The Study On The Design Method Of Precision Chain For NC Machine Tools

Based on the studies of tolerance design of mechanical system, reliability theoryof mechanical product, mathematical definition of tolerance modeling method and thetechnology of thermal error compensation, the design method of precision chain forNC machine tools was proposed in this dissertation. In concrete terms, the formationmechanism of geometrical error, the error transmission relationship and the criterionof machining tool output accuracy based on reliability theory were investigatedsystematically. The theory and model of thermal error transmission were alsodiscussed here. This dissertation is ground on the item of National Natural ScienceFund “Research on the Consistency Modeling Theory of Product Information and DesignMethod for Complex Equipment Precision Chain”(Grant No.51175161),“AccuracyOptimization Distribution of Complex Equipment based On MBD”(Grant No.51305132) and National Science and Technology Major Project“High-end CNC machine tools and basic manufacturing equipment”(Grant No.2011ZX04003-011). The main contents and innovations of this dissertationcould be summarized as follow:The first chapter summarized the research background and significance of thisdissertation. Firstly, the origin of the NC machine tool error was analyzed, and thedefinition and classification of the new generation GPS geometrical feature weregiven. The research advances of the design method of static and dynamic accuracy forNC machine tools, mechanical reliability theory and the tolerance modeling theorywere also presented. Then the definition of the design method of precision chain forNC machine tools was proposed, and finally the main contents and innovations of thisdissertation were given.In the second chapter, the multi-body system theory was introduced at first.Based on this theory and the coordinate transformation method, the geometrical errortransfer model, which contains21parameters, was constructed for the large CNCgantry guideway grinder. Through the nine-lines measuring method, this error transfermodel was proved to attain a good prediction precision.Chapter three introduced the mechanical reliability theory and the solvingmethod of reliability, and applied this theory to the precision design field of themachine tools. Taking the above geometrical error model of the large CNC gantry guideway grinder as the research object, the state function which meet the designrequirement of the machine precision reliability was built. Reliability approximatemodel was established by response surface method with error samples from25groupsof positions in the grinder working stroke, and the reliability of the machiningaccuracy at the each working position was analyzed. Taking the mean value andminimum value of reliability of these25groups data as the judging criteria of themachining accuracy performance, the sensitivity analysis was implemented at theworking positions of the grinder with the minimum reliability, and the geometricalerror parameters were sorted according to the importance degree that affect machiningaccuracy. On the basis of the analysis result, the distribution of error parameters wasoptimized gradually by virtue of the principle of accuracy proportionality until themean value and minimum value of the reliability meet the design requirements. Theoptimization of precision distribution for large CNC gantry guideway grinder wasverified feasible through numerical examples.The tolerance modeling method based on the mathematical definition canestablish the relationship between the bandwidth of variation interval of the tolerancefeatures and the corresponding tolerance. However, the explicit function betweenthem can not be obtained because of the constraint inequalities, and hence can not beused directly in the tolerance design. To solve this problem, a new method abouttolerance modeling based on Monte Carlo Simulation (MCS) and response surfacemethod was proposed in chapter four. Firstly, the constraint inequality and tolerancevariation inequality were established with tolerance analysis theory based onmathematical definition. And then simulations using MCS method were conducted tosimulate the variation of actual tolerance surface, and get the actual variation intervalof the tolerance features. With the samples of tolerance and the actual variationbandwidth of the tolerance features obtained above, the response surface modeling ofthe tolerance and bandwidth was constructed with the response surface method. Andthis method could be used to build the model of planar dimensional tolerance, flatnesstolerance, cylindricity tolerance and straightness tolerance of axes. Finally, a typicalexample of flatness tolerance was analyzed and the result indicated that the method inthis dissertation is consistent with engineering practice, and of better modelingprecision and technique economics contrary to the traditional methods.Based on the above work, the NC machine assembly was taken as the researchobject in chapter five, and the procedure of the design method of static precisionchain for the NC machine tools was introduced in detail. Firstly taking the junction surface of the planar assembly as instance, the error modeling method of junctionsurface based on the multi-body dynamical theory was illustrated, and the errortransfer model of assembly was established based on the error model above. The statefunction of the assembly accuracy reliability was built according to the actual demandand the error transfer model. Then the surfaces of the parts of the assembly errortransfer model were taken as the research object, and the response surface modelbetween the bandwidth of variation interval of the tolerance features and thecorresponding tolerance was constructed. With the state function of assemblyprecision reliability, the response relationship between the reliability of assemblyprecision and the tolerance were determined. Taking the processing cost of assemblyas objective function, the tolerance optimum model can be obtained under theconstraint condition of the reliability of assembly precision and selection criteria ofthe tolerance. Finally this method was verified feasible through a typical instance ofthe precision chain design of assembly. The design method of static precision chainfor NC machine tools covers the entire precision design procedure including thetolerance modeling of the machine tool parts, the error transfer modeling, thejudgment criterion of the output precision and the tolerance optimum modeling.Furthermore it improves the precision design theory for the NC machine tools andprovides a foundation for engineering application of the precision design theory.The thermal error of the dynamical errors was taken as the research object inchapter six. Based on the analysis of the origin mechanics of the thermal error, thethermal error modeling for the NC machine tools was established with the fuzzyneural control method. The structure and modeling theory of the thermal error fuzzyneural network were discussed clearly, and the contrast experiments proved that thismodeling method is of better robustness and predict precision. The error transfermodeling method based on multi-body dynamical theory was introduced into theprecision chain design of the thermal error, and the method of build thermal errortransfer model was proposed，and the study of dynamic accuracy chain design methodis attempted.Chapter seven summarized all the main contents of this dissertation, and somefurther discusses and outlooks were given for the precision design of the NC machinetools.