Research on Measurement and Evaluation Technology of Thread Pitch Diameter Based on Extended Virtual Three Ball Method

The thread pitch diameter is an important parameter to measure the thread manufacturing accuracy. In order to effectively measure and evaluate the thread pitch value which satisfies the actual thread fit effect by using machine vision, this paper presents an extended virtual three ball method combined with standard thread profile to measure thread pitch diameter. In this method, the middle diameter model of the extended virtual three ball method is constructed on the shaft section, and then the standard thread tooth profile matched with the actual thread profile is obtained. the extended virtual three ball method is applied to the standard tooth profile to obtain the center diameter value which is closer to the actual effect. Finally, the average relative error and standard deviation are used as evaluation indexes to evaluate the center diameter values of different experimental equipment. The experimental results show that the relative error between the middle diameter value measured by the image measuring instrument and the middle diameter value obtained by the comprehensive thread measuring instrument is stable at 3.3 μm, which provides an efficient and reliable measuring method for machine vision to measure the middle diameter of screw thread.