Surface finish (Ra) reference
N6 is an Ra of 0.8 µm (32 µin) — the everyday machined-and-ground finish, achievable by grinding, turning, milling, reaming, or broaching. This page lists the ISO 1302 preferred Ra grades (N1–N12) in both µm and µin, the processes that typically reach each one, and the ISO 1302 lay-direction symbols. Enter an Ra value to snap it to the nearest grade.
The N-grade series doubles the Ra at each step, so N7 is 1.6 µm and N5 is 0.4 µm. To convert units, Ra µin = Ra µm × 39.37 (0.8 µm × 39.37 ≈ 32 µin). Ra is an average-roughness parameter; a drawing may also call out Rz or a lay symbol alongside it.
| Grade | Ra (µm) | Ra (µin) | Typical processes |
|---|
Lay symbols
| Symbol | Name | Description |
|---|
What process gives a particular surface finish?
Each roughness grade maps to a band of manufacturing processes rather than one. An Ra of 0.8 µm (N6) is reliably reached by grinding and by good turning or milling; pushing below ~0.4 µm (N5) generally needs grinding, honing, or lapping. Casting and rough machining sit at the coarse end (N9–N12, Ra 6.3–50 µm).
The processes shown are typical achievable ranges from industry reference tables, not guarantees — the finish a given machine produces depends on tooling, feeds, and material. Treat the grade as the requirement and the process list as a starting point.
What do the lay symbols mean?
The lay is the dominant direction of the surface pattern left by machining. ISO 1302 defines symbols placed on the surface-texture callout — for example = for a lay parallel to the projection plane, ⊥ for perpendicular, and M for a multidirectional (non-directional) lay. Lay matters where sealing, sliding, or fatigue depends on scratch direction.