Air gap effects on dimensional accuracy in multi-material X-ray CT measurements

• Air gaps cause significant XCT measurement errors in multi-material assemblies • Multi-material setups reduce XCT accuracy even without air gaps between surfaces. • POM shafts show up to 0.4 mm error due to aluminium surroundings and air gaps • Smaller voxel sizes improve CT accuracy and reduce variability • Residual errors in multi-material XCT scans are up to 8 × higher than in single-material mode. Accurate dimensional measurement using X-ray computed tomography (XCT) is essential in quality control, especially for parts made of multiple materials. This study investigates how small air gaps between aluminium and polyoxymethylene (POM) components affect measurement accuracy in XCT scans. Using a full factorial experimental design, eight XCT scanning settings were tested, varying voxel size, binning/exposure time, and rotation mode. Measurements were compared to reference values from a coordinate measuring machine (CMM) across seven shaft segments with different air gap sizes from 25 μ m to 250 μ m. The results show that air gaps significantly increase measurement errors, in metal-polymer configuration. POM shafts showed measurement errors up to 0.4 mm, while aluminium bores had measurement errors up to 0.15 mm. Voxel size emerged as a critical factor: smaller voxel sizes (68.4 μ m) consistently yielded lower errors and reduced variability. Residual errors increased substantially in multi-material mode, reaching up to eight times those in single-material scans, especially for POM shafts. Importantly, even when no air gaps were present between surfaces, multi-material configurations still reduced XCT precision. The best achievable measurement errors were approximately 6 μ m for aluminium bores and 11 μ m for POM samples in single-material mode, increasing to 20 μ m and 50 μ m respectively in multi-material mode. These findings underscore the need for optimized XCT settings and material-aware calibration standards to ensure reliable metrology in multi-material assemblies.