Diameter grows with black anodize?

[Pontiff51]

Hello all. I machined some 7075-T6 aluminum parts and then had them anodized to a MIL spec with a black color. My dimensions were correct to the print, but my customer tells me that the anodize is flaking and the dimensions are .002" over the tolerance limit. Is it possible to "over anodize" aluminum and add .004" to a diameter? Any information would be welcome.
Thanks.

[ihavenofish]

Hello all. I machined some 7075-T6 aluminum parts and then had them anodized to a MIL spec with a black color. My dimensions were correct to the print, but my customer tells me that the anodize is flaking and the dimensions are .002" over the tolerance limit. Is it possible to "over anodize" aluminum and add .004" to a diameter? Any information would be welcome.
Thanks.

anodizing does grow the part thicker. hard anodizing it typically less than .001" in added thickenss, but .004" is entirely possible if done very heavy.

"fixing" the part will probably be a challenge, since the anodozing is basically sapphire so grinding it down isnt easy. stripping it off will disolve some of the original aluminium though, which might be worse.

[Geof]

....my customer tells me that the anodize is flaking and the dimensions are .002" over the tolerance limit....Thanks.

"Anodize is flaking" that is very unusual. Anodizing is not a coating it is a chemical reaction which creates an enhanced thickness of aluminum oxide on the surface of the metal; the anodized coating is actually part of the metal not a separate something that can flake off. As mentioned normally any growth is less than 0.001" but with hard coat anodizing it is fairly easy to have a growth of 0.002", maybe even more.

Have you seen the parts that are flaking and remeasured everything. I am loathe to accept a customer's word without verification.

[wildwestpat]

Hi

As you are discovering it is difficult to achieve a close tolerance on an anodised surface and the full advice given by the anodizing company should be sought prior to finishing the part for them to anodize.

Hard anodising is used in the electronics industry for giving ali heat sinks an insulated coating. The hard anodize will if subject to a LOT of stress part company with the base metal. Is the complaint about part of the surface that is used as a force fit onto some other customer part? a bearing?

If the close tolerance diameter is a bearing surface it would be conventional practice to mask this surface prior to anodising. A light surface treatment either anodize or alichrome the aluminum would protect the raw metal if that is an issue without adding a lot to the critical diameter.

Suggest you work with the anodizing company as the coating is highly dependent on the time / current / temperature in the baths. I recall seeing a MIL spec for various surface treatments but appear to have lost it. However the local shop will be well versed in the growth of their coatings.

Regards

Pat

[Danodizing]

Hello all. I machined some 7075-T6 aluminum parts and then had them anodized to a MIL spec with a black color. My dimensions were correct to the print, but my customer tells me that the anodize is flaking and the dimensions are .002" over the tolerance limit. Is it possible to "over anodize" aluminum and add .004" to a diameter? Any information would be welcome.
Thanks.

There are three types of ano covered under Mil spec, depending on which you requested dermines the exceptable range of thickness for the coating. Type II will range from .0005-.001, type III will range from .002-.004. I'm not fimilar with type I but I believe the range is closer to that of type II.

[Pontiff51]

Thanks all.

[Pontiff51]

My MIL spec was TYPE 2. I'll have to work more in tune with our plater in the future.