The harmonic fitting method for the assessment of the substitute geometry estimate error. Part I: 2D and 3D theory

The dimensional inspection of a part consists of the measurement of several points on the manufactured surface and the fitting of a substitute geometry. The estimated parameters of the substitute geometry, which are related to the position and dimensions of the measured surface, are then compared with the specifications, leading to the acceptance or rejection of the manufactured part. Consequently, the reliability of the dimensional inspection depends on the precision of the estimated substitute geometry. The error that affects the estimate (estimate error) is a function of several factors (number and distribution of sampled points, measurement error, machining deviations, etc.), but the analytic expression of this relationship is still unknown.The first part of this work presents the Harmonic Fitting Method (HFM) for the assessment of the estimate error. It will be demonstrated that with the HFM the closed analytic relationship between the estimate error and the most important factors can be determined. This relationship is extremely simple when using the matrix notation, since the estimate error can be expressed as the product of different matrices, each one accounting for a single factor. In the first part of the work the general theory of the HFM will be presented and discussed for 2D features (line and circle) and 3D features (plane, cylinder and cone). In the second part the statistic approach and the machining process analysis will be presented, which are the basis for the practical use of the HFM for the inspection plan optimisation. The HFM will be applied to real data to optimise the inspection of circular geometric elements, which are particularly relevant for the mechanical industry.     

Integrated Inspection and Machining for Maximum Conformance to Design Tolerances

Designers' intent for the form, fit and function of products is expressed by design tolerances the conformance to which is the main objective of manufacturing processes. A methodology for maximizing the adherence to the specified tolerances using an integrated machining and inspection system is presented. Considering the desired tolerance envelope of the part, an error decomposition technique is developed to model machining errors caused by the systematic and non-systematic errors in the machine tool. The model is used to adaptively plan the final machining cuts, based on inspection feedback, to enhance the geometric accuracy of the final product and is illustrated by an example. This approach reduces scrap and rework and their associated costs.     

精密卧式加工中心空间误差的建模、测量与补偿技术

采用空间误差补偿技术,可有效提高数控机床的空间定位精度。以一台精密卧式加工中心为对象,系统阐述其几何误差补偿中的关键问题及解决方案。通过三维误差建模与分析,得到该机床的21项几何误差中有17项需要测量和补偿,另外4项误差对机床定位精度的影响甚微。以此为依据,设计误差测量及补偿方案,并给出误差的具体测量方法和补偿结果。结果表明:经过一次系统地误差测量与补偿,精密卧式加工中心的空间定位精度可以提高50%～70%;合理规划和实施空间误差测量,可大幅提高测量效率。     

Adaptive support vector regression analysis of closed-loop inspection accuracy

This study investigates how closed-loop measurement error in CNC milling relates to two different inspection techniques. The on-line inspection of machining accuracy using a spindle probe has an inherent shortcoming because the same machine that produced the parts is used for inspection. In order to use the spindle probe measurement as a means of correcting deviations in machining, the magnitude of measurement errors needs to be quantified. The empirical verification was made by conducting three sets of cutting experiments at the state-of-the-art Cincinnati Arrow Vertical Machining Center. Three different material types and parameter settings were selected to simulate a diverse cutting condition. During the cutting, the cutting force and spindle vibration sensor signals were collected and a tool wear was recorded using a computer vision system. The bore tolerance was gauged by a spindle probe as well as a coordinate measuring machine. The difference between the two measurements was defined as a closed-loop measurement error and adaptive support vector regression analysis was used to predict these bore difference at various values of the explanatory variables. The results show the potential of improving production efficiency and part quality.     

5-Axis adaptive flank milling of flexible thin-walled parts based on the on-machine measurement

Deformation of the part and cutter caused by cutting forces immediately affects the dimensional accuracy of manufactured parts. This paper presents an integrated machining deviation compensation strategy based on on-machine measurement (OMM) inspection system. Previous research attempts on this topic deal with deformation compensation in machining of geometries in 3-axis machine tools only. This paper is the first time that concerned with 5-axis flank milling of flexible thin-walled parts. To capture the machined surface precision dimensions, OMM with a touch-trigger probe installed on machine׳s spindle is utilized. Probe path is planned to obtain the coordinate of the sampling points on machined surface. The machined surface can then be reconstructed. Meanwhile, the cutter׳s envelope surface is calculated based on nominal cutter location source file (CLSF). Subsequently, the machining error caused by part and cutter deflection is calibrated by comparing the deviation between the machined surface and the envelope surface. An iteration toolpath compensation algorithm is designed to decrease machining errors and avoid unwanted interference by modifying the toolpath. Experiment of machining the impeller blade is carried out to validate the methodology developed in this paper. The results demonstrate the effectiveness of the proposed method in machining error compensation.     

Enhanced virtual machining for sculptured surfaces by integrating machine tool error models into NC machining simulation

Sculptured surface machining is a time-consuming and costly process. It requires simultaneously controlled motion of the machine axes. However, positioning inaccuracies or errors exist in machine tools. The combination of error motions of the machine axes will result in a complicated pattern of part geometry errors. In order to quantitatively predict these part geometry errors, a new application framework ‘enhanced virtual machining’ is developed. It integrates machine tool error models into NC machining simulation. The ideal cutter path in the NC program for surface machining is discretized into sub-paths. For each interpolated cutter location, the machine geometric errors are predicted from the machine tool error model. Both the solid modeling approach and the surface modeling approach are used to translate machine geometric errors into part geometry errors for sculptured surface machining. The solid modeling approach obtains the final part geometry by subtracting the tool swept volume from the stock geometric model. The surface modeling approach approximates the actual cutter contact points by calculating the cutting tool motion and geometry. The simulation results show that the machine tool error model can be effectively integrated into sculptured surface machining to predict part geometry errors before the real cutting begins.     

Analysis–synthesis of dimensional deviation of the machining feature for discrete-part manufacturing processes

Dimensional deviation analysis has been an active and important research topic in mechanical design, manufacturing processes, and manufacturing systems. This paper proposes a comprehensive dimensional deviation evaluation framework for discrete-part manufacturing processes (DMP). A generic, explicit, and transmission model is developed to describe the dimensional deviation accumulation of machining processes by means of kinematic analysis of relationships between fixture error, datum error, machine tool geometric error, fixturing force inducing error and the dimensional quality of the product. The developed modeling technology can deal with general fixture configurations. In addition, the local contact deformations of the workpiece–fixture system are determined by solving a nonlinear programming problem of minimizing the total complementary energy of the frictional workpiece–fixture subsystem in machining system. Moreover, the deviation of an arbitrary point on machining feature can be also evaluated based on a point deviation model with prediction dimension deviation from the transmission model. The dimensional error transmission within the machining process is quantitatively described in this model. A systematic procedure to construct the model is presented and validated. This model can be also applied to process design evaluation for complicated machining processes.     

Improved positioning for side milling of ruled surfaces: Analysis of the rotation axis's influence on machining error

This article covers side milling of ruled surfaces using a milling cutter. Flank milling is useful for machining objects such as impellers, turbine blades, fan vanes and all workpieces defined by non-developable, ruled surfaces. In the present article, the influence of parameters defining improved positioning described in a previous study will be appraised. The general idea with improved positioning is to position the milling cutter at a tangent to the 2 directrices of the ruled surface while keeping a point of contact between the milling cutter and the rule considered. This is obtained by a rotation at a point about an imposed axis. Having defined calculation of error between the milled surface and the nominal surface, the influence of the point and the axis of rotation of improved positioning on error will be studied. From this, optimum improved positioning parameters allowing minimisation of error between the ruled surface and the milling cutter will be deduced.     

MVC850B型立式数控铣床误差分析与补偿试验研究

针对企业实际生产中铣床加工精度波动的问题,应用Renishaw XL-30激光干涉仪对MVC850B数控铣床的定位误差进行精密检测与补偿试验.利用环境参数对比试验,得出影响定位误差测量的因素;通过三因素双指标正交试验判断进给速度、加工时间以及测距等输入变量对反向间隙与螺距累积误差影响的主次关系;通过单因素对比试验获得反...     

加工夹具定位误差的计算

分析加工夹具定位误差产生的原因,以V形块为定位元件,按几何计算法讨论轴加工中的夹具定位误差的计算。     

提高轮廓加工误差联机检测精度的研究

在数控机床或加工中心上采用联机检测轮廓加工误差的方法，不用价格昂贵的坐标测量机，具有简单、省时、经济的特点。文章分析了数控机床或加工中心的直线运动误差对联机检测轮廓加工误差精度的影响，并测量出了加工中心的几何运动误差，提出了消除机床几何运动误差影响，提高轮廓加工误差联机检测精度的方法。实验结果表明，所采用的方法可以明显提高轮廓加工误差联机检测精度。     

An investigation of the power output and gap voltage during electrical discharge machining using microcomputer-based instrumentation

An analogue wattmeter has been developed for direct measurement of electrical power dissipation at the discharge gap during electrical discharge machining (EDM). A microcomputer samples power and corresponding gap voltage at regular intervals. Digitally encoded waveforms, which represent blocks of continuous power and voltage pulse trains, are stored on floppy disks for subsequent display and analysis. The advantage of using this microcomputer-based integrated measurement system is the ability to capture both typical and random events or phenomena in EDM and also to extract and process information from the digitized waveforms which represent characteristics of these events.In this investigation, samples of power and voltage pulse characteristics during four EDM tests have been recorded, analysed for unit and time-averaged variations and meticulously studied with the aid of the microcomputer-based system. Power and voltage waveforms have been reconstructed from the digital data and are suitably displayed. Random and systematic or cyclic variations as shown by the analysed data, together with the corresponding reconstructed waveforms, are presented. Reasons for the variations are discussed in the light of similar observations verified by other researchers.This paper reports part of a continuing programme involving the use of microcomputer-based measurement systems for both on-line monitoring and off-line analysis of the EDM process.     

Position error compensation of robotic contour end-milling

Three-dimensional cutting force dynamics of end-milling is developed, and position error sources of robotic machining is studied. A strategy to compensate cutting force in force/motion control of robotic machining is established in order to achieve better positioning accuracy. Position error is analysed in the cutting process through reference points by using force sensor. The tip position error estimated with the aid of wrist-force sensor is compensated in feedforward control form. The quantitative results are obtained from the digital computer simulations of the dynamic model of the studied system.     

A displacement measurement approach for machine geometric error assessment

It is complicated and time-consuming to evaluate the performance of a machine tool. In this research, a displacement method is proposed to shorten the measurement time and to simplify the measurement. By measuring the positioning errors along the 15 lines in the machine work zone, a total of 21 geometric error components can be determined. Among the 15 lines, seven of them are mandated by the ANSI/ASME standard for the performance evaluation of CNC machining centers. Therefore, only eight additional positioning error measurements are necessary to evaluate the machine's performance. The results from the experimental tests show that the method is feasible and accurate. This method shortens the calibration time and is beneficial, particularly to the reconfigurable machining system, which needs frequent calibration.     

基于改进模糊聚类和最大信息系数的数控机床温度测点选取北大核心

在加工过程中,机床会因热变形而产生误差,这将严重影响加工精度。减少加工过程的热误差是提高加工精度的有效途径,而确定关键温度测点不仅能提高计算效率,还可避免温度数据间复共线性问题,提高热误差模型的预测精度。提出基于改进模糊聚类和最大信息系数(MIC)的温度测点选择方法,通过改进模糊聚类对温度测点进行分类;根据MIC方法选择每类温度数据中的关键温度测点;使用BP神经网络对热误差进行建模。结果表明:与传统温度测点选择方法相比,利用所提方法改进的热误差模型精度更高。     

Design for manufacturing the elastic pivots with special reference to manufacturing error

The stiffness of an elastic pivot, a kind of torsional hinge, must usually be small in the direction of axial rotation and high in all other directions. In some applications, such as the gimbal suspension of the tuned gyroscope in an inertial guidance system, the size of the elastic pivot is small, and consequently the pivot becomes very sensitive to machining error. In this paper, we model the machining error in terms of the position error and roll-pitch-yaw angle error of the electrode in the electric discharge machining process. In the corresponding design problem, we recommend design procedures that allow a compromise between product performance and manufacturability. This paper introduces different types of elastic pivots, analyzes machining error due to machining inaccuracy, and treats the shape optimization. We believe that the design-for-manufacturability methodology developed here will be helpful in the design of many other precision mechanical components.     

面向汽车匹配主模型检具的大型薄壁件制造工艺

汽车匹配主模型检具中许多部件为大型薄壁件,技术性能高、制造精度高、加工难度大,针对这些问题,结合实际部件,从汽车主模型检具的材料选择、加工设备选择和工艺路线选择等方面,详细介绍加工中的难点及采取的措施。     

Forecasting compensatory control (FCC) of machining flatness

Forecasting Compensatory Control (FCC) was implemented to improve the flatness of three dimensional prismatic workpieces in an end-milling operation. The basic logic of the strategy is based on inprocess error measurement, stochastic modelling, and prediction and compensation of the cutting errors.The proposed FCC system consisted of a newly developed laser-based in-process flatness error measurement system, a hydraulic positioning servo system, and a digital microcomputer control system. Through the application of the FCC system to on-line cutting experiments, the workpiece flatness error was reduced by up to 80% during machining.     

基于模糊故障树的数控龙门铣床加工误差分析与试验

针对数控龙门铣床加工中出现的平面度误差问题进行研究与改进,分析工艺系统因素产生的误差,建立加工平面误差的模糊故障树,通过下行法对其进行简化分析,定性分析影响加工平面度误差的最小割集,运用模糊隶属函数定量分析顶事件的发生概率及底事件的发生概率重要度,确定主要因素为工作台形位误差、定位元件误差、热变形等。使用激光测量技术对安装的工作台平面进行矩形布点测量,根据采集数据建立基于最小二乘法的数学模型,运用Matlab软件定性分析平面度的评定,并对超差区域进行改进,最终实现了机床装配与使用要求。