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 — and where the cost steps are.
| N grade | Ra (µm) | Ra (µin) | Rz (µm) approx | Relative | 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; Rz measures peak-to-valley. 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 effectively free. Below Ra 1.6 you are into finishing passes and slower feeds. Below Ra 0.4 generally needs grinding, lapping or electropolishing — a step change in both cost and lead time.
Ra says nothing about direction
A turned surface and a ground surface can share an Ra and behave completely differently in a seal or a sliding fit, because the lay runs a different way. If lay matters, the drawing has to say so — there is a symbol for it.
Turned finish is predictable
On a lathe, theoretical Ra depends mostly on feed per rev and tool nose radius. Easing the feed or increasing the nose radius both improve finish — but a bigger radius pushes harder and will chatter a slender part.
Do not over-specify the whole part
A finish callout applies to every surface unless you say otherwise. Putting Ra 0.8 in the title block because one bore needs it costs money on every face. Call the fine finish on the feature that needs it.
Convert or predict it
Move between Ra, Rz, N grades and microinches with the surface finish converter, or work out achievable turned finish from feed and nose radius with the turned finish calculator, at dalloways.com/tools.
Typical reference values: the governing standard or your material certificate takes precedence.
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