ANPT plug gages are used for gaging internal ANPT threads. The process involves screwing up the plug gage by hand into the internal thread of the pipe or product at hand. In comparison to other gages of the ANPT thread family, the ANPT L3 plug gage is used to check the threads that will be engaged once the product is tightened with a wrench for practical application.
Besides the ANPT L3 plug gage, there are:
- ANPT L1 thread plug gage which is used to check the engagement of the thread elements when the product will be tightened by hand, and
- ANPT 6-step plain plug that is used for checking the minor diameter of the female fitting.
More On ANPT L3 Plug Gage
The process for gaging the internal thread of a product involves not just ANPT L3 plug gage, but all three gages mentioned above. First off, the thread is gaged with ANPT L3 plug gage, also called the limit-type L1 taper thread plug gage. The gaging notch that most nearly represents the size of the thread is noted.
Although the ANPT threads are similar to NPT threads, the difference in the application for aeronautical purposes dictates that they must be controlled very carefully for taper, thread form, and diameter. This is because up until the length of 2-inch size, the ANPT and NPT threads are almost identical and have a requirement of an L1 gage.
From the primary L1 threads that are measured with the L1 gage, the threads thereafter are referred to as the “L3 length” which is measured by the ANPT L3 plug gage. These are the threads that will be engaged when the threads of the product are tightened with a wrench. ANPT L3 plug gage is also a limit type gage whose total length equals L1 + L3.
But despite this, the gage only has four threads at the small end. This is why, for a thread to be accepted on the L3 gage, the position of the gaging notch must coincide with half turn of the position previously noted on the L1 gage. The ANPT threads have a distinctive design to make a true seal that eliminates the need for a sealant. They make a true seal by crushing in on themselves. The ANPT L1 and ANPT L3 threads are needed for measurements of the female threads.
Together, L1 and ANPT L3 plug gages measure the:
- Pitch diameter, and
- Major diameter
For measuring the minor diameter of the internal thread, the amount of the truncation is noted. Since the pitch diameter and truncation vary with limits, the minor diameter will also. The three-pitch diameter gaging positions are:
These pitch diameters necessitate 3 pairs of maximum and minimum truncation steps that amount to a total of 6 positions.
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