Measuring machine has accuracy taped

A British measuring tape manufacturer, Fisco Tools, has installed a laser measurement bench which, it believes, is the only one of its type in the world. The bench, used to verify and calibrate steel measuring tapes up to 50m long, is capable of measuring to an accuracy of 1.1µm per meter - around 1/75 the thickness of a human hair.

The bench (shown above), housed in a temperature-controlled room at Fisco`s Rayleigh factory, is used to prove that the company`s tapes comply with the EC Class 1 standard that demands an accuracy of ±0.2µm in the first metre.

Central to the measuring apparatus is a 32m-long linear rail system which carries the tape blade unwind and rewind carriage, and the camera carriage that houses the laser reflector. During trials, Fisco tested - and rejected - a linear slideway for this role. "Despite being very expensive, we couldn`t adjust the preload," explains Fisco`s engineering manager, Stan Burton. "I could push the carriage and move it by microns."

So the company opted instead for a linear motion system with variable preloaded bearings, supplied by Hepco. The slide was supplied in eight 4m sections and has been laid to within 0.1mm in both level and straightness. Hepco has guaranteed that it will maintain this accuracy for at least ten years. It was essential that the joints were near-perfect - a variation of just 10 microns would have been sufficient to cause the laser reflector carriage to vibrate, losing the beam and the measurement data.

"During a measuring cycle, when the carriages are at rest, we have total stability of less than 1 micron," Burton reports, adding that the system`s bearings are delivering a better rigidity than would be possible using the most expensive ball bearings.