Research On Toolpath Adaptive Optimization Driven By CNC Pre-interpolation Information

Complicated surfaces are widely used in many areas,such as aerospace,national defense,automation,and medical treatment.The fabrication of complicated surface with high precision and efficiency is the representation of advanced manufacturing,which is the frontier technology of the digital machining.With the development of the computer technique,the CAM/CNC systems provide the technological base for multi-axis complicated surface machining.However,the operation mechanism between the mainstream software systems is still not perfect.The existing machining procedure is not reasonable which prevents improving the machining quality and efficiency.Especially in five-axis surface machining,CAM system is comparative independent from the CNC system,so that the CNC system is only able to accept the existing toolpaths file to interpolate,which the toolpaths file is generated by CAM system by statistic standard.In fact,in five-axis surface machining,the shape of the toolpath and fixed position of the workpiece would affect the machining efficiency as well,which means they would decrease the machining efficiency.Unfortunately,in existing machining process,due to the lack of the information inverse communication mechanism from CNC system to CAM system,the downstream information in CNC system can't feedback to upstream.Therefore,the CAM system has no ability to improve the existing machining code.This doctoral thesis is intended to do some basic research studies on the above-mentioned problems to propose the inverse feedback mechanism from CNC system to CAM system based on the flaw of the existing surface manufacturing mechanism.The machine would reach its capacity if the interpolation information can be feedback to CAM system to help the upstream system improve the toolpath quality.The main contribution of the thesis is summarized as follows:The existing CAM/CNC system is comparative independent from each other.In order to overcome this deficiency,this thesis proposes an inverse feedback mechanism between the CAM/CNC system to bridge them together,which intend to break the traditional single information flow between CAM/CNC system.The downstream interpolation information would be feedback to CAM system to help the CAM system adjust the toolpath or change the toolpath generation scheme.Then the evaluation and selection mode of the inverse feedback mechanism would choose a most efficiency toolpath as the final toolpath.The toolpath efficiency is the important evaluation index when the inverse feedback mechanism bridges the CAM and CNC system.The feedrate schedule method is the basis of the efficiency evaluation.The classical feedrate schedule method doesn't take the axis dynamic characteristic into account when it plans the feedrate.Therefore,this thesis proposes an axis dynamic-based feedrate schedule method to make the feedrate profile match the axis drives'electromechanical characteristics,to improve the steadiness of the feedrate.This method decomposes the toolpaths into each axis based on the machine's kinematic model to acquire axis displacement profile.Then the feasible region of the feedrate profile can be resolved according to the limitation of each axis's velocity,acceleration,and jerk.The nominal feed of toolpath can be obtained in the feasible region which is used to approach the efficiency optimal feedrate profile under machine axes' limitation.The efficiency optimal feedrate profile would guarantee the feedrate profile is continuous in low order and the dynamic component on each axis should be under axis drives' saturation limit.Then,to overcome the influence of the toolpath' s shape on the feedrate schedule in three axis machining,this thesis proposes a toolpath generation method with feed sensitive zones inspection based on inverse evaluation mechanism.This method first gets the low velocity sensitive zones on the surface by pre-interpolation.Then the CAM system adjust the toolpath locally by the low velocity sensitive zones.The CAM system would evaluate the different schemes' efficiency to choose a time-optimal scheme at last.This method would generate a scallop-height constrained grid mesh on the surface according to the scallop-height limitation.The nominal feed of the grid nodes can be calculated by the surface geometry and the axis-dynamic-based feedrate schedule method.The low velocity sensitive zones would be found by enveloping the low velocity nodes.The toolpath would be modified according to these low velocity zones.This procedure would be repeated until the toolpath efficiency reaches requirement.The modified toolpath would send to CNC system to drive machine.To expand the application region of the inverse feedback mechanism form three-axis machining to five-axis machining,this thesis proposes a cutter's postures optimization method of five-axis toolpath when the machining iso-scallop trajectory is existing,in which situation the cutter's postures would affect the machining efficiency.This method generates the iso-scallop toolpath on the surface,and then projects the cutter's postures on the C-Space.The cutter's postures are obtained by the geometry property of the surface.According to the feedrate profile of the pre-toolpath,the low velocity zone is found in the C-Space as the feedback information.The CAM system changes the shape of the C-Space curve to optimize the cutter's posture.The new cutter location point would be calculated by the new C-Space curve and the feedrate profile would be re-scheduled.Furthermore,considering the rotary axes would affect the feedrate when the cutter moves along the large curvature zone in five-axis machining,this thesis proposes a five-axis toolpath optimization method by the rotary axes sensitive characteristic constrained.This method generates an iso-parameter grid mesh on the parameter zone at first,and then each grid would be given a rotary property according to the rotary axis displacement.The sub-zones would be found by gathering the grids with same property.The sub-zones would be feedback to CAM system as the feedback information.In toolpath generation stage,the CAM system would choose a suitable.scheme for each sub-zone to improve the whole machining efficiency.At last,the toolpath generated based on the inverse feedback mechanism is verified by machining experiments.The first experiment is comparing the machining efficiency between the workpiece fixed on different positions.Then the difference between toolpath generated by the statistic criterion and the toolpath optimized by the inverse feedback mechanism.The reasonability and the effectiveness of the inverse feedback mechanism are verified by the experiments.