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Fix I.D. threads tapped into aluminum part

Hi, we are making 6061 T6 aluminum parts with hard black anodize with a 21/2"-24 thd.on the inside of the part about 1/2' long . The thread was cut oversize, with the plating of .002 thk. the part barely accepts the no-go plug gage. I need a coating that can build up on the surface of the threads . Perhaps a dry lube?

Thank you,

Mark D. Fillingham
machine shop - LaVerne, CA, U.S.A.


Unfortunately a couple of typos came through on your posting, Mark, making it a bit difficult to read. Was that a 1/2"-24 UNS thread x 1/2" deep?

There are several problems here. Anodizing is not plating, and when you specify an anodizing thickness of .002", you'll get a build up of only roughly .001" since aluminum is consumed in the anodizing process; this may be why the hole accepts the no-go gauge. Second, some people say you must never anodize a thread. Third, a lubricant won't add thickness like you want. It is possible to strip the hard anodizing from the thread area and plate the threads with electroless nickel, but logistically it may not be easy, and the thread form may be too corrupted by the time the anodizing is stripped and the plating is done. Is there any chance that a helicoil could be fit and would be accepted?

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Good answer Ted, but remember that whatever type of build-up you use on threads, plating or anodize, you must multiply that build-up by a factor of 3-4 to measure how much the pitch diameter of the thread changes. A buildup of .001" on a smooth cylinder will produce a dimensional change of .002". A thread has two more surfaces to contend with.

Tim Hamlett
Tim Hamlett, CEF
- West Palm Beach, Florida, USA

Ed. note: The following letters seem to offer insight into thread allowances for plated parts: 2906, 22409, 12211.

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