High accuracy roundness measurement service

Accurate services for spheres and hemispheres

The NPL high accuracy roundness measurement service is primarily intended for the calibration of glass hemi-spheres used in the calibration of roundness measuring instruments. A novel error separation technique is used to separate the spindle error of the NPL instrument from the component error.

Roundness measurement facility

The NPL facility is based around a modified Talyrond 73 developed in collaboration with Taylor Hobson.

Fundamentally, the instrument’s design is to use a spindle with a highly reproducible rotation and then a novel error separation technique is applied to significantly reduce the errors associated with imperfect spindle geometry. The instrument used can collect 2000 data points per revolution.

In operation, the component to be measured is placed on a rotary stage and data collected at several orientations of the stage. The Fourier-series representations of the signal at every orientation are then calculated. These representations, along with an equivalent model in the frequency domain relating to the various Fourier representations, are solved to find the Fourier representations of the component and spindle errors.

Service details

• Measurement of roundness profile of spheres and hemi‑spheres (2000 data points collected per revolution).
• Roundness measurements traceable to the national standard of length
• Best measurement uncertainty for artefact calibration of ± 0.000 005 mm at a confidence level of 95% (calculated in accordance with ISO document Guide to the expression of uncertainty in measurement)
• NPL certificate of calibration states the peak-to-valley roundness value and deviations from roundness at fifty points around the circumference (deviations at all 2000 points available on request)
• Provides UKAS accredited calibration service (laboratory No. 0478)

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