Research And Application Of Motion Trajectory Smooth Technology In Five-axis Machining

Five-axis machining is the core function of high-end CNC machine tools, and isan effective means of realizing machining process of freeform surfaces such asimpellers, blades, and marine propellers. Most domestic high-end CNC systems useRTCP function for five-axis machining. Since tool orientation controlling isoverlooked in the whole procedure of machining, problems of large tool posture errors,overcutting, and frequent acceleration and deceleration of five axes may be caused andsmoothness of motion trajectory can not be guaranteed. High-end CNC systemsdeveloped by foreign countries have been equipped with motion trajectory smoothtechnology. However, considering the important role that five-axis machining plays inhigh-end CNC machine tools, developed countries still restrict the export of five-axisCNC machine tools to our country. Therefore, researches in preprocessing ofmachining information and tool path interpolation algorithms are in great demand andare necessary to develop high-end CNC machine tools equipped with functions of highspeed, high precision, and five axis linkage processing in order to provide a support fortransformation and upgrading of manufacturing industry.In this paper, motion trajectory smoothing technology is studied, and motiontrajectories are optimized and controlled by realizing smooth transition and tool pathinterpolation based on tool posture control. The impact that motions of rotation axeshave on accuracy and efficiency in five-axis CNC machining is firstly analyzed. Afterthat, smooth transition algorithms of command points and interpolation algorithmsbased on tool posture control are proposed. Finally, the effectiveness and practicality ofthese algorithms are verified.In the part of modeling of machining accuracy and efficiency, transformationrelationship between cutter location data in workpiece coordinate system and jointmovements of the machine tool is firstly analyzed. Depending on whether there is arotation axis that is parallel to the initial orientation of machine tool, various five-axismachining tools can be divided into two categories. Tool posture errors of the two partsare then calculated. Based on tool posture errors calculated above, the kinematicefficiencies of different machine tools are compared under the condition that identicalfinal rotational motion or final orientation is achieved. Finally, the relationship between the first and the second derivatives of tool orientation and the speed and theacceleration of rotation axes are obtained.Three interpolation algorithms based on tool posture control are then proposed tocontrol movement of the tool flexibly and to enhance machining accuracy. Toolorientations obtained by the interpolation algorithm based on plane tool posture controlalgorithm all locate on the plain determined by the start of the end tool orientation,which can not only avoid nonlinear errors but also improve machining accuracy. Aninterpolation algorithm based on smooth tool posture control is then given to keepcontinuous movement of rotation axes after obtaining the relationship betweenderivatives of tool orientation and the movement of rotation axes. Finally, a curveinterpolation algorithm based on conical tool posture control is proposed. In thisalgorithm, tool orientations obtained by conical tool posture curves all locate on therequired conical surface, which can avoid errors caused by deviations of tool posture.In the last part of this paper, aiming at solving problems of low efficiency andpoor quality caused by large variations of tool rotation axes, several smooth transitionalgorithms of command points are proposed. Taking different characters of face millingand frank milling into consideration, a smooth transition algorithm based on rotationaxes correction and a smooth transition algorithm based on tool orientations correctionare put forward. In the proposed smooth transition algorithm based on optimization ofrotation axes, large variations of rotation axes are corrected so that rotation axes arechanged in proportion to the movements of tool tips. Frequent acceleration anddeceleration are avoided. Less machining errors and higher machining efficiency areachieved. In the proposed smooth transition algorithm based on tool orientationscorrection, tool orientation vectors are segmented according to the inflection of the toolposture curve, and tool orientations in the same segment are fitted into a smooth curve.Tool orientations are therefore corrected by the value corresponding to their ownparameters. Results show that fitting error of the machining path is changed with thedegree of the fitting function.