Last month a quote package crossed my desk for a 6061-T6 mounting bracket — simple geometry, four bolt holes, one O-ring groove. Nothing exotic. But the drawing carried one note that nearly doubled the price: “Surface finish Ra 0.2, all surfaces.” The only face that touches anything is the one with the seal. Every other surface is either inside a pocket, hidden behind a cover, or bolted flat against another bracket. That blanket callout would have added hours of polishing work on faces no human will ever see.
Surface roughness is the quietest cost driver in CNC machining. Tolerances at least look intimidating on a drawing — a wall of ±0.01 makes everyone pause. Ra values slide by unnoticed, usually copied from the last project’s title block. Here is how I think about them, with the price tags attached.
What Ra actually measures
Ra, defined in ISO 4287, is the average absolute deviation of the surface profile from its mean line, in microns. It is a deliberately blunt number: a surface covered in shallow random scratches and one with evenly spaced tool marks can share the same Ra while feeling completely different. That is also why Ra gets confused with Rz, which measures the height from the deepest valley to the highest peak over a sampling length. On milled surfaces Rz tends to land four to seven times higher than Ra, so “Rz 6.3” and “Ra 6.3” are nowhere near the same surface. I once saw a buyer accept a supplier’s “Ra 0.8” that turned out to be Rz 0.8 — a much rougher part that passed incoming inspection because nobody checked which symbol was on the drawing.
What each step costs
Start with the baseline. A well-maintained 3-axis mill running fresh carbide at sensible feeds leaves Ra 1.6 to 3.2 µm as-machined. A careful finishing pass with a light stepover gets you to Ra 0.8 on external faces without much drama. So, working upward:
Ra 3.2 is the default. Free, or as close to free as machining gets. If a face is not sealing, sliding, or being looked at, this is the number.
Ra 1.6 costs almost nothing extra when the shop expects it — it is mostly about tool condition and feed rate. On a quote I usually fold it into the finishing pass we already planned.
Ra 0.8 is where the first real money appears: slower feeds, dedicated finishing tools, and profilometer checks on the first article instead of “looks fine.” Expect 10 to 25 percent added on the affected features. Static sealing faces usually live here, and that is money well spent — an O-ring leaking because someone wanted to save forty seconds of finishing is a worse trade.
Ra 0.4 means abrasive work after machining: sanding, tumbling, or a secondary grinding op on flat faces. Internal corners and pocket floors never quite get there, because the tool-mark pattern follows the cutter. Budget 30 to 60 percent more on the surfaces in question, and expect the lead-time question to come up.
Ra 0.2 and below is polishing and sometimes lapping, much of it by hand. Price depends entirely on geometry — an open flat face is manageable; a deep ribbed cavity is where quotes double. Two things people miss here. Polishing removes stock, so it drags your tolerance stack with it. And hand-polished surfaces vary from part to part in a way a machined finish does not. If a drawing asks for Ra 0.2 with ±0.01mm on the same face, the two specs fight each other, and the shop quietly resolves the conflict by loosening one of them.
Where the smoothness earns its keep
Four places, in my experience. Sealing faces — static seals are generally fine at Ra 0.8 to 1.6, while dynamic seals running against a shaft want closer to 0.4. Fatigue-loaded parts, where surface valleys act as crack starters; brackets with real load paths live here. Bearing seats and sliding contact, where roughness eats your clearances and eventually the bearing. And anything cosmetic that sits behind clear anodize — enclosure faces especially.
Everything else? Let it go. Weight-relief pockets, clearance holes, the back face of a flange nobody sees. I have seen Ra 0.4 called out inside a cable channel that was never in contact with anything but air.
A note on coatings
Type II anodize will not hide machining marks — if anything it makes the tool pattern more visible under raking light. If the drawing says “clear anodize, Ra 0.4” on a cosmetic face, the honest paths are bead blasting first for a uniform matte look, or accepting the marks and stating that up front. Settling this before the first batch ships beats settling it after.
How to write the callout
Put the finish symbol only on faces that earn it, indicated the ISO 1302 way, and leave a general note in the title block — usually “Ra 3.2 unless otherwise specified.” Then do the thing most people skip: ask the shop which of your critical faces are actually reachable with a finishing tool. A face a small end mill can touch polishes cheaply. The inside of a Ø6mm pocket 40mm deep does not, and no amount of note-writing changes that.
And if your drawing carries a blanket smooth-finish callout inherited from an older project, ask where it came from. In my bracket story the answer turned out to be “the previous supplier’s template.” The revised drawing went out with Ra 3.2 general and Ra 0.8 on the seal face, and the price came back about 35 percent lower. Same part, same function.
Finish and tolerance interact more than most people expect — our earlier piece on how tight tolerances really need to be covers the other half of that trade-off. The materials page explains how alloy choice changes the achievable finish, and our capabilities page lists the finishing operations we run in-house.