Research On Turning Vibration Characteristics Of Thin-walled Shell Workpiece Under Time-varying Factors

The cutting process of thin-walled shell has always been the focus and difficulty of the industry.It is easy to produce cutting vibration which known as the chatter due to the length-to-diameter ratio or diameter-thickness ratio was too large.The time-varying of the cutting system is observed during the machining process for the continuous metal cutting and the change of the cutting position of the tool.Therefore,the dynamic model considering the influence of time-varying thickness and position was applied to analyze the turning vibration characteristics of thin-walled cylindrical workpieces to better evaluate and predict possible machining vibration problems before machining.The main contents of this paper are as follows:Firstly,the dynamic equations of the static and rotating state of the thin-walled shell are established.The natural frequency and mode ratio of the workpiece are obtained through the analytical method that the natural characteristics of the workpiece under clamped-clamped,clamped-simply supported,simply supported-simply supported and clamped-free boundary conditions were conducted.The modal analysis of the workpiece is carried out with the finite element software,and the natural frequencies and modal modes of the static and rotating shells are obtained.The traveling wave vibration of the shell is verified with the modal analysis of the rotating workpiece,and the relation between the traveling wave frequency and the rotational speed is obtained.Considering the influence of time-varying thickness in the cutting process,the paper analyzes the change rule of natural frequency of thin-walled shell workpiece after a complete cutting.The research shows that the natural frequency of the workpiece gradually decreases with the continuous reduction of the thickness,and the modal shape of each step changes with no certain regularity.Secondly,differential equations of vibration for static and rotating state of thin-walled shell workpiece are established.The displacement response of workpiece under the cutting force is obtained through the vibration characteristics investigation of workpiece under clamped-clamped,clamped-simply supported,simply supported-simply supported and clamped-free boundary conditions.The vibration response characteristics of the workpiece under the action of radial simple harmonic cutting force are obtained using the finite element software.In addition,the traveling wave vibration law is obtained in frequency domain.The results show that under the same vibration mode,the Coriolis force effect caused by workpiece rotation bifurcates its natural frequency,i.e.the backward traveling wave with increased frequency and the front traveling wave with reduced frequency.The vibration response of cantilever thin-walled shell workpiece at different cutting positions is analyzed in which the influence of time-varying position in cutting process was considered.The research shows that as the tool moves away from the chuck,the vibration response of workpiece increases accordingly.Therefore,chatter can be avoided by adjusting cutting parameters.Finally,a single-degree-of-freedom chatter model with a thin-walled shell workpiece as the main vibrating body was established.Based on the Nyquist stability criterion,the three-dimensional limit relation of cutting chatter,the cutting depth with speed,as well as the stiffness were obtained.Modal knock tests and turning tests were carried out on flexible slender shaft and thin-walled shell respectively to get the amplitude-frequency response characteristics and displacement vibration laws of the workpiece.The results show that thin-walled cylinder undergoes two stages of stabilization-chatter in the whole turning process.The area near the free end experiences the chatter stage,and the area near the fixed end experiences the stabilization stage.