| Precision is one of the main performance indexes of CNC machine tools,which determines the machining precision and quality of products.In order to improve the precision of CNC machine tools,the forming process of motion error of mechanical transmission system was analyzed based on Meta-action and Meta-action chain.The research work of optimal distribution of motion precision of the Meta-action chain,error modeling and motion precision optimization analysis of the Meta-action was carried out.The optimal distribution technology of motion precision of Meta-action chain was proposed and the allowable value of motion error of Meta-action was obtained.The main function of Meta-action and the structure of Meta-action unit were analyzed.An error accumulation path identification method based on the concept of degree of freedom constraint is proposed,and the assembly error model of Meta-action unit is established.The types of error sources in the Meta-action unit were analyzed,and the logical relationship between the motion precision of Meta-action and the error source is clarified,then the motion precision model of Meta-action was established.The influence of part assembly process on production cost was analyzed,and the comprehensive manufacturing cost model of Meta-action unit was established.Then,the optimization analysis of Meta-action motion precision was carried out.The specific research contents of this research were as follows:(1)The optimal distribution technology of motion precision of Meta-action chain was proposed.The mechanical system was structurally decomposed by FMA decomposition tree,and the series Meta-actions and Meta-action chain were obtained.Based on the Meta-action chain,the forming process of motion error of mechanical transmission system was analyzed.An optimal distribution method of motion precision of Meta-action chain was proposed.The motion precision of Meta-action chain was distributed and the allowable value of motion error of Meta-action was obtained.In order to improve the rationality and accuracy of the analysis results,the effects of cost factors and sensitivity factors were comprehensively considered.The cost function was established by calculating the comprehensive assembly complexity of the Meta-action unit.At the same time,the sensitivity analysis of motion precision of each Meta-action was carried out,and the robustness function was established.Then,the mathematical model of motion precision optimal distribution of Meta-action chain was established by taking the cost function and robustness function as objective constraints.Finally,the allowable motion error of Meta-action was obtained.Based on the forming process of motion error of mechanical transmission system,the proposed method realizes the optimal distribution of motion precision,and the distribution result was more in line with the actual situation.(2)A fast identification method of internal error accumulation path of Meta-action unit was proposed.The main characteristics of the parts and the error model of the joint surface were analyzed,and the small displacement torsor model was used for modeling.At the same time,the common forms of adjacent parallel joint surface and parallel support structure in mechanical system were analyzed and modeled.Based on the joint surface error model,the binary vector of the small displacement torsor model was used to represent the constraints of part degrees of freedom in assembly.Then,a new method for identifying the error accumulation path of Meta-action unit based on the concept of degree of freedom constraints was proposed.The proposed method avoids the tedious analysis process,and lays a foundation for the assembly error modeling and motion precision analysis of Meta-action.(3)The influencing factors of motion error of Meta-action were analyzed,and the motion precision model was established.According to the structural characteristics of Meta-action unit,the influencing factors of motion precision were analyzed.The error sources that affect the motion precision,namely the device error of the Meta-action unit,the inherent error of the power input part and the power output part were defined.The constituent elements of device error and the logical relationship between the motion error of the Meta-action and each error source were analyzed.The assembly error calculation model of Meta-action unit was established by using Jacobian torsor model,and the analysis and calculation of Meta-action unit device error were realized.Finally,considering the device error,the inherent error of the power input part and power output part of the Meta-action unit,the motion precision model of the Meta-action is established,which lays a foundation for the optimization analysis of the motion precision of the Meta-action.(4)The optimization technology of motion precision of Meta-action was studied.The motion precision model of Meta-action showed the mathematical relationship between part geometric error and motion precision.The optimization analysis of motion precision can be realized by optimizing the tolerance of part features.Therefore,the motion precision optimization analysis of Meta-action becomes a tolerance optimization problem constrained by motion precision.The influence of part assembly cost on the comprehensive manufacturing cost of Meta-action unit was studied.Various factors affecting the part assembly process were analyzed.And the comprehensive manufacturing cost model of Meta-action unit was obtained by solving the part assembly difficulty coefficient and modifying the traditional tolerance-cost model.Then,taking the minimum comprehensive manufacturing cost and the minimum quality loss cost of motion precision of the Meta-action unit as the goal,the Meta-action motion precision as the constraint condition,the part feature tolerance and the inherent error of power input part and power output part as the optimization variables,a multi-objective optimization model of motion precision of Meta-action was established.The Pareto frontier solution set was obtained by solving multi-objective optimization model.Finally,the candidate solutions in the Pareto frontier solution set were analyzed and sorted by using TOPSIS multi criteria decision-making method,and the optimal solution was determined.The motion precision of the Meta-action was guaranteed under the minimum manufacturing cost and mass loss of motion precision. |
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