Dynamic Behavior Of Machine Tool Research Articles

Concept for Individual and Lifetime-Adaptive Modeling of the Dynamic Behavior of Machine Tools

The increasing demand for personalized products and the lack of skilled workers, intensified by demographic change, are major challenges for the manufacturing industry in Europe. An important framework for addressing these issues is a digital twin that represents the dynamic behavior of machine tools to support the remaining skilled workers and optimize processes in virtual space. Existing methods for modeling the dynamic behavior of machine tools rely on the use of expert knowledge and require a significant amount of manual effort. In this paper, a concept is proposed for individualized and lifetime-adaptive modeling of the dynamic behavior of machine tools with the focus on the machine’s tool center point. Therefore, existing and proven algorithms are combined and applied to this use case. Additionally, it eliminates the need for detailed information about the machine’s kinematic structure and utilizes automated data collection, which reduces the dependence on expert knowledge. In preliminary tests, the algorithm for the initial model setup shows a fit of 99.88% on simulation data. The introduced re-fit approach for online parameter actualization is promising, as in preliminary tests, an accuracy of 95.23% could be reached.

Operational modal analysis used to identify dynamic behaviour of machine tools during milling

Operational modal analysis used to identify dynamic behaviour of machine tools during milling

Dimensionality Reduction of High-Fidelity Machine Tool Models by Using Global Sensitivity Analysis

Abstract Models that are able to accurately predict the dynamic behavior of machine tools are crucial for a variety of applications ranging from machine tool design to process simulations. However, with increasing accuracy, the models tend to become increasingly complex, which can cause problems identifying the unknown parameters which the models are based on. In this paper, a method is presented that shows how parameter identification can be eased by systematically reducing the dimensionality of a given dynamic machine tool model. The approach presented is based on ranking the model’s input parameters by means of a global sensitivity analysis (GSA). It is shown that the number of parameters, which need to be identified, can be drastically reduced with only limited impact on the model’s fidelity. This is validated by means of model evaluation criteria and frequency response functions which show a mean conformity of 98.9% with the full-scale reference model. The paper is concluded by a short demonstration on how to use the results from the GSA for parameter identification.

EFFECTS OF DIFFERENT EXCITATION MECHANISMS IN MACHINE TOOLS WHEN PERFORMING OUTPUT-ONLY MODAL ANALYSIS

Output-only Modal Analysis makes it possible to investigate the dynamic behaviour of machine tools, especially under process conditions. The differences between standstill and process state, which are important to be considered, result from process damping, gyroscopic moments of the rotating spindle, and changes in preloads and clearance in bearing and joints. Output-only Modal Analysis assumes a complete excitation of the structure by white noise characteristics. There are several mechanisms in a machine tool under process conditions that could cause a vibration response. Besides the cutting process itself, the movement of the NC-axes, as well as rotating fans of the control system and auxiliary units, can be excitation sources. This paper investigates to what extent several excitation mechanisms in machine tools fulfill the analysis assumption and in which way the identified modal parameters depend on the boundary conditions of the excitation characteristics.

Influence of the compensation method of machining errors of Bspline and Cspline

ABSTRACT Contemporary advances in manufacturing are moving towards the diversification of interpolation methods in Computer Aided Manufacturing (CAM) softwares thanks to the growing need of free-forms of high quality. New interpolations such as ‘Bspline’ and ‘Cspline’ provide accurate toolpaths, but they generate significant machining errors. The nodes insertion technique is one of compensation methods of those errors. This paper presents a study of the influence of this compensation method on the manufacturing process: dynamic behavior of machine-tools, errors of controller, and surface quality. To make this study, Bspline and Cspline interpolations, errors generated, and the compensation method are defined. In addition to that, feed rate, cycle time and roughness before and after compensation are measured in order to exanimate the impact of the nodes insertion method. The present work aims to deliver a framework that demonstrates how manufacturing companies with such specific parameters optimization can improve smartness and machining efficiency.

Development of a new method for joint damping identification in a bolted lap joint

Mechanical joints have considerable effects on dynamic behaviour of machine tools; thus, joint damping identification is important in studying the dynamics of mechanical structures. Due to the difficulties in analysis of microslip friction, the experimental prediction of microslip joint dynamics is of great importance. In this study, a new experimental approach is proposed to determine the damping of bolted lap joints. Because of the complex nature of the mechanical joints, the lap joint is isolated through the addition of a mechanical resonator, which consists of a lumped mass and spring, to the bolted structure. The frequency response function (FRF) of this system is used for joint damping identification. This approach is used for bolted structures under both translational and torsional excitations and overcomes difficulties associated with slip boundaries identification in the joint interface. The method is verified by comparing the obtained results with those of the hysteresis loop approach.

Identification of modal parameters of machine tools during cutting by operational modal analysis

The dynamic behaviour of machine tools differs between standstill and process state. Operational modal analysis (OMA) makes possible to identify modal parameter under process conditions. In this paper, it is shown, how far the assumption of a time-invariant behaviour as well as a broadband excitation can be fulfilled by exciting the machine with a cutting process. After describing the dynamic behaviour in the work space to determine a time-invariant tool path, a broadband excitation is generated by changing cutting parameters during milling. The modal parameters are identified by different OMA methods and compared to results of the experimental modal analysis.

Modeling of the dynamic behavior of machine tools: influences of damping, friction, control and motion

In the process of designing a machine tool virtual models are required to predict the dynamic behavior and optimize the machine tool performance. For this purpose, the different influencing factors mass, stiffness and damping properties as well as friction forces, feed drive controls and movements have to be considered in the simulation. However, usually no suitable models and modeling approaches are available for all of these various influencing factors. In this paper, models are provided for the mentioned influencing factors. Subsequently, a modeling approach is proposed, which allows to predict the dynamic behavior with high accuracy. By using this modeling approach, the influencing factors are investigated and evaluated with regard to their effects on the vibration behavior of a machine tool. The nonlinear friction forces and the linear dissipation sources have the greatest impact on the damping behavior. In comparison, the impact of the feed drive control on the vibration behavior is low. Movements can greatly influence the vibration behavior. Their effects are mainly restricted to the axial modes of the feed drives. At these modes, the damping ratios can vary under motion by up to ±35% compared to a standstill. With these insights and the proposed models and modeling approaches new possibilities arise to predict and optimize the dynamic behavior of a machine tool and thus to enhance the machine tool performance.

Friction in feed drives of machine tools: investigation, modeling and validation

Virtual simulation and optimization of the dynamic behavior of machine tools in the development phase is required to satisfy the increasing demands on machine tool performance. While mass and stiffness properties can be simulated with sufficient accuracy, often no suitable damping models are available for the components of machine tools. The commonly used linear damping models are predominantly linear hysteretic or viscous models. However, the linear damping models are often not appropriate to reflect the occurring nonlinear effects in machine tools with the required accuracy. The reason for these nonlinearities are predominantly the friction forces in feed drive components. To resolve these deficits, the friction in feed drive components is comprehensively investigated in this paper, models for friction forces are identified and coupled with a reduced, flexible multi-body system. With the identified friction models the measured friction curves can be reproduced very precisely. The coupled, reduced, flexible multi-body model allows to simulate the nonlinear effects and to predict the dynamic behavior of machine tools with high accuracy. Consequently, a further important step towards accurate virtual simulation of machine tools is made.

Development and analysis of a consciously designed Joint Interface Module for improvement of a machining system’s dynamic performance

Machining vibrations and dynamic instability of machine tools is an important consideration in machining systems. Common approaches for improving their dynamic performance target either the process, or intelligent, yet complex control systems with actuators. Given that machine tools’ dynamic characteristics are largely defined by the characteristics of the joints, this article proposes a novel concept, attempting to create a new paradigm for improving the dynamic behaviour of machine tools—introducing modular machine tools components (Joint Interface Modules—JIMs) with joints deliberately designed for increasing dynamic stiffness and enhancing damping with the use of viscoelastic materials. Through a systematic model-based design process, a prototype replicating a reference tool holder was constructed exploiting viscoelastic materials and the dynamic response of the machining system was improved as a result of its introduction; in machining experiments, the stability limit was increased from around 2 mm depth of cut to 4 mm depth of cut, without compromising the rigidity of the system or changing the process parameters. The article also includes the results of investigations regarding the introduction of such prototypes in a machine tool and discusses the shortcomings of the stability lobe diagrams as a method for evaluating the performance of machine tool components with viscoelastically treated joints.

Approach for a General Pose-dependent Model of the Dynamic Behavior of Large Lightweight Machine Tools for Vibration Reduction

Large-scale machine tools for the manufacturing of large work pieces, e.g. blades, casings or gears for wind turbines, feature pose-dependent dynamic behavior. Small structural damping coefficients lead to long decay times for structural vibrations that have negative impacts on the production process. Typically, these vibrations are handled by increasing the stiffness of the structure by adding mass. That is counterproductive to the needs of sustainable manufacturing as it leads to higher resource consumption both in material and in energy. Recent research activities have led to higher resource efficiency by radical mass reduction that rely on control-integrated active vibration avoidance and damping methods. These control methods depend on information describing the dynamic behavior of the controlled machine tools in order to tune the avoidance or reduction method parameters according to the current state of the machine.The paper describes the approach for a general pose-dependent model of the dynamic behavior of large lightweight machine tools that provides the necessary input to the aforementioned vibration avoidance and reduction methods to properly tackle machine vibrations. The paper starts with an overview of the state of the art of the pose-dependent dynamic behavior of machine tools followed by the most common methods for vibration avoidance and reduction. Based on the results of an experimental modal analysis of a lightweight machine tool structure, the relevant pose-dependency is shown and the relevant parameters to derive the dynamic behavior are deduced. Then, a general model structure to model the machine tool's dynamic behavior is introduced. After updating the model parameters to different discrete machine poses the dynamical behavior of the model and the real machine tool structure are compared. Finally, it is explained how the model contributes to the actual vibration reduction of lightweight machine tools.

Investigation of the Time-invariance and Causality of a Machine Tool for Performing Operational Modal Analysis

The dynamic behaviour of machine tools affects the work-piece quality and productivity. Generally, the difference between the dynamic properties at standstill and during machining is neglected by the commonly applied assessment methods for production systems. Operational modal analysis takes into account these cutting- and operation-dependent influences. However, operation conditions might cause violation of the common measurement assumptions regarding time-invariance and causality. The investigations consider the time-invariance by the measurement of frequency response functions over the working space including the evaluation of mode shapes. The causality is evaluated by dynamic measurements at TCP excited by feed motions.

A new approach to identifying the dynamic behavior of CNC machine tools with respect to different worktable feed speeds

The dynamics of the machine tool structure are important in high precision machining. Some researchers have studied that the dynamics are expected to change under different machining conditions. However, the dynamic behaviors of the machine tool at different worktable feed speeds are rarely studied. In this paper, an output-only modal identification available to predict the dynamics of the machine tool at different feed speeds is proposed. The excitation of this method uses the inertia force sequence caused by random idle running of the worktable. The first six modes of the entire machine tool structure are estimated using the proposed method. The results indicate that the running state of the worktable can influence the modes in which the worktable vibrates. The estimated natural frequencies and damping ratios decrease obviously as the feed speed increases. Furthermore, because this method enable to determine modal parameters by measuring the response of machine tool structure without using any artificial excitation, it can be used to predict the dynamic behaviors of the machine tool in entire working space effectively.

P217 Catheter-related bloodstream infections in oncologic patients: increasing role of fungi

To simulate the dynamic behaviour of machine tools, the stiffness, damping and inertia parameters of the structure are needed. While masses and stiffness parameters of structural parts can be obtained with a static measurement, the determination of damping parameters requires a thorough methodology. In this paper the common methodology for the identification of local damping parameters of machine components was extended by an additional step to isolate the damping of the test object more precisely. Furthermore test benches as well as the identified damping models for components of an exemplary linear axis are presented.

Evaluation of Dynamic Behavior of Machine Tools for Sculptured Surface Manufacturing

The production sector of molds and dies is under a continuous pressure for process optimization due the high lead time required for the production of complex tools. In recent years, to improve this sector, numerous studies have been aimed at optimization of the CAD/CAM/CNC chain. However, despite recent technological innovations such as development of cutting tools, machine tools, and process simulation systems, the machining step still represents a major bottleneck in this process. The current generation of Numerical Control (NC) programs that represents the tool path do not consider the dynamic characteristics of the machine tool in the decision-making process to choose the most appropriate cutting strategies, resulting in NC programs that do not reach levels of excellence. The main objective of this paper is to present the development of a reference index based on the dynamic characteristics of the Machine Tool to support the NC programming with a commercial CAM system. This index considers the Machine Response Time (MRT) of NC controller, the acceleration and deceleration on the axes of the machine tool and the tool path segment (size and inclination). The index makes possible an experimental evaluation of relationships between the programmed feed rate for the tool path and dynamic performance due to movements of the machine axes, thus becoming an important decision-making factor in choosing the best machining strategy and consequently a better plan of the production process in die and mold industries.

Damping models for machine tool components of linear axes

To simulate the dynamic behaviour of machine tools, the stiffness, damping and inertia parameters of the structure are needed. While masses and stiffness parameters of structural parts can be obtained with a static measurement, the determination of damping parameters requires a thorough methodology. In this paper the common methodology for the identification of local damping parameters of machine components was extended by an additional step to isolate the damping of the test object more precisely. Furthermore test benches as well as the identified damping models for components of an exemplary linear axis are presented.

Research on the Cutting Force Induced Error Measurement Method of the CNC Machine Tools

The cutting force induced error has been more important as the hard cutting and hard cutting material used. This paper proposed the direct measurement method of cutting force induced error, which realized the corresponding directly of the cutting force and the value of error in the working process. According to the discrimination of machine error characteristics reflected by the different measuring methods, it puts forward to building the mixed error model based on the measuring technology of one dimension and two dimension. The results could provides the test data and theoretical basis for the mechanism of the processing performance affected by the dynamic behavior of machine tools.

Identification of weak spots in the metrological investigation of dynamic machine behaviour

Both the measurement of frequency response functions and the modal analysis are established tools for the measurement of the dynamic compliance of machine tools. State of the art measurement technique enables an allocation of dynamic weak points of the machine structure and optimization of the machine design on the basis of these measurement results. In recent applications the metrological investigation is used for the parameterization and the verification of simulation models to describe the dynamic behaviour of machine tools, which demands high standards regarding to the quantitative quality of measurement results. This paper describes the interlaboratory comparison test, which has been carried out at well-known German research institutes and was initiated by the Laboratory for Machine Tools and Production Engineering (WZL) of the RWTH Aachen. In this regard, the question is discussed, to what extent the compliance behaviour of machine structures under real process conditions corresponds to behaviour, determined in metrological investigation under idealized test conditions.

Prediction of the dynamic behaviour of machine tools during the design process using mechatronic simulation models based on finite element analysis

Increased productivity, higher velocities and acceleration for feed and cutting motions are requirements for innovative machine tools. At the same time the production process must achieve reduced form and position deviations of the work-piece. Therefore knowledge of the dynamic behaviour of machine tools during the design process is essential to develop high-performance machines. Using finite element analysis and mechatronic simulation, taking the mechanical, electrical and control systems into account, is the first step for optimisation. Developing the control parameters using these simulation techniques is one of the major steps in detecting the mechatronic characteristics. This paper presents a method for developing the control parameters concerning tool to work-piece deviations of mechatronic simulation models including disturbance variables. As an example a 2-axis CNC test stand for feed drive axes will be visualised with its simulation and measurement results in the time and the frequency domain.

Control concept for piezo-based actuator-sensor-units for uniaxial vibration damping in machine tools

Additional piezo-based components in drive trains can significantly improve the dynamic behaviour of machine tools. In this article we present a piezo-based actuator-sensor-unit that is able to reduce uniaxial vibrations in ball screw driven feed axis of machine tools. A complex model of a feed axis including ASU was developed to design a controller. The control concept is based on the direct velocity feedback. A modular test bench was designed, assembled and investigated to verify the ASU’s suitability for different feed axis configurations. In conclusion, the results for the different configurations where evaluated regarding economical aspects.