Once you have completed the precision machining of your aluminium parts, sending them off for hard anodising can be a complex process and requires some prior planning to avoid any rework, scrap parts or delays.
Allow for Coating Buildup on Dimensions
The aluminium oxide formed by a 50-micron hard anodising process builds into the substrate and forms a layer approximately half the total layer thickness outward from the surface. Therefore, to allow for coating growth on critical dimensions such as bores and shafts, allow for coating growth and machine to final size after coating, or pre-anodise parts to final size with due allowance for coating growth.
Mask Threads, Bores, and Press Fits
Masking of threads, boreholes and press fit parts must be specified. Hard anodising changes the surface hardness of a part, so care must be taken when using threads, boreholes and bearing seats. When specifying parts to be hard anodized it is essential that the processing company understands which features of a part need to be masked to what dimension.
More on Hard Anodising can be found at www.poeton.co.uk/surface-treatments/anodising/hard-anodising/.
Sequence Machining to Suit the Process
Try and rough machined a part before it is anodised and then finish machined after the anodising process has taken place. This will allow you to take into account the change in surface hardness of the part and potentially prevent damage to cutting tools.
Specify Pre-Anodise Surface Finish
The hard anodising process follows the surface of the base material closely, therefore a rough surface prior to anodising will result in a rough coated surface. Surface texture of the base material also affects the coating consistency and should therefore be specified to a maximum Ra value prior to anodising, typically Ra 0.8 or finer for sliding surfaces.
Mark Critical Dimensions Clearly on Drawings
In preparing your drawing for hard anodising ensure all critical bores, surfaces and fits are clearly marked. State which dimensions are to be measured post anodising and clearly indicate any areas which are to be masked. Such information must be clearly stated on the drawing to prevent misunderstanding by the processor.
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