Large Machine Tools Research Articles

On straightness measurements of large CNC machine tools

The CNC (computerized numerically controlled) machines are widespread in use due to their high capability of precise manufacturing in industrial production. They have a wide range of designs depending on the working capacity in manufacturing. The associated form errors in large-capacity CNC machines during production shall be identified and corrected or eliminated. This study presents an investigation of one of the main form errors that may affect the manufacturing precision of these machines. This error type is a straightness error with both two kinds of horizontal and vertical errors. The study is carried out for a vertical turning center CNC machine type. The straightness errors are measured for the X axis at different latches in the Z direction and for the Z axis at three positions in the X direction with multi-displacement steps. Different algorithms are used in the determination of straightness errors. The X-axis has minimum horizontal straightness errors at latch Nr. 3 and minimum vertical straightness errors at latch Nr. 5. For the Z axis, the minimum values for horizontal and vertical straightness errors exist when the spindle is located 1200 mm away from the machining center to the right. The displacement steps have a significant impact on the determination of straightness errors.

A novel domain decomposition-based model for efficient dynamic predictions of large composite machine tools

A novel domain decomposition-based model for efficient dynamic predictions of large composite machine tools

A Novel Methodology for Measuring Ambient Thermal Effects on Machine Tools.

Large machine tools are critically affected by ambient temperature fluctuations, impacting their performance and the quality of machined products. Addressing the challenge of accurately measuring thermal effects on machine structures, this study introduces the Machine Tool Integrated Inverse Multilateration method. This method offers a precise approach for assessing geometric error parameters throughout a machine's working volume, featuring a low level of uncertainty and high speed suitable for effective temperature change monitoring. A significant innovation is found in the capability to automatically realise the volumetric error characterisation of medium- to large-sized machine tools at intervals of 40-60 min with a measurement uncertainty of 10 µm. This enables the detailed study of thermal errors which are generated due to variations in ambient temperature over extended periods. To validate the method, an extensive experimental campaign was conducted on a ZAYER Arion G™ large machine tool using a LEICA AT960™ laser tracker with four wide-angle retro-reflectors under natural workshop conditions. This research identified two key thermal scenarios, quasi-stationary and changing environments, providing valuable insights into how temperature variations influence machine behaviour. This novel method facilitates the optimization of machine tool operations and the improvement of product quality in industrial environments, marking a significant advancement in manufacturing metrology.

FROM THE TRADITIONAL MULTILATERATION TO THE INTEGRATED MULTILATERATION APPROACH: A ROADMAP TOWARDS THE AUTOMATIZATION OF VOLUMETRIC ERROR MAPPING PROCESS FOR LARGE MACHINE TOOLS

Multilateration-based volumetric error mapping is gaining widespread adoption in the coordinate measurement machine and machine tool industry as the most effective approach for geometric characterization of large-size machines. In the traditional method, tracking interferometers are placed on the machine table while the measurement retro-reflector moves with the spindle. However, this approach has several limitations, including manual instrument manipulation and a sequential measurement scheme that hinders unattended execution. To overcome these limitations, the integrated multilateration method has been introduced, which addresses these challenges and reduces measurement uncertainty. This method offers the potential for fully autonomous volumetric error mapping of large-size machines and enables the development of new functionalities such as traceable coordinate measurement with large machine tools. This research presents the roadmap followed by the authors to automate the volumetric error mapping process for large machine tools. It outlines the current status of the technology, starting from the conceptual development and culminating in the experimental results obtained from real industrial machine tools.

PROCESS CONTROL COMBINING MACHINE LEARNING AND FINGERPRINT APPROACHES

Manufacturing operations in large machine tools often requires several hours per part. Ensuring output quality is vital to avoid time and financial losses. While quality assurance was always problematic and costly, the recent advent of Industry 4.0 brought a new perspective to the problem as cutting machines are now fully digitized. This paper proposes a process control framework that combines a fingerprint approach that detects deviations with respect to the validated process and a Long Short-Term Memory (LSTM) algorithm that predicts the upcoming signals. This paper demonstrates how combining these two methodologies surpasses the performance of previous purely learning-based algorithms.

COMPARISON OF THE DYNAMIC PROPERTIES OF THE UNDERFLOOR WHEEL LATHES BODIES

The research results presented in this article are important because information on machine tools for machining wheelsets is scarce in the literature, due to technical limitations related to large machine tool dimensions, a limited number of manufacturers and non-standard variants. This paper presents the findings of experimental and finite element method (FEM) simulation studies on the vibration of bodies used in Underfloor Wheel Lathes. Two bodies, varying in shape, dimensions, and weight, were examined, with their common characteristics being their purpose, material (cast iron), and casting technology. The bodies underwent experimental and numerical modal analyses to determine damping coefficients as a function of vibration frequency. The study was conducted on both free-suspended bodies and bodies positioned and leveled on a foundation. The results of the experimental and numerical tests were compared, showing good convergence in terms of natural vibration frequency and form.

Variable-Coefficient Dynamic Modeling Method for a Ball Screw Feed System in the No-Extra-Load Running State

In a ball screw feed system of high-speed/high-acceleration machine tools, large frictional and inertial forces may change the real contact state of the kinematic joints, resulting in changes in the contact and transmission stiffnesses and, hence, changes in the dynamic characteristics of the system. In this study, a variable–coefficient dynamic modeling method for a ball screw feed system is proposed, considering the influence of changes in the no-extra-load running states, such as position, speed, and acceleration. Based on Timoshenko beam elements with two nodes and four DOFs, an equivalent dynamic model of a ball screw feed system is established using the hybrid element method. The expression for the equivalent axial stiffness of individual kinematic joints is derived, considering the influence of the feed speed/acceleration under the no-extra-load running state of the system. In addition, the stiffness and mass of the screw shafts on both sides of the screw nut are calculated, considering the influence of the system’s feed position. Hence, we obtain the total stiffness and mass of the system in the no-extra-load running state and analyze the natural frequency. Finally, we conduct validation experiments on a ball screw feed system of a large gantry-type machine tool with different no-extra-load running states.

Feedforward compensation of contour errors in robotic machining system using compliance model

Industrial robots are receiving attention as alternatives for large machine tools owing to their high flexibility, large workspace, and cost-effectiveness. However, their low machining accuracy hinders their adoption for machining processes. The major error source is its compliance error caused by the cutting force resulting from the low stiffness of the robot manipulator. To increase the machining accuracy, an external measurement system and a compliance model were used to measure or estimate the compliance errors, and a control algorithm was subsequently applied to compensate for those errors. However, the control algorithm in terms of improving the dimensional accuracy of the workpiece has not been presented thus far. Unlike previous methods that deal with each axis compliance error separately, in this study, a contour control algorithm is proposed to effectively improve dimensional accuracy of the machined workpiece. The compliance model and a force torque sensor were used to predict the actual position of the cutting tool, and the contour errors were calculated using the interpolated reference positions and the predicted actual positions. A feedforward controller was designed to compensate for the estimate contour errors by modifying the reference positions. Experiments were conducted to evaluate the effectiveness of the proposed algorithm.

Traceable on-machine tool coordinate measurement through the integration of a virtual metrology frame in large machine tools

Traceable on-machine tool coordinate measurement through the integration of a virtual metrology frame in large machine tools

A multilevel recovery diagnosis model for rolling bearing faults from imbalanced and partially missing monitoring data.

As an indispensable part of large Computer Numerical Control machine tool, rolling bearing faults diagnosis is particularly important. However, due to the imbalanced distribution and partially missing of collected monitoring data, such diagnostic issue generally emerging in manufacturing industry is still hardly to be solved. Thus, a multilevel recovery diagnosis model for rolling bearing faults from imbalanced and partially missing monitoring data is formulated in this paper. Firstly, a regulable resampling plan is designed to handle the imbalanced distribution of data. Secondly, a multilevel recovery scheme is formed to deal with partially missing. Thirdly, an improved sparse autoencoder based multilevel recovery diagnosis model is built to identify the health status of rolling bearings. Finally, the diagnostic performance of the designed model is verified by artificial faults and practical faults tests, respectively.

Modeling the Static and Dynamic Behaviors of a Large Heavy-Duty Lathe Machine under Rated Loads

The static and dynamic performances of a machine tool structure are considered to constitute the primary factors affecting the load-carrying capacity, geometric accuracy and surface precision of the workpiece. The machining performance of a large machine tool under stable conditions is effectively determined by its dynamic response to the cutting force at low-frequency excitation. This study, therefore, investigated the static and dynamic characteristics of a large heavy-duty lathe machine tool in which the headstock and tailstock comprised critical component modules supporting a large workpiece during low-speed machining. Using a finite element model, the influences of the structural modules on the static and dynamic characteristics of the lathe were analyzed, considering the effects of the workpiece load. The results indicated that the fundamental vibration modes of the lathe were primarily dominated by headstock, tailstock, and workpiece behaviors. The maximum compliances of the lathe under the rated load were found to occur at relatively low frequencies (22, 40.7, and 82.7 Hz) and increase with the reduction in workpiece weight. Notably, these modal frequencies were significantly higher than the maximum rotational speed of the spindle (450 rpm). In addition, the dynamic rigidity corresponding to the rated speed was higher than that induced at the natural frequency. These results indicate that the subject lathe possesses sufficient capacity to sustain the cutting load during stable turning machining. This study can, therefore, help designers improve the performance of machine tools for future fabrication.

Analysis and test of conductive shaft of large horizontal NC ECM machine tool

In order to solve the problems of serious heating and easy ablation of conductive shaft in electrochemical machining of special-shaped deep hole with large aspect ratio, the two schemes of single-sided copper bar and symmetrical copper bar of conductive shaft are studied by ANSYS software. By establishing the thermoelectric coupling model of the conductive shaft, the distribution law of temperature field and thermal deformation of conductive shaft under different working conditions is analyzed. The conductive shaft structure based on symmetrical copper bar scheme is determined. Monitoring points are set at the key parts of the conductive shaft, and the temperature field distribution is verified by using the data acquisition card. The temperature acquisition results are consistent with the simulation results. Through the machining test of special-shaped inner spiral deep hole parts with large aspect ratio, the results show that under the conditions of working current of 15 000 A, feed rate of 5 mm/min, and continuous machining for 14 h, the forming accuracy of the workpiece is ± 0.15 mm and the surface roughness is better than Ra0.8 μm. The performance of the conductive shaft is stable and meets the actual processing requirements. The heat dissipation performance of the conductive shaft is optimized by providing air flow on the upper surface and side at the same time.

Performance balance oriented product structure optimization involving heterogeneous uncertainties in intelligent manufacturing with an industrial network

Optimization design of product structures is a critical point of intelligent manufacturing that is often overlooked, particularly their performance balance involving different uncertainties. It is urgent to connect product consumers, design experts, and manufacturing plants to balance the optimization of product structures. As an input of intelligent manufacturing, an industrial network based on software-defined networking (SND) for product optimization design including structure performance balance is established in this study, which breaks down the barriers between an industrial network and the performance balance optimization of product structures. Accordingly, the high-dimensional coupling clustering of product components is performed through a two-dimensional (2D) plane mapping with a design structure matrix (DSM) that aims at the entire product life cycle (PLC). A random chance-constrained programming model for the performance balance optimization of product structures is developed, where the fuzzy expert evaluation is considered to include heterogeneous uncertainties, and an integrated multi-objective discrete cuckoo algorithm nested with Monte Carlo simulation is designed to solve the model. The rationality and superiority of the proposed method are verified using a case study with a large hydraulic machine tool for sheet stamping where product modules with better comprehensive performance are generated.

Optimal cutting condition selection for high quality receptance measurements by sweep milling force excitation

In large machine tool applications, where machine's productivity is limited by structural chatter vibrations, an accurate dynamic characterization is vital for predicting reliable stability lobes. Motivated by the fact that the sweep milling force excitation method shows a great potential, this paper conducts a numerical stability analysis for optimizing the tool radial engagement employed during the characterization cutting tests. Additionally, an alternative spectra computation technique is proposed which considerably improves the signal to noise ratio over the existing technique. The receptance measurement is demonstrated using time domain simulations and applied in a large-scale three axis machining centre obtaining more accurate stability boundary predictions. The source code of the temporal milling simulation and the sweep milling signal processing is freely available to reproduce the simulation and experimental results. https://github.com/OierFranco/SMFE.

Research Status of Hydrostatic Bearing Technology in Machine Tool

Background: As the basis of mechanical manufacturing, large-scale machine tools are being developed for improving processing accuracy, load-bearing capacity and rigidity. Hydrostatic thrust bearing, hydrostatic guide and hydrostatic ram are important components of large machine tools; with the continuous improvement of product accuracy requirements, the research on the hydrostatic thrust bearing, hydrostatic guide and hydrostatic ram has become more important. Objective: This paper presents the current research on hydrostatic thrust bearing, hydrostatic guide and hydrostatic ram to highlight the importance of hydrostatic ram in improving the machining accuracy of machine tools. The study aims to lay a foundation for research and development of hydrostatic ram in the future. Methods: Firstly, the hydrostatic bearing technology is introduced. Secondly, the research status of hydrostatic thrust bearing, hydrostatic guide and hydrostatic ram is presented to provide theoretical basis for improving the processing performance of rest ram. Result: Deformation compensation and thermal deformation compensation of hydrostatic ram have been studied more. Most of the research study focuses on the accuracy of hydrostatic ram, but less on the bearing performance of hydrostatic ram. Conclusion: Breakthroughs have been made in the research on precision control and structural form of the hydrostatic bearing at home and abroad. Most scholars take the ram of Computer Numerically Control (CNC) boring and milling machines as the research object, but there is less research on the ram of vertical lathes. Therefore, the academic circle should pay more attention to the bearing performance of the hydrostatic ram of vertical lathes.

Adaptive compensation of the transmission errors in rack-and-pinion drives

Rack-and-pinion drive systems are commonly used in large machine tools. When considering the achievable path accuracy, the transmission errors of the gearing are of significant importance. Assembly and manufacturing tolerances coupled with load-dependent deformation of the gearing components lead to periodic position deviations, which cannot be satisfactorily suppressed by the position control.This paper describes a novel approach to minimize the individual errors of a drive by adaptive compensation in the control utilizing machine learning algorithms. Since both kinematic deviations and load-dependent deformations are considered, the path accuracy can be increased for a wide range of operating conditions.

METHOD FOR SAFE EXPERIMENTAL TESTING OF MACHINE TOOL USABLE SPINDLE POWER

The regenerative chatter during milling is a consequence of the machine tool dynamic compliance and the specific setting of the cutting process. The basic request on the machine structure is to enable a stable cut with dominant portion of the installed spindle power. In order to improve the dynamic properties of newly-designed machine tools, the machine tool structures are optimized using various advanced simulation methods and are made of various advanced materials. The final design should be tested experimentally. The test of usable spindle power provides important feedback on the machine tool design. The testing of large machine tools is time consuming due to the significantly varying machine dynamic compliance dependent on the working space. Due to the installed high power and large diameter tools used, the occurrence of chatter might be dangerous with a high risk of machine tool damage. The article presents a method for the safe and time-effective experimental testing of the machine tool usable spindle power. The method is based on the experimental identification of the cutting force coefficients and the experimental identification of machine tool behavior with the support of extrapolation models based on identified experimental data. The dynamic compliance is measured within a rough network of measurement points in the machine working space. The measured dynamic compliance results are characterized by a model based on modal identification data. Only a few cutting tests are done for the cutting force coefficient identification. Finally, a map of the usable spindle power is calculated. This approach makes it possible to calculate the results with respect to the real machining process and the current machine tool conditions across the whole working space in an acceptable time. The approach is demonstrated on a real case study.

A device to reduce positioning errors due to the machine tool compliance

One of the main problems that large machine tool designers must face is the onset of deflection and vibration due to the forces involved during the process: the own weight of the elements of the machine, the cutting forces and the inertial forces. The last ones are especially relevant in the case of large and heavy machine tools with cantilever elements as rams or columns. To counteract these forces, the present work proposes the use of an inertial device, based on the rotation of two eccentric mases around the same axis. Hence, a simplified dynamic model of a column is first developed, and tested during a standard face milling operation. Then, the dynamic model of the device is also developed, and a Computed Torque Control is proposed to govern the action of the device. The simulations show that the use of this device has the potential to reduce drastically the effect of the inertial forces and even the lower frequency components of the cutting forces.

Design of Two-Axial Actuator for Controlled Vibration Damper for Large Rams

Machine tool rams are important constructional elements found on vertical lathes as well as on many other machines. In most cases, a machine tool ram constitutes an assembly with significant dynamic compliance that affects the machine’s ability to achieve stable cutting conditions. There are various solutions for increasing a machine tool ram’s stiffness and damping. This paper describes an innovative concept of a two-axial electromagnetic actuator for controlled vibration dampers with high dynamic force values. The described solution is purposefully based on the use of standard electric drives. As a result, the size of the actuator is easier to scale to the required application. The solution is designed as a spacer between the end of the ram and the head. The paper presents the actuator concept, construction design, current control loop solution and experimental verification of the controlled vibration damper’s function on the test ram in detail. The presented position measurement concept will enable the use of non-contact position sensors for motor commutation as well as for possible use in vibration suppression control. Applications can be expected mainly in the field of vibration suppression of vertical rams of large machine tools.

Acceleration-based disturbance compensation for elastic rack-and-pinion drives

Rack-and-pinion drives are mainly used for large machine tools and are often operated with indirect position control. Due to the lack of state information on the output side, this results in reduced accuracy regarding the table position. In addition, the system can only react inadequately to disturbances outside the control loop, meaning that often insufficient results can be achieved in typical application scenarios such as milling. To meet the increasing dynamic and accuracy requirements of the modern manufacturing industry, this paper presents a highly dynamic acceleration-based disturbance compensation method. For this purpose, the table acceleration is estimated using a dynamical model of the drive train and compared to the signal from an additional acceleration sensor attached to the machine table. Based on the resulting difference, an additional compensation torque is provided, which suppresses the disturbance in counterphase. The approach is tested experimentally on an open control platform with industrial drive components and the behavior is investigated based on compliance frequency responses and externally applied milling forces. At the same time, a standardized parametrization methodology is developed and the robustness is evaluated by varying table masses. In summary, a considerable improvement of the dynamic disturbance behavior can be achieved compared to the conventional system without compensator.