Surface roughness chart: Ra values and N grades
Ra is the average roughness of a surface in micrometres. Each N grade halves the roughness of the one before.
| Grade | Ra (µm) | Typical process |
|---|---|---|
| N12 | 50 | Flame cutting, rough sawing |
| N11 | 25 | Sawing, rough turning |
| N10 | 12.5 | Rough milling and turning, drilling |
| N9 | 6.3 | Standard milling and turning, drilling |
| N8 | 3.2 | Standard CNC machined finish (most common default) |
| N7 | 1.6 | Fine milling and turning, reaming |
| N6 | 0.8 | Very fine turning, grinding, honing (bearing seats, sealing faces) |
| N5 | 0.4 | Grinding, honing, polishing |
| N4 | 0.2 | Fine grinding, lapping, superfinishing |
| N3 | 0.1 | Lapping, polishing |
Which value to specify
Ra 3.2 is the normal machined finish and a good default in the title block. Ask for Ra 1.6 on mating and sliding faces, and Ra 0.8 or finer only on bearing seats and seal faces. Read surface finish and what it costs.
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