Highprecision external screw measuring method based on doubletelecentric optical system

External screw parameters such as top diameter, effective diameter, bottom diameter and lead of the downhole explosive device body are needed to measure during producing. In order to improve the measurement automation, the external screw machine vision measuring method was studied on the basis of the device body structural features. A doubletelecetric optical measuring system was designed for the purpose of improving the measurement accuracy. The image region extraction method, quick feature points extraction method, and others parameters measuring methods were proposed. In addition, experiments were performed on the built platform. According to the results, the thread lead is 3 980.8 m, the large diameter is 65 435.6 m, the small diameter is 60 669.4 m and the toothform height is 2 383.1 m, meeting the demand of 4class threaded precision, which can prove the correctness and feasibility of the method.