Regenerative Chatter Vibration Research Articles

Inner Modulation Controlled Process for Suppression of Chatter Vibration in Double Inserts Turning

A novel cutting manner is presented to control regenerative chatter vibration in double inserts cutting, in which the forward and the backward inserts cut workpiece simultaneously with phase difference in the modulation of finished surface. In double inserts cutting, the forward insert is clamped above the backward insert. Both the inserts are positioned symmetrically with a height offset with respect to the workpiece rotation center. The forward insert mainly removes the material; while the backward insert cuts a part of the inner modulation to lose the regular excitation. The exciting force is also reduced with the cutting thickness of the forward insert after a workpiece revolution. Because the theoretical position offset of the inserts in the feed direction is small, the side cutting edges of inserts are aligned in the same position. The process parameters are determined by estimating the removal volume of the backward insert with the phase shift. The range of the cutting speed and the height offset are given by the frequency of the chatter vibration, which is nearly the same as the natural frequency of the workpiece in single degree of freedom. The proposed cutting manner is validated using comparison between the single insert and double inserts cuttings.

Fundamental investigation on process damping potentials of cutting tools with flank face chamfers

The effect of process damping in machining is an effective influencing factor for the suppression of regenerative chatter vibrations. Although process damping is typically associated with process configurations at low cutting speeds, the use of tools with flank face chamfers is a suitable strategy to specifically increase process stability over a wide range of cutting speeds. In this contribution, damping potentials of different chamfer designs were evaluated. Using an analogy setup providing defined, high-frequency tool oscillations, fundamental interrelations of the dynamic chamfer-workpiece contact were characterized. The results illustrate the impact of specific chamfer designs on process damping and the significance of the elastic material response on the tool-workpiece interaction.

Stability Analysis and Nonlinear Chatter Prediction for Grinding a Slender Cylindrical Part

A cylindrical plunge grinding process was modeled to investigate nonlinear regenerative chatter vibration. The rotating workpiece was a slender Euler–Bernoulli beam, and the grinding wheel was a rigid body moving towards the workpiece at a very low feed speed. A numerical method was proposed to provide the critical boundaries for chatter-free grinding. It was demonstrated that the intersection set surrounded by these critical boundaries was the chatter-free region for the considered parameters. When these parameters were outside of the chatter-free region, the stable grinding process underwent a supercritical Hopf bifurcation, resulting in the loss of the chatter-free behavior and the emergence of periodic chatter motions. Then, the periodic motions of both the grinding wheel and the workpiece were predicted analytically using the method of multiple scales, showing the effect of the regenerative force on the grinding process. We demonstrated that the analytical prediction was valid since it agreed with the numerical simulation. The results showed that there exist three kinds of nonlinear chatter motion, with different amplitudes and mode frequencies.

Control of chatter vibration in double inserts turning with phase difference of modulations

A double inserts turning with two inserts mounted on a tool shank is proposed to control regenerative chatter vibration in turning of workpieces with low rigidity. In the presenting turning, the forward and the backward inserts are placed with a position offset in the feed direction; and those inserts are also clamped with a height offset each other in the cutter height direction. A part of the waved surface formed in the preceding cut by the forward insert is concurrently removed by the backward insert placed at a higher position than the rotation center of the workpiece. The regenerative effect, then, is controlled to suppress the chatter vibration by cutting the outer modulation with a time delay. This paper describes the principle of the double inserts turning for control of chatter vibration based on phase difference of the outer and the inner modulations. An analytical model, then, is presented to associate the regenerative effect with the offset parameters of the inserts. The cutting tests were conducted to verify the double inserts turning of a slender workpiece with the offset parameters of the inserts obtained by the analytical simulation. The chatter vibration is controlled effectively by adjusting the offset parameters of the inserts corresponding to the turning parameters.

Elucidation of the mechanism of chatter mark formation during end-milling and inverse analysis of chatter vibration by two-dimensional discrete Fourier transform of chatter mark

Regenerative chatter vibration is a typical machining problem during cutting. The frequency and phase shift of regenerative chatter can be used as indicators to suppress regenerative chatter. However, direct measurement of chatter vibration during cutting is difficult for end milling because the tool blade rotates. Therefore, chatter vibration is generally measured indirectly, for example, from cutting noise or vibration transmitted to the spindle. The type and position of the installed sensors must be appropriately selected to accurately measure chatter vibration with these methods. This selection must account for the machine tool structure, cutting conditions, and the position where the chatter vibration occurs, making the measurement methods not universally applicable. When chatter vibration occurs during cutting, a periodic pattern called a chatter mark is formed on the machined surface. The chatter mark is formed by vibration during cutting, i.e., vibration information during cutting is directly recorded. Therefore, this paper proposes a method to analyze the frequency and phase shift of chatter vibration using a two-dimensional discrete Fourier analysis of the chatter mark based on the modeled formation mechanism of chatter marks, and further validates the proposed method by elucidating the mechanism of chatter mark formed by chatter vibration. First, a cutting simulation was performed based on the vibration displacement of the tool during chatter cutting obtained experimentally. This simulation elucidates the mechanism by which the vibration information of chatter vibration can be analyzed from the chatter mark. Subsequently, the chatter vibration parameters were analyzed from the chatter mark using the proposed method. The parameters were consistent with those analyzed from the vibration displacement of the tool, confirming the effectiveness of the proposed method.

Active Chatter Mitigation by Optimal Control of Regenerative Machining Process Dynamics

Chatter vibrations are a type of self-exited vibrations observed in many subtractive manufacturing processes. They limit achievable material remove rate (MRR), reduce tool life, resulting in poor surface finish, and even damage the manufacturing equipment. This article proposes a novel control system design for machine tool feed drives, which modulates cutting tool motion based on the in-process vibration measurements to mitigate chatter vibrations. The proposed approach has two key contributions. First, as opposed to introducing modal damping to the structure, the proposed approach stabilizes machining process dynamics by controlling its governing delay differential equation. H∞ control framework is used to design the controller and ensures robust process stability. The controller design problem is postulated as a convex optimization problem with linear matrix inequalities, which is solved to global optimality. Second, the proposed controller is implemented as an add-on compensator that works jointly with an existing servo motion control system to realize both positioning and chatter vibration control. Experimental validations are conducted on a lathe equipped with a fast tool servo actuator. It is shown that regenerative chatter vibrations in the orthogonal turning process can be suppressed robustly and up to five times increase in MRR (productivity) can be achieved.

Mechanism to suppress regenerative chatter vibration due to the effect of air-cutting and multiple regeneration during low frequency vibration cutting

Recently, the application of low frequency vibration has been attempted in the turning process. The tool is vibrated in the feed direction by numerical control to break up long continuous chips that adversely affect the surface quality and tool wear, etc., by generating a duration of cutting edge leaving workpiece. In order to achieve higher machining efficiency in vibration cutting, however, it is necessary to focus on chatter stability. When chattering occurs, a higher frequency chatter vibration is superimposed on the low-frequency tool vibration caused by the feed motion of the servo drive. However, the chatter amplitude decreases due to system damping, because the cutting force does not act in the air-cutting section of vibration cutting. In addition, tool displacement is regenerated not only before one spindle rotation, but also before two or more rotations in vibration cutting. Therefore, the application of low frequency vibration has an important influence on the chatter stability. The purpose of this study is to reveal the detailed effects of vibration conditions on the stability limit, and identify the vibration conditions that achieve high chatter stability. We consider the effect of the air-cutting and multi-rotation delayed regeneration described above, in which chatter displacement occurring more than two spindle rotations before is regenerated, and analyze regenerative chatter vibration during vibration cutting in the frequency domain. In this study, the phase difference φ of tool motion in vibration cutting is focused as an important vibration condition parameter that affects the stability limit. In the conventional condition (φ = π), the air-cutting ratio is limited to a relatively low value, and only the dynamic displacements before one and two spindle revolutions are regenerated. On the other hand, setting φ appropriately under the condition φ≠π, the air-cutting ratio is increased. Furthermore, the dynamic displacement is regenerated not only before one and two spindle rotations but also before three or more spindle rotations. These results indicate that proper setting of the phase difference φ is effective in suppressing regenerative chatter vibration due to the effects of both the air-cutting phenomenon and multi-rotation delay. Finally, cutting experiments show that the phase difference φ specified by the analysis was effective in suppressing chatter vibration. In summary, the application of low frequency vibration with an appropriately set phase difference φ is an effective strategy for suppressing chatter vibration without reducing the material removal rate in the turning process.

Hardware-in-the-loop simulator for emulation and active control of chatter

Machine tool cutting performance is limited by the onset of regenerative chatter vibrations. Chatter damages the machine tool system and must be avoided. Efficacy of models guiding circumvention of chatter in actual industrial contexts remains restricted due to cutting process nonlinearities and due to uncertainties in dynamics of machine tools. To verify chatter models in the presence of nonlinearities and uncertainties and to explore active control of chatter, a mechatronic hardware-in-the-loop (HiL) simulator is presented in this paper. Hardware layer of the simulator has a flexure approximating a flexible workpiece and two actuators to emulate cutting forces and apply active damping forces. Software layer involves control architecture that is to be emulated on the hardware layer. Since HiL simulators offer a non-destructive, repeatable, and safe platform for emulations and active control of chatter, and because this paper presents the design of a validated simulator with all accompanying source files along with build and operating instructions, the research community can build such simulators to investigate other complex chatter phenomena and can also use it as a pedagogical tool. Learnings from building and using such simulators can also be leveraged to characterize and solve problems across other engineering domains.

Development of generalized tool life model for constant and variable speed turning

In this research, a generalized tool life modelling for considering non-stationary cutting conditions was developed . In particular, for the first time in literature, the model was conceived for predicting the life of the tool when spindle speed variation SSV, one of the most effective techniques for suppressing regenerative chatter vibrations, is used. The proposed formulation takes into account the main cutting parameters and the parameters associated to the SSV. A dedicated experimental campaign of turning tests was executed and the data were used for modelling purposes. The model validation was carried out performing additional tool life tests. According to the analyzed technological scenario, it was found that the generalized formulation can be used for predicting the tool life both at constant spindle machining CSM and adopting SSV with the maximum estimating error of 6%.

Predicting regenerative chatter in milling with hardware-in-the-loop simulation using a dexel-based cutting model

This article proposes a new model for simulating the interaction between cutting process and machine tool in real-time. The purpose of the model is to be coupled with a real CNC (by using hardware-in-the-loop simulation) in order to consider process forces and to predict regenerative chatter vibrations during virtual commissioning. Therefore a dexel-based workpiece model with adaptive resolution is used for the computation of the chip thickness respectively the cutting forces based on the actual machine tool position and the machining progress on the workpiece. Several simulation experiments are performed to validate the model and to analyze its numerical limits, such as computational accuracy and efficiency. The capability of the model to predict chatter is proven by comparing the simulated critical depth of cut with an analytical solution of the stability lobes. Therefore the dynamics of the machine tool were approximated as a single degree of freedom (SDOF) oscillator. A concluding analysis of the real-time factor confirms the model’s ability to be integrated under hard real-time requirements and with cycle times of just a few milliseconds which are typical of CNCs.

Evaluation of a novel approach for considering damping effects in a process force model of a geometric physically-based milling simulation

Process damping in milling is a significant influencing factor in avoiding detrimental dynamic effects such as regenerative chatter vibrations, which can impair the workpiece quality and increase tool wear. In this context, the use of tools with flank face chamfers causes significant process damping effects over a wide range of spindle speeds. This way, processes with increased material removal rates due to high radial and axial tool immersions can be conducted without the occurrence of disturbing chatter vibrations. A precise prediction of the process stability is necessary to define efficient process strategies in advance. However, this is particularly challenging in the presence of process damping due to its nonlinear process behavior. This paper presents an evaluation of a new approach towards modeling dynamic forces in a geometric physically-based milling simulation for taking process damping effects caused by chamfered cutting tools into account. This model considers the vibration velocity present at the cutting edge in order to calculate a damping force caused by the dynamic tool-workpiece contact. The parameterization of the velocity-dependent force model was carried out a priori by conducting finite element simulations using an orthogonal cutting model as an analogy setup to determine the resulting force components. The proposed modeling approach was analyzed by comparing the simulation results to corresponding milling processes for validation. In contrast to established empirical force models, which calculate the process forces entirely as a function of the undeformed chip shape, the consideration of the dynamic tool-workpiece interaction led to a significant damping of the modeled process vibrations.

Active control of high frequency chatter with machine tool feed drives in turning

This paper presents a new active vibration control strategy to mitigate high frequency regenerative chatter vibrations using machine tool feed drives. Rather than modal damping, proposed approach aims to control regenerative process dynamics to shape the Stability Lobe diagram (SLD) and attain higher material removal rates. The controller is designed as a feedback filter whose parameters are optimized to compensate regeneration. The proposed strategy is applied to actively control orthogonal (plunge) turning dynamics where >2.5 [kHz] chatter vibrations are suppressed by a fast tool servo (FTS) drive system. Stability lobes are shaped locally to reach up to 4x higher material removal rates.

Optimization of cutting tool geometry and machining parameters in turning process

Vibrations play a major role in increasing the cost of production as they lead to a poor surface finish of the machined product and can also damage the cutting and machine tool. In this work, the influence of cutting tool geometry of a turning tool and machining parameters used in turning operations on regenerative chatter vibration during orthogonal cutting was studied. The optimum values of orthogonal rake angle, clearance angle, cutting tool overhang, feed rate and depth of cut are found by developing a model using design of experiments and the model was analysed using a statistical technique known as response surface method (RSM). The objective is to optimize cutting tool geometry and machining parameters of turning process by minimizing the real part of frequency response function (FRF) which is a response of a cutting tool subjected to a harmonic force. The harmonic analysis, carried out using software ANSYS, was used to get the peak value of real part of frequency response function (RFRF) of the cutting tool and its magnitude has to be minimized to improve the machining stability against regenerative chatter during machining. It may be inferred from the results that the tool rake angle, clearance angle, overhang of the cutting tool, feed rate and depth of cut are interacting with each other and influence the machining stability simultaneously. Hence, it may be advisable to pick an optimum set of machining parameters for the specific combination of cutting tool geometry to have vibration-free machining.

Robust time-delayed [formula omitted] synthesis for active control of chatter in internal turning

This research is motivated by the problem of regenerative chatter vibration, which is an extremely harmful phenomenon in machining in general, and in internal turning in particular. In addition to limiting the production rate and accelerating the tool wear, regenerative chatter deteriorates the surface finish, which is especially undesired in the context of precision machining. This paper presents an active control design method for slender bars with delayed excitation which can be applied to the problem of active control of regenerative chatter in internal turning. The presented control method is based on the delay-dependent output feedback controller synthesis. The contribution of the presented design approach stems from incorporating into the design procedure the delayed dynamics of the regenerative chatter, the flexible bar model uncertainty, and the hardware limitations. Furthermore, the delay dynamics is taken explicitly in the synthesis of an H∞ output feedback controller by linear matrix inequalities. The resulting controller is tested by simulations and experiments using a testbed imitating the dynamics of the internal turning process. The results demonstrate the effectiveness of the presented controller synthesis method in comparison to the commonly used approaches.

Identification and robust control for regenerative chatter in internal turning with simultaneous compensation of machining error

The motivation behind this study is the problem of regenerative chatter vibration in internal turning. Regenerative chatter is an extremely undesired phenomenon in machining. This is since, in addition to causing surface finish degradation and cutting tool wear, regenerative chatter imposes a fundamental limitation on the material removal rate. In internal turning, due to the relatively low internal damping of the boring bar, regenerative chatter is especially undesired. Another factor, which reduces the material removal rate, as well as surface accuracy, is machining error. Machining error is very common in internal turning due to low boring bar stiffness. At the same time, machining error compensation has received little attention in the literature hitherto. This study proposes a control methodology for chatter vibration suppression with simultaneous compensation of the machining error in internal turning. The proposed methodology consists of three components. The first component is a newly developed method for the identification of the cutting force model parameters. The second component, which utilizes the cutting force model parameters identified in the previous step, is a robust stabilizing controller synthesis developed to address the key issues arising in the context of regenerative chatter control in the internal turning process. In contrast to the corresponding control methods reported in the literature, the control approach developed in this study accounts explicitly for the time-delayed term associated with the regenerative feature of chatter, as well as uncertainty in both cutting force and the flexible boring bar models. The last third component of the presented control methodology is a novel approach for machining error compensation, which is based on a feedforward term designed to eliminate the bar tip deflection. Every single component of the proposed control methodology can be utilized separately in the context of active control in internal turning. The potential applicability of the control methodology presented in this study is demonstrated experimentally, using an active slender bar demonstrator imitating the internal turning process. In particular, it is demonstrated that: (i) given the identified model of the boring bar, the proposed method for cutting parameters estimation allows to obtain reasonable estimations of the cutting coefficient and the overlap factor, (ii) the presented robust control system synthesis procedure allows to obtain controllers which increases the chatter immunity of the actively controlled cutting process, and (iii) the presented method for machining error compensation counteracts successfully the error due to flexible bar deflection caused by the DC component of the cutting force model.

Optimization of vibration parameters for boring bar operation by RSM and ANN

In boring operation, the increase in length of the boring bar tool makesan instability ofthe metal removing process; this phenomenon leads to the reduction of tool life, increase in noise and poor quality of surface finish. To overcome these criteria, a radially arranged magneto rheological fluid damper assisted tool post is used to conduct the boring operation on a conventional lathe machine. Vibration signals were monitored with the aid of piezoelectric accelerometer (vibration sensor), which is recorded in online mode & evaluated with the use of DAQ card. MR fluid damper is modeled based on the Euler-Bernoulli’s beam model. Design of experiments were conducted based on Taguchi’s (L18) orthogonal array with the three level parameters(spindle speed, feed rate and depth of cut).The combination of optimum vibration parameters were selected with the use of response surface methodology (RSM) technique to obtain the optimum output.Performingthe machining operation with of optimum vibration parameters, reducesthe machining time and increase the productivity. Results obtained from the regression analysis shows that the regenerative chatter vibration is reduced effectively with the help of radial damper arrangement. Response surface methodology (RSM) and Artificial neural network (ANN) is applied to validate the results obtained during experimentation and to predict the behaviour of the system under any condition within the operating range. It is apparent from the analysis that the feed rate has a significant contribution for both the surface roughness and vibrational acceleration.

Analysis on milling stability of fuel tank's bottom combined with surface location error prediction

The bottom of fuel tank is a lightweight thin-walled part with large size and complex surfaces. Its processing quality directly determines the reliability of the whole rocket. This paper conducts the predictions of both milling stability and surface location error (SLE) based on the theory of regenerative chatter vibration. We proposed a new type of stability lobe diagram (SLD) combined with the contour map of SLE. Then, we studied the changes of the bottom's dynamic parameters through FEA method, which would be influenced by the removal of material during machining. By using the dynamic parameters as inputs, we obtained a series of SLDs and the processing parameters which will lead to no-chatter and low SLE under different processing periods. The efficient and high-precision milling of the fuel tank's bottom can also be guaranteed.

Symmetry breaking in milling dynamics

Abstract Regenerative chatter vibrations arising during machining operations are a severe problem affecting the manufacturing production of high-precision mechanical components. A relatively deep understanding of this phenomenon when the cutting tool has equally spaced cutting teeth without runout has been achieved by now. However, real cutting tools are always affected by teeth runout in practical applications. Moreoever, they can be characterized by irregular geometries designed on purpose for enhancing their cutting performance. The most advanced dynamic milling models compute the effective tool-workpiece engagement conditions by taking into account the exact tool geometry and the nominal milling trochoidal kinematics. Nevertheless, they exclude the forced vibrations of the machining system from the stability analysis. In this work a more complete milling model is introduced, which takes into account the influence of forced vibrations on the effective tool-workpiece engagement conditions. By doing so, milling dynamics are linearized around the true steady state solution and a more correct stability analysis is carried out. The dynamic coupling between tool-workpiece engagement conditions and the steady state vibrations may arise even for small tool asymmetries and it gives rise to a kind of spontaneous symmetry breaking mechanism. This yet unexplained physical mechanism may have an important impact on the stability diagrams, as it will be demonstrated theoretically and experimentally in this work.

Novel actuation approach for active control of slender bars with simulated delayed excitation

This research is motivated by the problem of regenerative chatter vibration, which is a pronounced problem in machining. The problem of chatter vibration is especially critical in the internal turning process, where the material is removed from the inner part of a rotating workpiece using a slender bar. Active mechatronic solutions to the problem of regenerative chatter vibration in internal turning are mostly given by means of piezoelectric actuators embedded into the boring bar. However, due to the volume needed for accommodation of the actuators, it is difficult to apply these solutions to narrow bars. This paper presents a novel actuation approach with a potential application for active control of regenerative chatter vibration in internal turning. This approach involves a bar whose clamping has an ability to move perpendicularly to the bar axis. In addition, it involves two actuators: one of them is placed near the bar clamping, and another one is placed at the bar clamping. The presented actuation approach is compared to the related approaches in the literature. Additionally, this paper presents a method for robustness analysis of the actively controlled cutting process, which is based on the structured singular value methodology. Experiments performed on a specially designed testbed imitating the internal turning process demonstrate the potential applicability of the introduced actuation approach, as well as the utility of presented robustness analysis method.

A machine-learning based solution for chatter prediction in heavy-duty milling machines

The main productivity constraints of milling operations are self-induced vibrations, especially regenerative chatter vibrations. Two key parameters are linked to these vibrations: the depth of cut achievable without vibrations and the chatter frequency. Both parameters are linked to the dynamics of machine component excitation and the milling operation parameters. Their identification in any cutting direction in milling machine operations requires complex analytical models and mechatronic simulations, usually only applied to identify the worst cutting conditions in operating machines. This work proposes the use of machine learning techniques with no need to calculate the two above-mentioned parameters by means of a 3-step strategy. The strategy combines: 1) experimental frequency responses collected at the tool center point; 2) analytical calculations of both parameters; and, 3) different machine learning techniques. The results of these calculations can then be used to predict chatter under different combinations of milling directions and machine positions. This strategy is validated with real experiments on a bridge milling machine performing concordance roughing operations on AISI 1045 steel with a 125 mm diameter mill fitted with nine cutters at 45°, the results of which have confirmed the high variability of both parameters along the working volume. The following regression techniques are tested: artificial neural networks, regression trees and Random Forest. The results show that Random Forest ensembles provided the highest accuracy with a statistical advantage over the other machine learning models; they achieved a final accuracy of 0.95 mm for the critical depth and 7.3 Hz for the chatter frequency (RMSE) in the whole working volume and in all feed directions, applying a 10 × 10 cross validation scheme. These RMSE values are acceptable from the industrial point of view, taking into account that the critical depth of this range varies between 0.68 mm and 19.20 mm and the chatter frequency between 1.14 Hz and 65.25 Hz. Besides, Random Forest ensembles are more easily optimized than artificial neural networks (1 parameter configuration versus 210 MLPs). Additionally, tools that incorporate regression trees are interesting and highly accurate, providing immediately accessible and useful information in visual formats on critical machine performance for the design engineer.