https://roymech.co/Useful_Tables/Surface_Texture/Sur_Fin_Values.html
ANSI B46.1 is used in USA (measurement units μinch(micro.inch) )
ISO 1302:1992 which uses N numbers…These are not included in BS EN ISO 1302:2002
The table below provide equivalent values for…
the Average Roughness R a , the USA R a Values, the approximate R t values, the N numbers, the obsolete triangular representations,
and some manufacturing finishes. There is no mathematical relationship between R a and R t The R t values shown in the table below are very approximate and should only be used for guidance…
For notes describing the meanings of R a, R t etc. reference links below and Surface Finish
Notes:
μm = 10 -6 metre
μinch = 10 -6 inch
| R a-μm - | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | 6.3 | 12.5 | 25 | 50 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| R a- μinch | 1 | 2 | 4 | 8 | 16 | 32 | 63 | 125 | 250 | 500 | 1000 | 2000 |
| R t (R y ) -μm | 0,25 | 0,5 | 1 | 2 | 4 | 8 | 16 | 32 | ||||
| N-Grade | N1 | N2 | N3 | N4 | N5 | N6 | N7 | N8 | N9 | N10 | N11 | N12 |
| Triangular | ||||||||||||
| Indication |  |
 |
 |
 |
||||||||
| Finish | Lapping/Superfinishing | Ground Finishes | Smooth Turned | Medium Turned | Rough Machined |
Rules of Thumb for surface finish equivalent of N grade and Ra values
- Rough turned with visible toolmarks…N10 = 12.5 μm (micro.m)
- Smooth machined surface…N8 = 3.2 μm (micro.m)
- Static mating surfaces (or datums)… N7 = 1.6 μm(micro.m)
- Bearing Surfaces… N6 = 0.8 μm (micro.m)
- Fine lapped surfaces… N1 = 0.025 μm (micro.m)
[Please visit the late Roy Beardmore site for additional details. Roy passed away, but we honor him by helping to promote his work building this excellent engineering reference site.]