Three-axis Machine Tool Research Articles

Investigation on Structural Mapping Laws of Sensitive Geometric Errors Oriented to Remanufacturing of Three-Axis Milling Machine Tools

Three-axis milling machine tools are widely used in manufacturing enterprises, and they have enormous potential demands for remanufacturing to improve their performance. During remanufacturing a three-axis milling machine tool, the structure needs to be reconstructed, so it is necessary to identify sensitive geometric errors of the remanufactured machine tool. In the traditional sensitive geometric error identification method, complex error modeling and analysis must be conducted. Therefore, professional knowledge is required, and the process of the identification is cumbersome. This paper focused on the quick identification of sensitive geometric errors for remanufacturing of three-axis milling machine tools. Firstly, sensitive geometric errors of a three-axis milling machine tool were identified based on the multi-body system theory and partial differential method. Then, mapping laws between the sensitive geometric errors and the machine tool structure were firstly presented. According to the proposed mapping laws, the sensitive geometric errors can be identified quickly and easily without complex error modeling and analysis. Finally, the simulation and experiment show that the straightness error of milling is improved up to 0.007 mm by compensating the sensitive geometric errors identified by the proposed mapping laws. The table lookup method based on the mapping laws can quickly identify the sensitive geometric errors of three-axis milling machine tools, which is beneficial for the efficiency and precision of remanufacturing of machine tools.

Fabrication of polygonal Fresnel lenses with a rotating cutting tool on three-axis ultraprecision lathes

Fresnel lenses are gaining wider applications due to their advantages of being lighter, thinner and suitable for increasingly complex and miniaturized optical systems. To increase the fill factor when combining multiple Fresnel lenses to form an array, polygonal Fresnel lenses have been designed and machined with the ultra-precision machine tools in industry. However, for the three-axis machine tool equipped with the X-, Z-, and C-axes, it is difficult to machine the straight edges of the polygonal Fresnel lens due to the lack of a Y-axis. Although the synchronized servo motions of the linear axis and the spindle can be utilized to fit the relative motion trajectory of the tool to a straight line, the cutting tool rotates relative to the workpiece and thus the tool rake face is not always perpendicular to the direction of the relative motion during machining. In this paper, a high-precision rotating platform is integrated into the three-axis ultra-precision machine tool to make the cutting tool track the rotation of the spindle and keep the tool rake face perpendicular to the direction of the relative motion at all times. Then the tool trajectory is generated and the influence of the tool nose radius is compensated. Finally, an experiment of cutting the hexagonal Fresnel lens is performed on a three-axis ultra-precision lathe, and the machined workpiece is inspected with a white light interferometer. The experimental results show that the profile accuracy of the machined polygonal Fresnel lens satisfies the requirements in industrial applications.

Machining with a Precision Five-Axis Machine Tools Created by Combining a Horizontal Parallel Three-Axis Motion Platform and a Three-Axis Machine Tools.

Five-axis working machines are applied in the high-precision machining of complex convex surfaces. Therefore, this study integrated a horizontal parallel three-axis motion platform and a three-axis machine tools to create a reconfigurable precision five-axis machine tools (RPFMT). A DELTA OPEN computer numerical control controller was used as the control system architecture. A human–machine interface and programmable controller were incorporated into the developed tool to achieve automatic online measurement. A suitable cutting tool was selected to calculate the five-axis NC machining code for a complex convex surface. The NC codes were input into the LabVIEW software for five-axis postprocessing conversion. A concave workpiece was cut through rough and finishing machining to verify the accuracy of the produced RPFMT.

Workshop-suited geometric errors identification of three-axis machine tools using on-machine measurement for long term precision assurance

In this paper we present a novel methodology for fast geometric errors identification of three-axis machine tools (MTs) using on-machine measurement. For this purpose, a new design of a three-dimensional artifact is proposed. The artifact allows to identify the linear positioning error and the two straightness errors of any linear axis of MTs with cartesian kinematics and the perpendicularity error between the axes. The prerequisite for the measurement is that the roll, pitch and yaw errors of all axes have already been measured and are negligibly small. The presented methodology is especially suitable for regular control of the machine accuracy in order to ensure the precision of the machining in the long term.As part of this work, an artifact was made of thermo-invariant material (Invar) and calibrated with an accurate coordinate measuring machine (CMM). This artifact was used to identify the geometric errors of an MT with three linear axes, with the raw data being collected using a 3D touch trigger probe. The measured errors were compared with results from laser interferometer and ballbar measurements. Moreover, the measurement uncertainty of presented methodology was analysed and compared with those of the laser interferometer and ballbar.

A Feedrate-Constraint Method for Continuous Small Line Segments in CNC Machining Based on Nominal Acceleration

When the CNC machining of continuous small line segments is performed, the direction of the machine tool movement will change abruptly at the corner of adjacent line segments. Therefore, a reasonable constraint on the feedrate at the corner is the prerequisite for achieving high-speed and high-precision machining. To achieve this goal, a feedrate-constraint method based on the nominal acceleration was proposed. The proposed method obtains the predicted value of acceleration during the machining process by the machining trajectory prediction and acceleration filtering. Then, the feedrate at the corner is constrained, according to the predicted acceleration. Specifically, for any corner of adjacent line segments, the proposed method assumes that the CNC machining of a short path centered on the corner is carried out at a constant feedrate. First, the actual machining trajectory is predicted according to the transfer function of the servo system. Then, the nominal acceleration, when the CNC machining is carried out to the corner, is calculated and processed by a low-pass FIR filter. Last, the feedrate-constraint value at the corner is obtained according to the nominal acceleration and the preset normal acceleration. The advantage of the proposed method is that it can be used for different machining paths consisting of long segments or continuous small segments and it has no special requirement for the accuracy of the machining path. As a result, the feedrate-constraint value obtained is reasonable and the smooth machining process can be ensured. The simulation results in both 2D and 3D machining paths show that the proposed method is insensitive to the length of the line segment and the angle of the corner, and the calculated feedrate-constraint value is close to the theoretical value, which has good stability and versatility. In contrast, the feedrate-constraint values obtained by conventional methods change abruptly along the machining path, especially in the 3D simulation, which will damage the machining quality. The experiment was performed on a three-axis CNC machine tool controlled by a self-developed controller, and a free-form surface workpiece was machined by a conventional feedrate-constraint method and the proposed method, respectively. The experimental results showed that the proposed method can make the feedrate of the machining process higher and more stable. Then, machining defects such as overcutting and undercutting can be avoided and the machining quality can be improved. Therefore, the article proposes a new method to constrain the feedrate at the corner of continuous small line segments, which can improve the machining efficiency and quality of the CNC machining.

Implementation of MQTT protocol based network architecture for smart factory

The core concept of smart manufacturing is based on digitization to construct intelligent production and management in the manufacturing process. By digitizing the production process and connecting all levels from product design to service, the purpose of improving manufacturing efficiency, reducing production cost, enhancing product quality, and optimizing user experience can be achieved. To digitize the manufacturing process, IoT technology will have to be introduced into the manufacturing process to collect and analyze process information. However, one of the most important problems in building the industrial IoT (IIoT) environment is that different industrial network protocols are used for different equipment in factories. Therefore, the information in the manufacturing process may not be easily exchanged and obtained. To solve the above problem, a smart factory network architecture based on MQTT (MQ Telemetry Transport), IoT communication protocol, is proposed in this study, to construct a heterogeneous interface communication bridge between the machine tool, embedded device Raspberry Pi, and website. Finally, the system architecture is implemented and imported into the factory, and a smart manufacturing information management system is developed. The edge computing module is set up beside a three-axis machine tool, and a human-machine interface is built for the user controlling and monitoring. Users can also monitor the system through the dynamically updating website at any time and any place. The function of real-time gesture recognition based on image technology is developed and built on the edge computing module. The gesture recognition results can be transmitted to the machine controller through MQTT, and the machine will execute the corresponding action according to different gestures to achieve human-robot collaboration. The MQTT transmission architecture developed here is validated by the given edge computing application. It can serve as the basis for the construction of the IIoT environment, assist the traditional manufacturing industry to prepare for digitization, and accelerate the practice of smart manufacturing.

Research of Integrated Impeller Modeling and Five-axis Machining Technology based on Reverse Engineering

Five-axis NC technology and reverse engineering provides critical support for industrial development and has been an important subject for scholars to study. This paper focused on aviation impeller and discussed the integrated application of NC machining technology and reverse engineering. It collected the shape point cloud data of the impeller through 3D scanner and Geomagic Wrap software and completed the reverse parametric modeling using Geomagic Design X software. The impeller model is complicated in structure which is hard to machine on three-axis and four-axis machine tools, hence developed a five-axis machining program by using the NC programming software of Hypermill and carried out actual machining test. The results showed that the error between the machined sample and the research model is less than 0.05 mm, which can satisfy engineering requirements. It also indicated that the NC program developed by Hypermill software has high machining accuracy and safety, and the integration of reverse engineering and five-axis technology presents positive significance to the innovative design and machining for complicated engineering surfaces.

Thermal Positioning Error Modeling of Servo Axis Based on Empirical Modeling Method.

In order to investigate the thermal effect of a servo axis’ positioning error on the accuracy of machine tools, an empirical modeling method was proposed, which considers both the geometric and thermal positioning error. Through the analysis of the characteristics of the positioning error curves, the initial geometric positioning error was modeled with polynomial fitting, while the thermal positioning error was built with an empirical modeling method. Empirical modeling maps the relationship between the temperature points and thermal error directly, where the multi-collinearity among the temperature variables exists. Therefore, fuzzy clustering combined with principal component regression (PCR) is applied to the thermal error modeling. The PCR model can preserve information from raw variables and eliminate the effect of multi-collinearity on the error model to a certain degree. The advantages of this modeling method are its high-precision and strong robustness. Experiments were conducted on a three-axis machine tool. A criterion was also proposed to select the temperature-sensitivity points. The fitting accuracy of the comprehensive error modeling could reach about 89%, and the prediction accuracy could reach about 86%. The proposed modeling method was proven to be effective and accurate enough to predict the positioning error at any time during the machine tool operation.

Prediction-Model-Based Contouring Error Iterative Precompensation Scheme for Precision Multiaxis Motion Systems

In order to achieve both accurate contouring error estimation and excellent contouring control performance for multiaxis motion systems, this article proposes an online iterative precompensation control method based on a third-order contouring error prediction model. Specifically, the closed-loop dynamical model of each axis is first used to predict tracking errors at future sampling instants. And then the related contouring error of the multiaxis system at future time can be expressed by the predicted tracking error and the theoretical third-order model of the reference contour. To reduce the influence of modeling error and uncertain disturbances on the prediction performance of contouring error, the numerical calculation method is utilized to obtain absolutely accurate contouring error, through which the deviation between the proposed contouring error model and the accurate one can be corrected accordingly. Based on the accurate contouring error prediction model, the optimal contouring error compensation can be generated through online iterative calculation. To achieve excellent contouring performance, the compensation is added to the original reference contour as a kind of trajectory precompensation. Comparative experiments are carried out on an industrial three-axis machine tool. A series of contour tracking experiments consistently demonstrate that the proposed method can predicate the contouring error accurately. Additionally, the contouring motion performance can also be significantly improved in the case of a variety of different trajectories.

Calibration of the laser displacement sensor and integration of on-site scanned points

On-site inspection with a laser displacement sensor installed on the spindle of a three-axis machine tool has found a wide range of applications in industrial processes. The calibration of the beam direction and the integration of the on-site scanned points are crucial for the measurement of free-form surfaces. This paper presents a method to calibrate the unit direction vector of the laser beam and integrate the on-site scanned points based on a standard ball. By scanning the fixed ball, a group of nonlinear equations is constructed. The nonlinear equations are solved by the Levenberg–Marquardt algorithm to get the unit direction vector of the laser beam and the center position of the standard ball. Then the coordinate transformation parameters between the world coordinate system and the sensor coordinate system are calculated. The scanned points from different coordinate systems are transformed into the world coordinate system by the coordinate transformation. The method simplifies the traditional calibration process and realizes the integration of the multi-angle scanned points on a three-axis machine tool. The feasibility and effectiveness of this method are verified through simulation and practical experiment. By comparing fitting value under different measuring conditions with the nominal, the measurement parameters including the incident angle and the scan depth are optimized. In the experiment, the measurement accuracy of this method reaches 0.02 mm.

Rapid calibration method for measuring linear axis optical paths of computer numerical control machine tools with a laser interferometer

The optical path calibration process using a laser interferometer is time-consuming and tedious, requiring repeated adjustment and experienced operators, leading to a significant reduction in measuring efficiency. A novel optical path calibration process of the linear axis of a three-axis machine tool using a Renishaw XL-80 laser interferometer is proposed in this paper. The homogeneous coordinate transformation error model of the measured axis is established. Using the established error model, only the initial distance from the laser transmitter to the beam splitter and the initial distance from the beam splitter to the linear mirror are required to accurately calibrate the laser beam. The relationship between the fixed distance from the center of the light target to the quarter point on the edge of the light target and the movement distance of the measured axis are obtained. Eight deviations of the optical path affected by the laser transmitter, the beam splitter and the linear mirror are solved. Calibrations based on the deviations can be achieved by adjusting the positions and orientations of the laser interferometer components, leading to a rapid and accurate calibration method for measuring the optical paths.

A high-precision laser method for directly and quickly measuring 21 geometric motion errors of three linear axes of computer numerical control machine tools

Error compensation has become an important means of improving the manufacturing and processing accuracy of computer numerical control (CNC) machine tools. Quick and precise measurement of the various geometric motion errors (GMEs) of CNC machine tools is crucial. We propose a novel laser method for the efficient and direct high-precision measurement of the 21 GMEs of a three-axis CNC machine tool, or three linear axes of a five-axis tool. A corresponding system was developed, comprising a fiber-coupled laser unit, sensor head, target unit, and beam-steering unit. The beam-steering unit was designed to perform high-accuracy 90° rotation of the measuring beam, and the target unit was designed to be sensitive to 18 GMEs of the three linear axes. Stability, repeatability, and comparison experiments were conducted to verify the performance of the proposed system. The results showed that the stability of the position error measurement is ± 6.3 nm. For straightness error measurement, the stability, repeatability error, and residual are within ± 60.3 nm, ± 0.5 μm, and ± 0.7 μm, respectively. These are within ± 0.12 arcsec, ± 0.5 arcsec, and ± 0.5 arcsec for the pitch and yaw measurements, and within ± 0.37 arcsec, ± 1.5 arcsec, and ± 1.0 arcsec for the roll measurements, respectively. For squareness error measurement, the repeatability error and residual are within ± 0.6 arcsec and ± 1.6 arcsec, respectively. Compared with a laser interferometer, the proposed system can measure the 21 GMEs of a three-axis machine tool with one-step installation. Without accuracy loss, the measurement efficiency is approximately 45 times higher than that of a laser interferometer, thus providing a new quick and accurate measurement method of GMEs and error compensation of CNC machine tools.

Efficient Dynamic Parameter Identification Framework for Machine Tools

Abstract Modeling the dynamic behavior of a machine tool accurately is a difficult but crucial task when optimizing a machine tool’s design. An accurate representation of the real behavior is essential to ensure the transferability of simulations from a virtual prototype to a physical prototype. Due to the complexity of modern machine tools, a large number of parameters have an influence on the dynamic behavior. The parameterization of the used dynamic models is still challenging, especially if intricate local models are used for the individual effects. This paper presents an efficient framework for parameterizing a dynamic model of a machine tool containing linear local damping and stiffness parameters. For parameter identification, measurements of single components on simple test rigs as well as measurements of the whole machine tool were carried out. Different numerical optimization algorithms as well as objective functions were compared and applied to a three-axis machine tool structure for parameter fitting. By using a parametric reduced-order flexible multibody model for the fitting, high accuracy can be combined with high computational efficiency. The use of the presented approach allows an efficient parameter estimation and lays the groundwork for an influence analysis and the targeted optimization of a machine tool.

Dynamic and static error identification and separation method for three-axis CNC machine tools based on feature workpiece cutting

CNC machine tools play an extremely important role in the equipment manufacturing industry. Because a machine tool has multiple geometric errors which are highly coupled, the error identification process is difficult. Most direct and indirect measurement methods do not consider the dynamic errors in the actual cutting process. This study aims to optimize the error identification process of three-axis CNC machine tools, and proposes an efficient identification and separation method for the dynamic error and the quasi-static error based on feature workpiece cutting. In this study, a stepped feature workpiece that can reflect geometric errors is designed, and the mapping relationship between the geometric error components and the features is established. After the feature workpiece is cut, it performs two tests of on-machine measurement and CMM calibration sequentially. It can realize the identification and separation of 7 geometric errors and corresponding dynamic errors based on the two sets of test data. This method takes the actual cutting workpiece as the error source and can identify and separate the dynamic error and the quasi-static error simultaneously. Thus, the measurement efficiency of this method is higher than direct measurements. The geometric error identification result is highly consistent with the laser interferometer measurement, which proves the method in this study is feasible, efficient, and accurate.

Thermally induced comprehensive error modeling and compensation for gantry machine tools with grating scale in large structure machining

In large structure machining by gantry machine tools, the long machining duration with large temperature variation affects the machining accuracy significantly. To reduce the machining error of large structure, a method to model and compensate thermally induced positioning error considering workpiece thermal deformation is proposed. The thermal expansion of the grating scale and the thermal deformation of the workpiece are first analyzed theoretically, and a mathematical model of the positioning error and workpiece deformation error is proposed. The comprehensive model is with regard to transient temperature data of particular sensors. Compensation experiment was conducted on a three-axis gantry machine tool, and the error compensation system was applied to large structure machining to verify the effectiveness of the comprehensive error model considering workpiece thermal deformation. And the machining accuracy after compensation was greatly improved over 60%, which indicates the practicability of the proposed modeling method. It shows great potential application in large structure machining.

A novel modeling of volumetric errors of three-axis machine tools based on Abbe and Bryan principles

The modelling of volumetric errors of machine tools has been widely used by the method of Homogeneous Transformation Matrix (HTM) based on rigid body kinematics analysis for a long time. Without spindle induced errors and thermal errors, to establish a closed-loop HTM for a three-axis machine tool, the total of 21 geometric errors are the primary elements to be known. It is well known that the measured points of translational errors are directly related to the volumetric error at the tool cutting point through rigid body kinematics. In the generalized HTM method, however, this relationship is missing. This report, therefore, proposes a new comprehensive approach to formulate the volumetric errors based on the famous Abbe principle in order to derive the error term in the motion direction, and Bryan principle to derive the error terms in orthogonal to the motion direction. The proposed methodology is simple in concept, rational in physical meaning and easy in implementation. Experimental results, including faster multi-degree-of-freedom error measurements, volumetric error analysis and compensation on a testbed of small machine tool, validate the correctness and effectiveness of this method.

Volumetric error measurement and compensation of three-axis machine tools based on laser bidirectional sequential step diagonal measuring method

This paper proposes an approach to the measurement, modeling, and compensation of the volumetric errors for three-axis machine tools, using laser bidirectional sequential step diagonal measurement. Disparate measuring tracks are specially arranged for forward and backward paths, and the conforming error decoupling method is demonstrated. By decoupling, all the geometric errors which can result in volumetric errors are distinguished simultaneously, including translational and angular errors. In comparison with the traditional unidirectional measuring method, the proposed bidirectional method distinguishes more geometric errors (the angular errors) by measuring the same four body diagonals. Uncertainty analysis based on the Monte Carlo simulation was conducted, followed by experimental validation on a vertical machining center. The result of the experiment indicates that the diagonal systematic deviation of positioning is reduced by 90% after volumetric compensation.

Development of a micro punching machine tool for micro lens array on steel mold

Micro lens arrays (MLAs) are widely applied in different areas to fulfill different functions. Ultra-precision diamond cutting technology can generate MLA with sub-micrometer form accuracy and nanometer surface roughness but mostly for nonferrous material. In this paper, a three-axis micro punching machine tool is developed to use diamond tool to produce MLAs on steel mold. The developed micro punching machine realizes high-feed and high-precision motions by a macro-micro composite structure. An on-site measurement system based on machine vision is designed to provide evaluation results for MLAs and compensation data for the PZT. The fabrication methodologies for MLAs are studied under the consideration of the output voltage of PZT, tool path generation, and machining parameters. Experiments are conducted to testify the machine tool and the on-site measurement system by machining of MLAs on mold steel. The experimental results show that the micro punching machine can produce MLAs on hard materials with sub-micrometer form accuracy, and the evaluation results from the on-site measurement are close to those from the white-interference profiler.

Robust Trajectory Generation for Multi-Axis Vibration Avoidance

Precision manufacturing equipment suffer from unwanted vibrations triggered in rapid motion. Unless mitigated, these vibrations limit achievable accuracy and productivity of motion systems. This article presents a novel technique to generate reference motion trajectories that can robustly avoid triggering unwanted inertial vibrations. The reference trajectory is defined in B-spline form, and its frequency spectrum is optimized to attenuate spectral energy in a desired frequency band to avoid excitation of unwanted vibrations. A novel time-domain smoothing objective is also proposed to improve tracebility of generated trajectories. The trajectory optimization problem is put in a convex form, and a novel windowing scheme is designed to solve it efficiently for real-time implementation. Robustness and effectiveness of the proposed method is validated experimentally on an industrial three-axis Cartesian machine tool. Experiments show that the proposed method can robustly avoid unwanted structural vibrations without introducing delay to the motion and outperform state-of-the-art prefiltering methods.

Planar tool radius compensation for CNC systems based on NURBS interpolation

This paper introduces the realization of a tool radius compensation algorithm for NURBS trajectory. First, a single-segment NURBS trajectory tool radius compensation algorithm is developed. Different from the straight line and arc trajectory, the self-intersection phenomenon is prone to happen when calculating a single NURBS tool center trajectory, and the self-intersection will cause the overcut of workpiece. To avoid this situation, the algorithm introduced in this paper can detect whether the NURBS tool center track has caused overcut, and deal with the self-processing. Second, the tool radius compensation algorithm with multi-segment NURBS trajectory is implemented. The focus of this part is the tool radius compensation of the trajectory transfer, and the trajectory transfer is divided into two types: the extension type and the shortened type. For the shortened type transfer, cross-processing is needed to avoid the overcut of workpiece at the transfer. When calculating the tool radius compensation of the shortened type, we not only need to find the intersection of the tool center trajectory of two adjacent NURBS curves, but also need to select the intersection we need when a number of intersections exist. For the extension type transfer, in order to ensure the continuity of the tool center trajectory, we need to extend the tool center trajectory or add arc-segment at the transfer. The proposed algorithm can automatically decide where to extend the tool center trajectory or add arc-segment to achieve the best efficiency. Finally, the algorithm can output the calculated NURBS tool center trajectory in the form of linear segment interpolation G code or NURBS interpolation G code according to the processing needs. Simulations on VERICUT and experiments on three-axis CNC machine tool shows the effectiveness and validation of the tool path compensation algorithm.