National Physical Laboratory

High accuracy roundness measurement

Roundness measurementMeasurement of a hemisphere 

NPL provides a high accuracy service for measuring the roundness of spheres and hemispheres up to 100 mm in diameter. This service, which is primarily intended for the measurement of glass hemispheres used to calibrate roundness measuring instruments, is based on a Talyrond 73 instrument that was specially developed in collaboration between NPL and Taylor Hobson. Key features of the instrument's design are a spindle with a highly reproducible rotation and a novel multi-step error separation technique which is used to separate the spindle error from the component roundness error. These features make it possible to measure departures from roundness with an uncertainty of ± 0.000 005 mm at a confidence level of 95%.

  • Measurement of roundness profile of spheres and hemispheres up to 100 mm in diameter.
  • Roundness measurements traceable to the national standard of length.
  • Accreditation for a best measurement uncertainty for measuring departures from roundness 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 stating the departure from roundness and the radial variations at fifty points around the circumference of the component under test (radial variations at up to 2000 points around the circumference can also be provided on request).
  • Graphical plot of the radial variations.

Principle of operation

The roundness measurement facility is based on a Talyrond 73 instrument that was specially developed in collaboration between NPL and Taylor Hobson. The fundamental basis of the instrument's design is to use a spindle with a highly reproducible rotation and then use a novel error separation technique to reduce significantly the errors associated with the lack of perfection of the spindle geometry. The instrument used to make the measurements is capable of collecting 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 representation of each measured trace is determined. A mathematical model which relates the Fourier representations of the component errors and the spindle errors to those of the traces is then solved. The resulting Fourier representation of the component error is used to determine the roundness of the component and to provide values of the component error at points around the circumference.

Contact

Customer Service tel: +44 20 8943 8631
E-mail: dimensional_enquiries@npl.co.uk

Last Updated: 18 Aug 2015
Created: 31 May 2007

Registration

Please note that the information will not be divulged to third parties, or used without your permission

Login