If you are specifying anodised aluminium parts then it is best to know how the anodising coats to prevent problems with the tolerance of the finished part.
The 50/50 Growth Rule
Anodising grows roughly 50/50 outward from the surface and into the substrate, therefore a 20 µm coating adds 10 µm to the outside dimension of a part, and also reduces the bore depth by 10 µm.
Sulphuric Versus Hard Anodising
Typical total thickness for sulfuric anodizing processes are from 5 to 25 µm while hard anodizing processes have thicknesses of from 25 to 100 µm. So as an example a hard anodized bore of 50 µm total thickness will increase in diameter by approximately 25 µm or 0.05mm per face.
Pre-Machining Bores and Shafts
For close tolerance parts, the part should be machined to an allowance prior to coating. Open up the bores slightly and reduce the shafts to a slightly smaller diameter than the allowance to allow for the growth on each face.
A useful reference for Anodising is poeton.co.uk/surface-treatments/anodising/.
Features That Need Masking
For threaded holes, even a 15 micron coating can seal the thread so it is advisable to mask off such features or alternatively have them tapped after anodizing. Other features that need to be masked are press fits, keyways and sharp internal radii as coating tends to wear thin at such sharp radii and can affect fit and corrosion resistance.
Mark Your Drawings Clearly
It is also important to note on your drawings the type of anodising, the finish class and the thickness range. If pre-machine allowance has been applied to certain dimensions it is also helpful to note this on the drawings so that machinists and surface treatment suppliers are aware of the parts tolerance prior to processing.
Getting the numbers right the first time costs nothing. Fixing parts that go out of tolerance after anodizing can cost a lot.
