非正交五轴数控机床曲面加工误差补偿研究

针对复杂曲面无法直接在数控机床中实现精加工问题,基于多体系统理论建立非正交五轴数控机床运动误差模型,该模型包含了五轴数控机床的静态误差和动态误差共计45项几何误差。同时将nurbs曲面重构技术运用到待加工曲面的数学表征中,通过三坐标测量机采集到曲面型值点后,通过曲面重构技术获得机床加工时所需的插值点坐标。最后结合数控机床精加工条件进行数控指令修正,能够达到误差补偿效果,一方面实现了复杂曲面加工同时又满足了其加工精度要求。     

基于多体系统理论的数控机床加工精度几何误差预测模型

针对多轴联动数控机床加工精度误差补偿问题,从分析数控机床误差产生机理和建立精度误差补偿模型的角度,提出了基于多体系统理论的数控机床加工精度几何误差预测模型。分析了B-A摆头五轴龙门数控机床的拓扑结构关系、低序体阵列、各典型体坐标变换,推导出了B-A摆头五轴龙门数控机床精度几何误差预测函数。采用平动轴十二线法误差参数辨识算法,测量并计算了某B-A摆头五轴数控机床21项空间几何误差,为精度几何误差预测函数提供有效参量。该几何误差参数建模方法,对不同拓扑结构和运动关系的数控机床具有通用性,为后续数控机床误差动态实时补偿并提高切削加工精度提供了理论依据。     

基于S型检验试件的数控机床动态性能辨识新方法

五轴数控机床动态性能是影响工件加工质量的重要因素,S型试件作为校验机床动态性能的检验试件,在实际应用中取得了一定成果.为进一步确定机床的加工状态,提高工件加工质量,提出了一种基于S型试件切削的五轴数控机床动态因素辨识方法.综合采用了多体运动学、模糊理论、BP神经网络理论对机床动态因素进行辨识.该方法可以用于评估数控机床的动态性能,通过误差溯源给出的机床动态因素,可用于指导机床的维修和调整,保障关键工件的数控加工质量.     

带摆角头五轴数控机床几何误差建模及补偿方法研究

研究带摆角头五轴数控机床几何误差建模方法及误差补偿技术.基于多体系统运动学理论,阐述带摆角头五轴数控机床的拓扑结构及其低序体阵列,建了五轴差模型.根据该误差模型研究了带摆角头五轴数控机床的几何误差软件补偿计算方法,分别建立理想条件与实际条件下刀具路线、数控指令及刀具轨迹三者间的相互映射关系,给出数控指令修正值的具体算法及迭代计算求解终止判别条件,通过该方法可获得修正后的数控指令,从而解决软件误差补偿的关键问题该补偿方法直接、简明,具有很好的通用性.     

五轴数控机床空间定位精度改善方法研究现状

五轴数控机床空间定位精度受机床静、动态误差的综合作用,是影响工件加工质量的重要指标。综述了基于不同误差源数值特征描述的几种主流误差建模方法,分析与建模方法对应的几种误差灵敏度分析方法的特点。根据五轴数控机床的构成,分别对平动轴和转动轴的误差检测和误差辨识方法进行了综述和分析。根据动、静态误差类型的不同,总结了现有几种主要误差补偿方法的特点。最终,综合误差建模、误差灵敏度分析、误差检测和辨识以及误差补偿五个方面,系统性地分析了现有五轴数控机床空间定位精度改善方法中尚需解决的问题,探讨了五轴数控机床空间定位精度改善方法未来的发展方向。     

五轴数控加工中心回转轴误差补偿技术的研究

针对五轴加工中心回转轴运动精度问题,基于多体系统理论,对回转轴运动拓扑结构进行描述．建立数控机床精密加工条件方位约束方程,研究了回转数控指令的迭代修正求解。在此基础上,利用C#语言编制误差补偿和仿真分析软件,通过试验检验,证明本文提出的补偿方法可使机床加工精度大大提高,为将来实际应用奠定了理论基础。     

CNC机床动态特性与S形试件轮廓误差映射关系分析

以双摆头五轴数控机床为研究对象建立了仿真模型,并对其准确性进行了实验验证。然后通过仿真分析揭示了S形试件轮廓误差在试件各分区的表现规律。基于机床动态特性与S形试件轮廓误差的映射关系,可以为实际加工中五轴数控机床动态性能参数的调整、机床性能评价及加工误差的溯源和辨识提供指导依据。     

一种多轴联动加工轮廓误差的向量计算方法

针对进给系统动态特性造成的轮廓误差,提出了一种多轴联动加工轮廓误差的向量计算方法,该方法通过实时读取数控机床的位置反馈和插补器的输入数据,实时精确计算数控机床加工过程中的轮廓误差向量。通过直线、圆和抛物线的Matlab三轴联动加工仿真对该方法进行了验证,结果表明,该轮廓误差的向量计算方法能够简单准确地计算多轴联动加工过程中由于进给系统动态特性造成的轮廓误差向量,该误差向量可用于评价数控机床多轴联动加工精度和提供误差补偿的参考数据。     

龙门五轴机床RTCP功能的新模型及其应用

旋转运动的存在使得五轴联动加工存在非线性运动误差。通过实时线性补偿,现有高档数控系统RTCP功能可有效补偿非线性运动误差,但并不能完全消除非线性误差。重点研究了现有RTCP模块设计和算法实现,总结了一种改进的RTCP模型,设计了相应的算法。利用MATLAB对一台A-C轴铣床进行编程仿真和加工验证,结果表明五轴联动数控机床的动态精度得到了提高。     

多功能复合五轴数控机床的综合误差建模研究

数控机床误差补偿是提高数控机床加工精度的有效方法,而建立快速准确的误差模型又是实施误差补偿的前提和基础。以多功能复合五轴数控机床为对象,阐述了五轴数控机床的综合误差建模过程,对传统建模过程中刀具、工件和参考坐标系之间的关系进行了优化处理,得到了包含方向误差在内的综合数学模型。     

A Cutter Orientation Modification Method for the Reduction of Non-linearity Errors in Five-Axis CNC Machining

In the machining of sculptured surfaces, five-axis CNC machine tools provide more flexibility to realize the cutter position as its axis orientation spatially changes. Conventional five-axis machining uses straight line segments to connect consecutive machining data points, and uses linear interpolation to generate command signals for positions between end points. Due to five-axis simultaneous and coupled rotary and linear movements, the actual machining motion trajectory is a non-linear path. The non-linear curve segments deviate from the linearly interpolated straight line segments, resulting in a non-linearity machining error in each machining step. These non-linearity errors, in addition to the linearity error, commonly create obstacles to the assurance of high machining precision. In this paper, a novel methodology for solving the non-linearity errors problem in five-axis CNC machining is presented. The proposed method is based on the machine type-specific kinematics and the machining motion trajectory. Non-linearity errors are reduced by modifying the cutter orientations without inserting additional machining data points. An off-line processing of a set of tool path data for machining a sculptured surface illustrates that the proposed method increases machining precision.     

五轴数控加工中旋转轴运动引起的非线性误差分析及控制

五轴数控(Computer numerical control,CNC)加工中,刀具路径规划阶段与实际加工阶段对旋转轴运动采用的插补方式存在差异,其中刀具路径规划阶段是根据零件的几何信息进行插补,而实际加工中则根据机床信息进行插补,这种差异将引起原理性加工误差。针对五轴数控加工中旋转轴的运动,分析采用线性插补方式控制两个旋转轴进行加工时刀具姿态变化引起的原理性误差,进一步研究不同加工情况下由此产生的在垂直于走刀方向的平面内的非线性误差。通过分析旋转轴运动过程中线性插补引起的刀轴偏差角,证明刀具在相邻两刀位点运动过程的中间时刻处刀轴偏差角取得最大值,并得到由该最大值的显式表达式,在此基础上分析最大偏差角的影响因素。提出通过限制相邻两刀位点间刀轴夹角来控制此非线性误差的方法,并给出实例验证。     

A new accurate curvature matching and optimal tool based five-axis machining algorithm

Free-form surfaces are widely used in CAD systems to describe the part surface. Today, the most advanced machining of free from surfaces is done in five-axis machining using a flat end mill cutter. However, five-axis machining requires complex algorithms for gouging avoidance, collision detection and powerful computer-aided manufacturing (CAM) systems to support various operations. An accurate and efficient method is proposed for five-axis CNC machining of free-form surfaces. The proposed algorithm selects the best tool and plans the toolpath autonomously using curvature matching and integrated inverse kinematics of the machine tool. The new algorithm uses the real cutter contact toolpath generated by the inverse kinematics and not the linearized piecewise real cutter location toolpath.     

考虑热误差的双转台机床工艺误差谱预测方法

五轴数控机床的几何误差和热误差是影响工件加工精度的两个重要因素,对这些误差因素进行分析可以有效提高薄壁件工件的加工精度。本文首先基于齐次坐标变换法,建立了双转台五轴数控机床的旋转轴几何误差模型;然后基于对标准球进行在机接触测量,辩识得出两旋转轴的12项几何误差,这些误差考虑了两旋转轴之间的相互影响和其热误差的影响;最后分析五轴数控机床加工空间的几何误差场,在该加工空间内几何误差从中心到外侧逐渐增加,当A轴旋转角度增加时,误差的最大值也随之增加。与其它位置误差辨识方法相比,本方法的测量精度符合加工要求,测量时间只需要30 min。     

An adaptive feedrate scheduling method of dual NURBS curve interpolator for precision five-axis CNC machining

Non-uniform rational b-spline (NURBS) tool path is becoming more and more important due to the increasing requirement for machining geometrically complex parts. However, NURBS interpolators, particularly related to five-axis machining, are quite limited and still keep challenging. In this paper, an adaptive feedrate scheduling method of dual NURBS curve interpolator with geometric and kinematic constraints is proposed for precision five-axis machining. A surface expressed by dual NURBS curves, which can continuously and accurately describe cutter tip position and cutter axis orientation, is first used to define five-axis tool path. For the given machine configuration, the calculation formulas of angular feedrate and geometric error aroused by interpolation are given, and then, the adaptive feedrate along the tool path is scheduled with confined nonlinear geometric error and angular feedrate. Combined with the analytical relations of feed acceleration with respect to the arc length parameter and feedrate, the feed profiles of linear and angular feed acceleration sensitive regions are readjusted with corresponding formulas and bi-directional scan algorithm, respectively. Simulations are performed to validate the feasibility of the proposed feed scheduling method of dual NURBS curve interpolator. It shows that the proposed method is able to ensure the geometric accuracy and good machining performances in five-axis machining especially in flank machining.     

A novel kinematic model for five-axis machine tools and its CNC applications

In conventional five-axis CNC machining, the machine structure is treated as a single kinematic chain just like a robotic manipulator while the cutter is treated as an end effector. In this paper, besides the machine kinematic chain, a cutter kinematic chain is introduced, and the two subkinematic chains are combined to form one machine–cutter kinematic chain. Forward, inverse kinematics and constrained inverse kinematics for the proposed machine–cutter kinematic chain are further put forward. Two applications are presented to demonstrate the advantages and effectiveness of the proposed kinematic model. The proposed kinematic model unifies the structure of the machine and cutter; therefore, the flexibility of the five-axis machine tool can be fully explored.     

五轴微段平滑插补算法

在五轴加工编程中,计算机辅助制造系统对曲面加工通常采用以折代曲,采用大量的微小G01直线段来加工曲面,在曲率半径较大的工件表面会出现明显折痕,严重影响工件表面的加工质量。为提高五轴数控加工工件的表面质量,提出一种五轴微段平滑插补算法。该算法考虑五轴加工中刀位数据的量纲差异,根据相邻数据点间的线性轴长度、线性轴的夹角和旋转轴角度变化量识别五轴数控加工程序中非连续微段和连续微段加工区域。对非连续微段加工区域按照原始直线段和旋转轴直接插补,从而保证加工精度。对连续微段加工区域,先通过五维变量获取节点参数,采用最小二乘法对指令点在允许的精度范围内进行修正;对修正后的指令点采用4点构造法计算二阶切矢,根据连续微段的指令点修正值,节点参数值和对应的二阶切矢值获取二阶连续的三次样条曲线;在二阶连续平滑的曲线上进行实时插补计算,控制机床进行五轴加工。试验结果表明:通过提出的五轴微段平滑压缩算法拟合后的路径要更加接近原始的曲面模型,平滑处理过的实际工件加工表面也要优于未进行处理的工件加工表面,提高了五轴自由曲面的表面质量。     

基于机床运动学约束球头刀多轴加工刀轴矢量优化方法

针对目前航空发动机叶片进排气边加工精度和表面质量较差的问题,提出了一种基于机床运动学约束球头刀多轴加工刀轴矢量优化方法。建立刀位优化变量与刀位数据之间的关系方程,同时建立刀位数据与机床回转轴角度之间的运动变换方程,从而推导出刀位优化变量与机床回转轴角度之间的关系方程。通过求解上述方程得到球头刀多轴加工复杂曲面的刀轴矢量计算公式。在此基础上,给出球头刀多轴加工刀轴矢量优化方法和刀轨生成方法。同时,以某航空发动机叶片为例,分析了本文算法和Sturz算法对机床回转轴角度的影响。分别利用本文算法和Sturz算法生成该叶片进气边加工的刀轨,并在五轴数控机床上进行加工试验。试验结果表明,该算法能够避免加工过程中机床回转轴的大幅波动,使机床轴运动更加平稳和光滑,从而提高曲面的加工质量和加工效率,具有一定的实际应用价值。     

Three-dimensional tool radius compensation for multi-axis peripheral milling

Few function about 3D tool radius compensation is applied to generating executable motion control commands in the existing computer numerical control (CNC) systems. Once the tool radius is changed, especially in the case of tool size changing with tool wear in machining, a new NC program has to be recreated. A generic 3D tool radius compensation method for multi-axis peripheral milling in CNC systems is presented. The offset path is calculated by offsetting the tool path along the direction of the offset vector with a given distance. The offset vector is perpendicular to both the tangent vector of the tool path and the orientation vector of the tool axis relative to the workpiece. The orientation vector equations of the tool axis relative to the workpiece are obtained through homogeneous coordinate transformation matrix and forward kinematics of generalized kinematics model of multi-axis machine tools. To avoid cutting into the corner formed by the two adjacent tool paths, the coordinates of offset path at the intersection point have been calculated according to the transition type that is determined by the angle between the two tool path tangent vectors at the corner. Through the verification by the solid cutting simulation software VERICUTwith different tool radiuses on a table-tilting type five-axis machine tool, and by the real machining experiment of machining a soup spoon on a five-axis machine tool with the developed CNC system, the effectiveness of the proposed 3D tool radius compensation method is confirmed. The proposed compensation method can be suitable for all kinds of threeto five-axis machine tools as a general form.     

A cutter orientation modification method for five-axis ball-end machining with kinematic constraints

Cutter orientation modification with kinematic constraints is very necessary and effective for five-axis machining especially machining at high speed. It is very helpful for achieving a smooth cutter motion and keeping the process steady. Therefore, a cutter orientation adjustment method is proposed to obtain an optimized tool path which makes best use of the kinematic characteristics of angular feed for five-axis machining. For the given five-axis cutter location path and the feed profile of cutter tip point both expressed by b-spline formats with the same parameterization, the analytic relations of angular feed, angular feed acceleration, and jerk with respect to the geometric and tangential feed parameters of the cutter tip trajectory are first derived. Then, the conditional inequalities of these kinematic constraints used for orientation adjustment are built. Subsequently, the determination method of feasible cutter orientation and detailed algorithm of orientation adjustment are given. Finally, illustrated examples are conducted to validate the proposed orientation adjustment method. The results show that the developed method is effective and can be applied to optimize geometrically complex five-axis tool path by taking the angular feed, angular feed acceleration, and jerk into account.