Research On Instability Characteristics Of Thin-walled Parts With Complex Curved Surface And Variable Thickness During Multi-axis Milling

The thin-walled parts with complex surface and variable thickness has been widely used in the field of aerospace, automotive, mold, energy and other fields because of its light weight, high specific strength advantage. The flutter phenomenon during milling process is a main limiting condition for it to achieve high speed, efficiency, and precision. This paper focuses on the modal of its multi-axis milling instability, and the goal is to improve the efficiency of process under the assurance of high process quality, and do some research on tool workpiece contact zone, cutting force, time-variable, and stability of the system based on the theoretical analysis and experimental research methods.Establish the modal of multi-axis milling tool workpiece contact zone, and the method of extract the multi-axis milling tool workpiece contact zone based on solid entity analysis is proposed. In the established model, in order to describe the motion of multi-axis milling cutter accurately, three coordinates are added, and respectively is:the global coordinate system GCS, machining coordinate system and tool coordinate system TCS, and derives the transformation between these various coordinate system coordinate. The proposed method of tool workpiece contact zone combines the solid entity method and analytic method, so as extract the tool workpiece contact zone, there is no need to construct the swept volume and update the process of the workpiece, with high speed and efficiency.Through the observation of the existing shape milling tool, a general form of universal tool milling process is presented, and a general expression of general machine tool geometric parameters is derived, and also the cutting force calculation model of universal tool in multi-axis milling process is established. The section of the proposed general tool is composed of a series of lines and most of the existing cutting tools can be divided into straight and circular arcs. The accuracy of the model is verified by the experimental results. In addition, the cutting force of different parts of the impeller in the machining process is simulated based on the calculation model of cutting force, which is of great significance for the optimization of machining parameters, and avoid excessive machining error caused by the cutting force.A time-variable parameter model of the system is established considering the effect of material removal and processing position on the dynamic characteristics of multi-axis milling system with complex curved surface. The removal process of the material is considered to be the change of the structure, and the whole machining process is discretized into finite elements along the tool path. The cutting process is considered to be a process of remove of the discrete unit one by one. The dynamic characteristics are obtained at the midpoint of the discrete element workpiece before and after processing through experimental modal or finite method, then predict the regularity of tool’s dynamic properties with the remove of materials based on the Sherman-Morrison-Woodbury formula. The accuracy of the method of obtain the dynamic characteristic in the process of machining based on the formula of Sherman-Morrison-Woodbury was verified by experiment, and the dynamic characteristics of the workpiece in the rough machining process were predicted, and is of great significance for the stability analysis.A multi-axis milling stability model for thin walled parts with complex curved surface is established, the control equation of the model is a time-variable parameters delay differential equation. The delay term of the equation is approximated with semi discrete method, and the original equations are conversed into ordinary differential equations. A series of ordinary differential equations is obtained by solving the obtained Floquet matrix, and the stability limit of the delay differential equation with time-variable parameters can be determined by the value of the characteristic multipliers of the matrix. In order to verify the accuracy of the stability limit diagram obtained by semi discrete, experimental research was carried out, in the experiment process different cutting parameters and the stability limit diagram are chosen for comparison, and there is a good agreement between them. That is to say, the milling stability limit diagram can be obtained by semi discrete method, optimization of machining parameters, avoid chatter occurring in the process, improve the machining precision and efficiency.