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Chatter happens due to the low rigidity of the machining system, and chatter identification is essential for improving the surface quality and productivity. More understanding on the dynamic characteristics of the machining system would help identify chatter more efficiently. This paper proposes a method for detecting chatter in the milling process of the thin-walled workpiece by monitoring and analyzing the cutting torque. A varying chatter detection threshold based on the workpiece geometries, tool path, and dynamic characteristics is utilized to identify chatter. A high-frequency filter and a median filter are utilized for signal denoising. The method is experimentally validated, which proved that it could predict chatter efficiently at its early infancy. The method could be a supplementary to other chatter identification methods. [ABSTRACT FROM AUTHOR]

Chatter is considered as one of the most important causes of instability in the precision grinding process. Suffering from the influence of nonlinearity and changed machining condition, regenerative chatter signal is nonlinear and non-stationary in nature, which makes the linear representations and stationary assumed methods not appropriate. Wavelet bicoherence is considered as an effective method for these nonlinear signals analysis. However, current wavelet bicoherence is often estimated by integrating over the finite time interval. This procedure may result to the loss of the time information; hence, it is not appropriate for chatter signal that cannot be assumed to be stationary. In this paper, an instantaneous nonlinearity indicator-based method is proposed for regenerative chatter identification in grinding by using the servomotor current signal. Firstly, the nonlinearity of servomotor current affected by regenerative chatter is discussed. After that, a non-stationary wavelet bicoherence is proposed for the quadratic nonlinear coupling detection of these nonlinear and non-stationary signals. On this basis, an instantaneous nonlinearity indicator is further established for regenerative chatter identification. The effectiveness of the proposed method is verified by simulations and experiments, and the results show that, compared with the commonly used wavelet bicoherence and continuous wavelet transform, this method can not only describe the non-stationary chatter signal in time-frequency domain but also extract the true nonlinear coupling component resulted from regenerative chatter. [ABSTRACT FROM AUTHOR]

Regenerative chatter vibrations are common in drilling processes. These unwanted vibrations lead to considerable noise levels, damage the quality of the workpiece, and reduce tool life. The aim of this study is to simulate torsional and axial chatter vibrations as they play important roles in dynamic behavior of indexable insert drills with helical chip flutes. While asymmetric indexable drills are not the focal points in most of previous researches, this paper proposes a simulation routine which is adapted for indexable drills. Based on the theory of regenerative chatter vibration, a model is developed to include the asymmetric geometries and loadings that are inherent in the design of many indexable insert drills. Most indexable insert drills have two inserts located at different radial distances, namely central and peripheral inserts. Since the positions of the central and peripheral inserts are different, the displacement and thereby the change in chip thickness differs between the inserts. Additionally, the inserts have different geometries and cutting conditions, e.g., rake angle, coating, and cutting speed, which result in different cutting forces. This paper presents a time-domain simulation of torsional and axial vibrations by considering the differences in dynamics, cutting conditions, and cutting resistance for the central and peripheral inserts on the drill. The time-domain approach is chosen to be able to include nonlinearities in the model arising from the inserts jumping out of cut, multiple delays, backward motions of edges, and variable time delays in the system. The model is used to simulate cutting forces produced by each insert and responses of the system, in the form of displacements, to these forces. It is shown that displacements induced by dynamic torques are larger than those induced by dynamic axial forces. Finally, the vibration of a measurement point is simulated which is favorably comparable to the measurement results. [ABSTRACT FROM AUTHOR]

Boring is one of the most common operations to enlarge or finish predrilled deep holes and is usually conducted in low spindle speed ranges. Due to low dynamic performances of conventional boring bar, boring operation is often hindered by drastic vibration. In this paper, a design and optimization method of a special boring cutter with constrained layer damping (CLD) bar is developed based on the CLD beam theory and finite element method (FEM). The boring cutter designed is composed of four modules: cutting head, substrate layer, damping layer, and constrained layer, which can greatly improve the dynamic performances in boring process. Meanwhile, the effectiveness and capability of boring cutter with CLD bar for forced- and chatter-vibration suppression are presented based on experimental and numerical methods. The results show that (1) the method for restraining forced vibration is different from that for chatter suppression. The former can be greatly avoided by shifting the natural frequency of boring system, e.g., changing dynamic stiffness. The latter, due to its complexity in low spindle speed range, on the other hand, cannot be entirely avoided and can only be eliminated by reducing the possibility of occurrence, e.g., increasing damping. (2) The boring cutter with CLD bar developed can change dynamic stiffness through constrained layer and improve damping performance through damping layer. So, it can avoid forced vibration and eliminate chatter to some extent. In the presented parameter domains, the capability of CLD bar for chatter suppression is improved five times corresponding to that of conventional bar. [ABSTRACT FROM AUTHOR]

Vibration-assisted metal forming is one of the most recent and beneficial techniques used to improve material flow of the forming process. However, some technical challenges, such as the large power requirement of vibration generation and technical complexity of the metal forming process control, are difficult to solve. The main objective of this work is to investigate the possibility of applying electric-hydraulic chattering in the cold extrusion process. A series of forward cold extrusion experiments and finite element simulations were performed with and without electric-hydraulic chattering assistance. The experimental results showed that the maximum extrusion load was decreased by 5.1 % when electric-hydraulic chattering with an oil pressure of 8 MPa and a vibration frequency of 200 Hz was superimposed on the lower die. The effect of the oil pressure was more significant than the vibration frequency for the reduction of the extrusion load. A significant improvement in the surface finish of the formed parts was observed using the proposed design. The velocity of material flow during chattering extrusion process was changed cyclically, and the maximum velocity was higher than that achieved from the conventional extrusion process. The material flow at the corner area was improved. The effective stress distribution was altered periodically due to the chattering. However, the maximum effective stress was approximately equal to that attained by conventional method. Moreover, the effective strain distribution was more uniform compared to that produced by the conventional extrusion process. [ABSTRACT FROM AUTHOR]

The boundary layer approach is the most popular method to reduce the chattering phenomenon in sliding mode control (SMC) for uncertain nonlinear systems. This paper applies the fuzzy sliding mode structure based on the boundary layer theory which is used as speed controller of an indirect field-oriented control (IFOC) of an induction motor (IM) drive. A fuzzy inference system is assigned for reaching the controller part of the fuzzy sliding mode controller (FSMC) to eliminate the chattering phenomenon in spite of the small and large uncertainties in the system. The applied fuzzy system acts like a saturation function technique in a thin boundary layer near the sliding surface so that the stability of the system is guaranteed. Also, the equivalent control part is estimated to avoid the computational burden by an averaging filter. On the other hand, the averaging filter assists to improve the tracking performance despite the possibility of large uncertainties in the system so that the stability of the system is guaranteed. The main advantages of the proposed chattering-free speed controller are robustness to parameter variations and external load disturbance. The simulation results are shown to verify the effectiveness of the proposed speed controller, and its advantages are shown in comparison with the FSMC system and the conventional SMC. [ABSTRACT FROM AUTHOR]

The first part of this study (Carnero et al., Mechanical Systems and Signal Processing, 24:1138-1160, ) analysed the influence of the process variables and work cycles on the quality of the bearings manufactured in an automotive bearing plant. The study was focused on the analysis of the overall vibration reading produced by the contact between the tool and the part. An analysis of variance was conducted on the overall vibration readings, which reflected that high-frequency vibration displacements are sensitive to process setup variables as well as the quality of products manufactured. Nevertheless, it was also observed that overall vibration values are not sufficient to analyse the relationship between the mechanical behaviour (vibration) and final quality obtained from high-precision machining processes. In this article, therefore, a new study is conducted based on spectral vibration measurements. A new experiment has been designed taking as input variables the diameter and the rotating speed of the tool. The selection criterion is based on the strong influence of these two variables on high-frequency vibration displacement and quality of parts (chattering, measured in terms of Lob A and Lob B). Two identical grinding machine tools were used during the experimental phase. Output variables are high-frequency displacements and high- and low-frequency chattering. The statistical analysis used in the new experiments determines spectral bands of the process in which vibrations induced by tool-part contact relates to development of lobes in the part to be identified. The study allows identification of vibration bands that are suitable for control in order to guarantee quality of the parts produced. To achieve that goal, the concept of spectral identity of the processes has been introduced to incorporate vibration induced by the process itself in the spectrums and to differentiate that process vibration from other mechanical vibration sources. [ABSTRACT FROM AUTHOR]

The dynamic interaction between different machine tool subsystems can be exploited to increase the machine cutting performance. Simplified models are proposed to define useful guidelines that maximize the material removal rate in milling processes in different industrial situations. Two different cases are presented: in the first one, the machine influence on spindle dynamics is considered and optimized. In the second case, the interaction between the control system and the machine tool mechanical structure is analyzed, suggesting control tuning criteria that maximize cutting process stability. The procedure is applied to a real industrial case: a five-axis machine tool with a bi-rotative head. Moreover, requirements for the applicability of the proposed approach are investigated and described by analytical formulas. [ABSTRACT FROM AUTHOR]

The concept of directional factor for chatter stability analysis has been used from a long time ago. The analysis of its evolution for different feed directions in milling processes provides a good way of selecting the best cutting conditions. For very stable cutting directions, corresponding to very low directional factors, the single frequency analysis gives unacceptable results and multi-frequency, or alternative solutions as semi-discretisation must be used. It is found also that the period doubling lobes extend at both sides of the tooth pass frequency equivalent to twice the natural frequency. In these cases, helix angle has a very important effect on the stability. For end milling processes, where the mill axial pitch can be of the order of the stable limit depth of cut, a very stable situation is found except in those areas corresponding to period doubling, where instability islands are found. Besides, a graphical construction for accurate estimation of the stable limit depth of cut and starting rotational speed of the period doubling lobes is proposed. [ABSTRACT FROM AUTHOR]

Chatter presents one of the main problems in quality of machined surfaces limiting tool life, productivity, and tolerances. Chatter in milling and turning operations has been extensively analyzed; however, drilling operations have been neglected due to the complexity of drilling tools and problems that develop in the modeling of the tool. In this paper, an overview of chatter vibrations and chatter suppression in drilling has been presented. Models such as torsional–axial model, bending model, and the combination of axial and bending models have been presented showing the different effects each considers. Influence of parameters such as drill geometry, chisel edge, drill flank, pilot hole, margin engagement, stick–slip interaction, gyroscopic effect, and rotary inertia effects has been incorporated into the various models. In addition, the problem of chatter analysis and suppression in micromachining processes has been investigated. The differences in macro- and micro-scale problems have been addressed giving an overview of the current research directions and future work for both areas. The paper presents experimental as well as simulation data with the comparison of the two for clear understanding of the complex problems of chatter in drilling operations. [ABSTRACT FROM AUTHOR]

This paper proposes a new machining strategy to realize better dimensional accuracy as well as surface finish. The strategy attempts to keep cutting force and the material removal rate approximately constant and force the tool to move along a so-called Pythagorean-hodograph curve, which is a group of analytic curves with special properties, in a milling operation. In conventional milling operations, a constant feed rate is frequently used. This would invariably lead to higher material removal rates in concave regions and lower material removal rates in convex regions, and affects dimensional accuracy. This work aims to establish the theoretical basis to show how the above-mentioned phenomenon can be predicted. Additionally, chattering is a well-known problem in machining. In this work, it is postulated that keeping certain machining conditions approximately constant would help in reducing chattering during machining. A numerical example is given to illustrate the proposed machining strategy. [ABSTRACT FROM AUTHOR]

The original version of this article contained a mistake. Dong Kyu Lee has been corrected to Dong Gyu Lee. This error is regrettable. [ABSTRACT FROM AUTHOR]