Tool Path Planning For Multi-axis NC Machining Based On Closed-loop Stiffness Model

With the development of national defense industry, transportation industry and energy industry etc., more and more requirements are brought forward for key parts of fundamental equipments such as ship propeller, turbine blade and air compressor impeller. Traditional methodology of tool-path planning, which is only followed by geometry constrain, is far from demand of high speed and precision of multi-axis NC machining. A new method by both geometry and physical constrain should be proposed. In this paper, the nonlinear and anisotropy of stiffness is deeply studied for the whole process system. The inherent relation of stiffness and tool-path planning is researched. Tool-path will be planned based on stiffness model in order to meet the demands mentioned above.Based on the assumption of small elastic deformation and principle of virtual work, a close-loop stiffness model for the whole process system is established, by concerning all the subsystems that affect the deformation of cutter tip, including machine joints, tool-spindle, flexible movement axis and workpiece with fixture. These subsystems will be modeled respectively. For stiffness model of machine joints, modal tests are conducted for joints stiffness matrix. For stiffness model of tool-spindle, analytical method is adopted to calculate the cutter-spindle stiffness matrix. For stiffness model of flexible movement axis, matrix displacement method is adopted to get the axis stiffness matrix. For stiffness model of workpiece, finite element tools are used to simulate and evaluate for the workpiece stiffness matrix. Finally the general stiffness model for the whole process system is obtained by transforming each sub matrix from local coordinate system to workpiece coordinate system with the help of Jacobi matrix.Recur to the concept of ellipsoid in classic analytical geometry, the stiffness matrix is decoupled and 3D force ellipsoid is achieved. Based on the characteristics of ellipsoid, a series of stiffness index are put forward to reflect the machining ability. Then tool-path planning is directed as function of stiffness index. Algorithm of tool engagement direction and tool posture are discussed successively. For each control point, local optimization is conducted. Integration of all local optimization results in global optimization. If there is a reluctant movement axis, farther work could be done by optimizing the preset value for best stiffness ability.Stiffness ability of 7-5 axis lathe & mill machine tools with standard mill for large-scale marine propeller is analyzed. A closed-loop stiffness model is established and simulated. By comparing multiple atlas figures, the bottleneck is spotted which play the most important role in general stiffness. The promotion is done relatively. stiffness ability of 7-5 axis lathe & mill machine tools with special mill for large-scale marine propeller is analyzed. A closed-loop stiffness model containing flexible movement axis is set up. Taking stiffness index as objective function, the preset value of reluctant axis is optimized to improve the tool posture. Some experiments are taken to validate the theory.Dual-table 5-axis high machining center for standard workpiece is analyzed. A closed-loop stiffness model is established. Taking stiffness index as objective function, the tool engagement direction is optimized to improve the machining stability. Some experiments are taken to validate the theory.The result indicates that experiment and theory accord very well. The theory is proved right.