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

ISO 1302:1992 which uses N numbers..These are not included in 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 = 12.5mm (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)