Research On Tri-NURBS Spline Interpolation In Five-axis Machining And Its Tool Contact Trajectory Tracking Control Method

After decades of development,China has finally become one of the world’s largest manufacturing and exporting countries.Five-axis numerical control technology is the key technology in the field of military and civil equipment such as aerospace,ship,mold,high-precision instrument,and it is also an important cornerstone for China to move from a world manufacturing power to a world manufacturing power.The emergence of five axis numerical control technology can give full play to the potential and advantages of numerical control technology in automatic processing,but it is also one of the more difficult technologies in the field of numerical control.Generally speaking,China’s five-axis CNC technology is still lagging behind developed countries,such as NURBS spline interpolation technology for five-axis NC machining.Therefore,the in-depth study of five axis CNC technology is of great significance for national strategic development.Compared with three-axis machine tool,five axis machine tool has two more rotating axes.The tool’s rotation and swing will cause the cutterworkpiece cutting contact(CC)point path to deviate from the preset machining path and cause the CC path nonlinear error.In this paper,a five-axis Tri-NURBS spline interpolation method with CC path nonlinear error control function is studied to realize the five axis high speed and high precision machining of complex curves and surfaces.The main contents of this paper are as follows:(1)Aiming at the problem that traditional double NURBS interpolation ignores the tool contact path interpolation,a five-axis Tri-NURBS interpolation method with three different interpolation parameters to obtain interpolation data points by adding tool contact spline path is studied,and the NC code format of the interpolation method in the five axis CNC system is studied.The interpolation method uses the S-type front acceleration and deceleration control law.On the basis of obtaining the interpolation parameters of the spline curve of the cutter center point,the interpolation parameters of the tool axis point and the CC point are obtained after the parameters of the node interval are equivalently allocated,and then the interpolation data points on the three spline paths are obtained.Using MATLAB software,the three interpolation parameters and their corresponding interpolation data points are calculated and verified.The results show that compared with the double NURBS interpolation method with the same interpolation parameters,the interpolation method proposed in this paper has better accuracy in maintaining the spatial position relationship of cutter center point,cutter axis point and CC point in the original tool position data.(2)Aiming at the non-linear error problem of the CC point deviating from the ideal CC path due to the rotation and swing of the tool during five-axis CNC machining,the nonlinear error of the CC trajectory in the five-axis Tri-NURBS spline interpolation method is first analyzed.The cause and mechanism of the error are studied and established a mathematical model for calculating the nonlinear error,and a method for compensation and repair for the nonlinear error of the CC point spline path during the five-axis Tri-NURBS spline interpolation is proposed.(3)In the five-axis CNC system,the process of implementing the five-axis TriNURBS spline interpolation method is studied.First,the target tool center point position interpolation parameters of the next interpolation cycle are calculated by using the tool center point spline curve,and at the same time the tool axis point nterpolation parameters and the CC point interpolation parameters are calculated,and then the position coordinates of the interpolation tool center point,tool axis point and CC point are obtained.Secondly,the real CC point is obtained bu using the above three points,and the deviation distance is obtained by the same way.The deviation distance is the distance between the real CC point and the ideal CC point path;For the real CC point with a large deviation distance,the error compensation vector is obtained.And the whole tool is translated through using error compensation vector,so that the real CC point coincide with the interpolation CC point through the interpolation calculation at the ideal CC point path,and the new interpolation tool center point,tool axis point and tool axis unit vector are calculated;Combined with the specific five-axis CNC machine tool,the new tool center point and tool axis unit vector are performed to real-time postprocessing calculation,the five machine tool coordinates used to control the movement position of each axis of the five-axis CNC machine tool are finally obtained,and the Tri-NURBS interpolation calculation process in one interpolation cycle is completed.