is entirely separate from dimensional tolerance: a part can be exactly on size yet feel rough, or feel smooth yet be out of tolerance.
The Ra ladder, what's realistic, and what it's for
| Ra (µm) | Ra (µin) | Typical process | Looks / good for | Cost |
|---|---|---|---|---|
| 6.3 | 250 | Roughing, bead blast | Coarse matte; non-critical faces | $ |
| 3.2 | 125 | Standard milling | Visible tool lines; general surfaces | $ |
| 1.6 | 63 | Finish milling / turning | Default machined finish; most faces | $ |
| 0.8 | 32 | Fine turning / finishing pass | Sliding fits, static seals, gaskets | $$ |
| 0.4 | 16 | Grinding / light polish | Fine & dynamic seals | $$$ |
| 0.2 | 8 | Grinding / polish / EDM skim | Bearing journals, near-optical | $$$ |
| ≤0.1 | ≤4 | Lapping / mirror polish | Mirror, optical, sealing-critical | $$$$ |
Why smoother costs more
Down to about Ra 1.6, the finish carries no added cost — it is simply how the part comes off the machine. Ra 0.8 typically requires one additional finishing pass, a modest cost. Below Ra 0.8, the cost reflects grinding, polishing or hand-work, and each step down multiplies the time. A mirror finish at Ra 0.1 can cost more than the machining itself.
Ra vs Rz: a key distinction
Ra is an average over the whole profile. Rz is the average peak-to-valley height, so for the same surface it reads roughly 4–7× higher. That means "Rz 6.3" describes a far smoother surface than "Ra 6.3." If your drawing uses Rz, please label it clearly so it is not read as Ra.
How to call it out on your drawing
ALL OTHER SURFACES: Ra 3.2 µm (general note)
Call out the fine Ra only on the faces that require it and leave the rest as-machined. This keeps the part cost-effective and indicates precisely where the finishing effort should be directed.
Three common mistakes
- A blanket fine Ra across the whole part — this incurs finishing cost that is not needed.
- Confusing Ra and Rz — Rz reads 4–7× higher for the same surface.
- Expecting plating or anodizing to smooth a rough surface — it only mirrors what's underneath.
