Oversized Parts After Anodizing

Question:

I am a manufacturing engineer at an aerospace company. Recently we have had several of our parts with critical diameters go oversize in the anodizing process. We send our parts to a vendor for anodizing. On a recent order we decided to document all of the critical diameters through our process before sending them to the anodizer. The parts came back with oversize dimensions. The anodizer claims that he does not perform any kind of operation that could cause this condition. Have you ever heard of this and, if you have, what was the cause? D.W.

Answer:

Anodizing makes outside diameters larger and inside diameters smaller. If a machined part is to be anodized, you must allow approximately the coating thickness dimension across the total of two outside or two inside surfaces when machining. Simply, the conversion of aluminum to aluminum oxide in the anodizing process adds dimension to the part because aluminum oxide occupies more space than the aluminum it replaces. This can be roughly one-third to one-half of the coating thickness per side anodized depending on the exact anodizing process. This means that a dimensional allowance must be made in the machining to offset the anticipated dimensional change caused by anodizing.