Understanding surface finish measurements
Ra (Roughness Average)
Ra is the average deviation of a surface from a mean line. Lower numbers indicate smoother surfaces.
- Ra 3.2 — standard as-machined. Suitable for most parts.
- Ra 1.6 — tighter finish. Used for sealing surfaces or aesthetic parts.
- Ra 0.4 — polished or ground. Adds significant cost.
Ra 3.2 is the default for standard CNC machining. This finish works for the majority of industrial components.
Rz (Average Maximum Height)
Rz is the average of the five highest peaks and lowest valleys. It is more sensitive to extreme surface features and is typically about four times Ra.
RMS (Root Mean Square)
RMS is an older standard similar to Ra. Most modern drawings use Ra. If you see RMS, convert to Ra using approximately RMS × 1.1 = Ra.
Typical surface finish by process
| Process | Typical Ra (μm) | Best Achievable |
|---|---|---|
| As-machined (milling/turning) | 1.6 – 3.2 | 0.8 |
| Swiss turning | 0.8 – 1.6 | 0.4 |
| Grinding | 0.4 – 0.8 | 0.1 |
| Polishing | 0.1 – 0.4 | 0.025 |
Bead blasting is a cost-effective way to improve appearance. It creates a uniform matte finish and hides tool marks at a fraction of the cost of polishing.
When does surface finish actually matter?
- Sealing surfaces — yes, surface finish matters for O-rings and gaskets
- Bearing surfaces — yes, smoother finish reduces friction
- Aesthetic parts — maybe, but consider bead blasting instead of polishing
- Hidden structural parts — no, standard as-machined is sufficient
- Threads — no, standard thread finishes are adequate
Specifying Ra 0.4 on every surface without understanding functional requirements. Tight finish adds cost and provides no benefit on non-critical surfaces.
How to specify surface finish effectively
- Use a general note: "All surfaces Ra 3.2 unless otherwise specified"
- Only call out tighter finish on critical surfaces
- Ask your supplier what finish is standard — typically Ra 1.6–3.2
Frequently Asked Questions
What is the standard surface finish for CNC parts?
Standard as-machined finish is Ra 1.6–3.2 μm. This is suitable for most applications.
Is a smoother finish always better?
No. Smoother finish costs more and doesn't add value unless the part requires it for sealing, bearing, or aesthetic purposes.
Can I specify different finishes on different surfaces?
Yes. Specify tight finish only where needed and leave the rest as-machined.
What's the cheapest way to improve appearance?
Bead blasting gives a uniform matte finish, hides tool marks, and costs much less than polishing.