Reference

Surface Roughness Chart

What Ra means, the typical roughness each machining process delivers, um-to-uin conversion, and the ISO N-grade scale, so you can spec a finish your shop can actually hold.

Ra (arithmetic average roughness) is the most common way to specify surface finish. It is the average deviation of the surface profile from its mean line, reported in micrometers (um) or microinches (uin). A smaller Ra is a smoother surface. Every process has a natural roughness band, and asking for a finer finish than the process delivers means a secondary operation and added cost. Spec the roughest Ra that still functions, and only tighten it on surfaces that seal, slide, or need a specific look.

What Ra means

Ra averages the height of the peaks and depth of the valleys along a measured length, so it describes overall texture rather than any single scratch. It does not capture waviness, lay (the direction of the tool marks), or the occasional deep flaw, which is why critical surfaces sometimes also call out Rz (average peak-to-valley). As a rule of thumb, 3.2 um (125 uin) is a standard as-machined finish, 1.6 um (63 uin) is a good general machined finish, 0.8 um (32 uin) needs care, and anything below 0.4 um (16 uin) usually means grinding, lapping, or polishing.

Typical Ra by process

Ranges are typical achievable finishes, not hard limits; the exact value depends on tooling, feeds, material, and setup. Finer finishes within a process are possible with slower finishing passes.

Process	Typical Ra (um)	Typical Ra (uin)	Notes
Milling (as-machined)	1.6-6.3	63-250	~3.2 um / 125 uin is a common default; finishing passes reach 0.8 um
Turning (as-machined)	0.4-6.3	16-250	Fine finishing turns reach 0.4-0.8 um on the right material
Drilling	1.6-6.3	63-250	As-drilled bores; ream or bore for finer or on-size holes
Reaming	0.8-3.2	32-125	Improves hole finish and size over drilling
Grinding	0.1-1.6	4-63	Commonly 0.2-0.8 um; precision surfaces and hardened parts
EDM (wire / sinker)	0.4-3.2	16-125	Finish set by power and passes; matte recast texture
Bead blasting	1.6-6.3	63-250	Uniform matte cosmetic texture, not a precision finish
Lapping	0.025-0.4	1-16	Seal faces, gauge surfaces, optical-grade flats
Polishing	0.05-0.4	2-16	Mirror and cosmetic surfaces; hand or machine

Typical ranges. Bead-blast and EDM textures vary widely with media and settings. Confirm a shop can hold your Ra before release.

Ra unit conversion (um to uin)

One micrometer equals 39.37 microinches. The values below are the standard rounded finishes used on drawings and their ISO N-grade equivalents.

Ra (um)	Ra (uin)	ISO grade
0.05	2	N2
0.1	4	N3
0.2	8	N4
0.4	16	N5
0.8	32	N6
1.6	63	N7
3.2	125	N8
6.3	250	N9
12.5	500	N10

uin values are the conventional rounded equivalents (e.g. 0.8 um = 32 uin, not 31.5).

ISO roughness grades (N1-N12)

The ISO N-grade scale gives each standard Ra value a grade number, from the smoothest N1 to the roughest N12. The scale is logarithmic, roughly doubling each step. Lower numbers are smoother.

N grade	Ra (um)	Ra (uin)
N1	0.025	1
N2	0.05	2
N3	0.1	4
N4	0.2	8
N5	0.4	16
N6	0.8	32
N7	1.6	63
N8	3.2	125
N9	6.3	250
N10	12.5	500
N11	25	1000
N12	50	2000

N6 (0.8 um) is a fine ground finish; N8 (3.2 um) is a standard as-machined finish.

Ra is defined by ISO 4287 and surface-texture callouts by ISO 1302; the N-grade values match those standards. Process ranges reflect common shop practice and vary with tooling, material, and setup. Confirm the finish a shop can hold on your part before release.

On OpenSpindle

Frequently Asked Questions

What does Ra mean in surface finish?

Ra is the arithmetic average roughness: the average deviation of the surface profile from its mean line over a measured length, in micrometers (um) or microinches (uin). A smaller Ra is smoother. It describes overall texture and does not capture waviness, lay, or isolated deep flaws.

What is a standard machined surface finish?

About 3.2 um (125 uin) Ra is a typical as-machined finish that most shops produce without extra effort, equivalent to ISO grade N8. A finer 1.6 um (63 uin) finish is achievable with finishing passes, and anything below roughly 0.4 um usually needs grinding, lapping, or polishing.

How do I convert Ra between um and uin?

One micrometer equals 39.37 microinches, so multiply um by about 39.37 to get uin. Drawings use rounded standard values: 0.8 um is 32 uin, 1.6 um is 63 uin, and 3.2 um is 125 uin. Those rounded numbers are the ones to put on a print.

What are ISO N surface roughness grades?

The N-grade scale (per ISO 1302) maps each standard Ra value to a grade number from N1 (0.025 um, smoothest) to N12 (50 um, roughest). Common ones are N6 (0.8 um), a fine ground finish, and N8 (3.2 um), a standard as-machined finish. Lower numbers are smoother.

Tell us your material, tolerances, and the surface finish you need, and set a budget. We source shops that can hold it and bring back a best-match quote.

Get quotes for machined parts