According to the requirements of out of roundness measurement​, the roundness tester must have three characteristics: (1) It has a high-precision spindle to realize the centering of parts and determine the center of the theoretical circle at the same time. (2) A sensor with high sensitivity. When it revolves around the center of the spindle, it can measure the small out of roundness error of the part. (3) A device with continuous indication or recording. In order to record the actual contour of the part for analysis and evaluation.