09/03/2026
When anodizing is specified on an engineering drawing, the coating requirement is only part of the equation. Coating thickness can affect dimensions, threads, edges, corners and even what is realistically achievable on certain aluminum alloys.
Understanding those effects before the part reaches the finishing stage can prevent dimensional issues, assembly interference, and unnecessary production questions.
Here are several practical considerations from MIL-PRF-8625F that engineers should keep in mind.
1. Know the coating thickness you're designing around.
For Type II anodizing, MIL-PRF-8625F identifies a required thickness range of 0.00007"–0.0010". A typical target range shown in our reference is 0.0004"–0.0007" (10–18 µm).
For Type III hardcoat anodizing, the specified thickness is 0.002" ±20%, or approximately 0.0016"–0.0024" (41–62 µm).
That difference matters when you're working with close tolerances.
2. Account for dimensional growth before machining is finalized.
An anodic coating doesn't simply sit on top of the original aluminum surface.
When specifying coating thickness—particularly with Type III hardcoat—allowance should be made for dimensional increase.
As a general design consideration, dimensional growth equal to approximately one-half of the applied coating thickness can be expected on each coated surface.
For example, with a 0.004" coating, approximately 0.002" per surface would need to be considered when establishing the pre-coating machining dimension.
If close fits are involved, ignoring anodic buildup can result in interference during assembly.
3. Don't overlook threads.
Anodizing affects both internal and external thread dimensions.
The major and minor diameters are affected by coating growth, and pitch diameter is affected even more significantly.
For a 60° thread, the pitch diameter changes by four times the amount of coating growth.
That means threaded features should be evaluated as part of the coating requirement—not after the parts have already been machined.
4. Sharp edges and corners can create coating problems.
Proper product design is particularly important with Type III hardcoat anodizing.
Because of the way anodic coatings form, voids can develop at sharp edges and corners.
In general, edges and inside corners should be radiused before anodizing.
As coating thickness increases, the recommended radius increases as well:
• 0.001" coating → approximately 1/32" radius
• 0.002" coating → approximately 1/16" radius
• 0.003" coating → approximately 3/32" radius
• 0.004" coating → approximately 1/8" radius
Chamfering alone may not solve the problem if the resulting geometry still creates a sharp edge.
5. Alloy matters, too.
Not every aluminum alloy responds to anodizing in exactly the same way.
One important example is high-silicon die-cast aluminum, such as alloys 360, 380, and 383.
Achieving a 2 mil or greater hardcoat on these materials can be extremely difficult, so alloy selection should be considered when establishing the coating thickness requirement.
6. More hardcoat does not automatically mean more abrasion resistance.
It's easy to assume that increasing coating thickness will continue to improve performance.
MIL-PRF-8625F notes that the abrasion resistance of Type III coatings decreases as coating thickness approaches 3 mils.
In general, abrasion resistance does not continue increasing simply because the coating gets thicker.
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The practical takeaway for engineers:
Before releasing a part for anodizing, evaluate more than the specification callout.
Consider:
Alloy + Anodize Type + Coating Thickness + Finished Dimensions + Threads + Fits + Edges & Corner Radii
When those requirements are established before production, your metal finisher has a much clearer target—and there is far less room for interpretation once the parts are on the shop floor.
At Global Metal Finishing, we encourage those conversations early. Clarifying the finishing requirements before processing can save questions, rework, and dimensional surprises later.
For more information, don’t hesitate and reach out to us by calling at…
📞(800) 407-6371 or (540) 362-1489
Or send us an email at 📧[email protected]
Reference: MIL-PRF-8625F
