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Before Plate NPT Gages

Before Plate NPT Gages

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.

BAD: Ignore the plating thickness. It is a taper thread. It will screw together and with enough pipe goo it may not even leak.

BETTER: 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.

OVERKILL: 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 gageguy@gagecrib.com.

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6701 Old 28th St SE, Suite B
Grand Rapids, MI 49546-6937
Phone: 001-616-954-6581 • Fax: 001-616-954-6583

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