Any kind of machinery would be incomplete without the use of the screw and the nut for tightening. However, one should also note that the simple screws one uses in the machines have to pass through difficult quality checks for determining their utility. The screws should have a tolerance factor. You need special kind of instruments to measure the pitch and the tolerance factor. Without determining the tolerance factor, no screw can move out of the factory into the market. This shows the importance of the screw. Therefore, the industry uses instruments known as the thread gauges for the determination of the tolerance quality of the threaded instruments.
Before coming to the issue of the thread gauges, one should also have an idea about the different kinds of threads in use in the industry. The most common threads would be the external threading and the internal threading. Both are complementary to each other. You cannot have a situation where you have an implement with external threads trying to fit into another instrument without threads. They should have the complementary internal threads to ensure a perfect fit.
The screws usually have the external threads. You can see some pipes and rods as well that have external threads. At the same time, you have the nuts having the internal threads. Certain pipes and joints have the internal threads as well. Just having threads would not be enough. These instruments should have the tolerance factor as well. In order to determine the tolerance factor of instruments having external threads, you make use of the thread ring gauges. At the same time, you would need the thread plug gauges for performing the same job on the instruments having internal threads.
In addition, you have the pipes having the NPT threads. Pipes have to be leak proof. Hence, there should not be any gaps while affixing the taps or joining the pipes. Therefore, you require them to have leak proof lubricants and pipe tape such as Teflon etc. These pipes have the NPT threads. In cases of high-risk pipes such as those carrying inflammable liquids or gasses, they do not use this Teflon as well. They use a higher quality of threads known as the NPTF threads. They do not corrode or leak easily.
Coming back to the tolerance levels, you can find two kinds of thread ring gauges. They are the Go, and the No Go type of ring gauges. The mechanism for testing is such that the screw should pass through the Go kind of ring quite easily without much effort. In the same vein, it should not go through the No Go type of ring. The quality controls can allow a maximum of two revolutions alone. Every screw has to pass through these tests.
The instruments having the internal threads need the services of the thread plug gauges. These instruments have the Go / No Go sides as well. The principle is the same. It should pass through the Go side while failing to do so on the No Go side.