Technical Articles

Thread Gages Information

Thread Gages Information

Thread gages are dimensional instruments for measuring and/or verifying thread size, pitch or other parameters.  A variety of thread gaging instruments and tools exist such as measuring wire, tri-roll comparators, thread plug gages, thread ring gages and thread micrometers.  The appropriate variable or fixed limit gage for an application should be selected based on internal and external thread type, specific thread designation (UNS, UNF, UNC, NPT, ACME, Buttress), part tolerances and gaging frequency (shop vs. high volume production).

Types of Gages

Thread gages can be one of any number of types of gages.  These in clued plug, ring, 3-wire, micrometer, tri-roll comparator, measuring wire, screw thread insert (STI), and thread gaging roll thread gages.  Thread plug gages measure go / no-go assessment of hole and slot dimensions or locations compared to specified tolerances.  Thread ring gages measure go / no-go assessment compared to specified tolerances of the dimensions or attributes of pins, shafts, or threaded studs.  3-wire thread gages are gages that use thread wires to gage thread size with one wire mounted in one holder and two wires mounted in a second holder.  The holders are placed in the measuring gage and brought in contact with the threads.  Thread micrometers are micrometers for measuring thread.  A tri-roll comparator is a specialized thread gage employing three thread rolls and a digital or dial display.  The thread gaging rolls can be interchanged to measure different thread sizes.  A measuring wire is a specialized wire manufacturing to precise gage sizes for measuring external threads.  The wire is wrapped or placed in the thread cavity and then a measurement is made with a micrometer or other OD gage.  STI gages, also referred to as helical coil or helicoil, are used where a screw thread insert will be used.  STI gages are widely applied in the automotive industry.  Thread gaging rolls are threaded rolls for use on roll thread comparators.

Thread Types, Profiles and Geometries

Different thread types, profiles ,and geometries provide different functionalities.  Thread designations include UNC, UNF, UNEF, UN, M/MJ (metric), NPT, NPTF, NPSF, ANPT, BSPT, BSPP, ACME, and buttress.  Thread gages measure the size or diameter of the feature being measured.  English pitch is the threads per inch that the gage can measure.  Metric pitch is the metric thread spacing that the gage can measure.

Thread verifiers, thread detection probes or thread verificaton systems detect or verify the presence of threads, threaded holes and/or specific thread pitches. Thread verifiers often use a high speed, non-contact technology to rapidly inspect parts. Eddy current thread verification probes are a common type of thread or thread pitch verifier.

Common shapes or geometries measured include cylindrical and tapered or pipe.  A go gage provides a precision tool for production comparative gaging based on a fixed limit.  Go gages consist of a fixed limit gage with a gaging limit based on the plus or minus tolerances of the inspected part.   No-go or not-go gages provide a precision tool for production comparative gaging based on a fixed limit. No-go gages consist of a fixed limit gage with a gaging limit based on the minimum or maximum tolerances of the inspected part.   Go / no-go gages configured with a go gage pin on one end and a no-go gage pin on opposite end of the handle. Go / no-go gages provide a precision tool for production comparative gaging based on fixed limits.  Go / no-go gages manufactured in the form of stepped pins with the go gage surface and the no-go gage surface on the same side of the handle.  The gage can save type in gaging since the gage does not have to be reversed for no go gaging.  Master gage blocks, master or setting discs, and setting rings are types of master gages used to calibrate or set micrometers, comparators, or other gaging systems.  Fixed limit or step gages are specialized thread plug gages for gaging taper pipe threads.  Notches or external steps indicate maximum and minimum allowable tolerances.  Tolerance classes for thread gages include Class XX, Class X, Class Y, Class Z, Class ZZ and thread Class W.

Measurement Units

Measurement units for thread gages can be either English or metric.  Some gages are configured to measure both.  The display on the gage can be non-graduated meaning that the gage has no display, dial or analog, digital display, column or bargraph display, remote display, direct reading scale, or Vernier scale.


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