The artefact for the key comparison will be a commercial steel ballplate with 25 balls, as it is routinely used industry. The size of the artefact still has to be determined according to the possibilities of the participants. The participants will be asked to calibrate the X,Y and Z centre positions of the balls in a fixed workpiece coordinate system. The probing points on the balls will be prescribed.The participant are generally free to use whatever equipment and strategy to deliver the best possible accuracy, but if the ball plate calibration is part of their standard calibration service, they must perform the calibration according to the procedures of their quality system. Only the expansion coefficient specified by the ballplate manufacturer will be given. The participants may use all information available for temperature compensation and uncertainty evaluation. This includes the measurement of the expansion coefficient.An uncertainty statement must be given:
(a) for the coordinates X,Y and Z at 20°C
(b) for the distances between the balls at 20°C The participants are allowed to state their expanded measurement uncertainty in a length dependent form, e.g. U=((0.5 µm) 2 L * 10-6)0.5 (k=2)
The Finnish institute TUT was former DI and participated as such in the project. The DI-status ended during the project.