Machine Tool System Research Articles

Real-Time Iterative Compensation Based Contouring Control Method for Polar Coordinate Motion Systems

For precision planar contouring control systems widely applied in industrial scenarios, additional contouring control action is usually added to two axes respectively to enhance contouring motion performance. In this article, planar motion is innovatively described in the polar coordinate system but not Cartesian one, and a novel definition of polar-radius error (PE) is proposed to evaluate contouring performance. Compared to traditional contouring error defined in Cartesian coordinate system, PE is more suitable for describing the deviation between the actual contour and the reference one, especially at contour vertexes where the traditional definition is defective. Based on the definition of PE, a real-time iterative compensation method is subsequently designed for radial motion axis to improve its trajectory tracking performance and to suppress the contouring error indirectly. The compensation mechanism is merely implemented on only one axis rather than both axes of planar motion, which makes it simpler for practical applications than other existing contouring control algorithms. Comparative experiments with different error definitions and control modes are carried out on machine tool systems to illustrate the effectiveness of the proposed contouring control framework.

Towards stable milling: Principle and application of active contact robotic milling

The low rigidity of industrial robots is a significant obstacle to high-performance robotic milling because it often leads to machining vibrations, which are undesirable. Typically studies on improving the stability of robotic milling rely on either maximising the stiffness of the robot in the direction of the milling force by optimising its configuration or reducing the milling force by using conservative milling parameters, which impose significant restrictions on the flexibility and productivity of the system. A new concept, called active contact robotic milling, is presented in this paper. It offers promise for use in robotic milling by ensuring system stability for the first time without sacrificing the flexibility and material removal rate. This is achieved by actively controlling the force between the robot and workpiece, that is, a force, in addition to milling force, is generated between the robot structure and workpiece. Theoretical analyses showed that the stability of robotic milling can be improved without requiring an elaborate robotic configuration or optimisation of the milling parameters. Therefore, flexibility and productivity of robotic milling are ensured. Based on the proposed principle, we develop a novel milling cutter that can control the contact force between the robot and workpiece. The experimental results confirm the validity of the proposed principle. The research findings presented in this study provide a concise and effective framework for a robotic milling system to avoid chatter in robotic milling. The proposed dynamic model is of great significance for understanding the influences of the interactions at the contact interface on system stability. Moreover, the proposed concept can inspire new tool designs for milling with flexible machine tool systems in the future.

NEW DEVICE FOR RAPID MEASUREMENT OF MACHINE TOOL GEOMETRIC ERRORS

Control systems of modern machine tools are equipped with a number of functions for volumetric accuracy enhancement. These functions can compensate for all the geometric errors of the axes, which are characterized by 21 errors for a 3-axis machine tool. Compensation data are stored in compensation tables and the control system calculates actual compensation based on these tables and actual machine tool position. Geometric errors are measured within the machine tool maintenance plan and it presents a time-consuming process. Measurement is mainly based on interferometric devices, where the precise setup of laser source and detector is challenging. This article introduces an original system for automatic interferometer set up to speed up the measurement of geometric errors. The system is designed for the interferometer Renishaw XM-60, which simultaneously measures all 6 degrees of freedom. The developed control software identifies interferometer position and transforms the measured data into a machine coordinate system. Numerical simulation verified the proposed solution.

Hybrid multibody system method for the dynamic analysis of an ultra‐precision fly‐cutting machine tool

AbstractThe dynamics of an ultra‐precision machine tool determines the precision of the machined surface. This study aims to propose an effective method to model and analyze the dynamics of an ultra‐precision fly‐cutting machine tool. First, the dynamic model of the machine tool considering the deformations of the cutter head and the lathe head is developed. Then, the mechanical elements are classified into M subsystems and F subsystems according to their properties and connections. The M‐subsystem equations are formulated using the transfer matrix method for multibody systems (MSTMM), and the F‐subsystem equations are analyzed using the finite element method and the Craig–Bampton reduction method. Furthermore, all the subsystems are assembled by combining the restriction equations at connection points among the subsystems to obtain the overall transfer equation of the machine tool system. Finally, the vibration characteristics of the machine tool are evaluated numerically and are validated experimentally. The proposed modeling and analysis method preserves the advantages of the MSTMM, such as high computational efficiency, low computational load, systematic reduction of the overall transfer equation, and generalization of its computational capability to general flexible‐body elements. In addition, this study provides theoretical insights and guidance for the design of ultra‐precision machine tools.

Iterative learning control for integrated system of robot and machine tool

AbstractThis study is concerned with the integrated system of a robot and a machine tool. The major task of robot is loading the workpiece to the machine tool for contour cutting. An iterative learning control (ILC) algorithm is proposed to improve the accuracy of the finished product. The proposed ILC is to modify the input command of the next machining cycle for both robot and machine tool to iteratively enhance the output accuracy of the robot and machine tool. The modified command is computed based on the current tracking/contour error. For the ILC of the robot, tracking error is considered as the control objective to reduce the tracking error of motion path, in particular, the error at the endpoint. Meanwhile, for the ILC of the machine tool, contour error is considered as the control objective to improve the contouring accuracy, which determines the quality of machining. In view of the complicated contour error model, the equivalent contour error instead of the actual contour error is taken as the control objective in this study. One challenge for the integrated system is that there exists an initial state error for the machine tool dynamics, violating the basic assumption of ILC. It will be shown in this study that the effects of initial state error can be significantly reduced by the ILC of the robot. The proposed ILC algorithm is verified experimentally on an integrated system of commercial robot and machine tool. The experimental results show that the proposed ILC can achieve more than 90% of reduction on both the RMS tracking error of the robot and the RMS contour error of the machine tool within six learning iterations. The results clearly validate the effectiveness of the proposed ILC for the integrated system.

An efficiency and quality based automatic drilling-riveting system for aero-structures assembly

Considering the constantly increasing demand for aircraft products, the development of automatic drilling-riveting machine tool systems is discussed in this paper. A structural conceptual design method considering both efficiency and quality is proposed. Particularly, a drilling-riveting machine tool system including single-column machine tool, light-weight end-effector, and assisting fixture system is built. The positioning error coordinates variation model is analyzed, and the ranges of reasonable processing parameters are determined. An experiment is conducted on the developed system to show satisfactory drilling-riveting quality and efficiency: compared with the position error of other similar systems, the position error with 0.082 mm is reduced by 41.42%, and the accuracy of aperture reaches H8, the rivets head height error ≤0.8 mm, and drilling and riveting efficiency can reach 6/min and 8/min.

Determination of the dynamic characteristics of a gear pump by the load variation method using special bench systems

Introduction. Some of the major factors in the occurrence of vibration of units, causing fatigue failure of the housings of elements, pipelines, and failure of pump elements, are pulsations of the working medium in the hydraulic systems of machine tools, fuel feed systems of aircraft engines and liquid-propellant engine supply. This study aimed at the implementation of a method for determining the dynamic characteristics of a volumetric pump using special bench systems, and the comparison of the calculation results to the experimental data. The stages of calculating the dynamic characteristics of a volumetric pump were described, taking into account the pre-developed special bench systems on the example of an external gear pump with a capacity of 14 cm3 /rev. The implementation of V. P. Shorin's load variation method using special bench systems developed by the authors with predetermined dynamic characteristics was shown. The main stages of the methodology for determining the dynamic characteristics of a gear pump were described.Materials and Methods. Methods of spectral analysis of pulsating pressure were applied in the work. Pulsations of fluid flow at the pump outlet were determined using the impedance method, the method of load variation, and special bench systems.Results. The paper implemented a technique for determining the dynamic characteristics of a gear pump in the drive shaft speed range of 500-2500 rpm for four harmonic components of the vibration spectrum in a wide range of dynamic loads (from inertial to capacitive nature). The bench systems yielding the calculation of the dynamic characteristics of the pump with a minimum error based on the condition of matching the dynamic load and the source of vibrations were analyzed. The developed approach to the evaluation of the dynamic characteristics of the pump was verified through comparing the calculated and experimental data of pressure pulsations in the bench systems with choke, cavity and an extended pipeline at the pump outlet.Discussion and Conclusions. The method for determining the dynamic characteristics of a volumetric pump was implemented using special bench systems developed by the authors. The research results show that the gear pump under study can be considered as an independent source of flow fluctuations, for which the deviation of its own dynamic characteristics from the average values does not exceed 10% for the first harmonic component.

Analysis and Design of Cooperative Control Solutions for Electrical Systems of Laser CNC Machines

In order to further improve the collaborative control efficiency of the electrical system of the laser CNC machine tool, an electrical system based on the ultrafast laser CNC machine tool is proposed. The design of the collaborative control scheme is discussed from three aspects: the overall control scheme design, hardware structure design, and control software implementation of the electrical system of the laser numerical control machine tool. The experimental results show that the length of the metal sheet processed by the electrical system of the laser CNC machine tool is 5.3 cm and the width is about 3.6 cm, and the actual processing aperture size will also change according to the set number of variable radius turns, and more precise processing can be enabled in the online test. The research results show that this electrical collaborative control system scheme has strong practicability and feasibility. With the help of system upgrades and optimization, the efficient integration of laser processing technology and numerical control technology can be better achieved. It should be noted that the machining process of the machine tool is also required to make in-depth improvements, which can maximize the operating efficiency and machining accuracy of the electrical system of the CNC machine tool. The laser CNC machine tool researched and developed in this paper verifies the rationality and feasibility of the collaborative control scheme through practical experiments, which can meet the processing requirements and realize the integration of laser processing technology and numerical control technology. High machining efficiency and machining accuracy.

Online monitoring of the axial thermal displacement of machine tools using a laser mouse sensor

We present a low-cost online system for monitoring the axial thermal displacement of machine tools. The proposed monitoring system includes an embedded optical sensor derived from a laser mouse; an image acquisition microcontroller; speckle patterns; and an edge computer that hosts software, including an image display module, a displacement calculation module, an image enhancement module, and a data visualization module. The proposed sensing system can measure the displacement in two orthogonal directions simultaneously by employing digital image correlation; thus, the proposed system is a two-dimensional displacement sensor. The sensing system is benefited from image enhancement techniques and customized optimal speckle patterns printed using a standard low-cost monochrome laser printer. Experimental results indicate that the proposed displacement sensing system has an accuracy and a precision of <5 μm in both orthogonal directions. The two-dimensional displacement sensing system was used for the online monitoring of the thermal deformation of a feed drive system for machine tools, and the performance was assessed experimentally. The complete procedure for measuring the displacement due to thermal deformation was conducted automatically and can be completed in 2 min using a program coded in a computer numerical control controller.

Machining cycle time prediction: Data-driven modelling of machine tool feedrate behavior with neural networks

Accurate prediction of machining cycle times is important in the manufacturing industry. Usually, Computer-Aided Manufacturing (CAM) software estimates the machining times using the commanded feedrate from the toolpath file using basic kinematic settings. Typically, the methods do not account for toolpath geometry or toolpath tolerance and therefore underestimate the machining cycle times considerably. Removing the need for machine-specific knowledge, this paper presents a data-driven feedrate and machining cycle time prediction method by building a neural network model for each machine tool axis. In this study, datasets composed of the commanded feedrate, nominal acceleration, toolpath geometry and the measured feedrate were used to train a neural network model. Validation trials using a representative industrial thin-wall structure component on a commercial machining center showed that this method estimated the machining time with more than 90% accuracy. This method showed that neural network models have the capability to learn the behavior of a complex machine tool system and predict cycle times. Further integration of the methods will be critical in the implantation of digital twins in Industry 4.0.

Cutting force prediction between different machine tool systems based on transfer learning method

Cutting force prediction between different machine tool systems based on transfer learning method

On-Machine Tool Condition Monitoring System Using Image Processing

An automation system of machine tools can free operators from simple and hard labor by the automatic loading and unloading of workpieces, and allowance of unmanned operation during nights and holidays. To achieve complete shop floor operations, various works that are manually performed by operators need to be automated as well. It is also crucial to accurately monitor tools to detect tool wear and chip winding during machining. In this study, we propose an on-machine tool condition monitoring system that measures tool length and diameter using images to detect tool breakage and winding chips.

Developing a Ball Screw Drive System of High-Speed Machine Tool Considering Dynamics

In order to provide guidance and reference for developing a ball screw drive system of the high-speed machine tool, this article investigates three basic issues with consideration of dynamics, including mechanical characteristics, inertia matching problem, and driving torque characteristics. The axial stiffness and natural frequency of the system are first calculated to verify the correctness of mechanical parts selection, and the stability of the system is ensured by designing an appropriate gain. vvNext, the inertia matching problem between the servo motor and load is analyzed based on the two-inertia-system, and the range of the inertia ratio is determined as an important principle in developing a ball screw drive system. The speed&#x2013;torque characteristics are further discussed, and a graphical method is proposed to quickly and accurately determine a suitable servo motor. Finally, a complete flow is described, and the ball screw drive systems of a high-speed machine tool for aeroengine casing manufacturing are developed as an application. The actual test results of speed, accuracy, cutting ability, and machining performance indicate that the developed ball screw drive systems have a satisfactory performance, which validate the effectiveness of the comprehensive study.

Design and Engineering Support of the Process of Deep Drilling of Heat-resistant Steel

Purpose of research. To attain the specified parameters of deep drilling accuracy by creating design and engineering support for the deep drilling process corresponding to the current level of technical and technological development.Methods. Analysis of machine tools and tool systems for drilling deep holes based on the monitoring of the conditions of machinery, equipment, cutting tools and tooling market of leading domestic and global manufacturers. Synthesis of a process system that ensures the creation of a product that meets the world level of quality with maximum process performance.Results. The design and engineering support of the process of deep drilling of high-alloy heat–resistant steel using modern equipment with a high-performance tool - a gun drill, was created. The equipment for the deep drilling process – an anti-vibration rest, an oil receiver, a clamping chuck, a guide bush are proposed. The conditions for the control of flow chips formed during deep drilling with a gun drill and its removal from the processing zone with a cooling mixture are determined.Conclusion. On the basis of the tooling for deep drilling of heat-resistant steel, the possibilities of using both existing at enterprises equipment and modern but expensive equipment, taking into account the series production, are shown. Both of these ways make it possible to ensure the specified accuracy and roughness of processing.To implement the deep drilling process, it is required to design a special tool - a gun drill, taking into account the properties of the material being processed. When implementing the deep drilling process applying specialized equipment, the machining pattern is provided by the tooling of the machine. When implementing the process applying universal equipment, the selected machining pattern is connected with the design and manufacture of the tooling – a clamping chuck, an oil receiver, a guide bush. These tasks are implemented in two ways: by the specialists of the enterprise or involving third-party specialized organizations to design attachments and cutting tools. The choice of the way to implement the deep drilling process depends on the availability of equipment, the size of the batch of parts and technical and economic assessment.

Investigation of chatter prediction in end milling using morphological studies on aluminum alloy (Al6061)

Future machine tools have to be highly powerful systems to maintain the needed intellectual performance and stability. The machine tool systems such as Spindle/Tool-holder/Tool assembly are necessary to be optimized for their functionality or cutting performance to meet the productivity and accessibility requirements of the user. Prediction of the dynamic behavior at the spindle tool-tip is, therefore, an important criterion for assessing the machining stability of a machine tool at the design stage. In commercial practice, the machine tool system is extensively influenced by the dynamic rigidity of the spindle system. In this work, a realistic dynamic high-speed spindle/milling tool holder/tool system model is elaborated based on rotor dynamics predictions. Using the finite element modeling with the Timoshenko beam theory, the frequency response at the tool-tip has arrived initially. Further, the numerical model is validated with 3D ANSYS and experimental modal testing. The theoretical stability lobe diagram (SLD) depends on the stiffness and damping properties of the cutting tool and distinguishes the stable and unstable cutting conditions. The mechanism of chatter formation is investigated experimentally during the end milling of Aluminum alloy (Al6061) by two different techniques. Experimental cutting tests are conducted at different depths of cuts, the corresponding vibration signals and cutting samples are examined using the Scanning electronic microscope (SEM) and optical microscope. These studies provide a detailed investigation to predict the stable and unstable cutting zones at different machining conditions.

Local smoothing of the piecewise linear tool path in a CNC system for high-speed machining

The basic algorithms of trajectory smoothing in control systems of machine tools and industrial robots in the case of linear-piece trajectory assignment are considered. The method of local contour smoothing by cubic B-spline in three-axis interpolation, which together with the algorithm of quasi-optimal feed planning with jerk limiting allowed to signifi cantly increase the machining productivity, is discussed in detail. The insertion of splines takes into account userdefi ned geometric constraints and converts the original trajectory into a composite curve that has geometric continuity G2, providing continuity of accelerations for each link of the machine. The defi nition of geometric continuity G1 and G2 is discussed, and its effect on the quality of the machined surface.

A Cloud-Based System for the Optical Monitoring of Tool Conditions during Milling through the Detection of Chip Surface Size and Identification of Cutting Force Trends

This article presents a cloud-based system for the on-line monitoring of tool conditions in end milling. The novelty of this research is the developed system that connects the IoT (Internet of Things) platform for the monitoring of tool conditions in the cloud to the machine tool and optical system for the detection of cutting chip size. The optical system takes care of the acquisition and transfer of signals regarding chip size to the IoT application, where they are used as an indicator for the determination of tool conditions. In addition, the novelty of the presented approach is in the artificial intelligence integrated into the platform, which monitors a tool’s condition through identification of the current cutting force trend and protects the tool against excessive loading by correcting process parameters. The practical significance of the research is that it is a new system for fast tool condition monitoring, which ensures savings, reduces investment costs due to the use of a more cost-effective sensor, improves machining efficiency and allows remote process monitoring on mobile devices. A machining test was performed to verify the feasibility of the monitoring system. The results show that the developed system with an ANN (artificial neural network) for the recognition of cutting force patterns successfully detects tool damage and stops the process within 35 ms. This article reports a classification accuracy of 85.3% using an ANN with no error in the identification of tool breakage, which verifies the effectiveness and practicality of the approach.

Performance Optimization of CNC Machine Tool System Based on Sensor Data

CNC machine tools have been popularized in the development of the manufacturing industry because of their high precision, high speed, high efficiency, and safe and reliable processing. The numerical control system and the measurement system of the machine tool are the key components of the modern numerical control machine tool, especially the measurement system of the machine tool, which is a prerequisite to ensure the high precision of the machine tool. The sensor is an important part of the measurement system. Sensors are devices that can sense the measurement objects within the measurement range and convert them into output signals and information according to certain rules. They are mainly used to detect the operating objects, system status, and operating environment status of the machine tool system and effectively control the normal operation of the machine tool system. Accurate and reliable information is provided. This study takes the CNC machine tool control system as the research object and studies the optimization measures of the sensor data to its performance. First, a model of the CNC machine tool control system was built based on MATLAB/Simulink, and then, based on this model, an open-loop (without sensor) and closed-loop (with speed and position sensors) control systems were built and were finally verified by simulation. The research results show that when there are sensors, the CNC machine tool control system has better stability and robustness.

Health status evaluation method of CNC machine tools based on grey clustering analysis and fuzzy comprehensive evaluation

In order to solve the problem of health evaluation of CNC machine tools, an evaluation method based on grey clustering analysis and fuzzy comprehensive evaluation was proposed. The health status grade of in-service CNC machine tools was divided, and the performance indicator system of CNC machine tools was constructed. On the above basis, the relative importance of each performance and its indicators were combined, and grey clustering analysis and fuzzy comprehensive evaluation was utilized to evaluate the health status of in-service CNC machine tools to determine their health grade. The proposed health status evaluation method was applied to evaluate the health level of an in-service gantry CNC machine that can be used for the machining propellers, and the results shown that the health status of the whole gantry CNC machine tool is healthy. The proposed evaluation method provides useful references for further in-depth research on the health status analysis and optimization of CNC machine tools.

Sliding mode control strategy based lead screw control design in electromechanical tracking drive system

This article presents the research synthesis method of electromechanical tracking system for lead screw control using alternating current (AC) servo motors applied in machine tools, based on the framework of sliding mode control method and the state observer. Control algorithms are capable of compensating for perturbation components, with a state observation to be established to evaluate and compensate for uncertain nonlinear components. The chattering reduction techniques of sliding mode surface are proposed to improve the control effectiveness. The research results are verified by simulation on MATLAB-Simulink, showing that the proportional integral derivative (PID) controller has not met the control quality, but requires the sliding mode control law and state observation as proposed. Then this tracking drive system will work with better quality and can be developed for lead screw system of machine tool. The research results in the paper are also the completed development for calculating, designing, and manufacturing intelligent electrical drive systems in industrial applications.