Integrated rough machining methodology for centrifugal impeller manufacturing

In mechanical engineering, most products or components, especially those for aerospace applications, are designed to fit the requirements of free-form surface features. The impeller often required by 5-axis machine operations is a key component of the aerospace industry. When 3-axis CNC machining center is used to manufacture the impeller, great difficulties, i.e., collisions between the cutting tool and impeller, need to be overcome. Presently most commercial CAM systems for 5-axis control lack generality, and functions for the rough tool-path generation are far from sufficient. Although the rough machining is the most important procedure influencing the machining efficiency and the condition for the following finishing process, many difficulties arise in performing 5-axis rough machining. The main objective of the present study is to overcome this problem by integrating the state-of-art machining technology, and consequently effective rough tool-paths are to be generated. This study aims to implement the algorithm of the constant scallop height method to improve tool-path planning of rough machining. As a result CL data based on the geometry model of blade and hub are generated. The CL data are confirmed by comparing them with original CAD model through software simulations and later by machining experiments. The verification results show that the machining methodology and procedure adopted turn out to be a successful case.     

基于五轴机床的叶轮实体建模与加工

由于整体叶轮的叶片是扭曲的,在建模和加工方面都有极大的困难。相比三轴加工机床,五轴机床具有较高的生产率和较好的加工质量等优点。根据实际工程数据,选用Pro/Engineer软件对实体叶轮进行造型,并对整体叶轮数控加工的走刀路径进行了分析和研究,针对干涉问题,选取刀轴自动避让功能,并定义刀轴限界功能。成功将NC程序从Powermill软件中导入到DEMAGE五轴机床。并以某离心式压缩机整体叶轮产品为例进行了加工验证,加工出了实体产品。实验结果表明选用的加工方法和工艺是非常成功的。     

Optimal sequencing of rotation angles for five-axis machining

In this paper, we propose two new algorithms to correct the trajectories of the tool tip of a five-axis milling machine by adjusting the rotation angles in such a way that the kinematics error is reduced. The first algorithm is based on the shortest-path optimization with regards to feasible rotations of the inverse kinematics. The cost function is represented in terms of the total angle variation. We show that such an optimization increases the accuracy of machining and is the most appropriate in the case of a rough cut. The shortest-path procedure applies to either the entire set of trajectories or to only the most inappropriate undercuts inside the workpiece. In the latter case, the algorithm generates an interesting family of solutions characterized by smaller undercuts obtained at the expense of increased overcuts. The second algorithm also exploits the idea of the minimization of the angle variation. It is based on the uniform distribution of the cutter contact points with regards to the rotation angles. The method inserts additional tool positions by numerically finding a grid of points distributed uniformly in the angular space. We prove that the proposed algorithm in the neighborhood of stationary points requires 3–4 times fewer additional points than the conventional scheme. Also, if a maximum angular speed has been exceeded, the controller detects this event and reduces the angular speed. Our correction algorithms minimize the total angle variation, thus, reducing the probability of such an event. Finally, the efficiency of the two algorithms has been verified by a five-axis machine MAHO600E at the CIM Lab of the Asian Institute of Technology of Thailand and HERMLE UWF920H at the CIM Lab of the Kasetsart University of Thailand.     

Three-half and half-axis patch-by-patch NC machining of sculptured surfaces

Despite the inbuilt advantages offered by five-axis machining, the manufacturing industry has not widely adopted this technology due to the high cost of machines and insufficient support from CAD/CAM systems. Companies are used to three-axis machining and their shop floors are not yet ready for five-axis machining in terms of training and programming. The objective of this research is to develop and implement a machining technique that uses the simplicity of three-axis tool positioning and the flexibility of five-axis tool orientation, to machine sculptured surfaces. This technique, 3½½-axis, divides a sculptured surface into patches and then machines each patch using a fixed tool orientation. This paper presents the surface partitioning scheme and the method of selecting an optimum number of sub-divisions along with actual machining experiments. For the example surface utilized in this study, the proposed hybrid method led to shorter machining time compared to traditional three-axis machining and comparable to simultaneous five-axis machining.     

Tool path planning for five-axis flank milling with developable surface approximation

This paper presents a novel approach that automatically generates an interference-free tool path for five-axis flank milling of a ruled surface. A boundary curve of the machined surface is subdivided into curve segments. Each segment works as a guide curve in the design method for developable Bézier surface that controls a developable patch for approximating the surface with available degrees of freedom. Geometric algorithms are proposed for calculating consecutive patches with G¹ continuity across the patch boundary. A tapered tool can move along the rulings of these patches without inducing local tool interference as a result of their developability. The machining deviation is controlled by the surface approximation error. A machining test is conducted with the generated CL data and the result verifies the feasibility of the proposed approach. This work successfully transforms avoidance of tool interference into a geometric modeling problem and provides a simple solution. It thus demonstrates a good potential for the developable surface theory of five-axis flank machining .     

<Emphasis Type="Bold">An Integrated Machining Approach for a Centrifugal Impeller</Emphasis>

When a 3-axis CNC machining centre is used for producing an impeller, great difficulties, i.e. collisions between the cutting tool and the impeller, can occur. The blade of an impeller is usually designed with a ruled surface. As the surface is normally twisted in design to achieve the required performance, it can cause overcut and collision problems during machining. The hub of the impeller is usually designed with an irregular surface, and is machined within a narrow and deep groove. The issues – how to satisfy the quality requirements of the part, reduce the machining time, and avoid the occurrence of collision – become an integral problem. This work develops an integrated 5-axis machining module for a centrifugal impeller by combining related machining technologies. As a result, cutter location (CL) data based on the geometric model of blade and hub are generated. Finally, the CL data are confirmed through software simulation. The results of verification show that the machining methodology and procedure adopted are successful.     

Three-half and half-axis patch-by-patch NC machining of sculptured surfaces

Despite the inbuilt advantages offered by five-axis machining, the manufacturing industry has not widely adopted this technology due to the high cost of machines and insufficient support from CAD/CAM systems. Companies are used to three-axis machining and their shop floors are not yet ready for five-axis machining in terms of training and programming. The objective of this research is to develop and implement a machining technique that uses the simplicity of three-axis tool positioning and the flexibility of five-axis tool orientation, to machine sculptured surfaces. This technique, -axis, divides a sculptured surface into patches and then machines each patch using a fixed tool orientation. This paper presents the surface partitioning scheme and the method of selecting an optimum number of sub-divisions along with actual machining experiments. For the example surface utilized in this study, the proposed hybrid method led to shorter machining time compared to traditional three-axis machining and comparable to simultaneous five-axis machining .     

五轴加工刀具路径生成的有效加工域规划方法

为复杂曲面五轴数控加工的刀具路径优化生成问题提出一种新的有效加工域规划方法。在对工件被加工表面和刀具的几何特征进行分析的基础上,得到在加工件表面上各处的最优可加工域宽度和刀具切削方向。通过采用离散采样和插值计算生成优化的有效加工域集,得到最优化的初始刀具路径;同时建立一种迭代搜索算法,用于解决最优加工域的选择规划问题。采用此算法生成优化的后续刀具路径,使得有效加工域最终完全覆盖整个被加工表面。给出的示例显示相对于传统的五轴加工刀具路径生成算法,有效加工域规划方法可以减少刀具路径的总长度和加工时间,得到更为优化的刀具路径和更好的工件表面质量,因此有效加工域规划方法可以被用于五轴数控加工实践以降低加工成本和提高产品质量。     

五轴数控加工中多刀加工自由曲面的刀具选择

针对自由曲面的五轴数控加工，提出一种新的刀具选择方法，此算法可以自动地选择出最优刀具组合，每个刀具对应一个或多个特定的加工区域。多刀具组合的加工方法既可以保证加工曲面的精确性，又可以实现大刀具的高效加工。此外，还提出了种五轴教控加工时间的近似算法，通过对比用刀具组合加工时间和单个刀具加工时间，从而证明了该方法的有效性。     

直纹面叶轮插铣加工关键技术

为提高整体叶轮的粗加工效率,弥补目前商用计算机辅助制造软件在五坐标插铣加工能力方面的不足,研究了直纹面叶轮五坐标插铣加工的关键技术。根据直纹面叶片的偏移边界矢量,利用四元数插值方法计算插铣加工的刀轴矢量,提出并推导了五坐标插铣加工的行距和步距计算公式,保证了插铣加工的材料去除率和加工效率。依据上述算法自主开发了整体叶轮五坐标插铣加工专用计算机辅助制造软件,并对该软件生成的刀具轨迹进行了仿真和实际加工验证。结果证明,该方法在一定程度上提高了叶轮零件的粗加工效率。     

Geometrical Error Analysis and Control for 5-axis Machining of Large sculptured surfaces

One of the important tasks in five-axis machining of large sculptured surfaces is to control and reduce the machined errors. This paper presents the methods to control geometrical errors based on the establishment of the link between geometrical errors and the parameters of tool path planning. Nonlinear errors, which are the majority of geometrical errors during five-axis machining, are is strictly analysed and formulated. An adaptive step length method is proposed to control effectively the cutter contact path error. The measures to reduce the scallop error in machining of the large sculptured surfaces are discussed also. With the combination of this research with CAM software, both large Kaplan and Francis hydroturbine blades have been successfully machined. It shows that the machined errors can be controlled effectively and the machining efficiency can be improved in the machining of the large sculptured surfaces by the proposed methods.     

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。     

基于广域空间的自由曲面宽行加工方法

为提高自由曲面环形刀五轴精加工的效率,在广域空间中分析刀具的干涉误差,对曲面宽行加工的刀具方位优化方法进行研究。首先根据环形刀刀具特性,定义曲面可能干涉的区域为广域空间,然后将广域空间离散点集变换至刀具坐标系中,计算刀具刀刃曲面的干涉误差,从而构建刀具的实际切削轮廓,并在此基础上精确计算有效的加工带宽。最后,通过建立刀具方位与加工带宽的函数关系,以加工带宽最大化为目标,实现环形刀五轴宽行加工刀具方位的自动优化。算例及试验表明,在相同的残留高度要求下,广域空间方法较之现有的二阶泰勒逼近方法,切触点轨迹长度减少了15.74%,说明基于广域空间的刀轴优化方法是实现自由曲面宽行加工的有效途径。     

整体叶轮五轴数控加工仿真与优化

根据整体叶轮的几何特点确定叶轮五轴数控铣削加工工艺路线,并通过UG/Post Builder后置处理器生成了NC代码;利用VERICUT数控加工仿真功能建立了五轴数控加工仿真平台,实现了加工过程的仿真、优化和NC代码验证,缩短了叶轮加工的生产周期。     

非典型叶轮数控5轴技术的研究

由于非典型叶轮结构复杂,无法使用主流CAD/CAM软件提供的叶轮5轴模块,在粗加工阶段采用3轴定向加工时,残余毛坯留量体积大形状不规则,严重影响了5轴加工的效率和质量。为此,一方面对于叶轮开式区域采用双圆柱面投影法获得驱动曲面,从而实现4轴联动粗加工,切削刚性大效率高;对于非开式区域则通过构建驱动曲面获得5轴联动刀轨,刀轨连续且刀轴矢量无突变。另一方面,提出了一种基于NC程序的刀轴矢量评价方法,可快速直观地评价5轴编程质量。实际使用结果表明:基于驱动曲面的刀轴矢量优化法能够显著提高复杂工件5轴加工的效率和质量。     

Optimal CNC plunge cutter selection and tool path generation for multi-axis roughing free-form surface impeller channel

Plunge milling is the most effective way for rough machining of impeller parts, but previous research had not considered the optimization of plunge cutter selection and tool path. In this paper, a new method for optimizing the plunge cutter selection and tool path generation in multi-axis plunge milling of free-form surface impeller channel is proposed in order to improve the efficiency in rough machining. Firstly, in the case of fixing a rotation axis at a certain angle in five-axis machine, a mathematical representation is formulated for the geometric model of the cutter interfering the impeller, and an optimization model of the cutter size is established at a cutter contact point on the impeller channel surface, so the largest tool could be determined. Secondly, by analyzing the machine tool movement characteristics, the geometric constraint model of the plunge tool path which relative to the largest tool, step distance, and row space is established, and a tool orientation calculation method of impeller channel machining is given, and then, the plunge tool path and tool orientation could be obtained. Finally, the generated tool path and tool orientation are simulated and verified in practical processing. Simulation and experimental result shows that the rough machining efficiency of the impeller part is improved up to 40 % with this method.     

五轴数控系统联动控制方法研究

提出了多轴联动控制的新思路,通过对五轴数控系统模型的分析,建立坐标系,根据坐标变换理论,推导出五轴数控系统的顺运动学数学模型。通过求解雅可比矩阵及广义雅可比逆矩阵,得到五轴数控系统各轴的运动量,实现联动控制。计算机仿真结果表明,此方法能够实现五轴数控系统的高精度联动控制。     

平底刀最优刀轴矢量规划算法

在进行自由曲面平底刀五轴数控加工刀轴矢量规划时,现有算法多以材料去除率最大作为优化目标,然而加工效率最高的刀轴矢量不一定是刀具最优姿态,还需要综合考虑加工质量、刀轴光顺等因素。从这几个方面建立单个刀轴矢量和相邻刀轴矢量光顺性的度量指标,并以归一化度量指标加权和最小作为优化目标建立刀轴矢量优化模型,经理论分析将多目标规划问题转化为力学平衡问题,弹簧模型中高斯球面上运动节点的平衡位置就是最优刀轴矢量。仿真结果表明SE权因子影响加工效率,DW、DM权因子影响机床运动平稳性,DF权因子影响切削条件,选择合适的权因子可以获得材料去除率大、加工效率高以及在工件坐标系、机床坐标系和进给坐标系下光顺的刀轴矢量。     

基于UG的叶轮5轴数控铣削编程与加工

叶轮零件结构复杂,其数控加工技术普遍存在编程复杂,困难且效率低下等问题.介绍使用典型的CAD/CAM软件UG7新增的叶轮加工模块,对深窄槽道、大扭角的滑轮发动机叶轮的五坐标加工程序刀路进行编制,大大提高了叶轮加工编程的速度和效率,为叶轮零件的编程加工提供了新思路和方法.     

高精度质量检测在航空器叶轮五轴加工中的应用

以航空器叶轮五轴高速加工为基础,针对叶轮复杂三维扭曲流道空间曲面检测困难的问题,研究了在机检测和智能修正技术、三维扫描检测技术,并利用工业级蓝光三维检测系统,对航空器叶轮进行五轴加工和准确、高效、可靠的三维数字化高精度检测。结果表明,该方法可快速实现高精度三维全尺寸检测,提高了测量效率。