National Physical Laboratory

Diameter of Plain Setting Rings

Plain setting ringsPlain setting rings 

Internal diameter measurement of plain setting rings

NPL provides a UKAS accredited service for measuring the diameter of grade AA plain setting rings up to 250 mm in diameter.

Certificates of calibration give the diameter at mid-bore, roundness at three specified positions and the variation of diameter along the bore.

Measurements of the diameter are made at three positions equi-spaced along the central half length of the bore in a single identified axial plane parallel with the supporting surface of the ring. The maximum variation of the three measured diameters is stated, together with the measured diameter at mid-bore.

Diametral measurements and the assessment of the geometrical quality of the ring are confined to the central half-length of the bore.

Roundness measurements in accordance with British Standard Specification 3730:1983 Assessment of departure from roundness are made at three positions. The maximum departure from roundness and the measured departure from roundness at mid-bore, using the minimum zone criterion, are stated.

Rings should be new or in new condition to benefit from the measurement precision available. Their geometries should comply with the requirements for Grade AA rings specified in British Standard Specification 4064:1966 Plain setting rings for use with internal diameter measuring machines (Metric Units) or 4065:1966 (Inch Units).


The best measurement capability expressed as an expanded uncertainty (k=2) is given by the formula: 0.07+(0.0005 x diameter in mm) micrometers.

Principle of Measurement

Setting rings ranging in size from 3 mm to 250 mm diameter are calibrated at one identified position by comparing their size with a calibrated master box standard using an NPL designed automatic measuring system. The box standard is formed from a pair of 10 mm fused silica blocks and two fused silica end pieces wrung to the ends of the 10 mm blocks. The size of the box standard is measured interferometrically. In operation a micro-displacement translator is used to position each inner face of the box standard, in turn, to produce a zero optical path difference relative to one or other of the reference reflecting planes in the fixed arm of the interferometer. This is achieved by observing the zero-order white light fringe. This fringe serves as a precise non-contact probe to detect the inner surfaces of the silica box. The displacement of the micro-displacement translator to obtain optical fiducial settings is equal to the separation of the inner surfaces of the box standard and is determined by a traceable laser interferometer.

The measuring system used to measure setting rings uses a ball-ended stylus mounted on an air slide. The worktable, on which the setting ring is lightly clamped, is driven by a stepping motor to bring one side of the ring into contact with the ball-ended stylus. Further movement of the worktable results in the pendulum being displaced by the air slide from its free hanging position giving rise to a reactionary force at the point of contact of the stylus with the ring. A vane passing a slotted opto-switch produces a signal to stop the motor when the air slide has been displaced to a predetermined position (approximately 12 mm from the neutral position). At this point the pendulum has produced the required measuring force. The measuring force is highly reproducible and is approximately 0.06 N. Relative displacement of the worktable (carrying the ring) and the air slide (on which the ball-ended stylus is mounted) are measured by a traceable, commercially available helium-neon laser displacement transducer. For the normal measuring sequence the stylus contacts the ring three times on one side and twice on the other. The difference between the mean displacement readings from each side of the ring is added to the stylus constant, derived from a similar set of measurements made on the box standard, to give the diameter of the ring. The stylus constant is approximately equal to the diameter of the ball-ended stylus but takes into account effects such as stylus stem deformation.


Customer Service tel: +44 20 8943 8631

Last Updated: 18 Aug 2015
Created: 31 May 2007


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