Investigation On Stability And Error Modelling During Milling Flexible Parts With Servo Support

Thin-walled workpieces are widely used in the aerospace industry.High speed milling technology is usually employed to manufacture these workpieces with high proficiency and precision.However,chatter and deformation easily occurs during milling process of the thin-walled workpiece due to its low rigidity.Henceforth,it is significant to investigate the chatter and deformation mechanism during milling process of thin-walled workpiece and thus propose effective approaches to suppress the chatter and deformation.So far,researchers have developed many approaches to suppress the chatter and deformation during milling process of the thin-walled workpiece.An approach that can suppress and deformation effectively is to strengthen the stiffness and damping of the workpiece.The methods to strengthen the workpiece stiffness and damping used in the real practice include die-by-mold support,multi-point matrix support as well as servo support.In all three methods,servo support has many advantages compared to other two methods.The servo support technology used during milling the thin-walled workpiece represents future technology trend in the field of manufacturing the thinwalled workpiece.The servo support technology during milling the thin-walled workpiece is realized by a milling machine and a support equipment.The milling machine and the support equipment are placed at two sides of the flexible workpiece.During milling process,the end effector of the support equipment will move with the cutting tool.This is the essential difference between the existed methods that strengthen the workpiece stiffness and damping during milling the thin-walled workpiece.The moving end effector of the support equipment will strengthen the local stiffness and damping of the workpiece at the cutting point,which will increase the stability characteristic of the milling process and thus guarantee the production efficiency.On the other hand,the moving supporting end effector can also decrease the workpiece deformation effectively and thus improve the dimensional accuracy of the final workpiece.Although this technology has been applied to manufacture some thin-walled workpieces in the real engineering,there exists no theoretical analysis of this newly developed technology.Henceforth,this doctoral dissertation aims to develop a theoretical analysis of this newly developed method under the grant of National Basic Research Project(973 Project)numbered 2014CB046603,and gained the following results and conclusions:Considering the influence of the time-varying supporting location of the end effector of the support equipment,based on the structure modification theory,mainly the so-called Sherman-Morrison-Woodbury formulation,a model that can compute the time varying frequency response function(FRF)of the workpiece is developed by the discretization of the milling process.The time varying FRF reflects the varying dynamic stiffness of the thin-walled workpiece under the effect of the supporting location of the moving end effector and the dynamic parameters of the end effector.It is the precondition to analyze the chatter stability of milling process with servo support.Besides,the effect of dynamic parameters of the end effector of the support equipment on the system stability can also be analyzed according to the time-varying FRF model.Based on the constructed time-varying FRF model and the machining dynamics,a chatter model that describes the chatter vibration of the workpiece during milling process with servo support is developed via considering the low stiffness of the thinwalled workpiece.The stability of the chatter model is solved based on the so-called Zero-order Approximation(ZOA)method proposed by Altintas et al..A threedimensional stability lobe diagram(SLD)that reflects system stability which is dependent on the cutter location and the cutting parameters was plotted.The three dimensional SLD was projected on the plane that normal to the axis represents the cutter location to obtain a two dimensional SLD.The two dimensional SLD can guarantee the stability of the whole milling process.From the two dimensional SLD,one can choose proper cutting parameters to realize maximum material removal rate yet to guarantee the stability of the cutting process.To validate the chatter model and the analyzed stability,a simple experimental platform was constructed to implement the validation experiments.The experimental results matched well with the theoretical ones,which demonstrated the correctness of the developed model.Based on the developed chatter model,the stability during milling the thin-walled workpieces with three different supports,i.e.die-by-mold support,multi-point support and servo support was compared.The comparison results showed that stability under these three cases are similar,and the milling process with servo support showed better stability under some locations.Under the stable cutting condition,a deformation model of the workpiece was developed to consider the coupling effect of the milling force and the servo support.A cutter-workpiece-support dynamic deformation model was constructed according to the thin shell vibration and deformation theory.Based on the mode superstition method,the dynamic deformation of the whole surface of the workpiece was solved.The model was validated experimentally.According to the deformation model,the machining error of the workpiece during milling with servo support can be obtained.The deformation along the milling path was extracted from the deformation of the whole surface that was obtained by solving the developed deformation model.The machining error was modeled based on the relationship between the error and the workpiece deformation.The machining error of the workpiece under different cutting condition,namely with servo support and without support was compared,the results showed that error of the workpiece under milling with servo support is much smaller than that under normal cutting,which illustrates that the milling process with servo support can decrease the form error effectively.Besides,the dynamic parameters of the support system on the machining error were also discussed based on the error model.The relationship between the machining error and the surface quality was also constructed.It revealed the effect of the workpiece deformation on the surface quality.This dissertation mainly analyzed the stability and the machining error of the workpiece during milling the thin-walled workpiece with servo support.The proposed model is expected to give some guidance to the designer of the servo support system and the processing engineers.