The efficiency of any machine depends on the efficiency and the tolerance levels of its most minute parts such as the screws and the nuts. It may appear irrelevant to many. However, you should ask an airplane pilot as to what could happen if one of the screws becomes loose and were to fall off. The chances of the same happening are extremely remote. Each screw that goes into the manufacture of an airplane or any machinery for the matter has to pass through certain quality checks. One of the first quality checks involves the use of the thread gauges for checking whether the tolerance levels are present.
You can safely state that the screw would be in a position to function properly in case it has the requisite tolerance levels. You might have noticed that there would be different kinds of threads for different implements. Naturally, if one implement or appliance has external threads, its compatible implement should have internal ones for the units to fit properly. Therefore, you would require different kinds of thread gauges to check the tolerance levels of each kind of thread.
In addition, you find the pipes having a different kind of thread known as the NPT threads. These threads are different in the sense that you might have to use lubricant or a pipe tape for ensuring there are no leaks. The pipes used for transporting inflammable materials like gasses and inflammable fuels require the dry seal. Therefore, they use the NPTF kind of threads. These threads do not use materials like Teflon for plugging the leaks. Any leak could be disastrous.
You have to use special kinds of thread gauges to check the tolerance levels of the screws and the pipes. The thread ring gauges would suffice for implements that have the external threads. The ‘Go’ and the ‘No Go’ kinds of thread gauges assume great importance under the circumstances. The screws should be able to go through the respective Go ring while not being able to pass through the No Go ring. The industry does this unique test for each screw that rolls out of the factory. No screw can reach the market without passing this test. In case the screw fails the test, you would have to either discard the same or return it for correction.
Similarly, the machines that have internal threads cannot use the ring gauges for checking the tolerance. You would need the services of the thread plug gauges for the purpose. These are simple tools working on same principles as the ring gauges. They have two ends, the Go end, and the No Go end. The plug has to pass through the threads from the Go end. At the same time, it should not do so from the No Go end. This is the test of tolerance for the tools having internal threads.
The tests are mandatory for every screw and nut. Not a single implement would be able to get a fit certificate unless they pass this test.