State-space analysis of mode-coupling in orthogonal metal cutting under wave regeneration

In this paper, mode-coupling in orthogonal metal cutting is analyzed in the state-space domain. Closed-form equations are obtained for eigenvalues and eigenvectors of the joint system formed by the cutting process and the mechanical structure, as a function of cutting conditions and structure modal parameters. System stability is analyzed and variations of the joint system natural frequencies are proposed for monitoring stability, forces and indirectly tool condition. Finally, experimental tests are presented to validate theoretical formulations in orthogonal aluminium cutting.By means of a state-space domain analysis of the joint system, formed by the cutting process and the machine structure, it is possible to obtain mathematical formulations that quantitatively relate joint system natural frequencies to system stability. Therefore, it is possible to detect chatter vibrations before work piece quality is affected by monitoring vibrations during machining. These formulations facilitate the development of robust active vibration control system which will improve machine tool stability in the future.     

Detection of chatter vibration in end milling applying disturbance observer

Suppression of chatter vibration is required to improve the machined surface quality and enhance tool life. For monitoring the chatter vibrations, additional sensors such as acceleration sensors are generally used, which results in high costs and low reliability of the machine tools. In this study, a novel in-process method to detect chatter vibrations in end milling is developed on the basis of a disturbance observer theory. The developed system does not require any external sensors because it uses only the servo information of the spindle control system. Self-excited and forced chatter vibrations are successfully detected.     

On-line drilling chatter recognition and avoidance using an ART2—A neural network

In the paper, an adaptive resonance theory (ART2-A) neural network is applied to on-line recognition and avoidance of drilling chatter. It is shown that the ART2-A neural network can adaptively learn the features of the thrust force spectrum in a drilling process. As a result, drilling chatter can be automatically detected when a chatter feature starts to appear in the thrust force spectrum. Once chatter is detected, a spindle speed regulation method to suppress chatter is used. Experiments show that this new developed system can monitor and suppress drilling chatter efficiently even under varying cutting conditions.     

An experimental analysis of damped coupled vibrations in broaching

The paper reports for the first time aspects related to dynamics of broaching when features with complex geometries are generated. It describes an experimental analysis of causes and outcomes of damped-coupled vibrations when broaching semi-closed profiles, i.e. dovetails of gas turbine engines disks. Singular indentations or groups of tilted chatter marks were found in particular zones on broached surfaces. Analysis of force and acceleration signals revealed that damped-coupled vibrations that result in tilted chatter marks mainly occur due to specific geometry of cutting edges that enable coupling of three-dimensional (3D) vibrations. A new method to detect the appearance of tilted marks as a result of damped-coupled vibrations with particular frequency has been proposed by monitoring the elliptical movement of cutting edge via time and frequency domain analysis of two acceleration signals.     

基于小波包分解和奇异谱分析方法的立铣加工颤振监测

在立铣加工过程中,颤振是加工过程失稳的一个最重要的原因。颤振将会严重影响工件表面质量和材料去除率,加剧刀具磨损和恶化工作环境。虽然大部分颤振监测系统可以监测到颤振发生,但颤振发生时已经对工件和刀具产生了严重的损伤,因此,需要提前监测到颤振特征。在颤振发生过程中,振动信号具有在时域中不断增大,在频域中能量频移的特性。考虑这两个振动信号特征,提出了一种颤振特征提取方法。提取颤振发生频带中振动信号的能量比和奇异谱熵系数作为两个颤振特征,并通过人工神经网络模型实现切削颤振的识别。文中提出的颤振监测系统包括特征提取和分类,能够精确辨识立铣加工中的稳定、过渡和颤振状态。     

Analysis of acoustic emission in chatter vibration with tool wear effect in turning

The progressive wear of cutting tools and occurrence of chatter vibration often pose limiting factors on the achievable productivity in machining processes. An effective in-process monitoring system for tool wear and chatter therefore offers the unique advantage of relaxing the process parameter constraints and optimizing the machining production rate. This research presents a dynamic model of the cutting RMS acoustic emission (AE) signal when chatter occurs in turning, and it determines how this motion is related to the RMS AE signal in the presence of tool flank wear. The tool wear effect on acoustic emission generated in turning is expressed as an explicit function of the cutting parameters and tool/workpiece geometry. The AE generated from the sliding contact on the flank wear flat during chatter is investigated based on the energy dissipation principle. This model offers an explanation of the phenomenon of chatter vibration in the neighborhood of the chatter frequency of the tool. It also sheds light on the variation of the RMS AE signal power in close correlation to the characteristic of the state of wear. Cutting tests were conducted to determine the amplitude relationship between RMS AE and cutting parameters. It is shown that RMS AE is quite sensitive to the dynamic incremental changes in the friction and the wear flat mechanism active in machining processes.     

A new on-line spindle speed regulation strategy for chatter control

A new on-line control method to suppress regenerative chatter vibration during the machining process by regulating spindle speed is proposed. The dynamic cutting force signal collected from a dynamometer is passed through a low pass filter, and then digitized. The fast Fourier transform is carried out to obtain the corresponding power spectrum. The chatter frequency is identified when the intensity at a certain frequency other than the spindle speed and tooth passing frequency exceeds a critical value. Based on the identified chatter frequency, a new spindle speed is computed by applying the principle of keeping the phase between the present and previous undulations to 90°. The new speed command is executed while the cutting proceeds. It is found from simulation that the chatter vibration can be suppressed by this approach in the shortest time. This method is also verified by experiments through actual cutting of various materials by a computer numerically controlled milling machine. The main feature of this approach is that the feed of the machine tool does not need to be halted during the change of spindle speed. Hence, tool wear can be reduced. Furthermore, no system identification of the machine tool structure is needed, and therefore it has great potential in actual applications.     

Novel ancillary device for minimising machining vibrations in thin wall assemblies

Milling thin wall structures is challenging due to their low stiffness and hence consequential vibration problems. Most of the research in this area was focussed on minimising chatter vibrations—either through generation of stability lobes or by employing targeted damping solutions such as piezoelectric damping; such solutions are suitable only for damping resonant vibrations. However, most of the thin wall structures also get poor surface finish due to forced vibrations either at tooth cutting frequency or tool natural frequencies. In this work, relying on the importance of improving mass and stiffness for greater vibration reduction in milling circular thin-wall components, an innovative articulated device pre-tensioned by torsion springs is proposed. The concept is novel in the sense that it is compact and light-weight and can be used on any shape of thin wall structure. Employing such a device does not alter the nature of dynamic characteristic of the structure thus ensuring better control of achieved dimension of structure. Significant (8 times) vibration reduction was observed using proposed device.     

The effect of tool geometry on regenerative instability in ultrasonic vibration cutting

Ultrasonic vibration cutting as a cutting process has been widely used in the precision machining of difficult-to-cut materials due to an enhanced cutting stability and increased productivity. The authors' previous researches have shown that chatter vibration prediction is made possible by the suggested cutting model. This paper is an attempt to determine cutting parameters based on regenerative chatter prediction in order to facilitate the machining objectives of high accuracy, high efficiency and low cost in ultrasonic vibration cutting. The machinability of SCM440 steel, called typical hardened steel, is investigated theoretically and experimentally. The cutting model is developed by introducing an experimental cutting database of SCM440 steel. The simulation and experimental results show that the workpiece material parameter has a direct influence on the occurrence of regenerative chatter. In order to achieve the chatter-suppressing dynamics in hard ultrasonic vibration cutting, a stability diagram is predicted based on the simulated work displacement for tool geometry changing. The stability diagram indicates that the regions of the chatter-suppressing dynamics expand with increasing tool rake angle and decreasing tool clearance angle. It is also found from the predictive results that regenerative chatter can be suppressed by a change of tool geometry. The determined tool geometry with the aid of the computer simulation is demonstrated through actual data of ultrasonic vibration cutting. By the use of the designed tool geometry, a good experimental result is achieved.     

An expert troubleshooting system for the milling process

Common problems experienced in milling processes include forced and chatter vibrations, tolerance violations, chipping and premature wear of the tools. This paper presents an expert system which attempts to troubleshoot the source of milling problems by utilising dynamics data coupled with the opinion of the operator and acoustic Fourier spectrum data taken from the cutting process. The expert system utilises a fuzzy logic based process to interpret the signals and information, and recommends possible alterations to the process to achieve high-performance milling operations.Specific inference engines were developed to assess the chatter stability, variation in cutting force coefficient, tool run-out and forced vibration characteristics of the system. Lastly, a stability lobe plot interpretation engine to automate the lobe selection process and recommend new, chatter free cutting conditions, was also developed. The chatter stability inference engine was tested with real cutting data, through acoustic measurements taken from various cutting conditions on an aluminium milling process. The chatter inference engine successfully determined the stability of the system for each sampled cutting condition. The robustness of the troubleshooting system depends on the accuracy of acoustic and frequency response measurements.     

The effect of tool nose radius in ultrasonic vibration cutting of hard metal

A tool edge with a small nose radius can alleviate the regenerative chatter. In general, it is important for conventional cutting to use the smallest possible tool nose radius. However, a sharp tool shape has an adverse effect on tool strength and the instability of machining process still occurs. Previous researches have shown that vibration cutting has a higher cutting stability as compared with conventional cutting. In the present paper, the influence of tool nose radius on cutting characteristics including chatter vibration, cutting force and surface roughness is investigated by theory. It is found from the theoretical investigation that a steady vibration created by motion between the tool and the workpiece is still obtained even using a large nose radius in vibration cutting. This article presents a vibration cutting method using a large nose radius in order to solve chatter vibration and tool strength problem in hard-cutting. With a suitable nose radius size, experimental results show that a stable and a precise surface finish is achieved.     

机床颤振的若干研究和进展

本文根据颤振的发生机理分别阐述了机床颤振的理论模型的研究方法和发展过程，并且着重讨论了近十几年在机床颤振的控制及在线监控领域内的动态和进展。从文中可以看出机床颤振的研究日益深入，并且与其它学科之间不断交叉发展。     

Dynamic analysis of a motor-integrated method for a higher milling stability

The productivity in High Speed Cutting is often limited by undesirable vibration effects in the main spindle (chatter). In many cases these limits are far below the technically possible cutting parameters provided by the machine technology. This paper presents a new approach to a motor-integrated milling spindle with an embedded electromagnetic actuator to actively reduce chatter vibrations and increase productivity. It is based on the non-contact application of highly-dynamic damping forces on the spindle shaft. That way the process stability can be increased significantly. By measurement and simulation-based analysis of spindle dynamics and transient and analytical approaches to process stability, the efficiency of the damping method is demonstrated in theory. Finally, a new, soft magnetic composite based motor-integrated electromagnetic actuator is introduced in this article.     

Experimental research on cutting force variation during regenerative chatter vibration in a plain milling operation

A plain milling operation is characterized by a transient and intermittent cutting process, in which undeformed chip thickness varies continuously. The reverse is the case in variations of undeformed chip thickness in the processes of up- and down-milling. In the present study, the property of regenerative chatter vibration in a plain milling operation is investigated from the viewpoint of cutting force variation. With primary chatter vibration, the vibration energy supply is closely related to the collision of the cutting tool flank against the workpiece surface during vibration, which is induced by the bending vibration or the torsional vibration of the arbor. In addition to this factor, the regenerative effect is considered to be one of the main causes of the chatter excitation in regenerative chatter vibration. The simulation result of the cutting force variation during regenerative chatter vibration agrees well with the experimental result, when considering these factors. It is shown that the regenerative chatter vibration in the-down-milling process occurs more easily than in the up-milling process.     

A new method for the identification of stability lobes in machining

This paper introduces a new method for identifying the stability lobes in milling. The method depends on ramping the spindle speed while monitoring the behaviour of a chatter indicator. Based on the pattern of this indicator, the stability lobes are located accurately. The lobes are identified on-line without stopping the machine. It is not necessary to calculate the frequency spectrum of any vibration signal. The method was tested successfully in immersion down-milling and was shown to be applicable to slotting. Experimental results showed that the frequency characteristics of the stability lobes identified using the developed method are the same as those of the lobes established using constant speed cutting.     

Analysis of chatter suppression in vibration cutting

The occurrence of chatter is strongly influenced by the tool geometry in conventional cutting. Therefore, the tool geometry is regarded as a very important factor. On the other hand, it is known that vibration cutting is capable of cutting hardened steels. However, the theoretical explanation for finish hard-cutting with vibration cutting is still unknown. In this paper, experimental investigations show that chatter is effectively suppressed without relying on the tool geometry, and the work displacement amplitudes are reduced from a wide range of 10–102 μm to the range of 3–5 μm by applying vibration cutting. In order to study the precision machining mechanism of vibration cutting, a new cutting model which contains a vibration cutting process is proposed. Simulations of the chatter model exhibit the main feature of chatter suppression in vibration cutting. The simulation results are in good agreement with the measurement values and accurately predict the work displacement amplitudes of vibration cutting.     

Maximizing Chatter Free Material Removal Rate in Milling through Optimal Selection of Axial and Radial Depth of Cut Pairs

Chatter vibrations in milling, which develop due to dynamic interactions between the cutting tool and the workpiece, result in reduced productivity and part quality. Several stability models have been considered in previous publications, where mostly the stability limit in terms of axial depth of cut is emphasized for chatter free machining. In this paper, it is shown that, for the maximization of chatter free material removal rate, radial depth of cut is of equal importance. A method is proposed to determine the optimal combination of depths of cut, so that chatter free material removal rate is maximized. The application of the method is demonstrated on a pocketing example where significant reduction in the machining time is obtained using the optimal depths. The procedure can easily be integrated to a CAD/CAM system or a virtual machining environment in order to identify the optimal milling conditions.     

Theoretical-experimental modeling of milling machines for the prediction of chatter vibration

Productivity of high speed milling operations can be seriously limited by chatter occurrence. Chatter vibrations can imprint a poor surface finish on the workpiece and can damage the cutting tool and the machine. Chatter occurrence is strongly affected by the dynamic response of the whole system, i.e. the milling machine, the tool holder, the tool, the workpiece and the workpiece clamping fixture. Tool changes must be taken into account in order to properly predict chatter occurrence. In this study, a model of the milling machine-tool is proposed: the machine frame and the spindle were modeled by an experimentally evaluated modal model, while the tool was modeled by a discrete modal approach, based on the continuous beam shape analytical eigenfunctions. A chatter identification technique, based on this analytical-experimental model, was implemented. Tool changes can be easily taken into account without requiring any experimental tests. A 4 axis numerically controlled (NC) milling machine was instrumented in order to identify and validate the proposed model. The milling machine model was excited by regenerative, time-varying cutting forces, leading to a set of Delay Differential Equations (DDEs) with periodic coefficients. The stability lobe charts were evaluated using the semi-discretization method that was extended to n>2 degrees of freedom (dof) models. The stability predictions obtained by the analytical model are compared to the results of several cutting tests accomplished on the instrumented NC milling machine.     

Chatter suppression based on nonlinear vibration characteristic of electrorheological fluids

Self-excited chatter is a serious problem in machining shops and its frequency is always near the resonant frequency of the machining system. This paper proposes a novel design method for a tunable-stiffness boring bar containing an electrorheological (ER) fluid to suppress chatter in boring. The ER fluid undergoes a phase change when subjected to an external electrical field and serves as a complex spring behaving nonlinearly in the structure. The deformation modes of the ER fluid are dependent on the applied electric field and the strain amplitude. As a result, the global stiffness and energy dissipation properties of the boring bar under an electric field change drastically at different amplitudes of vibration. It is found that the amplitude of chatter can be prevented from increasing by using the nonlinear vibration characteristic of the ER fluid. It is shown experimentally that the chatter can be suppressed under a certain range of the electric field related to the cutting conditions.     

基于小波包和倒频谱分析的颤振特征提取方法

立铣加工过程中的颤振会严重影响工件表面质量和材料去除率,加剧刀具磨损和恶化工作环境。虽然大部分颤振监测系统可以监测到颤振发生,但颤振发生时已经对工件和刀具产生了严重的损伤,因此,需要提前监测到颤振特征。由于加工过程的非线性导致振动信号频率成分复杂,单一的时频分析方法难于得到可靠的颤振特征。通过小波包分解确定颤振发生频段并重构该频段信号,通过颤振发生频段的倒频谱辨识稳定、过渡和颤振状态。研究结果表明,该方法可以有效识别立铣加工过程的稳定、过渡和颤振状态。