Off-line optimization on NC machining based on virtual machining

Virtual machining, based-on the model of a machining system, aims to simulate, evaluate and optimize the actual machining process with high sense of reality. It provides digital off-line optimization tools for NC machining. Taking advantage of virtual machining used in machining process simulation, one can build the framework of optimization system on NC machining so that the processes of reliability verification, cutting parameter optimization and error compensation can be integrated into one system to improve machining processes comprehensively. The optimization is realized via modifying NC programs. Several key issues such as virtual machining, cutting parameters optimization, error prediction and compensation are also highlighted. Optimization systems based on virtual machining have been developed to demonstrate the effectiveness of off-line optimization for different purposes. The results show that the machining process is obviously improved.     

采用深孔加工技术解决机床主轴深孔加工难题

立足企业实际情况,通过技术改造,在车床上实现了用DF系统深孔钻技术加工主轴深孔并取得了成功,解决了主轴批量化生产时效率低和劳动强度大的问题,同时也对主轴深孔的加工工艺和DF系统主要部件的设计要求做了较为详细的说明。     

Dynamic characterization of machining systems

In the working space model of machining, an experimental procedure is implemented to determine the elastic behaviour of the machining system. In this paper, a dynamic characterization and vibration analysis has long been used for the detection and identification of the machine tool condition. The natural frequencies of the lathe machining system are required (Ernault HN400??France) according to three different situations with no cutting process are acquired. The system modal analysis is used to identify the natural frequencies. These frequencies are then compared to the ones obtained on the spindle numerical model by finite element method. This work is validated by experimental tests based on measures of the lathe machine tool frequencies domain. The main objective is to identify a procedure giving the natural frequency values for the machine tool components, in order to establish a better condition in the cutting process of the machine tool.     

Digitization modeling and CNC machining for enveloping surface parts

Nowadays, the demand for high-quality enveloping surface parts is increasing in machinery, aviation, military, and many other fields. Given the complexity of the surfaces, the enveloping surfaces parts are usually processed by grinding method. This grinding process is low in precision and poor in efficiency for lack of accurate digital CAD model of these parts. In this paper, a new modeling method for the complicated enveloping parts has been developed. The digital CAD model is constructed by using vector representation, transformation matrix, and spatial meshing theory. Next, a better computerized numerical control (CNC) machining code is generated based on the accurate digital CAD model, and a high-quality envelope surface could be machined on the basis of the code in CNC machining center. The normal vector algorithm is adopted to detect and avoid collision and interference during the processing. Finally, the digital CAD models of a planar double-enveloping worm and worm wheel famous at difficulty in modeling and CNC machining have been conducted as an instance. The machining experiment results show that the machining precision of the enveloping surface parts have been increased because the part’s digital CAD model is extraordinarily accurate.     

Theoretical and experimental analysis of the effect of error motions on surface generation in fast tool servo machining

The fast tool servo (FTS) machining process provides an indispensable solution for machining optical microstructures with sub-micrometer form accuracy and a nanometric surface finish without the need for any subsequent post processing. The error motions in the FTS machining play an important role in the material removal process and surface generation. However, these issues have received relatively little attention. This paper presents a theoretical and experimental analysis of the effect of error motions on surface generation in FTS machining. This is accomplished by the establishment of a model-based simulation system for FTS machining, which is composed of a surface generation model, a tool path generator, and an error model. The major components of the error model include the stroke error of the FTS, the error motion of the machine slide in the feed direction, and the axial motion error of the main spindle. The form error due to the stroke error can be extracted empirically by regional analysis, the slide motion error and the axial motion error of the spindle are obtained by a kinematic model and the analysis of the profile in the circumferential direction in single point diamond turning (SPDT) of a flat surface, respectively. After incorporating the error model in the surface generation model, the model-based simulation system is capable of predicting the surface generation in FTS machining. A series of cutting tests were conducted. The predicted results were compared with the measured results, and hence the performance of the model-based simulation system was verified. The proposed research is helpful for the analysis and diagnosis of motion errors on the surface generation in the FTS machining process, and throws some light on the corresponding compensation and optimization solutions to improve the machining quality.     

Modelling and experimental investigation of spindle and cutter dynamics for a high-precision machining center

The geometric quality of high-precision parts is highly dependent on the dynamic performance of the entire machining system, which is determined by the interrelated dynamics of machine tool mechanical structure and cutting process. This performance is of great importance in advanced, high-precision manufacturing processes, including aerospace and biomedical applications. In this paper, the dynamics of the combined spindle/cutter system, a major component of any machine tool, is identified using impact testing techniques and is successfully approximated by a second-order linear model. Results of computer simulations of machining processes that include the identified spindle/cutter dynamics show a significant influence on the quality of the final product. From this, it is concluded that, for precision workpieces, the dynamics of the spindle and cutter system will have to be taken into account in order to improve future machining controls and processes.     

Analysis of the machining stability of a milling machine considering the effect of machine frame structure and spindle bearings: experimental and finite element approaches

Chatter vibration has been a troublesome problem for a machine tool toward the high precision and high-speed machining. Essentially, the machining performance is determined by the dynamic characteristics of the machine tool structure and dynamics of cutting process of spindle tool. Prediction of the dynamic behavior at spindle tool tip is therefore of importance for assessing the machining stability of a machine tool at design stage. This study was aimed to evaluate the machining stability of a vertical milling system under the interactive influence of the spindle unit and the machine frame structure. To this end, a realistic finite element model of a vertical milling tool was generated by incorporating the spindle-bearing model into the head stock mounted on machine frame. The influences of the dynamics of spindle-bearing system and the machine frame structure were investigated respectively. Current results show that the machine tool spindle system demonstrates different dynamic behaviors at different frequency ranges, which are also characterized as structural modes and spindle modes, respectively. In particular, the maximum compliance of spindle tool tip was found to occur at the bending vibrations of spindle shaft and vary with the preload amount of spindle bearing. The machining stabilities were predicted to different extent, depending on the exciting modes which could be related to the influences of machine frame and spindle unit.     

龙门加工中心主轴系统的热态性能分析技术研究

龙门加工中心主轴系统热态特性是提升机床性能的一个关键问题。针对龙门加工中心主轴系统热态特性分析的需要,建立有限元分析模型,计算主轴轴承的发热量以及主轴系统各个表面的对流换热系数,利用ANSYS WORKBENCH对其温度场、热变形状况以及热-结构耦合问题进行了分析计算,得到了不同位置轴承对总热变形的影响以及不同转速条件下主轴系统的稳态热态性能。实验测试数据表明分析计算结果的准确性较高,可用于指导该型龙门加工中心主轴系统结构优化设计和热变形补偿。     

曲轴复合加工中心主轴箱有限元分析

曲轴复合加工中心是一种高效加工曲轴的新型设备。建立曲轴复合加工中心主轴箱的三维模型,并分析其载荷及边界条件。以ANSYS软件作为分析工具,对曲轴复合加工中心主轴箱进行静力和模态分析,得到了应力云图、位移云图及前七阶模态。由分析可知,主轴箱的最大应力为18.272MPa,应变值为0.0044mm,应力和应变值均较小,固有频率大,动态特性好,满足机床设计要求且具有优化空间,为以后主轴箱结构改进、优化设计和动力修改提供了理论依据。     

On-machine 3D vision system for machining setup modeling

In computer numerical control machine tools, using machining simulation to prevent collision becomes more and more popular due to its efficiency and low cost. However, if the entire digital model of the machining setup does not exist, the simulation is not applicable. As a result, the operator has to manually check the numerical control program, which is a time-consuming and error-prone process. In this paper, an on-machine vision system is presented to quickly construct the digital model based on the actual machining setup. The total construction for a complex setup can be done within a few minutes. Several key technologies have been developed. First, a 2D edge feature detection algorithm has been designed which will extract the edges of the object of interest by processing both the real and virtual images. Second, a stereo vision system is developed which will obtain the three-dimensional (3D) edge data of the object of interest. A new algorithm is presented to solve correspondence, which is the key problem of the stereovision system. Furthermore, the 3D object recognition algorithm is developed to let the system intelligently search for the matched solid model in the database and import it into the virtual environment with an accurate pose. Finally, experiments are carried out to test the developed system.     

The effects of machining process variables and tooling characterisation on the surface generation

The paper presents a novel approach for modelling and simulation of the surface generation in the machining process. The approach, by integrating a dynamic cutting force model, regenerative vibration model, machining system response model and tool profile model, models the complex surface generation process. Matlab Simulink is used to interactively perform the simulation in a user-friendly, effective and efficient manner. The effects of machining variables and tooling characteristics on the surface generation are investigated through simulations. CNC turning trials have been carried out to evaluate and validate the approach and simulations presented. The proposed approach contributes to comprehensive and better understanding of the machining system, and is promising for industrial applications with particular reference to the optimisation of the machining process based on the product/component surface functionality requirements.     

Modal identification of spindle-tool unit in high-speed machining

The accurate knowledge of high-speed motorised spindle dynamic behaviour during machining is important in order to ensure the reliability of machine tools in service and the quality of machined parts. More specifically, the prediction of stable cutting regions, which is a critical requirement for high-speed milling operations, requires the accurate estimation of tool/holder/spindle set dynamic modal parameters. These estimations are generally obtained through Frequency Response Function (FRF) measurements of the non-rotating spindle. However, significant changes in modal parameters are expected to occur during operation, due to high-speed spindle rotation.The spindle's modal variations are highlighted through an integrated finite element model of the dynamic high-speed spindle-bearing system, taking into account rotor dynamics effects. The dependency of dynamic behaviour on speed range is then investigated and determined with accuracy. The objective of the proposed paper is to validate these numerical results through an experiment-based approach. Hence, an experimental setup is elaborated to measure rotating tool vibration during the machining operation in order to determine the spindle's modal frequency variation with respect to spindle speed in an industrial environment. The identification of natural frequencies of the spindle under rotating conditions is challenging, due to the low number of sensors and the presence of many harmonics in the measured signals. In order to overcome these issues and to extract the characteristics of the system, the spindle modes are determined through a 3-step procedure. First, spindle modes are highlighted using the Frequency Domain Decomposition (FDD) technique, with a new formulation at the considered rotating speed. These extracted modes are then analysed through the value of their respective damping ratios in order to separate the harmonics component from structural spindle natural frequencies. Finally, the stochastic properties of the modes are also investigated by considering the probability density of the retained modes. Results show a good correlation between numerical and experiment-based identified frequencies. The identified spindle-tool modal properties during machining allow the numerical model to be considered as representative of the real dynamic properties of the system.     

基于DEVS的机械加工过程碳排放多粒度动态模型构建及仿真

机械加工过程碳排放分析是低碳制造（Low carbon manufacturing,LCM）技术发展、创新及应用的基础。针对加工过程中的离散性、动态性及加工设备的高度非线性特点,同时为充分反映多设备、多工艺特征下的各类外部、内部事件对加工过程碳排放的影响,提出一种基于离散事件系统规范（Discrete event system specification,DEVS）的机械加工过程碳排放多粒度动态模型与仿真方法。基于对加工过程层级结构及碳排放特性的分析,采用DEVS分层阶梯建模方法,并结合信息控制下的加工运行机制及调度规则,构建设备层、工作组层、工艺链层的碳排放多粒度动态模型。以某全数控磨床尾架主轴的加工过程为例,基于Eclipse平台下的CD++Builder插件设计了各层次DEVS仿真模型,并通过所开发的仿真应用程序,对模型及方法进行了验证。该方法对于构建规范化、形式化的加工过程碳排放模型,实现加工过程各层级碳排放的实时动态分析与预测、定量评估低碳加工单元具有较广阔的应用前景。     

Manufacturing variation models in multi-station machining systems

In product design and quality improvement fields, the development of reliable 3D machining variation models for multi-station machining processes is a key issue to estimate the resulting geometrical and dimensional quality of manufactured parts, generate robust process plans, eliminate downstream manufacturing problems, and reduce ramp-up times. In the literature, two main 3D machining variation models have been studied: the stream of variation model, oriented to product quality improvement (fault diagnosis, process planning evaluation and selection, etc.), and the model of the manufactured part, oriented to product and manufacturing design activities (manufacturing and product tolerance analysis and synthesis). This paper reviews the fundamentals of each model and describes step by step how to derive them using a simple case study. The paper analyzes both models and compares their main characteristics and applications. A discussion about the drawbacks and limitations of each model and some potential research lines in this field are also presented.     

Non-linear mechanism in electrical discharge machining process

Electrical discharge machining (EDM) is an advanced non-traditional manufacturing technology that has many advantages over other machining methods. Many papers have discussed the machining mechanism and modeling of the EDM process. However, previous mechanism models have mainly been linear, which contradicts their precondition that EDM is a stochastic process. In this paper, a non-linear mechanism model is proposed for the EDM process. A threshold condition that leads to chaos is calculated using the Melnikov theory. The theoretical results indicate that the EDM system can generate varied chaos in the evolution of electrical discharge. To verify this conclusion, validation experiments are implemented. Several sets of complete EDM processes’ real-time series are analyzed by multiple chaotic numerical criteria, including power spectrum analysis, principle component analysis (PCA), correlation dimension analysis, and Lyapunov exponent analysis. The experimental results provide further qualitative and quantitative evidence that a complete EDM process has dynamical chaotic characteristics.     

Digital denture manufacturing-An integrated technologies of abrasive computer tomography, CNC machining and rapid prototyping

The orthodontic denture produced by the traditional method is heavily relied on the skill and experience of the technician. Its quality is depended on the accuracy of the technician’s subjective judgment. In addition, the manual process involves many steps that require a long time to complete. Most importantly, it does not preserve any quantitative information for future retrieval.In this paper, a novel device for scanning denture image and reconstructing 3D digital information of teeth models by abrasive computer tomography (ACT) is presented. The orthodontic denture is then to be produced by rapid prototyping (RP) or computer numerical control (CNC) machining methods based on the digital information. A force feedback sculptor (Freeform system, U.S.A.), using 3D touch technology, was applied to modify the teeth profile or features of the denture. It enables the dentist to perform digital manipulation of the denture profile with real-time and interactive operation. Due to its user-friendly human-computer interface, the dentist can directly access the 3-D model without relying on a CAD/CAM technician or denture technician. In this paper, the comparison between traditional manual operation and digital manufacture using RP and CNC machining technology for denture production is summarized.In this paper, a digital denture manufacturing protocol using an economic and harmless computer abrasive teeth profile scanning, computer-aided denture design, 3D touchable feature modification, and numerical denture manufacturing were proposed here. These proposed methods provide solid evidence that digital design and manufacturing technologies may become a new avenue for custom made denture design, analysis, and production in the 21th century.     

Dynamic characterization of machining robot and stability analysis

Machining robots have major advantages over cartesian machine tools because of their flexibility, their ability to reach inaccessible areas on a complex part, and their important workspace. However, their lack of rigidity and precision is still a limit for precision tasks. Innovations and design optimization of robotic structure, links, and power transmission allow robot manufacturers to propose business solutions for machining applications. Beyond accuracy problems, it is also necessary to quantify the vibration phenomena that may affect, as in machine tools, the quality of machined parts and the tools and spindle lifespan. These vibrations occurred at specific machining conditions depending on robot and spindle dynamic properties. The robot’s posture evolved significantly in its workspace and induces dynamic’s changes observed at the tool tip that in turn impact the stability of the machining process. The objective of this paper is to quantify the dynamic behavior’s variation of an ABB IRB 6660 robot equipped with a high-speed machining (HSM) spindle in its workspace and analyze the consequences in terms of machining stability. Through an experimental modal characterization, significant variability of modal parameters is observed at the tool tip and impacts the stability of machining. The results show that an adjustment of the cutting conditions must accompany the change of robot posture during machining to ensure stability.     

知识驱动的加工产品数字孪生拟态建模方法

对加工过程的实时观察、分析和控制是优化零部件加工策略的重要组成部分。通过融合几何状态、物理状态和设备状态变化等多维、实时的加工过程数据,可以实现对加工过程的建模和监控。数字孪生模型是数字孪生系统的核心与基础。但是,目前还缺乏一种系统且能自适应开发高保真、多尺度、多维加工数字孪生建模方法,以辅助系统进行分析与决策。提出了一种知识驱动的加工产品数字孪生拟态建模方法,可以在加工过程中自适应地构建产品数字孪生模型。该模型可根据加工过程自适应的变化,实时表达加工过程中的产品,为数字孪生决策系统提供数据支持。最后在一个航天零件的加工过程中测试了该方法,验证数字孪生拟态模型在加工中应用的可行性。     

An asperity interaction machining theory of friction

The understanding of friction for the simple case of two surfaces contacting under the influence of a normal force is a complicated problem for which there is currently no definitive answer. In this paper a brief review of previous models is given and the similarity of the static and dynamic conditions of asperity interaction to those of the machining process is shown.A new model of the friction process based upon a three-zone model of machining is proposed and the effects of material, normal load and velocity calculated based upon machining information obtained at low feeds. The new theory of friction is able to predict approximate values of μ and to account for its change as a function of material and sliding velocity. It can also explain the change in μ that occurs when normal force conditions increase to cause interfacial seizure.     

螺旋锥齿轮数控加工3维仿真研究

在分析了螺旋锥齿轮数控加工原理后，采用面向对象技术和计算机辅助设计系统中的3维建模功能，提出了CNC铣齿机床、齿轮毛坯和盘铣刀3维几何模型的具体构建方法；利用简化的齿轮毛坯实体模型和盘铣刀实体模型做布尔减运算，来实现模拟工件材料的去除过程；最终给出的加工仿真实例和实际加工结果，验证了加工仿真方法的正确性。所获得的仿真结果可为齿面接触分析、有限元应力分析和齿轮的数控加工等提供了精确的3维实体模型。