Achieving final dimensional accuracy on anodized aluminum parts requires precise pre-machining calculations to account for the growth of the anodic layer. Unlike a simple coating, anodizing converts the aluminum surface into aluminum oxide, which occupies more volume than the consumed base metal. This results in dimensional changes that must be factored into the initial machining tolerances.

For standard (Type II) anodizing, the oxide layer’s growth is approximately one-third of the total coating thickness, with the other two-thirds penetrating the material. For hard anodizing (Type III), the process is more aggressive, resulting in a 50/50 split between surface growth and material penetration. Therefore, for a 0.002″ (50 µm) hard coat, the surface will grow by 0.001″ (25 µm). Consequently, an external feature will become larger, and an internal feature like a hole will become smaller by twice the growth value (once per side). Proper dimensional control starts at the design stage, where engineers must specify pre-anodize dimensions to meet the post-process final tolerances, a key step in any aluminum finishing workflow.

Pros and Cons of Hard Anodizing Dimensional Impact

Pros:

• Creates an extremely hard, wear-resistant surface.
• The significant thickness can salvage parts that are slightly undersized.
• Offers superior corrosion resistance compared to regular anodizing.

Cons:

• The substantial dimensional change requires very careful pre-machining allowance calculations.
• Can increase surface roughness and negatively impact sharp corners.
• May reveal subsurface material defects not visible prior to the process.

How We Do It Our process begins by confirming the specified anodizing type and required coating thickness. We then apply the correct growth ratio (50% for hard coat, 33% for regular) to calculate the necessary anodizing allowance and adjust the pre-treatment machining dimensions accordingly.

Expected Outcome By implementing this precise dimensional management, the final part will meet all critical tolerance requirements after anodizing, ensuring proper fit and function in its assembly.