Automatic vision inspection and measurement system for external screw threads

Production of threaded parts in some manufacturing environments requires high-precision thread measurement and a reliable inspection method. This paper describes considerations and techniques used in the development of a vision system for measurement and inspection of male screw threads. A two-stage process was developed to calibrate the imaging system. This procedure uses a calibration gauge pin to calibrate the vision system's pixel scale and two pitch standards (max, min) to correct for lighting and image distortion. New and enhanced imaging techniques were used to determine outer and inner diameters for each thread pair. These techniques enable the system to determine the maximum, minimum, and average inner and outer diameters of each part. Pitch measurements were obtained using cross sections of each thread pair in the image. The dark and light (crest and root) areas were examined to deduce pitch diameter very precisely. Measurement techniques for overall thread length and full thread length were found to be beneficial to the part inspection process. Lab results as well as production line measurement results using the vision system are compared to optical comparator, micrometer, and gauge measurements. Overall accuracy was within ± 0.0005″ of gauge measurements. Some measurements exceeded this accuracy in lab tests and on the production line. The thread measurement system was demonstrated to be extremely beneficial during the manufacturing process.