Improvement of segmented bars for the verification of coordinate measuring arms

Abstract

In order to verify the performance of articulated arm coordinate measuring machines, the standards/guidelines that address this topic require the use of different length artefacts. For instance, the volumetric test of ISO 10360-12:2016 requires measuring five calibrated lengths in seven directions (three horizontal, three diagonal and one vertical). If a ball bar is used to perform the test with each sphere measured with five points, then the test requires measuring 1050 points (7 directions × 5 lengths × 2 spheres × 5 points × 3 repetitions).The aim of this paper is to optimize this type of test and adapt it to industrial environments, in terms of ergonomics and time of measurement. For this purpose, two solutions merging the concept of segmented ball bars with the concept of kinematic seats are proposed. The first solution involves using a passive self-centering probe and a regular segmented ball bar (with joining spheres). The second novel solution involves replacing the joining spheres of the ball bar by spherical mounts containing kinematic seat inserts and a regular spherical probe. The kinematic seats in both solutions are designed in a way to optimize their rigidity, machinability and their uncertainty of measurement. Both proposed solutions reduce considerably the time of measurement. In addition, it will be demonstrated in the paper that the first solution presents the advantage of decreasing the uncertainty of measurement whilst the second solution might increase it lightly, compared to a regular ball bar measured with a regular spherical probe.

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