Compensation Of Geometric Errors Of 5-axis Machine Tool And Deformation In Flank Milling Based On On-machine Measurement

With the superb characteristics of lightweight and high intensity,thin-walled parts with complex structures are commonly used in aviation aircraft,space launch vehicles and other fields.Owing to their unparalleled advantages in tool position and orientation adjustment,joint motion control and tool accessibility,5-axis machine tools are commonly used in complex thin-walled part machining.It is impossible for machine tool venders to prevent existences of geometric errors,because the manufacturing errors and assembly errors of motion axes components are unavoidable,as well as the wear and slight deformation errors during machine tool running.The situation is much more serious for 5-axis machine tools due to the containment of multiple axes,long transmission chain and nonlinearity of motion trajectory.It is difficult to ensure the high precision requirement in the whole movement space.Meanwhile,considering the physical factors of the thin-walled parts,like the low rigidity,the elastic-plastic deformations caused by cutting force and cutting heat will result in under-or overcutting in machining.As such,machining accuracy will be significantly decreased by these two types of errors.In order to improve the position accuracies of 5-axis machine tools and reduce the machining errors caused by cutting force induced deformations of thin-walled parts,this paper intends to compensate the geometric errors of the machine tools as well as the deformation errors in thin-walled workpiece machining based on on-machine measurement(OMM).The adverse effects of these two types of errors on machining accuracy are analyzed,respectively.On-machine measurement system is developed based on touch-trigger probe.Geometric errors of machine tools and deformation errors during thin-walled workpiece machining are separated and recognized according to the measurement results.As last,these two types of errors are eliminated or reduced by adjusting the toolpath.The main contents and innovative achievements are as follows.The definition and mathematic model of the position-dependent geometric errors of rotary axis of five-axis machine tool is discussed and standardized in this paper.Even though the definitions of geometric errors of rotary axis in existing literatures are seemingly identical,there are two different ways of definition in fact.It is extremely dangerous as it makes the comprehension of the geometric errors ambiguous and may make the geometric error identification and compensation less effective.This phenomenon has not been noticed so far.In this paper,two different commonly used geometric error definition and modeling methods are firstly identified and analyzed,and their features and relationships are analyzed.After a detailed comparison,a more suitable way to definite the geometric errors of rotary axis is determined.A position-dependent geometric error measurement and identification method for rotary axis is developed based on touch-trigger probe.Due to the character of the touchtrigger probe,the coordinate of the evaluation point that is used to evaluate the geometric errors of the rotary axis is acquired by fitting several corresponding points.As a result,the accurate coordinate of the evaluation point can be achieved.The coupling effect of the geometric errors are analyzed by the kinematic model of rotary axis.To eliminate the coupling effect,a strategy to determine the proper measurement points is developed in this paper.The geometric error identification experiment is conducted on a 5-axis machine tool with double tables of A-and C-axis.The result shows that the conducted method can identify the geometric errors of the two rotary axes effectively.An integrated post-processor with compensation of all geometric errors for 5-axis machine tools is established.Kinematic transformation model considering all the geometric errors of 5-axis machine tool is developed based on homogeneous coordinate transformations and multiple rigid body kinematics,to analyze the influences of geometric errors on machining accuracy.By establishing the differential motion relationship between the movement of the axes in machine coordinate system and the tool position movement in workpiece coordinate system,a geometric error compensation algorithm based on least squares method is developed by adjusting the toolpath.The algorithm can generate the machining code with the given cutter location source file(CLSF)and the identified geometric errors,which can be directly utilized as a post-processor with the function of compensating the geometric errors.The machining experiment result show that the developed algorithm can effectively compensate the geometric errors and significantly improve the machining accuracy by 76.9%.Method for compensation of machining errors in 5-axis flank milling based on OMM is developed for thin-walled parts.The machining accuracy is evaluated by comparing the envelope surface of tool path and the real machined surface which is measured and fitted by touch-trigger probe.5-axis adaptive flank machining is much more complicated as two rotary axes are introduced in and the cutting tool adds two more orientations,compared with the 3-axis milling.The toolpath is modified to compensate the deformation error by transforming the 3D tool position and orientation adjustment to 2D tool position parameter optimization.The experiment result on impeller indicates the developed strategy can improve the machining accuracy by 69.8%.In summary,in this paper,the geometric errors of the machine tools are measured and compensated to improve the accuracy of the space movement and touch-trigger probe based OMM.Furthermore,the deformation errors of thin-walled parts are detected by OMM,and compensated by adaptive machining strategy,which realizes a closed-loop control based machining system.