Research On The Measuring Method For Virtual Pitch Diameter Of A Thread Based On Interaction Effect

Virtual pitch diameter is an important technical parameter of the screw thread,which directly affects whether the internal and external screw threads can be assembled and determines the connection quality after assembling.The virtual pitch diameter of a screw thread is defined in 3D space,and its size is usually determined by a limited number of key points distributed in local areas of the interaction zones.In the existing measuring methods,only the geometrical thread contour is measured in a single shaft section,and the measured data is used to calculate the value of the virtual pitch diameter which is defined in 3D space.As a result,the important contour information in other shaft sections may be missed,and some irrelevant contour information with the virtual pitch diameter may be counted in data processing,which will weaken the influences of the key points further.Based on the definition of virtual pitch diameter,an intensive study of the measuring method for virtual pitch diameter was conducted in this paper.The main contents are as follows:(1)A measuring method for virtual pitch diameter was proposed,which is based on interaction effect.The probe is a slice with a whole profile of thread,and its rigid standard thread contour can simulate the spiral motion,then a virtual gauge for screw thread is obtained,which can measure tiny deviation.As the measurement is carried out in a range of 360°,the comprehensive effect of various errors on virtual pitch diameter will be directly reflected in the probe’s pose position and orientation in each axial section.Besides,the probe’s pose position and orientation naturally depends on the key points in local areas of the thread contour,which makes the innovative measuring method presented in this paper conform to the definition of virtual pitch diameter.(2)According to the measurement principle,a set of special probe mechanisms was invented(ZL201310036983.X),which make the probe be able to move with the spiral line of the thread to be measured,and then the real-time correlation between the situation of the thread profile and the probe’s pose position and orientation was established.The probe can engage with the thread to be measured automatically and realize the spiral motion determined by the thread to be measured.The engagement can be maintained throughout the measurement procedure.(3)A prototype was innovatively developed to verify the rationality of the measurement principle and the feasibility of the proposed measuring method.By establishing the quasi-rigid body model of the prototype,the error sources and their influences were analyzed.A customized standard apparatus was designed,which was used to calibrate the prototype’s key parameters,and the calibration accuracy was traceable.(4)A data processing method for determining the value of virtual pitch diameter based on interaction effect was proposed,and the uncertainty of the measurement results were evaluated.Thread Profile Tolerancing,a new concept was proposed to provide development ideas and theoretical foundations for expanding the function of the prototype and predicting the assembly quality of the internal and external screw threads.The results of the experiments and data processing showed that the method mentioned in this paper can measure internal and external screw threads with all classes mentioned in current national standards.Compared with the existing methods,the method presented in this paper can fulfill the requirements from the definition of virtual pitch diameter further,which verified the feasibility and necessity of the measurement method proposed in this paper.