Research On Error Compensation Technology For On-machine Measurement Of Curved Surface

With the rapid development of high-end manufacturing industries such as aerospace,defense industry and medical devices,the application of curved workpieces is becoming more and more widespread.How to ensure the problem of machining accuracy,machining efficiency,measurement accuracy and measurement efficiency of curved workpieces has become a hot spot for domestic and foreign scholars to study.In the face of increasingly complex machining workpieces in the manufacturing industry,the process of inspection can be realized by computer numerical control(CNC)machine tools for on-machine measurement(OMM).On-machine measurement can measured errors without disassembling the workpieces,so as to compensate and effectively improve the measurement efficiency.This paper focuses on the compensation of probe radius,and the machining error compensation based on a compliance error model reconstructed surface method,and the prediction and compensation of machining errors using a genetic algorithm optimized BP neural network algorithm.Specifically,these include the following.(1)When using the contact measurement such as on-machine measurement for inspection,the obtained is the data coordinates of the probe center point,and there is an error between the actual measurement points and probe center points.To address this problem,a method that combines the triangular mesh model method with the non-uniform rational B-spline(NURBS)surface reconstruction method is proposed to compensate for the probe radius.First,the sampling point of the probe center is fitted to a curved surface and then the surface is triangulated to solve for the triangular mesh vertex normal vector and calculate the actual measurement points.Then select the area where the Gaussian curvature is large or the angle between the surface normal direction and the coordinate plane is small,and perform NURBS surface reconstruction to calculate the reconstructed real points.Finally,the proposed method is to verify by MATLAB simulation experiment and comparison experiment before and after the actual surface compensation by coordinate measuring machine(CMM)measurement.(2)To reduce the errors caused by the overall compliance of the system and the machine tool wear during the machining process for curved workpieces,and to improve the machining accuracy of curved workpieces,a machining error compensation method using a compliance error model to reconstruct the surface is adopted.During a process involving multiple cuts,on-machine measurement is achieved by replacing the cutting tool with a touch probe.After the last semi-finishing cut,the error values after machining are measured and compensated for using a compliance error model that predicted the change in the overall compliance of the system and estimated the error at the finishing cut depth based on the measured machining error at the previous cut depth.In this paper,two methods,the surface reconstruction method based on the compliance error model and the overall modified machining allowance method based on the compliance error model,are used to experimentally verify the machining error compensation for two formula surface workpieces and two free-form surface workpieces,respectively.Then the final four machined workpieces are put into CMM for accuracy inspection to verify the compensation effect of the two methods.(3)To address the problems of inefficient inspection and relatively low machining accuracy of free-form surfaces,a prediction and compensation model of free-form surface machining error is developed.This model used a genetic algorithm(GA)to optimize the back propagation neural network(BPNN)for prediction and compensation of free-form machining error.Firstly,the machining error of some data points are measured on the free-form surface by CMM,and the errors are used to predict the machining error of other data points and to compare with the actual measurement results.The algorithm is then used to predict the machining error after semi-finishing,and the two free-form workpieces are compensated by the surface reconstruction method and the overall modified machining allowance method respectively.Finally,the on-machine measurement experiment and CMM accuracy measurement experiment are conducted to verify that both methods can improve the measurement efficiency and machining accuracy.