Roundness error evaluation by minimum zone circle via microscope inspection

Utilizing the convex hull theory, a novel minimum zone circle (MZC) method, named improved minimum zone circle (IMZC) was developed in this paper. There were three steps for IMZC to evaluate the roundness error. Firstly, with the convex hull algorithm, data points on the circle contour were categorized into two sets to determine two concentric circles which contained all points of the contour. Secondly, vertexes of the minimum circumscribed circle and the maximum inscribed circle were found out from the previously determined two sets, and then four tangent points for determining the two concentric circles were also found out. Lastly, according to the evaluation using the MZC method, the roundness error was figured out. In this paper, IMZC was used to evaluate roundness errors of some micro parts. The evaluation results showed that the measurement precision using the IMZC method was higher than the least squared circle (LSC) method for the same set of data points, and IMZC had the same accuracy as the traditional MZC but dramatically shortened computation time. The computation time of IMZC was 6.89% of the traditional MZC.