Thread gages are measuring tools used to identify pitch or turns per inch (TPI) on machined parts. Pitch gages are accurately notched metal blades that match the TPI of different types of threads and thread pitches. Ring gages are used to measure the accuracy of threads cut in the outer surface of rods or pipes. Plug gages check the conformity of threads tapped inside holes.
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Three basic types of thread pitch gages are needed for the identification of common machine parts. Separate gage tools are required for metric machine thread measurement and imperial thread measurement. A completely different thread gage is required to match the wider style of the Acme thread. Multi-blade gages are common, with several different patented designs available. If the teeth of the gage fit the threads, the pitch is the same as that stamped on the gage.
In pipe fitting, threads must be cut within a standard range to ensure a good seal. Gages for the National Pipe Thread (NPT) standard come in two types--ring and plug. These are go/no-go gages which provide a pass or fail rating of the part tested. Ring gages thread onto a pipe or rod, while plug gages thread into a tapped fitting. Each type measures the accuracy of four full turns of the thread. NPT fittings meeting this standard will provide a good seal if a sealing medium like Teflon tape is used in the joint.
National Pipe Thread Fuel (NPTF) fittings must meet a higher standard. The basic types of gages used to judge the accuracy of the threads is the same as for NPT--ring or plug. NPTF fittings are so precise that when tightened they form a dry seal. Checking the thread quality requires checking the accuracy of the thread crest and root--the high and low points of the thread--as well as thread pitch. Special gages called 6-steps are needed to perform this test.
Since gages wear out and can be damaged by improper use, the gages themselves must be regularly checked for accuracy. Corresponding sets of master thread gages are used to judge whether a gage is worn out or still usable. Standards for master gages are higher than for either standard thread gages or parts. A new part that passed inspection is not a substitute for a master gage.
Taps and dies should not be used in place of a pitch gage since they may damage the threads being checked. Thread gages of all types must be properly maintained. To ensure accuracy, gages should be cleaned before use. Gages are for measuring only--do not force them onto parts or use them to clean up or size marginal parts. If a gage is dropped or you suspect damage, check it with a master gage and replace it if needed.