Measurement Theory And Method Of Thread Functional Parameters

The screw thread is a complex geometric body.When the internal and external threads are assembled,it should be the contact of the spiral surfaces in theory.However,the actual threads have machining errors,and the expressions of their respective errors and the distribution in three-dimensional space are different.It is difficult to quantify the actual assembly effect.For the sake of simplicity,the actual thread is assembled with a virtual ideal thread,which reflects some characteristics of the actual thread after assembly.The thread standard ISO 5408: 2009 defines that the functional diameter is the pitch diameter of a virtual ideal thread that can fit the actual thread perfectly and does not interfere with the crests and the roots.In this paper,according to the definition of the functional diameter,the concepts of the functional boundary and the functional axis are proposed.By theoretical research,it solves key problems such as the measurement of the functional boundary,the extraction of the thread profile features,the correction of the measurement errors and the evaluation of the thread assembly effect.A novel comprehensive measurement method is applied to provide a new technology for improving the thread inspection level and perfecting the thread inspection system.The main research contents of this paper are as follows:(1)The measurement theory and method of thread boundary is proposed.The contour feature extraction model is established with axial sectional profile as the real feature by measuring the whole thread.The associated derived feature is obtained as the reference axis,and then the position and geometric dimension of the boundary are determined.(2)The compliant mechanism design theory is applied to establish the pseudorigid-body model of the threaded probe,which ensures that the measuring tip can comprehensively reflect the contour features of the measured thread in real time.The measurement error source of the measuring head and the measurement uncertainty caused by the motion errors are analyzed.(3)For the structure of the measuring tip on the prototype,the error compensation method of the lead angle factor is proposed.A new type of measuring tip is invented,which can directly eliminate the error source of the lead angle effect.(4)Establish a size transfer system based on the axis of the measuring instrument turntable,and propose a rapid calibration method for the measuring tip.(5)The precision and accuracy of the measuring instrument are verified by experiments;the comprehensive evaluation system of the thread based on the functional boundary measurement is preliminarily established,and the thread workpieces measurement experiments are carried out;the uncertainty of the measuring instrument is evaluated according to the error source analysis.