Accuracy of X-ray computed tomography for dimensional metrology with employment of a new threshold selection method.

The article presents and discusses results of an experimental study regarding the accuracy of measurements performed with X-ray CT scanners. One of the main factors affecting the measurement result is definition of the threshold value between the air and the material. In this study, the results of measurements using two methods to determine the threshold value namely, the newly developed method and a well-known and commonly used method of ISO50%, were compared. A novel method is based on the measurement of an additional reference workpiece. The correction value, based on the measurement, is determined, and then it takes account when determining the threshold value between the air and the material. The study was performed using two types of gauges. These were workpieces of a specified volume of material (consisting of reference balls) and with the defined values of linear dimensions (in the form of cubes and cylinder). Using five different CT scanners, the conducted study yielded increase in the accuracy of measurements by applying the developed method in relation to the ISO50% automated method. In the case of volumetric test workpieces, relative measurement errors were reduced by 3 to 10 times for metrological CT scanners and from 50 to 30% for defectoscopy CT scanners. In the case of linear dimension measurements, average measurement errors were reduced twice.