Plug gauges, ring gauges and special gauges are fixed-limit dimensional instruments used for GO/NOGO checks on bores, shafts, grooves, railway profiles and dedicated geometries. Unlike measuring instruments proper, they do not provide a numerical value for the dimension but verify whether the part falls within the specified tolerances, ensuring a rapid, repeatable check that is independent of the operator’s judgment. They are irreplaceable in series inspections and in the acceptance testing of standardised components.
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Main types
Plug gauges take the form of precision cylinders (in hardened steel or tungsten carbide) with two ends of different diameter: the GO end corresponds to the lower tolerance limit (the part must pass), the NOGO end to the upper limit (it must not pass). They are used to check bores. Ring gauges perform the opposite function on the external diameters of shafts and pins: the GO end must fit, the NOGO must not. GO/NOGO thread gauges check the conformity of internal and external threads, with standardized profiles. There are also special gauges for railway profiles (fixed loading-gauge templates), for grooves, for clearance thicknesses (feeler gauges) and for pipe threads.
Tolerances and classes
The tolerances of plug and ring gauges are governed by ISO 1938-1 and ISO 1938-2: the manufacturing tolerance of the gauge is a fraction (5-10%) of the tolerance of the part to be checked, in order to ensure that wear of the gauge does not compromise acceptance of the part. The classes provide GO and NOGO with “Type 1” or “Type 2” precision levels depending on the severity of the check. Thread gauges comply with ISO 1502 for ISO metric threads. Periodic calibration and wear checks are essential.
Applications
Plug and ring gauges are used in series production of mechanical components (shafts, gears, pins, bushings), in in-line quality control, in the acceptance of supplies and in the periodic checking of tools. GO/NOGO thread gauges are widespread in the inspection of screws, nuts, studs and fittings. Special gauges for railway profiles (for example those for checking train wheels and rail/wheel contact) are required by dedicated standards of the railway sector.
Standards and calibration
The main reference standards are ISO 1938 (plain cylindrical), ISO 1502 (metric threads) and ANSI/ASME B89.1.6 (imperial systems). Calibration of gauges is performed on a CMM or with high-precision micrometers, in accordance with ISO 17025, with verification of the flatness of the faces, the effective diameter, squareness and surface roughness. Periodic calibration is critical, because a worn gauge accepts parts that are out of tolerance.
Selection criteria
The choice depends on the type of geometry to be checked (bore, shaft, thread, profile), on the nominal tolerance of the part, on the required accuracy class, on the frequency of use (tungsten carbide gauges last longer but cost more) and on the need for certified gauges with a calibration report. For high-volume production it is advisable to have redundant gauges, so that rotation and periodic calibration are possible without interrupting the inspection.
The use of GO/NOGO gauges forms part of a statistical process control approach: the inspection frequency, the sample size and the acceptance criteria are defined by sampling plans (UNI ISO 2859) that balance the quality of the check against the cost of carrying it out.
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