Anodizing ruined a perfectly machined part

A few weeks ago, a customer came back to us: Their 6061 aluminum parts were machined exactly to print - but after anodizing, the press-fit no longer worked.

We measured the diameter. +0.05 mm over spec.
 
The issue wasn't machining. It was anodizing.

A common mistake:
Engineers specify anodizing for hardness and appearance, but forget that the oxide layer grows both outward and inward. If you don't allow for that thickness in advance, your tolerances can disappear after finishing.

After seeing this happen more than once, I've realized how often this detail is underestimated in real projects. So here are a few practical reminders from the shop floor:

- Every step — cleaning, etching, anodizing, sealing - can influence final dimensions
- Type I, II, III anodizing all behave differently in thickness and tolerance impact
- 6061 is relatively stable
- 7075 may surprise you with color variation
- 5052 usually forms a thinner coating
- Pre-adjusting critical dimensions before anodizing can save an entire batch

Anodizing doesn't “ruin” parts - But ignoring its dimensional impact can. If you're designing tight-fit components, especially press fits or bearing seats, it's worth planning finishing into your tolerance stack from day one.