How it works
Ra is the average deviation of the profile from its mean line and is the most widely used roughness parameter. Rz is the average of the largest peak-to-valley heights in five sampling lengths, so it reacts more strongly to isolated scratches. There is no exact conversion between them: the Rz/Ra ratio is typically 4 to 7 depending on the process, so the calculator gives a range. When starting from Rz, Ra is estimated with the middle ratio (Rz/5.5).
Formula
Ra = 1/l · ∫|z(x)| dx Rq = √(1/l · ∫z(x)² dx) ≈ 1.25 · Ra Rz ≈ 4…7 · Ra (no exact conversion) 1 µm = 39.37 µin
ISO 1302 N grades encode nominal Ra values (N7 = 1.6 µm, N8 = 3.2 µm); the calculator shows the finest grade the surface still meets. The old triangle symbols (DIN 3141) depended on the series used and are obsolete; specify Ra or Rz on drawings. For non-periodic profiles, ISO 4288 selects the cut-off wavelength from the Ra value. Process ranges are typical values for guidance.
Chart
| N grade | Ra µm | Ra µin | Rz ≈ µm | λc mm |
|---|---|---|---|---|
| N1 | 0.025 | 1 | 0.1–0.2 | 0.25 |
| N2 | 0.050 | 2 | 0.2–0.4 | 0.25 |
| N3 | 0.100 | 4 | 0.4–0.7 | 0.25 |
| N4 | 0.200 | 8 | 0.8–1.4 | 0.80 |
| N5 | 0.400 | 16 | 1.6–2.8 | 0.80 |
| N6 | 0.800 | 31 | 3.2–5.6 | 0.80 |
| N7 | 1.600 | 63 | 6.4–11.2 | 0.80 |
| N8 | 3.200 | 126 | 12.8–22.4 | 2.50 |
| N9 | 6.300 | 248 | 25.2–44.1 | 2.50 |
| N10 | 12.500 | 492 | 50.0–87.5 | 8.00 |
| N11 | 25.000 | 984 | 100.0–175.0 | 8.00 |
| N12 | 50.000 | 1,968 | 200.0–350.0 | 8.00 |
Typical roughness by process
| Process | Ra range µm | N grade |
|---|---|---|
| Sawing | 1.6 – 25.0 | N7 – N11 |
| Flame cutting | 12.5 – 25.0 | N10 – N11 |
| Sand casting | 6.3 – 25.0 | N9 – N11 |
| Forging | 1.6 – 12.5 | N7 – N10 |
| Drilling | 1.6 – 6.3 | N7 – N9 |
| Milling | 0.80 – 6.3 | N6 – N9 |
| Turning | 0.40 – 6.3 | N5 – N9 |
| EDM | 0.80 – 6.3 | N6 – N9 |
| Die casting | 0.80 – 3.2 | N6 – N8 |
| Reaming | 0.40 – 3.2 | N5 – N8 |
| Cold rolling | 0.40 – 3.2 | N5 – N8 |
| Grinding | 0.10 – 1.6 | N3 – N7 |
| Honing | 0.10 – 0.8 | N3 – N6 |
| Lapping | 0.050 – 0.4 | N2 – N5 |
| Polishing | 0.050 – 0.4 | N2 – N5 |
| Superfinishing | 0.025 – 0.2 | N1 – N4 |
Rz range: empirical estimate (4–7 × Ra). Cut-off: ISO 4288, non-periodic profile.
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