A thread gage is a device that is used to check the acceptance of an internal or external threaded part. Each gage type is used to monitor the tolerance, strength, reliability and durability of each component to ensure overall quality. With a handful of different thread gage types, it can be hard to determine which is used for specific measuring. This comprehensive guide will help walk you through the application and usage of each specific gage.
TYPES OF THREAD GAGES
THREAD PLUG GAGE – GO AND NO-GO
This type of thread gage is used to check the acceptance of an internal threaded part, otherwise referred to as a nut. For smaller threaded parts, the thread plug gage will consist of two ends, one end with a GO gage and another with a NO-GO gage. Larger threaded parts will usually require a gage with two separate pieces. This particular gage is designed to check the correctness of pitch diameter. In order for a threaded part to be deemed accepted, the GO gage should pass through the entire nut without using too much force. The NO-GO gage should not go beyond 2 turns.
THREAD GAGE RING – GO AND NO-GO
There are two types of thread ring gages, solid and adjustable. Both gages are designed to check the correctness of thread, including thread depth and pitch diameter.
A solid thread ring gage is used to check the acceptance of an external threaded part, otherwise known as a screw. Similar to the thread plug gage, a solid ring gage will consist of a GO and NO-GO gage, however, the rings are normally separate pieces. For acceptance of a part, the GO gage must pass through the entire length of the screw without much force – particularly looking for smooth and easy transition. The NO-GO gage should not go beyond 2 turns.
An adjustable thread ring gage is more complex than the solid thread ring gage. Although similar in shape, an adjustable thread gage consists of a split with an adjusting and locking screw facility. This facility is used to adjust the right gage sizes. These gages require setting plugs in order to correctly set the size. For accuracy, two setting plugs are used, one for the GO side and one for the NO-GO side.
THREAD CALIPER GAGES – GO OR NO GO
Similarly to a thread ring gage, a thread caliper gage is used to check an external threaded part. Although these gages are not as accurate as a thread ring gage, they are considerably faster. A thread caliper gage is designed with a roller type GO element, which simply slides over the threaded part using gravity. These gages require setting plugs in order to correctly set the inspecting size. Acceptance is deemed when the GO roller smoothly passes through the gage using gravity alone.
THREAD PITCH GAGE
A thread pitch gage is not a typical inspection device. It does not decide acceptance of a threaded part. This type of gage is used for visual checking to identify a threaded parts pitch. It will usually come as a set of thin plates with teeth precisely machined to a given thread pitch (threads per inch/TPI). The plates must closely match up with the threaded part. When all the teeth match up properly with the part being measured, with absolutely no gaps, the pitch size engraved in the plate becomes readable. Notably, a thread pitch gage is not completely foolproof and unless the inspector is an expert, there is a possibility of making an incorrect determination of a pitch/thread type.
IN-HOUSE VS. OUTSOURCED CALIBRATION
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