Double Ball Bar Test Research Articles

A systemic approach to identify the volumetric and dynamic errors for five-axis machine tools with double ball-bar test

A systemic approach to identify the volumetric and dynamic errors for five-axis machine tools with double ball-bar test

A New Kinematic Synthesis Model of Spatial Linkages for Designing Motion and Identifying the Actual Dimensions of a Double Ball Bar Test Based on the Data Measured

This paper presents the new synthesis models of spatial linkages for designing measurement motion functions and ranges and identifying the actual dimension parameters. The spatial five-bar linkage is first introduced for the kinematic model of a double ball bar test of a two-axis rotary table. To design the ideal measurement motion and motion range of the double ball bar test, a novel saddle synthesis model of a spatial four-bar linkage RRSS is readily presented. Based on the output data measured from the double ball bar test, a new saddle synthesis model of a spatial five-bar linkage RRSPS is logically proposed for identifying their actual dimensions. Finally, three test cases and their results indicate that the new synthesis models presented in the paper can conveniently and efficiently calculate the measurement motion function and range and accurately identify the actual dimensions of the double ball bar test, which provides a suitable mathematical model for improving the accuracy of the double ball bar tests of a two-axis rotary table of machine tools.

Identification method for the dynamic motion error of the rotary axis based on circular and elliptical test piece cutting

The rotary axis is crucial for the machining accuracy of five-axis machines, and its motion accuracy is significantly influenced by the dynamic motion error. A test of circular and elliptical test piece cutting is proposed to identify and analyze the dynamic motion error of the rotary axis. The influence of the feed rate on the dynamic motion accuracy is explained. The dynamic motion error can be divided into two types, depending on its source and manifestation: dynamic error inside the servo loop (DEIS) and dynamic error outside the servo loop (DEOS). At different feed rates, elliptical test piece cutting and kinetics analyses are conducted to identify the DEIS, and circular test piece cutting together with double ball bar (DBB) tests are conducted to identify the DEOS. On-machine inspection and the coordinate measuring machine (CMM) calibration are performed. By combining the DBB test and kinetics analysis results, the DEIS and DEOS are identified and analyzed. The experimental verification shows that the DEIS and DEOS change according to the feed rate and can be determined from the machining error of the test piece. Consequently, the proposed method offers an effective reference for improving the motion accuracy of the rotary axis.

A reconfigurable mechanism model for error identification in the double ball bar tests of machine tools

A unified approach is presented in this paper for error identification of machine tools by means of double ball bar (DBB) tests and a reconfigurable mechanism model. The mechanism model consists of a kinematic chain describing motion of the tested object and a spherical-prismatic-spherical (SPS) chain describing motion of the DBB. Depending on the types of motion of a machine tool measured, five different mechanisms, designated as PP-SPS, PPP-SPS, R–SPS, RR-SPS, and PPRRR-SPS, are employed to deal with (i) two linear axes, (ii) three linear axes, (iii) one rotary axis, (iv) two rotary axes, and (v) three linear and two rotary axes, respectively. Treating the whole DBB test loop as a mechanism allows for the measured data of the DBB, the structure parameters of the mechanism and the motion parameters to be coupled together by the displacement equations. The structure parameters are obtained using the measurements with the help of kinematic synthesis of mechanisms and saddle point programming. The position-independent geometric errors and the mounting position of the DBB, and the error motions caused by the position-dependent geometric errors of the machine tool are also discerned successfully with the proposed mechanism concept for any DBB mounting position and direction. The experimental results confirm that the proposed mechanism approach is valid for error identification of machine tools composed of the linear and rotary axes.

Dynamic performance test under complicated motion states for five-axis machine tools based on double ballbar

During five-axis CNC machining, the dynamic tracking error caused by imperfect servo dynamic performance is becoming the major factor affecting the accuracy during high-speed and high-precision manufacturing. The double ballbar (DBB) test is one of the commonly used dynamic performance tests, but existing DBB test methods do not have satisfactory test capability for the requirement of complex freeform surface machining. In this paper, on the basis of summarized of dynamic tracking error’s mechanism and characteristics, an improved dynamic performance test based on DBB, which is named the circle-8 test, is proposed. In this test, a provenly efficient RTCP test trajectory is used to rebuild the DBB test process, which includes complex movements of all the five motion axes. To exhibit the improvement of the circle-8 test, this test and the BK3 test of ISO standard, which is considered as a comparison, are conducted in a five-axis machine tool with a tilting rotary table. According to the simulation and experiment results, for common dynamic inaccuracy situations, the circle-8 test always has better sensitivity of dynamic performance test than BK3 test. The above-improved dynamic performance test can be applied to provide effective data for the research of modeling and error reduction of five-axis machine tools.

The application of a regularization method to the estimation of geometric errors of a three-axis machine tool using a double ball bar

Geometric accuracy is crucially important for machine tools. Identification of geometric errors, especially position-dependent geometric errors, is still a challenging issue. This paper presents a systematic and fast approach to identify the geometric error components of a precision machine tool using double ball bar (DBB). The approach can be implemented in three steps: (1) polynomial based error modeling that relates the DBB radius error directly to the geometric error parameters of machine tool; (2) spatial measurement trajectory planning with a single installation of DBB in order to avoid producing extra setup errors; (3) error identification with regularization method that can solve the ill-posed identification problem effectively. Simulations and experiments show the accuracy and effectiveness of the proposed identification approach. The results of the DBB test show that, utilizing the proposed identification method, the roundness errors of the three circular paths in xy-, yz- and xz-plane are reduced from 27.3 μm, 20.7 μm and 24.1 μm to 9.2 μm, 12.3 μm and 7.8 μm, respectively, with error compensation.

Prediction and compensation of geometric error for translational axes in multi-axis machine tools

This paper proposes an integrated geometric error prediction and compensation method to eliminate the positioning inaccuracy of tool ball for a double ball bar (DBB) caused by the translational axes’ geometric errors in a multi-axis machine tool (MAMT). Firstly, based on homogeneous transform matrix (HTM) and multi-body system (MBS) theory, the positioning error model only considering the translational axes of a MAMT is established. Then, an integrated error parameter identification method (IEPIM) by using a laser interferometer is proposed. Meanwhile, the identification discrete results of geometric error parameters for the translational axes are obtained by identification experiments. According to the discrete values, the optimal polynomials of 18 position-dependent geometric errors (PDGEs) are founded. As a basis, an iterative compensation method is constructed to modify the NC codes generated with the ordinary compensation method in self-developed compensation software. Finally, simulation verification is conducted with these two compensation methods. Simulation results show the positioning errors for test path of tool ball calculated with the iterative compensation method that are limited within 0.001 mm, and its average accuracy and accuracy stability are improved by 79.5 and 52.2%, respectively. In order to further verify the feasibility of the presented method, a measuring experiment is carried out in XY plane of a five-axis machine tool by using DBB. The experiment results show that the maximum circularity error with the iterative compensation method is reduced about 40.4% than that with the ordinary compensation method. It is therefore reasonable to conclude that the proposed method in this paper can avoid the influence of the translational axes’ geometric errors on rotary ones during a DBB test.

SUM OF SINUSOIDAL PLUS NOISE MODEL TO EXTRACT COMPONENT ERROR FROM A DOUBLE BALLBAR MEASUREMENT ERROR MAP

The double ballbar (DBB) test, which captures actual data from multiple error origins of axes interaction, was defined as sinusoid error map model plus noise. When the number of sinusoids is the same as the number of individual errors of moving axis in the test map, we can extract a single source geometric error value from the DBB error map by modeling the well-known geometric error of linear axis. We considered the “noise” as mixed errors from other sources than geometric errors. This method is applicable to both a full circle and a truncated DBB test path.

Extracting Single Source Geometric Error Value from a Double Ballbar Measurement Error Map

The double ballbar (DBB) test is a well-known way to check the geometric error of axis interaction. The DBB test captures actual data from multiple error origins. Here, we define the DBB measurement result as the sinusoid error map model plus noise. Using this concept, we extract a single source geometric error value from the DBB error map by LS fitting. We considered the “noise” as mix error from other sources. To ensure the quality of a numerical fitting, we used a sinusoid model of each geometric error that was generated by simulation of axis movement based on homogeneous transformation matrices (HTMs) as general best-fit curve. To verify the proposed method, we extract a well-known geometric error of linear axes and compare it with the result from a commercial measurement system. This method is applicable to both a full circle and a truncated DBB test path. Then, we use the method to estimate the geometric error of axis interaction between linear and rotary axes in a five-axis machine. A sequence of DBB tests is arranged based on linear-linear and linear-rotary simultaneous motions. The tests contain seven DBB test runs with two setups, and are able to identify eleven geometry errors of interaction of axes in less time, and with less human “intervention” error.

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 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.

NC工作機械の運動精度に関する研究(第6報)

Generating mechanism of the stick motion, which occurs on the motion error during circular interpolation motion for NC machine tools, has been studied The motion errors including the stick motion were measured by Double Ball Bar test. Then simultaneously feedback values of the servo motor, the encoder, the linear scale were measured to compare the relationships between them, and the stick motion is generated due to the time lag at servo motor reversing, where lost motion is due to the elastic deformation of ball-screw driving system. A feed-forward control method was developed to compensate the stick motion error.

ＮＣ工作機械の運動精度に関する研究 Ｖ 回転誤差原因の診断法

Diagnosis of the angular motion error origins in NC machine tools has been studied by using double ball bar test. Angular motion errors of the machine is represented by error vector, and the relation between error vector and motion error trace obtained at DBB test, was theoretically analyzed. Based upon this analysis characteristic trace patterns were defined in individual angular motion error origin. Diagnosis is carried out on by extracting these characteristic patterns from motion error trace which were measured by DBB test. The results show that this method has sufficient accuracy and convenience for machine shop use.

誤差ベクトルによる３次元測定機の補正とＤＢＢ検定法の評価

A method to improve the accuracy of 3-dimensional coordinate measuring machine and a convenient method to test its measuring accuracy were studied. Error vector of the measuring position was defined in the machine, and the relation between it and volumetric error obtained by the double ball bar test was theoretically analyzed. Double ball bar test equipment was fabricated and its performance was strictly examined. The results show that it has sufficient accuracy and conveniency for machine shop use to evaluate the accuracy of the 3-dimensional coordinate measuring machine. The volumetric error obtained by double ball bar test was compensated using error vector which was reduced from the measured straight motion errors of the machine. It was found that the volumetric error obtained by the double ball bar test was significantly decreased by the compensation.