自由曲面数控加工刀具轨迹曲线的一种生成算法

提出一种自由曲面数控加工轨迹曲线计算方法。根据这一方法,可由自由曲面上一定数量的任意分布型值点来 计算曲面加工时的刀具曲线。算法的基本思想是,型值点影响并决定着其控制区域内的曲面形状,这种影响的大小与到 型值点的距离及权指数相关。分别给出了沿X向进给及Y向进给时刀具轨迹曲线的生成算法。讨论了算法中参数选取 对加工曲面形状及拟合精度的影响。     

Correcting and compressing interpolation algorithm for free-form surface machining

The existing interpolation algorithm cannot meet the need of high-speed and high-accuracy machining of a free-form surface. So this paper proposed a correcting and compressing interpolation algorithm. Depending on the distance and angle evaluated from the adjacent command points, the machining path of free form can be divided into two machining types. For those regions where the accurate figure is critical such as corners, the convention linear interpolation is performed exactly between the adjacent command points. For those regions having a large radius of curvature where the smooth figure is critical, firstly, the interior point selection method based on circle transition is derived to reduce the tolerance between the machining path and the original surface; secondly, the interior point correction method based on the least-square method is proposed to reduce the calculation error and round-off error in the interior point and estimate the first- and second-order derivative vectors of the interior point; thirdly, the shape-defining point is selected by the bend direction of the machining path and fitted to a quintic spline curve which has the C2 continuity; fourthly, the fitting accuracy controlling method is proposed to ensure the machining accuracy; lastly, the curve interpolation is performed on the fitted smooth curve. Machining tests carried out on a vertical machining center show that the proposed algorithm can improve the machining efficiency and machining quality of a free-form surface.     

Iso-parametric tool path generation from triangular meshes for free-form surface machining

The polyhedral model is widely used in the manufacturing industry. However, apart from the iso-planar method, the tool path generation methods for polyhedral machining are very limited. In such a case, the given tool paths are no longer boundary-conformed or efficient. This paper presents a new approach to iso-parametric tool path generation for triangular meshes. The strategy proposed herein first parameterizes the triangular faces via a harmonic map. The cutter-contact (CC) points and the path interval are then calculated based on the machining tolerance requirements and the iso-parametric tool paths are finally generated. The method is implemented on a computer and some illustrative examples are provided to show the effectiveness of the developed algorithm. The main advantage of the proposed method is that the tool paths can be generated naturally along the boundary of a polyhedral model, thus eliminating internal tight-radius corners in conventional paths. This leads to substantial reductions of tool wear and machining time. In addition, the proposed method can also be used in other non-iso-parametric tool path planning methods for triangular meshes and compound surfaces machining.     

A simplified mathematical model development for the design of free-form cathode surface in electrochemical machining

High-performance machining of free-form surfaces is highly critical in automotive, aerospace, and die–mold manufacturing industries. Therefore, electrochemical machining (ECM) process has been used in such cases in that sense. The most important challenges of using ECM process are the lack of accuracy and difficulty in designing proper machining tool (cathode) surfaces. In this article, a simplified mathematical model is presented to obtain a cathode surface for ECM of free-form surfaces which have high curvatures. In this theoretical approach, the finite-element method (FEM) is used to solve the 3-D Laplace equation and to determine the potential distribution between the anode (workpiece) and cathode (tool) surfaces. A compact and simple program was developed to obtain a proper cathode surface that only requires some nodal coordinates on the anode surface and boundary conditions. In this work, a trial cathode surface is constructed for a given gap distance. For the determined ECM parameters, cathode shape that satisfies the boundary conditions is obtained for the 45^th layer. The results are compared with the literature and ANSYS Workbench for verification. The developed theoretical approach benefits simpler and faster FEM solutions, accurate cathode surface, and consequently correct form of machined surface.     

A new free-form surface fitting method for precision coordinate metrology

Free-form surfaces are widely used in advanced optical and mechanical devices. In order to assess the form accuracy of free-form surfaces, it is needed to fit the measurement surface with the design template. To improve the fitting efficiency and accuracy, the fitting procedure is divided into two stages, rough fitting and final fitting. A new rough fitting method, called structured region signature is proposed. Structured region signature is a generalised global feature which represents the surface shape by a one-dimensional function. The template location occupying the best fitting signature is considered to be the correct initial position of the measurement surface. In the final fitting, the motion parameters are calculated iteratively using the Levenberg-Marquardt algorithm. Simulation and experiments are given to identify the validity of this method. It is demonstrated that the fitting accuracy is as high as 1:10⁶.     

Adaptive sampling method for laser measuring free-form surface

In order to achieve high accuracy in the laser measuring of free-form surfaces, equal step-length sampling usually involves much more measurement data than necessary. An equal arc-length sampling method based on tangent extrapolation of cubic spline curves is introduced to solve this problem. By this method, step-length is adjusted adaptively according to the slope variation of the sampled curve with arc-length maintained to be constant, thus the small slope region is sampled sparsely while the large slope region is sampled densely. Sampling simulation of a typical surface proves that the data amount of equal arc-length sampling is much less than that of equal step-length sampling under the same circumstance.     

自由曲面数控加工刀具轨迹生成的约束条件分析

在提出自由曲面数控加工过程和刀具轨迹的规划原则情况下,分析无干涉控制、恒表面速度进给、伺服能力控制和相邻加工曲线影响等轨迹控制的约束条件,实时对轨迹的生成进行系统优化控制。通过系统分析刀具轨迹生成的约束条件,为刀具轨迹优化控制提供决策,对提高自由曲面的切削质量和加工效率具有重要意义。     

自由曲面五轴变步长数控加工刀轨生成方法

为了在满足逼近误差要求的同时最大程度减少冗余刀轨,对自由曲面提出了一种五轴变步长数控加工刀轨生成方法.首先对刀触点轨迹基于线性误差计算出初始刀触点点集,再以局部干涉调整前倾角的方式计算出无干涉刀位点和刀轴矢量;以最大非线性误差刀位处到刀触点轨迹的最小值作为相邻刀位点之间的逼近误差,并基于数据点自适应离散法计算逼近误差;...     

基于改进遗传算法求取加工自由曲面的最大刀具尺寸

为选取合适的加工刀具尺寸以对自由曲面实现无曲率干涉加工,提出了改进遗传算法并应用于自由曲面全局最大主曲率求取,确定无曲率干涉最大刀具尺寸.根据主曲率特点设计了一种新的非线性自适应度函数,避免算法过早收敛于局部最优值,并给出了新的复制原则.改进遗传算法应用算例分析表明,该算法在500代内能有效求取自由曲面全局最大主曲率,比传统的离散法效率高,选择的最大刀具尺寸满足自由曲面加工无曲率干涉加工的要求.     

AutoCAD环境下工程自由曲面的一种三维重建方法

提出一种AutoCAD环境下工程自由曲面的三维重建方法 ,根据投影栅相位法测得的三维离散数据 ,将 3DMESH命令与脚本文件相结合 ,实现自由曲面三维模型的自动重建     

Feedrate scheduling strategy for free-form surface machining through an integrated geometric and mechanistic model

In free-form surface machining, it is essential to optimize the feedrate in order to improve the machining efficiency. Conservative constant feedrate values have been mostly used up to now since there was a lack of physical models and optimization tools for the machining processes. The overall goal of this research is the integration of geometric and mechanistic milling models for force prediction and feedrate scheduling in five-axis CNC free-form surface machining. For each tool move, the geometric model calculates the cut geometry, and a mechanistic model is used along with a maximum allowable cutting force to determine a desired feedrate. The results are written into the part NC program with optimized feedrates. When the integrated modeling approach based feedrate scheduling strategy introduced in this paper was used, it was shown that the machining time can be decreased significantly along the tool path.     

大型自由曲面零件的机器人视觉快速定位方法

针对大型自由曲面零件缺少规则特征导致定位成本高、效率低且精度难以保证的问题,提出一种机器人视觉快速定位方法.设计了自适应分辨率的超体素聚类算法,对不同模型点云进行均匀分割;提出以分块质心点的曲率、夹角以及快速点特征直方图(FPFH)描述子为依据,快速识别出完整CAD模型点云与局部测量数据的重叠区域并求解粗配准矩阵.改进了分支定界(BNB)嵌套迭代最近点(ICP)算法,提出基于关键点的快速精配准方法.以冲压件为对象进行实验验证,结果表明本方法节省了30％以上的配准时间,并显著提高了配准精度,能够解决大型自由曲面零件的定位难题.     

A novel iso-scallop tool-path generation for efficient five-axis machining of free-form surfaces

Weld cladding is a process of depositing a thick layer of a corrosion resistance material over carbon steel plate to improve the corrosion resistance properties. The main problem faced in stainless steel cladding is the selection of process parameters for achieving the required clad bead geometry and its shape relationships. This paper highlights an experimental study carried out to develop mathematical models to predict clad bead geometry and its shape relationships of austenitic stainless steel claddings deposited by gas metal arc welding process. The experiments were conducted based on four-factor, five-level central composite rotatable design with full replication technique. The mathematical models were developed using multiple regression method. The developed models have been checked for their adequacy and significance. The direct and interaction effects of process parameters on clad bead geometry and its shape relationships are presented in graphical form.     

基于网格曲面上空间填充曲线刀具路径生成算法研究

在细分网格曲面上,用最短哈密顿回路法通过连接网格节点去寻找最优路径,以形成填充曲线刀具路径。将空间曲面细分成有限四边形网格后,结合无向网上最短哈密顿回路求解算法,通过构建代价树的方法求解最短路径。应用了邻接矩阵的形式描述图形,及基于矩阵法数据存储的度数消减算法判断和处理图形,构建了空间网格曲面上最短哈密顿回路生成算法。通过一个曲面填充实例验证了构建算法的正确性,及用此方法生成曲面加工刀具路径的可行性。     

螺旋面数控加工刀具运动轨迹计算方法研究

在数控机床上加工螺旋面多采用平面啮合理论,但实际在工件表面的切削轨迹为空间曲线,针对平面编程方法存在不可消除的理论误差,对截面包络法数控加工螺旋面时刀触点和刀具运动轨迹进行研究,提出一种新的计算刀触点"最小有向距离算法"的理论并进行论证.     

用圆柱铣刀与不可展直纹面的二阶切触及蒸汽机车排汽叶轮加工

把微分几何中曲线与曲面之间的“切触”概念应用于不可展直纹面的数控加工，提出用圆柱铣刀的外圆在五坐标联动中加工的最不可展直纹面最佳切触条件。并应用于蒸汽机车排汽叶轮加工。     

A novel robust Gaussian filtering method for the characterization of surface generation in ultra-precision machining

A lot of research work has been focused on the study of the surface generation mechanisms in order to predict the surface topography and provide the optimal machined parameters based on the experiential understanding of relationship of machined conditions and surface features. Although the formation of novel geometrical product specification (GPS) and verification framework system promotes the relevant research work to new characterization methods and draft of international standards, relative little research work was conducted on the application of surface characterization techniques to ultra-precision machining which is very important to evaluate the surface quality. In this paper, a novel robust Gaussian filtering method (RGF) is proposed and used to characterize the surface topography of ultra-precision machined surfaces. Cubic B-spline and M-estimation are used to make the method reliable and robust. Based on the property comparisons of classical weighting functions, a novel auto-developed robust weighting function (ADRF) is defined to improve the robustness of RGF. To verify the characterization feasibility of the proposed method, computer simulation is used and then the real ultra-precision machined surfaces are analyzed. The experimental results indicate that the RGF method cannot only separate the surface components effectively on the whole measured area and but also eliminates the influence of freak outliers.     

Study of magnetic abrasive finishing in free-form surface operations using the Taguchi method

This study employed magnetic abrasive finishing (MAF) to conduct free-form surface abrasion of stainless SUS304 material operations. The operations were demonstrated using a permanent magnetic finishing mechanism installed at the CNC machining center. The operations were performed using the Taguchi experimental design, considering the effects of magnetic field, spindle revolution, feed rate, working gap, abrasive, and lubricant. Furthermore, the experimental data was collected using the Taguchi experimental design. The optimal parametric conditions for processing stainless SUS304 material were applied in a two-stage process comprised of rough finishing that involved MAF followed by a precise finishing of the surface. Prior to rough finishing, the R_max value was 2.670 μm; after rough finishing, the value was 0.158 μm. Precise finishing yields an even lower value of 0.102 μm similar to that of the mirror surface. Therefore, the results revealed that MAF provides a highly efficient way of obtaining surface finish.