National Physical Laboratory

Diameter of Plain Setting Plugs

Plain setting plugsPlain setting plugs 

External diameter measurement of plain setting plugs

NPL provides a UKAS accredited service for measuring the diameter of plain setting plugs, pins, rollers and thread measuring cylinders in the range 0.1 to 150 mm diameter. To benefit from our high accuracy measurements, plugs should be new or in new condition and have a geometry complying with the requirements for Grade AA rings specified in British Standard Specifications; 4064:1966 Plain setting rings for use with internal diameter measuring machines (Metric Units) or 4065:1966 (Inch Units).

The diameters of setting plugs are measured by comparison with calibrated gauge blocks of similar size. A plug is compared with a gauge block in a machine having flat, parallel measuring faces whose separation is determined either interferometrically or by using a capacitance gauging system. The measured size of the plug is corrected to zero measuring force. The diameter is measured midway along the length of the plug in a single identified plane and this result is stated together with the maximum variation in diameter obtained from diametral measurements made along and around the gauging surface. The departure from roundness (MZC) at the mid-axial position is measured and stated.


The best measurement capability expressed as an expanded uncertainty (k=2) is given by the formula:

  • Thread measuring cylinders
    • From 0.1 mm up to 5 mm diameter: 0.08 + (0.001 x diameter in mm) μm

  • Plain plug gauges (parallel) reference cylinders and rollers
    • From 0.1 mm up to 100 mm diameter: 0.07 + (0.0011 x diameter in mm) μm
    • Above 100 mm up to 150 mm diameter: 0.05+(0.0014 x diameter in mm) μm


Customer Service tel: +44 20 8943 8631

Last Updated: 18 Aug 2015
Created: 31 May 2007


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