Calculate Anodizing Thickness and Dimensional Changes
Estimate how Type II and Type III anodizing affect external dimensions, hole diameters, and precision fits. Calculate finished sizes, reverse-calculate machining allowances, and check dimensional tolerance risks.
Anodizing Thickness Calculator: Inputs
Estimate coating build-up and material consumption for Type II or Type III aluminum anodizing. Compare the predicted finished dimensions with drawing tolerances, then calculate the required pre-anodize machining size.
Coating Thickness Range
The initial 10–25 µm (Type II) and 25–50 µm (Type III) ranges are editable planning examples, not universal process guarantees. Specify your actual minimum and maximum coating thickness.
Machining Dimensions and Drawing Limits
Dimensional Change and Tolerance Check
Forward: Before → After Anodizing
Reverse: Finished → Pre-Anodize
Outside dimensions increase as oxide grows outward on both surfaces.
This is a geometric allowance model, not an anodizing process certification, fit-class calculation, or guarantee of final tolerance. Confirm actual coating growth and the dimensional inspection plan with your anodizing supplier.
Calculation Method and Design Guidance
Per-surface outward growth = anodic coating thickness × outward growth fraction. For an outside diameter, add growth from both opposite faces. For an anodized hole diameter, subtract growth from both interior faces. For a single coated surface, apply the change once.
The calculator uses the minimum and maximum specified film thickness and the pre-anodize machining tolerance to estimate the full range of finished dimensions. Its reverse calculation centers a suggested pre-anodize dimension on your finished drawing nominal; the associated tolerance check shows whether the complete modeled range fits.
- Type II anodizing thickness: Start with a process-specific coating requirement. A common nominal growth estimate is one-third of coating thickness outward and two-thirds into the substrate; actual values vary.
- Type III hardcoat: A common nominal estimate is half outward and half into the substrate. More demanding holes, sliding fits, and precision bores need part-specific verification.
- Anodizing thickness measurement: Check film thickness using a suitable calibrated eddy-current instrument (ASTM B244) or validated cross-section measurement (ASTM B487), as appropriate. Confirm before-and-after sizes separately.
- Masking and finishing: Specify masked holes, threads, datum surfaces, and any post-anodize finishing operations on the drawing. Do not assume every recess receives a uniform coating.
Anodizing Thickness and Dimension FAQs
Does anodizing change dimensions?
Yes. The oxide film forms partly above and partly below the original aluminum surface. External dimensions typically increase while fully coated bores decrease. The amount depends on the coating thickness, growth ratio, pretreatment, and process conditions.
Why is the full coating thickness not added to an external diameter?
Coating thickness includes the oxide extending into the consumed aluminum substrate. Only outward build-up changes the surface position. For a diameter, count outward build-up at both opposite faces.
How should a tight-tolerance anodized bore be specified?
Define the finished hole size and tolerance after anodizing, identify any masking requirements, and agree on coating thickness verification. For critical fits or threads, use a process-qualified dimensional plan instead of relying only on nominal film growth.
Is the Type II or Type III outward growth percentage guaranteed?
No. The default percentages are engineering rules of thumb. For production planning, replace them with values measured on relevant alloys, features, and actual anodizing process conditions.
For related process capabilities and drawing requirements, see aluminum surface finishing services.
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