Usually, both ends of a pipe are threaded with external pipe threads. Notice, in all the figures showing pipe threads, that they are V-shaped. The standard 3/4M taper per foot of pipe threads is equal to 1/16" per inch. Therefore, the taper of the threads on each side of the pipe is 1/32" taper per inch. This taper cannot be changed. This produces a tight joint. The angle between sides of the threads is 60°, and several threads on the end of the pipe are perfect threads. The next few have V-bottoms but flat tops, and the last few threads have both flat tops and bottoms. Each size of pipe has a certain number of threads per inch, built into the pipe taps and dies.
Adjustable pipe dies have a reference mark on each die which, when lined up with the corresponding reference mark on the die stock, will give a standard-size thread. You adjust the dies one way or the other from the reference mark to cut a thread with the fit you want.
To cut external threads on iron pipe, first determine its nominal size. Nominal size means the "name size" of the pipe such as 1/8", 3/4", and so on. Except in the sizes below 1", nominal sizes correspond closely to inside diameters. For 1" pipe and larger, measure the inside diameter (IŁ>.) with your rule, to the closest 1/32", and you will have nominal size. For sizes below 1", you can determine nominal size by measuring the outside diameter (O.D.) to the nearest 1/32" and reading the corresponding nominal size in Table 6-4. This method can also be employed for sizes 1" and above in lieu of the I.D. measurement.
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