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Rework on internal thread of size N 10 UNF as gauge not pass after plating



I have the components having threading of N°10UNF and the threading length is just 2 to 3 threads. Now after plating of Zinc having plating thickness 10 to 15 microns we found that the thread plug gauge 10 * 32 UNF - 2B can't pass.

Kindly advice will it require any oversize tap & of what size
OR it require any other gauge to check after such plating.

Kundan Deore


Your problem should be expected. There are 2 sides to a thread, so it gets twice as much plate as a flat surface. Then the top of the thread is a high current density area, so it will get about 4 times as much plate. Then the 1st thread is a very high current density area, so it will get 5 - 10 times the amount of plate as the last thread.The last thread will have very little plate on it.
If you only have 2-3 threads to hold, you can not get your design strength if you oversize before plate.
I honestly think that you will need to re-tap and then use a heavy oil after taping.Consider using plug masking on future parts and the heavy oil.

James Watts
Navarre, Florida


Adding to what James properly explained, please remember that the flanks of the threads are at an angle, so the pitch diameter actually is reduced with the cosine inverse of that angle (which is over 1) times the plating thickness times 2.

Guillermo Marrufo

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