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. Enter an Ra (or pick an N grade) to see which processes give it on the classic roughness-by-process chart, what a profile at that Ra looks like, the feed and nose radius that give it in turning (and the stepover with a ball nose), and how it is written on a drawing, with the lay symbols.
Surface texture is now ISO 21920-1:2021 (indication and lay symbols) and ISO 21920-2:2021 (parameters, including Ra), which replaced ISO 1302:2002 and ISO 4287:1997 in December 2021; ISO 21920-1 also makes the max rule the default acceptance rule, in place of the 16 % rule. The N grades are legacy notation from the withdrawn ISO 1302 editions: still found on older drawings, but a current ISO callout states Ra in µm.
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.
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- Ra
- Ra in µin
- 31µin
How it’s calculated
Ra is the average height of the stylus trace above and below its mean line (ISO 21920-2:2021). The process chart is the classic chart of roughness by production method (ASME B46.1, as reproduced in Machinery's Handbook): the dark bar is the usual result, the light bar what the process reaches less often, with care or special conditions. The profiles are typical shapes scaled to an exact Ra, not measurements. Each N grade doubles the Ra; the N numbers are legacy notation, and a current callout states Ra in µm (or µin on inch drawings). On the drawing the symbol stands on the surface, or on an extension line or leader from it, and is read from the bottom or the right of the sheet.
Ra grades: N grade, µm and µin
| N grade (legacy) | Ra (µm) | Ra (µin) | Typical processes |
|---|
Lay symbols
| Symbol | Name | Description |
|---|
Turning: feed and nose radius
Each revolution the insert's round nose moves on by the feed f, and leaves a scallop between one pass and the next. A bigger nose radius or a finer feed makes the scallops shallower.
Insert code 08
- Theoretical Ra ≈ f²/(32·rε)
- Expect to measure (×1.2–2, stable cut, steel)
- 1.88 – 3.13µm
- Rt ≈ f²/(8·rε)
- Exact scallop height
- 6.27µm
- Ra in µin
- 62µin
- Feed for your Ra 0.8 µm at this rε (theory)
Milling: ball-nose stepover and face-mill wipers
A ball nose leaves cusps between passes exactly as a turning nose does between revolutions, with the stepover ae in place of the feed. A face mill's finish comes from its wiper flat, which must span the feed per revolution.
Ball nose on a flat surface
Ø10.00 tool
- Cusp height h
- Theoretical Ra ≈ ae²/(32R)
- 1.56µm
- Expect to measure (×1.2–2)
- 1.88 – 3.13µm
Face mill with a wiper insert
From the insert data
One wiper covers the surface each revolution.
- Feed per revolution fn = fz · z
- Wipers needed, equally spaced
- 1
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 21920-1 (formerly 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.