Research Of Local Path Smoothing And Look-Ahead Control Algorithm For High Speed Machining Of Continuous Shortline Segments

High-speed and high-precision numerical control(NC) machining is mainly applied to machining a series of continuous line segments that are discretely formed by complex curves and surfaces. Five-axis tool path is composed by linear path which is tangency discontinuities, so that the velocity and acceleration change abruptly around the corner of the adjacent segments, and effect the machining quality of the parts. This paper studies the local tool path smoothing transition in three-axis maching, and presents a NURBS arc transition path in the adjacent segments. On the basis of the local smoothing transition in three-axis linear tool path, a local path smoothing algorithm for five-axis linear tool path based on dual cubic NURBS which is curvature-continuous and satisfies the given accuracy constraint is proposed to achieve the local corner smoothing transition at the adjacent linear path of five-axis in work coordinate system(WCS). For any given five-axis machine tools and the axis servo capacity constraints, the relationship between the machine tool’s speed, acceleration and feed rate is derived at dual NU RBS interpolation to make the output command of interpolation device not exceed the axis machine tool servo ability. Synthetically using the local tool-path smoothing algorithm, the acceleration/deceleration(ACC/DEC) algorithm and the look-ahead control algorithm to realize the local tool path smoothing transition and implement the continuous smooth change of velocity and acceleration and satisfy the demand of high efficiency and high precision machining for the five-axis.The main contents of this paper and the innovatory results are listed as follows:Firstly,the theory and implement of the NURBS curve, S acceleration/deceleration algorithm and look-ahead control algorithm are researched, and they lays the theoretical foundation for the next sections that use NURBS curve to smooth the linear tool path and propose the look-ahead control S-curve ACC/DEC algorithm.Secondly, a quadratic NURBS arc transition path in the adjacent segments, which was everywhere curvature-continuous, was proposed to solve the prob lem of the machine vibration and shocks caused by abrupt change of velocity and acceleration, and then obtain the high-speed smooth transition. Based on the arc model, a look-ahead control algorithm was established. Under the conditions of the chord error and machine dynamics, the algorithm aimed to achieve optimal transition velocity, and control ACC/DEC by using the 7 section and 5 section mixed bidirectional S-curve algorithm, and avoid the motor starting/stopping frequently, then realize the high-speed transition smoothly. The example demonstrates that the proposed algorithm can achieve more stable transition velocity, improve the machining efficiency and quality, and satisfy the demands of high-speed machining.Thirdly, five-axis tool path is composed b y linear path which is tangency discontinuities, so that the velocity and acceleration change abruptly around the corner of the adjacent segments, and effect the machining quality of the parts. So a smooth transition algorithm based on dual cubic NURBS, which is curvature-continuous and satisfies the given accuracy constraint, is proposed to achieve the local corner smoothing transition at the adjacent linear path of five-axis. On the basis of the transition model, the parameters synchronous relationship of the dual NURBS curves and the translation path are constructed respectively to realize the smoothing variation of the tool translation and the tool rotation. The simulation and example show that the prosed transition algorithm can satisfy the error constraint, generate the curvature-continuous dual N URBS tool path, make local corner smooth transition and improve the machining quality.Finally, through analysising the motion trajectory of dual-NURBS tool path in five-axis machining, the speed distribution o f cutter rational ruled surface is derived at any instant and a kinematic model of cutting edge element is established. For any given five-axis machine tools and the axis servo capacity constraints, the relationship between the machine tool’s speed, accele ration and feed rate is derived at dual NURBS interpolation to make the output command of interpolation device not exceed the axis machine tool servo ability.