Surface finish chart — Ra, N grades and Rz
Roughness grades side by side with the processes that typically achieve them, so you can see what a drawing callout actually implies for the shop.
| N grade | Ra (µm) | Ra (µin) | Rz (µm) approx. | Typical process |
|---|---|---|---|---|
| N1 | 0.025 | 1.0 | ≈0.13 | Lapping, superfinishing |
| N2 | 0.05 | 2.0 | ≈0.25 | Lapping, honing |
| N3 | 0.1 | 3.9 | ≈0.50 | Honing, fine grinding |
| N4 | 0.2 | 8 | ≈1.0 | Grinding, electropolish |
| N5 | 0.4 | 16 | ≈2.0 | Grinding, fine turning |
| N6 | 0.8 | 31 | ≈4.0 | Fine turning, milling |
| N7 | 1.6 | 63 | ≈8.0 | Turning, milling — good |
| N8 | 3.2 | 126 | ≈14 | Turning, milling — typical |
| N9 | 6.3 | 248 | ≈28 | Milling, drilling |
| N10 | 12.5 | 492 | ≈56 | Rough machining |
| N11 | 25 | 984 | ≈113 | Rough turning, casting |
| N12 | 50 | 1968 | ≈225 | Flame cutting, sand casting |
Ra to Rz is not a reliable conversion. Ra averages the profile and Rz measures peak-to-valley, so two surfaces with identical Ra can have very different Rz depending on how they were made. The values here are indicative only — if a drawing specifies Rz, measure Rz.
Where the cost steps are. Ra 1.6–3.2 µm is normal as-machined and free. Below about Ra 1.6 you are into finishing passes and slower feeds. Below Ra 0.4 generally needs a secondary process such as grinding, lapping or electropolishing, which is a step change in both cost and lead time.
Convert between systems with the surface finish converter, or work out achievable turned finish from feed and nose radius with the turned finish calculator.
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