Surface Finish Chart

Surface Texture Equivalents

Disclaimer: The information on this page has not been checked by an independent person.   Use this information at your own risk.

ANSI B46.1 is used in USA (measurement units minch(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

μm = 10 -6 metre
μinch = 10 -6 inch

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
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
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 = 12.5mm (micro.m)

  • Smooth machined surface....N8 = 3.2mm (micro.m)

  • Static mating surfaces (or datums).... N7 = 1.6mm(micro.m)

  • Bearing Surfaces.... N6 = 0.8mm (micro.m)

  • Fine lapped surfaces.... N1 = 0.025mm (micro.m)

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