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

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 = 12.5mm (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)

Surface roughness comparator

Grit to Ra Micron Conversion

Ra Micron (µm) Grit
0.10 Mirror
0.23 400
0.30 320
0.38 240
0.48 220
0.76 180
1.06 150 or Bead Blast
1.32 120
1.8 80
60
3.61 36