Double ballbar test for the rotary axes of five-axis CNC machine tools

In this paper a new method that uses the double ballbar to inspect motion errors of the rotary axes of five-axis CNC machine tools is presented. The new method uses a particular circular test path that only causes the two rotary axes to move simultaneously and keeps the other three linear axes stationary. Therefore, only motion errors of the two rotary axes will be measured during the ballbar test. The theoretical trace patterns of various error origins, including servo mismatch and backlash, are established. Consequently, the error origins in the rotary block can be diagnosed by examining whether similar patterns appear in the motion error trace. The method developed was verified by practical tests, and the servo mismatch of the rotary axes was successfully detected.     

Feed optimization for five-axis CNC machine tools with drive constraints

Real time control of five-axis machine tools requires smooth generation of feed, acceleration and jerk in CNC systems without violating the physical limits of the drives. This paper presents a feed scheduling algorithm for CNC systems to minimize the machining time for five-axis contour machining of sculptured surfaces. The variation of the feed along the five-axis tool-path is expressed in a cubic B-spline form. The velocity, acceleration and jerk limits of the five axes are considered in finding the most optimal feed along the tool-path in order to ensure smooth and linear operation of the servo drives with minimal tracking error. The time optimal feed motion is obtained by iteratively modulating the feed control points of the B-spline to maximize the feed along the tool-path without violating the programmed feed and the drives’ physical limits. Long tool-paths are handled efficiently by applying a moving window technique. The improvement in the productivity and linear operation of the five drives is demonstrated with five-axis simulations and experiments on a CNC machine tool.     

基于球杆仪的数控机床误差识别与补偿

论述了数控机床几何误差的球杆仪识别及软件补偿技术。提出了从Renishaw球杆仪测量数控机床的联动误差数据中识别反向间隙、直线度、垂直度、定位误差的一种方法;建立了机床结构的每个误差元和切削刀具相对工件位置误差相联系的通用数学模型;用球杆仪在数控机床上进行补偿前后加工轨迹的测量实验表明该方法效率高、效果显著。     

Smooth trajectory generation for five-axis machine tools

This paper presents a smooth spline interpolation technique for five-axis machining of sculptured surfaces. The tool tip and orientation locations generated by the CAM system are first fitted to quintic splines independently to achieve geometric jerk continuity while decoupling the relative changes in position and orientation of the cutter along the curved path. The non-linear relationship between spline parameters and displacements along the path is approximated with ninth order and seventh order feed correction splines for position and orientation, respectively. The high order feed correction splines allow minimum deviation from the reference axis commands while preserving continuous jerk on three translational and two rotary drives. The proposed method has been experimentally demonstrated to show improvements in reducing the excitation of inertial vibrations while improving tracking accuracy in five-axis machining of curved paths found in dies, molds and aerospace parts.     

Mapping the effects of positioning errors on the volumetric accuracy of five-axis CNC machine tools

Fixe-axis capabilities on machine tools are becoming increasingly common. However, the problem of modeling the propagation of errors of individual axes through the kinematic chain, and their effect on the position and orientation errors of the cutting tool in the machine's work space has not been addressed. To increase the accuracy capabilities of such machines it is crucial to be able to study this propagation and develop approaches to minimize their effects on the errors at the tool tip. This paper discusses an approach to model the effects of the positioning errors of a machine's axes on the accuracy (positioning and orientation) of the cutting tool in its work space. Computer programs are developed for implementing the models and generating error contours or maps showing the variation of the different components of a machine's volumetric errors in its work space. This is a useful tool that can be used in machine tool design for the budgeting of errors and for optimization of a machine's accuracy.     

一种三维五轴数控激光切割机

论述了一种三维五轴数控光纤激光切割机,该设备配置有激光头旋转装置,可以对三维金属板进行高速切割加工,制成任何所需轮廓的产品,具有双工位切换加工功能,是三维复杂结构钣金件柔性加工设备。     

A new compensation method for geometry errors of five-axis machine tools

The present study aims to establish a new compensation method for geometry errors of five-axis machine tools. In the kinematic coordinate translation of five-axis machine tools, the tool orientation is determined by the motion position of machine rotation axes, whereas the tool tip position is determined by both machine linear axes and rotation axes together. Furthermore, as a nonlinear relationship exists between the workpiece coordinates and the machine axes coordinates, errors in the workpiece coordinate system are not directly related to those of the machine axes coordinate system. Consequently, the present study develops a new compensation method, the decouple method, for geometry errors of five-axis machine tools. The method proposed is based on a model that considers the tool orientation error only related to motion of machine rotation axes, and it further calculates the error compensations for rotation axes and linear axes separately, in contrast to the conventional method of calculating them simultaneously, i.e. determines the compensation of machine rotation axes first, and then calculates the compensation associated with the machine linear axes. Finally, the compensation mechanism is applied in the postprocessor of a CAM system and the effectiveness of error compensation is evaluated in real machine cutting using compensated NC code. In comparison with previous methods, the present compensation method has attributes of being simple, straightforward and without any singularity point in the model. The results indicate that the accuracy of positioning was improved by a factor of 8–10. Hence, the new compensation mechanism proposed in this study can effectively compensate geometry errors of five-axis machine tools.     

A new sampled-data drive for CNC machine tools

A mathematical analysis of a sampled-data servo-drive system controlled by the step control algorithm for computer numerical control (CNC) is presented. The step control algorithm provides for rapid and precise positioning and is ready for practical implementation. Its performance, including stability, damping ratio, maximum overshooting, steady-state error for both the step and ramp input, and bandwidth for accurate circular motion, is studied and depicted in design charts. The design procedure proceeds by using the design charts to choose the weighting factor and the sampling period to satisfy the specifications of the system. The effectiveness of the proposed control algorithm is demonstrated by simulation and experiment.     

数控机床的PLC编程方法

PLC是数控机床系统与机床主体之间连接的关键中间环节,本文以西门子802系列数控系统为例,分析了数控机床中PLC的控制功能以及接口信号和地址的分配,通过实例介绍了数控机床的PLC编程方法。     

Dynamic calibration of CNC machine tools

Machine calibration has become an important tool for assessing and maintaining machine accuracy, and for providing a measure of production quality. The calibration techniques supported by the calibration standards and the available metrology technology use static measurement cycles [BS3800, Part 2 (1991); ISO 230-2, Part 2 (1988)]. Static calibration cycles can be time-consuming to perform, and with coarse step sizes cannot give a complete picture of the machine performance. Recent advances in the laser interferometer technology used for machine calibration allow data to be captured dynamically. Dynamic data-capture technology provides the potential for dynamic machine calibration. Dynamic calibration overcomes the inherent problems of static calibration, being quick to perform and providing detailed information on machine performance. This paper describes the concept of dynamic machine calibration. In particular a novel dynamic calibration technique, conceived at the University of Huddersfield, is described. This technique utilizes existing calibration technology. Example dynamic calibrations, that highlight the potential of the technique, are presented and discussed.     

DOG-I型并联数控机床加减速控制

针对五轴并联数控机床的特点,文章在已有的研究基础上提出一种加减速控制方法.首先,在程序段的插补周期内均分其刀具姿态改变量,解决了刀具姿态变化问题;其次,根据并联机床的运动学特性,限制其刀具进给速度,并对伺服输入进行检查,通过细分进给量来控制伺服输入速度在一个合理的范围之内.这种加减速控制方法简单易行,具有较大的实用价值.     

NURBS-based fast geometric error compensation for CNC machine tools

In this paper, a novel method for the compensation of geometric errors of CNC machine tools is presented. The key idea is to use the basis functions of the setting NURBS path to approximate its error compensation function and to generate a new compensated NURBS path. In this way, both the setting and the compensated NURBS path have the same NURBS form. More importantly, the control points of the error compensation function can be obtained by simply calculating the positioning deviations of the control points of the setting NURBS path using the error model. A high compensation accuracy can be achieved through the systematic insertion of new knots, which creates new control points and raises the flexibility of NURBS in representing the error compensation function. The real-time interpolation of the compensated NURBS path completes the error compensation automatically. Simulations and experiments have shown that the new method delivers the same positioning accuracy as a model-based real-time geometric error compensation method does, but without additional real-time CPU loading. The proposed method can also be implemented in the post-processor of a CAM system for off-line compensation.     

Fast real-time NURBS path interpolation for CNC machine tools

In this paper, a novel fast real-time non-uniform rational B-spline (NURBS) path interpolation method is presented. This method efficiently integrates the data processing of a NURBS path in a CNC controller, from pre-processing to real-time interpolation. In the calculation of the total length of the NURBS path, the numerical adaptive quadrature method adapts to the integrand, i.e. the first derivative of the length function, automatically, dividing the parameter interval into subintervals with fine or coarse spacing according to the varying condition of the integrand. This new method takes full advantage of the subdivision scheme. The key point is to generate inverse length functions (ILF) for each resulting subinterval. In the real-time NURBS path interpolation, the new setting path parameter can be calculated directly using the ILF without the need for any time-consuming computation of NURBS derivatives and iteration. The proposed method is extremely fast, accurate and suitable for real-time implementation, and simulations and practical tests proved its effectiveness.     

基于PowerMILL的五轴数控机床仿真与应用

通过虚拟仿真技术,基于PowerMILL软件自带的机床仿真功能,以双转头五轴数控加工中心为例,介绍五轴机床仿真模型的建立和仿真机床参数设置,并以轴流风叶片进行数控刀路仿真、优化和试切加工,验证仿真系统的有效性,确保五轴数控机床的加工效率和可靠性。     

Configuration analysis of five-axis machine tools using a generic kinematic model

Five-axis machine tools are designed in a large variety of kinematic configurations and structures. Regardless of the type of the intended analysis, a kinematic model of the machine tool has to be developed in order to determine the translational and rotational joint movements required to achieve a specified position and orientation of the cutting tool relative to the workpiece. A generic and unified model is developed in this study as a viable alternative to the particular solutions that are only applicable to individual machine configurations. This versatile model is then used to verify the feasibility of the two rotational joints within the kinematic chain of three main types of five-axis machine tools: the spindle rotating, rotary table, and hybrid type. A numerical measure of total translational joint movement is proposed to evaluate the kinematic performance of a five-axis machine tool. The corresponding kinematic analyses have confirmed the advantages of the popular machine design that employs intersecting rotational axes and the common industrial practice during setup that minimizes the characteristic rotating arm length of the cutting tool and/or workpiece.     

利用PMC进行数控机床故障诊断和维修

PMC是数控机床的控制中心,机床的很多控制指令都要经过PMC处理后才能控制执行机构。通过对PMC的检查,利用PLC可以快速、准确地判断数控机床故障。     

Modal parameter identification for CNC machine tools using Wavelet Transform

The paper presents a new use of the Continuous Wavelet Transform for modal parameter identification applied to CNC machine tools. Firstly, the resonant frequencies and damping ratios of the CNC machine tool axis drive are estimated in the frequency domain using the transmissibility relation at resonance. The experimental Bode diagrams are determined using a novel measurement practice for the decoding of signals generated by a position encoder.This paper focuses on a novel application of the Continuous Wavelet Transform to identify the resonance frequencies and corresponding damping ratios of the CNC machine tool axis drive. The proposed method has the ability to detect variations in the amplitude levels of weak components embedded in strong noise and non-stationary processes. The superior ability of the Wavelet Transform to identify accurately modal parameters is demonstrated by comparing the results of the two different methods.     

多轴CNC机床耦合轮廓误差补偿方法

介绍了CNC机床耦合轮廓误差补偿方法的原理及设计方法。首先分析了非耦合的轮廓控制系统,给出了轮廓误差的定义及算法,引入了耦合轮廓控制的概念。以两轴CNC机床为例,介绍了耦合轮廓控制系统的结构及设计方法。最后讨论了耦合轮廓控制的效果及需要进一步研究的问题。     

Augmented Taylor's expansion method for B-spline curve interpolation for CNC machine tools

In computer numerical control (CNC) systems, parametric curves can be used instead of a large amount of linear blocks to describe tool paths for freeform surface or curve machining. However, existing parametric curve interpolation methods may cause large feed fluctuations or even a failure of the machining process near the sharp corners of a parametric curve. Therefore, a parametric curve interpolation method with an error correction and failure prevention scheme is required. In this paper, the augmented Taylor's expansion (ATE) method for computing B-spline curve parameters is proposed. A group of calibrators consisting of the knots and the arc lengths between adjacent knots are pre-computed before the interpolation starts. The parameter is computed based on Heun's method in a prediction–correction manner, and the accumulated errors caused by the cut-off errors of Taylor's expansion are eliminated by the calibrators at the knots. To cope with the extreme cases that usually occur near the sharp corners of a curve, a linear parametric interpolation between the previous parameter and its next calibrator is carried out when Heun's method fails to obtain a parameter in the domain. Simulation and experimental results show that, when the arc length increments are kept small enough near the sharp corners, the ATE method attains high accuracy and robust computation. The proposed method is also applicable to the NURBS curves.     

Circular tests for HSM machine tools: Bore machining application

Today's High-Speed Machining (HSM) machine tool combines productivity and part quality. The difficulty inherent in HSM operations lies in understanding the impact of machine tool behaviour on machining time and part quality. Analysis of some of the relevant ISO standards [230. Acceptance code for machine tools. Part 4, Circular tests for numerically controlled machine-tools, April 1998, 10791. Test conditions for machining centres. Part 6, Accuracy of feeds, speeds and interpolation, September 1998, 10791. Test conditions for machining centres. Part 7, Accuracy of feeds, speeds and interpolation, September 1998] and a complementary protocol for better understanding HSM technology are presented in the first part of this paper. These ISO standards are devoted to the procedures implemented in order to study the behaviour of machine tool. As these procedures do not integrate HSM technology, the need for HSM machine tool tests becomes critical to improving the trade-off between machining time and part quality. A new protocol for analysing the HSM technology impact during circular interpolation is presented in the second part of the paper. This protocol, which allows evaluating kinematic machine tool behaviour during circular interpolation, was designed from tests without machining. These tests are discussed and their results analysed in the paper. During the circular interpolation, axis capacities (such as acceleration or Jerk) related to certain setting parameters of the numerical control unit have a significant impact on the value of the feed rate. Consequently, a kinematic model for a circular-interpolated trajectory was developed on the basis of these parameters. Moreover, the link between part accuracy and kinematic machine tool behaviour was established. The kinematic model was ultimately validated on a bore machining simulation.