Research On Parametric Interpolator And Linear Segments Smoothing Methods For Five-axis Machine Tools

Five-axis machine tools is one of the most important equipments for complex workpieces machining,and it plays a pivotal role in the field of advanced equipment manufacturing such as aerospace,transportation,national defense and energy,etc.The flexible kinematic characteristics of five-axis machines can promote the machining efficiency and stabilize the machining quality,meanwhile,the challenges in the tool path generating and scheduling are arising.In the CAM(Computer Aided Manufacture)system,NURBS(Non-Uniform Rational B-Splines)and the micro linear segments are used to represent the tool path of the sculptured surfaces,and the tool path will be transmitted to the CNC(Computer Numerical Control)system to finish the scheduling and interpolation process.NURBS tool path can preserve the full geometry information of the surfaces,and improve the machining efficiency and quality.Meanwhile,the micro linear segments/tool path has been provided a long-term support from the CNC and CAM systems,and it is still the most widespread representation form in sculptured parts manufacturing.However,the linear tool path with only position continuity leads to acceleration oscillation,long machining time and low surface quality,which are urgent to solve.Thus,it is of great significance to realize NURBS interpolation and overcome the shortcomings of micro linear tool path for high-speed and high-precision CNC machining.Aiming at the target of promoting the machining efficiency and accuracy,this paper developes the NURBS interpolator and corner smoothing methods.The major research works and results can be concluded as following:1)The constant feedrate scheduling method with keg regions splitting based on the extreme feedrate and the breakpoints is described.In this method,the geometric and kinematic contraints are considered to generate the limited feedrate profile associated with geometric properties of NURBS curves.Then,the key regions will be splitted and the constant feedrate will be determined.Meanwhile,the S-type feedrate shcheduling with lookahead function is applied to finish the global-constrained feedrate profiles.Besides,the average feedrate calculating method based on displacement difference and the parametric interpolation compensation method based on cosine theorem are established,which ensure the continuous outputs of feedrate and reduce the feedrate fluctuation.Accordingly,the high-accuracy NURBS interpolator is developed.2)The geometry-based corner smoothing method for five-axis machine tools with double asymmetric quartic B-spline curves is presented.The symmetric and asymmetric B-spline curves are used to smooth the tool tip path and tool orientation path,separately.And,the possible machining error is considered to control the interpolation accuracy,with which the smoothing errors,curvature extremum and the synchronization of tool tip and tool orientation are analytical limited,leading to the continuity of the tool tip and tool orientation.Besides,the tool tip feedrate adjustment and smooth feedrate scheduling are applied with second-order Taylor’s method to finish the feedrate scheduling and interpolation of five-axis linear segments.With the proposed method,the continuous motion and smooth feedrate are generated,and the higher machining efficiency and accuracy are realized.3)The one-step corner smoothing method of five-axis machine tools is depicted based on feedrate blending algorithm.In this algorithm,the mapping relation of geometric error and dynamic performance between the machine coordinates system and workpiece coordinates system are constructed,which realize the optimal dynamic parameters determination and effective axial performance controlling.And,the feedrate scheduling of the adjacent linear segments will be applied.Then,the feedrate blending processes are implemented in the machine coordinates system and workpiece coordinates system with the specified tool tip and tool orientation smoothing errors,as a consequence,the one-step corner smoothing algorithm is established.This proposed method can avoid the construction of the complex parametric splines and the separation between the curve fitting and feedrate planning in two-step smoothing methods,in the meantime,the calculating efficiency in smoothing process and the tracking accuracy of physical axes are improved.