Accuracy enhancement of five-axis CNC machines through real-time error compensation

Although error modeling and compensation have given significant results for three-axis CNC machine tools, a few barriers have prevented this promising technique from being applied in five-axis CNC machine tools. One crucial barrier is the difficulty of measuring or identifying link errors in the rotary block of five-axis CNC machine tools. The error model is thus not fully known. To overcome this, the 3D probe-ball and spherical test method are successfully developed to measure and estimate these unknown link errors. Based on the identified error model, real-time error compensation methods for the five-axis CNC machine tool are investigated. The proposed model-based error compensation method is simple enough to implement in real time. Problems associated with the error compensation in singular position of the five-axis machine tool are also discussed. Experimental results show that the overall position accuracy of the five-axis CNC machine tool can be improved dramatically.     

Accuracy test of five-axis CNC machine tool with 3D probe-ball. Part II: errors estimation

To improve the accuracy of CNC machine tools, error sources and its effects on the overall position and orientation errors must be known. Most motional errors in the error model of five-axis machine tool can be measured with modern laser interferometer devices, but there are still some not measurable geometric errors. These not measurable errors include constant, inaccurate link errors of components such as rotary axes block, main spindle block and tool holder. After setting all measured errors in the error model, a reduced error model is defined, which describes the influence of each unknown and not measurable link error on the overall position errors of the five-axis machine tool. On the other hand, the newly developed probe-ball device can measure the overall position errors of five-axis machine tools directly. Based on the reduced model and the overall position errors, the link errors can be estimated very accurately with the least square estimation method. The error model is then fully known and can be used for advanced purposes such as error prediction and compensation.     

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.     

A method of testing position independent geometric errors in rotary axes of a five-axis machine tool using a double ball bar

Ensuring that a five-axis machine tool is operating within tolerance is critical. However, there are few simple and fast methods to identify whether the machine is in a “usable” condition. This paper investigates the use of the double ball bar (DBB) to identify and characterise the position independent geometric errors (PIGEs) in rotary axes of a five-axis machine tool by establishing new testing paths. The proposed method consists of four tests for two rotary axes; the A-axis tests with and without an extension bar and the C-axis tests with and without an extension bar. For the tests without an extension bar, position errors embedded in the A- and C-axes are measured first. Then these position errors can be used in the tests with an extension bar, to obtain the orientation errors in the A- and C-axes based on the given geometric model. All tests are performed with only one axis moving, thus simplifying the error analysis. The proposed method is implemented on a Hermle C600U five-axis machine tool to validate the approach. The results of the DBB tests show that the new method is a good approach to obtaining the geometric errors in rotary axes, thus can be applied to practical use in assembling processes, maintenance and regular checking of multi-axis CNC machine tools.     

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.     

Accuracy improvement of miniaturized machine tool: Geometric error modeling and compensation

A novel capacitance–sensor based multi-degree-of-freedom (DOF) measurement system has been developed for measuring geometric errors of a miniaturized machine tool (mMT) overcoming the size limitations. In the present work five geometric error components of a three-axis mMT are measured simultaneously along each axis and the squareness errors are determined by the slopes of straightness error profiles. Least-squares fitting method is used to represent the analytical models of geometric errors. A kinematic chain consisting of various structural members of mMT is introduced to establish the positional relationships among its coordinate frames. Based on this kinematic chain a general volumetric error model has been developed to synthesize all geometric error components of a miniaturized machine tool. Then, a recursive compensation method is proposed to achieve error compensation efficiently. Test results show that the positioning accuracy of miniaturized machine tool has been improved with compensation.     

Identification and compensation of geometric errors of rotary axes on five-axis machine by on-machine measurement

The geometric errors of rotary axes are the fundamental errors of a five-axis machine tool. They directly affect the machining accuracy, and require periodical measurement, identification and compensation. In this paper, a precise calibration and compensation method for the geometric errors of rotary axes on a five-axis machine tool is proposed. The automated measurement is realized by using an on-the-machine touch-trigger technology and an artifact. A calibration algorithm is proposed to calibrate geometric errors of rotary axes based on the relative displacement of the measured reference point. The geometric errors are individually separated and the coupling effect of the geometric errors of two rotary axes can be avoided. The geometry error of the artifact as well as its setup error has little influence on geometric error calibration results. Then a geometric error compensation algorithm is developed by modifying the numeric control (NC) source file. All the geometric errors of the rotary errors are compensated to improve the machining accuracy. The algorithm can be conveniently integrated into the post process. At last, an experiment on a five-axis machine tool with table A-axis and head B-axis structure validates the feasibility of the proposed method.     

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。     

Measurement and verification of position-independent geometric errors of a five-axis machine tool using a double ball-bar

In this study, position-independent geometric errors, including offset errors and squareness errors of rotary axes of a five-axis machine tool are measured using a double ball-bar and are verified through compensation. In addition, standard uncertainties of measurement results are calculated to establish their confidence intervals. This requires two measurement paths for each rotary axis, which are involving control of single rotary axis during measurement. So, the measurement paths simplify the measurement process, and reduce measurement cost including less operator effort and measurement time. Set-up errors, which are inevitable during the installation of the balls, are modeled as constants. Their effects on the measurement results are investigated to improve the accuracy of the measurement result. A novel fixture consisting of flexure hinges and two pairs of bolts is used to minimize set-up error by adjusting the ball's position located at the tool nose. Simulation is performed to check the validation of measurement and to analyze the standard uncertainties of the measurement results. Finally, the position-independent geometric errors of the five-axis machine tool (involving a rotary axis and a trunnion axis) are measured using proposed method.     

Accuracy enhancement of five-axis machine tool based on differential motion matrix: Geometric error modeling,identification and compensation

This paper presents the precision enhancement of five-axis machine tools according to differential motion matrix, including geometric error modeling, identification and compensation. Differential motion matrix describes the relationship between transforming differential changes of coordinate frames. Firstly, differential motion matrix of each axis relative to tool is established based on homogenous transformation matrix of tool relative to each axis. Secondly, the influences of errors of each axis on accuracy of tool are calculated with error vector of each axis. The sum of these influences is integration of error components of machine tool in coordinate system of tool. It endows the error modeling clear physical meaning. Moreover, integrated error components are transformed to coordinate frame of working table for integrated error transformation matrix of machine tools. Thirdly, constructed Jacobian is established using differential motion matrix of each axis without extra calculation to compensate the integrated error components of tool. It makes compensation easy and convenient with reuse of intermediate. Fourthly, six-circle method of ballbar is developed based on differential motion matrix to identify all ten error components of each rotary axis. Finally, the experiments are carried out on SmartCNC500 five-axis machine tool to testify the effectiveness of proposed accuracy enhancement with differential motion matrix.     

三维动态模拟在机床设计中的应用

主要对三维动态模拟在机床设计制造中应用的可行性及其意义进行探讨，通过介绍SolidWorks及IPA软件在机床设计阶段进行虚拟设计制造的应用方法，并说明CAD与三维动态模拟结合可以直观，有效地描述产品设计及试制过程，有利于制造部门降低风险和成本，缩短产品制造周期，从而提高企业的市场竞争力。     

三维数控气动左右弯管机的开发设计

三维数控气动左右弯管机以伺服电机为动力源,气缸行程磁性开关为检测器件,摆臂、主轴和气缸为执行器件,对管件施加夹紧力和抽芯力;利用伺服电机驱动的送料小车,可进行不同长度送料,既能进行平面弯管,又能进行三维空间弯管;在同一台机器上,使用一个机头、一个摆臂、一个靠模部件的情况下,通过对靠模机构的巧妙设计,对摆臂部件的灵活运用,使其既能进行左弯,又能进行右弯。与其他数控左右弯管机相比较,该机结构比较简单,性价比高,设备的柔性和自动化程度高。     

Identification of strut and assembly errors of a 3-PRS serial–parallel machine tool

In a serial–parallel type machine tool, the parallel spindle platform plays the key role in manipulating three directions of movement. Spatial symmetry of the 3-PRS loops is essential to the machine’s systematic accuracy. Currently, however, there is no effective instrument capable of measuring the symmetrical errors of the corresponding joints and strut lengths during structure assembly. In this study, an experimental method is proposed to identify the mechanism symmetric errors of a 3-PRS serial–parallel machine tool during the test run. It is based on the differentiation of the inverse kinematics equations. The mechanism errors could be derived by an identification model. With the aid of a developed 3D laser ball bar to detect the spatial position and orientation of the spindle platform, and three laser Doppler scales to measure three sliders’ positions simultaneously, the length errors of three struts and the symmetrical errors of the R-joints and S-joints can be identified by the optimization technique. This technique can help shop floor engineers to tune the symmetrical errors of the 3-PRS mechanism during machine assembly.     

A new method and device for motion accuracy measurement of NC machine tools. Part 2: device error identification and trajectory measurement of general planar motions

This paper describes in two parts a new method and device for measuring motion accuracy of NC machine tools. In the first part, the measurement principle and the characteristics of the prototype device have been presented and discussed. In the second part, an efficient and practical approach to identifying the errors of the proposed device after assembly is developed and evaluated. The approach ensures realising the aim of the investigation, i.e. to measure the most items of the motion accuracy, especially, to measure and assess the trajectory accuracy of a general planar motion of NC machine tools. The result of the identification experiment by using the prototype device on a machining centre for the prototype device is presented and it well verifies the validity and practicality of the approach. Some measurement results for the general planar motions of the machining centre are also shown, which sufficiently demonstrate the desirable capability of the proposed method and device.     

Analytical modeling of spindle–tool dynamics on machine tools using Timoshenko beam model and receptance coupling for the prediction of tool point FRF

Regenerative chatter is a well-known machining problem that results in unstable cutting process, poor surface quality and reduced material removal rate. This undesired self-excited vibration problem is one of the main obstacles in utilizing the total capacity of a machine tool in production. In order to obtain a chatter-free process on a machining center, stability diagrams can be used. Numerically or analytically, constructing the stability lobe diagram for a certain spindle–holder–tool combination implies knowing the system dynamics at the tool tip; i.e., the point frequency response function (FRF) that relates the dynamic displacement and force at that point. This study presents an analytical method that uses Timoshenko beam theory for calculating the tool point FRF of a given combination by using the receptance coupling and structural modification methods. The objective of the study is two fold. Firstly, it is aimed to develop a reliable mathematical model to predict tool point FRF in a machining center so that chatter stability analysis can be done, and secondly to make use of this model in studying the effects of individual bearing and contact parameters on tool point FRF so that better approaches can be found in predicting contact parameters from experimental measurements. The model can also be used to study the effects of several spindle, holder and tool parameters on chatter stability. In this paper, the mathematical model, as well as the details of obtaining the system component (spindle, holder and tool) dynamics and coupling them to obtain the tool point FRF are given. The model suggested is verified by comparing the natural frequencies of an example spindle–holder–tool assembly obtained from the model with those obtained from a finite element software.     

A new method and device for motion accuracy measurement of NC machine tools. Part 1: principle and equipment

This paper describes in two parts a new method and device for measuring motion accuracy of numerical control (NC) machine tools. In the first part, the measurement principle and the characteristics of the prototype device are proposed. The device consists of a double-bar linkage with two rotary encoders. The working range of the device is disc-shaped with a radius of almost the double the link length, except a small area around the centre of the disc and an outer area both around the change points of the linkage at the centre and the circumference of the disc. Because the method has high resolution for any measuring point within the working range, it can be applied to measure most items of motion accuracy of NC machine tools. The method is particularly suitable to measure the trajectory accuracy of circular motions. The device has a compact structure and its installation on a machine tool to be measured is simple and quick. The experimental results show that the prototype device has very good response to small displacement and good repeatability with high precision to the measurement of circular motion trajectories. The influence of measurement noise is hardly observed in the experimental results.     

Phase stability and fcc/hcp martensitic transformation in Fe–Mn–Si alloys: Part II. Thermodynamic modelling of the driving forces and the MS and AS temperatures

This paper presents the second part of a study of the fcc/hcp relative phase stability and the fcc↔hcp martensitic transformation (MT) in the Fe–Mn–Si system. In part I, an experimental database was built up using dilatometric measurements, which covers the composition range in which the fcc↔hcp MT is detected. This new information is analysed in the present work using models for the molar Gibbs energy (G_m) of the various phases. Hcp is a metastable phase in the system, but we show that its properties can be inferred from selected pieces of experimental data, using the concept of T₀ temperature. The assessed G_m functions of the ternary fcc and hcp phases are used to evaluate the so-called resistance-to-start-the-transformation energy (RSTE). According to our results the RSTE in this system vary smoothly with composition, and we account phenomenologically for that using low-order polynomials in the atomic fractions. Finally, the optimum G_m and RSTE functions are also used to calculate the M_S and A_S temperatures for arbitrary compositions in the ternary system. The extensive comparisons between calculations and measurements presented in this work show a very good agreement, which adds to the credibility of the present approach.     

Bandsawing. Part I: cutting force model including effects of positional errors, tool dynamics and wear

This article presents a mechanical cutting force model for bandsawing. The model describes the variation in cutting force between individual teeth and relates it to initial positional errors, tool dynamics and edge wear. Bandsawing is a multi-tooth cutting process, and the terminology of the cutting action is discussed and compared with other cutting processes. It will also be shown that the setting pattern and the preset feed govern the cutting data.     

Influence of the borohydride concentration on the composition of the amorphous Fe–B alloy produced by chemical reduction of synthetic, nano-sized iron-oxide particles: Part I: Hematite

Amorphous Fe–B alloys can be prepared at room temperature by reduction with borohydride of iron-oxide particles in suspension. By varying the borohydride concentration, amorphous Fe–B alloys with boron contents between 2 and 13 at.% have been produced by reduction of synthetic (nano-sized particles) and natural (micro-sized) hematite (α-Fe₂O₃) using sodium borohydride (NaBH₄). The results presented in this paper were obtained from a systematic study of the effect of borohydride concentration on the resulting reaction products using a variety of experimental techniques, such as X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mössbauer spectroscopy (TMS) and integral low-energy electron Mössbauer spectroscopy (ILEEMS). Three distinct NaBH₄ concentrations have been applied. Beside unreacted hematite, amorphous Fe_1−xB_x alloys have been identified from the TMS spectra recorded at various temperatures between 15 K and room temperature. The amount of Fe_1−xB_x increases strongly with increasing NaBH₄ concentration, and for a given concentration with increasing specific surface area (SSA). Thermal analyses have suggested that for any given reduction condition, the boron content x in the formed amorphous alloy has a bimodal distribution. This is found to be consistent with the finding that the contribution of the Fe_1−xB_x phase to the total Mössbauer spectra consists of a superposition of a broad sextet and doublet. ILEEMS has further revealed that especially the surface layers of the hematite grains are affected by the reduction processes.