A novel chatter stability criterion for the modelling and simulation of the dynamic milling process in the time domain

The modelling of the dynamic processes in milling and the determination of chatter-free cutting conditions are becoming increasingly important in order to facilitate the effective planning of machining operations. In this study, a new chatter stability criterion is proposed, which can be used for a time domain milling process simulation and a model-based milling process control. A predictive time domain model is presented for the simulation and analysis of the dynamic cutting process and chatter in milling. The instantaneous undeformed chip thickness is modelled to include the dynamic modulations caused by the tool vibrations so that the dynamic regeneration effect is taken into account. The cutting force is determined by using a predictive machining theory. A numerical method is employed to solve the differential equations governing the dynamics of the milling system. The work proposes that the ratio of the predicted maximum dynamic cutting force to the predicted maximum static cutting force can be used as a criterion for the chatter stability. Comparisons between the simulation and experimental results are given to verify the new model.     

Prediction of simultaneous dynamic stability limit of time�Cvariable parameters system in thin-walled workpiece high-speed milling processes

A method for predicting simultaneous dynamic stability limit of thin-walled workpiece high-speed milling process is described. The proposed approach takes into account the variations of dynamic characteristics of workpiece with the tool position. A dedicated thin-walled workpiece representative of a typical industrial application is designed and modeled by finite element method. The curvilinear equation of modal characteristics changing with tool position is regressed. A specific dynamic stability lobe diagram is then elaborated by scanning the dynamic properties of workpiece along the machined direction throughout the machining process. The results show that, during thin-walled workpiece milling process, material removing plays an important part on the change of dynamic characteristics of system, and the stability limit curves are dynamic curves with time?Cvariable. In practical machining, some suggestion is interpreted in order to avoid the vibrations and increase the chatter free material removal rate and surface finish. Then investigations are compared and verified by high-speed milling experiments with thin-walled workpiece.     

Prediction of regenerative chatter in the high-speed vertical milling of thin-walled workpiece made of titanium alloy

With the wide application of high-speed cutting technology, high-speed machining approach of titanium alloy has become one of the most effective ways to improve processing efficiency and to reduce the processing cost, but the cutting chatter which often occurs in the cutting process not only affects the machining surface quality but also reduces the production efficiency. Regenerative chatter is a typical phenomenon during actual cutting, and it has the greatest impact on the cutting process. With the purpose of avoiding regenerative chatter and selecting appropriate cutting parameters to achieve a steady cutting process and a high surface quality, it is necessary to determine the critical boundary conditions where chatter occurs. Built on the work of previous theoretical researches of regenerative chatter, this paper utilized Visual C++ software to calculate the chatter stability domain during the finish machining of titanium alloy. It was shown that the border between a stable cut and an unstable cut can be visualized in terms of the axial depth of cut as a function of the spindle speed. Using the result, it can find the specific combination of machining parameters, which lead to the maximum chatter-free material removal rate. In order to verify the result, the high-speed milling experiment of an I-shaped thin-walled workpiece made of titanium alloy was conducted. It revealed that the actual machining result was consistent with the calculation prediction. This study will offer a useful guide for effective parameter selection in future CNC machining applications.     

Three-dimensional stability lobe and maximum material removal rate in end milling of thin-walled plate

Chatter phenomenon often occurs during end milling of thin-walled plate and becomes a common limitation to achieve high productivity and part quality. For the purpose of chatter avoidance, the optimal selection of the axial and radial depth of cut, which are decisive primary parameters in the maximum material removal rate, is required. This paper studies the machining stability in milling of the thin-walled plate and develops a three-dimensional lobe diagram of the spindle speed, axial, and radial depth of cut. Through the three-dimensional lobe, it is possible to choose the appropriate cutting parameters according to the dynamic behavior of the chatter system. Moreover, this paper studies the maximum material removal rate at the condition of optimal pairs of the axial and radial depth of cutting.     

Low-frequency chatter genesis during inclined surface copy-milling with ball-end mill: Experimental study

In this study low-frequency chatter during machining of inclined surfaces with ball-end mills is experimentally investigated. An explanation of genesis of low-frequency vibrations have been proposed for various conditions: cutting direction, lead angle values, spindle speed, depth of cut. As a result, it has been proven that low-frequency chatter has more significant effect on machined surface than usual chatter. Low-frequency chatter occurs during downward milling, rather than upward milling, especially when lead angle increases. Furthermore, low-frequency chatter takes place in the beginning of cutting process, thereafter develops into steady state of usual chatter, which has no such significant effect on machined surface, as it has been shown. The results are in line with the supposition that low frequency vibrations are caused by sudden and irregular nature of shearing process, when magnitude is small.     

MODELING WORKPIECE DYNAMICS USING SETS OF DECOUPLED OSCILLATOR MODELS

During the machining of thin-walled components, the dynamic behavior of the workpiece has a significant influence on the machining process and on the quality of the machined surfaces. In this article, a hybrid simulation concept for modeling regenerative workpiece vibrations is presented, which couples a geometric workpiece model with sets of decoupled harmonic oscillators to take the workpiece dynamics into account.     

Investigation on chatter stability of thin-walled parts considering its flexibility based on finite element analysis

High-speed milling of thin-walled part is a widely used application for aerospace industry. The low rigidity components, large quantities of material removed in machining progress, are in the risk of the instability of the progress. In this paper, the thin-walled parts have the similar characteristics with the tools. Therefore, the dynamic model and the stability critical condition determined by the relative dynamic behavior between tool subsystem and workpiece subsystem are put forward. The thin-walled parts’ dynamic character varies greatly with time when machining. The whole workpiece has been divided into several stages by finite element analysis (FEA) so that its various modal parameters in the milling progress can be obtained gradually; thus, the variation due to metal removal has been accurately taken into account. The stability critical condition is predicted by frequency domain method based on the dynamic behavior of the two subsystems. With the respect to time-varying critical stability condition, a three-dimensional lobe diagram has been developed to show the changing conditions of chatter. Finally, the proposed methods and models were proven by series milling experiments.     

Finite strip modeling of the varying dynamics of thin-walled pocket structures during machining

The structural model of the workpiece is required for modeling, analysis, and avoidance of forced and regenerative (chatter) vibrations in machining of thin-walled parts. Finite element models (FEM) provide a versatile means for modeling the workpiece dynamics, but such models need to be updated frequently as the mass and stiffness of the workpiece varies continuously during machining. The computational time and power that is needed for re-meshing the FEM and then re-computing the natural modes of the workpiece is prohibitive. In this paper, a new approach based on Finite strip modeling (FSM) is presented for modeling the structural dynamics of thin-walled structures during pocket milling operations. The substantially higher computational efficiency of the FSM approach in predicting the varying dynamics of thin-walled pocket structures is verified by comparing its performance against FEM and the multi span plate (MSP) approach presented in (J Manuf Sci Eng 133:021014, 2011). Additionally, the accuracy of the presented approach in analyzing the stability of vibrations and determining the extent of dynamic deflections is verified using experimental results.     

Breakthrough of regenerative chatter modeling in milling by including unexpected effects arising from tooling system deflection

Self-excited anomalous vibrations called chatter affected milling operations since the beginning of the industrial era. Chatter is responsible for bad surface quality of the machined part and it may severely damage machining system elements. Although the significant advances of recent years, state of the art dynamic models are not yet able to completely explain chatter onset even when some conventional cutting tools are applied for conventional milling operations. In this work, a more general model of regenerative chatter is presented. The model takes into account some additional degrees of freedom and cutting forces which are neglected in the classical approach. By so doing, a more accurate representation of milling dynamics is obtained, especially when considering large diameter cutters. An improved mathematical formulation of regenerative cutting forces is provided with respect to a very recent publication where the new model has been first outlined. This approach allows ?45 % of computation time. Moreover, here a new, independent, and stronger experimental validation is provided, where the new model successfully predicts an increase of about +(50 ÷ 100) % of the stability boundaries with respect to the classical prediction, thus showing the potential breakthrough of the new approach.     

Chatter stability of micro end milling by considering process nonlinearities and process damping

The micro end milling uses the miniature tools to fabricate complexity microstructures at high rotational speeds. The regenerative chatter, which causes tool wear and poor machining quality, is one of the challenges needed to be solved in the micro end milling process. In order to predict the chatter stability of micro end milling, this paper proposes a cutting forces model taking into account the process nonlinearities caused by tool run-out, trajectory of tool tip and intermittency of chip formation, and the process damping effect in the ploughing-dominant and shearing-dominant regimes. Since the elasto-plastic deformation of micro end milling leads to large process damping which will affect the process stability, the process damping is also included in the cutting forces model. The micro end milling process is modeled as a two degrees of freedom system with the dynamic parameters of tool-machine system obtained by the receptance coupling method. According to the calculated cutting forces, the time-domain simulation method is extended to predict the chatter stability lobes diagrams. Finally, the micro end milling experiments of cutting forces and machined surface quality have been investigated to validate the accuracy of the proposed model.     

Incomplete mesh-based tool path generation for optimum zigzag milling

The majority of mechanical parts are manufactured by milling machines. Hence, geometrically efficient algorithms for tool path generation and physical considerations for better machining productivity with guarantee of machining safety are the most important issues in milling tasks. In this paper, we present an optimized path-generation algorithm for zigzag milling, which is commonly used in the roughing stage as well as in the finishing stage, based on an incomplete two-manifold mesh model, namely, an inexact polyhedron that is widely used in recent commercialized CAM software systems. First of all, a geometrically efficient tool path generation algorithm using an intersection points-graph is introduced. Although the tool path obtained from geometric information has been successful to make a desirable shape, it seldom considers physical process concerns like cutting forces and chatter. In order to cope with these problems, an optimized tool path that maintains constant MRR in order to achieve constant cutting forces and to avoid chatter vibrations at all times is introduced and the result is verified. Additional tool path segments are appended to the basic tool path by using a pixel-based simulation technique. The algorithm was implemented for two-dimensional contiguous end-milling operations with flat end mills and cutting tests were conducted by measuring the spindle current, (which reflect machining situations) to verify the significance of the proposed method.     

实时振动数据驱动的薄壁件平铣工艺参数自适应优化

为减小加工振动对薄壁件平铣（端面盘铣）加工质量及效率的影响,提出一种实时铣削振动数据驱动的平铣工艺参数自适应优化方法。首先根据再生效应原理建立薄壁件平铣颤振稳定性模型。其次将薄壁件平铣过程中前一个工步内的实测振动数据分为若干段,以此模拟其材料去除过程,对各段铣削振动数据进行分析,由有限元单位力法和优化STD法分别识别出薄壁件刚度和各材料去除阶段模态频率及阻尼比,并由此导出薄壁件单模态频响函数,将其代入颤振稳定性模型求解稳定域叶瓣图并做插值处理后即可确定包含材料去除信息的薄壁件三维颤振稳定域叶瓣图。基于此,以避免铣削颤振、共振和满足机床性能要求为约束条件,以材料去除率最大为目标,利用遗传算法计算薄壁件下一个工步较优的工艺参数,如此循环进行,直到完成薄壁件加工。最后,通过某型飞机垂尾薄壁装配界面平铣试验验证该方法的可行性和有效性。由试验结果可看出,采用优化后的加工工艺参数,能使薄壁装配界面粗加工过程表面粗糙度从Ra 3.2提升为Ra 1.6,加工效率提高33%。     

Robust regenerative chatter stability in machine tools

The expeditious nature of manufacturing markets inspires advancements in the effectiveness, efficiency and precision of machining processes. Often, an unstable machining phenomenon, called regenerative chatter, limits the productivity and accuracies in machining operations. Since the 1950s, a substantial amount of research has been conducted on the prevention of chatter vibration in machining operations. In order to prevent regenerative chatter vibrations, the dynamics between the machine tool and workpiece are critical. Conventional regenerative chatter theories have been established based on the assumption that the system parameters in machining are constant. However, the dynamics and system parameters change due to high spindle speeds, tool geometries, orientation of the tool with respect to the rest of the machine, tool wear and non-uniform workpiece material properties. This paper provides a novel method, based on the robust stability theorem, to predict chatter-free regions for machining processes, by taking in account the unknown uncertainties and changing dynamics for machining. The effects of time-variant parameters on the stability are analyzed using the robust stability theorem. The experimental tests are performed to verify the stability of SDOF and MDOF milling systems. The uncertainties and changing dynamics are taken into account in order to accommodate the optimal selection of machining parameters, and the stability region is determined to achieve high productivity and accuracy through applications of the robust stability theorem.     

Three-dimensional stability prediction and chatter analysis in milling of thin-walled plate

Stability lobe diagram can be used for selecting proper milling parameters to perform chatter-free operations and improve productivity during milling of thin-walled plates. This paper studies the machining stability in milling of thin-walled plates and develops a three-dimensional stability lobe diagram of the spindle speed, tool position, and axial depth of cut. The workpiece-holder system is modeled as a 2-degree-of-freedom system considering that the tool system is much more rigid than the thin-walled plate, and dynamic equations of motion described for the workpiece-holder system are solved numerically in time domain to compute the dynamic displacements of the thin-walled plate. Statistical variances of the dynamic displacements are then employed as a chatter detection criterion to acquire the stability lobe diagram. The experimentally obtained stability limits correspond well with the predicted stability limits. In addition, influence of feed rate on stability limits is also investigated. By performing frequency analysis of the measured cutting forces to judge if chatter occurs, it is found that feed per tooth has little influence on the stability limits. However, feed per tooth impacts the machined surface quality. The results show that the surface quality drops by increasing feed per tooth.     

Stability charts with large curve-flute end-mills for thin-walled workpieces

The development of advanced performance materials by foundrymen allows new designs of components with similar mechanical resistance but lower thicknesses. This accentuates the well-known problems in machining cutting systems (static form errors, forced vibrations, and chatter), which appear increasingly early. These problems are particularly severe in the case of rotating parts in the field of aeronautics. Currently, the machining time for impellers and blisks can be improved so that new cutting tools with innovative features are continually developed. Barrel type or curved end mills will have a significant impact on the manufacturing of aircraft components during incoming years. In this paper, a time domain method is proposed for the first time to predict stability in milling operations when using these tools. The dynamic cutting forces are first obtained and then used to plot stability contour maps. Key aspects, such as tool orientation angles, several dynamic modes, and runout were also considered. Finally, chatter tests were performed for a low-stiffness mechanical system to validate the model. The presented maps indicate chatter severity and may be used for chatter prediction and productivity improvement.     

切削颤振研究的关键技术与进展综述

切削颤振是金属切削加工过程中的一种非常复杂的机械振动现象,影响零件加工质量并限制生产率提高.颤振产生的原因和发生、发展规律与切削过程本身以及切削机床动态特性都有着内在的本质联系,长久以来被众多研究者关注.本文就切削颤振在颤振机理、建模、稳定性分析、颤振识别预报以及颤振控制领域的国内外研究成果进行了详细介绍,讨论和分析了...     

High-efficiency smooth-surface high-chatter-stability machining of thin plates with novel face-milling cutter geometry

This paper presents a new technology to realize high-efficiency, smooth-surface, and high-chatter-stability machining of flexible thin plates with the proposed face-milling cutter geometry. Radius end mills are widely utilized for high-efficiency and smooth-surface machining, but they generally have low machining stability, i.e. chatter vibrations often occur. It is well known that the chatter stability depends on the structural flexibility and the cutting conditions, whereas it is less known that it strongly depends on the cutter geometry. In this study, a novel face-milling cutter geometry is proposed to improve chatter stability, especially for regenerative chatter, without sacrificing the high efficiency and the surface smoothness in radius end milling. The validity of the milling technology utilizing the proposed cutter geometry is verified analytically and experimentally.     

A novel deep groove machining method utilizing variable-pitch end mill with feed-directional thin support

A novel method for deep groove machining is developed which utilizes a long-shank variable-pitch end mill with a feed-directional thin support in this research. Recently, thin and tall ribs are required for many parts to reduce their weight and material consumption without sacrificing their stiffness and strength. It leads to necessity of deep and narrow grooves in dies and molds for their mass production. However, such deep groove machining is difficult, since long flexible end mills cause severe chatter vibrations induced by regeneration and mode-coupling. There have been many studies on the regenerative chatter vibration, while there have been few studies on the mode-coupling chatter vibration. Both chatter vibrations need to be suppressed in the deep groove machining. In this study, the regenerative chatter vibration is suppressed by employing one of the conventional methods, i.e. a variable-pitch end mill. On the other hand, there is not a good method to suppress the mode-coupling chatter vibration in the deep groove machining. Therefore, a new suppression method is proposed in which the long-shank variable-pitch end mill is supported with a thin plate in the feed direction. The support device and the long-shank variable-pitch end mill are developed and applied to machining of hardened die steel. Validity of the proposed method is verified both analytically and experimentally in this study.     

Fault detection and classification in automated assembly machines using machine vision

The geometry of rotary aircraft engine components is usually defined by thin mechanical elements and complex surfaces that are only achievable by 5-axis machining due to either limited access or the design itself. Such thin-walled characteristics make these components susceptible to vibrations while machining and usually require careful manipulation of the toolpath parameters to minimize cutting forces and vibration. Moreover, the tool suppliers’ approach leans towards the feature-build design of cutter geometry to increase the productivity and quality of a machined surface. Some examples of those recent improvements for rotary aircraft engine components are barrel-shaped tools that attempt to increase the contact radius on the tool-part interface to minimize step-over while conserving the scallop height to meet roughness tolerances. This research aims to fill a gap in the current literature by proposing a stability model for barrel-shaped tools. Stability contour maps make use of a mechanistic dynamic force model for barrel-shaped tools. The force model is also capable of including tool runout and orientation angles, tilt and lead, as named in most CAM software. By simulating dynamic forces on the time domain, a contour map is presented to address unstable vibrations. Since forced vibrations and surface location error (SLE) may also appear when milling aircraft parts, SLE and surface roughness are also determined. Finally, given the complexity and number of parameters, validation of the stability maps is performed through experimental chatter tests using a thin wall component.     

Chatter analysis of a parallel mechanism-based universal machining center

Extensive researches have been carried out on machine tool chatter to obtain assessment procedure and improvement measures. In this study, chatter limit is predicted on a newly fabricated universal machining center by the combination of structural dynamic characteristics and cutting mechanics. We showed the unstable cutting conditions, and from them we could plot the unstable borderlines. From the chatter simulations we could say that the newly built universal machining center can be well used in the finishing machining of steel as other common machine tools.