| Development of high-level CNC machine tools,especially precision and ultra-precision multi-axis CNC machine tools,is not only the key technology towards the improvement of the international competitiveness,but also the important signs for the assessment of a country’s manufacturing level.Since 1990 s,the rapid development of aerospace,defense,marine,micro-electronics,communication,automobile manufacturing and other industrial technology puts forward higher requirements for the high precision complex parts,components surface integrity and manufacturing precision.Therefore,the development of CNC machine tools with high efficiency and high quality,high precision and high intelligence are more urgent.The thirteenth five-year plan of China clearly put forward 100 major projects and engineering that will be implemented within five years,which relates to science and technology,equipment manufacturing,transportation,environmental protection and energy and other fields.Among them,the twenty-eighth project is to develop the high-level CNC machine tools.So it can be seen that the development of high-level CNC machine tools not only manifest the national strategic intent,but also have a profound impact on economics,society and human life and so on.There are two main methods to improve the accuracy of machine tools.The first one is the "hard technology" error prevention method.Although it can reduce the error of the machine,the economic cost is high.The other is the "soft technology" error compensation method,which can achieve a high processing accuracy hardly reached in "hard" at a very small price.Nowadays,a lot of experiments and research have been conducted on many precision manufacturing equipment,precision machine tools and precision instruments based on this method.However,the commercialization realization of this method is still unreachable.The goal of this topic is to improve the positioning precision of CNC machine tools,the machining precision and efficiency.The research object is the two sets of self-built multi-axis CNC machine tools.The geometric,thermal and force measurement error of CNC machine tools are analyzed and researched.The precision of the integrity machine and the main parts of the machine and the relationship between them are studied.The comprehensive experimental error model and error compensation experiments are carried out for geometric,thermal and force error.The main research content and achievements are as follows:(1)Based on the theory of multi body system,the ideal motion and its motion characteristics of adjacent typical bodies are described by using homogeneous transformation matrix.The kinematic analysis and research on the geometric error,thermal error,force error and other error elements of the adjacent typical bodies are carried out;Kinematic description,analysis and research are carried out on the error of the adjacent typical bodies in the actual movement,and the model of the machine tool error is built.The model can synthetically consider and analyze the error elements of the machine tool,and can analyze and decouple the coupling relationship between them.The steps of error modeling and the significance of modeling methods are combed.(2)The large number of error data were measured and analyzed by using laser interferometer instrument on the self-built five-axis precision CNC machine tools.It is verified that the different feed speed of the machine tool has no significant influence on the displacement error and angle error of the moving axis,and the influence of different measuring distance on the linear displacement error is slight.It is found that the linear displacement error is the most important factor among the ones affecting the compensation error,and the rotation angle and the straightness error has lesser influences on it.To classify and analyze the error elements of the machine tool,we can know that there are 126 items of geometric,thermal and force error elements,which are simplified and combined into 42.Based on the multi-body system theory,geometric,thermal and force error elements kinematic synthesis error models of machine tools are established,and the correctness of the model is verified.The kinematic synthesis error model of machine tool geometric error is constructed,and the geometric error variation rule of each moving axis and rotation axis of five-axis precision CNC machine tools is analyzed and studied.Obtain the comprehension error variation rules of axes movement along axis X,Y,or Z and angle error in the case of two-axis linkage.(3)The relationship between the main parts of the machine,and the precision of each other were studied with the self-built four-axis precision CNC experiment platform served as the research object.The geometric errors of the moving shaft and the rotating shaft of the precision CNC experiment platform are measured and analyzed,and the curve of the positioning,straightness and angle error are obtained,and it can be founded that the positioning error is the main influence factor of the error of the machine tool’s geometry.The comprehensive geometric error model of four-axis precision CNC experiment platform is established.The geometric error variation rules of six axes movements along axis X,Y,and Z and angle error in the case of two-axis linkage and coupling of the geometric error on the four-axis precision CNC experiment platform are obtained through the systemic research,demonstration and analysis.Through further analysis,it is found that the component curves of comprehensive error in translational axial and the positioning error is basically consistent,excluding a small difference.It is proved sufficiently that the positioning error is the key factors determining the geometric error.At the same time,it is found that the influence of angle and straightness error on geometric comprehensive error is minimal,excepting in ultra-precision machining.When the machining accuracy is not high,the optimal working space is selected by the optimization algorithm,and the machining accuracy can be achieved with the positioning error compensation.This method has many advantages,such as simple operation,fast and high efficiency and so on.(4)This paper laid special stress on analyzing four-axis precision CNC experiment platform’s variation law between temperature of the measured key points and position error during the process from normal temperature to the thermal steady state.It is found that the position error of the moving axis of the experiment platform increased with the increase of the temperature of the platform until the experimental stage reached the thermal state and the positioning error fluctuated in a small range.Therefore,it was concluded that the relationship and the variation law between the position error and the temperature change of the moving shaft of the four-axis precision CNC experiment platform.Also,based on the grey theory,the temperature measuring point,which has the largest correlation degree with the thermal error of the machine tool,was selected as the thermal error modeling variable;the geometric and thermal error of moving axes of precision experiment platform was modeled by orthogonal polynomial and Newton interpolation algorithm.The integrated model has the advantages of high precision,strong robustness,simple calculation,easy programming,and higher efficiency and so on.The compensation experiment was conducted by using the compensation module of four-axis precision experiment platform,and the correctness and validity of the model are verified.(5)Measurement and characteristic analysis on positioning error of moving axis of five-axis CNC Machine Tools in the simulated processing state was carried out.And through the analysis on a large number of experiment data,it is found that the spindle speed,feed speed and axial polishing depth had no significant effect on the positioning error of moving shaft of machine tool.Three modeling methods-BP,RBF neural network and supporting vector regression were comprehensively compared and analyzed and the thermal error model of five-axis CNC machine Tools and four-axis precision CNC experiment platform were established using three modeling methods respectively,which concludes that SVR has the best performance,which has the most narrow belt deviation,the minimum degree of dispersion and the slowest trend off center value.Compensation experiment was carried on five-axis CNC machine tools and moving shaft of four-axis precision CNC experiment platform respectively according to the forecasting data of SVR model,and the compensation experimental results showed that prediction ability,compensation effect and robustness of supporting vector regression modeling method were better than those of RBF and BP neural network method. |
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