Research On Algorithms Of Interference-free Tool Path Generation In Free-form Surface Machining On CNC

Free-form surfaces have been used in a wide variety of applications in the national defense, aerospace, automotive and aesthetically pleasing products. It is difficult to be machined with high accuracy due to its complicated shape. Five-axis machining offers some advantages and is widely applied for free-form surfaces. The research and development of five-axis machining lag behind developed countries. Therefore pursuing in-depth research and development in high-speed and high-accuracy five-axis machining technology has far-reaching implications.According to the latest research achievements, some research and exploration in several key technologies of five-axis NC machining of free-form surfaces are conducted in this dissertation, including tool orientation planning, global tool interference avoidance, RTCP and the high speed machining of consecutive micro lines. A five-axis embedded NC system with self-owned intellectual property rights is designed.A multi-objective algorithm to determine the optimal tool orientation for five-axis flat-end cutter machining is proposed aiming at high efficiency, stable machining and smooth movement. Most existing algorithms only pursue the maximum material removal rate without taking into account other factors, such as the machining quality, cutter rigidity and the smoothness between adjacent tool orientations, etc. The metrics of tool orientation considering all above are established in this dissertation. Aiming at the minimal weighted metrics, a spring model on Gaussian Map (SMGM) is established, and the problem of finding optimal tool orientation is treated as the mechanical equilibrium problem. The fixed point at Gaussian Map will converge to locus of the optimal tool orientation.A global tool interference detection algorithm based on artificial potential field (APF) is proposed with robustness and high performance. The minimum distance between two Non-Uniform Rational B-Splines (NURBS) surfaces is calculated to determine interference or not. By constructing APF model, simulating the movements and obtaining the equilibrium positions, the solution of the nonlinear equations is converted to the kinematics problems. It can be applied to the computation of the minimum distance among points, curves, and surfaces represented by NURBS, triangles and the exact expressions. Finally, an interference avoidance method is given.A rotation tool center point (RTCP) algorithm based on angle interpolation is proposed. It can effectively reduce the nonlinear error, ensure the rotation axes moving smoothly, improve the machining efficiency by ensuring that CNC codes needn’t to be generated by postprocessing software when tool length changes. The global interference may occur while interpolating two tool orientations which change largely. RTCP interpolation error factor formula is derived to avoid collision.A real-time look-ahead direct interpolation algorithm for small lines is proposed aiming at the problems of frequent acceleration/deceleration and the interpolation error of endpoint. A uniformly accelerated DDA interpolation algorithm is proposed which can achieve uniform variable motion in an interpolation period according to the start velocity and end velocity of a line. It can significantly reduce the impact of motor and enhance the machining quality. Rough interpolation is made up of two parts: velocity planning and real-time looking-ahead. It calculates the maximum allowable transition velocity, adjusts the actual velocity in accordance with look-ahead data, and then uses the uniformly accelerated DDA interpolation algorithm to output pulses.A five-axis embedded NC system with self-owned intellectual property rights is designed. Examples of actual cutting are presented to demonstrate the validity and effectiveness of the proposed algorithms of interference-free tool path generation in free-form surface machining.