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Setting up ball plates and hole plates

Investigation of deformation due to the dead weight of large standards

PTBnews 1.2021
07.01.2021
Especially interesting for

calibration laboratories and metrology institutes

manufacturers and users of coordinate measuring machines

Ball plates and hole plates are measurement standards typically used to ensure the traceability of coordinate measuring machines. To arrange the plates vertically, one single supporting stand is often used that is placed laterally to the standard. Numerical simulations based on the finite-element method that were performed at PTB have shown that such an arrangement no longer fulfills the ever stricter requirements for the calibration of modern, high-precision coordinate measuring machines.

Deformation of a ball plate made of tool steel with a mass of approx. 30 kg and an edge length of 620 mm

In the past few years, high-precision coordinate measuring machines have been developed to ensure the quality of ever more accurate manufacturing processes in production. These coordinate measuring machines achieve uncertainties of 0.3 μm + 1 μm/m for length measurement errors. The traceability, correct adjustment and checking of such coordinate measuring machines require standards that have been calibrated with a much lower measurement uncertainty. This is the reason why influences that could previously be neglected, now have to be taken into account when calibrating ball and hole plates typically used as standards as their considerable dead weight leads to a deformation of the standards, depending on how they were set up on the coordinate measuring machine. A simulation study using the finite-element method (FEM) was carried out in order to describe these influences quantitatively and to allow the measurement and calibration procedures used to be improved. Here, the central question was to determine the relation between the deformations undergone by the plates and typical measurement uncertainties and whether these deformations are negligible.

The results of the FEM investigations have shown that the vertical supporting stands currently used in industry for ball plates (consisting of a single supporting stand applied laterally to the standard) are particularly prone to plying under the influence of the dead weight of the ball plates. This causes an asymmetric deformation of up to 0.37 μm, which is not negligible. In this scenario, the deviations from the calibration values of the standards are particularly large in the horizontal plane, since spherical calotte plates are usually calibrated horizontally but also used in various positions.

The obvious alternative involving two supporting lateral stands was also investigated. It showed that this arrangement reduces deformation to such an extent that the influence becomes acceptable, even to ensure the traceability of high-precision coordinate measuring machines.

Contact

Daniel Heißelmann
Department 5.3
Coordinate Metrology
Phone: +49 531 592-5321
Opens local program for sending emaildaniel.heisselmann(at)ptb.de

Scientific publication

Publication in the “Technisches Messen” specialist journal (DeGruyter) in preparation