Analysis of thermal errors in a high-speed micro-milling spindle

Thermally induced errors account for the majority of fabrication accuracy loss in an uncompensated machine tool. This issue is particularly relevant in the micro-machining arena due to the comparable size of thermal errors and the characteristic dimensions of the parts under fabrication. A spindle of a micro-milling machine tool is one of the main sources of thermal errors. Other sources of thermal errors include drive elements like linear motors and bearings, the machining process itself and external thermal influences such as variation in ambient temperature. The basic strategy for alleviating the magnitude of these thermal errors can be achieved by thermal desensitization, control and compensation within the machine tool.This paper describes a spindle growth compensation scheme that aims towards reducing its thermally-induced machining errors. The implementation of this scheme is simple in nature and it can be easily and quickly executed in an industrial environment with minimal investment of manpower and component modifications.Initially a finite element analysis (FEA) is conducted on the spindle assembly. This FEA correlates the temperature rise, due to heating from the spindle bearings and the motor, to the resulting structural deformation. Additionally, the structural deformation of the spindle along with temperature change at its various critical points is experimentally obtained by a system of thermocouples and capacitance gages.The experimental values of the temperature changes and the structural deformation of the spindle qualitatively agree well with the results obtained by FEA. Consequently, a thermal displacement model of the high-speed micro-milling spindle is formulated from the previously obtained experimental results that effectively predict the spindle displacement under varying spindle speeds. The implementation of this model in the machine tool under investigation is expected to reduce its thermally induced spindle displacement by 80%, from 6 microns to less than 1 micron in a randomly generated test with varying spindle speeds.     

A study of pre-compensation for thermal errors of NC machine tools

Thermally-induced errors are major contributors to the overall accuracy of machine tools. In this paper, an error pre-compensation system is developed to correct the thermal errors of the spindle and lead screws. A simple gauge 1-D ball array is used to accelerate and simplify the error measurement. An auto-regressive model based on spindle rotation speed is then developed to describe the thermal errors. Using the model, the thermal errors can be predicted without measuring the temperature field of the machine tool as soon as the workpiece NC machining program is made. By correcting the program, the errors can be pre-compensated before machining. Thus the process of compensation is greatly simplified and the cost is reduced. The test results on a vertical machining center show that a 70% reduction of thermal errors has been gained after compensation.     

立式加工中心主轴系统热变形试验及误差补偿

立式加工中心经过长时间的运行之后,主轴箱及主轴系统组成的单元会产生热变形,这影响到被加工零件轴向尺寸的加工精度。以VMC750立式加工中心为试验对象,测量主轴箱多点温度及主轴变形伸长量,确定主轴变形的主要原因,建立误差补偿模型,通过对立式加工中心加工过程中的热误差进行了实时补偿实验,结果表明：通过热变形补偿,主轴系统热变形实测为0.28～0.33 mm,其误差可减少75％左右,验证了该模型的有效性。     

A review on spindle thermal error compensation in machine tools

Thermal error caused by the thermal deformation is one of the most significant factors influencing the accuracy of the machine tool. Among all the heat sources which lead to the thermal distortions, the spindle is the main one. This paper presents an overview of the research about the compensation of the spindle thermal error. Thermal error compensation is considered as a more convenient, effective and cost-efficient way to reduce the thermal error compared with other thermal error control and reduction methods. Based on the analytical calculation, numerical analysis and experimental tests of the spindle thermal error, the thermal error models are established and then applied for implementing the thermal error compensation. Different kinds of methods adopted in testing, modeling and compensating are listed and discussed. In addition, because the thermal key points are vital to the temperature testing, thermal error modeling, and even influence the effectiveness of compensation, various approaches of selecting thermal key points are introduced as well. This paper aims to give a basic introduction of the whole process of the spindle thermal error compensation and presents a summary of methods applied on different topics of it.     

加工中心主轴系统的热变形分析与有限元计算

以TH6350卧式加工中心为研究对象,构建了一套基于虚拟仪器系统的加工中心主轴系统温度场和热误差测量系统,测出了加工中心主轴系统的温度场和各项热变形.建立了基于I-DEAS的加工中心主轴系统的温度场和热变形有限元模型,得到了主轴系统的温度场和热变形分布及其计算结果,计算结果与实测值得到了较好的吻合,研究结果为加工中心的改进设计、温度控制和误差补偿提供了理论依据.     

卧式加工中心机械镗轴热膨胀及补偿的研究与应用

对镗轴伸缩式卧式加工中心机床机械镗轴在高速范围(2 000~3 000 r/min)加工时,就其热变形量和热变形的原因进行研究,并找出行之有效的补偿方法,以满足机械式镗轴卧式加工中心在加工精度高要求的条件下使用。经检测发现:热影响的各参数中,镗轴后轴承位置的温升明显,且与镗轴热伸长有较好的线性拟合关系。选用PTl00型温度传感器进行温度数据采集并反馈到数控系统中进行轴向补偿,最终以满足加工的需求。     

滑枕综合热补偿研究

新设计的滑枕热伸长补偿机构消除了滑枕达到热平衡之前因热变形造成的瞬态热误差。通过试验,测出机床达到热平衡后主轴的温度误差和机床对应的温度场,并利用最小二乘法拟合出该误差和温度值之间的数学模型,将数学模型输入数控系统中进行机床主轴的稳态热补偿,即温度误差补偿。这两种热补偿相结合的方式进一步提高了机床的加工精度,保证了数控龙门柔性生产线各种零件的加工精度要求。     

Measurement of spindle thermal errors in machine tool using hemispherical ball bar test

The assurance of top-quality products in machining processes requires improved machine tool accuracy. Among the various errors related to machine tools, thermal errors of a spindle have a significant effect on machining accuracy and directly influence both the surface finish and the geometric shape of the finished workpiece. Accordingly, the current paper proposes a new measurement method for spindle thermal errors in a machine tool based on the use of a ball bar system instead of the conventional capacitance sensor system. The novel measurement method is more efficient and easier to use compared to conventional measurement systems. Furthermore, a single ball bar system is sufficient for the simultaneous measurement of both geometric and thermal errors.     

数控车床热误差建模与补偿研究

数控机床热变形引起的误差通常占到总体误差的40%~70%。以某公司生产的某型卧式数控车床为研究对象,检测主轴热误差和X进给轴热误差,基于最小二乘法对该机床主轴X、Y、Z向和X进给轴分别建立热误差模型。考虑到实测环境温度相对参考温度20℃时滚珠丝杠伸长的因素,对主轴热误差实测值进行了修正。根据主轴X向修正后的热误差模型和X进给轴热误差模型建立了X轴综合热误差模型,并采用西门子840D系统进行了热误差补偿试验,热误差降低了54.5%,CP值由1.34提升至1.88,证明此该建模与补偿方法有效、可行。     

减少机床热变形方法的研究

机床在各种热源的作用下,产生热变形,影响工件与刀具之间的相对位移,造成加工误差,从而影响零件的加工精度。所以,减少机床热变形对提高机床加工精度是极其重要的。本文对机床工艺系统的主要热源进行了分析,并对减少机床热变形的方法及国内外研究的现状进行了讨论。     

精密卧式加工中心主轴轴向热误差补偿方法

热误差是精密机床最主要的误差源之一。主轴是机床的关键部件,其热误差直接影响机床的加工精度。文章以某型号精密卧式加工中心主轴为对象,对其温度场和热变形进行了仿真分析。根据仿真结果发现主轴轴向热变形更严重,并结合机床结构确定温度传感器布置位置。在此基础上,对不同转速下主轴部分位置温度和轴向热误差进行现场测试。运用最小二乘法建立热误差补偿模型,直接结合机床FANUC数控系统实施主轴轴向热误差补偿。经实验验证,补偿后主轴轴向热误差减小了85%以上。     

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.     

A machining test to calibrate rotary axis error motions of five-axis machine tools and its application to thermal deformation test

This paper proposes a machining test to parameterize error motions, or position-dependent geometric errors, of rotary axes in a five-axis machine tool. At the given set of angular positions of rotary axes, a square-shaped step is machined by a straight end mill. By measuring geometric errors of the finished test piece, the position and the orientation of rotary axis average lines (location errors), as well as position-dependent geometric errors of rotary axes, can be numerically identified based on the machine׳s kinematic model. Furthermore, by consequently performing the proposed machining test, one can quantitatively observe how error motions of rotary axes change due to thermal deformation induced mainly by spindle rotation. Experimental demonstration is presented.     

高速电主轴热误差实验与预测建模

数控加工中心采用高速电主轴,由于电主轴发热导致主轴工作端明显热延伸,严重影响加工精度。以数控精雕机用永磁同步电主轴为研究对象,通过建立热特性实验平台,测试电主轴在不同转速、不同工作条件下的特征温度和热误差数据,建立基于自然指数函数的电主轴轴向热误差预测模型。在不同的工况下对模型的补偿结果进行实验验证,验证结果表明:该预测模型简单、精度较高,且建模成本较低,可以快速应用到实际的加工环境中。     

A generalized modelling methodology for optimized real-time compensation of thermal deformation of machine tools and CMM structures

Thermal deformation of the machine tool structure plays a critical role in controlling the level of machining accuracy. Since thermally-induced errors cannot completely be eliminated at the design stage, the use of indirect compensation systems emerges as the most economical and inevitable course of action. These systems are based on two different, but yet complimentary, approaches; the use of empirical compensation function and the on-line execution of numerical simulation models. Recognizing the existence of a mathematical similarity between the real process and a simplified dynamic model, the generalized modelling approach is proposed to resolve the main problems associated with existing indirect compensation methods. It is demonstrated that the proposed methodology can be used to identify the true global optimum position and minimum number of the temperature sensors with the highest contribution to the compensation function. These issues have been the focus of a recent research activity world-wide and remain the main obstacles limiting the acceptance of the empirical compensation approach by industry. The use of the generalized modelling method to improve the thermal deformation predictions, through filtering out random temperature measurement errors, is also demonstrated. The application of the generalized modelling method to compensation systems based on numerical simulation is discussed. It is shown that the method can reproduce the accuracy of the finite-element solution, but two orders of magnitude faster, for on-line prediction of the transient temperature field in the machine tool structure. The use of the generalized modelling method to test and to improve the accuracy of finite-element models is also discussed.     

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.     

卧式加工中心热变形控制与热误差补偿技术研究

从减少发热、控制温升、热误差补偿3个层次对卧式加工中心产品的热变形控制与热误差补偿技术进行了研究。第一层次从高精度功能部件的选用和润滑技术两方面研究减少机床的发热;第二层次从主轴温升控制、中空丝杠温升抑制、结构基础件温度控制等方面研究了冷却在控制关键部件的温升和均衡温度场中的作用;第三层次从数据采集、分析、建模等方面研究了热误差补偿技术。提出的各个层次控制热变形及热误差的具体技术和措施在实际产品中获得了应用,取得了优良的应用效果。     

基于总线的数控系统嵌入式热误差补偿实现方法

热误差是影响机床加工精度的重要误差源。简要介绍THK6370卧式加工中心主轴热变形的测量及建模方法,详细阐述热误差补偿在基于总线的数控系统上嵌入式集成的方法,提出热误差补偿模块的软硬件设计方案,并开发补偿模块。在THK6370上进行实验验证,结果表明,加工精度提高了30%。     

机床热变形及热误差优化研究

为减少热变形对精密加工精度影响,对夏冬两季节机床主轴箱上温升和热变形及环境温度的影响进行了测试分析,并采用BP神经网络模型化的Volterra级数非线性系统实现热误差建模。分析结果表明：夏天环境温度受主轴箱散热影响而温度迅速升高;冬季机床散热较快,主轴箱上温升比较明显,环境温度几乎不变;同一台机床在夏季和冬季的热变形规律相似而变形量稍有不同。通过实验验证了该模型具有预测精度高的优点,为数控机床热误差实时补偿提供了参考。     

基于ANSYS的直线导轨热变形分析

对直线导轨副滑块往复运动过程中导轨摩擦热变形进行模拟,考虑导轨和滑块之间的相对位置变化,分析摩擦热在往返运动中对直线导轨热变形的影响。首先分析直线导轨副热载荷分布情况,建立热应变有限元模型,计算加载摩擦热源下导轨变形量,对加工过程中热误差补偿具有指导性作用。