超精密加工误差补偿技术研究综述

超精密加工技术是高端制造领域的一项关键技术,当前超精密加工已进入纳米尺度,掌握超精密加工误差控制关键技术、保障并提高数控机床的加工精度,已经成为提高加工制造水平的研究热点。系统总结了超精密加工误差补偿技术研究现状及发展趋势,重点介绍了对超精密加工影响最大的几何误差、力诱导误差、热诱导误差及其补偿方法。在此基础上,深入探讨了超精密加工在几何误差分离,切削力、热诱导误差测量与补偿等方面存在的一系列问题,进一步指出超精密加工误差补偿技术还应关注其向高效、高精,通用化,模块化,智能化及柔性化的发展方向。     

Prediction and compensation of machining geometric errors of five-axis machining centers with kinematic errors

Kinematic errors due to geometric inaccuracies in five-axis machining centers cause deviations in tool positions and orientation from commanded values, which consequently affect geometric accuracy of the machined surface. As is well known in the machine tool industry, machining of a cone frustum as specified in NAS979 standard is a widely accepted final performance test for five-axis machining centers. A critical issue with this machining test is, however, that the influence of the machine's error sources on the geometric accuracy of the machined cone frustum is not fully understood by machine tool builders and thus it is difficult to find causes of machining errors. To address this issue, this paper presents a simulator of machining geometric errors in five-axis machining by considering the effect of kinematic errors on the three-dimensional interference of the tool and the workpiece. Kinematic errors of a five-axis machining center with tilting rotary table type are first identified by a DBB method. Using an error model of the machining center with identified kinematic errors and considering location and geometry of the workpiece, machining geometric error with respect to the nominal geometry of the workpiece is predicted and evaluated. In an aim to improve geometric accuracy of the machined surface, an error compensation for tool position and orientation is also presented. Finally, as an example, the machining of a cone frustum by using a straight end mill, as described in the standard NAS979, is considered in case studies to experimentally verify the prediction and the compensation of machining geometric errors in five-axis machining.     

工件分特征下的五轴数控机床关键几何误差分析与补偿方法

提出了工件分特征下的五轴数控机床关键几何误差分析与补偿方法,将复杂工件进行特征分解,通过灵敏度分析辨识工件分特征下的关键几何误差并补偿,从而提高工件整体加工精度。以某一复杂工件为例,首先,将其分解为平面、斜面、圆柱和圆锥台四个典型特征;然后,基于灵敏度分析分别辨识出各典型特征对应的关键几何误差;最后,分特征地进行误差补偿。在AC双转台五轴数控机床上进行了实验验证,实验结果表明,辨识得到的关键几何误差灵敏度系数之和占比均大于90%,补偿后工件四个典型特征的加工精度提高了20%~30%。研究结果表明,所提方法能有效辨识不同工件分特征下的关键几何误差,从而提高复杂工件的加工精度。     

Modeling and compensation of geometric errors in simultaneous cutting using a multi-spindle machine tool

A volumetric error compensation method for a machining center that has multiple cutting tools operating simultaneously has been developed. Due to axis sharing, the geometric errors of multi-spindle, concurrent cutting processes are characterized by a significant coupling of error components in each cutting tool. As a result, it is not possible to achieve exact volumetric error compensation for all axes. To minimize the overall volumetric error in simultaneous cutting, a method to determine compensation amount using weighted least squares has been proposed. This method also allows tolerance distribution of machining accuracy for different surfaces of a workpiece. A geometric error model has been developed using an arch-type, multi-spindle machine tool, and the error compensation simulation results based on this model are presented. The simulation results demonstrated effectiveness of the proposed error compensation algorithm for use with multi-spindle simultaneous cutting applications.     

Volumetric error modeling and sensitivity analysis for designing a five-axis ultra-precision machine tool

The five-axis machine tools are increasingly popular for meeting the demand of machining the workpiece with growing geometric complexity and high accuracy. This paper studies the volumetric error modeling and its sensitivity analysis for the purpose of machine design. The volumetric error model of a five-axis machine tool with the configuration of RTTTR is established based on rigid body kinematics and homogeneous transformation matrix, in which 37 error components are involved. The sensitivity analysis of volumetric error regarding 37 error components is carried out respectively. The analysis results are successfully used for the accuracy design and manufacture of a five-axis ultra-precision machine tool. The preliminary experiment of machining sine grid surface testifies the high accuracy and effectiveness of the designed five-axis machine tool.     

Repetitive Measurement and Compensation to Improve Workpiece Machining Accuracy

Achieving workpiece high accuracy at low cost is one of the greatest challenges in the manufacturing industry. A repetitive error measurement and compensation scheme to improve the workpiece diameter accuracy for machining centres is des-cribed. The scheme entails an on-machine measurement and error compensation technology between machining processes. The workpiece diameters are measured along the workpiece length by using a fine touch sensor. The workpiece diameters in the compensation program are modified for implementation of next pass error correction. The technology is realised on a CNC turning centre. This method works well in hard machining and turned workpieces with large length–diameter ratios where the machining process induced errors are significantly greater than errors from other sources. It demonstrates that the work-piece can obtain maximum possible machining accuracy by this repetitive measurement and compensation technique.     

Application of projection pursuit regression to thermal error modeling of a CNC machine tool

Thermal errors are the major contributor to the dimensional errors of a workpiece in precision machining. Error compensation technique is a cost-effective way to reduce thermal errors. Accurate modeling of errors is a prerequisite of error compensation. In this paper, a thermal error model was proposed by using projection pursuit regression (PPR). The PPR method improves the prediction accuracy of thermal errors in the computer numerical control (CNC) turning center. A thermal error compensation system was developed based on the PPR model, and which has been applied to the CNC turning center in daily production. The results show that the thermal drift in workpiece diameter has been reduced from 34 to 5???m.     

Quasi-static kinematics model for motion errors of closed hydrostatic guideways in ultra-precision machining

Aspheric optics has been widely employed in some high-tech industries for its superiority. In order to achieve the ultra-precision machining of these aspheric surfaces, the large optical ultra-precision grinding machine becomes crucial because it determines the efficiency of the whole process. As the key functional unit in ultra-precision machine tool, the hydrostatic guideways are commonly adopted for the excellent performance. However, the motion errors of hydrostatic guideways have a direct influence on the accuracy of the machined workpiece, and some analysis approaches have been reported correspondingly. Although the existed analysis models do work, their imperfections also can be easily captured. Accordingly, a novel analysis model with less imperfection is deserved to be developed. In this paper, the kinematic theory is utilized to establish the quasi-static analysis model for motion errors in closed hydrostatic guideways. Through the large ultra-precision grinding machine designed by our research group, the consistently good agreement between the predicted results and the measured experimental data are obtained. Furthermore, it is found that the motion accuracy is more sensitive to the profile error of the guide rail bearing the external load rather than that of the other guide rail in the closed hydrostatic guideway. The presented research is supposed to be valuable to the peer designers.     

复杂曲面零件在线检测与误差补偿方法

复杂曲面零件的高精度加工与精密检测一直是数字化制造领域的研究热点。为提高复杂曲面零件的加工精度、检测精度,提出一种集数控机床在线检测、加工误差分解与补偿加工为一体的集成化方法。介绍集成化在线检测方法及补偿系统的基本原理,分析数控加工后曲面零件测点数据的误差组成,提出一种基于空间统计分析的加工误差分解方法,在建立基于B样条曲面的确定性曲面回归模型的基础上,对回归模型残差进行空间独立性分析,分解出系统误差和随机误差,进而通过数控代码的修改,实现零件加工过程的系统误差补偿。列举一个曲面零件的加工与检测实例,进行方法有效性验证。通过加工工件的在线检测、误差分解、代码修改及补偿加工等环节,实例零件的加工精度有了大幅提高,而该系统的检测精度也通过与三坐标测量机(Coordinate measuring machine, CMM)检验结果的对比,得到了有效验证。     

机械加工派生误差的可行性分析

通过分析控制工件的内应力、拱度、位移滞留来减小派生误差;大多数派生误差是可控、可调的,只要在加工过程中配合合理的工艺系统,充分发挥自位补偿功能和形位补偿功能,就可以有效的减少或消除工件在机械加工过程中的派生误差,来达到工件加工精度的要求。     

Study on the influence of geometric errors in rotary axes on cubic-machining test considering the workpiece coordinate system

Evaluating the influence of geometric errors in rotary axes is a common method used by a five-axis machine tool for improving the machining accuracy. In conventional geometric error models, the table coordinate system is considered as the final workpiece coordinate system. In this study, an additional workpiece coordinate transformation was proposed to identify the influence of geometric error. First, a cubic machining test was conducted. Second, the necessity of workpiece coordinate transformation was analyzed, and a method for coordinate transformation was proposed. In addition, both machining simulation and an actual machining experiment of the cubic machining test were conducted to verify the efficiency of the proposed method. The results indicate that the workpiece coordinate transformation is an essential part of the geometric error model for accurately simulating the geometric error influence. The method for identifying the geometric error influence considering the workpiece coordinate transformation is applicable in manufacturing.     

Simultaneous geometric error identification of rotary axis and tool setting in an ultra-precision 5-axis machine tool using on-machine measurement

This paper presents a method to identify the position independent geometric errors of rotary axis and tool setting simultaneously using on-machine measurement. Reducing geometric errors of an ultra-precision five-axis machine tool is a key to improve machining accuracy. Five-axis machines are more complicated and less rigid than three axis machine tools, which leads to inevitable geometric errors of the rotary axis. Position deviation in the process of installing a tool on the rotary axis magnifies the machining error. Moreover, an ultra-precision machine tool, which is capable of machining part within sub-micrometer accuracy, is relatively more sensitive to the errors than a conventional machine tool. To improve machining performance, the error components must be identified and compensated. While previous approaches have only measured and identified the geometric errors on the rotary axis without considering errors induced in tool setting, this study identifies the geometric errors of the rotary axis and tool setting. The error components are calculated from a geometric error model. The model presents the error components in a function of tool position and angle of the rotary axis. An approach using on-machine measurement is proposed to measure the tool position in the range of 10 s nm. Simulation is conducted to check the sensitivity of the method to noise. The model is validated through experiments. Uncertainty analysis is also presented to validate the confidence of the error identification.     

Application of ACO-BPN to thermal error modeling of NC machine tool

Thermal errors are the major contributor to the dimensional errors of a workpiece in precision machining. Error compensation technique is a cost-effective way to reduce thermal errors. Accurate modeling of errors is a prerequisite of error compensation. In this paper, four key temperature points of a NC machine tool were obtained based on clustering method. A thermal error model based on the four key temperature points was proposed by using ant colony algorithm-based back propagation neural network (ACO-BPN). The ACO-BPN method improves the prediction accuracy of thermal deformation in the NC machine tool. A thermal error compensation system was developed based on the proposed model, and which has been applied to the NC machine tool in daily production. The results show that the thermal drift in workpiece diameter has been reduced from 33 to 8 μm from its center of tolerance.     

SOFTWARE-CONTROLLED SYSTEM OF ULTRA-PRECISION MACHINING AXISYMMETRIC ASPHERIC MIRROR

In order to improve machining accuracy and efficiency, a software-controlled system of ultra-precision machining for axisymmetric aspheric mirror, using techniques of error compensation, remote transmission and modularization, is designed based on industrial PC, Windows 2000 work platform and Visual Basic 6.0. By experiments, this system realizes functions of ultra-precision machining, machining error compensation, remote data transmission and automatic data transformation among first machining, compensation machining and accuracy measurement. The actual application shows that error compensation improves machining accuracy, remote transmission improves machining efficiency while modularization avoids repeated work and improves design efficiency. Therefore, the system has met ultra-precision machining need for aspheric mirror.     

Critical success factors for Kaizen implementation in manufacturing industries in Mexico

To machine a noncoaxial nonaxisymmetric aspheric lens, a new parallel grinding method that employs a fixture with an adjustable gradient (AGF) is proposed. The AGF is developed for a three-axis computer numerically controlled grinding machine. The grinding method is presented according to the proposed grinding system. To ensure the machining accuracy, the main machining errors and the compensation algorithm are discussed for the grinding method using the AGF. Simulation results show that the AGF rotation errors are crucial factors affecting the profile error of the machined workpiece. Experimental results show that employing the compensation algorithm increases machining accuracy.     

考虑热误差的双转台机床工艺误差谱预测方法

五轴数控机床的几何误差和热误差是影响工件加工精度的两个重要因素,对这些误差因素进行分析可以有效提高薄壁件工件的加工精度。本文首先基于齐次坐标变换法,建立了双转台五轴数控机床的旋转轴几何误差模型;然后基于对标准球进行在机接触测量,辩识得出两旋转轴的12项几何误差,这些误差考虑了两旋转轴之间的相互影响和其热误差的影响;最后分析五轴数控机床加工空间的几何误差场,在该加工空间内几何误差从中心到外侧逐渐增加,当A轴旋转角度增加时,误差的最大值也随之增加。与其它位置误差辨识方法相比,本方法的测量精度符合加工要求,测量时间只需要30 min。     

Error Compensation of Thin Plate-shape Part with Prebending Method in Face Milling

Low weight and good toughness thin plate parts are widely used in modern industry, but its flexibility seriously impacts the machinability. Plenty of studies focus on the influence of machine tool and cutting tool on the machining errors. However, few researches focus on compensating machining errors through the fixture. In order to improve the machining accuracy of thin plate-shape part in face milling, this paper presents a novel method for compensating the surface errors by prebending the workpiece during the milling process. First, a machining error prediction model using finite element method is formulated, which simplifies the contacts between the workpiece and fixture with spring constraints. Milling forces calculated by the micro-unit cutting force model are loaded on the error prediction model to predict the machining error. The error prediction results are substituted into the given formulas to obtain the prebending clamping forces and clamping positions. Consequently, the workpiece is prebent in terms of the calculated clamping forces and positions during the face milling operation to reduce the machining error. Finally, simulation and experimental tests are carried out to validate the correctness and efficiency of the proposed error compensation method. The experimental measured flatness results show that the flatness improves by approximately 30 percent through this error compensation method. The proposed method not only predicts the machining errors in face milling thin plate-shape parts but also reduces the machining errors by taking full advantage of the workpiece prebending caused by fixture, meanwhile, it provides a novel idea and theoretical basis for reducing milling errors and improving the milling accuracy.     

立式加工中心空间几何误差辨识解析及定位误差补偿研究

为大幅提升立式加工中心加工精度,满足当代数控机床对高精度的需求,针对立式加工中心3个运动轴,深入分析了其轴向运动空间几何误差,提出了可有效辨识运动轴轴向运动空间6项几何误差的辨识方法.建立了空间6项几何误差辨识模型,并针对关联轴联动垂直度误差进行了有效分析,建立了垂直度误差辨识解析模型.同时,针对3个独立运动轴轴向定位...     

Error analysis and compensation for the volumetric errors of a vertical machining centre using a hemispherical helix ball bar test

Machining accuracy is directly influenced by the quasi-static errors of a machine tool. Since machine errors have a direct effect upon both the surface finish and geometric shape of the finished workpiece, it is imperative to measure the machine errors and to compensate for them. A laser measurement system to identify geometric errors of a machine tool has disadvantages, such as a high cost, a long calibration time and the usage of a volumetric error synthesis model. In this study, we proposed a novel analysis of the geometric errors of a machine tool using a ball bar test without using a complicated error synthesis model. Also, a statistical analysis method was employed to derive geometric errors using a hemispherical helix ball bar test. According to the experimental result, we observed that geometric errors of the vertical machining centre were compensated by 88%.     

离轴非球面超精密磨削加工几何模型的探讨

在用来加工轴对称非球面的二轴联动超精密数控机床的工件主轴上安装可以精确给出旋转角度的码盘，采用柱坐标加工方式进行离轴非球面的超精密磨削加工。在给出离轴非球面方程的基础上，给出数控加工离轴非球面的相应的几何模型。计算机仿真验证了该加工方法的简便性与可靠性。该加工方法可以降低设备投入，提高生产效率。