Dynamic Modelling And Chatter Stability Prediction Of The Milling Process Of Thin-Walled Workpiece

In order to minimize the weight of the aircrafts and spacecrafts as well as meet mechanical requirements of the structures,thin-walled workpieces are substantively used in aeronautic and astronautic industries.Due to their characteristics of large size,complex shape and low rigidity,chatter often occurs under the cutting forces and heat when milling thin-walled workpieces.This leads to several negative effects,such as decrease of machining efficiency,surface accuracy and surface quality,increase of tool wear and damage,even damage of the workpiece.Thus,it is of great significance to investigate the mechanism during cutting of thin-walled workpieces,solve the critical issues arising from stability prediction of cutting of thin-walled workpieces and develop dynamic model of the thin-walled workpiece cutting process.This thesis aims at investigating critical issues for stability prediction of milling process of thin-walled workpieces from the perspective of the cutting techniques and computational mechanics.To reveal the dynamic behavior,i.e.vibration of the workpiece and tool during milling processes,an efficient method to extract cutter-workpiece engagement maps,a generalized method for the analysis of receptance of tool-holder-spindle assembly,a bi-distributed joint interface model for collet chuck holder,an efficient method to identify the in-process workpiece dynamics and a new dynamic model for stability analysis of peripheral milling of workpiece with curved surfaces are developed in detail in this thesis.The main research works and contributions will be introduced as follows:1)Development of method to extract cutter-workpiece engagement maps.Based on Brep solid modeler,a solid trimming method to extract cutter-workpiece engagement maps for multi-axis milling is developed.The removal material is generated by analytical tool swept volume and trimmed by the feasible contact surfaces.Both simulation of the material removal procedure and extraction of cutter-workpiece engagement maps are implemented on the removal material.This scheme can control the growth of data structure of the workpiece,avoid abundant surface/surface intersections and improve the computing efficiency and accuracy.Comparisons of calculated and experimental results show that the the proposed method has the advantages of high accuracy and efficiency.2)Modelling of tool-holder-spindle assembly for receptance analysis.The generalized geometries of rotating tools,such as helix end mill,twist drill bit and tap,are summarized since they have similar geometrical and assembly features.Based on the generalized geometries,Timoshenko beam theory is adopted to comprehensively model the translational and rotational dynamic responses related to all axes(X,Y and Z).The influences of shear deformation on the receptances of tool at high frequency,the effect of the actual geometry of the tool's fluted part on its receptance as well as the distributed damped-elastic feature of the joint interface are considered.Based on structural dynamic modification scheme,correction algorithm for the elimination of the adapter's mass effect on torsional and axial measurements is proposed.A method to measure torsional and axial receptances of tools in small size is proposed.Comparisons of predicted and experimental results show that the proposed model for receptances analysis can predict the bending,torsional and axial receptances accurately and the proposed method is valid for measuring tool point torsional and axial receptances.3)Modelling of tool-holder-spindle assembly using bi-distributed joint interface model for collet chuck holder.Considering the effect of collet on the tool point receptances,a model to predict the tool point receptances is developed.The tool-collet and holder-collet joint interfaces are regarded as two distributed damped-elastic layers with varying stiffness.A method to calculate receptance matrix of spindle-holder subassembly is proposed by using the receptance coupling substructure analysis.Comparisons of predicted and experimental results show that by taking the actual effect of the collet into consideration,the proposed model can give accurate prediction no matter which thickness of the collet is used.The method to calculate receptance matrix of spindle-holder subassembly can avoid repeated impact tests on different spindle-holder combinations.4)Development of method to identify in-process workpiece dynamics.Based on structural dynamic modification scheme,an efficient method is developed to determine in-process workpiece dynamics.The effect of material removal,different tool positions and different elevations along the tool axis upon the in-process workpiece dynamics are taken into consideration.An extraction procedure for in-process workpiece dynamics based on the FEM model of the initial workpiece is proposed.It allows meshing the initial workpiece,building the FEM model and performing modal analysis of the FEM model of the initial workpiece only once.As a result,the in-process workpiece dynamics can be obtained by using structural dynamic modification scheme based on the initial workpiece dynamics.Comparisons of predicted and experimental results show that the proposed method can accurately and efficiently determine the in-process workpiece dynamics.5)Modelling of peripheral milling of thin-walled workpieces with curved surfaces for stability prediction.Considering the effect of time-varying engagement and feed direction on the cutting force and stability of peripheral milling of thin-walled workpieces with curved surfaces,dynamic cutting force model is developed.Based on the cutting force model,a new dynamic model of tool and workpiece system is proposed to consider the varying dynamic behavior of tool and workpiece caused by material removal,different tool positions and different elevations along the tool axis.3D stability lobe diagrams are evaluated in terms of axial depth of cut,spindle speed and tool position by using the proposed model.Comparisons of predicted and experimental results show that the proposed model can accurately predict the stability for peripheral milling of thin-walled plate and workpiece with curved surfaces.The research works in this thesis can supply some principles for stability prediction of milling process of thin-walled workpieces and be applied in optimizing milling process of thinwalled workpieces.