参数曲面五轴加工轨迹优化方法

非线性误差是五轴加工中加工误差的重要来源,不可避免。针对五轴加工中的非线性误差问题,分析了非线性误差产生的原因,提出了一种减小机床非线性误差的轨迹优化算法。该方法直接计算各实际刀轨与工件参数曲面之间的误差,找出非线性误差过大的刀位,通过递归插入新刀位来减小非线性误差,直到满足精度。插入刀位点时,结合曲面凹凸性及刀具路径的弦弧逼近误差,同时将过切、欠切情况考虑在内。仿真分析结果显示该方法能避免插入不必要的刀位点,可显著减小非线性误差。     

最小非线性插补误差约束的多轴侧铣刀轴矢量优化

机床运动轴的线性插补会导致刀具位置及刀轴矢量的非线性插补,造成的非线性误差是影响曲面侧铣精度的重要因素。为减少刀轴矢量非线性插补造成的侧铣加工误差,建立转台旋转-刀轴偏差角模型,证明当机床旋转轴线性联动时刀轴矢量的插补一定是非线性的,并提出刀轴矢量线性插补的条件。在加工坐标系中建立球面坐标系,讨论相邻插补刀轴矢量在球面坐标系中的经纬度差异,以及对刀轴偏差角的影响。通过计算知相邻刀轴矢量的夹角与刀轴最大偏差角不呈单调关系。提出相邻刀轴矢量平均纬度最小化的优化目标;通过改变工件装夹姿态,满足优化条件。以典型叶片曲面类零件的曲面侧铣加工为例,验证刀轴矢量线性插补的约束条件。利用BC轴转台式机床进行加工,借助优化刀轴矢量在转台旋转-刀轴偏差角模型中的平均纬度,验证所提方法的有效性。     

摆头转台五轴数控机床RTCP算法的研究

在五轴数控机床加工中,由于旋转运动的影响,机床各轴线性插补的合成运动会使实际刀位运动偏离编程直线,造成编程直线和机床实际运动轨迹之间产生了误差,该误差被称为非线性误差。在对摆头转台五轴数控机床运动求解进行研究的基础上,基于机床的运动求解模型,分析了五坐标加工中的非线性误差的数学模型,提出了一种RTCP功能的插补算法,并通过MATLAB实例仿真验证该RTCP算法可以有效减小非线性误差,显著提高加工精度。     

五轴数控加工后处理关键技术分析与实现

从后处理的角度,分析了五轴数控加工的非线性误差,指出了其影响因素;提出了利用齐次变换矩阵和正向、逆向运动学相结合的误差计算方法以及相应的误差补偿策略;同时,引入了刀轴矢量"平滑化"的概念,提出了基于四元数向量插值的新算法;最后,以一个离心压气机叶轮为例,进行了计算机仿真加工和五轴机床试切加工实验.实验结果表明,所提出的方法改善了曲面加工精度和表面质量.     

五轴数控机床非线性误差建模及补偿方法

针对五轴数控机床后置处理中由于平动轴和旋转轴的联动产生的非线性误差,提出一种基于误差建模的非线性误差在线预测与补偿方法.根据任意两个相邻刀位数据点产生的非线性误差,获得误差的分布特征,建立起误差分布模型;利用最小二乘法求解出非线性误差的数学表达式,经与误差许用值相比较来确定新的刀位点,从而实现非线性误差的在线预测及补偿...     

五轴加工奇异问题及其处理方法

在五轴机床加工中,当刀轴矢量接近或平行于某一旋转轴(即奇异轴)而产生奇异问题,文章在分析奇异问题及其产生原因的基础上,提出一种奇异域检测的几何方法.这种方法可以根据始末位置刀轴矢量在未经插补的情况下,提前检测出刀轴是否经过奇异域及其类型,然后针对每种类型指定一种相应的插补算法,并且通过仿真实验.仿真结果表明该算法能够有效检测出加工中会出现的奇异问题并改善加工精度.     

刀具摆角对复杂曲面轮廓精度的影响研究

目的通过优化五轴联动加工中刀具摆角参数,基于后置处理技术提高复杂零件表面加工的轮廓精度。方法针对回转轴非线性运动造成的刀具姿态误差过大会导致零件轮廓精度低,提出了一种摆角优化方法。首先,对回转轴线性插补产生的刀具姿态误差进行分析,控制回转角的摆动幅度大小和初始位置;其次,将线性插补后的刀轴矢量投射到理论上始末两点矢量构成的平面上获得新的插补矢量,通过线性插补刀轴矢量来优化刀具空间姿态;最后,以某叶轮试件通过仿真及实际加工实验进行了验证。结果通过摆角优化方法后,叶片轮廓与理论轮廓的轮廓误差由0.08 mm减小到0.04 mm,最大过切量也由0.03 mm减小到0.01 mm。刀具摆角优化后,能大大提高复杂曲面零件的轮廓精度。结论基于后置处理技术对五轴机床回转轴摆角进行优化,在通用算法基础上加载角度优化算法,开发专用的后置处理器处理G代码程序,是一种提高复杂曲面加工轮廓精度的可行措施。实验验证了该方法的有效性。     

瑕疵刀位点几何特征分析及其对插补指令的影响

由于CAD曲线建模的误差以及CAM算法的精度问题，刀位点中存在着瑕疵。通过对典型加工零件的刀位数据点进行分析，归纳瑕疵的类型，分析瑕疵的几何特征，研究瑕疵对数控插补指令的影响。结果表明：瑕疵类型包括密集、突起、折返、重复加工和相邻不一致；密集瑕疵造成线段长度的减小，突起瑕疵造成转角的增大，折返瑕疵在开始和结束位置造成曲率变化率的增大，重复加工在零件同一位置多次加工，相邻轨迹不一致造成点的分布不均匀；密集、突起、折返瑕疵刀位点造成指令速度的降低，以及重复加工对同一位置的多次加工，影响加工效率，突起、折返瑕疵造成指令加速度和加加速度的波动增大，引起机床振动，相邻轨迹不一致在加工同一特征时速度和加速度不同，影响加工表面质量。     

细长轴的尺寸误差补偿研究

为了减小细长轴加工中产生的尺寸误差,建立了细长轴在径向力作用下的力学模型、车刀受切削热时的热变形模型,推导了一个与细长轴参数相关的尺寸误差公式。介绍了一种细长轴尺寸误差补偿法,将该误差补偿法与让刀量尺寸误差公式相结合,运用于细长轴数控车削实验。实验结果表明,通过该补偿法能显著减小加工尺寸误差,提高了细长轴的加工精度。     

细长轴车削加工工艺路线的研究

详细分析正向走刀和反向走刀加工细长轴时,在切削力作用下产生的变形,并考虑压杆稳定性,建立了误差大小的数学计算公式,从而为减小细长轴的加工误差、提高加工精度提供了理论上的依据.     

A new compensation method for geometry errors of five-axis machine tools

The present study aims to establish a new compensation method for geometry errors of five-axis machine tools. In the kinematic coordinate translation of five-axis machine tools, the tool orientation is determined by the motion position of machine rotation axes, whereas the tool tip position is determined by both machine linear axes and rotation axes together. Furthermore, as a nonlinear relationship exists between the workpiece coordinates and the machine axes coordinates, errors in the workpiece coordinate system are not directly related to those of the machine axes coordinate system. Consequently, the present study develops a new compensation method, the decouple method, for geometry errors of five-axis machine tools. The method proposed is based on a model that considers the tool orientation error only related to motion of machine rotation axes, and it further calculates the error compensations for rotation axes and linear axes separately, in contrast to the conventional method of calculating them simultaneously, i.e. determines the compensation of machine rotation axes first, and then calculates the compensation associated with the machine linear axes. Finally, the compensation mechanism is applied in the postprocessor of a CAM system and the effectiveness of error compensation is evaluated in real machine cutting using compensated NC code. In comparison with previous methods, the present compensation method has attributes of being simple, straightforward and without any singularity point in the model. The results indicate that the accuracy of positioning was improved by a factor of 8–10. Hence, the new compensation mechanism proposed in this study can effectively compensate geometry errors of five-axis machine tools.     

Inverse kinematics of five-axis machines near singular configurations

In five-axis milling, singular configurations of the machine axes may cause tool path errors or collisions between the tool and parts of the milling machine. This paper presents an algorithm for calculating the inverse kinematics of five-axis machines close to singular configurations. The algorithm modifies the exact inverse kinematics in order to give robustness to singularities at the expense of a small tool orientation deviation. The kinematics of a five-axis machine with non-orthogonal rotary axes is analyzed. The forward kinematics is developed, and a closed form solution of the inverse kinematics is presented. The kinematics and the singularity algorithm are implemented in a postprocessor, and machining tests are conducted to verify the algorithms.     

A machining test to calibrate rotary axis error motions of five-axis machine tools and its application to thermal deformation test

This paper proposes a machining test to parameterize error motions, or position-dependent geometric errors, of rotary axes in a five-axis machine tool. At the given set of angular positions of rotary axes, a square-shaped step is machined by a straight end mill. By measuring geometric errors of the finished test piece, the position and the orientation of rotary axis average lines (location errors), as well as position-dependent geometric errors of rotary axes, can be numerically identified based on the machine׳s kinematic model. Furthermore, by consequently performing the proposed machining test, one can quantitatively observe how error motions of rotary axes change due to thermal deformation induced mainly by spindle rotation. Experimental demonstration is presented.     

Modeling and improvement of dynamic contour errors for five-axis machine tools under synchronous measuring paths

In this paper, a contour error model of the tool center point (TCP) for a five-axis machine tool is proposed to estimate dynamic contour errors on three types of measuring paths. A servo tuning approach to achieve five-axis dynamic matching is utilized to improve contouring performance of the cutting trajectory. The TCP control function is developed to generate measuring trajectories where five axes are controlled simultaneously to keep the TCP at a fixed point. The interpolation method of the rotary axes with S-shape acceleration/deceleration (ACC/DEC) is applied to plan smooth five-axis velocity profiles. The contour error model for five axes is derived by substituting five-axis motion commands into servo dynamics models. The steady state contour error (SSCE) model is demonstrated to illustrate three particular dynamic behaviors: the single-circle with amplitude modulation, double-circle effect and offset behavior. Furthermore, the model is also utilized to investigate the behaviors of dynamic contour errors change in 3D space. The factors that affect dynamic contour errors, including the initial setup position, feedrate and five-axis servo gains, are analyzed. With the developed servo tuning process under the measuring paths (CK1, CK2 and CK4), the contour errors caused by servo mismatch are reduced remarkably. Finally, experiments are conducted on a desktop five-axis engraving machine to verify the proposed methodology can improve dynamic contouring accuracy of the TCP significantly.     

Accuracy enhancement of five-axis CNC machines through real-time error compensation

Although error modeling and compensation have given significant results for three-axis CNC machine tools, a few barriers have prevented this promising technique from being applied in five-axis CNC machine tools. One crucial barrier is the difficulty of measuring or identifying link errors in the rotary block of five-axis CNC machine tools. The error model is thus not fully known. To overcome this, the 3D probe-ball and spherical test method are successfully developed to measure and estimate these unknown link errors. Based on the identified error model, real-time error compensation methods for the five-axis CNC machine tool are investigated. The proposed model-based error compensation method is simple enough to implement in real time. Problems associated with the error compensation in singular position of the five-axis machine tool are also discussed. Experimental results show that the overall position accuracy of the five-axis CNC machine tool can be improved dramatically.     

五轴联动数控机床非线性误差的研究

五坐标插补过程中,旋转轴运动的影响使实际刀位运动偏离编程直线,产生了非线性误差。在深入分析双摆头五坐标机床运动原理和非线性误差的产生机理的基础上,介绍了一种集成RTCP(旋转刀具中心点)功能的插补算法。RTCP功能可使数控系统自动对旋转轴的运动进行实时线性补偿,从而保证插补点始终位于编程轨迹上。通过MATLAB仿真计算验证了该算法可以有效减小非线性误差。     

Identification and compensation of geometric errors of rotary axes on five-axis machine by on-machine measurement

The geometric errors of rotary axes are the fundamental errors of a five-axis machine tool. They directly affect the machining accuracy, and require periodical measurement, identification and compensation. In this paper, a precise calibration and compensation method for the geometric errors of rotary axes on a five-axis machine tool is proposed. The automated measurement is realized by using an on-the-machine touch-trigger technology and an artifact. A calibration algorithm is proposed to calibrate geometric errors of rotary axes based on the relative displacement of the measured reference point. The geometric errors are individually separated and the coupling effect of the geometric errors of two rotary axes can be avoided. The geometry error of the artifact as well as its setup error has little influence on geometric error calibration results. Then a geometric error compensation algorithm is developed by modifying the numeric control (NC) source file. All the geometric errors of the rotary errors are compensated to improve the machining accuracy. The algorithm can be conveniently integrated into the post process. At last, an experiment on a five-axis machine tool with table A-axis and head B-axis structure validates the feasibility of the proposed method.     

Error map construction for rotary axes on five-axis machine tools by on-the-machine measurement using a touch-trigger probe

Position-dependent geometric errors, or “error map,” of a rotary axis represent how position and orientation of the axis of rotation change with its rotation. This paper proposes a scheme to calibrate the error map of rotary axes by on-the-machine measurement of test pieces by using a contact-type touch-trigger probe installed on the machine's spindle. The present scheme enables more efficient and automated error calibration, which is crucial to implement periodic check of rotary axes error map or periodic update of its numerical compensation for five-axis machine tools. The uncertainty analysis of the error calibration is also presented with a particular interest in the influence of error motions of linear axes. The experimental demonstration is presented.