Compensation of thread profile distortion in image measuring screw thread

The thread measuring method possesses an efficient and easy to install with image measurement of axial section but results in thread profile distortion. Through the geometric analysis of the axial section projection image of the screw thread, it is confirmed that phenomenon of thread profile distortion always exist in measuring screw thread by projection image, meanwhile, the requirements of depth of focus is put forward with the method, calculation formula of thread profile distortion is deduced, and the corresponding compensation algorithm is given; through the simulation, analysis of coarse thread, it found the distortion has influence in thread angle and pitch diameter significantly, reaching 2.3% and 3.36% respectively, the unilateral shaded proportion of the thread profile in the root region is up to 7.12%, but the root diameter itself remains unaffected mostly. Take metric coarse thread gauge as measurement object, and compared the results with tool microscope, experiments show that the compensation effect is more than 85% in pitch diameter, more than 70% in thread angle.