Research On Measurement And Evaluation Of Surface Machining Error Based On Reconstruction Of NURBS Control Points

With the development of auto, mold and aerospace industries, more and more parts with free-form surface are used, thus it is important to measure and analyze the machining error of these parts when high-precision parts with free-form surface are needed. With a lot of advantages in the free-form surface presentation, NURBS is widely used to reconstruct machined surface. Therefore, research on the measurement technology of parts with free-form surface has vital significance to improve the processing precision of parts based on the NURBS surface theory. On the basis of further study of NURBS surface theory and measurement technology of free-form surface, this thesis proposed a method based on on-machine measurement data and reconstruction of NURBS control points, which can effectively analyze the machining error of free-form surface.Firstly, the background and significance of this thesis are elaborated. Besides, the research status is introduced, include coordinate measuring technology, on-machine measurement technology and NURBS surface reconstruction technology, and the framework and the main research content are concluded. In addition, the technology of reconstructing NURBS surface by modifying the surface is analyzed. Considering the optimization of the constraint, reconstruction technology based on the control points of NURBS surface is proposed. The problem of input/output and displaying the NURBS surface is solved by using VC++and OpenGL technology.Secondly, on-machine measurement technology is proposed based on the reconstruction of NURBS control points. Compared with traditional coordinate measuring technology, parts can be directly measured on the machine tools without moving by on-machine measurement, so the error caused by relocation is reduced and it is important to measure the machining error of machined surface precisely. After analyzing the probe radius compensation technology during surface measuring, a new probe radius compensation technology based on the normal vector of NURBS surface is proposed and the problem of measure point planning is simply analyzed. When the machined surface is reconstructed, the minimum distance between theoretical surface and machined surface is calculated by optimization algorithm, which can present the normal error of free-form surface. After calculating the normal error, the machining error of surface is evaluated by means of surface profile tolerance.Finally, the system method proposed in this paper has been carried out in the measuring and analysis of a NURBS surface workpiece. After the workpiece is machined, the machined surface is reconstructed by making use of coordinate of measure points of the machined surface. Then the machining error of surface is gained by comparing the reconstructed surface and theoretical surface, and the surface profile tolerance is evaluated. The experimental results verify the feasibility and reliability of the proposed method.