Milling Of Thin-walled Structures Research Articles

Online Monitoring Dynamic Characteristics in Thin-Walled Structure Milling: A Physics-Constrained Bayesian Updating Approach

Accurate estimation of modal parameters of thin-walled structures during the milling process is crucial since it determines the accuracy of deformation and vibration prediction and then the feasibility of selected machining parameters. However, time-varying dynamic characteristic due to material removal brings a great challenge in the pursuit. This article presents an online monitoring strategy for the estimation of modal parameters of thin-walled structures with measured cutting forces and single acceleration signal. The detailed physical constraints are presented, and the analytical form of the modal shape of thin-wall structures is derived. A Bayesian filter framework with slack variables is established for updating the modal parameters by simultaneously considering the physical constraints and the measured signals. Validation experiments are conducted on two kinds of thin-walled structures with different structures. The results of the proposed method are compared with the results of the standard modal test at certain stages. It indicates that the proposed approach is effective for accurately online monitoring of the spatiotemporally varying modal parameters in milling of thin-walled structures.

Dimensional Deviations of Horizontal Thin Wall of Titanium Alloy Ti6Al4V Determined by Optical and Contact Methods

Thin-walled structures are used in many industries. The need to use such elements is dictated by the desire to reduce the weight of the finished product, as well as to reduce its cost. The most common method of machining such elements is the use of milling, which makes it possible to make a product of almost any shape. However, several undesirable phenomena occur during the milling of thin-walled structures. The main phenomenon is a deformation of the thin wall resulting from its reduced stiffness. Therefore, it is necessary to control the dimensional and shape accuracy of finished products, which is carried out using various measuring instruments. The development of newer measuring methods such as optical methods is being observed. One of the newer measuring machines is the 3D optical scanner. In the present experiment, thin-walled samples in horizontal orientation of Ti6Al4V titanium alloy were machined under controlled cutting conditions. During machining, the cutting speed and feed rate were assumed constant, while the input factors were the tool and cutting strategy. This paper presents graphs of deviations in the determined cross-section planes of thin-walled structures using a 3D optical scanner and a coordinate measuring machine. A correlation was made between the results obtained from the measurement by the optical method and those determined by the contact method. A maximum discrepancy of about 8% was observed between the methods used.

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.

Optimization of material removal rate in milling of thin-walled structures using penalty cost function

The high flexibility of thin-walled structures makes their machining process prone to chatter and excessive vibrations. This challenge is compounded by the continuous varying dynamics of the workpiece as material is removed from the workpiece. As a result, the common optimization methods that are used for maximizing material removal rate in the milling processes of rigid parts are not applicable to machining of thin-walled structures. In this paper, an optimization method is presented to maximize the material removal rate in milling of thin-walled structures without violating forced vibration and chatter stability constraints. The cost function of the optimization problem is formulated using penalty terms to impose the vibration constraints. Moreover, a resampling strategy is implemented to minimize the risk of convergence to a local minimum. The performance of the presented approach is studied through planning of the semi-finishing stage of machining a thin-walled pocket structure. The dynamics of the workpiece is modelled using finite strip modelling to ensure high computational efficiency. The numerical simulations and experimental studies in this work show that the presented optimization approach can be effectively used to plan an optimum machining process for thin-walled structures which does not cause chatter or excessive vibration amplitudes.

Predicting the dynamics of thin-walled parts with curved surfaces in milling based on FEM and Taylor series

The effect of material removal on workpiece dynamics has a great influence on suppressing resonance and regenerative vibrations in milling of thin-walled structures. Although some works have been done to obtain the varying dynamics of the workpiece, many researchers take plate structures as research objects. In this paper, a method based on FEM and Taylor series is proposed to predict the time-varying dynamics of thin-walled blade structures in milling. The input data in the method is the mass and stiffness matrices of the initial workpiece and the change of the matrices caused by material removal. The initial matrices can be obtained through the FEM model established by the mechanical analysis of the eight-node shell element, and the change of the matrices can be obtained by a third-order Taylor series. In comparison with three-dimensional cube elements, the shell elements can reduce the number of degrees of freedom of the FEM model by 74%, which leads to about 9 times faster computation of the characteristic equation. Meanwhile, the proposed method eliminates the necessities of re-building and re-meshing the FEM model at each cutting position, then the computational efficiency of workpiece dynamics is improved. Finally, the result of experiments shows that the maximum prediction error of the proposed method is about 4%.

Possibilities of using stability lobe diagram for stability prediction of high speed milling of thin-walled details

High-speed milling is a cost and time effective process, which becomes more popular nowadays. It especially available at milling of thin-walled structures of airfoil components. Manufacturer often faced with problem of producing thin-walled parts due to vibration and stability lobes theory in a lot of cases does not allow to avoid this problem by choosing the appropriate cutting conditions. In practice cutting condition for milling of thin walled detail is usually obtained by experimental way. In this paper authors present main differences of thin-walled end milling from classical end milling and show features of thin-walled end milling process and aspects, which effect on quality of surface finish. The aim of the paper is to validate that at high speed thin-walled end milling not only chatter is the reason of unstable cutting condition. Evaluation of stability, obtained by experimental tests, was compared with stability lobe diagram.

Stability Prediction for High-speed Milling of Thin Walled Structures and Experimental Validation

In view of the frequency response characteristic of small stiffness and dynamic variation and the dynamic behaviour of high order for high-speed milling system of thin walled structures, stability limits prediction theory is studied. In connection with the small-stiffness frequency response characteristics of thin walled structures, the dynamic model and the stability critical condition determined by the relative dynamic behaviour between tool subsystem and workpiece subsystem are put forward. In the light of the high-rotation characteristics of high-speed milling, the stability limit prediction model based on the high-order dynamic behaviour of multi-DOF system is built. In view of the time variability of thin walled structures’ dynamic behaviour during the machining process, the method to predict the stability limit by tracing the dynamic behaviour variation of workpiece during diverse machining stages is suggested. The stability prediction model and method proposed are validated by the experiments carried out to cut the typical thin walled structures.

Impact Dynamics in Milling of Thin-Walled Structures

The development of reliable high-speed spindles and motioncontrol systems has led to an increase in the industrial use ofhigh-speed milling. One of the primary applications of this newtechnology is the manufacture of thin-walled aluminum components foraircraft. The flexibility of the tools and workpieces, the high spindlefrequencies, and the inherent impact nonlinearities in the millingprocess can lead to complicated dynamic tool-workpieceinteractions. An experiment was constructed to study the vibrations ofa thin-walled part during milling. Time series, power spectra,autocorrelations, auto-bispectra, and phase portraits were examined.From this data, it is inferred that stiffness and damping nonlinearitiesdue to the intermittent cutting action have a pronounced effect on thedynamics of the workpiece. Delay space reconstructions and pointwisedimension calculations show that the associated motions arecharacterized by a fractal geometry. The auto-bispectra suggestquadratic phase coupling among the spectral peaks associated with thecutter frequency. A mechanics-based model with impact-nonlinearities wasdeveloped to explain the observed results. The predicted results agreewell with the experimental observations. The model predictions indicatethat aperiodic motions are possible over a large range ofcontrol-parameter values. These analytical and experimental results haveimplications for the prediction and control of vibrations in milling.