Developing an Integrated Approach for the Performance Verification for Large Coordinate Measuring Machines (CMM) at the Standards and Calibration Laboratory (SCL)

Abstract

To support local industries, the Standards and Calibration Laboratory (SCL) in Hong Kong has developed a performance verification service for Coordinate Measuring Machines (CMM) meeting the requirements of ISO 10360-2:2009 (“the ISO standard”). In the ISO standard, a non-contact laser-based approach may be used in generating long test lengths. In that approach, additional measurement steps for an artifact with a normal coefficient of thermal expansion (CTE) (“Requirement A”) and bi-directional measurements (“Requirement B”) are required. When the time and effort spent on the laser alignment are taken into consideration, using the laser-based approach with a laser interferometer for on-site CMM verification might not be justified. To address these issues, SCL has developed an integrated approach. In this approach, most of the test lengths are measured with a step gauge. For test lengths exceeding the coverage of the step gauge, the step gauge is first fitted and the step lengths measured. Test lengths longer than the step gauge are then measured using laser interferometer. The benefit of such approach is three-fold. First, Requirement A may be fulfilled by measuring a longer distance that the step gauge provides. For Requirement B, any of the 20 mm step lengths on the step gauge may be used in lieu of the short gauge block. Second, the step gauge may guide the fitting of the laser optics thereby greatly minimizing the alignment time and effort, in particular for retro-reflectors on some measuring heads that do not support automatic angle setting. Finally, since the measurements for Requirements A, B, and the unidirectional laser interferometer measurements all lie on the same axis, they may be cross-checked to minimize any setting and cosine errors. The approach might promote the use of the ISO standard by not just complying with its requirements but also providing a more effective, comparable, and rigorous way in carrying out the CMM performance verification.