Double Ball Bar Research Articles

A methodology for error characterization and quantification in rotary joints of multi-axis machine tools

A novel methodology was developed and implemented for error characterization, evaluation, and its quantification in a rotary joint of multi-axis machine tools by using a calibrated double ball bar (DBB) system as a working standard. This methodology greatly simplified the measurement setup and accelerated the error quantification. Moreover, the methodology is observed to be highly economical by reducing the tooling and instrumentation. However, considerable time and little more efforts are required in measuring and computing the errors. The developed methodology is capable of evaluating the five degrees of freedom (DOF) errors including two radial errors, one axial error, and two tilt errors out of six DOF error components of rotary joints. A DBB system with point accessory was used for direct measurement by change of length at multi-points and various locations, whereas mathematical modeling technique was implemented, and equation solvers were exercised for error evaluation and its quantification. Validation and authentication of the methodology and results obtained were carried out by simulation process prior to implementing the methodology on rotary joints of a five-axis turbine blade grinding machine. The results obtained from “A” and “B” rotary joints of the turbine blade grinding machine were presented by applying cubic spline technique for error modeling and its further utilization. The methodology was found extremely pragmatic, quite simple, efficient, and easy to use for error characterization and its quantification in rotary joints of multi-axis machine tools.

Identification of Motion Error Sources on Five-axis Machine Tools by Ball-bar Measurements (1st Report)

The inclusion of the application of the double ball bar (DBB) measurement to the accuracy calibration of five-axis machine tools into the revision of ISO standards is currently under the discussion. This paper presents the modified DBB measurement device, referred to as “DBB5” in our study, where master balls are supported from the 45°direction to the spindle axis. It can perform all the circular tests on XY, YZ, and ZX planes without changing the setup. This paper first presents the classification of motion error components of a five-aixs machine tool into location and component errors. Experimental application examples of the DBB5 to the calibration of location and component errors assciated with rotary axes on a five-axis machine tool are then presented.

Kinematic calibration of a hexapod of simple design

To benefit from the advantages of parallel kinematic machines (PKM) in industrial applications an efficient kinematic calibration is of particular importance. In this paper a new approach for kinematic calibration of PKM is presented that has been developed and successfully tested on a hexapod at the IWM Dresden. The approach particularly respects the concept of simple design that aims at a cost-effective and economic over-all solution. Aspects of calibration that are in many cases not explicitly accounted for are considered. A simple and robust double-ball-bar (DBB) is used for data acquisition while continuously moving the platform on a specific trajectory in 6 degrees of freedom (dof). The measuring trajectory is optimized to ensure the identifiability of kinematic parameters, respecting their sensitivity and orthogonality in the workspace. Parameter identification is based on Genetic algorithms with customized genetic operators as well as real and simulated measurements. A significant rise of motion accuracy was obtained in the whole workspace.

Study on Checking and Measurement of Double-Ball Bar for Thermal Error of CNC Machine Tools

Thermal errors of CNC machine tools was checked and measured by double-ball bar in this paper. Mathematical model of measuring thermal error of machine tools by DBB on the principle of its basic measurement was derived. Combining it with kinematics model of machine tool, the identification law of error component parameters was established when thermal error was checked and measured by DBB. Thus, all of the thermal error component would be separated from the synthetical error. Finally, the entire analysis was provided to adjust machine tool and actualize error compensation of machine tool through grasping main factors influencing the machining precision of machine tool.

A17 DBBによる5軸マシニングセンタの幾何偏差測定における留意点(OS-1 最新工作機械(4))

To clarify the point to note in measuring geometrical deviation of five axis MC by simultaneously three axis movement using the double ball bar, we carry out following evaluation. 1. Reproducibility of measurement. 2. Reproducibility of alignment accuracy between the center of ball on the spindle side and center of C axis. 3. The influence of C-axis rotation to the alignment accuracy. 4. The influence of range of measurement angle on the calculated value of track center. As a result the notes to be considered in measuring are made clear. The factor of influence to the measurement precision is shown, and the note to high accuracy measurement is examined.

A12 テーパコーン加工動作の軌跡誤差を用いた5軸制御工作機械の幾何誤差同定方法(OS-1 最新工作機械(3))

Cone frustum machining with simultaneous five-axis control, which is prescribed in NAS979, is one of the most general method for the accuracy test of five-axis controlled machine tool. And instead of the machining, the measurement with double ball bar device has been proposed. In our previous work, we showed that the geometric errors between feed axes in five-axis controlled machine tool influence the accuracy of machined cone frustum, and it depends on the location, apex angle, tilt angle or diameter of the cone frustum. It indicates that all geometric errors might be identified with the profiles of cone frustum. In this paper, we suggest a method to identify the geometric errors in five-axis controlled machine tool with tilting table using the multiple motion trajectories of cone frustum machining measured by double ball bar device.

Non-Contact Measuring Modeling and Simulation of CNC Machine Tools under High Feed Rate Condition

This article sets forth a new precision measurement method of CNC machine tools. The method incorporates the laser interferometer and the measurement principle of the double ball bar (DBB), and can measure the machine tool precision under the high feed rate condition. The generalized measurement model of three-axis has been built. Data analysis software is developed to simulate the run condition of machine tools. The repeatable experiment results proves that the method is feasible and its precision is higher than DBB`s.

Development of a three-dimensional contouring measuring system and error compensation method for a CNC machine tool

The traditional method of dynamic performance inspection for a machine tool is to use a double ball bar (DBB) and grid encoder for contouring tests. With the increasing demand for miniaturization and fast machining in industry. DBB has an inadequate dynamic performance for inspection in conditions that have a small working range, high speed, 3D measurement, and so on. The present paper describes the development of a high-speed dynamic performance measurement system with position-sensitive detectors (PSDs). It consists of two PSDs and two laser diodes. After calibration and actual measurement, the system was verified to be competent for measuring the static and dynamic error of the linear axis in one, two, or three dimensions with a system resolution of 0.5 μm and a repeatability of 1 μm. The current paper focuses on measuring the machine tool controller performance in three-dimensional (3D) high-speed motion. When the machine tool moves with high-speed, machining errors can easily occur on the workpiece; 3D error compensation methods are put forward in this paper to solve this problem. The methods were verified as significantly decreasing the contouring error.

A New and High Precision Measurement Method of Spindle Rotation Induced Error and its Performance Research

The accuracy of spindle rotation is one of the most important precision indexes of CNC machine tools. And the spindle rotation induced error is one of the main influence factors of cutting accuracy. It will greatly affect the shape precision and surface roughness of workpiece. A new measurement method of the spindle radial rotation induced error is proposed using the cross grid encoder. The Spindle rotation induced error of machining center is analyzed in frequency domain and time-frequency domain separately to comparably study the measurement results by KGM 182 grid encoder and Double ball bar. The results indicate that the performance of the cross grid encoder test for measuring the CNC machine tool motion error is better than that of the Double ball bar, especially in the details of motion errors because of its unique characteristics. A wavelet packet algorithm is developed to analyze the singularity of signal. On machine measuring experiment has been carried out on a new Cincinnati 750 Arrow II vertical machining center. The measuring experiment result shows that the spindle radial rotation induced error of the machining center is about 0.6 μm and the cycle time is every rotation time.

Kinematics Calibration of Parallel Kinematic Machine Using a Developed Double-Ball Bar Gauge

In this paper a novel constraint calibration technique for Stewart platform based Parallel Kinematics Machine(PKM) is presented. A commercial trigger probe and a developed double-ball bar gauge are employed to aid the calibration. In the measuring process the characterized errors can be obtained by comparing the nominal characterized size calculated by the function using the measuring data with the real size of the double-ball gauge. An optimization approach is used to calculate the PKM structural errors, and these errors can be finally compensated into the control model. The error transferring matrix with regard to the structural error of PKM can be formulated. The calibration efficiency can be improved by the automatic calibration operation with the doubleball bar gauge. The developed calibration system has been applied in the industry where the PKM is used for machining the turbine vane, and the machining quality can meet the requirements of the specifications. The presented calibration scheme is generic to be applied for the wide range of PKMs.

Kinematic calibration of the parallel mechanism using double-ball-bar system

Since that it’s difficult to realize full closed-loop feedback control,kinematic calibration becomes a method to heavily gain economic value and effectively enhance the accuracy of parallel kinematic machine.In general,kinematic calibration consists of four steps,i.e.error modeling,measurement,identification and compensation.A double-ball-bar(DBB) based kinematic calibration of the 5-DOF reconfigurable hybrid robot named TriVariant is prespnted.The first order error mapping function is formulated to link the measured data to the geometric source errors affecting the compensatable pose accuracy.The issue to place a DBB mount is also investigated in order to achieve sufficient robustness of parameter identification.The effectiveness of the proposed approach is verified by both computer simulation and experiments.Some key issues for the further enhancement of the pose accuracy are also addressed by examining the experiment results.

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.

Motion error estimation of 5-axis machining center using DBB method

In order to estimate the motion errors of 5-axis machine center, the double ball bar (DBB) method is adopted to realize the diagnosis procedure. The motion error sources of rotary axes in 5-axis machining center comprise of the alignment error of rotary axes and the angular error due to various factors, e.g. the inclination of rotary axes. From sensitive viewpoints, each motion error is possible to have a particular sensitive direction in which deviation of DBB error trace arises from only some specific error sources. The model of the DBB error trace is established according to the spatial geometry theory. Accordingly, the sensitive direction of each motion error source is made clear through numerical simulation, which is used as the reference patterns for rotational error estimation. The estimation method is proposed to easily estimate the motion error sources of rotary axes in quantitative manner. To verify the proposed DBB method for rotational error estimation, the experimental tests are carried out on a 5-axis machining center M-400 (MORISEIKI). The effect of the mismatch of the DBB is also studied to guarantee the estimation accuracy. From the experimental data, it is noted that the proposed estimation method for 5-axis machining center is feasible and effective.

Assessment of machine tool trunnion axis motion error, using magnetic double ball bar

A method is proposed in this paper to assess the axis motion errors of a trunnion-type A-axis using the magnetic double ball bar (DBB) as the measuring instrument. The proposed method consists of five DBB tests with a single setup for all of the tests and the exclusive motion of the trunnion axis during data acquisition. The single setup helps to reduce non-productive time by limiting the intervention of the operator within the machine workspace whereas the exclusive trunnion axis motion prevents the data from being contaminated by other axes motion errors within each test. Simulations show that setup errors cause eccentricities and radius changes of the ball bar data when viewed on a polar plot. Finally, the proposed method is applied to a VL30 Mitsui-Seiki vertical machine tool to identify its trunnion axis motion errors. The results show the effectiveness of the proposed method as well as its ease of use and the short time required.

A model based method for centering double ball bar test results preventing fictitious ovalization effects

The telescoping magnetic double ball bar (DBB) is widely used to assess the motion and link errors of machine tools through circular tests. Despite a careful setup, some slight eccentricity of the balls relative to the circular trajectory often remains. Removing the eccentricity from the DBB results is essential due to its effects on the estimated trajectory. However, if the centering is not performed with great care, a fictitious ovalization effect may appear in the processed data. The source of the fictitious ovalization is discussed in this paper and a method is proposed to quantify the eccentricity and adequately process the data so that the fictitious ovalization is avoided. The results obtained by applying the proposed method both in simulation and in practice demonstrate its effectiveness.

Identification System of Machining Errors Based on Analysis of Machined Shape(Digital design and digital manufacturing)

The DBB (Double Ball Bar) method is able to analyze motion errors of a machine tool in the un-loaded condition. However, the method cannot be applied in the loaded condition. Researches on machining error analyses proposed so far have successfully obtained precise estimation results, but applications of such analyses are extremely limited. In order to realize high accuracy machining, it is necessary to identify the factors of machining errors in the loaded condition and eliminate these errors from machined parts. The objective of this study is to develop a system to analyze and identify the error factors of machining by comparing the measurement data of a machined part with CAD data. The usefulness of the developed system was verified through tests.

A DBB-Based Kinematic Calibration Method for In-Parallel Actuated Mechanisms Using a Fourier Series

For kinematic calibration of robots, we proposed using a low order Fourier series obtained by transforming the data for circular measurement paths. The errors of the realized paths were measured using a Double-Ball-Bar (DBB) system. Two nondimensional indices were proposed for evaluating the orthogonalities of the measurement paths and calibration parameters. An index was also proposed to evaluate the accuracy of the calibration with regard to measurement error. An algorithm for determining adequate measurement paths and an optimal set of a specified number of paths using these indices was also proposed. The effectiveness of the proposed method, indices, and algorithm was investigated through simulations and experiments with an experimental 6 dof in-parallel actuated worktable that we developed.

Development of a measuring method for several types of programmed tool paths for NC machine tools using a laser displacement interferometer and a rotary encoder

Many circular motion measuring methods for NC machine tools have been proposed, however, the drawback common to many of these methods is the restriction on the radius size due to the short measuring range of the displacement transducers used. Moreover, most of these measurement tools are specialized, and can only perform circular test path measurements. A circularity test method using a laser displacement interferometer and a rotary encoder has been developed. The measuring method features a much longer range of motion than ordinal circular test methods such as the double ball bar (DBB) method and, therefore, the radius restriction on these measurements is greatly reduced. Moreover, this measuring system can also be used for the evaluation of positioning accuracy and other more complex test paths. The proposed device consists primarily of a laser displacement interferometer and a rotary encoder. The holders for the interferometer head and the retroreflector are connected with a stainless steel rod. The retroreflector holder has a synthetic resin linear bearing allowing it to move relative to the interferometer head so that both optical components are always facing each other. The laser interferometer measures the change in distance between the interferometer head and the retroreflector, and the rotary encoder measures the rotation angle of the stainless steel rod. In this paper, the background, measuring principle and apparatus structure are briefly described. The experimental setup is also presented. The apparatus was employed in several measuring experiments, including circularity tests for a vertical machining center. The results from these experiments support the validity of this measurement apparatus.

A Study on the Improvement of Motion Accuracy of Hexapod-type Parallel Mechanism Machine Tools (2nd Report)

This paper presents a methodology to calibrate kinematic parameters in a Hexapod-type parallel mechanism machine tool. In a conventional procedure of the DBB (Double Ball Bar) test to measure a machine's contouring accuracy in a circular operation, the center deviation in an error profile is ignored to cancel setup errors. In such a case, the machine's positioning error can be evaluated only locally. We propose to use a specialized jig plate to fix a ball of a DBB device in order to evaluate the machine's positioning error in the global coordinate system. Kinematic parameters in a Hexapod-type parallel mechanism can be calibrated based on contouring error profiles. Experimental comparison shows the machine's global positioning error is significantly improved by using the proposed calibration method. In particular, the parallelity error and the straightness error are reduced by 89% and 76%, respectively, compared to the conventional calibration.

パラレルメカニズムを用いた三次元座標測定機の校正に関する研究 ダブルボールバーとタッチプローブを用いたキャリブレーション

This study has proposed a new coordinate measuring machine (CMM) based on a three degree-of-freedom parallel mechanism. This paper describes a kinematic calibration method using a double ball bar (DBB) system with fixed ball distance. The CMM with a touch trigger probe measures the moving ball of the DBB, and calculates position errors of the ball using kinematic parameters. The least square method using Jacobian matrix corrects gradually the kinematic parameters so that the position errors are minimized. The above calculations have been repeated in both numerical simulations and experiments. The position error approaches a zero level, but has never reached it. The calibration with 19 parameters decreased the position errors of the DBB from 168 μm to 9μm in the experiments. Furthermore, accuracies of both length measurement with block gauges and profile measurement with an optical flat have been improved after the calibration. The measurement accuracy most improves when the repetitive operation is discontinued after three or five calculations.