A team from the National Physical Laboratory (NPL) and Taylor Hobson Precision (THP) and the are in the final stages of a DTI contract to provide a roundness measurement service as part of the National Measurement System capable of measuring the departure from roundness of hemi-spheres and spheres with an uncertainty of measurement (k =2) of 5 nm.

The system is based upon Taylor Hobson's highly successful Talyrond 73 HPR (High Precision Roundness) system, which was modified by Taylor Hobson to incorporate a new non-influencing spindle drive. To achieve the specified uncertainty, the system incorporates an automated rotary stage to enable a number of roundness measurements to be made with the component rotated relative to the instrument spindle between each measurement. Software developed by NPL enables separation of the instrument spindle and component errors from the data. This software along with the instruments repeatable spindle has enabled a high degree of accuracy to be obtained. The error separation technique used avoids the problems of harmonic suppression associated with other techniques.

Currently, Taylor Hobson is the UK's only laboratory accredited by UKAS (NAMAS) for the calibration of the roundness of spheres and hemispheres. This project has enabled NPL to provide, and seek UKAS accreditation for, a high accuracy roundness measurement service.

Chief Mechanical Engineer, John Garratt of Taylor Hobson said - "In the past we have completed a number of successful projects working with NPL and in the process developed a good working relationship with them. We have a common objective to explore ever-higher precision measurement capabilities".

The collaboration of Taylor Hobson's many years of expertise in the field of precision roundness measurement coupled with NPL's research capabilities has developed the technology to measure roundness to a greater degree of accuracy than has probably ever been achieved before.

Head of Dimensional Metrology, Dr. Graham Peggs said
"NPL's aim was to provide an easy to operate, high accuracy measurement service and it was felt that this could be best achieved by basing the new service around a commercial instrument with the appropriate resolution and repeatability. The UK manufactured Taylor Hobson Talyrond 73 met these requirements."

"This project has improved NPL's capabilities; it now has a world class roundness measurement facility which will provide a service to UK industry. The measurement service builds upon NPL's R&D capabilities in roundness measurement and Taylor Hobson's expertise in providing roundness instruments with repeatable characteristics."