A Hull Normal Approach for Determining the Size of Cylindrical Features

In coordinate metrology, discrete data is sampled from a continuous form to assess the manufactured feature’s deviation from its design specifications. Although coordinate measuring machines have a high degree of accuracy, the unsampled portion of the manufactured object cannot be completely described. The definition of cylindrical size for an external feature as specified by ASME Y14.5.1M-1994 [1,2] matches the analytical definition of a minimum circumscribing cylinder (MCC) when Rule #1 is applied. Even though the MCC is a logical analysis technique for size determination, it is highly sensitive to the sampling method and any uncertainties encountered in that process. Determining the least-sum-of-squares solution is an alternative method commonly utilized in size determination. However, the least-squares formulation seeks an optimal solution not based on the cylindrical size definition [1,2], and hence has been shown to be biased [6,7]. This research presents a novel Hull Normal method for size determination of cylindrical bosses. The goal of the proposed method is to recreate the sampled surface using computational geometry methods and determine the cylinder’s axis and radius based upon the reconstructed surface. Through varying the random sample size of data from an actual measured part, repetitive analyses resulted in the Hull Normal method having a lower bias and distributions that were skewed towards the true value of the radius.