How Hard Anodising Affects Aluminium Dimensions
When anodising an aluminium part the anodic coating can change the dimensions of the part. For most decorative anodizing jobs this is of little concern but for a precision component it can be critical. The coating can move a feature outside of its tolerance if not planned for.
How the Oxide Layer Grows
Half of the anodic coating penetrates into the aluminium, and the other half grows outward from the surface of the part. For this reason, when a part is hard anodized to a 50-micron thickness, 50-micron coating of oxide will build out approximately 25-micron on each surface of the part (total 50-micron). In addition, the bores in the part will shrink approximately 25-micron.
Typical Thickness and Net Change
Typical thickness of hard anodizing is between 25 and 75 microns with 50 microns being the most common wear resistant coating. Remember that 50 microns of coating grows inward from the surface as well as outward from the surface, so 50 microns of coating adds 25 microns to each external dimension, and 25 microns to the reduction of each bore.
Pre-Compensating Your Drawing Dimensions
When specifying pre-treatment dimensions for a component, work backwards from the finished tolerance for each dimension. 95 mm. If you are looking for Hard Anodising, see poeton.co.uk/surface-treatments/anodising/hard-anodising.
Features That Need Special Attention
Internal threaded holes, bearing seats, and closely tolerance bores are critical to dimensional accuracy. In hard anodizing these features can become fouled by the build of the coating within the hole. To prevent this all such features are usually masked prior to processing. In some instances where such features are not critical to dimensions, the threads may be chased after anodizing with a thread tap.
Post-Treatment Machining
Where possible features such as threaded holes are to be masked during anodizing, should this not be possible then post treatment grinding or lapping can be used to bring features back to exact dimension. It is essential that this process is included in the manufacturing process plan before work is commenced.
Make a check list for all critical features on your drawing and use them to calculate the build up prior to signature off.
