Correlative Mechanism Study Between Bolt Connection And Accuracy Retention Of High Precision Machine Tool

Compared with foreign high-precision machine tools,domestic high grade Computerized Numerical Control(CNC)machine tools generally exposed poor accurary retention problems in the service period,which has become a bottleneck to restrict their competence in the market.Most of beds,as the general basic large component in heavy duty machine tool,use the ways of anchor bolt group connection,and the structural stability after site installation will directly affect the entire CNC machine tool machining precision and accuracy retention.During the pre-tightening process of the anchor bolt group,the elastic interaction between bolts and the creep-relaxation effect of the bolt based on the time-hardening cause the change of the bed geometry and the stress distribution,thereby affecting the geometrical precision and accuracy retention of the machine tool.In this paper,researches on theoretical modeling of the elastic interaction between bolts,bolt creep relaxation mechanism and bolt creep force are conducted regarding multi-bolt joints assembly as the research object,and the results are applicated to achieve anchor bolts pre-tightening process quantitative analysis and pre-tightening program optimization in five-axis gantry machining center,the main research contects include the following aspects:(1)Beginning with the theoretical modeling of the elastic interaction stiffness in single-bolt connection structure,the elastic stiffness expression of the single-bolt connection is calculated and based on this,the theoretical model of the four-bolt connection structure is generalized by simplifying the bolt-hole unit of the connector as the "spring-node" structure,and then generalized to a general n-bolt elastic interaction model,and the residual pre-tightening force of each bolt after the bolt group is preloaded in a certain order is obtained.(2)A general power-law model based on age-hardening is used to describe the creep relaxation behavior of bolts,the mathematical model of bolts' axial creep deformation is established and generalized to a general model of the general interaction effect of n-bolt elasticity and creep relaxation effect,and the residual pre-tightening force of each bolt after the bolt group is preloaded in a certain order is obtained.In addition,the concept of bolt creep force is introduced to accurately describe the axial creep behavior of bolts during the creep period.Combined with the finite element simulation technology,the creep force's general formula of each anchor bolt in five-axis gantry machining centeris is established.(3)The influence of the elastic interaction between bolts on the residual pre-stress distribution of the bolt group is calculated by means of comprehensive theoretical derivation,numerical simulation and experimental test,and the general model's accuracy of elastic interaction is verified.Based on the experimental test,the influence of different bolt pre-tightening schemes on the distribution of the residual pre-tightening force uniformity of the bolts in the bolt group was analyzed.(4)Based on the design criteria of the minimum installation deformation error of the rail mounting surface on both sides of the bed and the stress distribution of the uniform bolt,and how these factors,including pre-loading order,pre-loading steps' number and pre-loading steps' ratio,influence the geometrical accuracy of rail mounting surface,uniformity of the pressure distribution and precision retention of the gantry machining center are analyzed quantitatively.The results show that the corresponding pre-loading effect of the anchor bolt group is best when the bolt load is applied in two steps with the initial step pre-loading ratio of 50%,following the order from two sides to the middle.Combined with the theoretical modeling,the differences of the creep force of anchor bolts and the deformation of the rail mounting surface on both sides on the bed between origine program and optimized program areanalyzed,and the geometrical accuracy and accuracy of the machining center are quantitatively demonstrated.