How Anodizing Affects CNC Part Dimensions
Contents
- What the Coating Actually Does
- Thickness by Type
- Where It Shows Up on a Real Part
- How to Compensate
- Threads, Inserts and Masking
- Raise It Before Cutting, Not After
Type II and Type III anodizing growth rates, how to allow for them in your design and when to raise it with your machinist before cutting.
What the Coating Actually Does
Anodizing converts the surface of the aluminium into an oxide layer — it is not a paint or a plate that sits on top. The coating builds both outward and into the base metal, so the finished part is slightly larger than the machined part on every anodized surface. As a working figure for fits, allow roughly 0.01–0.03 mm per surface depending on the type and thickness specified.
Thickness by Type
Type II clear and black anodizing is typically 5–25 µm and adds a relatively small amount of growth — usually manageable with a normal tolerance. Type III hard anodizing runs 25–75 µm and is the case that catches people out, especially on aluminium 7075 where the process produces a darker, harder coating. Any feature that has to fit another part after coating needs to be considered before the first cut.
Where It Shows Up on a Real Part
Bores get smaller. Shafts get larger. A 30 mm jaw gap closes. A precision pin that was a light push fit before coating may not enter the hole afterwards. Threads can tighten enough to be difficult to assemble. None of these are defects in the machining; they are the predictable result of a coating applied at the wrong allowance.
How to Compensate
There are four normal approaches, and they can be combined: machine the critical feature undersized (or oversized) by the expected growth; mask the surface so it is not coated; finish-machine the feature after coating; or specify the finished dimension and let the process plan absorb the difference. For gripping faces, masking datums and machining tight are the most common combination.
Threads, Inserts and Masking
Threads are usually masked, chased after coating, or protected with a thread relief. Where aluminium threads will be assembled and disassembled repeatedly, a thread insert — Helicoil, Keensert or E-Z Lok — avoids both thread wear and coating build-up in the thread. If a face must stay electrically conductive, say so on the drawing so it is masked; masking also leaves a small uncoated witness mark, which matters on cosmetic parts.
Raise It Before Cutting, Not After
The anodizing allowance has to be in the CAM program, which means it has to be decided at quote stage rather than discovered at final inspection. Put a note on the drawing — for example "machine grip faces 0.05 mm tight for Type III hard anodize" — or simply tell us which interface has to fit after coating. We will confirm the allowance before machining and check the finished feature on the CMM.
Get Engineering Articles in Your Inbox
No marketing. Technical content only — published when we have something worth sharing.
Ready to Quote?
Need a quote for a similar component?
Upload your CAD files for an engineering review and quote within one business day.
Upload CAD Files