Ball Bar Research Articles

Identification and verification of location errors of rotary axes on five-axis machine tools by using a touch-trigger probe and a sphere

As a foundation to enhance the machining accuracy of five-axis machine tools, a robust, efficient, and precise method to measure the location errors of rotary axes on five-axis machine tools has been proposed in this study. This precise identification and calibration methodology for on-machine measurement of location errors of rotary axes is achieved by using a touch-trigger probe and a precise sphere installed on a tilting rotary table. Compared to commercially available devices, such as the double ball bar and R-test, the proposed measurement method has the advantages of efficient and automated calibration procedures in each periodical measurement. This proposed calibration algorithm builds a kinematic error model and measurement equations by using a forward and inverse kinematic approach and estimates the location errors by applying the least squares method. Moreover, the proposed calibration algorithm defines the location errors of the two rotary axes so they can be estimated and separated individually to avoid coupling effects. All the location errors of the rotary axes measured using the proposed measurement method were identified after compensation to improve the accuracy of the five-axis machine tool. A simulation was implemented to inspect the influence of uncertainties on the identified location errors of the rotary axes. Finally, an experimental demonstration on a five-axis machine tool with a tilting rotary table validates the feasibility of the proposed measurement method.

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.

Analiza dokładności pięcioosiowej frezarki CNC z użyciem standardowego i zmodyfikowanego testu okrągłości

In analysis of accuracy of 5-axis CNC milling machine measuring system with telescopic kinematic ball bar was used. Standard measuring test was made. The modified test was proposed with application of investigated inclined basis. The obtained results were compared. The accuracy of machine tool in indirect way was evaluated.

Position-dependent geometric errors measurement and identification for rotary axis of multi-axis machine tools based on optimization method using double ball bar

Geometric errors measurement and identification for rotary table are important. However, precisely adjustment for the setup position of a double ball bar in each measurement pattern is inconvenient. This study proposes a novel optimization identification method using a double ball bar to recognize the position-dependent geometric errors (PDGEs) of rotary axis. A mathematical model for ball bar measurement is firstly constructed to map the relationship between measurement direction and position of the double ball bar. And then, the setup positions of the double ball bar for PDGEs identification are analyzed. According to analysis for setup positions of the double ball bar, simplified measurement patterns would be conducted by adjusting only two setup positions for the ball bar and thus reduce the procedure of accurate adjustment for the ball bar. The PDGEs can be fitted as an nth B-spline curve, on the account of its being smooth and continuous. To identify the PDGEs, an optimization method, by computing the suitable value of control points of nth B-spline curve of errors to minimize the optimal value of the target function between the actual measured value and the value derived from a theoretical measurement model, is proposed. Moreover, in order to obtain the accurate value of control points of the error curve, the sensitivity analysis is conducted to acquire the sensitivity matrix with respect to control points of errors. The PDGEs are able to be identified simultaneously after calculating the appropriate values of control points of errors. The proposed identification method is validated by simulation and experiment. The results prove the effectiveness of the proposed method.

Identification of position independent geometric errors of rotary axes for five-axis machine tools with structural restrictions

Abstract Some of five-axis machine tools, which are widely used for micro-machining, have structural restrictions, such as the center line of tilting rotary axis being close to the surface of worktable or beneath the worktable. In this paper, simultaneous motions of both rotary and translational axes are combined with double ball bar to establish a measurement and one installation-based method for identifying the position independent geometric errors (PIGEs) of rotary axes for this kind of five-axis machine tools. Due to the existence of structural restrictions, the tool ball usually cannot be placed in the center line of the tilting rotary axis, and thus, actual spatial motion of ball bar is considered in the proposed identification procedure. For each rotary axis, two test configurations, i.e. the one along the radial direction of rotary axis and the other along the axial direction, are proposed to carry out the designed measurement and identification. Especially, to well avoid the potential negative problems induced by structural restrictions, rotary and translational axes are simultaneously controlled to carry out the measurement, and in the identification procedure, eccentricities together with angle shift and setup errors are comprehensively involved. Based on this basic consideration, spatial plane, on which the actual trajectory of table ball lies, is accurately constructed to integrate the actual movement of ball bar into the PIGEs identification procedure. Novel fitting strategies are developed to mathematically model both the actual trajectory and the spatial plane of table ball’s motion, which are then adopted to formulate the expressions for identifying location and orientation errors. Both simulation and experimental verifications show that the proposed method is effective and helpful for checking and improving the status of five-axis machine tools.

New Method Based on Improved Double Ball Bar for Measuring Geometric Motion Errors of Coordinate Measuring Machine

In this paper, an optimized method of measuring the geometric motion errors of a coordinate measuring machine (CMM) is proposed. The method is based on an improved double ball bar (DBB) that acquires the motion and link errors of the CMM and its actual rotation angles through simultaneous circular tests. The improved DBB has embedded a ring encoder system to the bottom of a commercial DBB on an auxiliary platform. In addition, an improved motion and link error separation algorithm is established by considering the difference angle Δθ between the actual rotation angle and the theoretical rotation angle of the DBB. Both influential factors of the center offset of the DBB and Δθ are discussed through simulations. When geometric motion errors are compensated for and measured on a 400 mm × 400 mm × 150 mm CMM, the standard deviations of the roundness errors decrease to 1.9 μm and 1.5 μm on the XY and ZX planes, respectively.

A novel geometric error identification methodology for the tilting head of five-axis machine tools based on double ball bar

The paper proposes a novel geometric error identification methodology for the tilting head of five-axis machine tools using double ball bar(DBB).Firstly, based on the motion condition of the tilting head of five-axis machine tools, three measurement patterns in Y direction, X direction and Z direction are proposed respectively. Then, the relative displacement equations of two balls of DBB in three measurement patterns are established respectively on the basis of homogeneous transform matrix (HTM) and multi-body system (MBS) theory. Finally, the geometric error parameters of the tilting head are identified totally. The presented method is universal and provides a reference for the error identification for the tilting head of five-axis machine tools.

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.

Identification of geometric errors of rotary axis on multi-axis machine tool based on kinematic analysis method using double ball bar

Abstract Accuracy identification of geometric errors of rotary axis is important for error compensation of the multi-axis machine tool. However, it is not easy because of the influence of setup error of measurement instrument, and there exists angular errors and displacement errors need to be identified simultaneously. In this paper, a decoupled method based on double ball bar is proposed to identify the geometric errors of rotary axis including both position independent geometric error (PIGE) and position dependent geometric error (PDGE). A formulation for ball bar measurement is derived from kinematic analysis to reveal the relationship between sensitivity direction and setup position of ball bar. The angular errors and displacement errors can be identified separately when choosing the appropriate setup position and direction of ball bar. It can effectively reduce the interaction influence between them and improve the accuracy. To identify the 4 PIGEs, a method by averaging the measured results of ball bar to compute the eccentricity and slant of rotary axis is proposed. And, the PDGEs are leaved to mainly describe the oscillation of geometric errors of rotary axis. It is useful to correct the deviation error resulting from setup error of ball bar. For the identification of 6 PDGEs, by means of adjusting setup position and direction of ball bar, they can be identified one by one along the sensitivity direction of ball bar. Furthermore, in order to reduce the influence of setup error of ball bar, the sensitivity analysis is performed to obtain the sensitivity of measured results of ball bar with respect to setup error. According to the sensitivity characteristics of setup error, a method is presented to correct the PDGEs. Finally, several numerical simulations and experiments are conducted to verify the theoretical model and the proposed identification method. The results from the simulations and experiments demonstrate that the method can identify the geometric errors of rotary axis effectively and accurately.

Development and evaluation of the machining performance of a CNC gantry double motion machine tool in different modes

Conventional methods of building CNC machine tools involve using linear motors and ball screw drives to obtain table motion. The double opposite sync motion design is an improvement over traditional CNC machines. In this work, an enhanced CNC gantry machine design is proposed, which exhibits a double motion mechanism. The new design is based on a rack and pinion system such that both the gantry tool and worktable are movable. The gantry’s natural frequency was designed at 202 Hz in the first vibration mode, enabling it to work at higher speeds of up to 11,530 rpm, which makes the gantry suitable for both rough cutting and fine finishing. A prototype of the multi-mode motion CNC gantry milling machine was developed to investigate the machining performance and efficiency of the double opposite sync system. Performance analysis was done using ball bar precision tests on the different modes of CNC gantry operation. Validation tests were carried out to determine the effects of the motion of the machine parts on the dimensional accuracy and surface finish of the machined parts. The results indicated that the straightness of the developed machine was reduced from 176.3 to 114.6 μm, which occurred due to the reduced total distance travelled by the tool and worktable. Moreover, the circularity increased from 338.7 to 667.0 μm. This increase could be attributed to the combination of errors arising from both the gantry and table.

Error calibration of controlled rotary pairs in five-axis machining centers based on the mechanism model and kinematic invariants

The mechanism model of ball bar testing for a two-axis rotary table of 5-axis machining center is discussed, and a new ball bar method to measure the three-dimensional motions of the rotary pairs in multi-axis machining center is developed based on the mechanism model. Then, the fixed axes and moving axes of the rotary pairs are identified by using spherical image circle fitting and striction circle fitting, according to the kinematic invariants of nominal rotation and the measured motions. The structure errors and kinematic pair errors of the rotary pairs are defined and identified by using the fixed and moving axes, and the kinematic model of the two-axis rotary table is deduced with those errors. The simultaneous two-axis motions of the rotary table are measured to verify the proposed calibration method. The experimental results show good agreements with the predicted results calculated by the calibrated kinematic model. Furthermore, the accuracy of the simultaneous multi-axis motions of the machining center is improved when the identified errors are corrected.

Ball Bar Measurement of Motion Accuracy in Simulating Cone Frustum Cutting on Multi-Axis Machine Tools

The ISO accuracy test standard for five-axis machining centers was revised recently. A cone frustum cutting test by end milling, well-known for testing multi-axis controlled machine tools for aircraft part processing, is adopted in the ISO standard as a similar and precise test. It considers both the accuracy of the finished test piece and an interpolation accuracy test measured by ball bar in the same-feed motion of cone frustum cutting. Although it is possible to apply the ISO test methods to various structures of five-axis machining centers, the application of the cone frustum test to multi-axis machine tools with a rotary axis on the workpiece side is rare; thus, it is difficult to evaluate the test results. In this report, the ISO test method was outlined. Next, the ball bar cone frustum test was performed on a five-axis machining center whose two rotary axes were located on the workpiece side. The evaluation method of the test results is discussed. Moreover, the ball bar cone frustum test was also performed on turn-mill complete machining centers to confirm the effectiveness.

Linked Ball Bar for Flexible Motion Error Measurement for Machine Tools

A measuring instrument, Linked Ball Bar (LBB), is developed to measure machine tool motion errors quickly, flexibly, and robustly. The LBB employs the concept of double ball bar (DBB) and measures the distance between two balls attached to the spindle and table. The problem of short measurement range, the drawback of the DBB, is solved using a link. The measurement accuracy of the LBB is investigated. The analytical resolution of displacement measurement using the LBB is under 30 nm when the displacement direction coincides with the sensitivity direction. The difference between the LBB and the laser interferometer is less than 1 μm in the center measurement range of 75 mm. The repeatability of the LBB is ±0.4 μm and is at the same level as the interferometer. The kinematic error of a five-axis machine tool is measured using the LBB to demonstrate its validity. The parallelism between the C-axis and Z-axis identified using the LBB agrees with the result measured using the cylindrical square. The difference between the LBB and the cylindrical square is about 10 μm/m at the maximum. The LBB can provide quick and flexible measurements of the motion errors of five-axis machine tools.

Reproducibility of machine tools’ circularity test according to ISO 230-4 with respect to testing position

Abstract In this paper circularity of three different machine tools by Virtual Machine was analyzed. To get circularity we make a ball bar test according to ISO 230-4. The test is very common in engineering for a quick diagnostic Computerized Numerical Control (CNC) technical conditions. We implemented earlier calculations of Volumetric Error in our Virtual Machine. Then we simulated testing of circularity of CNC machine tools. The place to take the test was chosen randomly from Uniform Distribution in three different kinds of machine tools. Those machines had different characteristics of kinematic errors and squareness and also different sizes of working space. We observed significant differences in the indicator (circularity) depending on the place where the test was taken. Moreover we showed that there was no reason to take the test in the center of working table.

Error Compensation for 3-axis Machine Tools

This paper proposes an efficient and economic way to improve the accuracy of machine tools by error compensation. The paper focuses on the basic research on error compensation, including seeking out the machine tools’ geometric errors, modelling the errors and presenting the measurements methods. The experiments are done by the laser interferometer and the ball bar, which prove the feasibility.

Virtual Distances Methodology as Verification Technique for AACMMs with a Capacitive Sensor Based Indexed Metrology Platform.

This paper presents a new verification procedure for articulated arm coordinate measuring machines (AACMMs) together with a capacitive sensor-based indexed metrology platform (IMP) based on the generation of virtual reference distances. The novelty of this procedure lays on the possibility of creating virtual points, virtual gauges and virtual distances through the indexed metrology platform’s mathematical model taking as a reference the measurements of a ball bar gauge located in a fixed position of the instrument’s working volume. The measurements are carried out with the AACMM assembled on the IMP from the six rotating positions of the platform. In this way, an unlimited number and types of reference distances could be created without the need of using a physical gauge, therefore optimizing the testing time, the number of gauge positions and the space needed in the calibration and verification procedures. Four evaluation methods are presented to assess the volumetric performance of the AACMM. The results obtained proved the suitability of the virtual distances methodology as an alternative procedure for verification of AACMMs using the indexed metrology platform.

Experimental Evaluation of Ball Bar Standard Thermal Properties by Simulating Real Shop Floor Conditions

Experimental Evaluation of Ball Bar Standard Thermal Properties by Simulating Real Shop Floor Conditions

An algorithm for circular test and improved optical configuration by two-dimensional (2D) laser heterodyne interferometer.

Circular test is an important tactic to assess motion accuracy in many fields especially machine tool and coordinate measuring machine. There are setup errors due to using directly centring of the measuring instrument for both of contact double ball bar and existed non-contact methods. To solve this problem, an algorithm for circular test using function construction based on matrix operation is proposed, which is not only used for the solution of radial deviation (F) but also should be applied to obtain two other evaluation parameters especially circular hysteresis (H). Furthermore, an improved optical configuration with a single laser is presented based on a 2D laser heterodyne interferometer. Compared with the existed non-contact method, it has a more pure homogeneity of the laser sources of 2D displacement sensing for advanced metrology. The algorithm and modeling are both illustrated. And error budget is also achieved. At last, to validate them, test experiments for motion paths are implemented based on a gantry machining center. Contrast test results support the proposal.

Accuracy evaluation of rotary axes of five-axis machine tools with a single setup of a double ball bar

A double ball bar (DBB) is used extensively to evaluate the geometric and dynamic performance of three-axis machine tools by means of the XY, YZ and XZ planar circular tests. However, research using a DBB to test the rotary axes of five-axis machine tools simply, quickly and effectively is scarce. In this paper, a method having two steps to identify the imprecision of the rotary axes caused by the position-independent geometric errors (PIGEs) is presented for a tilting rotary type five-axis machine tool using a DBB. The first step is designed to evaluate two rotary axes with one setup. Its advantage of fast diagnosis effectively reduces the machine down time, and thus can be employed as a quick testing approach of the machine tool. However, if some of the diagnosed errors fall outside their tolerances, a more accurate but slower check needs to be carried out due to the limitation of the first step. The second step aims to test the two rotary axes separately, each in two sub-steps. By means of varying the position of the pivot, the A- and C-axes can be tested individually. Both steps are performed with only one axis moving, thus simplifying the error analysis. Implementation of the proposed methods was carried out on a Hermle C600U five-axis machine tool. To show the validity of the method, the identified PIGEs are compensated for in each step, which suggests that the first step can be used as a fast and preliminary indication of a five-axis machine tool’s performance, whilst the second can be carried out if a more thorough evaluation is needed.

Kinematic calibration of a 3-DOF spindle head using a double ball bar

This paper presents a simple and effective approach for kinematic calibration of a 3-DOF spindle head developed for high-speed machining. This approach is implemented in three steps, (i) error modeling that allows the geometric errors affecting the compensatable and uncompensatable pose accuracy to be classified; (ii) identification of the geometric errors using a set of distance measurements acquired by a double ball bar (DBB) with a single installation; (iii) design of a linearized error compensator for real-time error implementation. Experimental results on a prototype machine show that the compensatable pose accuracy can significantly be improved by the proposed approach.