Before coating pipe threads are a bit difficult to figure out.
Before coating UN series threads have a straight forward calculation
for determining the before plating machine size. But the taper of
pipe threads gives a different slant to the equation. There are
four approaches to solve the problem.
Ignore the plating thickness. It is a taper thread. It will screw
together and with enough pipe goo it may not even leak.
Machine the thread toward the minimum material condition. Something
like: Plus 1 turn/Minus 0 turn. This is the most popular method
because it is the easiest way to prepare a part to accept a coating
without exceeding the finished product parameters. It seem that,
given the broad pipe product tolerance, this will work in most situations.
Best to try a few parts first to assure that this fits in your application.
Calculate the before plate condition and translate that into adjustment
in the number of turns used in gauging. Now we get technical. For
every 0.0001" of coating thickness the point along the center
line of the product where a specific pitch diameter measurement
is taken will move 0.0064" in the maximum material direction.
The formula works this way: 1"/TPI=Pitch; 64xCoating=Movement;
Movement/Pitch=Turns. Using an example of 11-1/2 Threads-Per-Inch
we get the following results:
0.0000" coating thickness = Plus 1 turn/Minus 1 turn
0.0001" coating thickness = Plus 1.0736 turn/Minus 0.9262 turn
0.0010" coating thickness = Plus 1.7360 turn/Minus 0.2640 turn
A quick evaluation of the numbers calculated will show you that
using the better option above will usually work just fine.
ULTIMATE: Buy Before-Coating
Gages. An option that is good for my business. Few gage makers will
even quote this option. The gages are expensive, like twice the
standard gage price. Master gages add more to the cost, are strongly
suggested for future calibration of the working gages, and master
plugs may be required by the gage maker for the ring gages. The
gages are long lead-time to get, like six to eight to ten weeks.
This option may be appropriate if the production quantity is large
or it is difficult to communicate gage requirements to your workforce.
The data provided is accurate to the best of my knowledge. Please
use it at your own risk. Refer comments/corrections to email@example.com.