Two different experimental approaches for the uncertainty determination of X-ray computed tomography dimensional measurements on lattice structures

X-ray computed tomography (CT) is increasingly used in industrial dimensional metrology. However, the establishment of metrological traceability can be challenging for CT dimensional measurements, as the evaluation of their uncertainty is generally non-trivial. Moreover, uncertainty determination becomes even more difficult when dealing with complex measurement tasks, such as in the case of lattice structures produced by additive manufacturing, characterized by features not accessible from the outside and/or by complex surface topographies, which are problematic to be calibrated using other measuring principles to ensure sufficiently low calibration uncertainty. This paper presents a critical investigation of two different experimental approaches for the uncertainty determination of CT dimensional measurements performed on components with inaccessible and/or complex geometries, as well as complex surface topography. The first approach, the “substitution method”, is applied in this work using a newly developed reference object as the basis for the uncertainty assessment procedure through substitution measurements. The second one, the “multiple measurements approach”, is based on repeated measurements both on the uncalibrated workpiece in multiple orientations and on simple calibrated standards. This second approach, initially introduced some years ago for tactile coordinate measuring machines, is now investigated in this work for its application to CT metrology. The two approaches are discussed and compared through an industrial case study: the dimensional verification of lattice structures fabricated by metal additive manufacturing. For the selected case, both approaches yielded comparable measurement results and uncertainties. Finally, the multiple measurements approach was further investigated by applying it to the CT measurement of the calibrated object. The paper also discusses the open issues of both methods and outlines directions for future investigations.