Guides / Finishing
The Coating Is Not Free, It Takes Up Space
September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked
An anodized layer is not paint sitting on the surface. It is aluminum oxide converted from the metal itself, and it occupies more volume than the aluminum it came from. That extra volume has to go somewhere.
The 50/50 rule
Roughly half the coating thickness grows into the part and half grows outward from the original surface. It is an approximation, not a law, but it is close enough to design with.
What that means in practice, for a coating of thickness t:
| Feature | Change |
|---|---|
| Outside diameter of a shaft | Grows by about t |
| Inside diameter of a bore | Shrinks by about t |
| A flat face | Grows by about t/2 |
| Slot width | Shrinks by about t |
| Overall length across two coated ends | Grows by about t |
Put numbers on it. A Type II coating at 20 um closes a 20 mm H7 bore by about 0.020 mm. The H7 tolerance band at that size is about 0.021 mm. The coating alone consumes the entire tolerance.
A Type III hardcoat at 50 um closes the same bore by 0.050 mm, which is more than twice the tolerance band. The part is not marginal, it is scrap.
Threads are worse than bores
A thread has flanks on both sides of every groove, so coating builds from two directions into the same space. A 50 um hardcoat can make an M4 screw refuse to start. Threads in aluminum covers the rest of what goes wrong with tapped holes.
Standard practice is one of three things:
- Mask the tapped holes.
- Tap after anodizing.
- Tap oversize deliberately, which weakens the joint and is rarely the right answer.
For most parts, masking is simplest. Say so on the drawing.
The three ways to handle it
Dimension post-anodize. Write that the tolerance applies to the finished, coated part. The shop machines the feature oversize or undersize by the right amount and the part lands in tolerance after coating. This is the cleanest approach when a lot of features are affected.
ALL DIMENSIONS APPLY AFTER ANODIZING.
Mask the feature. The bore or thread is plugged or taped so no coating reaches it. The feature stays exactly as machined. Bare aluminum in a masked bore will oxidise naturally, which is normally fine inside an assembly.
MASK BORE O20 H7 AND ALL TAPPED HOLES. ALL OTHER SURFACES TYPE II, 15 um NOMINAL.
Open the tolerance. If a hole is a clearance hole, give it enough room that 25 um either way does not matter and stop worrying about it. This is free, and it is the same logic as not over-tolerancing in the first place.
Specify the thickness, not just the finish
A drawing that says “black anodize” has not specified the growth, because Type II covers 5 to 25 um. That is a 20 um range of uncertainty on every coated surface, which is wider than a lot of fits.
Name a nominal and let the anodizer work to it:
ANODIZE PER MIL-A-8625 TYPE II CLASS 2, BLACK, 15 um NOMINAL (12-18 um).
Now the shop knows how much to leave and the anodizer knows what to hit. That single line prevents more anodizing rework than any other thing you can write on a print.
One last thing to tell the shop
Say the part is being anodized before it is machined, not after the quote comes back. The machinist needs to know while they are still choosing how to cut the feature, because the compensation goes into the program. Finding out afterwards means re-machining, and on a part that is already coated it usually means starting again.
Sources
Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.
Frequently asked questions
How much does a bore shrink when anodized?
Roughly the full coating thickness across the diameter. The coating grows about half its thickness outward from each wall, and a bore has two walls facing each other, so a 20 um Type II coating closes the bore by about 20 um. A 50 um hardcoat closes it by about 50 um.
Do I dimension before or after anodizing?
State which one you mean. Either is workable and shops handle both. What causes scrap is a drawing that gives a tight tolerance and says nothing, because the shop machines to the number, the anodizer adds coating, and the part is now out of tolerance through nobody's fault.
Is masking expensive?
It is hand work, so it costs more per part and it does not get cheaper in quantity the way machining does. On a handful of features it is cheap insurance. If you find yourself masking a dozen features per part, it is usually cheaper to dimension post-anodize instead.
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