Milling Of Thin-walled Workpiece Research Articles

Chatter prediction for parallel mirror milling of thin-walled parts by dual-robot collaborative machining system

Thin-walled parts with double-sided features are widely used in the aerospace industry, but the machining of such parts still faces great challenges due to their weak rigid structure and the positioning errors caused by turning/re-clamping of the workpiece. For this purpose, a novel machining approach for parallel mirror robotic milling of thin-walled parts is proposed. Different from conventional single-sided milling, it can simultaneously machine both sides of the workpiece while avoiding the above problems and improving machining efficiency. However, the introduction of multiple cutting excitations makes the vibration behavior of the workpiece more complicated, thus making it difficult for the dynamic modeling of the dual-robot machining system. This paper aims to uncover the dynamic mechanism of chatter phenomenon in parallel mirror robotic milling process in consideration of the interaction of tool-workpiece-tool system. A dual-robot collaborative machining system is first developed for the milling of thin-walled workpiece, and the method of synchronous motion planning for dual robots is described. The slave robot synchronously follows the master robot and the two milling cutters mounted on tool flange of the robot are kept in mirror position. Over a tooth passing period, the engagement state of the cutters and the workpiece is analyzed considering the effect of rotation phase lag (RPL) between the twin cutters. After that, the vibration state and the corresponding instantaneous displacement of the workpiece can be expressed explicitly, then the dynamic resultant cutting force acting on the workpiece is calculated according to the mechanistic force model. Comprehensively considering the dynamic interaction of the tool-workpiece-tool system, multi-mode and position-dependent modal characteristics of thin-walled part, a time-varying delay differential equation is constructed and solved to explore the dynamical behavior of parallel mirror milling process. The predicted stability lobe diagram is validated by a series of parallel mirror milling tests on thin-walled PMMA plates. Comparisons of numerical and experimental results show that the dynamic model successfully predicts the chatter stability for the parallel mirror milling of thin-walled parts.

Top burr thickness prediction model in milling of thin-walled workpiece considering tool and workpiece deformation

Top burr thickness prediction model in milling of thin-walled workpiece considering tool and workpiece deformation

Vibration and deformation suppression in mirror milling of thin-walled workpiece through a magnetic follow-up support fixture

Large thin-walled workpieces have large deformation and vibration during end milling due to their weak rigidity, which can lead to degradation of the surface quality of workpiece. To this end, this paper proposes a new method to suppress vibration and deformation during mirror milling of thin-walled workpieces by using a magnetic follow-up support fixture. The main idea is to design a fixture that can clamp around the milling area of the workpiece in real time, follow the tool motion and continuously provide a controlled air pressure force to the opposite side of the milling area without providing the mirroring support fixture that occupies a large space to the opposite side of workpiece. First, to accurately characterize the motion of the fixture, a mathematical model of the axial magnetic force considering the lateral offset is developed. On this basis, to efficiently investigate the capability of the fixture in improving the dynamic characteristics and dynamic response of workpiece, the finite element method (FEM) is used to analyze the frequency response functions (FRF) and the mode shapes of workpiece under the action of the fixture. Then, the vibration amplitude and deformation of workpiece on the milling path are simulated and studied by considering the variation of the air pressure and the position of the fixture. Finally, the prototype of the fixture is developed and then the effectiveness and feasibility of the proposed method are verified under the experimental tests with different machining parameters.

Optimal deformation error compensation process in flank milling of thin-walled workpieces

Optimal deformation error compensation process in flank milling of thin-walled workpieces

Chatter suppression for milling of thin-walled workpieces based on active modal control

Owing to the low stiffness of workpiece, chatter easily occurs in milling of thin-walled structures, which may cause poor surface quality and uncontrolled machining accuracy. For suppressing milling chatter of thin-walled workpieces, this paper presents an active control method with a piezoelectric patch actuator. As a flexible body, thin-walled workpieces have more than one modals participating in milling vibration, and its dynamic characteristics vary with tool position. This paper establishes dynamic model of the thin-walled workpiece with a semi-analytical method. Considering three order modals of workpiece, actuator position is optimized and active controller is designed. The varying dynamic characteristics of workpiece are overcome by designing controller with the parameters on the maximum vibration position to stabilize the whole process. Experiments verify the necessity of considering more than one modals of workpiece in active control through various chatter characteristics dominant by different modals. With the designed control method, the stable cutting depth improves from 0.2 mm to 1 mm.

Dynamic characteristic reconfiguration of a fixture-workpiece system for vibration suppression in milling of thin-walled workpieces based on MR damping fixture

Dynamic characteristic reconfiguration of a fixture-workpiece system for vibration suppression in milling of thin-walled workpieces based on MR damping fixture

Forced vibration mechanism and suppression method for thin-walled workpiece milling

Thin-walled workpieces are widely used in aerospace applications, but their weak stiffness characteristic leads to forced vibration during milling, which reduces the quality of the milled surface. To solve the forced vibration problem in milling thin-walled workpieces, this paper proposes a forced vibration mechanism for the thin-walled workpiece under the influence of various factors and a corresponding suppression method. First, the response characteristics of the thin-walled workpiece excitation features correlated with the mode shape are given. Then, the vibration shapes of the thin-walled workpiece are combined with the cutting position, and the effects of the cutting force frequency and cutting force magnitude are also considered. Finally, the different vibration characteristics are associated with the time constant properties of the shear thickening fluid (STF), and the corresponding forced vibration effects are suppressed by the STF. After a series of experimental modal analyses and cutting experiments, the results show that the proposed theory can explain the forced vibration phenomenon of the thin-walled workpiece, and the vibration suppression effect of the STF is consistent with the theory. The method is also applicable to vibration suppression at the weak position during the machining of the large workpiece.

An Explicit Coupling Model for Accurate Prediction of Force-Induced Deflection in Thin-Walled Workpiece Milling

Abstract Cutting force-induced vibrations in thin-walled parts milling may cause violation of dimensional tolerance while accurate modeling of the milling error distribution is still a challenging work because of the coupling effect between the dynamic cutting forces and the resulting steady-state vibrations. It greatly increases the computational complexity to capture the true cutter–workpiece engagement with classic time domain or iteration method. This paper proposes a novel explicit model to predict the error distribution considering this coupling relationship without iterative calculation. A new cutting force model with variable coefficients with respect to the deflections is developed to describe the dynamic cutting forces. The effectiveness of the force model is verified by a group of calibration experiments. The analytical solution of the dynamic model is discussed and a semi-analytical method is constructed to predict the error distribution directly. Machined surface as well as the deformation errors are derived and thin-walled workpiece milling experiments for verification are conducted. Comparisons between simulations and experiments show that the proposed method is accurate and efficient.

Chatter Suppression and Machining Quality Improvement in the Robotic End Milling of Thin-Walled Workpieces Based on a Magnetic Follow-Up Support Device

Chatter Suppression and Machining Quality Improvement in the Robotic End Milling of Thin-Walled Workpieces Based on a Magnetic Follow-Up Support Device

Early chatter detection in thin-walled workpiece milling process based on multi-synchrosqueezing transform and feature selection

The onset of chatter is extremely disadvantageous to machining process, which will induce poor surface quality, low processing efficiency and severe tool wear, especially in the milling of weakly rigid thin-walled workpieces, so it should be detected as early as possible to avoid further damage. To handle this issue, a multi-synchrosqueezing transform (MSST) based early chatter detection method as well as a novel feature selection algorithm is proposed in this paper to monitor the occurrence of chatter in thin-walled parts machining. MSST is firstly conducted on the sampled vibration signal to generate a time–frequency (TF) representation. The periodic components associated with spindle rotation are removed from the TF matrix and the chatter-related signal is recovered by multi-ridge detection. Then, features in time and frequency domains are extracted from the reconstructed chatter signal. A feature selection algorithm combining multi-feature distance evaluation technique (MFDET) and distance correlation analysis is subsequently presented to determine the optimal feature subset. Compared with other chatter indicators given in existing works, the selected compact feature set is more discriminative and has low redundancy. Furthermore, random forest (RF) is utilized to intelligently diagnose the machining stability. Several experiments under constant and variable conditions are performed to evaluate the performance of the proposed method in terms of early chatter detection. The experimental results demonstrate that the presented chatter monitoring method is highly efficient and robust, which can recognize the chatter onset timely and accurately for thin-walled parts milling under different machining conditions.

Operational modal analysis based dynamic parameters identification in milling of thin-walled workpiece

The workpiece dynamics are an important factor in the planning of machining strategy. Usually, the structural dynamic parameters of workpiece are obtained by experimental modal analysis (EMA). However, the dynamics of thin-walled workpiece are varying due to material removal and tool-workpiece coupling. Operational modal analysis (OMA) provides a route to estimate the structural dynamic parameters during operation, but the input excitation of milling system is mainly periodic milling force, which violates the premise of OMA. In this paper, an output-only modal identification method for the dynamic parameters of thin-walled workpiece is proposed. The milling force is analyzed and the analysis results show the milling force contains white noise for OMA. However, strong harmonic components in milling force seriously interfere the identification of dynamic parameters. To remove the harmonic components in response signal, a revised least-squares (LS) method is proposed to fit harmonics with multiple fundamental frequencies. After the harmonic removal, OMA is conducted in frequency domain using the poly-reference least squares complex frequency (p-LSCF) method based on positive power spectral density (PSD). To verify the feasibility of the proposed method, the simulation case is conducted by exciting a 3-degree of freedom structure using measured milling force signal, and the results show a good agreement with the theoretical values both in frequencies and damping ratios. Furthermore, a series of milling tests under different cutting parameters are conducted on a thin-walled workpiece, where thin-film sensors are adopted to measure the structural response. The results show the method can be successfully applied to extract the workpiece dynamic parameters. Compared with results of EMA, the structural frequencies increase slightly, while the damping ratios show a large promotion due to the process damping.

Position-varying surface roughness prediction method considering compensated acceleration in milling of thin-walled workpiece

Position-varying surface roughness prediction method considering compensated acceleration in milling of thin-walled workpiece

Multi-pass adaptive tool path generation for flank milling of thin-walled workpieces based on the deflection constraints

In multi-pass flank milling of thin-walled workpieces, deflections of both the workpiece and tool caused by the cutting force jeopardize the geometrical accuracy of the machined part, which is particularly problematic when cutting hard materials, such as titanium super-alloy used for blades on jet engines, as the due cutting force will be exceedingly large. Traditionally, to mitigate the deflection, machining parameters such as feed rate and depth-of-cut are set as constants and selected extremely conservatively, thus severely prolonging the total machining time. In this paper, aiming at reducing the total machining time while refraining the deflection of both the tool and in-process workpiece, we present a new multi-pass tool path generation method for flank milling of thin-walled workpieces at semi-finish and finish machining stages. In our method, both the feed rate and depth-of-cut are allowed to vary in each pass, and they are simultaneously maximized while subject to the given limits on the deflections of both the tool and in-process workpiece as well as the kinematical limits of the machine tool itself. A practical semi-greedy algorithm is then proposed to solve the formulated global optimization problem which includes both the finite element analysis for calculating the workpiece deflection and the meshing operation. Both computer simulation and physical cutting experiments are performed to demonstrate that a substantial saving (over 20%) in total machining time could be realized by the proposed method.

Influence of the Cut Axial Depth on Surface Roughness at High-Speed Milling of Thin-Walled Workpieces

The paper presents the results of research on the dynamics of end milling of thin-walled work-pieces having complex geometric shapes. Since the milling process with shallow depths of cut is characterized by high intermittent cutting, the proportion of regenerative vibrations decreases, and the effect of forced vibrations on the dynamics of the process, on the contrary, increases. The influence of axial depth of cut on the vibrations arising during processing, and roughness of the processed surface have been studied in paper. The experiments have been carried out in a wide range of changes in the spindle speed at different axial cutting depths. Vibrations of a thin-walled work-piece have been recorded with an inductive sensor and recorded in digital form. Then an oscillogram has been used to estimate the amplitude and frequency of oscillations. The profilograms of the machined surface have been analysed. Roughness has been evaluated by the parameter Ra. The results have shown similar relationships for each of the investigated axial cutting depths. The worst cutting conditions have been observed when the natural vibration frequency coincided with the tooth frequency or its harmonics. It is shown that the main cause of vibrations in high-speed milling is forced rather than regenerative vibrations. Increasing the axial depth of cut at the same spindle speed increases the vibration amplitude. However, this does not significantly affect the roughness of the processed surface in cases when it comes to vibration-resistant processing.

Active vibration control of thin-walled milling based on ANFIS parameter optimization

In view of the time-varying dynamic characteristics of thin-walled milling, an optimization method of thin-walled milling control parameters based on the ANFIS system is proposed. Firstly, the control system strategy design and equipment selection construction were carried out, and the fuzzy inference model was built based on matlab platform; Subsequently, the fuzzy inference model of the thin-walled workpiece milling process was obtained by collecting fuzzy inference training data set of and the ANFIS system training. The proportional control coefficient of the controller was optimized based on the input and output relation of the model. Finally, the dynamic response and surface roughness of the controlled thin-walled panel milling process before and after optimization were obtained through cutting experiments, and the effectiveness of the ANFIS method to optimize the control parameters and the reliability of the fuzzy reasoning model were verified through analysis of the reasoning results. The results show that the ANFIS system can predict the vibration response performance of the target by setting the input and output reasonably, and optimize the related variable parameters according to the predicted results, so as to make the thin-wall milling system more stable.

Position-Dependent Stability Prediction for Multi-Axis Milling of the Thin-Walled Component with a Curved Surface

Time-varying dynamic behaviors are essential to investigate the stability in the thin-walled workpiece milling process, which is usually affected by material removal and position-dependent characteristics of the workpiece along with the tool feed direction. To predict the milling stability with position-dependent, thin-walled component multi-axis milling, an improved structural dynamic modification method with variable mass is proposed in the paper. Firstly, the extraction of multi-axis milling material and the removal process of thin-walled parts with a complex curved surface and variable thickness is completed with CAM software. Then, the material removal of one cutting path as a modification of the structure is divided into multi-cutting steps with equal length to obtain the corrected FRFs in the machining process on the basis of the extended Sherman-Morrison-Woodbury formula. Furthermore, the dynamic characteristics of the initial un-machined workpiece and final machined workpiece are calculated by both experimental modal analysis and FEM. Finally, the multi-axis milling stability is predicted using the extended numerical integrated method, and an aero-engine blade is used to validate the accuracy and effectiveness of the proposed method for multi-axis milling molding parts.

In-Process Monitoring of Changing Dynamics of a Thin-Walled Component During Milling Operation by Ball Shooter Excitation

During the milling of thin-walled workpieces, the natural frequencies might change radically due to the material removal. To avoid resonant spindle speeds and chatter vibration, a precise knowledge of the instantaneous modal parameters is necessary. Many different numerical methods exist to predict the changes; however, small unmodelled effects can lead to unreliable results. The natural frequencies could be measured by human experts based on modal analysis for an often interrupted process; however, this method is not acceptable during production. We propose an online measurement method with an automatic ball shooter device which can excite a wide frequency range of the flexible workpiece. The method is presented for the case of blade profile machining. The change of the natural frequencies is predicted based on analytical models and finite element simulations. The measurement response for the impulse excitation of the ball shooter device is compared to the results of impulse modal tests performed with a micro hammer. It is shown that the ball shooter is capable of determining even the slight variation of the natural frequencies during the machining process and of distinguishing the slight change caused by different clamping methods. An improved FE model is proposed to include the contact stiffness of the fixture.

Dynamic modeling and stability prediction in milling process of thin-walled workpiece with multiple structural modes

Regenerative chatter can easily occur in the milling process of thin-walled workpiece due to the inherently low stiffness. This article aims to predict the stability of thin-walled workpiece in the milling process with a complete dynamic model. First, multiple structural modes of thin-walled workpiece are taken into consideration, and a complete dynamic model of thin-walled workpiece milling system is developed. Then, a numerical integration method is used to achieve the stability lobe diagrams of the milling system and identify the chatter frequency. Besides, the major structural mode, which is responsible for the occurrence of thin-walled workpiece chatter in the milling process, is predicted. A series of milling tests concerning a general cantilever plate are conducted, and the test results agree well with the predicted results, which shows the effectiveness of the proposed method. Finally, the effects of milling tool and structural modes on milling stability are discussed separately, which could provide theoretical basis for the dynamic modeling of thin-walled workpiece in milling process.

A prediction method based on the voxel model and the finite cell method for cutting force-induced deformation in the five-axis milling process

When milling a thin-walled workpiece, knowledge of machining error due to the workpiece deformation induced by cutting force is crucial to ensuring the quality of the final machined part. In the current research of deformation prediction, the finite element method (FEM) is the dominant tool for calculating the deformation induced by the cutting force. However, FEM needs tremendous manual effort to generate a mesh of the in-process workpiece (IPW) and cannot be directly applied to discrete structured models such as the voxel or dexel models that are widely used for its advantage on parallel computation. In addition, a very fine mesh is needed for FEM to preserve the geometry of IPW, but it will lead to a huge global stiffness matrix. Therefore, it calls for a new method of fast deformation prediction in the milling of thin-walled workpieces. In this paper, aiming at the fast prediction of the deformation error of five-axis milling directly and efficiently on the voxel model, we present a prediction algorithm based on a combination of the finite cell method (FCM) and the voxel model. Specifically, in this study, a pipeline of FCM combined with the voxel model is specially designed for the efficient analysis of IPW deformation along a continuous tool path in the five-axis milling of thin-walled workpieces. By introducing a technique of partially updating the global stiffness matrix and the cells’ stiffness matrices, the proposed algorithm can tremendously reduce the FCM calculation time per sample point on the tool path, by as much as 19 times in our test examples. Both computer simulation and physical cutting experiments performed by us have convincingly verified the accuracy of the proposed deformation prediction algorithm. To our best knowledge, this is the first work of introducing FCM on the voxel model to the prediction of deformation induced by cutting force in milling problems, which enables customized tool path design and optimization in the five-axis milling of free-form surfaces.

Improving dynamic process stability in milling of thin-walled workpieces by optimization of spindle speed based on a linear parameter-varying model

In milling of thin-walled workpieces, like aero engine blades, the reduction of vibrations is of central importance to reach a more economical and reliable process as well as an improved workpiece surface quality. However, the dynamic behavior of the workpiece continuously varies due to changes in workpiece stiffness and mass, caused by the moving position of the excitation force as well as the material removal. In this paper, the simulation of the changing workpiece dynamics for a simplified blade geometry using FE-modal analysis is demonstrated. All extracted workpiece dynamic states are combined in a reduced LPV-model (Linear Parameter-Varying model). The LPV-model is able to describe the varying process dynamic behavior and makes the selection of advantageous spindle speeds possible.