多轴联动数控加工刀具轨迹的优化算法的研究和实验验证

提出一种新的刀具轨迹优化算法,使相邻加工轨迹间的残留高度值都等于允许残留高度值(规定精度),从而达到最大的切削带宽,消除了干涉.同时采用刀位显示验证方法对加工某叶片模型所得到的刀具轨迹进行验证.实验数据表明,在保证精度的前提下,轨迹数和轨迹长度明显减少,大大提高了多轴联动数控加工的效率.     

多轴联动数控加工乃具轨迹的优化算法的研究和实验验证

提出一种新的刀具轨迹优化算法，使相邻加工轨迹间的残留高度值都等于允许残留高度值（规定精度），从而达到最大的切削带宽，消除了干涉。同时采用刀位显示验证方法对加工某叶片模型所得到的刀具轨迹进行验证。实验数据表明，在保证精度的前提下，轨迹数和轨迹长度明显减少，大大提高了多轴联动数控加工的效率。     

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.     

Generating tool-path with smooth posture change for five-axis sculptured surface machining based on cutter’s accessibility map

In five-axis high speed milling, one of the key requirements to ensure the quality of the machined surface is that the tool-path must be smooth, i.e., the cutter posture change from one cutter contact point to the next needs to be minimized. This paper presents a new method for generating five-axis tool-paths with smooth tool motion and high efficiency based on the accessibility map (A-map) of the cutter at a point on the part surface. The cutter’s A-map at a point refers to its posture range in terms of the two rotational angles, within which the cutter does not have any interference with the part and the surrounding objects. By using the A-map at a point, the posture change rates along the possible cutting directions (called the smoothness map or S-map) at the point are estimated. Based on the A-maps and S-maps of all the sampled points of the part surface, the initial tool-path with the smoothest posture change is generated first. Subsequently, the adjacent tool-paths are generated one at a time by considering both path smoothness and machining efficiency. Compared with traditional tool-path generation methods, e.g., iso-planar, the proposed method can generate tool-paths with smaller posture change rate and yet shorter overall path length. The developed techniques can be used to automate five-axis tool-path generation, in particular for high speed machining (finish cut).     

曲面实时插补无干涉刀轨的生成方法

提出一种数控加工中曲面实时插补无干涉刀轨生成方法。该方法既能在加工中实时产生无干涉刀位轨迹 ,同时也能方便地确定因避免产生干涉而出现的切不足区域 ,以便该区域的后续加工。     

基于插铣加工的非等参数刀具轨迹生成方法

针对复杂多曲面通道的多坐标数控粗加工,提出了一种新的刀具轨迹生成算法.该算法以传统的插铣加工方法为基础,提出适合于复杂多曲面通道的新插铣方法;利用曲面参数线,使用B样务曲线正算反算方法,在流道划分的多个截面上做由初始孔到截面轮廓的渐变曲线;通过计算满足残留高度的走刀行距,在各个截面的渐变曲线上提取参数点并拟合出相应曲线,进而得到刀具接触点轨迹.     

A New Tool-Path Generation Method Using a Cylindrical End Mill for 5-Axis Machining of a Spatial Cam with a Conical Meshing Element

The geometry of a spatial cam is normally generated using a form-milling cutter and a special multi-axis machine and software. The general purpose CAD/CAM software can generate the cutter location file of this class of product only by the sculpturing method. Because of the limited choice of cutting tool when using the generating method, this paper presents a new toolpath generating method which combines the advantages of the generating and sculpturing methods for machining a spatial cam. In the machining process, the cutter location is derived for rough and finish machining using a cylindrical end mill. The guidelines for choosing cutter diameter for the machining process are discussed. To avoid interference, the principal curvatures and the principal induced normal curvatures are analysed. The mathematical errors including chordal deviation and scallop height are used as the basis for generating appropriate tool-paths. Cutting simulation of a solid model was performed to verify the proposed tool-path generation method.       

基于Z-Buffer理论的三坐标无干涉刀位轨迹的生成

针对当前无干涉刀位轨迹生成算法存在的问题，提出一种基于Z-Buffer理论的用于三坐标加工的无干涉刀位轨迹生成算法。该算法已成功地应用于洗衣机波轮、水轮机叶片等复杂曲面零件的NC加工。实践证明该算法具有可行性，计算方法简单，并且能很好地解决干涉问题，对于在三坐标加工中的刀位轨迹生成具有很高的实用价值。     

Selecting a milling strategy with regard to the machine tool capabilities: application to plunge milling

The last generation of machining equipment has made innovative milling strategies, such as plunge milling tool-paths, to spread in the industry. However, it is still difficult to evaluate whether the implementation of plunge milling will be beneficial for a specific combination of machine tool and part to machine. The optimal parameterization is difficult to be found. Hence, this paper gives a new approach to enhance the implementation of plunge milling tool-paths by computing the achievable material removal rate according to the tool-path parameterization, the machine tool dynamics, and the machined feature properties. Machine tool key properties identification is enabled as well. A simulation tool was developed accordingly. The chosen modeling and the main functions are introduced in the first section. The main use cases are then discussed in the next sections of the paper: implementing a plunging tool-path for a given machine tool, selecting the most suitable equipment among a group to meet the requirements, and defining the functional specifications for a new machine tool.     

Tool-path generation for sheet metal incremental forming based on STL model with defects

Tool-path generation is a key issue in sheet metal incremental forming process, and existing approaches either are prone to computationally expensive or cannot be applied to stereolithography (STL) model including defects. Thus, a new tool-path generation method is presented by adopting the thought of generating cutter-location (CL) data directly from corresponding cutter-contact (CC) data. By analyzing the interference characteristics between fillet-end tool and model surface and considering tangential case and intersection case comprehensively, a discrete computational model is proposed to calculate single-layer interference-free CL contour, instead of computing a precise CL point by checking potential interferences from candidate facet, vertices, and edges, respectively. So this method is more efficient and easier for program implementation. Additionally, a fast recursive search algorithm is developed to identify and extract flat area features, and an efficient 2D invalid loops removal algorithm based on decomposition thought is presented to obtain valid CC contours and CL contours with a near linear time-complexity. Implementation tests prove that the new method is effective and robust for STL model with defects, and tool-path achieved is highly precise. It is also applicable to various tool shapes and suitable for planning various types of tool-paths to meet different SMIF process requirements.     

Tool-path planning for rough machining of a cavity by layer-shape analysis

In the manufacture of parts with sculptured cavities from prismatic stock, rough machining usually constitutes most of the machining time owing to the significant difference between the stock and the part shape. When using 2 1/2-D milling or a contour-map approach to do the rough machining, the appropriate selection of tool-path pattern for each cutting layer can significantly reduce rough machining time and hence increase productivity. In this paper, the commonly used toolpath patterns are summarised. A knowledge-based parametric approach for optimising the toolpath pattern of a given cutting layer is proposed. Then, a novel methodology is developed to calculate an arbitrary polygon area and locate the concave cavities in the polygon. Procedures for cutting-layer-shape analysis and the optimal comprehensive tool-path pattern generation are also built and proposed in this paper. These procedures can not only be applied to sculptured cavity parts with simple islands, but also to parts with arbitrarily-shaped islands. Finally, an example is given to illustrate the reasoning process.     

复杂曲面五坐标数控铣削表面粗糙度预测的关键技术研究

针对CAD/CAM复杂曲面数控加工刀位点曲率与走刀行距关系进行分析,给出了不同刀具路径下走刀行距的计算方法,并且对影响表面粗糙度的因素进行了讨论,该方法可用于对表面粗糙度的一个范围进行预测.并提出一种基于主曲率匹配的等残留高度刀具路径规划改进算法,该方法能够提高加工效率,改善加工表面粗糙度.     

CNC Tool-Path Planning for High-Speed High-Resolution Machining Using a New Tool-Path Calculation Algorithm

Reduced machining time and increased accuracy for a sculptured surface are both very important when producing complicated parts, so, the step-size and tool-path interval are essential components in high-speed and high-resolution machining. If they are small, the machining time will increase, whereas if they are large, rough surfaces will result. In particular, the machining time, which is a key factor in high-speed machining, is affected by the tool-path interval more than the step size. The conventional method for calculating the tool-path interval is to select a small parametric increment or small increment based on the curvature of the surface. However, this approach has limitations. The first is that the tool-path interval cannot be calculated precisely. The second is that a separate tool-path interval must be calculated in three separate cases. The third is the requirement of a conversion from the Cartesian domain to the parametric domain or vice versa. Accordingly, for high-speed and high-resolution machining, the current study proposes a new tool-path interval algorithm, that does not involve a curvature or any conversion, plus a variable step-size algorithm for NURBS.     

NC machining of free-form Kriging surfaces

An algorithm for three-axis NC tool path generation on sculptured surfaces is introduced when the free-form surface is modeled parametrically by free-form surface Kriging. The flexibility of dual Kriging that easily defines the intersection of the surfaces with a set of parallel planes and Cartesian method are combined to generate the tool-paths automatically. The presented algorithm can simultaneously generate the tool path with a predefined machining accuracy and remove gougings along the tool path. The algorithms are validated by three experiments in rough and finish machining and the results prove its reliability. Since Kriging is based on the interpolation of data points, the proposed algorithm can be used for reverse engineering applications. The system was executed on a standard micro-computer and the software was structured to offer a library of machining functions for endusers.     

Interference-free tool-path planning for flank milling of twisted ruled surfaces

Flank milling is the key feature that the five-axis NC machine offers. Compared with bottom-edge based machining (e.g. flat/fillet endmilling), the machinability can be greatly enhanced by flank milling where the side cutting edge is mainly used. As far as tool-path planning is concerned, the conventional method of the cutter-axis parallel to the ruling lines of the generator curves imposes interference problems including overcut/undercut. In this paper, a new interference-free tool-path planning method fortwisted ruled surfaces is presented. Our strategy is to change the tool orientation and offset distance such that the undercut volume is minimised without global tool interference. The validity and effectiveness of the algorithm is demonstrated via several illustrative examples.     

A Reverse Engineering Approach to Generating Interference-Free Tool Paths in Three-Axis Machining from Scanned Data of Physical Models

An NC tool path is usually generated by sweeping parametric surfaces of a CAD model. In modern design, freeform or sculptured surfaces are increasingly popular for representing complex geometry for aesthetic or functional purposes. Traditionally, a prototype is realised by machining the workpiece using the NC codes generated from a CAD model. The machined part can then be compared with the CAD model by measurement using a coordinate measuring machine. In this paper, a reverse engineering approach to generating interference-free tool paths in three-axis machining from the scanned data from physical models is presented. There are two steps in this procedure. First, a physical model is scanned by a 3D digitiser, and multiple data sets of the complex model are obtained. A surface registration algorithm is proposed to align and integrate those data to construct a complete 3D data set. A shortest-distance method is used to determine the connecting sequence of the neighbouring points between two adjacent scan lines, such that the scanned data are converted into triangular polygons. Tool paths are then generated from the tessellated surfaces. Using the Z-map method, interference-free cutter-location data are calculated, relative to the vertex, edges, and planes of the triangles. The algorithms for tool-path generation are usually different for cutters of different geometries. Some algorithms found in the literature require complex numerical calculations and are time consuming. In this paper, an efficient algorithm is developed to calculate interference-free cutter-location data by easy geometric reasoning without complex computation. This robust method is suitable for generally used cutters such as ball, flat, and filleted end mills, and the time taken to obtain full tool paths of compound surfaces is short. Some real applications are presented to validate the proposed approach.     

A five-axis rough machining approach for a centrifugal impeller

A clear trend shows that most products or mechanical components, especially those regarding aerospace applications, are designed to fit the requirements of free form surface features. When a 3-axis computer numerical controlled (CNC) machining centre is used to produce a typical centrifugal impeller, great difficulties, i.e., collisions between the cutting tool and impeller, need to be overcome. In this case, sophisticated five-axis machines have to be utilised. Presently, most commercial computer-aided manufacturing (CAM) systems for five-axis control are lacking generality, and functions for the rough tool-path generation are far less than required. The rough machining is recognised as the most important procedure influencing the machining efficiency and is critical for the success of the following finishing process. However, great difficulties are expected to arise in performing five-axis rough machining. The main objective of the present study is to overcome this problem by combining related machining technology. As a result, CL data based on the geometry model of blade and hub of the impeller are generated. Finally, the CL data is confirmed through software simulation. The results of verification prove the machining methodology and procedure to be successful.     

Generating NC tool paths from random scanned data using point-based models

This paper presents a new approach for the generation of NC tool paths from random scanned data. Instead of using smooth or triangulated surfaces reconstructed from raw data, which is usually a time-consuming reverse engineering approach, the point-based surfel models computed by a GPU (graphics processing unit) are used to generate NC tool paths. The tool-path generation is highly efficient and still maintains the advantage of having accurate and smooth machining result. The word “surfel” itself is the combination of the two words “surface” and “element”. It is originally applied to the rendering of scanned data. In this paper, the point-based model is created using an elliptical Gaussian re-sampling filter that is based on a signal re-sampling algorithm. Since the input scanned data is of discrete and random nature, the warping process is utilized to transform the input data into a continuous surface and then re-sample the continuous surface by using GPU. Because the re-sampled data can accurately represent the original surface, tool paths can be generated based on the point data set. For cutting tools with various sizes, adaptive re-sampling schemes are employed to generate sufficient sampled points for the generation of accurate and smooth tool-paths.     

Engagement Angle Modeling for Multiple-circle Continuous Machining and Its Application in the Pocket Machining

The progressive cutting based on auxiliary paths is an effective machining method for the material accumulating region inside the mould pocket. But the method is commonly based on the radial depth of cut as the control parameter, further more there is no more appropriate adjustment and control approach. The end-users often fail to set the parameter correctly, which leads to excessive tool load in the process of actual machining. In order to make more reasonable control of the machining load and tool-path, an engagement angle modeling method for multiple-circle continuous machining is presented. The distribution mode of multiple circles, dynamic changing process of engagement angle, extreme and average value of engagement angle are carefully considered. Based on the engagement angle model, numerous application techniques for mould pocket machining are presented, involving the calculation of the milling force in multiple-circle continuous machining, and rough and finish machining path planning and load control for the material accumulating region inside the pocket, and other aspects. Simulation and actual machining experiments show that the engagement angle modeling method for multiple-circle continuous machining is correct and reliable, and the related numerous application techniques for pocket machining are feasible and effective. The proposed research contributes to the analysis and control tool load effectively and tool-path planning reasonably for the material accumulating region inside the mould pocket.     

Optimal selection of machining direction for three-axis milling of sculptured parts

In the field of free form surface machining, CAM software allows management of various modes of tool-path generation (zig-zag, spiral, z-level, parallel plan, iso-planar, etc.) leaning on the geometry of the surface to be machined. Various machining strategies can be used for the same shape. Nevertheless the choice of a machining strategy remains an expert field. Indeed there are no precise rules to facilitate the necessary parameter choice for tool-path computation from analysis of the numerical model of a part and the quality requirements. The objective of this paper is to provide a method to assist in the choice of the machining direction for parallel plane milling of sculptured parts. The influence of the tool-path on final quality according to the intrinsic geometrical characteristics of the latter (curves, orientation) was studied. Directional beams are introduced and defined from the local surface parameter. Finally, a methodology to optimize machining time while guaranteeing a high level of quality was developed and applied to examples.