Double Ball Bar Research Articles

A Standard Procedure for Development Performance Map of CNC Machining Centers by Using Double Ball-Bar

Currently, CNC machining centers have been designed to provide flexibility for a wide range of applications. They are widely used in the metal cutting industry with many advantages; their accuracy, reliability, repeatability, and productivity are the important factors. Generally, for CNC machining centers, the working area of the machine table is larger than the size of the workpiece. In case of machining small workpieces, the operators can produce several workpieces at the same time by setting several fixtures at different positions on the table. This can reduce setup time for cutting a new set of workpieces. However, the performance of a machine varies throughout the workspace. It is thus possible that two finished parts produced from the different positions of the table will have different values in geometric dimensioning and tolerance (GD&T). Therefore, in this study, the researchers developed a new procedure for establishing the performance map of CNC machines. As a result, the operators are able to know which area can produce workpieces with positional tolerances that do not exceed a given threshold. By using a CNC model DMC 635V DECKEL MAHO together with double ball-bar model Renishaw QC10 ball-bar as a case study, experimental results indicated that this procedure can provide a performance map of the machining centers efficiently.

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.

Techniques for measuring and compensating for servo mismatch in machine tools using a laser tracker

Servo mismatch, which affects positioning accuracy in computer numerical control (CNC) machine tools, is an important source of error. Dynamic measurements of simultaneous movements in two axes are essential in determining servo mismatch. A circular test using a double ball-bar (DBB) is used commonly to assess such errors. However, servo-mismatch measurements of some machine tools, such as miniaturized machine tools, remain difficult due to structural restrictions of the measuring devices. In this study, a technique for servo-mismatch measurement via a bi-directional circular test using a laser tracker is described. The laser tracker is easy to set up experimentally and has fewer measuring range restrictions than other measuring devices. In order to ignore the effects of quasi-static errors, a bi-directional circular test is conducted using the laser tracker. Additionally, a compensation procedure, with a proportional equation, is proposed. The validity of the technique described was confirmed by comparison with the DBB result. In a machining test after servo-mismatch compensation, by adjusting the position loop gain, the roundness of a cylinder part was improved by ∼47.9%.

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.

A New Decoupling Measurement Method of Five-axis Machine Tools’ Geometric Errors Based on Cross Grid Encoder and DBB

In order to obtain all the geometric errors of five-axis CNC machine tool, a measuring strategy with cross grid encoder and double ball-bar (DBB) is proposed. According to the theory of multi-body systems, by using the homogeneous coordinate transformation method the model of geometric errors is established. The cross grid encoder is used to detect the translation geometric errors and the DBB is used to measure the rotation ones. Based on the kinematic chain and the properties of geometric errors, the translation geometric errors are measured before the rotation ones. For each axis, both PIGEs (position-independent errors) and PDGEs (position-dependent errors) are under consideration. Then a complete compensated model can be established.

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.

Parallelism error measurement for the spindle axis of machine tools by two circular tests with different tool lengths

In this study, parallelism errors between the spindle axis and the linear axis of machine tools were measured using a double ball-bar. Specifically, two circular tests were performed to measure the parallelism errors using tool balls with different tool lengths and a workpiece ball fixed to a workpiece table. Parallelism errors were calculated by analyzing, simultaneously, the measured double ball-bar (DBB) data from the circular tests. The proposed method provides accurate measurements due to a large offset between the circular tests within the working space of machine tools. Additionally, the approach only requires a DBB and adjustment/extension fixtures; thus, it simplifies measurements and is cost-effective.

Kinematic calibration of parallel manipulator with full-circle rotation

Purpose The purpose of this paper is to develop a means of the kinematic calibration of a parallel manipulator with full-circle rotation. Design/methodology/approach An error-mapping model based on the space vector chain is formulated and parameter identification is proposed based on double ball-bar (DBB) measurements. The measurement trajectory is determined by the motion characteristics of this mechanism and whether the error sources can be identified. Error compensation is proposed by modifying the inputs, and a two-step kinematic calibration method is implemented. Findings The simulation and experiment results show that this kinematic calibration method is effective. The DBB length errors and the position errors in the end-effector of the parallel manipulator with full-circle rotation are greatly reduced after error compensation. Originality/value By establishing the mapping relationship between measured error data and geometric error sources, the error parameters of this mechanism are identified; thus, the pose errors are unnecessary to be measured directly. The effectiveness of the kinematic calibration method is verified by computer simulation and experiment. This proposed calibration method can help the novel parallel manipulator with full-circle rotation and other similar parallel mechanisms to improve their accuracy.

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.

Accuracy design of high precision machine tools using error sensitivity analysis methodology

Geometric accuracy is a crucially important performance factor for machine tools. Theoretically, the effects of source errors on pose accuracy (positional and angular accuracy) of 3-, 4- or 5-axis machine tools cannot fully be compensated by software, and only those pose errors associated with the permission motions are compensatable by means of error compensation. Therefore, the uncompensatable pose errors should be strictly guaranteed in the processes of design and manufacture. In this paper, after the geometric error model is established, the source errors affecting the uncompensatable pose accuracy are identified out of all the source errors. In order to enhance the understanding of which source errors have more influences on the pose accuracy, a probabilistic sensitivity analysis method is proposed, and the global sensitivity index is defined to evaluate the influence in the overall workspace. According to the sensitivity analysis results, the uncompensatable pose accuracy index is allocated to each source error. And then, assembly accuracy acceptance criteria are proposed as a guideline for machine assemblers. As an application example, the presented approaches are applied to the accuracy design and manufacture of a 4-axis machine tool, and double ball bar measurement and machining test are carried out to check the accuracy of the designed machine tool.

A comprehensive error analysis method for the geometric error of multi-axis machine tool

In this paper, a comprehensive error analysis method is proposed to discover how the geometric error propagation through every motion axis, and to find out which error parameters have greater impact on the tool posture error at the end of the kinematic chain. As the geometric error of a motion axis can be regarded as the differential movement, an error model for a four-axis machine tool is established to calculate the tool posture error with all the geometric error parameters. Then a cumulative process of the differential movements of every axis is proposed to describe the error propagation process when moving the tool to the given position. Moreover, the workspace of the machine tool is discretized into an amount of points with a uniform sampling method on the measured positions of the geometric error. Then, a Spearman rank correlation method is presented to find out how closely linked between a single error parameter and the tool posture error all over the sampling workspace. Hence, the ten key error parameters are selected according to the analysis results in the three-axis and four-axis sampling workspace. Finally, an experiment is conducted on the four-axis machine tool with a three-axis controlled trajectory to verify the effectiveness and correctness of the proposed method using a double ballbar.

Accurate identification and compensation of geometric errors of 5-axis CNC machine tools using double ball bar

Five-axis CNC machine tools are widely used in manufacturing of parts with free-form surfaces. Geometric errors of machine tools have significant effects on the quality of manufactured parts. This research focuses on development of a new method to accurately identify geometric errors of 5-axis CNC machines, especially the errors due to rotary axes, using the magnetic double ball bar. A theoretical model for identification of geometric errors is provided. In this model, both position-independent errors and position-dependent errors are considered as the error sources. This model is simplified by identification and removal of the correlated and insignificant error sources of the machine. Insignificant error sources are identified using the sensitivity analysis technique. Simulation results reveal that the simplified error identification model can result in more accurate estimations of the error parameters. Experiments on a 5-axis CNC machine tool also demonstrate significant reduction in the volumetric error after error compensation.

A ballbar test for measurement and identification the comprehensive error of tilt table

This paper proposes a comprehensive geometric error measurement and identification method for a tilt table equipped in a five-axis machine tool with two turntables using the double ballbar. Three positional errors of the socket, caused by both the volumetric error and linkage error of the tilt table, are measured in the sensitive directions with simple circular paths. A mathematics model is applied to map the relationship between the night results that are from three installation positions of the socket and all the errors of the tilt table. Firstly, all the measured results are applied to establish the average axis that is just with the linkage error of the tilt table using the newly developed circular fitting method, rather than solving the large-scale linear equations. Then, the linkage error is treated as the trend item in the measured results for it keeps constant during the rotation of the tilt table. Hence, the volumetric error is identified by the remaining part of the measured results, which are obtained by subtracting the part caused by the linkage error, using the pseudo invert matrix method. Meanwhile, an error model is established to analyze the influence of the installation errors of the socket and the tool cup. Two simple correction procedures, using a low-cost electron clamp and an easily operated circular path, are developed to eliminate these two installation errors before the ballbar test. Finally, a verification experiment, by comparing the predicted value from the identified errors with the measured results by the ballbar, is carried out to validate the effectiveness and correctness of the proposed method.

Geometric characterisation and simulation of position independent geometric errors of five-axis machine tools using a double ball bar

A double ball bar (DBB) is extensively used to evaluate the geometric and dynamic performance of three-axis machine tools by means of the XY, YZ and XZ planar circular tests. Errors can be estimated by comparing them with known error profiles. However, such geometric interpretation of error plots of five-axis machine tools is limited. In this paper, a five-axis machine tool model is established with Homogeneous Transformation Matrices (HTMs), laying the foundation for characterising particular geometric shapes induced by various Position Independent Geometric Errors (PIGEs) of all five axes. A testing scheme is proposed to evaluate the target five-axis machine tool in two major steps: testing the rotary axes individually and testing the linear-rotary axes couples. In the first step, each rotary axis is tested with two substeps, with and without the extension bar on the DBB. The second step requires each linear and rotary axes combination to move simultaneously. Both approaches are performed with only one setup, thus simplifying the setup procedure and reduce the machine down time. To show the validity of the method, PIGEs for each axis are simulated with the given machine tool model. Several DBB trajectories are simulated using the machine tool model. Compared with the actual testing plots, the simulated DBB error plots are helpful to diagnose the PIGEs of linear and rotary axes based on their particular geometric shapes. The results suggest that the proposed method along with the given error characteristics can be used as a fast indication of a five-axis machine tool’s performance.

Prediction and identification of rotary axes error of non-orthogonal five-axis machine tool

This paper proposes an efficient and automated scheme to predict and identify the position and motion errors of rotary axes on a non-orthogonal five-axis machining centre using the double ball bar (DBB) system. Based on the Denavit-Hartenberg theory, a motion deviations model for the tilting rotary axis B and rotary C of a non-orthogonal five-axis NC machine tool is established, which considers tilting rotary axis B and rotary C static deviations and dynamic deviations that total 24. After analysing the mathematical expression of the motion deviations model, the QC20 double ball bar (DBB) from the Renishaw Company is used to measure and identify the motion errors of rotary axes B and C, and a measurement scheme is designed. With the measured results, the 24 geometric deviations of rotary axes B and C can be identified intuitively and efficiently. This method provides a reference for the error identification of the non-orthogonal five-axis NC machine tool.

Latest development of a new standard for the testing of five-axis machine tools using an S-shaped test piece

It has been generally recognized that it is very important for industrial users to test the performance of high-accuracy numerical control machine tools before ordering them. Methods based on special devices such as Double Ball Bar and R-test are currently used for kinematic error measurement and compensation of five-axis machine tools. However, these methods need special devices and software, and industrial machine tool users prefer testing machine tools by physically machining a special workpiece, due to its simplicity and intuition, as doing such machining tests are just like manufacturing parts in real production. On the other hand, since existing machining tests such as cone frustum test piece fail to reflect the dynamic performance of five-axis machine tools, it has drawn a lot of attention to the development of a new machining test method to address this issue. This article introduces a new machining test standard that has been proposed for testing the accuracy of five-axis machine tools using an S-shaped test piece. This method has already been accepted by the International Standard Organization and is now under development for inclusion in ISO 10791 Part 7. The advantages of the S-shaped test method in testing machining performance of five-axis machine tools are analyzed and demonstrated in this article. Furthermore, new development has been made to simplify the definition, reduce the testing workload and improve the measurement accuracy of the proposed testing method.

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.

The Use of DBB Measurement in Horizontal Machine Tool

The double ballbar(DBB) is an instrument to measure roundness error of the machine tools. Traditionally, DBB is used in the vertical machine tool which has a vertical spindle, so the DBB can be easily installed to measure without additional jigs. But the condition in the horizontal machine tool is different. Because the horizontal machine tool has no vertical spindle, additional jigs are needed for the measurement. Not only the differences in the using DBB but also in the forming circular trajectory exist. Center of the circular trajectory in the vertical machine tool is stationary, but not in the horizontal machine tool. The trajectories of two balls of the DBB are not same in the two kinds of the machine tools. The method that the DBB measurement in the horizontal machine tool has the same effect comparing with the measurement in the vertical machine tool is presented.

Identification of error sources in a five-axis machine tool using FFT analysis

The use of five-axis machine tools is increasing because of their flexibility and high throughput. However, having more components in motion, these machines are more prone to failure than their three-axis counterparts. In this paper, using previously published tests of the double ball bar (DBB), the deviations of the rotary axis are measured and then treated to remove the eccentricity due to setup errors. Fast Fourier transform (FFT) analysis is applied to the centered readings, in order to obtain the machine frequency spectrum. From the latter, the contribution of error sources and their amplitude are obtained. This method can be used on a regular basis as a predictive maintenance (PM) tool. The novelty of this work falls in the realm of condition monitoring (CM) of rotary-axis components using a commercial instrument, with limited angular velocity and number of turns.