Surface Finish (Ra) Guide
A print calling out "32 Ra" doesn't mean much until you know what that actually looks and feels like. Here's the range you'll run into most.
| Ra (μin) | Ra (μm) | Look & feel | Typical use |
|---|---|---|---|
| 125 | 3.2 | Visible tool marks, rough to the touch | As-cast, as-forged, or rough-milled non-mating surfaces |
| 63 | 1.6 | Visible machining marks, slightly rough | General rough machining, non-critical surfaces |
| 32 | 0.8 | Smooth to the touch, faint marks visible up close | Standard "commercial" finish for most general machining |
| 16 | 0.4 | Smooth, marks hard to see without magnification | Close-fitting parts, moderate-precision bearing surfaces |
| 8 | 0.2 | Fine, satiny finish | Precision fits, static seal surfaces |
| 4 | 0.1 | Fine ground or lapped, near-reflective | Precision bearing races, gauge surfaces |
| 2 and below | 0.05 and below | Mirror or optical finish | Optical components, superfinished bearing surfaces, seal running surfaces |
What Ra actually measures
Ra (roughness average) is the average distance between a surface's peaks and valleys and the mean line through them, measured over a sampling length — it's a statistical average, not a peak-to-valley number, which means two surfaces with the same Ra can still feel or look somewhat different depending on whether the roughness comes from many small, even marks or fewer, deeper ones.
Getting a finer finish costs more, every time
Moving from a standard 32 Ra finish down to 8 or 4 Ra usually means slower feeds, lighter finishing passes, a sharper or higher-quality tool, and sometimes a completely separate finishing operation (grinding, lapping, polishing) after the initial machining is done. Specifying a finer finish than a part actually needs is one of the more common ways to quietly add cost and cycle time without adding real function — call out the finish the part's job actually requires, not a "safer" number.
Reading a finish off the machine, roughly
Experienced machinists can often judge a finish within a factor of two or so by eye and by running a fingernail across the surface, but "roughly right by feel" isn't the same as a certified number on an inspection report — anything with a Ra callout on the print that actually matters for form, fit, or function should be checked with a profilometer, not eyeballed.