Study On Contact Characteristics Of Spindle-toolholder Conical Joint Surface At Macro-micro Scale

High-speed motorized spindle is one of the key components of highperformance machining centers.As an important component in its internal structure,spindle-toolholder is a typical application of conical interference fit.The contact stiffness of the spindle-toolholder tapered joint is influenced by the speed,tool pulling force and interference of the high-speed electric spindle,and in turn,the contact stiffness of the joint has a certain impact on the rotary accuracy of the electric spindle.In this paper,the fractal model of the normal load and tangential/normal contact stiffness of the spindle-shank conical joint surface is established from the microscopic scale by applying the fractal theory,and the normal load fractal model is proposed as a bridge to correlate the pressure values of the spindleshank conical joint surface at the macroscopic scale to obtain the three-dimensional equivalent of the spindle-shank joint surface.On this basis,the influence of rotational speed,tool pulling force,interference and friction factor on the contact characteristics of spindle-shank cone surface is investigated,and the accuracy of this research method is indirectly verified through spindle rotation accuracy experiments.The results of the study provide theoretical reference for the design of spindle-to-shank taper coupling.The main research contents and results of this paper are as follows:(1)Establishment of a two-dimensional fractal contact stiffness model for the conical bond surface.In order to characterize the contact problem of the conical bond surface under actual working conditions,the contact coefficient of the conical bond surface is constructed,and the two-dimensional fractal model of the normal/tangential contact stiffness of the conical bond surface considering the micro-convex body interaction and domain expansion factor is established by applying the fractal theory.The accuracy of the model is verified by taking the high-speed spindle-toolholder conical bond surface as the application object,and the influence of parameters such as fractal dimension,characteristic scale and material property coefficient on the normal/tangential contact stiffness under the action of axial force is analyzed.(2)The three-dimensional contact stiffness fractal model of the spindleshank conical joint surface is established.In order to better meet the actual contact condition of the bond surface,the three-dimensional normal load model P and normal/tangential contact stiffness model of the spindle-shank cone bond surface are established based on the two-dimensional contact stiffness model and considering the friction factor,in which the normal load model P is used as a bridge parameter connecting the macroscopic and microscopic scales.(3)The contact characteristics of the spindle-shank conical joint surface are analyzed.The equivalent radial stiffness/torsional stiffness of the spindle-shank conical joint surface was obtained by equivalence processing after extracting the pressure value of any bus node on the conical joint surface with the help of the spindle-shank finite element model and bringing it into the normal load model P.The influence law of different rotational speeds,broaching force,interference amount and friction coefficient on the equivalent stiffness of the conical joint surface was analyzed.It was found that the equivalent stiffness of the spindle-shank conical joint surface decreased the most when the rotational speed was from 9000 r/min to 11000 r/min,which would affect t he radial displacement of the spindle,and the contact characteristics of the spindle-shank conical joint surface were better when the ultimate rotational speed was11000 r/min,the broaching force was from 11 k N to 17 k N,the interference amount was from 8 μm to 20 μm and the friction coefficient was less than 0.5.(4)Experimental verification.From the experiments,it is found that when the spindle speed is at 9000 r/min,the spindle slewing error is the largest and the vibration displacement offset in the y direction is stronger;while the slewing error is the smallest when the speed is at 11000 r/min;this phenomenon is in line with the spindle speed mentioned in(3)when the equivalent stiffness reduction is the largest at 9000 r/min～11000 r/min and the limit speed is 11000r/min.min.The experimental results indirectly demonstrate the accuracy and rationality of the contact characteristics of the spindle-shank tapered surfaces at macro and micro scales proposed in this thesis.