Machine it at
25.050 mm
A 25 mm bore closes by about 0.050 mm on diameter at 50 micrometres, so cut it 0.050 mm larger before coating.
First pass figure, not a substitute for a trial part. Coating thickness varies across a part, more on edges and less inside deep pockets, so tight fits should be proved on the first article. Tell us the finished dimension and who does the anodising and we will decide the machining allowance with the processor.
The rule underneath
Anodising converts aluminium into aluminium oxide, which takes up more room than the metal it replaces. Roughly half the coating thickness grows outward from the original surface and half eats into it.
| Feature | Change | At 50 micrometres |
|---|---|---|
| Bore diameter | Closes by about one coating thickness | 0.050 mm smaller |
| Shaft diameter | Grows by about one coating thickness | 0.050 mm larger |
| Flat face | Rises by about half a coating thickness | 0.025 mm higher |
| Slot or gap width | Closes by about one coating thickness | 0.050 mm narrower |
| Thread pitch diameter | Closes by about four coating thicknesses | 0.200 mm smaller |
The thread line is the one that catches people. Four times comes from the geometry of a 60 degree thread form, where coating on both flanks moves the pitch diameter much more than it moves a flat surface. It is why a hard anodised M3 hole often will not start a screw.
Three ways to deal with it
- Machine to allow for it. Cut the feature at the pre coating size this tool gives you. Put the finished dimension on the drawing and note that it applies after coating.
- Mask the feature. It stays bare aluminium. Right for ground paths, bearing seats and threads, wrong for surfaces that need the corrosion protection.
- Finish after coating. Ream or hone the bore once the part is anodised. Exact, and it leaves a bare ring where the coating was removed.
Put this on the drawing
- Process and type, for example hard anodise type III
- Thickness range in micrometres
- Whether dimensions are before or after coating
- Masked features listed by number
- Sealed or unsealed, if the part is bonded
Questions
How much does anodising change a dimension?
About half the coating thickness grows outward from each surface. A bore closes by roughly the full coating thickness on diameter, a shaft grows by roughly the same, and a flat face rises by about half. A thread is the exception, where pitch diameter closes by roughly four times the coating thickness.
Why do hard anodised tapped holes reject their screws?
Because of that four times rule. At 50 micrometres of hard anodise a thread loses roughly 0.2 mm on pitch diameter, which is more than the clearance most screw and hole combinations have. Mask the holes, tap after coating, or size the hole for an insert.
Can a bore be machined after anodising instead?
Yes, and it is common on bearing seats and located bores. The consequence is a bare aluminium ring where the coating was cut away, so it is the wrong answer on a part that sees salt or spray unless that surface is sealed another way.
Longer version with the colour and masking detail: anodising drone parts, and the dimensions it quietly changes. Related tool: thread engagement calculator.
Send the part and let us do the allowance
Give us the finished dimensions and the finish you want. We work out the machining allowance with the processor and confirm it on the first article, which comes back measured before the lot runs. Quote within 24 business hours.