Does Anodizing Change Part Dimensions?

The Number to Plan Around

Anodizing converts the aluminum surface into oxide, so the coating is grown out of the part rather than laid onto it. Approximately half the coating thickness builds outward from the original surface and approximately half penetrates into it.

Across a dimension, though, both sides change at once. A shaft gains about the full coating thickness on its diameter. A bore loses about the full coating thickness on its diameter. A plate gains about the full thickness overall. So the practical number is the whole coating thickness, not half of it.

A Worked Example

Take a 0.500 inch bore that has to accept a dowel pin, on a part called out for standard Hard Coat at 0.002 inch nominal. The coating grows on every surface of that bore at once, so the diameter closes by roughly the full 0.002 inch, landing near 0.498. That is well outside a press-fit tolerance and comfortably enough to stop the pin entering at all.

The same bore on a Type II Class 2 part carrying, say, 0.0005 inch of coating closes to about 0.4995. Whether that matters depends entirely on the fit, which is exactly the point: the number is small but it is not zero, and only your tolerance tells you which case you are in.

Three ways out, in the order we see them used: plug the bore so it stays bare, machine it oversize by the expected build, or dimension the drawing after finishing so the machinist undercuts. All three are ordinary. Silence is the only option that reliably produces scrap.

When It Matters and When It Does Not

A typical Type II coating sits in the ten-thousandths of an inch, which most parts absorb without anybody noticing. Close-tolerance bores, press fits and gauged threads are the exceptions.

A standard Type III Hard Coat at around 0.002 inch nominal changes almost any fit on the part. If your drawing has a Hard Coat callout and tight tolerances with no coating allowance stated, that is the most reliable predictor of a first article problem we see.

What to Do About It

The cleanest approach is to dimension after finishing and put a note on the drawing saying so, which gives the machinist and the finisher the same target. The alternative is to dimension before finishing and state the coating thickness clearly.

For features that cannot move at all, mask them. Threads, precision bores and sealing faces can all stay bare, and that is ordinary work rather than a special request. The full numbers by type and class are in the thickness and tolerances guide.

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