Strategies and Processes to Measure the Dimensional Geometry of Sheet Metal Parts for Exact Laser Cutting

This paper describes strategies and processes to measure the dimensional geometry and deformations of sheet metal parts, in order to improve accuracy in laser cutting. A robust circular triangulation laser probe was integrated into a CNC machining centre. For the development of the measuring system a non-contact laser sensor and a personal computer (PC) were integrated information-technically with the NC-controller of the machining centre, as introduced in this study. To meet the demands of the measuring tasks in laser machining, the meas-uring uncertainty of the selected laser probe with respect to different measuring procedures, such as the fixed height procedure, surface tracing procedure, and profile-orientated procedure for steel and plastic materials, was investigated in this study. When measuring uncertainty is considered, the profile-orientated procedure best meets the accuracy requirements. According to the experimental results, the uncertainty could be minimised to within ±20 μm in the middle measuring range. To shorten the measuring time, a twofold measuring strategy was developed. The first step checks for existing deviations quickly. If necessary, in the case of deviations of the actual machining path from the theoretical machining path, these can be quantified in the second step. A nine-point measuring strategy was proposed in this study in order to calculate the surface normal of the deformed sheet metal. An algorithm for calculating the path correction using these deviations and the calculated surface normal with respect to the coordinates and the surface normal of theoretical path position, was also presented. The algorithms were verified using the developed automatic measuring system on a 4-axis machining centre with satisfactory results. One specific characteristic of the strategy presented is that the machining path can be verified for the case of laser cutting for workpieces without edges or marks.     

Effect of slice thickness on the profile accuracy of model maker rapid prototyping measured by a circular triangulation laser probe

The objective of this research is to investigate the effect of slice thickness on the profile accuracy of the model maker (MM) rapid prototyping (RP) system, layer by layer, through non-contact laser probe measurement. A circular triangulation laser probe, model OTM-3A20, made by Wolf & Beck Co., was mounted on a coordinate measuring machine (CMM), as the non-contact sensor. An adjustment device for the laser probe was designed to minimise the cosine error caused by assembly inaccuracy. The alignment test of the measuring laser beam was carried out using a calibrated specimen. The systematic accuracy of the circular triangulation laser probe with respect to the surface roughness and the surface slope of the RP workpiece was investigated using a HP5529A laser interferometer system. The maximum error of 21/2D RP part profile accuracy can be improved from 220 μm to 131 μm, and the average error can be improved from 78 μm to 46 μm as the slice thickness changed from 0.127 mm (0.005 in.) to 0.0127 μm (0.0005 in). However, the machining time increases by about seven fold based on the experimental results. An overall error of 197 μm as measured by the laser probe is attainable using the finest slice thickness 0.0127 mm (0.0005 in.) for the 3D profile accuracy. To verify the accuracy of non-contact laser probe measurement, the 3D profile of the RP part was also measured by a CNC CMM, with good consistency.     

Improving the machine accuracy through machine tool metrology and error correction

Improving both the positioning accuracy and contouring accuracy of a vertical machining centre has been studied by using a machine tool metrology and in-house error correction techniques. Contouring errors caused by the servo lag and friction of servomechanisms were measured by the circular test and then reduced by off-line parameter tuning of the CNC and servo-driver. The quasistatic thermal errors were predicted online using a neural network based model which was calibrated in advance via a quick set-up and multiple-error measurement system consisting of a spindle-mounted probe and artifacts. Positioning errors caused by both the static geometric errors and thermal effects were eliminated in real-time by a PC based software error compensation scheme integrated with the CNC controller through digital communication. An error reduction of 70% was achieved after error compensation and CNC tuning.     

Neural network-based modelling and error compensation of thermally-induced spindle errors

This research is concerned with enhancing the accuracy of a machining centre by compensating for thermally induced spindle errors in real-time. A neural network model was developed for on-line thermal error monitoring. A PC-based error compensation scheme was also developed to upgrade a commercial CNC controller for real-time thermal error compensation without any hardware modifications to the machine. The spindle thermal errors of a vertical machining centre were reduced by 70% after compensation.     

Real-Time Prediction of Workpiece Errors for a CNC Turning Centre, Part 1. Measurement and Identification

This paper analyses the error sources of the workpiece in bar turning, which mainly derive from the geometric error of machine tools, i.e. the thermally induced error, the error arising from machine–workpiece–tool system deflection induced by the cutting forces. A simple and low-cost compact measuring system combining a fine touch sensor and Q-setter of machine tools (FTS FQ) is developed, and applied to measure the workpiece dimensions. An identification method for workpiece errors is also presented. The workpiece errors which are composed of the geometric error, thermal error, and cutting force error can be identified according to the measurement results of each step. The model of the geometric error of a two-axis CNC turning centre is established rapidly based on the measurement results by using an FTSFQ setter and coordinate measuring machine (CMM). Experimental results show that the geometric error can be compensated by modified NC commands in bar turning.     

NON-CONTACT MEASUREMENT OF SCULPTURED SURFACE OF ROTATION

A method for measuring the sculptured surface of rotation by using coordinate measuring machine (CMM) and rotary table is proposed. The measurement is realized during the continuous rotation of the workpiece mounted on the rotary table while the probe moves along the generatrix of the surface step by step. This method possesses lots of advantages such as simplicity of probe motion, high reliability and efficiency. Some key techniques including calibration of the effective radius of the probing system, determination of the position of axis of rotation, auto-centering of the workpiece, data processing algorithm, are discussed. Approaches for determining the coordinates on measured surface, establishing workpiece coordinate system and surface fitting are presented in detail. The method can be used with contact or non-contact probes. Some fragile ceramic and plaster parts are measured by using the system consisting of a CMM, rotary table, motorized head and non-contact laser triangulation probe. The measurin     

激光在机测量系统的实现

为了能在加工航空发动机关键零部件(如叶片)等复杂曲面零件的过程中实现快速在机测量,研制了非接触式激光在机测量系统。分别介绍了测量系统的工作原理,机械结构和电控系统。该系统主要由激光测头、无线传输电路、可充电锂电池、转接基座、刀柄和外壳等部分组成。为了实现机床的加工模式与测量模式之间的快速切换,其采用刀柄式的安装方式,从加工叶片切换到在机测量时,机床只要运行换刀程序,即可实现叶片加工到叶片测量的转换。此外文中还针对在机测量系统的电控部分研制了通过无线传输的数据采集系统。为了验证所研制的在机测量系统的实用性和有效性,在五轴叶片加工中心上进行了叶片截面测量实验,结果显示其测量精度为20μm,测量时间为10min。验证结果表明所研制的激光在机测量系统能够高效精确地完成叶片型面的测量任务。     

激光三角法扫描测头特性的研究

在分析了激光三角法位移测量原理的基础上,以ＩＢ－１２型激光三角法位移传感器为实例,研究了激光三角法位移传感器的使用特性,补偿非线性误差、倾斜角误差,提高了测量精度,使其能够作为非接触式扫描测头用在三坐标测量机上,完成空间自由曲面的非接触扫描测量。     

数控机床几何误差非接触圆轨迹检测方法研究

几何误差是评定数控机床精度的主要指标之一。本文提出一种基于圆测法,利用两台激光干涉仪及可控移动平台实现数控机床几何误差检测的新方法。介绍了该方法的基本原理,完成了测量系统的设计,并推导出单项误差分离的模型。使用该方法与球杆仪法分别在MCV-510数控加工中心上进行模拟测量。试验表明:两种方法测得数据的最大绝对误差是0.8μm,从而证明该方法的可行性和正确性。     

Optimal Control of a Multistage Machining Operation on a Computer Numerically Controlled Machine

A multistage machining operation ( MMO) model is proposed for a computer numerically controlled (CNC) machine to minimise the production cost of a deterministic order quantity under deadline constraint. Using the Lagrange method, the optimal machining time for each cutting tool is then achieved and the cost-related property of the Lagrange multiplier is also verified. In addition, a computer simulation of a numerical case using the MATLAB program is fully discussed, and the cost function of the machine for machining such operations with respect to the production deadline, is then obtained. Through this study, the project scheduling, production cost estimating, and even the contract negotiating of a deterministic quantity scheduled on the CNC machine can be further accomplished. This paper not only contributes an applicable operational strategy for a machine, but also provides a valuable approach for minimising the production cost for manufacturing engineers. ID="A1"Correspondance and offprint requests to: Chun-Hsiung Lan, 90-2,Nanya W. Road Sec. 2, Panchiao, Taiwan 220. E-mail: clan@mail-iem.tnit.edu.tw     

基于激光位移传感器的非接触三坐标测量机的设计

针对采用接触测头进行测量的三坐标测量机的测量效率低、不能对易变形物体进行测量等问题,设计了一种基于激光位移传感器的非接触三坐标测量系统,以PLC和工控机为主控,通过运动模组带动激光位移传感器定位,实现对工件表面特征点尺寸和数据的快速测量,且用户可通过触摸显示屏对设备进行操作.通过对待测工件进行实测表明,该设备能满足生产...     

数控机床误差的多项式预报与补偿

建立了数控机床空间误差预报的多项式模型，该模型把机床的空间误差表示为机床运动坐标的多项式函数；提出了用激光球杆仪直接测量机床空间误差的方法；由机床工作空间有限定位点误差的测量数据，利用最小二乘法来决定误差预报模型的系数。在XK713数控加工中心上进行了误差的多项式建模和补偿实验，结果表明本误差预报和补偿方法省时，效果显著。     

激光位移传感器在自由曲面测量中的应用

以点激光位移传感器(HL-C211BE)为对象,研究它在自由曲面测量中的应用。针对激光位移传感器因测点倾角代入的测量误差,提出了一个可以量化的倾角误差模型。基于直射式点激光三角法原理,分析了激光光路的几何关系,从会聚光斑光能质心发生的偏移推导出倾角误差模型。随后,用高精度激光干涉仪和正弦规对激光位移传感器进行校对实验,并用误差模型对测量结果进行补偿。结果显示,补偿后激光位移传感器的测量精度得到明显提高。对一非球面凸透镜进行了实验测量,得到了自由曲面测点倾角的计算方法,并用倾角误差模型修正了测量数据。实验结果表明,量化的倾角误差模型可以将激光位移传感器的测量误差控制到小于10μm,满足激光位移传感器在自由曲面测量中应用的要求。     

基于激光测距的三维激光切割加工法矢获取系统

为提高激光加工中示教录返编程的精度和效率,克服原有系统在结构上的限制和操作上的不足,采用借机测量系统的概念,提出了基于激光测距的三维激光加工示教编程系统,通过保证三维曲面上点的精确测量来实现其所需加工信息的测量,并对三维曲面上自由轨迹的示教编程进行了详细的探讨。     

A method for evaluating spindle rotation errors of machine tools using a laser interferometer

This paper presents a method for assessing radial and axial error motions of spindles. It uses the Hewlett Packard 5529A laser interferometer. The measurement is made using reflection directly from a high-precision sphere. Such object is used as the optical reflector. The sphere is affixed at the end of a wobble device, which is clamped in the spindle. The principle of measurement is similar to that of a linear interferometer, except that the high-precision sphere is used in place of the usual retroreflector. A convergent lens is utilized to focus the laser beam to a small spot on the sphere surface. This minimizes the dispersion of the beam due to the reflection on the spherical surface. A software package has been developed for data acquisition and presentation of the error motion polar plots of the spindle. Application of this spindle error calibrator on a CNC machining centre is undertaken. The results are presented and discussed.     

基于双NURBS参数混合插补算法的五轴联动交叉耦合控制器研究

针对零件复杂曲面高速高精度数控加工的需求,分析五轴联动数控加工系统刀具运动轨迹中的轮廓误差和方向误差,建立了该轮廓误差和方向误差的实时计算模型,从而得出系统跟踪误差计算模型;在此基础上设计出一种基于双NURBS参数混合插补算法的五轴联动数控系统交叉耦合控制器,并对其进行仿真,仿真结果表明,使用该交叉耦合控制器能有效提高五轴联动数控系统的零件加工精度。     

Premachining Computer Numerical Control Contour Validation

The current procedure followed to manufacture a new part by computer numerical control (CNC) machining is to write the part program, machine a test part, and measure the test part for conformance to the required dimensions and tolerances. If the test part dimensions are incorrect, the part program is modified, and the process is repeated until a successful part is machined. In many applications, such as the aerospace industry, where material cost and machining time are high, this iterative process becomes economically unacceptable. Research has been conducted to test the feasibility of using the laser ball bar (LBB), a spatial coordinate measuring device, to measure dynamic continuous path contours of CNC part programs to micrometer accuracy before machining. In this way, a virtual test part can be measured and compared to the design drawings to validate the CNC part program. This reduces or eliminates the costly and time-consuming steps involved in the machining of physical test parts. This paper outlines the testing method and results acquired using one LBB to measure dynamic part paths employing sequential trilateration. A circular contour was measured using an encoder trigger for data capture. The radial error motions of the spindle used to generate the circular contour were also measured using a capacitance probe nest to verify the LBB results. Comparable error waveforms between the LBB and cap probe measurements verified the possibility of using the LBB to measure dynamic continuous path contours. Future work using three LBBs simultaneously is also outlined.     

在机测量线激光传感器安装位姿的全局标定

针对三轴数控机床激光测头安装位姿误差造成测量误差且不易调整和校准的问题,提出了一种在机测量线激光传感器安装位姿标定方法。建立了线激光在机测量系统的数学模型,通过机床运动带动线激光测头对标定基准点的空间位置进行测量,基于手眼标定原理给出了关于测头安装位姿参数的线性求解算法,完成了对测头安装误差的全局标定。考虑了机床定位误差对于标定结果精度的影响,采用蒙特卡洛模拟进行了误差分析。采用半径为35 mm的圆孔进行测量验证,实验结果表明,标定后圆孔测量误差为0.051 6 mm,测量精度提高了约96%,实验结果验证了该标定方法的有效性和可行性。     

IDEF0 Model of the Measurement Planning for a Workpiece Machined by a Machining Centre

A comprehensive analysis of the entire measuring operation for a workpiece machined by a machining centre should be conducted systematically before the application of a planning technique. For this purpose, an IDEF0 model is used in this paper to plan the measuring sequence and operation of a coordinate measuring system for a workpiece machined by a machining centre. Generally speaking, a machining centre can machine workpieces having complicated shapes. Therefore, the planning of measurement procedures and operations on a workpiece machined by a machine centre requires consideration of issues such as the position and measuring sequence of the measurement points, how to avoid probe collision during measuring, which fixture elements are required for measuring fixtures, and where the support points of fixtures and position of fixture locations should be.     

在机测量激光测头位姿的线性标定

针对在机激光扫描测量中激光测头安装位置和姿态引起的测量误差,提出了一种适用于在机激光测量的测头标定方法。构造了在机激光扫描测量原型系统,建立了激光测头随机床运动的测量模型;通过多角度扫描标准球球面拟合球心,给出了一种线性求解测头安装位姿参数的算法,避免了非线性优化求解中的大量计算和不稳定问题。分析了测量过程中机床各个轴的运动误差对测量结果的影响,建立了误差模型,并给出补偿机床系统误差的方法。实验显示,对直径已知的标准球进行测量时,测头在不同摆角测得的标准球直径误差小于0.05 mm,误差补偿后球心位置误差减小了83%。实验结果验证了该标定方法的可行性,以及机床误差对测量精度影响的模型及补偿方法的正确性。