| Five-axis CNC(computer numerical control)machining is an important way to process complex curved surface parts in current industrial manufacturing.Linear segment path(G01)is a common path command in multi-axis CNC machining process.However,the sharp corner of the line segments leads to discontinuous feed speed,which causes large vibration of machine tool during operation and affects the surface quality.In addition,contour error caused by mismatch of multi-axis dynamic performance and insufficient tracking bandwidth in CNC machining directly affects the accuracy of CNC machining.Therefore,it is important to study the high-order smoothing and contour error control method of tool path to improve the efficiency and accuracy of CNC machining.At present,there are the following main problems in multi-axis tool path smoothing and control:(1)B-spline and Bezier spline are commonly used in corner smoothing.The arc length can not be calculated analytically.The efficiency and accuracy of path smoothing and interpolation are low.However,the arc length of the Pythagorean-Hodograph(PH)spline can be calculated analytically,and the existing research can only beC~2 continuity,can not guarantee continuous jerk.(2)In the existing method of tool path smoothing for five-axis CNC machining,the synchronization of tool tip position and tool orientation can not be calculated analytically,and the calculation efficiency is low and the jerk of each feed axis can not be guaranteed continuously.(3)The existing general contour error estimation and control model for five-axis CNC machining does not take into account the characteristics of tool path after local corner smoothing,and the estimation efficiency and accuracy of contour error need to be improved.Aiming at the above problems,this dissertation studies on on-line corner smoothing of five-axis tool path and on-line estimation and control of contour error after path corner smoothing for five-axis tool path.The main innovative research work and results are as follows:(1)AC~3 continuous corner smoothing algorithm with analytic calculation of arc length is proposed.Firstly,PH splines satisfyingC~3 continuous conditions and smoothing deviation constraints are constructed.In this algorithm,the position of spline control points,and curvature of arbitrary points and corresponding arc length can be calculated analytically,which is very suitable for on-line corner smoothing.The continuity of spline and linear connection makes the acceleration and jerk of each axis of machine tool continuous in the course of movement.Compared with theC~2 continuous corner smoothing algorithm,it has better dynamic characteristics.This method is applied to the on-line corner smoothing and interpolation experiment of three-axis CNC machining.The on-line corner smoothing and interpolation of three-axis CNC machine tools are realized,which greatly improves the machining efficiency.(2)The proposed three-axis corner smoothing algorithm is extended to five-axis machine tools.Firstly,the corner smoothing of tool tip position and tool orientation is carried out respectively,in which the smoothing deviation of tool tip position and tool orientation is constrained within the maximum allowable error tolerance set in the workpiece coordinate system.In order to solve the problem of discontinuous feed speed caused by the unsynchronization of tool tip position and tool orientation after the decoupled smoothing,an analytic algorithm for synchronizing the tool tip position and tool orientation is proposed,which can make the axes of the machine tool reach jerk on the whole five-axis smoothed path.This method has been applied to the on-line corner smoothing and interpolation experiment of five-axis CNC machining.The on-line corner smoothing and interpolation of five-axis CNC machine tools have been realized,which greatly improves the machining efficiency.(3)A method for estimating and controlling the contour error of five-axis tool paths after local corner smoothing is proposed.The smoothed five-axis tool path is composed of straight lines and splines,and the expression is clear.It is easy to find the nearest reference position and posture point on the contour from the actual position and posture.In this paper,a fast and accurate model for estimating contour errors is established by utilizing the feature of five-axis path after local smoothing.Compared with the existing methods,the proposed method has higher accuracy and efficiency.This method is applied to the on-line corner smoothing,interpolation and contour error estimation and control of five-axis CNC machining.The on-line corner smoothing,interpolation and contour error estimation and control of five-axis CNC machine tools are realized and the contour error is reduced,which greatly improves the processing efficiency and accuracy. |
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