Research On Corner Smoothing Methods Based On Drive Constraints For NC Machining Along Linear Tool Paths

With the rapid development of aerospace,automobile manufacturing and precision machinery,tool path smoothing technology for complex curves and surfaces plays an increasingly important role in high-speed and high-quality NC machining.The path used for machining usually obtains a series of G01 points according to the given machining accuracy on the theoretical curves.The linear trajectories composed by discrete G01 points are widely used in high-speed machining based on the simplicity and convenience of the algorithm and the strong adaptability to the complex curve and surface.However,the slope and curvature discontinuity of adjacent segments will inevitably result in the feedrate fluctuation and excessive acceleration,thus deteriorating the machining accuracy and machining quality.Curvature-continuity tool path is the base for high speed and quality NC machining,this thesis takes the G~2 continuity,approximation error and drive limits into consideration when constructing the smoothing tool path and calculating the interpolation points.A circumscribed local interpolation methodology with double cubic Bezier curves for a three-axis linear tool path is proposed.Firstly,the polygons where the control points of the Bezier curves located are constructed according to the maximum approximation error.Then,the control points of the double cubic Bezier curves are derived with the G~2 continuity at the intersection of two curves and the polygon length limits.Finally,a smoothing cornering trajectory is obtained and consists of linear segments and Bezier curves.Compared with inscribe smoothing methods,the simulation results show that the method can effectively reduce the fluctuation of the X and Y axes acceleration in the constant speed machining,which can be benefical to motion characteristcs.Moreover,the total motion time in the proposed smoothing method is shorter than the point to point motion,and experiment is performed to verify the efficiency of the proposed method.At the same time,a circumscribed smoothing method based on double cubic B-splines for a five-axis linear tool path is proposed.A configuration of control points for the circumscribed corner transition spline is first given to make the constructed transition curve satisfy the limits of approximation error and G~2 continuity.The five-axis tool path is composed of the trajectories of the tool tip point and the second point on the tool axis in the workpiece coordinate system,and the two trajectories are smoothed by the cubic B-spline respectively.Then,based on the parametric synchronization between the bottom and top trajectories,the first and second geometrical derivative continuous conditions of the tool orientation are derivative at the junctions between the remaining trajectory and the transition trajectory.The simulation results show the transition curves generated by the circumscribed smoothing method have smaller curvatures compared with commonly used inscribed smoothing methods.The two smoothing methods are used to smooth the rhombic tool path,and compared with the inscribed tool path,the total cycle time in the circumscribed tool path saved 6.4%,which can enhance machining efficiency at sharp corners.For five-axis tool path,the geometric derivative continuity of the tool orientation can be guarantee by adjusting the positions of the control points,which can suppress the fluctuation of the feedrate.