Surface roughness: Ra, Rz, N grades, µin
An old Russian drawing says ▽7, a new one Ra 1.6, an imported one N7 or 63 µin. Enter any of them to see the rest and how such a finish is usually produced.
Quick answer
Ra 1.6 µm = N7 = 63 µin — a normal finish-turned surface. Ra 0.8 µm = N6 = 32 µin, Ra 3.2 µm = N8 = 125 µin.
- Ra
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- In microinches
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- Class per GOST 2789-59
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- Nearest N (ISO 1302)
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- Rz of that class
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- Usually produced by
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Summary chart
| GOST class | Ra, µm | Rz, µm | N | Ra (N), µm | µin | Typically |
|---|---|---|---|---|---|---|
| ▽1 | 80–40 | 320–160 | N12 | 50 | 2000 | very rough cutting |
| ▽2 | 40–20 | 160–80 | N11 | 25 | 1000 | rough turning, sawing |
| ▽3 | 20–10 | 80–40 | N10 | 12.5 | 500 | rough turning |
| ▽4 | 10–5 | 40–20 | N9 | 6.3 | 250 | semi-finish turning |
| ▽5 | 5–2.5 | 20–10 | N8 | 3.2 | 125 | finish turning |
| ▽6 | 2.5–1.25 | 10–6.3 | N7 | 1.6 | 63 | finish turning, reaming |
| ▽7 | 1.25–0.63 | 6.3–3.2 | N6 | 0.8 | 32 | fine turning, grinding |
| ▽8 | 0.63–0.32 | 3.2–1.6 | N5 | 0.4 | 16 | fine turning, grinding |
| ▽9 | 0.32–0.16 | 1.6–0.8 | N4 | 0.2 | 8 | fine grinding, honing |
| ▽10 | 0.16–0.08 | 0.8–0.4 | N3 | 0.1 | 4 | lapping, superfinishing |
| ▽11 | 0.08–0.04 | 0.4–0.2 | N2 | 0.05 | 2 | lapping, polishing |
| ▽12 | 0.04–0.02 | 0.2–0.1 | N1 | 0.025 | 1 | polishing |
| ▽13 | 0.02–0.01 | 0.1–0.05 | — | — | — | mirror polishing |
| ▽14 | 0.01–0.005 | 0.05–0.025 | — | — | — | mirror polishing |
Ra is the arithmetic mean deviation, Rz the ten-point height of irregularities. There is no exact conversion between them — it depends on the profile shape. Classes 1–5 and 13–14 were specified by Rz, 6–12 by Ra.
The Russian ▽ classes were dropped by GOST 2789-73 but appear on old drawings all the time. N grades were likewise removed from ISO 1302:2002 and show up on older drawings.
Sampling length (GOST 2789-73)
| Ra, µm | Rz, µm | Length, mm |
|---|---|---|
| ≤ 0.025 | ≤ 0.10 | 0.08 |
| 0.025–0.4 | 0.10–1.6 | 0.25 |
| 0.4–3.2 | 1.6–12.5 | 0.8 |
| 3.2–12.5 | 12.5–50 | 2.5 |
| 12.5–100 | 50–400 | 8 |
Classes follow the GOST 2789-59 / 2789-73 correspondence table from engineering handbooks (two sources cross-checked, value series per GOST 2789-73). N grades and microinches follow ISO 1302 (before the 2002 edition); µm/µin pairs are the conventional rounded ones. Sampling lengths per GOST 2789-73, appendix 1. Machining methods are typical and depend on machine, tool and cutting data.
What Ra your feed and nose radius give — in the surface finish calculator.