Basic concept of thread ring gauges and plug gauges
What is the need to test the tolerance of the threads? This can be the question in the minds of a layman. It is important for a particular screw to fit tightly into the hole or the socket provided for it. Not only should it fit properly but also it should not pose problems while you remove it. This is why you require the threads to have adequate tolerance. They should be able to withstand the loads they are entrusted with.
Imagine an aircraft or a railway locomotive. Have you seen that they comprise of various parts joined together by screws, nuts and bolts. Some of them might be visible whereas some might be camouflaged. In case the screws were to fall off midway through your journey, imagine the consequences you will face. This proves that the screws should fit in properly while the machine works at full capacity.
How do you test the tolerance levels of the screws and bolts? The time tested method is by using thread gauges. You have thread ring gauges to test the pitch of the externally threaded instruments. The thread plug gauges serve to check out the tolerance levels of the internally threaded equipment.
Checkinh the tolerance levels is a very simple task. Each gauge has a GO side and a NO GO side. The thumb rule is that the equipment must pass through the GO side of the gauge with ease. You should apply the normal pressure. Of course, it should not just slip through without engaging the threads. The threads should engage in the normal manner. At the same time, the threads should not pass through the NO GO side. The standard rule is that you are allowed to make a couple of turns. The equipment should not proceed beyond the first three turns. In case it does, the screw or nut is unfit for use.
Does this measure the pitch of the screw. It does not. This is an attributive test and not a quantitative inspection. The result is a very simple Pass or Fail. There is no in-between kind of a situation. Once the instrumens are rejected, they have to go back to the industry for recalibration.
This is a very simple test that every screw, nut, or bolt is required to clear to be fit for use in the industry. Is this not a simple test? Yes, it is.