Tools / Surface finish
Surface Finish Conversion Chart
Ra, Rz, RMS and ISO N-grade side by side, with the one column most charts leave out: what each band actually costs on aluminum.
| ISO N | Ra um | Ra uin | Rz um | RMS uin | How it is reached | On aluminum |
|---|---|---|---|---|---|---|
| N12 | 50 | 2000 | 200 | 2200 | Rough turning, flame cut | Not typical on a machined aluminum part |
| N11 | 25 | 1000 | 100 | 1100 | Heavy roughing | Rough pass only |
| N10 | 12.5 | 500 | 50 | 550 | Roughing | Rough pass only |
| N9 | 6.3 | 250 | 25 | 275 | Medium milling, turning | Cheap, clearly tool marked |
| N8 | 3.2 | 125 | 12.5 | 138 | Milling, turning | Roughing pass, no finish pass. Cheapest usable |
| N7 | 1.6 | 63 | 6.3 | 69 | Finish milling, turning | The default. A normal finish pass, effectively free |
| N6 | 0.8 | 32 | 3.2 | 35 | Fine finishing, reaming | Slower pass and sharp tooling. Modest cost |
| N5 | 0.4 | 16 | 1.6 | 18 | Grinding, fine boring | Dedicated strategy or a second operation. Significant |
| N4 | 0.2 | 8 | 0.8 | 9 | Fine grinding | Specialist. Expensive on aluminum |
| N3 | 0.1 | 4 | 0.4 | 4.4 | Lapping, honing | Specialist |
| N2 | 0.05 | 2 | 0.2 | 2.2 | Lapping, polishing | Specialist, optical work |
| N1 | 0.025 | 1 | 0.1 | 1.1 | Superfinishing | Specialist, optical work |
N7 (Ra 1.6 um) is highlighted because it is the default a machined aluminum part arrives at without a callout. Rz values are the common engineering approximation, not an exact conversion: the Ra to Rz ratio depends on the profile shape.
Convert a value
Nearest gradeN7
Ra1.6 um
Rz approx6.3 um
Specify it the cheap way
A general note plus named exceptions, rather than a fine finish across the whole part:
SURFACE FINISH Ra 1.6 um UNLESS NOTED. FACE 'A' AND O-RING GROOVE: Ra 0.8 um MAX. BEAD BLAST ALL EXTERNAL SURFACES BEFORE ANODIZING.
Applying Ra 0.8 um to a whole part because two faces need it is one of the most expensive habits in machined part buying.
Questions
What surface finish do I get if I do not specify one?
On aluminum, typically Ra 1.6 um (N7) on milled faces, because that is what a normal finishing pass produces. You will also see tool marks and a cutter path pattern. If that is acceptable, write it down so the shop is not guessing.
Is Rz just Ra multiplied by four?
No, and treating it that way causes arguments. Ra is an average deviation and Rz is an average peak to valley height, so the ratio depends on the shape of the profile. The values in this chart use the common engineering approximation, which is fine for selecting a band but not for converting an inspection result.
Does bead blasting improve Ra?
Usually not. Bead blasting replaces directional tool marks with an even matte texture, which looks more uniform while the measured Ra often goes up slightly. It is a cosmetic treatment, not a precision one.