Tools / Anodizing
Anodize Growth Calculator
Anodizing is aluminum oxide converted from the part itself, and it takes up more room than the metal it came from. This works out how much.
Bore closes to19.985 mm
Shaft grows to20.015 mm
Flat face gains0.008 mm
How to use the result
You have three ways to handle the growth, and the drawing has to say which one you mean.
- Dimension post-anodizeState that the tolerance applies to the coated part. The shop machines oversize or undersize to land on it. Cleanest when many features are affected.
- Mask the featureThe bore or thread is plugged so no coating reaches it. Costs hand work, and it does not get cheaper in quantity.
- Open the toleranceIf it is a clearance hole, give it room and stop worrying. Free, and right more often than people expect.
Threads are the worst case, because coating builds from both flanks into the same space. A 50 um hardcoat can stop an M4 screw entering at all.
Questions
How much does anodizing close a bore?
Roughly the full coating thickness across the diameter. The oxide 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.
Does the same apply to an outside diameter?
Yes, in the other direction. A shaft grows by about the full coating thickness across the diameter, because each side gains roughly half.
Is the 50/50 split exact?
No, it is the standard engineering approximation. The real split varies with alloy, bath chemistry and coating type, and it is usually close enough to design with. For a critical fit, agree the compensation with the shop and the anodizer rather than relying on a rule of thumb.