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Where errors are normal

A new multiwave standard refines the calibration of form measuring instruments

12.05.2000

[es]A disk the size of the palm of one's hand, made of metal, with tiny waves on the surface - this is what the object looks like which tells form measuring instruments what they have to indicate. For the departures from flatness on a surface of the so-called multiwave standard are fabricated with great precision and have exactly defined properties. These properties must be determined by the measuring instrument. If the instrument is successful, it may be regarded as calibrated. With the aid of the new multiwave standard which has been developed at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, the calibration of form measuring instruments has become much simpler, much safer and more versatile. The PTB presented this new standard at the "Control Fair," the international fair for quality assurance at Sinsheim from May 16 to 20.

How is it possible, for example, to make German screws and Brazilian nuts match? The best way would be to bind the two manufacturers to apply the same international standard specifying that the measuring instruments used for quality control during manufacture must have a specific accuracy. Checking of this accuracy is the aim of calibration. The measuring instrument has to cope with an object whose data are exactly known, and it is checked whether these are the data which are indicated by the measuring instrument. In the language of metrology, the art of measurement, such an object is referred to as "standard."

To the many standards already existing, the PTB - which as the National Metrology Institute has the task to develop ever better measuring procedures - has added a new one. The multiwave standard for the calibration of form measuring instruments is a round disk 80 mm in diameter the surface of which is provided with up to 1000 tiny waves in the form of a sinusoidal curve. These waves differ in frequency or wavelength, respectively (in the millimeter range), and in amplitude (in the micrometer range). "We have superposed these waves with different frequencies and amplitudes," says Otto Jusko, one of the development scientists. "This is more or less the same thing as if two waves meet in water and develop into a higher wave. We practically have frozen such wave movements." A procedure which requires nanotechnology for the fabrication. The Fraunhofer Institute for Production Technology in Aachen which fabricates these standards is one of the small number of institutes which are able to achieve this precision. So it is possible now to replace the flick standards which were used up to now and whose disadvantages had already been known for a long time - for example, they contained only one deviation from ideal roundness instead of several departures the novel multiwave standard shows. This novel standard allows the calibration of the different devices employed for form measurement to be carried out much more rapidly and at much lower costs - and in addition it is much less susceptible to faults: dirt or scratches on the surface cannot so strongly falsify the result. And what is most important: The new standard allows also the dynamic properties of form measuring instruments to be checked.

At the "Control," one of the most important fairs for quality assurance and quality management, these advantages were demonstrated to the interested public by two prototypes: one specimen with waves on the outside and - an absolute novelty - another specimen on which trough and crest of the wave rise and fall on the inside of the (hollow) cylinder. Thus it is possible for the first time to calibrate measuring instruments whose task it is to measure hollow forms.

Further information:
Dr. Frank Lüdicke, tel. (05 31) 592-5310, e-mail:frank.luedicke(at)ptb.de
Dr. Otto Jusko, tel. (05 31) 592-5211, e-mail: otto.jusko(at)ptb.de
Laboratory "Geometrical Standards"
Physikalisch-Technische Bundesanstalt (PTB)