Dynamic Characteristics Of Machine Tool Research Articles

Coupling effect of machine tool dynamic characteristics and cutting conditions on the cutting process vibration and high-speed micro-planing surface mid-frequency waviness

In addition to the dynamic characteristics of the machine tool, cutting conditions significantly influences the vibration of the cutting system and mid-frequency waviness of the workpiece surface in ultra-precision machining (UPM). In this work, the effects of cutting conditions on machining vibration and surface waviness were investigated by high-speed micro-planning experiments. The origin of vibration and waviness were elucidated by frequency-domain processing of the vibration signal and surface waviness analysis. The dynamic characteristics of machine tool are determined using modal test and analysis. The results revealed that the cutting force has different contributions to the workpiece vibration and tool vibration. In the time domain, distinct differences can be found in stability and attenuation of various waviness components. The divergent effects of cutting force components in different vibration forms are confirmed through a novel cutting force model of micro-planing. The investigation can further reveal the formation mechanisms of cutting system vibration and surface waviness during the micro-cutting process, which can provide practical guidance for suppressing mid-frequency waviness on machined surfaces.

Optimization of the mechanical properties of machine tool with steel reinforced mineral concrete

High damping and high static stiffness are essential to improve the static and dynamic characteristics of machine tools. Traditionally, the structural parts of machine tools are usually made of cast iron and steel, which may lead to poor surface finish and inaccurate dimensions of finished products. Therefore, many scholars have investigated other alternatives of structural material for machine tools, such as concrete, polymer concrete and epoxy granite. Although epoxy granite has better damping properties, its structural stiffness (as measured by Young’s modulus) is only 1/3 of the gray cast iron. Thus, this study would introduce several steel bar designs into epoxy granite to improve the static stiffness and quantitatively review the performance by finite element analysis. The results showed that the equivalent static stiffness and natural frequency were about 12 to 20% higher than the structural material of cast iron. Therefore, the proposed finite element model of vertical machining center (VMC) column in epoxy granite could serve as a feasible alternative to achieve higher damping or static stiffness, as it was also more environmentally friendly to the manufacturing process.

Construction Method of Digital Twin System for Thin-Walled Workpiece Machining Error Control Based on Analysis of Machine Tool Dynamic Characteristics

In the intelligent optimization process of aerospace thin-walled parts, there are issues such as solidification of core knowledge base, high system coupling degree, and real-time evaluation and optimization feedback required for the knowledge base. These problems make it difficult to expand the functions of the digital twin system and meet the growing processing needs, ultimately hindering the application of digital twin technology. To address these issues, a digital twin system for controlling processing errors in thin-walled parts was built using a microservices architecture. In addition, a method for building a digital twin system at the processing unit level with the best coupling degree was proposed, mainly targeting the dynamic characteristics analysis knowledge base of thin-walled parts. Furthermore, to meet the requirements for backward compatibility of the processing unit level digital twin system, a comprehensive solution including the construction, operation, evaluation, optimization, and visualization of a knowledge base for the dynamic characteristics of the processing unit was proposed, providing guidance for the digital transformation and upgrading of CNC machine tools and the optimization of processing technology based on digital twin technology.

Study on Method for Avoiding Chatter Vibration by Changing Machine Tool Rigidity

Chatter vibration is a major problem in machining. At present, chatter vibration is avoided by changing the cutting conditions based on the stability limit diagram such that chatter vibration does not occur. However, changing the cutting conditions may reduce the productivity. The stability limit is estimated from the relationship between the dynamic characteristics of the machine tool and the cutting conditions. Therefore, we propose a method for avoiding chatter vibration by changing the machine tool rigidity. The dynamic characteristics of a desktop milling machine tool developed in a previous study can be changed by changing the tensile load of the brace bar attached on the frame. We report the transition of the dynamic characteristics and stability limit diagram with changes in the rigidity of the desktop machine tool, and confirm the presence or absence of chatter vibration through machining tests.

Influence of spatial position of double turntable swing on dynamic characteristics of five-axis machine tool

In the machining of a five-axis machine tool with a double turntable, the dynamic characteristics of the machine tool are affected by different spatial positions of workpieces. Predicting the dynamic characteristics of machine tools at different spatial positions can effectively improve the machining accuracy of workpieces. In the present study, a five-axis CNC machine tool (Tuopu VMC-C50, China) is considered as the research object. Firstly, the bearing stiffness model of the double turntable rotary system is established, and the influence of the joint stiffness of the turntable bearing on the dynamic characteristics of the five-axis machine tool table rotary system is studied. Based on the established three-dimensional model and rigidity of the sliding joint surface of the machine tool, the machine tool modes at 81 working positions are analyzed. Accordingly, the influence of different swinging positions of the double turntable on the dynamic characteristics of the five-axis machine tool is studied. Finally, based on the three-dimensional Kriging prediction model, variations of the natural frequency and vibration amplitude of the system in the machining space for different swing table angles are analyzed.

Static and Dynamic Characteristics of Machine Tools with an Adhesive–Mechanical Support System

Static and Dynamic Characteristics of Machine Tools with an Adhesive–Mechanical Support System

Research on the commonness and dissimilarity of group machine tools based on BP and SAE algorithms

The use of group machine tools has become one of the important production methods for large-scale manufacturing of high-precision parts. Due to the impact and wear of the group machine tool for long-term machining, the dynamic characteristics of the machine tool structure change significantly, resulting in a decrease in product quality; at the same time, the random errors in manufacturing and assembly make the group machine tool structure dynamics more different. Studying the dynamic differences of group machine tools will help improve the processing of group machine tools. This paper proposes a deep learning algorithm based on Back-propagation (BP) neural network and Stacked Auto Encoder (SAE) network to study the dynamics of the machine tool structure under static and dynamic conditions. The BP neural network is used to train the characteristics of some machine tools, and the dynamic characteristics of other groups of machine tools are predicted by changing the parameters. Then SAE network is used to find out the influence rule of group machine tool structure, changing on group machine tool dynamic characteristics. Experiments have proved that this method has certain significance for the study of dynamic characteristics of group machine tools.

Holistic multi-scale model of contact stiffness considering subsurface deformation

The stiffness of mechanically fastened joints influences static and dynamic characteristics of machine tools. The conventional contact stiffness model considers only the deformation of roughness asperities based on uncertain topographic parameters. This study presents a holistic contact stiffness model considering the subsurface deformation and avoiding assumptions of topographic characteristics, which tend to introduce uncertainty. Subsurface deformation was also investigated via stress distribution measurements and finite element simulations. Using the proposed model, the stiffness of jointed parts was estimated with an error <15%. Moreover, the subsurface deformation's influence on contact surface design was discussed.

Lightweight heavy-duty CNC horizontal lathe based on particle damping materials

With the development of high-precision, high-speed, and green computer numerical controlled (CNC) machine tools, the advantages of lightweight versions of such tools are becoming increasingly obvious. Taking a ZN-XK2840 CNC lathe as an example, in this study a lightweight design method is proposed for CNC machine tools based on particle damping technology. The static performance of conventional CNC machine tools is analyzed using a finite element method, and the wall thickness is optimized. Then, the dynamic characteristics of conventional CNC machine tools, such as the modal characteristics and harmonic response, are analyzed. The particle damping technology can effectively reduce the vibration of the machine structure in the vibration transmission path. In this paper, the discrete element dynamic method is used for the finite element model of a lathe. Based on a traditional machine tool, the dynamic characteristics of the machine tool structure with a particle damper are analyzed. The lathe with a particle damper is manufactured according to the lightweight design. The acceleration and displacement amplitudes of lightweight machine tools are smaller than those of traditional machine tools. The third-order natural frequency of lightweight CNC machine bed is increased by about 28.49%, and the vibration amplitude of harmonic response is reduced by 3.38%. Compared with the similar machine tools used in industry, the light CNC lathe with particle damper reduces the weight by 50.8% on average.

An adaptive grinding chatter detection method considering the chatter frequency shift characteristic

At present the majority of the research works on machining chatter detection are based on energy level monitoring of certain chatter frequencies which are premised to be invariant. However, chatter frequencies are very complicated with multi-frequency/frequency band, time-variant characteristics and are also affected by many factors in the machining process. These characteristics make the traditional chatter detection methods vulnerable under the practical complex machining conditions. In this study, the complexity of grinding chatter frequency was investigated firstly. The influence from change of machining parameters and machine tool dynamic characteristics on grinding chatter frequencies were analyzed theoretically. Then a high accuracy and robust chatter detection method adaptable for dealing with complex machining conditions was proposed. Specifically, information fusion of feed motor current and vibration signal based on cross wavelet transform was utilized to extract chatter information adaptively and completely, whereas the harmonics of forced vibration and other noises were eliminated. To assess the grinding stability, two chatter indicators, i.e. normalized spectral entropy and logarithmic spectrum distance based on the cross wavelet transform spectrogram were devised. The efficacy of the proposed method was successfully verified through grinding operations in which both constant and variable machining were carried out. The results show that the developed method is capable of robustly detecting chatter in all the tested machining conditions and both the indicators have suitable properties to enable them for adaptive chatter detection. By comparison, spectrum distance has a monotonic relationship with the severity of chatter vibration, which makes it a comparatively better indicator.

Analysis and structural optimisation of bed structure for CNC lathe based on modal strain energy sensitivity

Based on the modal analysis of the finite element model of the bed, the position of the modal strain energy (MSE) concentration in the first mode is selected, and the sensitivity analysis method is used to optimise the design size of the bed. The optimisation results show that the first-order MSE can be distributed evenly after optimisation, and the quality of the bed can be reduced. At the same time, the blindness of structural optimisation can be reduced and the optimisation efficiency can be improved. This optimisation design process and method can be extended to other machine tool parts, and provide the important approach and theory reference for machine tool dynamic characteristics improvement and structure optimisation.

Reliability and Modal Analysis of Key Meta-Action Unit for CNC Machine Tool

Dynamic characteristics have an important impact on the reliability of computer numerical control (CNC) machine tool. As the basis of dynamic characteristics analysis, the modal analysis should be conducted. However, the current modal analysis methods have some shortcomings. For example, the selected research object cannot reflect the dynamic characteristics of the CNC machine tool, the selection of research object lacks a scientific basis, and the influence of coupling factors is also often ignored. To address these problems, the concepts of meta-action failure mode and the latest research results about meta-action were given herein. Meta-action unit is the smallest structural unit to ensure the normal operation of the machine tool, and it can fully reflect the dynamic characteristics of a machine tool, so it is taken as the research object. In order to reduce the blindness of selecting the analytic object when implementing the reliability improvement measures, we put forward the concept of a key meta-action unit and gave the corresponding extraction method. Meanwhile, electromechanical coupling was considered when built the dynamic model of a key meta-action unit. A CNC machine tool made in China was taken as an example, and a key meta-action unit was extracted and analyzed, then the corresponding reliability improvement measures were also given. The results verify the applicability and effectiveness of this method. The proposed method can solve the shortcomings of the current methods, which lays a foundation for further research of mechanical properties and reliability based on machine tool meta-action unit.

Dynamic Characteristic Optimization of Ball Screw Feed Drive in Machine Tool Based on Modal Extraction of State Space Model

The accurate prediction and research about the dynamic characteristic of machine tool during the design process are essential to develop high-performance machines. In order to consider the effect of mechanical modal characteristics on the whole dynamic performance and servo controller of ball screw feed drive fully, the modal characteristics of mechanical transmission are extracted and solved based on reduced state space matrix of ball screw feed system, which is also easy for integration with servo control model. On this basis, the step-by-step optimization method from the global to the local is proposed to optimize the servo controller and overall dynamic performance of ball screw feed drive. This step-by-step optimization method is as follows: first, the global initial optimal tuning of servo controller parameters under the influence of structure modal characteristics is originally realized through the analytic derivation of servo controller based on the extraction of state space modal characteristics. Second, the integrated model of the feed drive system is established. Finally, based on the above global optimization results, the overall dynamic performance, and the servo parameters of the feed drive system under the influence of the modal characteristics are further studied and optimized locally. Taking the ball screw feed drive studied in this paper like the example, its rise time and the settling time of step response are 0.089 and 0.104s before optimization, while these reduce to 0.038 and 0.092s after optimization. Its dynamic response performance after optimization is much better. The optimization result is also validated by the test: the rise time and the settling time by the test are 0.042 and 0.094s, which are close to that from the simulation. Also, the consistency of step response curves from experiment and simulation as well. All these show that: this prediction and optimization method of overall dynamic performance considering the modal characteristic influence of mechanical transmission fully is correct and reliable.

Mechanical and dynamic properties of alternate materials for machine tool structures: A review

Machine tool structures find large-scale applications in various machining industries due to their necessity to design several kinds of machines. However, the vibration produced on machining is a significant problem which has to be overcome. To suppress the vibration, several researchers have attempted to enhance the machine tool structure’s dynamic characteristics in the recent past. The composite materials have risen up as a new kind of material for the manufacture of machine tool structures with producing lesser vibrations since the past few decades. To increase the production of precision products, machining should be done at high speeds without producing vibration. The dynamic characteristics such as natural frequency as well as damping of machine tool structures are very important parameters. Hence, the improvement of these parameters is nothing but the improvement of dynamic characteristics, as said earlier. Therefore, the present review elaborates various available reports on the improved dynamic characteristics of machine tools. The review focused mainly on mechanical and dynamic properties of alternate materials for machine tools with different composite materials. Furthermore, conflicting conditions of suitable alternate material for the preparation of machine tools are also focused in the present review. To the best of our knowledge, the review on the use of alternate materials for the manufacturing of machine tool structures has not been reported elsewhere and hence the present review will provide useful information for subsequent researchers to enhance the scope of research work in the area of manufacturing machine tool structures.

Prediction of cutting force in milling process using vibration signals of machine tool

This paper presents a method based on the theory of Kalman filter (KF) to predict cutting force through vibration signals in milling process. The acceleration signal and displacement signal of the machine tool spindle vibration are acquired and used for the prediction of the cutting force. In actual machining process, different cutting parameters lead to different time and frequency domain characteristics of cutting force. As system output, the vibration of machine tool is influenced by the input signal of force and machine tool dynamic characteristics. The bandwidth of the transfer function between the cutting force and acceleration should be considered when adopting indirect cutting force measurement method by using vibration sensors. Because of the influence of structural modes of the spindle-tool system, the measurement bandwidth is limited and as a result, it leads to the distortion of measurements containing high frequencies. Consequently, it is necessary to compensate the disturbances caused by structural modes and the bandwidth of the system should be increased properly according to machining conditions. In order to implement the compensating strategy for the measurement, a Kalman filter is designed for the test system. Before the construction of the filter, the frequency response functions (FRFs) of the spindle-tool system are tested through experimental modal analysis (EMA). With the implementation of the compensation, the bandwidth of the testing system is increased significantly. Besides, a strong tracking filter (STF) is designed and applied to the system to improve filter performance. Application of the proposed method has been demonstrated experimentally through cutting tests under different experimental conditions on a three-axis vertical machining center.

Key Technologies of Finite Element Modeling of Machine Tools Based on ANSYS Platform

In order to establish the joint interface elements more quickly, and to analyze the dynamic characteristics of machine tools under different states (such as headstock at different position), the CAD model of machine tool must be divided effectively, so that the hexahedron elements with specified size can be generated in the area of the joint interfaces, so that the nodes on the joint surfaces can be coincident. Using APDL technology, the linear spring-damped elements are quickly established on the fixed joint (bolt joint interface between the bed and base) and the moving joint (guideway-slider, screw-nut). The experimental and simulation results show that the method is effective.It has considerable accuracy, and can greatly speed up the machine tool modeling, especially the processing speed of the joint interfaces.

Study on Construction Method of Geometry Simulation Environment of Eight-axis with Five-axis Linkage Machining Center

In order to solve the machining geometry simulation of an eight-axis with five-axis linkage boring and milling machining center produced by JOBS company in Italy, in this paper, the structure of geometry simulation environment was designed based on the analysis of functional requirements, the geometric constraint relations and motion relations of machine tool was analysed, the machine model, control model and measurement analysis model was built, and the machining geometry simulation environment was established that had the same all functions, characteristics and behaviours with real machine tool. The practical application shows that the simulation environment can factually reflect the static and dynamic characteristics of machine tool; the measurement analysis function can effectively ensure the quality of processing products and the safety of machine tool. The construction method of geometry simulation environment can provide a technology and method reference for constructing numerical control machining geometry simulation environment of other types multi-axis machine tool.

A method of measuring the frequency response function of the tool point based on the surface of feeding system

In the existing method of predicting the frequency response function of the tool point, the effect of lathe feed system is considered in the tool point frequency response function. The junction surface affects the tool point frequency response function, and the junction surface include: bearing–lead screw, lead screw–nut, working bench-guide rail, tool holder–turning tool. The lathe machining system is divided into four substructures, and the lead screw–nut junction surface and workbench-guide rail junction surface have a significant effect in the tool point frequency response function, so we mainly considered the impact of these junctions surface. Through the junction surface stiffness and junction surface damping to obtain a small response at the junction surface. And the frequency response function of the substructure is obtained by hammering method and Timoshenko beam model. Finally, the frequency response function of the substructure end is coupled with the vibration response of the lead screw–nut and workbench-guide rail by the response coupling method, and finally we get the tool point frequency response function. The CA6140 lathe was used as the experimental object, and the frequency response function of the tool point is predicted by above method. The experimental results show that the predicted tool point frequency response function has a high degree of coincidence. The method proposed in this paper can quickly get the frequency response function of the tool point which laid a foundation for the accurate prediction of static and dynamic characteristics of machine tool.

Analysis of the machine tool dynamic characteristics in manufacturing space based on the generalized dynamic response model

Machine tool dynamic characteristics are seriously affected by the changes of the machining poses and the spindle bearing joints dynamic properties. As these changes contribute much more complexity and uncertainty for predicting the machine tool dynamic characteristics accurately, a new method is developed to research the changing regularity of the whole machine tool dynamic characteristics in generalized manufacturing space. In this method, the dynamic flexibilities at the spindle nose of x, y, and z directions in the focused frequency range are taken to represent the whole machine tool dynamic characteristics. The response surface method (RSM) and the orthogonal experiment design method are combined to establish the generalized dynamic response model, which contains the information of the spatial poses and the spindle bearing dynamic parameters. To establish this model, the simulations arranged by the orthogonal design are conducted by utilizing the dynamic model modified approach based on the validated finite element model (FEM) of the whole machine tool. With evaluating the fitting degree of this generalized dynamic response model, it can be used to predict the dynamic characteristics in the manufacturing space. Furthermore, an algorithm based on the established model is proposed to calculate the effect factors acting on the whole machine tool dynamic characteristics, which are caused by the changes of the spindle bearing dynamic parameters. The proposed analytical method has been applied in a three-axis vertical machining center to establish its generalized dynamic response model. The dynamic flexibilities at the spindle nose in the generalized manufacturing space are predicted, and they are validated by the dynamic experiments. With the calibrated model, effect factors of the spindle bearing joints are also obtained. All the predicted results support a theoretical basis on the optimal process routes planning, and the proposed analytical method can lay a foundation for further study on the dynamic information of the tool point.

Evaluation of tool steel alloy performance in a milling operation through operational dynamic parameters

Dynamic characteristics of machine tools and cutting tools have gained much attention from researchers and industry as it is one of the major factors limiting productivity due to excessive vibrations such as chatter during the cutting process. Numerous factors have to be taken into consideration when selecting material of the cutter body. This paper presents a comparison between two cutter bodies with the same geometry but made out of different alloys. Pre-hardened steel and conventional tool steel were investigated in order to highlight the advantages of selecting correct material to achieve high performance tools with respect of chatter resistance. The experimental part of this paper consists of impact testing, machining tests and surface integrity measurements. Operational dynamic parameters obtained through auto-regressive moving average model estimates from machining tests under stable and unstable conditions was used to characterise the performance. The findings are in correlation with material damping research and chatter analysis, thus giving a strong coupling to material selection in tool holders for enhanced process stability. The research also shows that operational dynamic properties obtained through indirect measurements is a valuable tool for process stability characterisation.