Chatter Stability Research Articles

Chatter Stability Prediction and Process Parameters’ Optimization of Milling Considering Uncertain Tool Information

Stability is the prerequisite of a milling operation, and it seriously depends on machining parameters and machine tool dynamics. Considering that the tool information, including the tool clamping length, feeding direction, and spatial position, has significant effects on machine tool dynamics, this paper presents an efficient method to predict the tool information dependent-milling stability. A generalized regression neural network (GRNN) is established to predict the limiting axial cutting depth, where the machining parameters and tool information are taken as input variables. Moreover, an optimization model is proposed based on the machining parameters and tool information to maximize the material removal rate (MRR), where the GRNN model is taken as the stability constraint. A particle swarm optimization (PSO) algorithm is introduced to solve the optimization model and provide an optimal configuration of the machining parameters and tool information. A case study has been developed to train a GRNN model and establish an optimization model of a real machine tool. Then, effects of the tool information on milling stability were discussed, and an origin-symmetric phenomenon was observed as the feeding direction varied. The accuracy of the solved optimal process parameters corresponding to the maximum MRR was validated through a milling test.

Validation of variable helix milling instability islands

During machining, it is well-known that unstable self-excited vibrations known as regenerative chatter can limit productivity. There has been a great deal of research that has sought to understand regenerative chatter, and to avoid it through modifications to the machining process. One promising approach is the use of variable helix tools. Here, the time delay between successive tooth passes is intentionally modified, in order to improve the boundary of instability. Previous research has predicted that such tools can offer significant performance improvements whereby islands of instability occur in the stability lobe diagram. By avoiding these islands, it is possible to avoid regenerative chatter, at depths of cut that are orders of magnitude higher than for traditional tools. However, to the authors’ knowledge, these predictions have not been experimentally validated, and there is limited understanding of the parameters that can give rise to these improvements. The present study seeks to address this shortfall. A recent approach to analysing regenerative chatter stability is modified, and its numerical convergence is shown to outperform alternative methods. It is then shown that islands of instability only emerge at relatively high levels of structural damping, and that they are particularly susceptible to model convergence effects. The model predictions are validated against detailed experimental data that uses a specially designed configuration to minimise experimental error. To the authors’ knowledge, this provides the first experimentally validated study of unstable islands in variable helix milling, whilst also demonstrating the importance of structural damping and numerical convergence on the prediction accuracy.

Transmissibility Enhanced Inverse Chatter Stability Solution

Abstract The structural dynamics of a machine tool at the tool center point characterizes its vibration response and machining stability which affects productivity. The dynamics are mostly dominated by the spindle-holder-tool assembly whose main vibration mode can change during machining due to centrifugal forces, thermal expansion, and gyroscopic moments generated at high spindle speeds. This paper proposes the identification of the spindle's in-process modal parameters: natural frequency, damping ratio and modal constant, by using a limited number of vibration transmissibility and critical chatter stability measurements. The classical inverse stability solution, which tunes the modal parameters to minimize prediction errors in chatter stability limits, is augmented with vibration transmissibility under two methods: (1) transmissibility-enhanced inverse stability solution: the modal parameters are updated to minimize prediction errors in chatter stability, and vibration transmissibility; (2) artificial neural network (ANN)-integrated inverse stability solution: the ANN uses vibration transmissibility to estimate the natural frequency and damping ratio, such that the inverse stability solution only needs to identify the modal constant. While both methods are experimentally validated, it is shown that the transmissibility-enhanced inverse stability solution is a more effective approach than the time-consuming ANN alternative for the estimation of in-process spindle dynamics.

Three dimensional prediction of stability lobes in end milling of thin-walled structures based on tool and workpiece dimensions

Nowadays, researchers are very interested to investigate the dynamic behavior of the thin-walled structures during the machining process due to their broad application in aerospace, automotive industries and etc. One of the main problems in machining of thin-walled structures is unstable chatter vibrations, which causes the poor machined surface quality and decreases the system life span. In this regard, the main aim of this paper is to propose a practical method to solve the chatter instability problem during the milling process of thin-walled components. To this end, first the effects of geometrical parameters like workpiece height, thickness, and tool overhang, diameter and their ratios on the chatter stability are investigated. Then three dimensional stability lobe diagrams (SLDs) base on the mentioned parameters are presented for the first time. In which one can implement the mentioned diagrams to switch the unstable machining process to stable one by changing the value of the system parameters. Finally, the results obtained by the experimental test show that the presented three dimensional diagrams can be utilized to avoid chatter instability in the milling process.

Research on the milling stability of thin-walled parts based on the semi-discretization method of improved Runge-Kutta method

Chatter is a very common phenomenon in the milling process. The occurrence of chatter will cause chatter marks on the processed surface, cause the tool to jump, and even bring sharp and harsh noise, making the processed surface unable to meet the accuracy requirements. Thin-walled parts are more prone to chatter due to their lower stiffness. In order to avoid the occurrence of chatter, this paper adopts the method of combining theoretical analysis with milling experiments to conduct in-depth research on the prediction of chatter. Considering the first-order and second-order modal parameters of the tool and thin-walled part as well as the specific processing parameters, based on the milling dynamics model, a semi-discretization method based on the improved Runge-Kutta method (IRKM-SDM) is used to draw the chatter stability lobe diagram. Through simulation and experiments, it is proved that in terms of simulation accuracy and calculation speed, compared with the zero-order analysis (ZOA), multi-frequency solution (MFS), and the traditional semi-discretization method (SDM), the stability lobe diagram obtained by the IRKM-SDM is more advantageous. This study is of great significance for optimizing cutting parameters and suppressing chatter in the actual machining process.

Chatter stability and surface quality in milling of unidirectional carbon fiber reinforced polymer

Carbon fiber reinforced polymers (CFRPs) are typical difficult-to-machine materials. Due to their anisotropic material property, chatter and poor surface quality are easy to occur during machining. In this paper, the mechanical cutting force model and dynamic milling system model considering the anisotropy of unidirectional CFRP (UD-CFRP) were established. The cutting force coefficients and the edge force coefficients in the cutting force model were calculated as functions of fiber cutting angle. The cutting force model can accurately describe the variation laws of milling force in different fiber directions. The chatter stability of milling UD-CFRP was analyzed and the influence of the fiber direction on it was studied. The influence laws of the fiber directions and chatter stability on surface quality were discussed, and a variable related to cutting stability, chatter factor, was defined, which provides a novel method for predicting surface quality of machined UD-CFRP.

Can mode coupling chatter happen in milling?

In milling, two different self-excited vibrations have been reported; regenerative and mode coupling chatter. The regenerative chatter mechanism has been extensively studied and validated with tests whereas mode coupling chatter mechanism was reported a long time ago for threading operations. The presented mode coupling chatter models were based only on current vibrations of the system but not on the delayed vibrations. With the increase in the research carried out on robotic milling operations, the low frequency severe self-excited vibrations at high spindle speeds were claimed to be mode coupling chatter by many researchers. However, the justification of mode coupling chatter mechanism is absent from predicted stability boundaries and a strong evidence distinguishing it from regenerative chatter mechanism is not present. Additionally, mode coupling chatter models applied to milling was based on threading operations and hence they were not capturing the characteristics of intermittent milling process. Therefore, this paper focuses on the diagnosis of mode coupling chatter in robotic milling. Mode coupling chatter principles are applied to milling process considering its intermittent characteristics. The zeroth order approximation for mode coupling chatter mechanism is adapted for milling and extended to multi frequency approximation. Mode coupling chatter stability boundaries are calculated, explored with tests and compared with regenerative chatter stability boundaries. Results show that mode coupling chatter stability boundaries are very low and vaguely dependent on the spindle speed. This contradicts the stability observed in the experimental tests. Hence, it is concluded that mode coupling chatter in milling is not possible, because the assumption of the chip thickness depending only on current vibrations does not apply to milling operations. The novelty of the presented paper is the theoretical and experimental justification that mode coupling chatter is not possible in milling operations.

Chatter suppression in micro-milling using shank-mounted Two-DOF tuned mass damper

The tuned mass damper (TMD) has been used in machining processes for reducing forced vibration, suppressing chatter, and improving machined surface quality. In micro-milling process, the tiny size of the cutting tool-tip and the high rotating speed bring challenges in implementing the TMD. Besides, the TMD needs to have two degrees-of-freedom (DOFs) for reducing vibrations of micro-mill in two orthogonal directions. This paper presents the chatter suppression for micro-milling by attaching a two-DOF TMD to the tool shank and rotates with the cutting tool. The frequency response function (FRF) at the tip of the micro-mill clamped by an aerostatic spindle is predicted using receptance coupling analysis. A two-DOF TMD is designed via graphical approach based on the FRF result at the tool-tip. The natural frequencies and damping ratio of the TMD are optimized under different spindle speeds in order to enhance the cutting stability. The chatter stability of micro-milling is predicted considering the gyroscopic and centrifugal effects of the TMD structure. Modal tests and micro-milling experiments are conducted to validate the effect of the TMD on chatter stability. The results show that the TMD is able to improve the critical depth of cut by 13 folds, and satisfy the compact design requirement for micro-milling.

Analytical prediction of chatter stability for modulated turning

This paper proposes an analytical chatter stability prediction for low frequency vibration assisted turning process, i.e. the modulated turning. Introducing low frequency tool modulations in machining is initially proposed to transform continuous cutting process into a discrete one so that chip-entanglement/jam could be eliminated. However, tool modulation assistance has a fundamental effect on chatter stability. This paper is the first attempt to uncover the regenerative chatter stability of the modulated turning process by a frequency-domain analysis. Tool engagement kinematics and the dynamic chip generation mechanism are modeled. It is understood that unlike conventional turning or milling processes, modulated turning process exhibits distinct stabilizing effects due to 1) short-time tool engagement and 2) multiple regenerative loops as the dynamic chip thickness is affected by past two revolutions. Considering the average immersion of the cutting tool (cutting duty cycle), a zero-th order frequency domain chatter stability model for the process is proposed. The developed model is used to quantify the effect of multi-delay regeneration and tool modulation parameters on the stability loop formation. The analytical stability solutions are benchmarked against the time domain digital simulation results, and both predictions are validated through orthogonal cutting experiments. It is shown that the proposed solution can accurately generate the Stability Lobe Diagram (SLD) of modulated turning for typical modulation conditions.

Improved semi-discretization method based on predictor-corrector scheme for milling stability analysis

Chatter has a detrimental effect on the milling process, which makes the chatter stability prediction very critical in milling. This paper presents a new semi-discretization method (SDM) based on predictor-corrector scheme to predict the stability of milling process. Firstly, the dynamic of the milling system is modelled by delay-differential equation (DDE), and the forced vibration duration is divided into many parts. Secondly, the DDE is integrated on the small time interval. The time-delay term and the periodic coefficient matrix are taken as an operator and approximated by second-order interpolation polynomial. And then, the state transition matrix is constructed based on predictor-corrector scheme. The proposed method is validated by comparing it with the benchmark. In general, the proposed method is superior compared with the benchmark in terms of the rate of convergence. Besides, the proposed method is also a robust method. The computational efficiency of the proposed method is proved to be good. For a one degree of freedom (1-DOF) milling system under full immersion condition, the stability lobe diagrams obtained by the proposed method are much closer to the reference than those obtained by the PCHDM and UNIM, especially in the peak of the lobes. The proposed method also can be used to predict the stability for a 2-DOF milling system accurately. It is also indicated from this study that the prediction accuracy of the SDM can be improved by combining the predictor-corrector scheme.

Theoretical and experimental study of the chatter vibration in wet and MQL machining conditions in turning process

Turning is one of the most commonly used cutting processes for manufacturing components in production engineering. The turning process, in some cases, is accompanied by intense relative movements between tool and workpiece, which is called chatter vibrations. Chatter has been identified as a detrimental problem that adversely impacts surface finish, tool life, process productivity, and dimensional accuracy of the machined part. Cooling/Lubrication in the turning process is normally done for some reasons, including friction and force reduction, temperature decrement, and surface finish improvement. Wet cooling is a traditional cooling/lubrication process that has been used in machining since the past. Besides, a variety of new cooling and lubricating approaches have been developed in recent years, such as the minimum quantity lubrication (MQL), cryogenic cooling, nanolubrication, etc., due to ecological issues. Despite the importance of cooling/lubrication in machining, there is a lack of research on chatter stability in the presence of cutting fluid in cutting processes. In this study, the chatter vibration in turning process for two cooling/lubrication conditions of conventional wet and MQL is investigated. An integrated theoretical model is used to predict both the metal cutting force and the chatter stability lobe diagram (SLD) in turning process. This model involves deriving a math equation for predicting metal cutting force for both wet and MQL conditions using experimental training force data and a Genetic Expression Programming (GEP)-based regression model. Also, the traditional single degree of freedom chatter model is used here for predicting the SLDs. The chatter model is discussed and verified with experimental tests. Then, the experimental results of the tool's acceleration signal, work surface texture, surface roughness, chip shape, and tool wear are presented and compared for wet and MQL conditions. The results of this study show that the cooling/lubrication systems such as wet or MQL have a considerable effect on the SLDs. Also, the predicted results of metal cutting force and SLD for both wet and MQL techniques are in good agreement with the experimental data. Therefore, it is recommended that for each lubrication condition including wet, or MQL, the SLD be determined to achieve higher machinability.

Comparative analysis of chatter in micro end milling operations

The advancement in microfabrication in pursuit of miniaturization is cutting a dynamic role for micro-milling. Micro-milling is a fast and cost-efficient threedimensional fabrication method with the ability to achieve good accuracy, low surface roughness and extraordinary material removal rate (MRR) compared to some other micro manufacturing processes. Given the need for components with high dimensional integrity, identification of appropriate machining conditions is very vital to getting good surface finish. It is also required to couple different complex dynamics which are associated with this machining technique. Chatter during milling and micro-milling process bring adverse effects in surface quality, dimensional accuracy and in tool life. There is need for versatile modelling of micro end-milling dynamics in order to understand how chatter affects the micro end milling operation and to advance the synthetic application of several chatter stability parameters to overcome the limitation imposed by the single chatter stability criterion. This study performs a linear and non-linear analysis of chatter in micro end milling based on the major influential parameters. In addition, a detailed time domain simulation for micro end milling was performed to predict the force a displacement resulting from variable cutter geometries. This dynamic simulation results have significant impact on the actual micro milling process towards the manufacturing of miniaturized parts.

Simpson’s 3/8–based method stability analysis for milling processes

Chatter stability prediction based on linear time-periodic delay differential equations (DDEs) of the milling dynamic process is an important issue in achieving high-performance processing operations. To obtain efficient and precise regenerative chatter prediction, a novel method for milling stability analysis based on Simpson’s 3/8 rule is proposed. First, the dynamics model of milling processes, considering the regenerative effect, is modeled by an n-dimensional delay differential equation. Second, a set of algebraic equations is obtained to determine the transition matrix by converting and discretizing periodic DDE into a system of integral equations at the neighboring sampling grid points. Then, the eigenvalues of the transition matrix are utilized to determine stability based on Floquet theory. Finally, the comparisons of lobe diagrams calculated by different methods and discretization interval number for one and two degrees of freedom milling models are discussed, and the consistency of the predicted and experimental stability boundaries is analyzed. Results show that the proposed method achieves high computation accuracy with high efficiency.

A chatter detection method in milling of thin-walled TC4 alloy workpiece based on auto-encoding and hybrid clustering

Chatter is a typical fault in milling, which is a self-excited vibration. Chatter can be diagnosed by using several methods, such as Delay Differential Equation (DDE) modelling methods, Artificial Neural Network (ANN) supervised learning methods, etc. These methods are effective. But they have some limitations, such as susceptible to measurement errors, require all data to be labeled, etc. In this paper, an unsupervised method to diagnose the chatter stability in milling according to massive unlabeled measured dynamic signals is proposed. The proposed method is not susceptible to measurement errors, don’t require labels, and is robust. The dynamic signals are acquired from several milling trials. In the proposed method, the measured signals are compressed by using auto-encoding based method. Then, a hybrid clustering method based on both density metric and distance metric is used to cluster the compressed signals. The proposed method achieves a detection accuracy of 95.6033% on the experimental measured dynamic signals.

Dynamics-based optimization of rolling schedule aiming at dual goals of chatter suppression and speed increase for a 5-stand cold tandem rolling mill

In the process of cold tandem rolling, chatter instability leads to serious impacts on enhancing rolling speed, improving product quality, reducing production cost and realizing intellectualization. Chatter occurs with the rolling speed up to a certain threshold value, but the critical speed is determined by both product specifications and rolling schedules. A 5-stand cold tandem rolling mill whose first three stands and subsequent two stands, respectively, have four and six rolls was investigated by formulating its dynamic equations with the corresponding structure–process coupling. By applying the stability-based calculation model about the critical rolling speed in each stand, the system dynamic responses around the critical rolling speed were simulated, and the system eigenvalues which represent instability and characteristic frequencies were figured out. Thereafter, via combining the critical rolling speeds with the system dynamic behavior, a dynamics-based optimization model of rolling schedule for the 5-stand cold tandem system was proposed for the purposes of both the chatter suppression and rolling speed increase. In the optimization model, eight rolling technique parameters (four strip thicknesses and four tensions between the upstream and downstream stands) were taken as design variables, and the constraint conditions were set as no chatter instability in all five stands, and the optimization goal was to maximize the outlet speed of the final stand. The pattern search method was introduced to solve the optimization model. By applying such a dynamics-based optimization model for the 5-stand cold tandem rolling process, the chatter instability was suppressed effectively and the rolling efficiency was improved considerably; therefore, such an optimization model is expected to be valuable for intelligent manufacturing of rolling process.

A 3D Deep Learning Model for Rapid Prediction of Structural Dynamics of Workpieces During Machining

Chatter stability in machining flexible parts depends directly on the structural dynamics of the workpiece. This paper proposes a novel two-stage framework that combines finite element (FE) and data-driven deep learning techniques to rapidly predict the varying dynamics of workpieces during machining. An automated framework is developed to create a large training dataset of CAD models with gradually-changing geometries. A deep 3D Convolutional Neural Network (3D-CNN) is developed to “learn” the variations in dynamic parameters as a function of geometry. The current model has been successfully implemented for prediction of natural frequencies of workpieces during turning operations. The proposed framework can be used as a computationally efficient tool in online process monitoring and automated correction applications.

Milling stability prediction based on the hybrid interpolation scheme of the Newton and Lagrange polynomials

The stability lobe diagram (SLD) is commonly used to determine the suitable cutting parameters of the machining system in order to achieve a chatter-free machining process. An improved full-discretization method (FDM) is proposed to predict the SLD based on the hybrid interpolation scheme of the Newton and Lagrange polynomials. In order to solve the SLD, a third-order Newton polynomial is employed to interpolate the state term of the physical space equation of the system. Meanwhile, to investigate the influence of the interpolation order on predicting the SLD, the delayed term is estimated using the Lagrange polynomials of orders one to four successively. Subsequently, after constructing the transition matrix, a series of calculation for the stability prediction are carried out by applying Floquet theory. The calculated results from these constructed methods demonstrate that the FDM with a second-order Lagrange polynomial is optimal, and further has better computational performance compared with some existing discretization methods. Lastly, the influences of the dynamic parameters on chatter stability are analyzed based on the proposed FDM. When the stiffness and damping ratio increase, the limit cutting depth will be enhanced. The increasing natural frequency not only causes an obvious shift of the lobes to the right, but also raises the limit cutting depth to some extent. These theoretical analyses can guide the prediction and improvement of the chatter stability of a machining system.

Chatter Stability Prediction in Low Immersion Milling Based on Time-domain Simulation

Chatter Stability Prediction in Low Immersion Milling Based on Time-domain Simulation

Design optimization of tool holder extension for enhanced chatter stability by using component mode tuning method

Tool overhang length tuning by matching the modes of the tool-holder extension-holder-spindle (TEHS) assembly components creating dynamic absorber effect is one of the rare chatter suppression techniques. In some milling applications, a tool-holder extension may have to be used to increase the overall tool overhang length for reaching far sections of the workpiece. In this work, design parameters of the holder extension and the tool are optimized simultaneously to maximize the dynamic absorber effect resulting from modal interactions among the components of the TEHS assembly. Improvements in the dynamic rigidity and chatter free Material Removal Rate (MRR) are demonstrated through simulation results.

Research on machining technology of complex structure parts of high-speed train body

According to the high-speed train body chassis connecting plate and the side wall key piece structure is complex, the processing difficulty is big, the processing is easy to deform, the precision is not easy to control, the processing efficiency is low, carries on the processing craft characteristic analysis, produces the processing craft analysis report. The dynamic milling forces in X, Y and Z directions during milling process are tested by means of testing and analyzing the dynamic characteristics of the machine tool, and the dynamic performance of the system can be tested by means of modal test, the transfer function of the system is obtained, and the modal parameters such as natural frequency, damping ratio and modal stiffness are obtained by mode fitting on the basis of the transfer function data, complete the dynamic characteristic analysis of the machine tool. Based on the modal analysis of the whole process system and the dynamic model analysis of the milling process, the chatter stability region of the milling process is simulated by using the theory of regenerative chatter, the results of chatter stability region related to milling process parameters are given.