Investigation of a 2-D planar motor based machine tool motion system

A new 2-D planar motion system for precision machining operations is investigated. Characteristics of the 2-D planar motor system are studied to gain better understanding of its capabilities, limitations and interactions with machining processes. The planar motor system is applied to an end milling process, where experimental data on cutting force and surface finish are in agreement with simulation results. Of further interest is that the motor's fast response and ability to perform simultaneous motion in two directions potentially provides a simpler means to compensate for errors such as runouts. Issues associated with such compensation motions to improve surface finish are discussed.     

Damping models for machine tool components of linear axes

To simulate the dynamic behaviour of machine tools, the stiffness, damping and inertia parameters of the structure are needed. While masses and stiffness parameters of structural parts can be obtained with a static measurement, the determination of damping parameters requires a thorough methodology. In this paper the common methodology for the identification of local damping parameters of machine components was extended by an additional step to isolate the damping of the test object more precisely. Furthermore test benches as well as the identified damping models for components of an exemplary linear axis are presented.     

Repetitive control design and implementation for linear motor machine tool

The goal of this paper is to eliminate the period tracking error via the design of discrete-time domain repetitive controller. It increases the stabilizing range and enhances the robust performance by adopting the prototype repetitive controller design principle to compensate repetitive control. Furthermore, with the concept of command feedforward, it introduces the feedforward gains of speed and acceleration, which can forcefully enhance the tracking ability of the repetitive controller and improve on the errors of the system. Finally, it puts into practice the theory on a gantry type machinery platform with linear motors. The results prove that the theory can reduce period tracking error successfully.     

Simulation of thermal behavior of a CNC machine tool spindle

The thermal deformations of a CNC machine tool spindle are the major contributor of thermal error. It is very significant both theoretically and practically to study how to accurately simulate the thermal error of the spindle. Firstly, this paper proposes a method for computing the coefficient of convection heat transfer of the spindle surface by referencing the theory on computing the coefficient of convection heat transfer of a flat plate when air flows along it. Secondly, the temperature field and thermal errors of the spindle are dynamically simulated under the actions of thermal loads using the finite element method. Thirdly, the characteristics of heat flow and thermal deformation within the spindle are analyzed according to the simulation results. Fourthly, the selection principle of thermal key points, which are indispensable for building a robust thermal error model, is provided based on the thermal error sensitivity technology. At last, a verification experiment is implemented on a CNC turning center, and the results show the simulation results are satisfying to replace the experiment results for further studies.     

直线电机在数控机床上的调试

本文主要介绍了直线电机的工作原理及其驱动技术、直线电机进给驱动技术及在数控机床上的调试,直线电机进给驱动技术将是高速数控机床未来发展的方向。     

Thermal analysis for the feed drive system of a CNC machine center

A high-speed drive system generates more heat through friction at contact areas, such as the ballscrew and the nut, thereby causing thermal expansion which adversely affects machining accuracy. Therefore, the thermal deformation of a ballscrew is one of the most important objects to consider for high-accuracy and high-speed machine tools. The objective of this work was to analyze the temperature increase and the thermal deformation of a ballscrew feed drive system. The temperature increase was measured using thermocouples, while a laser interferometer and a capacitance probe were applied to measure the thermal error of the ballscrew. Finite element method was used to analyze the thermal behavior of a ballscrew. The measured data were compared with numerical simulation results. Inverse analysis was applied to estimate the strength of the heat source from the measured temperature profile. The generated heat sources for different feed rates were investigated.     

An improved thermal model for machine tool bearings

Thermal model for machine tool spindle is of great importance to machine tool design. Traditionally, the thermal contact resistance between solid joints and the change of the heat generation power with the bearing temperature are often ignored when thermal characteristics of a machine tool spindle are analyzed. This has caused inaccuracies in the thermal model. With the heat source models and the heat transfer models from Bossmanns and Tu [Journal of Manufacturing Science and Engineering 123 (2001) 495–501, International Journal of Machine Tools & Manufacture 39 (1995) 1345–1366], a model including the thermal contact resistance at solid joints based on a fractal model and the change of the heat generation power, viz. the amount of the heat generation per second, with the bearing temperature increases is developed. The complete thermal model is used to simulate the temperature distribution in grinding machine housing with a conventional spindle bearing. Compared with experiment, it is shown that the completed model is much more accurate than the traditional model which ignores the two important factors above. The thermal expansion of the housing system is analyzed.     

Dynamic neural network modeling for nonlinear, nonstationary machine tool thermally induced error

This paper presents a new modeling methodology for nonstationary machine tool thermal errors. The method uses the dynamic neural network model to track nonlinear time-varying machine tool errors under various thermal conditions. To accommodate the nonstationary nature of the thermo-elastic process, an Integrated Recurrent Neural Network (IRNN) is introduced to identify the nonstationarity of the thermo-elastic process with a deterministic linear trend. Experiments on spindle thermal deformation are conducted to evaluate the model performance in terms of model estimation accuracy and robustness. The comparison indicates that the IRNN performs better than other modeling methods, such as, multi-variable regression analysis (MRA), multi-layer feedforward neural network (MFN), and recurrent neural network (RNN), in terms of model robustness under a variety of working conditions.     

Estimation of machine tool feed drive inclination from current measurements and a mathematical model

Leveling is an important part of a machine tool installation process because it significantly influences the product quality, machine tool accuracy, and machine lifetime. Conventional leveling procedures are performed by skilled engineers using leveling instruments such as spirit or electronic levels. It is difficult to monitor the level of a machine tool because an accurate leveling instrument is expensive and difficult to install. Therefore, a novel methodology for estimating the inclination angle of a machine tool feed drive is proposed in this paper to overcome the difficulties of leveling. The proposed methodology utilizes motor current measurements and a new mathematical model of the machine tool feed drive that considers the inclination. Experimental results showed that the proposed method successfully estimates the inclination angle and enhances the accuracy of the machine tool feed drive model by considering the inclination effects.     

Application of a two-level optimization process to conceptual structural design of a machine tool

This paper proposes a modified two-level optimization approach for the concept design of a machine tool. The lower level of optimization is applied to each structural part of the tool and the upper level to the machine tool as an integrated system. When frequent modification of design specifications is required and re-use of previous design experience is preferred, the proposed approach is efficient because of the approximation functions used. Examples presented in the paper show that the principal dimensions of all the structural parts can be determined, minimizing the weight of the machine while maintaining sufficient stiffness.     

基于能量输入与耗散反馈原理的机床热变形预报方法的研究

热变形是影响数控机床尤其是精密和超精密机床加工精度进一步提高的主要障碍。在研究一维钢轴热伸长理论模型的基础上,从能量角度入手,提出一种新的基于能量输入与耗散反馈原理的机床热变形预报模型的建立方法,并通过实验证明该模型具有较高的预报精度。     

Digitally controlled output filters for improvement of temperature behavior of machine tool main spindle drives

Variable speed main spindle drives of machine tools are usually fed by three-phase two-level inverters. Because of the switching operating mode of these inverters, the motor voltages and currents contain harmonics which do not contribute to torque formation but solely to an undesirable warming of motor spindle elements. This warming results in thermal elongation of the spindle shaft and therefore to a deviation in dimensional accuracy. The increased wear of the spindle bearings through high temperature differences between the inner and outer rings of the bearings is another drawback. This paper proposes a novel high-performance control system of an interior permanent magnet synchronous motor (IPMSM) using an inverter with an LC output filter. The combination of inverter and filter reduces the motor losses by compensating the switching ripples and therefore leads to an evident improvement of the temperature behavior of the main spindle drive.     

数控机床一种直流变换器拓扑研究

针对数控机床电器系统应用场合,对一种直流变换器拓扑进行研究,该拓扑以电感、电容为储能元件,应用电感伏秒平衡、电容充放电平衡原理进行理论分析,得到输入输出电压数学关系,通过仿真软件验证了理论分析的正确性。     

On-line estimation of time-variant thermal load applied to machine tool structures using a s-domain inverse solution

Thermal deformation of machine tools is of prime concern due to its significant effect on machining errors. To compensate for residual thermal errors, the control system has to incorporate an inverse heat conduction problem IHCP solver to estimate the transient heat input to the structure in real-time. The limitations of available IHCP solutions to applications involving real-time state estimation of complex, multi-dimensional structures are identified. A new s-domain IHCP solution has been developed to resolve these problems and to ensure stability of the solution without using future data. An accelerated inverse numerical Laplace transformation procedure is developed to improve the computational efficiency of the solver. Analysis of results showed the proposed method to be inherently stable and that the random fluctuation in the temperature measurements is considerably attenuated by the numerical nature of the solution. The results also show that the computational time has been improved by at least one order of magnitude over available methods, without compromising the accuracy and stability requirements. Due to the generalized nature of the formulation and the solution algorithm, this method can readily be applied to all classes of machine tools without any further developments by the end user.     

Thermal stability and electronic specific heat of GaN

The thermal stability of GaN single crystals obtained by Li-based flux was investigated by DTA–TG, XRD, Raman and infrared spectroscopy. The results evidence that pure GaN becomes unstable above 900 °C under N₂ of 0.1 MPa. The specific heat of GaN was determined at temperatures ranging from 1.9 K to 80 K. Its electronic specific heat coefficient is 0.47 mJ K⁻² mol⁻¹ and the Debye temperature is 278 K.     

ICA based thermal source extraction and thermal distortion compensation method for a machine tool

Thermal distortion in machine tools is one of the most significant causes of machining errors. One of the difficult issues in developing a system to compensate for thermal distortion is to select the appropriate temperature variables and to obtain an accurate thermal distortion model. This paper presents a new thermal distortion compensation method based on the Independent Component Analysis (ICA) method. The ICA method was used to extract the thermal sources from the temperature variables. The Optimal Brain Surgeon (OBS) algorithm was used to reduce the temperature variables with insignificant information. Using the extracted sources, a new thermal distortion model and a compensation method is proposed and is implemented in real-time hardware. In these experiments, the proposed method was shown to be capable of compensating for thermal distortions to a few micrometers.     

Development of a polymer impregnated concrete damping carriage for linear guideways for machine tools

Linear guideway systems of CNC machine tools are replacing the conventional slideways to achieve higher machining speed which has resulted in lower damping capacity. Damping carriages made of steel have been introduced to improve damping, however, that did not render significant improvement. Consequently, an attempt has been made in this study to improve damping in a CNC machine table by replacing the current steel damping carriage with one made of concrete since it has much higher internal damping. To select a suitable material for the carriage, the damping characteristics and strength of normal concrete, cellular concrete and polymer impregnated normal and cellular concrete have been investigated. It has been found that polymer impregnated cellular concrete (PC) is the most suitable material for the purpose. Consequently, a new damping carriage has been designed and fabricated using PC and has been incorporated into the table of a CNC machine table. This damping carriage has been found to improve the damping capacity and dynamic characteristics of the CNC table system equivalent of the slideway system.     

一种快速选配机床挂轮的方法

通过计算器简单、快速的计算，可快速选配机床挂轮。     

直线电机驱动型工作台热态特性分析

以精密螺母磨床用直线电机驱动型工作台为研究对象,考虑滚动导轨摩擦和直线电机等热源对工作台上台面的影响,应用ANSYS有限元分析软件对上台面进行了热-力耦合分析.结果表明:机床磨削过程中,上台面达到稳态时的温升为3℃左右,最大变形值为35.2 μm,中间部位为其结构设计的薄弱环节;与静力分析比较,两者的最大变形值虽相差不大,但热分析时的整体变形量和等效应力值均大于静力分析结果,等效应力最大值增加了128％.研究结果为同类型结构的热态特性分析提供了参考.     

Thermal oxidation behavior of WC-Co hard metal machining tool tip scraps

The thermal oxidation behavior of scraps of WC-Co hard metal machining tool tips, which have worn in high speed machining of metals or alloys, is investigated with a view toward recycling WC-Co tool tip scraps into W and Co resources. Oxidation is a preliminary process in eco-friendly dry recycling of scraps including rare metals. High oxidation rate of the WC-Co tool tip scraps is achieved when they are fragmented into many pieces to avoid the resistance to oxidation of a protective CrAlN coating. WC-Co compound scrap is oxidized to WO₃ and CoWO₄ with a weight gain of 20% compared to its initial weight during isothermal oxidation at 900 °C. It experiences a large volume expansion of more than 200%, producing a porous microstructure. Fragmented WC-Co scraps are oxidized in a parabolic-like behavior, whereas the initial gradual oxidation of mechanically untreated WC-Co scraps is followed by a rapid oxidation after 1.5 h due to the breakdown of the CrAlN coating.