Surface Finish, Ra, and Coatings: What to Specify and Why
Surface finish is one line on a drawing and one of the easiest to get wrong in both directions: too vague and the shop guesses, too tight and you pay for passes nobody needed. Here is what the numbers mean and where they actually matter.
What Ra is actually measuring
Ra is the arithmetic average roughness of a surface, in micrometres, measured across a sampling length. A lower number is a smoother surface, and smoother is slower and more expensive to produce, not automatically better. A cosmetic panel and a hydraulic seal face can have wildly different Ra requirements even though both look “smooth” to the eye.
The mistake worth avoiding is treating Ra as a proxy for quality. A rough surface can be perfectly functional, and a mirror finish on a feature nobody touches is a cost with no purpose behind it.
What a process gives you without asking
Every machining operation leaves a characteristic finish. Knowing the typical range for free lets you decide whether you need to specify anything at all.
| Process | Typical Ra | Notes |
|---|---|---|
| Rough milling | 6.3 to 12.5 µm | Stock removal. Not a finish you would specify on a visible face |
| Finish milling | 1.6 to 3.2 µm | The default as machined finish on most parts |
| Fine finish milling | 0.8 to 1.6 µm | Extra passes at slow feed. Specify only where it matters |
| Turning | 1.6 to 3.2 µm | Comparable to finish milling on a cylindrical face |
| Fine turning | 0.4 to 0.8 µm | Sharp tooling and light cuts |
| Reaming | 0.8 to 1.6 µm | The normal way to hold a bore at plus or minus 0.02 mm |
| Boring | 0.8 to 3.2 µm | Used where a reamer of that size is not practical |
| Grinding | 0.2 to 0.8 µm | A separate operation and a separate cost line |
| Bead blast | 1.6 to 3.2 µm | Cosmetic and uniform. Changes appearance more than measured roughness |
If your drawing says nothing about finish, most shops quote as machined, which lands in the finish milling or turning range above. That is a reasonable default for the majority of features on most parts.
Where a tighter finish earns its cost
A finer Ra is worth specifying on:
- Sealing faces, where a gasket or O-ring needs a consistent surface to seal against
- Bearing bores and shafts, where roughness affects fit and wear
- Sliding surfaces, where friction and wear depend on the finish
- Anything explicitly cosmetic and visible in the final assembly
Everywhere else, as machined is the cheaper and equally functional choice. Calling out Ra 0.8 across a whole part because one bore needs it is the same mistake as tolerancing every dimension tight because one feature matters, covered in the tolerances guide.
Coatings and platings: what they add, in real numbers
A coating is not just a colour or a corrosion barrier. It is a dimensional change, and on a toleranced feature that change has to be planned for.
| Finish | Typical thickness | What it changes |
|---|---|---|
| Anodise type II | 5 to 25 µm | Roughly half grows outward, so tight bores need masking or post machining |
| Hard anodise type III | 25 to 50 µm | Significant dimensional growth. Account for it on the drawing |
| Passivation | No measurable build | Chemical treatment for stainless. Removes free iron, does not coat |
| Zinc plating | 5 to 15 µm | Corrosion protection on steel. Masking needed on threads with a tight fit |
| Powder coat | 60 to 120 µm | Thick. Never apply it to a mating face without masking |
| Black oxide | 1 to 2 µm | Minimal build. Appearance and mild corrosion resistance |
Powder coat is the one buyers most often forget to mask. At 60 to 120 micrometres it can turn a free-running thread into one that has to be chased, or a locating pin into one that no longer fits its hole.
Anodising, specifically
Anodising is the most commonly specified coating on aluminium and the one most likely to cause a dimensional surprise, because roughly half its thickness grows outward from the original surface rather than consuming it. On a part with no toleranced features this is irrelevant. On a bore that has to accept a bearing, or a shaft that has to slide, it is the difference between a part that fits and one that has to be reworked.
Two ways to handle it: mask the feature before anodising, or size the feature to land in tolerance after the coating grows. Say which one you want on the drawing. Leaving it unstated means the shop decides, and the shop’s default is usually to mask nothing unless told to.
Passivation is not a coating, and that confuses people
Passivation is a chemical treatment for stainless steel that removes free iron from the surface to improve corrosion resistance. It adds no measurable thickness and changes no dimension. If your drawing calls for passivation and a Ra value in the same breath as an anodise callout, be clear that the two are not related the way you might expect: passivation is closer to a cleaning step than a finish.
Specifying it so a shop can quote it, not guess it
A finish callout that gets a fast quote states three things: the Ra value if one is needed, which faces it applies to, and any coating or plating by name and class. “As machined, no coating” is a complete, quotable instruction. “Nice finish” is not.
The drawing checklist covers this alongside every other note a shop needs before it can quote without a question. For the roughness parameter itself, Ra as defined by ISO 4287 is the underlying standard behind every number in the tables above.
Once your drawing states the finish plainly, send it here and up to three shops in Vietnam will quote it without a clarifying email.
Frequently asked questions
What Ra should I specify for a part with no cosmetic requirement?
For most machined parts, leaving the surface as machined and stating no requirement is enough, since finish milling already gives a Ra of roughly 1.6 to 3.2 micrometres without any extra passes. Specify a tighter Ra only where a mating or sealing surface actually needs it.
Does anodising change the size of the part?
Yes. Standard anodise adds a measurable layer, roughly half of which grows outward from the original surface, and hard anodise adds more. On a toleranced bore or a press fit, that growth has to be accounted for on the drawing or masked off before coating.
Is bead blasting a surface finish or a cosmetic step?
Both, but it is closer to cosmetic. It changes the visual appearance to a uniform matte texture more than it changes the measured roughness number, and it does not add meaningful dimensional build the way a coating does.
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