An anodic coating does not sit on top of the part like paint. It converts the aluminium surface itself, so part of the coating thickness grows above the original surface while the rest eats into the base metal below it. That split decides whether a shaft ends up oversize or a bore closes up, and it is the number you need before you set a machining dimension.
This calculator takes the finished dimension you need after anodizing and returns the dimension to machine beforehand. It also reports how much base metal the process consumes, which matters when you are working to a tight wall thickness or a shallow thread depth.
Anodizing dimension calculator
Enter the finished dimension you need after anodizing. The calculator returns the dimension to machine before the part goes into the tank.
Selecting a coating type sets both a typical thickness and an outward fraction: 50 percent for Type II, 40 percent for Type III. Your alloy, electrolyte and current density shift that split, so treat the result as a starting point and confirm it on the first article.
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Quick reference: coating thickness against dimensional change
The table assumes 50 percent outward growth for Type II and 40 percent for Type III. At 50 percent the change on a diameter comes out equal to the coating thickness, which is the rule most machinists carry in their head. Hard coat puts more of the layer below the original surface, so the diameter moves by roughly 0.8 times the thickness.
| Coating type | Coating thickness | Change on a diameter | Machine compensation |
|---|---|---|---|
| Type II | 5 micron | 5 micron | Shaft down 5 micron, bore up 5 micron |
| Type II | 10 micron | 10 micron | Shaft down 10 micron, bore up 10 micron |
| Type II | 20 micron | 20 micron | Shaft down 20 micron, bore up 20 micron |
| Type III | 25 micron | 20 micron | Shaft down 20 micron, bore up 20 micron |
| Type III | 50 micron | 40 micron | Shaft down 40 micron, bore up 40 micron |
| Type III | 75 micron | 60 micron | Shaft down 60 micron, bore up 60 micron |
| Type III | 100 micron | 80 micron | Shaft down 80 micron, bore up 80 micron |
Where the outward fraction comes from
Aluminium oxide takes up more volume than the aluminium it replaces, so the coating cannot grow purely inward. In practice the layer divides between outward build-up and inward penetration. Conventional sulphuric acid anodizing sits close to half and half; hard coat runs at low temperature and high current density and pushes more of the layer inward, nearer 60 percent, which is why selecting Type III drops the outward field to 40 percent. Change the alloy, the acid concentration or the bath temperature and the split moves again, so the calculator leaves the field open rather than locking it in.
Two consequences follow from the split. The finished diameter grows by twice the outward portion, because there is coating on both sides of the part. And the substrate loses depth equal to the inward portion, which is what erodes thin walls and shallow features.
Where the 50/50 rule breaks down
- Sharp corners and edges. Current density concentrates at corners, so coating builds thicker there than on a flat face. A sharp external corner also tends to chip in service. A small radius, typically 0.4 mm or larger, evens out both problems.
- High-copper and high-silicon alloys. Cast alloys such as A380 and free-machining grades such as 2011 carry alloying elements that do not convert cleanly, giving softer, more porous coatings and a growth split that drifts from the wrought 6061 norm.
- Hard coat at high thickness. Type III runs at low temperature and high current density. As the layer thickens it resists current flow, so the growth rate falls off and the effective outward fraction changes over the cycle.
- Sealing. Hot water or nickel acetate sealing hydrates the outer layer and swells it slightly. The dimensional effect is small next to the coating thickness itself, but on a 5 micron tolerance it is worth measuring rather than assuming.
Verifying the number on your own parts
Use the calculator to set the drawing, then confirm it on the first article before you release a batch. Coating thickness on production parts is normally checked with an eddy current gauge as described in ASTM B244, which reads non-destructively on aluminium. When you need to see the inward penetration as well as the total thickness, a cross-section under the microscope is the method, covered by ASTM B487. Background on how alloy and process variables interact is available from ASM International.
Record the measured finished dimension against the machined dimension on that first article and feed the difference back into the outward fraction field. After two or three jobs you will have a fraction that matches your actual tank, and the calculator stops being an estimate.
Frequently asked questions
Does anodizing change part dimensions?
Yes. Part of the coating builds above the original surface, so external diameters grow and internal diameters close. Under the usual 50 percent split, a diameter changes by roughly the coating thickness.
How much does hard anodizing add to a shaft?
Work from the specified coating thickness and take about 40 percent of it as build above the original surface. A 50 micron Type III coat puts roughly 20 micron on each side, growing the diameter by about 40 micron in total.
Should bores be machined oversize before anodizing?
Yes, by twice the outward portion of the coating. If you spec 25 micron on a bore that must finish at 20.000 mm, machine it to roughly 20.025 mm and let the coating bring it back down.
Do threaded holes need the same compensation?
Threads are a special case. Coating builds on the flanks and can pull a thread out of tolerance faster than a plain bore, which is why threaded features are often masked or left unthreaded until after coating. See our guide to anodizing masking for threads and holes.
Does sealing change the dimension?
Sealing hydrates the outer layer and swells it slightly. The change is small relative to the coating thickness, but it is measurable, so confirm it on the first article if your tolerance is tighter than about 10 micron.
Why does the coating build thicker on corners?
Current density concentrates at edges and points, driving faster local conversion. A generous radius reduces the effect and also makes the edge far less likely to chip.
Related resources
- Anodizing thickness tolerance impact on CNC parts
- Type II anodize thickness guide
- Hard anodize (Type III) service for CNC aluminium
- MIL-A-8625 Type II anodize for CNC aluminium parts
- Custom aluminium anodizing services
- CNC machining services
Have a part with a coating thickness and a tolerance that have to agree? Send the drawing to our engineering team and we will confirm the compensation before cutting metal.
