Milling Of Thin-walled Workpiece Research Articles

Multi-modal method for chatter stability prediction and control in milling of thin-walled workpiece

Chatter stability in milling can be predicted by analytical methods or numerical methods. The system should be considered as multi-modal in milling of thin-walled workpiece. This paper proposes a numerical difference method based on Adams-Bashforth scheme. Moreover, multi-modal scheme of numerical methods is proposed. Analytical methods and numerical methods are verified by performing a series of milling trials. The experimental results are consistent with the predicted critical stability boundaries. Moreover, a new method for analyzing the computational time of analytical methods and numerical methods, which is based on the time complexity modeling, is presented. Computational time can be expressed as exact mathematical expression. By using the expression, the rate of increase of computational time can be derived.

Vibration suppression of thin-walled workpiece milling using a time-space varying PD control method via piezoelectric actuator

The vibration suppression and workpiece surface improvement of thin-walled workpiece milling are highly concerned due to the weak stiffness of the workpiece and the time variant of dynamic characteristics during milling process. In this paper, an active vibration control system is developed to suppress the vibration of thin-walled workpiece milling. The dynamic model of plate-actuator milling system is derived using Hamilton’s principle and solved using FEM method. Acceleration sensor and piezoelectric patch are selected as the sensor and actuator in the control system. Considering the time variant of dynamic characteristics and the position limit of sensor and actuator during milling process, a time-space varying PD (VPD) control method is presented and utilized. The VPD control method applies time varying control parameters. Some simulation and experimental validations are carried out to validate the effectiveness of the control system. The overall results indicate that the proposed control system perform well in suppressing the vibration of thin-walled workpiece milling process.

Stability Research Considering Non-Linear Change in the Machining of Titanium Thin-Walled Parts.

Aiming to solve the problem whereby the damping process effect is significant and difficult to measure during low-speed machining of titanium alloy thin-walled parts, the ploughing coefficient of the flank face is obtained based on the frequency-domain decomposition (FDD) of the measured vibration signal and the energy balance principle, and then the process-damping prediction model is obtained. Aiming to solve the problem of non-linear change of dynamic characteristics of a workpiece caused by the material removal effect in the machining of titanium alloy thin-walled parts, a prediction model of dynamic characteristics of a workpiece is established based on the structural dynamic modification method. Meanwhile, the effect of material removal on the process-damping coefficient is studied, and the internal relationship between the process-damping coefficient and the dynamic characteristics of the workpiece is revealed. The stability lobe diagram is obtained by the full discretization in the titanium alloy milling process. The correctness of the model and stability prediction is verified by experiments under different working conditions. It is found that the coupling characteristics of process-damping and workpiece dynamic characteristics control the stability of the milling process. The research results can provide theoretical support for accurate characterization and process optimization of titanium alloy thin-walled workpiece milling.

High-Quality Machining of Edges of Thin-Walled Plates by Tilt Side Milling Based on an Analytical Force-Based Model

The milling of thin-walled workpieces is a common process in many industries. However, the machining defects are easy to occur due to the vibration and/or deformation induced by the poor stiffness of the thin structures, particularly when side milling the edges of plates. To this problem, an attempt by inclining the tool to a proper tilt angle in milling the edges of plates was proposed in this paper, in order to decrease the cutting force component along the direction of the lowest stiffness of the plates, and therefore to mitigate the machining vibration and improve the machined surface quality effectively. First, the milling force model in consideration of the undeformed chip thickness and the tool-workpiece engagement (TWE) was introduced in detail. Then, a new analytical assessment model based on the precisely established cutting force model was developed so as to obtain the optimum tool tilt angle for the minimum force-induced defects after the operation. Finally, the reliability and correctness of the theoretical force model and the proposed assessment model were validated by experiments. The methodology in this paper could provide practical guidance for achieving high-quality machined surface in the milling operation of thin-walled workpieces.

Time-Varying Chatter Frequency Characteristics in Thin-Walled Workpiece Milling With B-Spline Wavelet on Interval Finite Element Method

As the most significant material removal method, milling plays a very important role in the manufacturing industry. However, chatter occurs frequently in milling, which will seriously affect the production efficiency. The accurate prediction of chatter frequency can contribute to chatter monitoring and the design of the controller for chatter mitigation. During thin-walled workpiece milling under chatter, a new phenomenon of time-varying chatter frequency is discovered and explained in this paper. This phenomenon can be explained as follows, with the workpiece material removal, the modal parameters change during thin-walled milling, which can cause the continuous change of chatter frequency. In order to predict the varying modal parameters, this paper provided an efficient tool, the B-spline wavelet on interval finite element method (BSWIFEM), which can possess the material removal problem more accurately and more rapidly. Based on the calculated modal parameters, the time-varying chatter frequency can be obtained with the chatter frequency calculation formulas. To verify the calculated results, a number of milling tests are implemented on thin-walled parts. The experimental results show that the calculated chatter frequency is in good agreement with the measured chatter frequency, which validates the effectiveness of the proposed method.

Forced vibration response during the milling of thin-walled workpieces

Forced vibration response during the milling of thin-walled workpieces

Milling Process Monitoring Based on Vibration Analysis Using Hilbert-Huang Transform

Vibration analysis is one method of machining process monitoring. The vibration obtained in machining is often nonlinear and of a nonstationary nature. Therefore, an appropriate signal analysis is needed for signal processing and feature extraction. In this research, vibrations obtained in the milling of thin-walled workpieces were analyzed using the Hilbert-Huang transform (HHT). The features obtained by the HHT served as machining-state indicators for machining process monitoring. Experimental results showed the effectiveness of the HHT method for detecting chatter and tool damage.

Mechanism of process damping in milling of thin-walled workpiece

Abstract Existing ploughing mechanism and velocity-dependent mechanism, which were used for explaining the process damping in milling, did not consider the feedback influence of vibrations in normal direction, and thus, they cannot be used to reveal the underlying cause of process damping in milling process of thin-walled workpiece. This paper systematically studies the generation mechanism of process damping in thin-wall milling, and turns out that the relative vibration of cutter-workpiece system rather than the ploughing indentation is the main source of process damping in this kind of process. Theoretically, the actual cutting velocity and the relative vibrations in both feed and normal directions are combined to formulate the expression of instantaneous uncut chip thickness, which is then used to derive the equations of dynamic cutting forces. Taylor series expansions are carried out around harmonic response to linearize the equations of dynamic cutting forces. Derivations show that the dynamic cutting forces are composed of two items, i.e. the vibration displacements-induced force and the vibration velocities-induced force. The latter, which is actually an additional dissipative force being inversely proportional to the cutting velocity, constitutes a source of process damping. It is subsequently integrated into the process's governing equation to estimate the stability lobe diagrams (SLDs). Verification shows that the SLDs predicted by the proposed velocity-dependent mechanism reasonably agree with the experimentally observed results (among 672 sets of detailed comparisons, agreement rate is more than 95%.). Especially, thin-wall milling experiments with various values of spindle speeds show that the SLDs predicted using the proposed velocity-dependent mechanism are much closer to the experimental observations than those obtained by the ploughing mechanism (among 36 sets of detailed comparisons, agreement rate between the predictions and measurements is more than 94% for the proposed velocity-dependent mechanism, while the agreement rate is only 47% for the ploughing mechanism.).

Chatter detection and stability region acquisition in thin-walled workpiece milling based on CMWT

Milling chatter is one of the biggest obstacles to achieve high performance machining operations of thin-walled workpiece in industry field. In the milling process, the time-varying and position-dependent characteristics of thin-walled components are evident. So, effective identification of modal parameters and chatter monitoring are crucial. Although the advantage of chatter monitoring by sound signals is obvious, the milling sound signals are nonstationary signals which contain more stability information both in time domain and frequency domain, and the common analytical transformation methods are no longer applicable. In this paper, short time Fourier transform (STFT) is taken as an example to compare the processing results with cmor continuous wavelet transform (CMWT). This article concerns the chatter detection and stability region acquisition in thin-walled workpiece milling based on CMWT. CMWT combines the advantages of the cmor wavelet and continuous wavelet transform which has good locality and the optimal time-frequency resolution. Therefore, CMWT can be adaptively adjusted signal by the window, which is very suitable for processing nonstationary milling signals. Firstly, the model and characteristics of thin-walled workpiece during the cutting process are presented. Secondly, the CMWT method for chatter detection based on acoustic signals in thin-walled component milling process is presented. And the chatter detection results and stability region acquisitions are analyzed and discussed through a specific thin-walled part milling process. Finally, the accuracy of the method presented is verified through the traditional stability lobe diagram predicted using the exiting numerical method and the machined surface morphologies at different cutting positions obtained through the confocal laser microscope.

Error compensation in the five-axis flank milling of thin-walled workpieces

Five-axis flank milling is the most commonly used processing method in the aviation industry for the machining of thin-walled parts with complex ruled surfaces. During machining, the tool/workpiece deformations caused by the cutting force often lead to surface errors on the machined components that severely affect the accuracy of the machining results. This article presents an iterative compensation strategy to reduce the tool/workpiece deformation-induced surface error during the five-axis flank milling of thin-walled workpieces by modifying the tool tip position and tool axis orientation. This approach can be implemented in four steps. First, a highly integrated cutter-workpiece engagement extraction method is developed for the construction of a flexible cutting force model that can follow changes in the process geometry. Second, the tool/workpiece deformations are predicted by the cantilever beam model and finite element model, respectively. Third, an off-line error compensation scheme is performed at each cutting location of the tool path to obtain the modified tool position. Fourth, the machined surface of the workpiece model is reconstructed, and the compensated machining code, which can be used directly for actual machining, is generated. A case study is presented at the end of this article, and the effectiveness of the present compensation strategy is verified by machining experiments.

Chatter mitigation for the milling of thin-walled workpiece

During the milling process of thin-walled workpiece, chatter is the major obstacle in achieving desired accuracy and productivity of the final part due to its low rigidity. In this paper, a novel method is proposed to mitigate the chatter of the thin-walled workpiece by submerging the milling system in viscous fluid. To evaluate the effectiveness of the presented method, the influence of the viscous fluid on the milling force coefficients is investigated, and the modal parameters of the thin-walled workpiece are identified under dry and viscous conditions respectively. It is found that compared with the dry milling, the milling force coefficients reduce significantly under viscous fluid condition due to the lubrication. Besides, the damping of the milling system increases substantially under viscous fluid condition, and the natural frequencies of the milling system decrease owing to the added mass of the viscous fluid. The milling stability lobe diagrams are predicted based on the numerical integration method by taking multiple modes of the thin-walled workpiece into consideration. A series of milling tests are conducted to verify the stability lobe diagrams and confirm the effectiveness of the proposed approach. The vibration displacement signals are measured and then analyzed in time-frequency domain to demonstrate the state of the milling process. Moreover, the surface finishes of the thin-walled workpieces are compared. The results show that the thin-walled workpiece chatter could be suppressed greatly with the proposed method and higher stability limit can be achieved.

Optimization and improvement of stable processing condition by attaching additional masses for milling of thin-walled workpiece

Abstract Light weight is the main design requirement for minimizing costs or fuel consumption in mechanical equipments, and it, together with the material removal rate (MRR) requirement, also brings an important source of chatter, which still remains as an essential phenomenon to be suppressed in the future. This paper investigates the stable cutting region optimization problems in milling of structures with low rigidity. An effective method is proposed to improve the chatter stability by attaching appropriate additional masses to the workpiece, and thorough studies are also carried out to reveal the effect of additional masses on chatter stability. An efficient method based on structural dynamic modification scheme is developed to calculate the varying dynamics of the in-process workpiece under the combined effect of additional masses and material removal during milling process. Typical characteristic of this method lies in that only one modal analysis is needed to be performed on the finite element (FE) model of the initial workpiece, and the mode shape and natural frequency of the workpiece after attaching additional masses and removing material at each tool position can be calculated without the requirement to rebuild the FE model of the in-process workpiece. Based on the proposed dynamic modification scheme, an optimization algorithm is established to obtain the optimized combination of additional masses and the suitable stable cutting region for the achievement of maximum MRR. The proposed method is verified by milling process of a set of thin-walled workpieces, and comparisons of predictions and measurements show the validity and reliability.

Formulating a numerically low-cost method of a constrained layer damper for vibration suppression in thin-walled component milling and experimental validation

Due to its low stiffness and time-varying modal parameters, thin-walled workpiece milling is still a challenging problem. In order to reveal the influence of the relative position between the cutter and workpiece on the dynamic response of the flexible thin-walled component, and optimize the parameters of the constrained layer damper applied to suppress the machining vibration in thin-walled workpiece peripheral milling process, with assumption of chatter-free milling operation, a dynamic model of thin plate peripheral milling with constrained layer damper subjected to moving cutting force using Lagrange equation is proposed based on the first shear deformation theory. Considering the complexity and variability of the boundary conditions of workpiece in practical machining processes, a mixed Rayleigh-Ritz solution together with Courant's penalty method and differential quadrature method is presented to evaluate the damping performance and dynamic response. Courant's penalty method is used to handle the complex and changing boundary conditions, Rayleigh-Ritz method is employed to deal with the spatial partial derivatives, and differential quadrature method is applied to treat the temporal derivatives. The influence of the relative position between cutter and workpiece on dynamic response is investigated, and the response of milling position are calculated using presented technique and compared with the results measured through milling tests. The results show that an acceptable agreement between numerical and experimental results can be obtained and the present technique is efficient to estimate the dynamic response of thin plate milling. Additionally, the parametric study of constrained layer damper is performed through evaluating the root-mean-square of response, and the results can be used to optimize the parameters of the constrained layer damper for suppressing machining vibration and improving surface finish accuracy.

An Analytical Approach for Machining Thin-walled Workpieces

An analytical approach is developed to predict the dynamic chip thickness variation in ending milling of thin-walled workpieces. The proposed model considers the mechanism of calculating the stiffness of removed material by the end- mill from the overall stiffness of the workpiece to work out the changing stiffness of the workpiece which in turn is used to determine the displacement occurring. The displacement of the workpiece influences the radial depth of cut and thus yields a dynamic variation. The generalized model of the volume of removed material is computed by considering the discrete axial depth of cut, the radial depth of cut and the circumferential section of the tool radius contact for each tool step taken. The features of this model cater for the helical tool geometry used. The stiffness of removed material is updated by subtraction from the overall stiffness of workpiece. The overall stiffness of the wall is updated by considering the removed material at each time step. The feedback of the displacement amplitude is linked with a cutting forces model to address the influence of the dynamic displacement of the workpiece from the acting chip load. The cutting forces is modeled based on the Oxley variable flow stress machining theory. The predictive capabilities of the proposed model are verified with the experimental results. The comparison of the results is encouraging and reasonable correlation is achieved.

Chatter prediction utilizing stability lobes with process damping in finish milling of titanium alloy thin-walled workpiece

Machining chatter often becomes a big hindrance to high productivity and surface quality in actual milling process, especially for the thin-walled workpiece made of titanium alloy due to poor structural stiffness. Aiming at this issue, the stability lobes are usually employed to predict if chatter may occur in advance. For obtaining the stability lobes in milling to avoid chatter, this article introduces an extended dynamic model of milling system considering regeneration, helix angle, and process damping into the high-order time domain algorithm which can guarantee both high computational efficiency and accuracy. Via stability lobes, the reasonability and accuracy of the proposed method are verified globally utilizing specific examples in literature. More convincingly, the time-domain numerical simulation is also implemented to predict vibration displacement for partial stability verification. In this extended model, process damping is well-known as an effective approach to improve the stability at low spindle speeds, and particularly, titanium alloy as typical difficult-to-machine material is generally machined at low spindle speeds as well due to its poor machinability. Therefore, the proposed method can be employed to obtain the 3D stability lobes in finish milling of the thin-walled workpiece made of titanium alloy, Ti-6Al-4V. Verification experiments are also conducted and the results show a close agreement between the stability lobes and experiments.

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.

A time-space discretization method in milling stability prediction of thin-walled component

The stability prediction of thin-walled workpiece milling is an awkward problem due to the time variant of dynamic characteristics during milling process. Integrating the time discretization method for stability prediction mentioned in many articles, a novel time-space discretization method for thin-walled component milling stability prediction is proposed based on thin plate theory and mode superposition principle, which includes the effects of the engagement position between cutter and workpiece and multi-modes of the system. The results show that the method presented is very reliable and efficient, and its accuracy is also in good agreement with experimental results. Additionally, the method can be used to handle various complex boundary conditions by means of the updated Rayleigh-Ritz solutions together with the penalty method. Two case studies are performed to explain the validation of the method as well as milling experiments of a half-clamped thin plate.

Chatter prediction for the peripheral milling of thin-walled workpieces with curved surfaces

Workpiece dynamics is the dominant factor which should be taken into consideration in chatter prediction of peripheral milling of thin-walled workpieces. Usually, material removal, tool position and varying dynamic displacements of the workpiece along the tool axis influence the workpiece dynamics. However, these three aspects were not considered simultaneously in the existing researches.This paper comprehensively investigates the effect of varying workpiece dynamics on the stability in peripheral milling of thin-walled workpieces with curved surfaces. A new dynamic model of tool and workpiece system is proposed to consider the dynamic behavior of tool and workpiece as well as the influences of engagement and tool feed direction. Interaction between tool and thin-walled workpiece is modeled at discrete nodes along the axial depth of cut. An efficient method based on structural dynamic modification scheme is developed to characterize the effect of material removal upon the in-process workpiece dynamics. This is done by only performing modal analysis on the FEM model of initial workpiece, while mode shapes and natural frequencies of the in-process workpiece can be calculated without re-building and re-meshing the instant FEM model at each tool position.The proposed model and method are verified by the milling process of two thin-walled workpieces concerning a plate and a workpiece with curved surface. Comparisons of numerical and experimental results show that chatter can be accurately predicted for the peripheral milling of thin-walled workpieces.

Stability improvement and vibration suppression of the thin-walled workpiece in milling process via magnetorheological fluid flexible fixture

In aerospace industry, thin-walled workpiece milling is a critical task. Also, the machining vibration is a major issue for the accuracy of the final part. In this study, a new dynamic analytical model is proposed to determine the effect of damping factor on the dynamic response of thin-walled workpiece in machining. A complex structure workpiece is equivalent to a thin plate. The fixture constrains and the damping factor are crucial elements of this thin plate. Therefore, the magnetorheological fluid flexible fixture is designed to suppress the machining vibration in machining process. Then, the general dynamic cutting force model and the damping force model are proposed for the key dynamic equation for the prediction of dynamic response to evaluate the stability of the milling process with and without the damping control. Finally, the feasibility and effectiveness of the proposed model is validated by machining tests. The predicted values match on the experiment results.

Stability prediction of thin-walled workpiece made of Al7075 in milling based on shifted Chebyshev polynomials

With the rapid development of aerospace technology, Al7075 has been widely used for structural components. High-speed milling is one of the most effective ways to improve machining efficiency of Al7075. During the milling process, regenerative chatter which restricts the milling quality and productivity often occurs. With the aim of avoiding regenerative chatter, stability lobe diagram (SLD) is widely used to obtain chatter-free parameters. This work presents a stability prediction method by using shifted Chebyshev polynomials. The milling dynamics with consideration of the regenerative effect is described by time periodic delay-differential equations (DDEs). The transition matrix of the milling system is constructed with the help of Chebyshev–Gauss–Lobatto (CGL) points. In order to demonstrate the accuracy of the proposed method, the rate of convergence of the proposed method is compared with that of the classical benchmark methods. On the other hand, in the process of thin-walled workpiece milling, the dynamic behavior of the workpiece depends on the tool position. To study the influence of the tool position dependent dynamics on the chatter stability of the thin-walled workpiece, a three-dimensional SLD is obtained. The verification experiments are conducted to verify the reliability of the proposed method. The results show that the experimental results are consistent with the predicted results.