数控回转工作台的几何误差测量和建模

作为五轴数控机床的关键零部件,数控回转工作台能够实现C轴和A轴的回转进给与定位,其精度在很大程度上决定了数控机床的加工精度。以此为研究背景,建立了回转工作台的空间误差模型,对回转工作台的几何误差进行测量,并建立了误差补偿的数学模型,为最终进行机床的误差补偿提供依据,从而实现数控加工中心加工精度的提高。     

Accurate tool position for five-axis ruled surface machining by swept envelope approach

This paper presents a swept envelope approach to determining tool position for five-axis ruled surface machining. The initial tool position is traditionally located to contact with two directrices of a ruled surface. The swept profile of the tool is then determined based on the tool motion. By comparing the swept profile with the ruled surface, the tool position is corrected to avoid machining errors. The cutter's swept envelope is further constructed by integrating the intermediate swept profiles, and applied to NC simulation and verification. This paper presents the explicit solution for the swept profile of a taper-end cutter in five-axis ruled surface machining. The relation of the ruled surface geometry, the tool motion and the machining errors is developed. Therefore, the error sources can be detected early and prevented during tool path planning. The explicit swept envelope indicates that the machined surface is not a ruled surface in five-axis ruled surface machining. Manufacturing industries should take extra care in high precision ruled surface machining. Computer illustrations and example demonstrations are shown in this paper. The results reveal that the developed method can accurately position tool location and reduce machining errors for five-axis ruled surface machining.     

Adaptive learning control for thermal error compensation of 5-axis machine tools

The research presented in this paper shows an adaptive approach for long-term thermal error compensation of 5-axis machine tools (MT). A system of differential equations is used to compute the model based compensation values. The model can predict thermal displacements of the tool center point (TCP) based on changes in the environmental temperature, load-dependent changes and boundary condition changes and states, like machining with or without cutting fluid. The model based compensation of the rotary axis of a 5-axis MT is then extended by on-machine measurements. The information gained by the process-intermittent probing is used to adaptively update the model parameters, so that the model learns how to predict thermal position and orientation errors and to maintain a small residual error of the thermally induced errors of the rotary axis over a long time. This approach not only increases the MT accuracy but also reduces the amount of time spent on preproduction model parameter identification. Additionally an algorithm has been developed to dynamically adjust the length of the on-machine measurement intervals to maintain a high productivity and a constant deviation of the machined parts.Experimental results confirm that the adaptive learning control (ALC) for thermal errors shows a desirable long-term prediction accuracy.     

混联复合机床转台定位精度的激光测量与补偿

传统的数控机床进给轴定位精度的检测方法,存在精度低、方法落后、检验重复性差等缺点。因此,采用目前国际先进的激光干涉法检测混联复合机床转台的定位精度,以评价和提高其精度。对干涉仪的测量原理、测量方法和数据处理进行了分析,认为空气折射率是干涉仪应用中的主要误差来源,并分析了其原因和补偿方法。最后,给出了基于激光干涉仪的机床定位精度检测和补偿方案。     

Process intermittent measurement of tools and workpieces

This paper describes the application of surface sensing probes in unmanned machining. The aim is to automate tool setting, workpiece alignment, machining allowance test, and also to provide workpiece inspection capability. Special emphasis is given to the mathematical aids for the compensation of the machine tools systematic errors. A hardware-software system has been designed to solve this problem. It is an integrated part of the control unit and executes the necessary transformations of the subsequent numerical control sentences.     

Improving machining accuracy in precision line boring

There is an ever-growing demand for high precision machining to obtain increased accuracy and surface finish, as they are key factors in product quality and performance. Machining operations, in general, are associated with errors of varying magnitude originating from different sources. As a result, the sizes of the machined features usually deviate from their desired, nominal values. Identification of error sources, techniques of measurements (on/off line), and efficient strategies for their compensation are the steps required to minimize, and, in some cases eliminate process errors. This paper focuses on modeling and compensation of geometric errors in machining operations specific to the line boring process. It is part of an undergoing research project focused on design and development of an agile precision line boring station for machining of long bores. After a brief overview of sources of geometric errors and their components, a methodology for their calculation is introduced. In this regard, error equations reflecting the effects of machine tool geometric errors at the tool tip are derived. It is shown that these equations can be further simplified without significantly affecting computational accuracy of the results. This makes the approach more attractive for real-time applications. A set of experimental data obtained from a prototype of the machine is used to study the effectiveness of the proposed approach and the results are reported. The paper concludes with discussions and presentation of different methods and available tools for real time compensation of these errors.     

Continuity-preserving tool path generation for minimizing machining errors in five-axis CNC flank milling of ruled surfaces

Five-axis CNC flank machining has been commonly used in the industry for shaping complex geometries. Geometrical errors typically occur in five-axis flank finishing of non-developable surfaces using a cylindrical cutter. Most existing tool path planning methods adjust discrete cutter locations to reduce these errors. An excessive change in the cutter center or axis between consecutive cutter locations may deteriorate the machined surface quality. This study developed a tool path generation method for minimizing geometrical errors on finished surfaces while preserving high-order continuity in the cutter motion. A tool path is described using the moving trajectory of the cutter center and changes in two rotational angles in compact curve representations. An optimization scheme is proposed to search for optimal curve control points and the resulting tool path. A curve subdivision mechanism progressively increases the control points during the search process. Simulation results confirm that the proposed method not only enhances the computational efficiency of tool path generation but also improves the machined surface finish. This study provides a computational approach for precision tool path planning in five-axis CNC flank finishing of ruled surfaces.     

Improving the dynamics of five-axis machining through optimization of workpiece setup and tool orientations

Existing works in optimization of five-axis machining mainly focus on the machining efficiency and precision, while the dynamic performance of the machine tools has not been fully addressed, especially in high-speed machining, in which the rotary actuators have limited dynamic ability. In this paper, a study is reported on how to generate a tool path so that the maximal angular accelerations of the rotary axes of the five-axis machine can be reduced. Two independent methods are proposed for this task: (1) by optimizing the setup of the workpiece on the machine’s table, and (2) by finding better tilt and yaw angles for the tool orientations. In this paper, the setup parameters of the workpiece are incorporated into the inverse kinematic equations, and angular acceleration functions are established according to the numerical solutions of those equations. While varying the tool orientations unquestionably would affect the surface quality of the machining, we introduce the so called Domain of Geometric Constraints that will restrict the allowable tilt and yaw angle of the tool at the cutter contact points on the part surface, so to ensure the satisfaction of the requirement of both local-gouging-free and cusp-height. For the first method–finding the optimal workpiece setup–a heuristic-based approach, i.e., the Genetic Algorithm (GA), is adopted, whereas for the second method–the constrained optimization of tool orientations–we present an elaborate algorithm based on the results from the analysis conducted by the authors. At the end of the paper, computer simulation experiments are reported that demonstrate the effectiveness of our proposed methods and algorithms.     

Virtual machining considering dimensional,geometrical and tool deflection errors in three-axis CNC milling machines

Virtual manufacturing systems can provide useful means for products to be manufactured without the need of physical testing on the shop floor. As a result, the time and cost of part production can be decreased. There are different error sources in machine tools such as tool deflection, geometrical deviations of moving axis and thermal distortions of machine tool structures. Some of these errors can be decreased by controlling the machining process and environmental parameters. However other errors like tool deflection and geometrical errors which have a big portion of the total error, need more attention. This paper presents a virtual machining system in order to enforce dimensional, geometrical and tool deflection errors in three-axis milling operations. The system receives 21 dimensional and geometrical errors of a machine tool and machining codes of a specific part as input. The output of the system is the modified codes which will produce actual machined part in the virtual environment.     

Machine models and tool motions for simulating five-axis machining

This paper presents a method of determining the tool motion of a five-axis machine. The method is motivated by the imprint point method, where points on the machined surface are computed based on the tool position and tool motion. While simple linear motion can be used as a coarse approximation to this motion, this paper looks at more accurate models of tool motion based on machine kinematics that can be generalized and applied to any CNC machine with one tool head. A kinematic chain is created by decomposing the linear motion of a machine’s translational and rotational axes into parameterized affine transformations, and a hierarchical model of the machine combines the transformations to provide an accurate model of machine motion.     

An accurate and efficient approach to geometric modeling of undeformed chips in five-axis CNC milling

Many important and complex parts, such as aero-engine compressors and automotive punch dies, are often machined in five-axis computer numerically controlled (CNC) milling. To machine the parts with accurate dimensions and shapes and low machining costs it is necessary to construct 3D models of the finished parts in the geometric simulation and in-process workpiece models of the parts in the physical simulation of their five-axis milling. A kernel technique of the geometric and the physical simulations is to accurately and efficiently model the geometry of the workpiece material removed at every moment of the machining, which is the instantaneous, undeformed chip geometry. Although in the past decades much research has been conducted on modeling cutter swept volumes in CNC milling to represent the finished part geometry in the geometric simulation, it is very time consuming to calculate the instantaneous, undeformed chip geometries using the cutter swept volumes. Besides, the existing method of modeling undeformed chip geometry in three-axis milling cannot be used for that in five-axis milling. To address this problem, our work proposes an accurate and efficient approach. In this article, a generic theory about the boundary of the area covered by the instantaneous cutting edges on a workpiece layer at any moment is established, which is called the boundary theory. A simple diagram of determining the boundary is invented, which is called boundary construction diagram. This approach lays a theoretical foundation for the geometric and the physical simulations of five-axis milling and will significantly promote them for high performance machining in industry.     

A prediction and compensation method of robot tracking error considering pose-dependent load decomposition

Industrial robots are widely used because of their high flexibility and low cost compared with CNC machine tools, but the low tracking accuracy limits their application in the field of high-precision manufacturing. To improve the tracking accuracy and solve the complex modeling problems, a prediction and compensation method of robot tracking error is proposed based on temporal convolutional network (TCN), where the pose-dependent effect of load on joint tracking error is considered. The terminal load is decomposed to joint load by using Jacobian matrix and then used as the pose-dependent information of the data-based model. A prediction model based on TCN is used to predict the tracking error of joints. Finally, a pre-compensation method is adopted to improve the joint tracking accuracy based on the predicted errors. Experimental results show that the model presents good prediction and compensation accuracy. The mean absolute tracking errors are increased by more than 80% in the test path. This method can effectively compensate the tracking errors of the robot joints and therefore greatly improve the tracking accuracy of the tool center point and tool orientation in the Cartesian coordinate system.     

The stencil buffer sweep plane algorithm for 5-axis CNC tool path verification

A new algorithm based on the sweep plane approach to determine the machined part geometry in 5-axis machining with general APT tools is presented. Undercut and overcut can be determined. Collision detection between the toolholder, workpiece and workpiece fixture can also be detected. The subtraction of the removed material is obtained for each sweep plane by using a stencil buffer. A flat plane is swept through the blank part, fixture and tool swept volume geometry. The intersections of sweep planes and the swept tool volume are computed based on the canonical representation of a cone, torus and sphere. The necessary data to compute all the intersections is stored in a text file, here called the M-Plane file (Memory Plane). The equations of the intersections are approximated by a polygon with variable accuracy. The resulting APT tool intersection in each sweep plane is then clipped against the blank workpiece intersection with the current sweep plane. The stencil buffer provides automatically the union of all tool intersections and the subtraction from the blank workpiece. This algorithm provides a 3D geometric model of the tool swept volume. The display algorithm is based on the Painter's algorithm, but there is no time consuming sorting from back to front required, as the sweep proceeds from back to front. The accuracy of the algorithm can be varied as a function of the requirements by changing the polygon approximation and the distance between the sweep planes.     

基于D-H 修正标记法的五轴加工后处理研究

在工业生产中,需要将CAD/CAM 软件生成的CL 数据通过后处理程序转换为NC 加工代码。针对自由曲面五轴加工的数据转换问题,应用D-H 修正标记法开发五轴加工机床后 处理程序,对D-H 修正标记法的参数定义过程进行了描述并提出改进,针对DMU 80 monoBlock 型五轴加工机床开发了专用后处理程序,最后通过VERICUT 进行了加工仿真,验证该程序的 正确性。该程序对提高自由曲面五轴加工数据后处理的效率具有重要意义。     

基于压缩体素模型的鼓形刀空间扫描体构造方法及其应用

首先给出了鼓形刀空间扫描体构造公式并构造出其表面模型,利用Ray Casting方法将该表面模型进行离散,转化为压缩体素模型,该模型采用沿X,Y,Z3个坐标轴方向相互垂直的Dexel模型表示,各个Dexel模型之间按体素模型大小均匀分布．刀具空间扫描体模型和仿真工件模型之间的布尔运算转化为Dexel模型之间的一维布尔运算,简化了布尔操作并提高了操作速度．通过MarchingCubes方法提取数控加工仿真工件表面三角网格模型并进行图形显示,提高了仿真工件显示质量,NC编程人员可实时地从任意方向观察、验证仿真结果．该方法已成功地应用于基于压缩体素模型的五坐标数控加工仿真系统中,克服了现有五坐标数控加工仿真方法和商品化软件系统的不足．     

Machining accuracy reliability analysis of multi-axis machine tool based on Monte Carlo simulation

Although machine tool can meet the specifications while it is new, after a long period of cutting operations, the abrasion of contact surfaces and deformation of structures will degrade the accuracy of machine tool due to the increase of the geometric errors in six freedoms. Therefore, how to maintain its accuracy for quality control of products is of crucial importance to machine tool. In this paper, machining accuracy reliability is defined as the ability to perform its specified machining accuracy under the stated conditions for a given period of time, and a new method to analyze the sensitivity of geometric errors to the machining accuracy reliability is proposed. By applying Multi-body system theory, a comprehensive volumetric model explains how individual geometric errors affect the machining accuracy (the coupling relationship) was established. Based on Monte Carlo mathematic simulation method, the models of the machining accuracy reliability and sensitivity analysis of machine tools were developed. By taking the machining accuracy reliability as a measure of the ability of machine tool and reliability sensitivity as a reference of optimizing the basic parameters of machine tools, an illustrative example of a three-axis machine tool was selected to demonstrate the effectiveness of the proposed method.     

Arc–surface intersection method to calculate cutter–workpiece engagements for generic cutter in five-axis milling

Calculating cutter–workpiece engagements (CWEs) is essential to the physical simulation of milling process that starts with the prediction of cutting forces. As for five-axis milling of free form surfaces, the calculation of CWEs remains a challenge due to the complicated and varying engagement geometries that occur between the cutter and the in-process workpiece. In this paper, a new arc–surface intersection method (ASIM) is proposed to obtain CWEs for generic cutter in five-axis milling. The cutter rotary surface is first represented by the family of section circles which are generated by slicing the cutter with planes perpendicular to the tool axis. Based on the envelope condition, two grazing points on each section circle are analytically derived, which divide the circle into two arcs. The feasible contact arc (FCA) is then extracted to intersect with workpiece surfaces. Using arc/surface intersection and distance fields based approach, the boundary of the closed CWEs is accurately and efficiently calculated. Compared with the solid modeler based method and the discrete method, the ASIM has higher computational efficiency and accuracy. Moreover, an analytical solution for calculating CWEs can be obtained with this method in five-axis milling of the workpiece merely comprising of flat and quadric surfaces. Finally, two case tests are implemented to confirm the validity of the ASIM and comparisons have been made with a Vericut based system which utilizes the Z-buffer method. The results indicate that the ASIM is computationally efficient, accurate and robust.     

Computer-based estimation and compensation of diametral errors in CNC turning of cantilever bars

This paper aims to introduce a computer-based estimation and compensation method for diametral errors in cantilever bar turning without additional hardware requirements. In the error estimation method, the error characteristics of workpieces are determined experimentally depending on cutting speed, depth of cut, feed rate, workpiece diameter, length from the chuck and the geometric error sum of CNC lathe. An Artificial Neural Network (ANN) model is trained using these experimental error characteristics for estimation of the error. The ANN model estimated the workpiece dimensional errors with a good accuracy. Error correction is realised via turning of workpieces with a CNC part program which modified based on the estimated error profile. The dimensional errors are reduced approximately by 90% with the proposed method.     

ACWGAN-GP for milling tool breakage monitoring with imbalanced data

Tool breakage monitoring (TBM) during milling operations is crucial for ensuring workpiece quality and minimizing economic losses. Under the premise of sufficient training data with a balanced distribution, TBM methods based on statistical analysis and artificial intelligence enable accurate recognition of tool breakage conditions. However, considering the actual manufacturing safety, cutting tools usually work in normal wear conditions, and acquiring tool breakage signals is extremely difficult. The data imbalance problem seriously affects the recognition accuracy and robustness of the TBM model. This paper proposes a TBM method based on the auxiliary classier Wasserstein generative adversarial network with gradient penalty (ACWGAN-GP) from the perspective of data generation. By introducing Wasserstein distance and gradient penalty terms into the loss function of ACGAN, ACWGAN-GP can generate multi-class fault samples while improving the network's stability during adversarial training. A sample filter based on multiple statistical indicators is designed to ensure the quality and diversity of the generated data. Qualified samples after quality assessment are added to the original imbalanced dataset to improve the tool breakage classifier's performance. Artificially controlled face milling experiments for TBM are carried out on a five-axis CNC machine to verify the effectiveness of the proposed method. Experimental results reveal that the proposed method outperforms other popular imbalance fault diagnosis methods in terms of data generation quality and TBM accuracy, and can meet the real-time requirements of TBMs.     

Analysis and optimization of fixture under dynamic machining condition with chip removal effect

Dynamic interactions in the tool–workpiece and workpiece–fixture systems significantly impinge on the quality of finished workpieces. The presented simulation system integrates the effects of workpiece fixture dynamics with the other factors contributing to the machining process dynamics. It provides more accurate prediction of the process output which helps in the design of the optimum fixture configuration prior to the production stage. Modelling of the frictional contact behaviour between the fixture element and the workpiece helps to improve the prediction accuracy of the simulation system which accelerates the convergence to the optimum fixture configuration design and consequently improves the machined part dimensional accuracy and geometric integrity. The developed simulation is capable of modelling complicated part geometries by interfacing with commercial ANSYS.V10^® packages. This research work minimizes the deformation of workpiece using integrated optimization tool of Genetic algorithm (GA) and ANSYS Parametric Design Language (APDL) of finite element analysis. The same layouts given by the above optimization tool are used in the experimental setup and it is found that the improved geometric tolerance of squarness and flatness of the given workpiece. The chip removal effect and frictional contact between the workpiece and the fixture elements are taken into account based on element death technique and nonlinear finite-element analysis. A Case study of an open slot milling process illustrates the application of the proposed improved geometric tolerance approach.