A critical investigation into the accuracy of a parallel coordinate measuring machine

[摘要] ENGLISH ABSTRACT: The increasing demand for manufacturing high quality and high precision productsimplies that special methods are required to ensure the quality and precisionof these manufacturing processes and techniques (Lin and Chen, 2001;Küng et al., 2007). As a result, coordinate measuring machines (CMM's) arewidely used as a means of precision measurement and verification of qualitycontrol. Ordinary CMM's have matured in their technology, yet are still lackingin their ability to measure at a sub-micron level (10-9 m). Furthermore,existing systems with sub-micron accuracy have a limited measurement rangeand are extremely costly (Sudatham et al., 2016).The process of designing and manufacturing micro-coordinate measuringmachines (CMM's) is kept as a trade secret by existing market suppliers(Bommakanti, 2012). This has a negative effect on micro-manufacturing companies,since quality verification through the use of costly speciality CMM's,proves to be disadvantageous (Kruth et al., 2002). Introducing a CMM thatcan achieve a measurement accuracy in the sub-micron level has shown evidenceto be beneficial for the market of micro and nanomanufacturing systems(Wu et al., 2012). However, the lack of theory supporting the challenging design and research of CMM's present great challenges in manufacturing andcommercializing these systems.This thesis endeavoured to investigate the accuracy of the current systemdeveloped by Rugbani (2014) to fashion improvements. Rugbani's (2014)system is unverified and does not yet achieve the desired accuracy. Therefore,investigating the accuracy of the CMM designed and manufactured byRugbani (2014) provided useful insight into the current state of the machine.Furthermore, the Rugbani (2014) system illuminated accuracy influencing factorsof the CMM such as the design, layout, and measurement equipment.The initial hypothesis envisioned improvements through temperature compensation.Therefore, the study endeavoured to improve measurement accuracy,perform baseline accuracy study on a calibrated artefact, and to incorporateand test thermal error compensation. However, in the course of the study,various fundamental design problems of the system were identified.This thesis details the measurement test that was performed to examinethe accuracy state of the CMM. Post-processing of the measurement showedunfavourable results with the original calibrated parameters. These parameterswere tweaked, however, the results did not improve. Further tests wereinitiated to simplify methods of calibration to sanction improved regulation ofthese parameters. The mentioned tests evidenced that: the machine requiredlarge displacements for the laser measurement to change significantly; the ratioof probe movement to laser measurement is 20:1 and the tetrahedron hasa position settling error. The study recommends an investigation into designimprovement of the CMM to address fundamental design problems.

[发布机构] Stellenbosch University