Accuracy Of Machine Tools Research Articles

Spatial Expression of Assembly Geometric Errors for Multi-axis Machine Tool Based on Kinematic Jacobian-Torsor Model

Assembly geometric error as a part of the machine tool system errors has a significant influence on the machining accuracy of the multi-axis machine tool. And it cannot be eliminated due to the error propagation of components in the assembly process, which is generally non-uniformly distributed in the whole working space. A comprehensive expression model for assembly geometric error is greatly helpful for machining quality control of machine tools to meet the demand for machining accuracy in practice. However, the expression ranges based on the standard quasi-static expression model for assembly geometric errors are far less than those needed in the whole working space of the multi-axis machine tool. To address this issue, a modeling methodology based on the Jacobian-Torsor model is proposed to describe the spatially distributed geometric errors. Firstly, an improved kinematic Jacobian-Torsor model is developed to describe the relative movements such as translation and rotation motion between assembly bodies, respectively. Furthermore, based on the proposed kinematic Jacobian-Torsor model, a spatial expression of geometric errors for the multi-axis machine tool is given. And simulation and experimental verification are taken with the investigation of the spatial distribution of geometric errors on five four-axis machine tools. The results validate the effectiveness of the proposed kinematic Jacobian-Torsor model in dealing with the spatial expression of assembly geometric errors.

A multiple test arbors-based calibration method for a hybrid machine tool

Kinematic calibration is a necessary way to guarantee the accuracy of hybrid machine tools. The traditional calibration methods have high requirements for the measuring instruments and the measurement environment, and the measurement is extremely complex. The contradiction between measurement complexity and identification accuracy is an important problem in calibration. In this paper, a multiple test arbors-based calibration method for a hybrid machine tool is presented. The tool center point (TCP) position errors of multiple test arbors are measured sequentially by virtual TCP position constraints. The error parameters can be accurately identified based on these position errors without orientation measurement. The corresponding measurement scheme is described in detail according to whether the ball diameters of the test arbors are the same. The influence of the length and number of test arbors on the calibration results is investigated, and the basic principle for the selection of test arbors is given. Finally, the proposed method is validated by simulations and experiments. The proposed method can achieve overall high-accuracy calibration with simple measurement devices and convenient measurement steps, which provides a basis for automated calibration.

Effect of the Stiffness of the Turntable Bearing Joint on the Dynamic Characteristics of the Five-Axis Machine Tool Rotary System

The positioning accuracy and machining performance of the five-axis machine tool are significantly impacted by the dynamic characteristics of the rotary system of the two-axis rotary table machine. In this study, first, the stiffness of the joint bearing of the rotary table system is calculated, and the effect of bearing clearance and external load on the stiffness is analyzed. Second, considering the stiffness characteristics of the joint, the natural frequency and mode shapes of the turntable system are calculated. Finally, the influence of turntable angle and bearing stiffness on the dynamic characteristics of the turntable system is analyzed. The results show that the natural frequency of the rotary table system does not change obviously with the axial stiffness of the turntable bearing joint, but increases significantly with the increase in the radial stiffness. The first order natural frequency of the turntable decreases with the increase in the swing angle, and the change in the first order natural frequency is 77.43 Hz. The research results provide theoretical basis and guidance for machine tool design and use.

Research on optimal multivariate thermal error modeling based on finite-element analysis

To improve the nonlinear fitting ability of multiple linear regression model in thermal error prediction of machine tools, an optimized thermal error modeling method based on test and simulation is proposed in this paper. Firstly, the finite-element analysis is used to complete the thermal-structure coupling analysis of the CNC machine tool. Based on the simulation results of temperature and deformation field of the machine tool, temperature-sensitive points (TSPs) are selected with fuzzy cluster analysis and correlation analysis. According to the simulation results, TSPs of the machine tool are fitted by quadratic polynomial from temperature to deformation. Finally, taking the deformation value of TSPs as the intermediate variable, the multiple linear regression model is established, and the optimized quadratic multiple regression thermal error model is obtained. The results show that the prediction curve of the optimized thermal error model based on on test and simulation is closer to the test curve, and the fitting index is better than that obtained from the traditional model. This method can effectively improve the nonlinear fitting ability of the thermal error model of CNC machine tools. Based on the thermal error model established in this paper, the compensation value of thermal error can be obtained by inputting the real-time measured temperature data of TSPs, which can effectively improve the machining accuracy of CNC machine tools.

Research on Acceleration Segmented Compensation Strategy for Backlash of CNC Machine Tools

For the problem of backlash in the semi-closed-loop linear feed servo system of CNC machine tools, which seriously affects the accuracy of machine tools, an acceleration segmentation backlash compensation strategy is proposed. In the reverse motion of the servo motor, the servo motor is controlled to compensate for the backlash error during a period of accelerated motion and a period of decelerated motion. Based on the established backlash transmission model, the parameters of the compensated motion process are calculated. In order to ensure accurate and quick compensation of the gap compensation value for the transmission gap, the backpropagation method is used to backpropagate the acceleration calculation based on whether the acceleration process time is an integer multiple of the servo interpolation period. Finally, the feasibility of the method is verified through experiments, and the results show that the method improves the positioning accuracy of machine tools.

A novel method for measuring radial and axial errors in a cylindrical coordinate system using a combined double ball bar

Multiaxis machine tools are widely used in manufacturing. The measurement of motion error is important to improve the machining accuracy of machine tools. A double ball bar (DBB), as a high-precision single-dimensional position reading device, is widely used to measure the motion error of the circular path of machine tools. However, the motion errors contain the cylindrical radial and axial errors with respect to the circular path plane. Therefore, the measurement results of the DBB are incomplete because they do not contain axial errors. To measure the motion error of the circular path accurately, completely and concisely, a novel measurement method based on the combined DBB (C-DBB) is proposed in this paper, which can directly measure the radius and axial motion error of the circular path in one measurement. First, the measuring principle of the C-DBB method is introduced, and the measurement model of the radius and axial motion error of the circular path is established. Then, a measuring device using the C-DBB method was designed based on the lever principle, and the error parameters of the designed device were analyzed. According to the error analysis results, the measurement results of the C-DBB method were compensated and corrected. Finally, the C-DBB method is used to measure the circular path on the vertical CNC milling machine, and the results are compared with the measurement results using the DBB method. It is shown that the proposed C-DBB method can obtain high-precision multidimensional motion error in a single measurement, and the designed measuring device based on the C-DBB method has high repeatability and good circular path measuring ability. The proposed method provides an effective method for machine tool multidimensional motion error measurement and compensation.

Dimensional Error Compensation Based on Data-Driven Sliding Mode Terminal Iterative Learning Control for CNC Batch Grinding

Dimensional error of workpiece is an important index that indicts the grinding accuracy of machine tools. Conventional error compensation strategies focus on modeling one or several error factors for a particular CNC machining type, which are time consuming and costly. Based on data-driven sliding mode terminal iterative learning control, a new dimensional error compensation method is proposed in this paper. The compensation algorithm based on an iterative sliding surface and an objective function is established using the terminal experimental data. A modified NC program is fed to the machine tool to push the workpiece dimension towards desired. The theoretical analysis and experimental results show the proposed compensation method can effectively improve the batch grinding accuracy of the machine tool, which shows good perspective in manufacturing industry.

Look-ahead prediction of spindle thermal errors with on-machine measurement and the cubic exponential smoothing-unscented Kalman filtering-based temperature prediction model of the machine tools

Reducing the thermal equilibrium time is one effective way to improving the efficiency and utilization and ensuring the accuracy of machine tools. This paper proposed the look-ahead prediction of spindle thermal errors based on CES-UKF temperature prediction model using on-machine measurement. It can predict the thermal errors during the warm-up stage just with the measured temperature data in the required on-machine measurement time (ROMT). With the thermal error compensation function, the machine tool only needs warm-up for ROMT to improve the efficiency and ensuring the accuracy of machine tools. The look-ahead temperature prediction is realized combining UKF and CES including real-time adjustment of exponential smoothing coefficients and ROMT calculation. Temperature-sensitive point selection based on self-attention mechanism is established. Gate recurrent unit (GRU)-based thermal error model is proposed considering the thermal hysteresis effects. Verifications and cutting experiments are carried out to testify the effectiveness and robustness of the method.

A new method to identify the position-independent geometric errors in the rotary axes of five-axis machine tools

Position-independent geometric errors (PIGEs), caused by assembly imperfection, affect the machine tool's accuracy. Moreover, the rotary axes of five-axis machine tools introduce more PIGEs during parts machining, hence, the need to identify them to improve the accuracy. However, the identification of rotary axes centerlines and the use of extension bars, using double ball bar (DBB), introduce additional setup errors and uncertainties during error measurements and sometimes make the process tedious. This work proposes a new method to identify the eight (8) PIGEs in the rotary axes of a five-axis machine tool with a tilting table. The homogeneous transformation matrix (HTM) was used to construct the relative motion matrix to the established coordinate systems and then deduced the error model. Three different DBB installation setups were employed for the identification of the PIGEs for both rotary axes using the planar test, the recorded DBB length data was substituted into the model to obtain the PIGEs. The results show that the proposed PIGEs identification method is accurate and can improve the accuracy of the five-axis machine tool effectively, and this method has some advantages over some existing methods of identification in terms of simplicity which shows a good improvement in the manufacturing industry.

십자형 교정체를 이용한 5축 공작기계 회전축의 기하학적 오차 추정에 관한 연구

Estimation and compensation of geometric errors for rotary axes are among methods to improve machining accuracy of five-axis machine tools. Studies have been conducted on various methodologies for estimating geometric errors for rotary axes, which are essential for improving machining accuracies of five-axis CNC machine tools. This paper presents a method for estimating geometric errors of a rotating/tilting table using a cross-shaped calibration artifact with a touch trigger probe. The proposed method includes rotary axes error estimation equations for angles of each rotary and tilt axis based on locations of probing points. Computer simulations were performed based on a MATLAB/Simulink and ADAMS cosimulation system using the probing cycle process to verify the proposed method. Computer simulation results confirmed the usefulness of the proposed method in terms of volumetric errors.

Data Fusion Approach for Extracting Spindle Key Performance Features Under Different Workloads

The spindle is the core component of CNC machine tools, and its performance features are the main factors affecting the overall accuracy and efficiency of machine tools. This study proposes a method for extracting key performance features of machine tool spindles under different workloads. First, it develops a spindle performance test device, which can obtain the performance features of the spindle at different speeds and radial and axial forces. Then, it proposes a spatial vector model for predicting radial runout at any axial position. Test results show that the maximum error between the test and the prediction model is 2.85% at the first prediction position. At the second prediction location, the maximum error is 2.25%. Lastly, this study proposes a data fusion method to calculate the bearing stiffness by combining the measured data, mechanical theory, and finite element analysis. Test results show that the bearing stiffness feature can be simplified to a constant value in the milling process. In the typical working load range, the bearing stiffness fluctuates within 20% of the axial force. The method is applicable and instructive for engineering applications.

Virtual sensor for monitoring the geometric and kinematic accuracy of machine tools

The performance of machine tools depends on their working accuracy. The paper presents a new approach for in-process monitoring the geometric and kinematic accuracy of a machine tools. This approach utilizes a virtual sensor for capturing displacements at the tool center point (TCP). The virtual sensor is set up by using a data-driven approach based on extended transmissibility functions. The input data for monitoring are the NC internal data. The transmissibility functions are established via standard experimental procedures such as double ball bar (DBB) tests tests and estimation of frequency response functions (FRF). The data from the virtual sensor can be used for process monitoring and troubleshooting when launching a new process.

"INCREASING THE POSITIONING ACCURACY OF THE FEED KINEMATIC LINKAGES OF CNC MACHINE TOOLS THROUGH THE CALIBRATION METHOD "

The accuracy of a CNC machine tool is one of the most important parameters that make the geometry and dimensions of the work pieces comply with the tolerance limits specified through the designer’s technical drawing. When a machine tool is handed over to the customer it is compliant in terms of the accuracy limits specified in the documentation but, after a time interval of usage, because of the wear of the components in relative motion and machine exploitation conditions. This work suggests an innovating method of testing the accuracy of a CNC machine tool by using the “calliper”, with a view to enhancing the positioning accuracy of the feed kinematic linkages. After designing and manufacturing this device, the measurement of the target points on the coordinate measuring machine is necessary, so that this device becomes a “calliper”. For inspecting the machine tool, the “calliper” is located on the machine table, being measured at the target points by means of the probe with the tactile shooter that belongs to the machine. The measurement results are stored and compared to the results of the measurements performed through the coordinate measuring machine. In function of the two measurements the related corrections of the errors will be done in the software of machine tool numerical control.

Pivotal errors identification of the face gear worm grinding machine tool with a piecewise sensitivity analysis

As one of the main error sources, geometric errors greatly affect the machining accuracy of the face gear worm grinding machine tool. To reveal the mapping rules between the geometric errors and the tooth surface errors (TSEs) of face gear, the tool comprehensive error model (TCEM) and the tooth surface error model (TSEM) were successively established. The TCEM expressed the relationship between 41 geometric errors of the machine tool and 6 tool comprehensive errors (TCEs) of the tool while the TSEM expressed the relationship between 6 TCEs and 3 TSEs. Then, using the improved Sobol method, a piecewise sensitivity analysis was carried out by calculating the sensitivity indexes of the input of the TCEM and TSEM individually. Finally, the joint sensitivity indexes were calculated and the pivotal geometric errors were identified. By modifying the pivotal geometric errors in each TSE direction separately, all the main error reduction rates (ERR) are greater than 70%. Besides, by modifying all the pivotal geometric errors, the maximum values of 3 TSEs are reduced by 73.7%, 74.7% and 86.96%, respectively, and the minimum values are reduced by 57.1%, 71.79% and 76.19%, respectively, confirming the validity of the identified results.

COMPENSATIONOF WORKTABLEMOTIONERROR BYANACTIVELY CONTROLLEDHYDROSTATIC GUIDEWAY

The accuracy of machine tools is limited by many factors,such as geometric and thermal errors. Current error reduction approaches include e.g. volumetric compensation. However, if a machine tool has only linear axes, angular errors cannot be compensated since there is no controllable axis that could change the tool or workpiece inclination. Therefore, an actively controlled hydrostatic guideway was developed to compensate for angular and other geometric errors. This paper aims to experimentally verify the hybrid use of constant resistance and the pilot stage of an industrial proportional pressure relief valve for regulation of oil flow through hydrostatic pockets and control of these pockets as well as active compensation of machine tool geometric errors. The error reduction of the tested linear axis is presented.

INFLUENCE OF COMPOSITE SPINDLE RAM ON MACHINE TOOL DYNAMIC STIFFNESS

Demands for improving machine tool productivity and accuracy can be addressed using alternative material structures with the potential to reduce the mass of moving bodies and decrease machine tool dynamic compliance. A useful option is to apply composite materials because they offer high damping and low density in comparison with steel or cast iron. The key question is how the stiffness and damping of a single composite or hybrid metal + composite component influences the behaviour of the machine tool. In this paper, simulation models for the prediction of machine tool dynamic compliance were prepared for a detailed analysis of a use case study using a hybrid and ductile iron spindle ram for a portal milling centre. A simplified model for the damping matrix formulation was assembled and the influence of the spindle ram damping on the dynamic compliance was tested along with stiffness and mass change, leading to conclusions about the effect of a single component redesign. The minor influence of the material damping of the hybrid structure is noted, and factors influencing the final dynamic compliance are discussed in detail.

An approach to enhancing machining accuracy of five-axis machine tools based on a new sensitivity analysis method

An approach to enhancing machining accuracy of five-axis machine tools based on a new sensitivity analysis method

Effect of Coolant Temperature on the Thermal Compensation of a Machine Tool

Machine tool (MT) accuracy is an important factor in the industry and is affected by heat generation through internal and external moving parts; the electrical components used; and variable environmental temperatures. Thermal errors lead to 40–60% of all MT errors. To improve MT accuracy, efficient techniques to minimize thermal errors must be identified. This study investigated the coolant temperature effects under different rotating speeds of a standalone built-in spindle system and computer numerical control (CNC) machine with a direct-drive spindle on the accuracy of thermal deformation prediction. The z-axis thermal deformation of the standalone built-in spindle system and CNC machine with a direct-drive spindle was conducted at different spindle rotating speeds and coolant temperatures at a constant coolant flow rate of 5 LPM. All experiments were conducted in a steady and dynamic operation according to ISO 230-3. For the standalone built-in spindle system, in comparison to the Mares model, the developed new model based on the coolant temperature effect on the Mares model (Mares CT model) can improve the thermal deformation prediction accuracy by 18.17% to 39.50% at different coolant temperatures of 12 ∘C to 26 ∘C and the accuracy can be controlled within the range of 0.03 μm to 5.24 μm, while the supply coolant temperature is above 16 ∘C. However, the thermal compensation analysis of the Mares CT model for a CNC machine with a direct-drive spindle shows a thermal deformation prediction accuracy improvement of 58.30% to 66.35% at different coolant temperatures of 22 ∘C to 28 ∘C and the accuracy can be controlled within the range of 0.14 μm to 4.05 μm. To validate the feasibility of the compensation model in real machining processes, dynamic operational analysis was performed for a standalone built-in spindle system and a CNC machine with a direct-drive spindle, and the thermal deformation prediction accuracy improved by 12.19% to 35.53% with the standalone built-in spindle system and 40.25% to 60.33% with the CNC machine with a direct-drive spindle. The compensation model analysis shows that the coolant temperature has a high impact on thermal deformation prediction and markedly affects system accuracy within certain limits.

Thermal Error Analysis of Machine Tool Spindle Based on BP Neural Network

With the continuous improvement of the accuracy of machine tools, the proportion of the thermal error of machine tools in the total error is increasing. In this paper, the thermal error of horizontal machining center is analyzed by finite element simulation, a three-dimensional model is established in SolidWorks, and some details (such as bolt holes) are simplified and imported into ANSYS Workbench to determine the heat generation model, heat dissipation model, convection heat transfer coefficient and other boundary conditions. On the basis of the temperature field, the thermal-structural coupling analysis is carried out, and the temperature cloud field of the whole machine tool is obtained through the analysis, which provides a theoretical basis for the experimental design. Then, the data are divided into four categories by fuzzy cluster analysis, and finally, the thermal error model is established by BP neural network based on time series. It provides a theoretical reference for the compensation of thermal error.

One-step measurement method of five-axis machine tools using a laser tracer

To improve the motion accuracy of five-axis machine tools, it is important to develop a simple methodology to calibrate all geometric errors. This paper proposes a one-step measurement method to calibrate the five-axis machine tool using a laser tracer. First, a simplified geometric error definition of the five-axis machine tool is presented, which is sufficient and necessary to determine the tool error relative to the workpiece. Subsequently, a one-step measurement method using a laser tracer is proposed, and joint movements of five motion axes are adopted to simultaneously identify all geometric errors. Besides, a measurement path generating method is presented to save the trouble of manually planning the complex measurement path. Compared with the traditional two-step measurement method, the proposed method only needs four laser tracer positions and two reflector offsets, simplifying the measurement procedure and enhancing the measurement accuracy of geometric errors of rotary axes.