Research On The Modelling,identification Of Volumetric Error Considering Error Coupling And Traceability Of Multi-level Geometric Errors For CNC Machine Tools

The geometric error of the machine tool is the inherent systematic quasi-static error which is one of the most important factors to determine the overall accuracy of the machine tool.The effect of each geometric error on the end error can be investiga ted and then the overall volumetric error compensation can be implemented to improve the accuracy of the machine tool.On the other hand,by research on the relations hip between the volumetric error and the error in manufacturing and assembly process,the volumetric accuracy can be improved by proper design and manufacturing at a low cost.At present,as the coupled geometric errors(the positioning error,straightness errors and angular errors are coupled together)are not considered in present volumetric error models,the results of volumetric error compensation in machine tools are not perfect.There are still some problems in measurement and identification of geometric errors as the accuracy,efficiency and feasibility cannot meet the strict actual applicatio n commands.The error transfer mechanism in the key components such as the moving carriage component is unknown that makes the assembly errors difficult to be considered in process of inspecting the origin of the volumetric error.This paper takes the volumetric accuracy of the NC machine tool as the core and focuses on research on key problems of the volumetric error modelling,compensation,error measureme nt,identification and error traceability analysis in NC machine tools.This is of important value in theory and application on improving the accuracy of the machine tool with proper design,manufacturing and compensation.The main contents are as follows:(1)The theory and method of volumetric error modelling for the machine tool.The screw theory in the application of volumetric error modeling is firstly analyzed,as well as the comparison and analysis of differences with defects of the traditiona l homogeneous coordinate transformation in modelling.As the coupling errors are not ful y considered in the traditional volumetric error model that would introduce unnecessary error in volumetric error model when implementing volumetric error compensation.To handle the problem,a method for modelling new optimized volumetric error model is proposed considering the geometric error coupling based on screw theory,and the rotation center parameter for angular errors is introduced in.The experiment on a 3-axis NC machine tool verifies the effectiveness of the optimized volumetric error model and the accuracy for compensation of the traditional model has been improved by 57%.Moreover,for a multi-axis large NC machine tool includ ing the rotary axis,the volumetric error model in the global coordinate system based on the screw theory has been established.(2)The method of measurement and identification of geometric errors in machine tools.For the six position dependent geometric errors in rotary and translational axis in the multi-axis large machine tool that cannot be measured directly,the two measuring types and identification methods by the laser tracker are proposed,which improve measurement efficiency and guarantee low uncertainty of the results.Moreover,there are other geometric errors coupling in measurement of the squareness error between every two translational axes,which cause it difficult to obtain the accurate squareness error.To handle the problem,a new measuring and identification method for the squareness error considering geometric error coupling effects has been proposed based on the measurement with the laser interferometer.The experiment,in which differe nt traditional measuring methods including traditional ball bar,squareness reference with dial gauge and laser diagonal method for squareness errors are adopted to give comparisons to the proposed measuring and identification method considering the coupled errors,has proved that more accurate straightness errors can be obtained and it would be with better forecasting precision for the overall model.(3)The theory and method for multi-level error traceability of the volumetric error.To reasonably control the volumetric accuracy of the machine tool by design and manufacturing,the relationship of errors in different levels in the volumetric error traceability should be analyzed.The assembly error transfer relationship in the translational axis is the key problem to restrict the error traceability and analysis.An assembly geometric error transformation model has been proposed after analyzing the assembly structure and geometric errors of the translational axis based on screw theory and stress and deformation analysis,which establishes the relationship between the base,slider,guideway and sliding table in quantity.It has been proved feasible in the experiment on a translational axis of the precise machine tool.Based on the proposed volumetric error model and error transfer model,a multi-level error traceability analys is method for volumetric error based on contribution ratio is proposed,and a distributio n result of error traceability analysis is also presented in the workspace.In order to analyze the dynamic influence of geometric error adjustment on the volumetric error variance,a multi-level sensitivity analysis method based on partial differential and numerical simulation is proposed.On this basis,an assembly geometric error allocatio n method and algorithm flow are proposed.The simulation results verifies that under the condition of guaranteeing accuracy,the assembly geometric error values are reasonably alocated and the cost is reduced,which can be used to guide the design,manufactur ing and assembly process for geometric errors of machine tools.(4)Based on the proposed volumetric error model and error identification method,the 3-axis NC machine tool and multi-axis large machine tool have been adopted in experimental application to verify the identificatio n and compensation.The optimized volumetric error model considering coupled geometric errors has been applied in volumetric error compensation in two types of 3-axis machine tool named ?2000/800 H and HMC63.The results show that the accuracy of the machine tools is largely improved by 20%~76% except small errors.The volumetric error compensation has also been implemented in the multi-axis large NC machine tool.The trajectory precision is improved by 54%~87% based on the proposed geometric error measurement and identification method,as well as the inverse solution of volumetr ic error compensation.The geometric error identification has been well accomplished and the volumetric accuracy has been largely improved,which verifies the feasibility and validity of the proposed model and method.