Gantry-type Machine Tool Research Articles

Active prediction compensation strategy: A new positioning compensation technique for improving the accuracy of the dual ball screw feed mechanism under load

The dual ball screw feed mechanism (DBSFM) is commonly employed in high-end machining applications. However, its accuracy is challenging to enhance due to variations in the axial stiffness of the dual actuators under load, making geometric error compensation (GEC) difficult. To address this, our research establishes a comprehensive error model that integrates geometric error and load-induced error (LIE) to improve feeding accuracy. The axial stiffness matrix of the DBSFM is identified using motor current information, and an active prediction error matrix is defined based on the geometric error model, milling force model, and axial stiffness matrix. We propose and analyze the active prediction compensation (APC) strategy as a new method to enhance accuracy under load. Experimental validation using a gantry-type machine tool demonstrates that the APC strategy can reduce the positioning error of the DBSFM to ±2 μm under load, representing significant improvements over the established GEC method. Specifically, the APC strategy achieves a 53.07% and 57.04% enhancement in accuracy along the two axes. This work illustrates the effectiveness of the APC strategy in reducing positioning errors and improving feeding accuracy in DBSFM machining processes.

게임엔진 기반 갠트리형 공작기계의 디지털 트윈 구현

With the advent of the Fourth Industrial Revolution, digitalization has been accelerated, and interest in digital twins has grown in tandem with technological advancements. In this study, we developed a realistic and interactive digital twin system using a game engine. We focused on a dual-servo gantry-type machine tool and employed the Unreal Engine to create a virtual environment, while using Arduino as a gateway for data communication. Our system facilitates the transfer of virtual machine tools using real machine tool data and allows for the emergency stop of real machine tools through virtual machine tool emergency stop signals. By ensuring real-time data communication and bi-directionality between the real and virtual systems, we successfully produced a highly realistic simulation system.

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.

Modeling and optimization of B-axis hydraulic delay for gantry-type CNC machine tool

Due to the limitation of dimension, structure and material, aviation structural workpiece is usually processed by AB rotary axis five-axis CNC gantry-type machine tools with large torque. However, the B-axis mechanical structure of gantry-type machine tools is complex and bulky, and hydraulic system is adopted for gravity balance. After the long production period of the machine tool, the hydraulic balance force of the B-axis is lost immediately and the spindle goes out of control when it emergency stops, which seriously endangers the quality of workpiece. In view of this phenomenon, this paper takes the hydraulic control system of machine tool as the breakthrough point, and analyzes the hydraulic system without adjusting the mechanical structure. And the mathematical model of hydraulic system is established. The hydraulic control system of B-axis is optimized by the simulation of MATLAB/Simulink. On the premise of improving the stability of the system, the duration of hydraulic balance force is extended, and the deflection time of 2 seconds is delayed, which effectively reduces the processing risk of workpiece.

Study on the dynamic identification method of the weak part of the bar-shaped combined structure

The weak part of the stiffness of machine tool combined structure is the key to improve the stiffness of machine tool. To overcome the static deformation with difficulty acquisition, the paper chooses machine tool combined structure which can be equivalent to one-dimensional bar structure, and a weakness index (WI) is proposed to identify the weak part of the stiffness by means of the dynamic hammer test method. Based on the bar structure as a numerical example, the weak parts are modeled as EA reduction in stiffness while the mass is maintained at a constant value. Thorough finite element (FE) method simulations are performed to assess the robustness and limitations of the method in several scenarios with single and multiple weakness. On the crossbeam of gantry type machine tool, the sensors are used to collect vibration data, the structural modal parameters are obtained by singular value decomposition (SVD) technique, and the dynamic characteristics are systematically reconstructed by using modal state space method to obtain stiffness data at zero-frequency. Then, the weak part of the structural stiffness is identified by the weakness index. Finally, the comparison of FE simulations and experiment results are provided to illustrate the working of the method.

A Non-Delay Error Compensation Method for Dual-Driving Gantry-Type Machine Tool

The drive at the center of gravity (DCG) principle has been adopted in computer numerical control (CNC) machines and industrial robots that require heavy-duty and quick feeds. Using this principle requires accurate corrections of positioning errors. Conventional error compensation methods may cause vibrations and unstable control performances due to the delay between compensation and motor motion. This paper proposes a new method to reduce the positioning errors of the dual-driving gantry-type machine tool (DDGTMT), namely, a typical DCG-principle-based machine tool. An error prediction method is proposed to characterize errors online. An algorithm is proposed to quickly and accurately compensate the errors of the DDGTMT. Experiment results verify that the non-delay error compensation method proposed in this paper can effectively improve the accuracy of the DDGTMT.

Thermal error modeling and compensation of a heavy gantry-type machine tool and its verification in machining

In order to derive a thermal error model that can better reflect the physical model of the machine tool to improve the model’s robustness and also has an explicit and simpler expression for real-time thermal error compensation, a new thermal error model of a heavy gantry-type machine tool was established. Firstly, multi-body system (MBS) was used to transform the thermal error modeling of the machine tool into the thermal error modeling of the discrete parts of the machine tool. Then, as for the thermal error modeling of the slider-ram structure, by analyzing the characteristic of the thermal expansion of a one-dimensional rod, power function was considered to better reflect that relationship in which the index number can represent the distance between heat source and the temperature measurement location. Then, its thermal error model was expressed as the superposition of two power functions according to the thermal superposition principle and the distribution of the heat sources. As for the thermal error modeling of gantry structure, it was simplified to a frame with −3 DOF. And its thermal error model was derived by virtual work principle (VWP). Then, the whole thermal error model of the machine tool was embedded into the Simens840d NC system and S7-300 programmable logic controller to establish a thermal error compensation system. Finally, the error model and compensation was verified by machining a rhombus-shaped workpiece of which different zones reflect the thermal error in different directions. And the workpiece was measured by a CMM. The results show that the average thermal error can be reduced from around 101 to 13.5 μm after the compensation, indicating the effectiveness of the thermal error modeling and compensation for the practical application.

Dynamic modeling and analysis for gantry-type machine tools considering the effect of axis coupling force on the slider–guide joints’ stiffness

As for the gantry-type machine tools, the forces of the slide blocks supporting the beam are different for different saddle locations, which would further affect the stiffness of the slider–guide joints and the dynamic characteristics of the whole machine tools. Therefore, the beam kinematic joints (the slider–guide joints) on the gantry-type machine tools are crucial for the dynamics prediction of traveling bridge systems. In this article, considering the effect of axis coupling force on the slider–guide joints’ stiffness, an equivalent dynamic model of traveling bridge systems for the gantry-type machine tools is established using hybrid element method. The additional load variation in the four slider blocks is analyzed for different locations of the saddle, and the variation in the slider–guide joints’ stiffness and traveling bridge system natural frequency are also studied. Finally, validation experiments are conducted on the traveling bridge systems of the gantry-type milling machine tools, and the results show that the dynamic modeling proposed in this article can reach a higher accuracy.

An investigation on modeling and compensation of synthetic geometric errors on large machine tools based on moving least squares method

This article intends to provide an efficient modeling and compensation method for the synthetic geometric errors of large machine tools. Analytical and experimental examinations were carried out on a large gantry-type machine tool to study the spatial geometric error distribution within the machine workspace. The result shows that the position accuracy of the tool-tip is affected by all the translational axes synchronously, and the position error curve shape is non-linear and irregular. Moreover, the angular error combined with Abbe’s offset during the motion of a translational axis would cause Abbe’s error and generate significant influence on the spatial positioning accuracy. In order to identify the combined effect of the individual error component on the tool-tip position accuracy, a synthetic geometric error model is established for the gantry-type machine tool. Also, an automatic modeling algorithm is proposed to approximate the geometric error parameters based on moving least squares in combination with Chebyshev polynomials, and it could approximate the irregular geometric error curves with high-order continuity and consistency with a low-order basis function. Then, to implement real-time error compensation on large machine tools, an intelligent compensation system is developed based on the fast Ethernet data interaction technique and external machine origin shift, and experiment validations on the gantry-type machine tool showed that the position accuracy could be improved by 90% and the machining precision could be improved by 85% after error compensation.

Modeling and Synchronous Control of Dual Mechanically Coupled Linear Servo System

This paper presents a system modeling technique for a high-speed gantry-type machine tool driven by linear motors. One feed axis of the investigated machine tool is driven by the joint thrust from two parallel linear motors. These two parallel motors are coupled mechanically to form the Y-axis while another standalone motor fixed to a support forms the X-axis. The components in the X-axis, which is treated as the mechanical coupling, are carried by the slides of the Y-axis motors. This configuration is applied to improve the dynamic stiffness of the system and operation speed/acceleration. However, the precise synchronous control of the two parallel and coupled motors would be the major challenge. To overcome this challenge, a multivariable system identification method is developed in this paper. This method is used to construct an accurate system mathematical model for the target coupled system. A synchronous control scheme is then applied to the model obtained using the proposed technique. The performance of the system is experimentally verified with a high-speed S-curve motion profile. The results substantiate the constructed system model and demonstrate the effectiveness of the control scheme.

Position-dependent Dynamic Analysis of Multi-axis Machine Tool Considering Axial Coupling

Because of the changing of the machine tool configuration and the axis coupling forces in multi-axis machine tools motion, the stiffness of the kinematic joints will change nonlinearly, therefore the amount of the inertia matrix, stiffness matrix of the system will vary, resulting in the variation of the dynamic characteristics of the system. A typical three axis gantry-type machine tool is taken as an object of investigation, using the lumped-parameter method to set up the dynamic model of machine tool and derive the dynamic equation. The system matrices with respect to position are established. Based on the dynamic model, the variations of the frequencies and frequency response functions with respect to position parameters are calculated. Modal testing is carried out to validate the accuracy of the model. The simulation and experimental results show that: the dynamic characteristics of the machine tool are greatly affected by axial coupling, the natural frequencies and the reacceptance of the tool cutting point(TCP) are greatly changed when the TCP is moving along x-axis, y-axis or z-axis. Comparing two extreme positions, the maximum changing of natural frequencies is up to 10% and response magnitude up to 2 times.

Non-synchronous Error and Modeling of Dual-drive System in Gantry-type Machine Tools with Travelling Bridge

According to the configuration movement feature of dual-drive system,the behavior of the dual-driving system is divided into two parts.The dynamic model of each one part is developed.Based on the dual-driving system model,the analytic formula of the non-synchronous error is derived.The quantitative relation between the structural parameters,the acceleration,velocity,and position input of the dual-driving system and the non-synchronous error is obtained.The influence law of stiffness ratio,friction damping ratio,the shift of mechanical coupling part's orthocenter on the non-synchronous error is developed.The experiments are taken on the DK-1200 machine tool with several different input signals,and the experiment results agree with the theoretical calculations.The accuracy of the calculation formula of the non-synchronous error is proved.

A method for thermal characterization and modeling of large gantry-type machine tools

In this paper, a method specially designed for the assessment of repeatability and accuracy of large machine tools is proposed, along with results for a large gantry-type milling machine, recorded in a medium-term period. For this purpose, temperatures were recorded and a metrological frame was used along with inductive sensors in the tool tip, performing repetitive measurements. As initial results, origin, weight and influence of every heat source on this kind of machines were found. Afterwards, high precision measurements of thermal deformations were obtained. Mechanism of errors was found, discerning between main thermal error in the vertical Z-axis and secondary error in longitudinal X-axis towards out of the plane of the gantry bridge. Finally, a finite element model was developed which showed main thermal behavior identified experimentally. This will permit to make simulations of the thermal response of the machine and to choose machining strategies for future parts. Proposed methodology was therefore proved satisfactory for thermal characterization of this kind of big machine tools. This knowledge will make possible to improve thermal design of machines and to develop error compensation procedures. This method can also be applied on workshop conditions for recalibration purposes.

Dynamic Characteristic Analysis for Machine Tools Based on Concept of Generalized Manufacturing Space

Aiming at studying the variation of the machine tools' dynamic characteristics in generalized manufacturing space,a concept of generalized modal and dynamic stiffness is put forward. Finite element method is employed to set up dynamic models for a three-axis gantry-type machine tool and a four-axis vertical machine tool. Modal and dynamic response analysis are carried out through out the whole manufacturing space with dynamic model modified method,in order to establish generalized modal function and stiffness map function respectively. By means of generalized modal analysis for three-axis gantry machine tool,the first five order natural frequencies of the machine tool through out the manufacturing space are obtained. The results show that the maximum change of natural frequencies can reach up to 25% during machining process,so complete modal information is obtained to provide data for performance optimization of machine tool. With the analysis of generalized dynamic stiffness for four-axis vertical machine tool,it can be concluded that the best working state in some frequency is not the best one for another,and then the theoretical support is present for the process optimization and accuracy promotion of machine tools.

Tracking control for a synchronized dual parallel linear motor machine tool

This paper analyses and compares several different synchronized controllers and motion command schemes, and implements them using a PC-based gantry-type machine tool. First, an integral-proportional (IP) controller and a proportional controller are applied to the velocity and position loop of a single-axis system respectively. The tracking control accuracy of the motor is advanced by the fastened response and improved lag. Next, for the dual-axis synchronized system, the equation of the cross-coupled controller (CCC) parameters can be acquired by analysing the tracking error and the synchronized error of the dual linear motors. The equation can be further applied to integrate the synchronized motion and reduce the synchronized error. The experiment results have proved these theories to be successful in reducing synchronized error and realizing the performance demand of tracking control.

Modeling and synchronous control of a single-axis stage driven by dual mechanically-coupled parallel ball screws

This paper presents a synchronous control scheme and system modeling technique for a single-axis stage driven by dual parallel ball screws/servo motors. The slide tables of the two ball screws are bridged with an aluminum bar, and each end of the bar is firmly fixed to one slide table. The thrust can, thus, be jointly output via this coupling. Such an arrangement aims to solve the potential skew problem that would occur on large scale machine tools whose stages have a long span between the two linear guides. This layout can also improve the system dynamics, speed and accuracy for large gantry-type machine tools. However, the precise synchronization of the two coupled parallel ball screws is a major challenge to overcome. Hence, in this paper, a system identification technique is first developed to construct a mathematical model of the coupled system, which is experimentally verified to be effective in improving performance. Three types of synchronous control schemes are then employed for control of the system. The performances of the three control schemes are experimentally compared in terms of positioning accuracy, tracking ability and reduction of the synchronization error between the two ball screws. Appropriate control schemes are suggested based on the experimental results.