ISO compliant reference artefacts for the verification of focus varation-based optical micro-coordinate measuring machines

Demand form micro-coordinate measuring machines (micro-CMMs) within industry is increasing due to the need for accurate measurement of the geometry of small-scale objects. Optical micro-CMMs have the advantage over traditional stylus-based CMMs of being non-contact instruments, and have the ability to acquire large amounts of data, with high resolution, in a relatively short period of time. The focus variation (FV) technique is typically used in the field of surface topography measurement, but has the potential to be implemented as a sensor technology for optical micro-CMMs. Exploring the possibility of the FV technique as an optical micro-CMM requires that the instrument and measuring procedure can be performance verified. Consequently, a prototype FV based optical micro-CMM should have a verification route that is traceable to the definition of the metre. The ISO 10360 specification standard for acceptance testing and verification of CMMs has several parts, all specific to different groups of instruments and configurations. Each section of ISO 10360 identifies methods and artefacts best suited for the acceptance testing and verification of each group and configuration. ISO/DIS 10360-8.2 (due for ISO/FDIS publication in 2013), is a verification standard written for CMMs with optical distance sensors. There are four main parts to the acceptance and re-verification tests: length measurement error, probing form error measurement, probing size error measurement and flat form error measurement. The probing form and size error tests require a calibrated reference sphere that has a diameter of at least 10 mm. FV instruments are potentially covered by this standard but the recommended minimum size of the calibration sphere is too large to fully fit within one field of view of a typical FV instrument.. Optical distance measuring instruments similar in operation to FV systems, such as confocal microscopes and coherent scanning interferometers are, therefore, also currently excluded from the application of this standard by default, unless the user, and the instrument manufacturer, can potentially agree to use a smaller calibrated reference sphere for the assessment of the probing size and form error. A prototype FV based optical micro-CMM should, therefore, be verified with calibrated reference spheres of similar size to objects for which the technique has been designed to measure. The research reported here considers the use of 0.5 mm, 1 mm and 2 mm diameter reference spheres as suitable components for a reference artefact, with the surfaces of the spheres roughened using bespoke micro-roughening techniques. The research presents a novel verification artefact composed of multiple small-scale spheres, specifically designed to evaluate probing size error, probe form error, and dimensional accuracy of a prototype FV based optical micro-CMM, compliant to ISO/DIS 10360-8. The results suggest that the artefacts and procedures detailed in ISO/DIS 10360-8 can, and should, be applied to optical micro-CMMs.