Investigation of measuring strategies in computed tomography

Computed tomography has entered the industrial world in 1980’s as a technique for non-destructive testing and has nowadays become a revolutionary tool for dimensional metrology, suitable for actual/nominal comparison and verification of geometrical and dimensional tolerances. This paper evaluates measuring results using different measuring strategies applied in different inspection software. The strategy influence is determined by calculating the measuring uncertainty. This investigation includes measurements of two industrial items, an aluminum pipe connector and a plastic toggle, a hearing aid component. These are measured using a CT scanner and compared with reference measurements on tactile coordinate measuring machine (TCMM) and optical CMM (OCMM), to obtain traceability of measurement. Results have shown that diameter measurements of cylindrical features for both parts resulted in small bias (difference between measurements using CT scanner and reference instruments) compared to distance and height measurements. It was found that bias values as well and uncertainties of all measurands calculated in ATOS for the pipe connector were generally bigger compared to measurements in Calypso CT and VGStudio MAX. Bias values of all measurands for the toggle were in the same range for all the three software and uncertainties were in the range of calibration uncertainties. Uncertainties connected with measurement of the distance between two surfaces on the inner flange of the pipe connector from CT scanner were found bigger compared to uncertainties obtained from reference measurements performed on tactile CMM. Uncertainties for measurements of the pillar height on the toggle from CT scanner were found to be in the same range as uncertainties obtained from reference measurements performed on optical CMM.