Letter 41095

Protecting threaded holes during anodizing [Minnesota] 

June 12, 2006

I have multiple parts that have a threaded hole that are black anodized. The problem is that the threads are becoming oversized after they have been tapped. Could this be a result of the anodizing process.

Brian Smith
Contract Metal stamper - Mankato, MN, USA

NO. It is a result of a very agressive etch being used in the preprocess steps. Silicon pull plugs will protect the threads, but to really seal the threads, they need to be slightly oversize which will leave a tiny ring around the hole that is not anodized. If the anodize is thin, you could tap the hole, with a water alcohol lubricant, after the anodize. Helicoils or PEM fasteners are possible alternatives.

James Watts
- FL

Yes, it could be. Anodizing a part almost always results in a change in dimensions - between the etching that decreases the size of the part and the build-up of the anodize that increases it the part is rarely exactly the same size afterward. In my experience - and with the parts I work with - it is usually smaller (making any holes slightly larger).

Get the parts as clean as possible at your facility, and then tell your anodizer to give them a much lighter etch - it might be exactly what you need. The more difficult answer is to make the parts slightly larger, so the anodizing process drops them into specification, but that is a heck of a lot more complicated and likely to fail.

Good luck!

Jim Gorsich Accurate Anodizing Inc. Compton, CA, USA

Absolutely....

If the Anodizer you are using is etching the parts to long or at to high of a temprature the etch will most certainly remove enough material to oversize a hole on a part..

You should speak with your plater about the necessity of the tight tolerance of the holes and arrange to either plug the holes, mask, or reduce/eliminate the etch time completely..

Ryan Cook Team Metal Finishing , Inc. Toccoa, Georgia

On first glance the first two answers might look confusing and appear to contradict each other - so, to clear up the confusion... When I said "yes" I was interpreting "anodizing" to mean "I sent this to a shop to get anodized" and inherently include the pre-process steps. James Watts' answer is completely correct - the material loss is due to the etch, not the actual anodizing step - but the problem would be occuring during the anodizing process as a whole.

Sorry for the confusion.

Good luck!

Jim Gorsich Accurate Anodizing Inc. Compton, CA, USA

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