Metal finishing Q&As since 1989
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Oversize tap for anodizing
We have various products that require anodizing after the part has been tapped. Currently we use a labor intensive process to plug the holes so they don't get anodized. Is there a tap that would cut 4-40 threads slightly oversized so that when they are anodized the screw would thread correctly?
Brent PetersonME - Salt Lake City, Utah, USA
2004
Threads should NEVER be anodized. The "cutting edge" of the thread is turned into pure Aluminum Oxide which is brittle, has no strength , and will crack right off when the mating threads are turned against it.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
2004
While I don't doubt that there are MANY threads for which what Mr. Probert said is correct, Accurate Anodizing has bulk anodized thousands of pounds of threaded products per year and never had any complaints regarding loss of functionality after anodizing. To answer Mr. Peterson's question, they do make oversized taps to allow for added material post-processing, they are marked by the Gage Number (H#) and are measured in 0.5 thousandths increments. According to one of my customers you should probably look into a H3 or H4 for that size of threads.
On a less related note, depending on the size and design of your parts, it may be possible to bulk anodize them at a significant savings to you as well. Although it sounds as though your part has a blind hole in it, which can cause some problems due to liquid and air entrapment, Accurate Anodizing has many methods to try to relieve or remove that issue and still maintain the cost benefit of bulk processing. Feel free to contact me privately through the link below if you are interested in having a sample run.
Compton, California, USA
2004
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