Milling Chatter Research Articles

Chatter detection in milling process based on VMD and energy entropy

Abstract This paper presents a novel approach to detect the milling chatter based on Variational Mode Decomposition (VMD) and energy entropy. VMD has already been employed in feature extraction from non-stationary signals. The parameters like number of modes (K) and the quadratic penalty (α) need to be selected empirically when raw signal is decomposed by VMD. Aimed at solving the problem how to select K and α, the automatic selection method of VMD’s based on kurtosis is proposed in this paper. When chatter occurs in the milling process, energy will be absorbed to chatter frequency bands. To detect the chatter frequency bands automatically, the chatter detection method based on energy entropy is presented. The vibration signal containing chatter frequency is simulated and three groups of experiments which represent three cutting conditions are conducted. To verify the effectiveness of method presented by this paper, chatter feather extraction has been successfully employed on simulation signals and experimental signals. The simulation and experimental results show that the proposed method can effectively detect the chatter.

Investigation on milling chatter identification at early stage with variance ratio and Hilbert–Huang transform

Chatter occurs as an unexpected and unstable phenomenon in milling process, imposing an extremely negative effect on the workpiece and machining system. Therefore, a method that can identify chatter at an early stage is desperately needed. However, some aspects in terms of effectiveness, robustness, and practicality with existing methods deserve further improvement. In this paper, the characteristics of chatter in different stages are investigated. Considering the properties of a vibration signal when the onset of chatter occurs, an adaptive filter is designed to remove the spindle speed-related frequency components, and the chatter-related components can be amplified simultaneously with the filter. Next, the variance ratio (VR) of the filtered signal series to the original signal series is defined as the chatter indicator, which is very sensitive to chatter. After chatter is detected, a time frequency analysis method based on ensemble empirical mode decomposition (EEMD) and the Hilbert–Huang transform (HHT) is introduced to estimate the dominant chatter frequency. Milling experiments with different configurations of cutting conditions are performed and the results show that all the chatter can be detected at an early stage. In addition, the transients of the vibration signal caused by discontinuity of workpiece geometry or inhomogeneous material can be distinguished from the chatter.

Chatter detection in milling based on singular spectrum analysis

Chatter is a frequently encountered problem in metal cutting field which reduces the machining efficiency and surface quality. Therefore, a reliable and robust chatter detection method is necessary to improve the machining performances. In this work, a novel milling chatter detection approach based on singular spectrum analysis (SSA) is proposed. SSA is applied to process the cutting force signal and extract the feature that is closely related to the machining state. The singular value spectrum obtained by SSA is used to describe the energy distribution of the principal modes in the signal. On the basis of frequency domain chatter theory, singular value entropy (SVE) is adopted to evaluate the variation of energy distribution in the signal and the milling chatter is detected accordingly. Milling experiments under different cutting conditions are performed out to verify the effectiveness of the proposed method. Experimental results demonstrate that the proposed method can accurately identify the onset of chatter. This method is simple in operation and fast in calculation, which makes it have great potential for online chatter detection.

Machining Chatter Analysis for High Speed Milling Operations

Chatter in high speed milling is characterized by time delay differential equations (DDE). Since closed form solution exists only for simple cases, the governing non-linear DDEs of chatter problems are solved by various numerical methods. Custom codes to solve DDEs are tedious to build, implement and not error free and robust. On the other hand, software packages provide solution to DDEs, however they are not straight forward to implement. In this paper an easy way to solve DDE of chatter in milling is proposed and implemented with MATLAB. Time domain solution permits the study and model of non-linear effects of chatter vibration with ease. Time domain results are presented for various stable and unstable conditions of cut and compared with stability lobe diagrams.

CCT-based mode coupling chatter avoidance in robotic milling

Currently, large aerospace structures are machined using large multi-axis CNC machining centers. In comparison, milling with a multiple degree-of-freedom (dof) articulated robotic arm has several advantages due to its lower cost and versatility. The low stiffness of an articulated arm robot, however, gives rise to severe low frequency mode coupling chatter during machining. Previous studies have shown that such chatter can be suppressed by minimizing the angle between the average resultant cutting force direction and the direction of maximum principal stiffness of the robot. This approach limits the range of permissible robot motion, and therefore its flexibility of use. This paper presents a new method for avoiding mode coupling chatter in robotic milling using the Conservative Congruence Transformation (CCT) stiffness model, which does not require changing the tool feed direction or the workpiece orientation. Robotic milling experiments show that mode coupling chatter is significantly reduced when using this approach.

Rolling mill chatter mechanism based on the unsteady lubrication performance

The unsteady lubrication is one of the major causes for self-excited vibration in rolling. But it is difficult to build a rolling chatter model which considers the unsteady lubrication state in roll bite. Using the regression exponential function model of rolling friction coefficient, a dynamic rolling process model was built. Coupling the dynamic rolling process model with a mill stand structure model, a chatter model which considers the unsteady lubrication state at roll bite was established. Based on the proposed chatter model, the friction coefficient, rolling force and critical velocity of a rolling mill stand under different working conditions were calculated. The computed results were compared to the test results. And it proves the validity of the proposed model. With the proposed chatter model, the effects of rolling process parameters and emulsion lubricant characteristic parameters on friction coefficient and critical velocity were both analyzed and discussed. The chatter model successfully introduces the unsteady friction model into the rolling mill chatter model. It better explains the negative damping effect of the rolling chatter and reflects the rolling mill dynamic vibration characteristic.

EEMD-based online milling chatter detection by fractal dimension and power spectral entropy

Chatter is a kind of self-excited unstable vibration during machining process, which always leads to multiple negative effects such as poor surface quality, dimension accuracy error, excessive noise, and tool wear. For purposes of monitoring the processing state of milling process and detecting chatter timely, a novel online chatter detection method was proposed. In the proposed method, the acceleration signals acquired by sensor were decomposed into a series of intrinsic mode functions (IMFs) by the adaptive analysis method named ensemble empirical mode decomposition (EEMD), and the IMFs which contain the feature information of milling process were selected as the analyzed signals. The two indicators power spectral entropy and fractal dimension which is obtained by morphological covering method are introduced to detect the chatter features. Then, both the frequency characteristic and morphological feature of the extracted signals can be reflected by the two indicators. To verify the approach, milling experiments were performed; the experiment results show that the proposed method can detect chatter timely and effectively, which is important in the aspect of improving the milling quality. And finally, in order to detect milling chatter timely, an online milling chatter monitoring system was developed.

Rolling interface friction dynamics of hot strip continuous rolling and its effect on mill chatter

According to the fluid mechanics and rolling theory, the rolling interface friction dynamic characteristics and their influence on rolling speed, rolling load, roller material, rolling temperature and oil properties were studied respectively with lubrication or without lubrication in hot rolling. The two-dimensional Reynolds equation was established for steady and unsteady rolling friction interface. Through the research of rolling interface friction characteristics influence on the mill vibration, we found that when the lubrication film thickness in the roll gap was thicker, the friction coefficient was lower, its damping effect and system stability was worse. And if the rolling speed is greater than a certain value, the rolling interface friction coefficient will fall sharply with the increase of rolling speed and produces the self-excited vibration caused by negative damping. So the contrast test with different rolling interface has been carried out, such as the emulsion is open or close, high chromium cast iron/high speed steel roller is put to use, high chromium cast iron roller is grinded finely or coarsely, rolling speed is normal/reduced. And the results showed that it had an obvious effect on the mill vibration suppression by the adoption of the emulsion close and high speed steel roller.

Chatter identification in milling of the thin-walled part based on complexity index

In order to avoid chatter in milling of thin-walled part, chatter prediction based on the complexity of vibration signals in the milling process is proposed. According to the nonlinear characteristics of vibration signals in milling process, the complexity which is a nonlinear dynamics index and has higher computational efficiency is used to as the chatter prediction index in milling of thin-walled part, and the physical meaning of the complexity index is explained in detail. Firstly, different types of chatters in the milling process are simulated by numerical calculation, and the causes that cause different types of chatter are discussed. Secondly, according to the chatter types in the experiment of the paper, the complexity is used to analyze the vibration signals simulated by tending to the flip bifurcation, the result show that complexity index is valid in the chatter prediction for the simulated vibration signals by tending to chatter. In order to verify the validity of the complexity for the chatter prediction, the experimental platform in milling of the thin-walled part was set up. The complexity is used to analyze the actual vibration signals caused by tending to the chatter, and the results show that the index is effective in the actual milling chatter prediction.

Tool posture dependent chatter suppression in five-axis milling of thin-walled workpiece with ball-end cutter

In aerospace industry, five-axis ball-end milling is widely used for flexible thin-walled aerospace parts by considering the process mechanics. A new analytical method is proposed to investigate the effects of the change of tool posture, especially lead and tilt angle, on chatter stability in milling. In this method, the dynamic cutter-workpiece engagement regions, which are essential to predict dynamic cutting forces and investigate chatter stability of the thin-walled workpiece, are determined considering the dynamic machining conditions. Then, according to dynamic cutter-workpiece engagement region data, the dynamic cutting force model considering the effects of lead and tilt angle is developed in machining the thin-walled aerospace parts with ball-end cutter. Subsequently, the milling chatter stability model is established based on the dynamic cutting force model, and the milling stability limits are determined in different lead and tilt angles. Finally, the feasibility and effectiveness of the proposed method are verified by experiments with ball-end cutter in five-axis machining center. The results are shown that the predicted values well match with the experimental results.

Effect of the cutter path and the workpiece clamping position on the stability of the robotic milling system

The milling robots have many advantages compared with the traditional numerical control machine, but the chatter phenomenon in the robotic milling obstructs its popularization. Therefore, the chatter problem in the planar milling by robot is discussed in the way of cutter path and clamping position to give directions for chatter suppression and quality improvement. Firstly, the dynamic behaviour of the robotic milling system is studied by the models of the chatter stability domain analysis, which include robot stiffness model and dynamic milling force model. The mode coupling effect is considered in chatter stability lobe diagram of robotic milling, which is different with the machine tool. Then, the robotic milling experiments are conducted at six different clamping positions relative to robot by two ways of cutter path. Based on the obtained modal parameters under the different milling conditions, the corresponding chatter stability lobe diagrams are plotted. By the comparative analysis about the experiment results, it is found that the surface quality of workpiece with the feed motion of milling tool along the Y-axis direction of robot world coordinate system is better than the X-axis direction and the chatter is more likely to occur in the down milling than in the up milling. Furthermore, a proper workpiece clamping position can availably prevent the chatter in the robotic milling. These conclusions can be references for designing a suitable machining scheme in the robotic milling to ensure the application of robot in the metal cutting.

Generalized design method of highly-varied-helix end mills for suppression of regenerative chatter in peripheral milling

Peripheral finishing of hardened steel by milling has recently been introduced to the machining field, but often causes severe chatter vibration due to high specific cutting force and low stiffness of slender end mills. It was shown in the previous papers that highly-varied-helix end mills were effective for the suppression of regenerative chatter, and a low radial immersion was applied to avoid the effect of mode coupling. Especially for the highly-varied-helix end mills, it was proved experimentally that they were effective across a wide range of cutting conditions, making it possible for cutting with a relatively large axial depth of cut to be carried out and thus realizing a high surface generation rate in peripheral milling of hard materials. However, the milling tests were carried out under just one specific condition, i.e., tool diameter 8 [mm], projection length 36 [mm], and workpiece of 60 HRC hardened steel, etc., and the helix angles of the tool were varied large enough to suppress regenerative chatter but with no quantitative discussion for their determination. In this paper, a generalized design method for varied-helix end mills to suppress regenerative chatter is proposed. Namely a design index apalim, in which ap is the distance between 2 adjacent regenerative effect cancellation lines and alim is the asymptotic stability limit in the targeted cutting process, is introduced to design the helix angle difference of a tool, and milling experiments are carried out in several conditions to verify the validity of the proposed index.

Chatter detection in milling process based on the energy entropy of VMD and WPD

This paper presents a novel approach to detect the milling chatter based on energy entropy. By using variational mode decomposition and wavelet packet decomposition, the cutting force signal is decomposed into two group of sub-signals respectively, and each component has limited bandwidth in spectral domain. Since milling chatter is characterized by the change of frequency and energy distribution. Therefore the energy features extracted from the two group of sub-signals are considered and the energy entropies are obtained, which can be utilized to demonstrate the condition of the milling system synthetically. Several milling tests are conducted and the results show that the proposed method can effectively detect the chatter at an early stage.

Chatter reliability of milling system based on first-order second-moment method

In this paper, reliability analysis for dynamic structural system is presented to predict chatter vibration in a milling system. Chatter reliability is defined to represent the probability of stability (no chatter occurs) of milling system. Probability model (reliability model) of chatter vibration is established to predict milling chatter vibration, in which structural parameters and spindle speed are considered as random variables. Choosing chatter frequency as an intermediate variable, the reliability model is built. The first-order second-moment method is adopted to solve the reliability model of the milling process system to obtain the reliability level of the system. The reliability lobe diagram (RLD), which is a contour line with a specified reliability level as a function of spindle speed and cutting depth, is presented to designate the reliable region for chatter vibration prediction. A numerical example is used to demonstrate the method for reliability analysis. The reliability of milling chatter system was calculated using first-order second-moment (FOSM) method and compared to the Monte Carlo simulation method. The results from the FOSM method and Monte Carlo method were found to be similar. Comparing the results with the traditional stability lobe digram (SLD) method, chatter reliability of milling process system can be used to judge the probability of stability of milling process system. It can be concluded that RLD can be efficiently used to predict reliability in workshop applications.

Determination of the stability lobes with multi-delays considering cutter’s helix angle effect for machining process

As a common phenomenon, chatter is one of the most important factors that inhibit the improvement of productivity and deteriorate the machined surface quality in milling process. In this article, the mathematical model of the dynamic machining process is first constructed with multi-delays, in which the effect of the cutter’s helix angle on the chatter is considered. And a new integral interpolation method is proposed to predict the stability lobes. Based on this method, the mathematical model which is divided into two parts is calculated respectively with an interpolation method and an integration method. Using the Floquet theory, the stability limits of the dynamic milling system for an arbitrary point can thus be precisely predicted. Subsequently, the convergence rate of this integral interpolation method is analyzed. In order to verify the mathematical model, the stability lobes with uniform and variable cutter pitch angle are computed and the results show a good agreement with published experimental data; also one experiment is conducted to validate the proposed model and the method. Simultaneously, the simulation of a stability lobe with non-zero helix angle is also performed and the results validate the fact that the proposed method is capable of predicting stability limits with high accuracy. Finally, the influences of the cutter and process parameters such as helix angle, pitch angle, down-milling or up-milling and radial cutting depth on the milling chatter are analyzed.

Intelligent Fixtures for High Performance Machining

Fixtures are an essential element of the machining system, being part of the precision path and force flux between process and machine tool. Intelligent fixtures enable the identification of critical process conditions, a compensation of error influences and the minimization of defective parts. At first, this contribution presents a study in which the influence of the clamping setup and of the workpiece characteristics at various steps of the machining process is analyzed. Experimental and theoretical results regarding the dynamic process behavior reveal the relevance of these influences with respect to machining performance and workpiece quality. Secondly, the European research project INTEFIX is introduced. Representative approaches of intelligent fixtures, reducing workpiece vibrations and distortions, and improving workpiece alignment, are described and prototypes are shown. In a third part, two examples of intelligent fixtures are presented and discussed more in detail. The first example concerns a fixture for the identification and active mitigation of chatter in milling of thin walled workpieces. The second example is related to the compensation of workpiece distortions which occur in machining of large thin walled structural parts.

Critical depth of cut and asymptotic spindle speed for chatter in micro milling with process damping

This paper presents formulas for determining the critical depths of cut and asymptotic spindle speed for stable micro milling processes with process damping. The asymptotic spindle speed of a theoretically infinite stable depth of cut is shown to be proportional to the modal natural frequency, radial ploughing constant and radial immersion angle, but inversely proportional to the shearing related cutting constants and tool diameter. These formulas enable identifying the asymptotic speed, absolute stability limit, and in-process radial ploughing constant from experimental stability limits without requiring modal parameters. The presented model is validated by comparison with prior works and verified by experiments.

Experimental Study of Thin Wall Milling Chatter Stability Nonlinear Criterion

The nonlinear dynamic behavior of milling process has been accompanied by the entire cutting process. In order to accurately determine and predict chatter stability of machining process, this article studied at both ends of the fixed thin part nonlinear criterion of milling chatter stability with experimental method. The experiment takes the vibration signal of thin part as the study object. And it analyses the vibration signal of different processing parameters based on the phase plane method, Poincare method and spectral analysis. Then, the relationship between the maximum Lyapunov exponent and the spindle speed and milling depth changes is discussed. Finally, taking the largest Lyapunov exponent as the criterion, the study determines the chatter stability domain of milling by using contour method. The comparative analysis is based on the milling chatter stability domain which obtained from the full discrete method. The experiments obtained the nonlinear stability criterion of aviation aluminium alloy 7075-T6 thin part.

Influence of damping coefficient from permanent magnets on chatter formation during end milling of titanium alloy (Ti-6Al-4V)

In machining operations, chatter is undesirable due to its adverse effects on the product quality, operation cost, machining accuracy and machine tool life. It is also responsible for reducing output. Chatter is a self-excitation phenomenon occurring in machine tools, in which the cutting process tends to lower the damping capacity of the machine structural components ending in an unstable behavior of the system. Chatter arises due to resonance when the vibrations of the instability of chip formation and the natural vibration modes of the machine-system components coincide. This paper focuses on the influence of damping coefficient from permanent magnets on chip serration frequency as an approach of minimizing chatter in end milling of Titanium alloy (Ti6Al4V). The method consists of two ferrite permanent magnet bars (dimensions: 1" × 6" × 3"), mounted 5mm from the cutting tool using a specially designed fixture which provided a uniform magnetic field of 2500-2700 Gausses. A titanium alloy Ti6Al4V block was then end milled using uncoated WC-Co inserts. A sequence of 15 experimental runs was conducted based on a small Central Composite Design (CCD) model in Response Surface Methodology (RSM). The primary (independent) parameters were: cutting speed, feed, and depth of cut. The data acquisition system comprised a vibration sensor (accelerometer) and a signal conditioning unit was used to measure the vibration data. The resultant vibrations were then analyzed using the DASYLab 5.6 software. Machining tests were conducted for two different conditions - with and without the application of magnets. Scanning Electron Microscope (SEM) was used to measure the chip segmentations. The SEM analysis of chip serrations demonstrated that the chip serration frequency were more stable while cutting under the presence of permanent magnets due to lower intensity of chatter.

Identification of chatter in milling of Ti-6Al-4V titanium alloy thin-walled workpieces based on cutting force signals and surface topography

In vertical milling process of Ti-6Al-4V titanium alloy thin-walled workpieces, chatter as a thorny problem occurs easily, which can lead to the instability of the machining process, tool wear, and poor surface finish. It has become a focus issue to identify and avoid chatter in machining process. In this paper, the milling force signals in the direction perpendicular to the machined surface were analyzed by fast Fourier transform and wavelet transform for detecting chatter. The combination of the surface topography and the prediction theory of regenerative chatter were put forward to further verify chatter. The results show that fast Fourier transform in some cases, wavelet transform, and the combination of surface topography and the prediction theory of regenerative chatter can obtain better results for the detection and verification of chatter, and the stable cutting zone was obtained using a stability lobe diagram, which can assist in the selection of reasonable cutting parameters to avoid chatter.