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.     

Investigation on chatter stability of thin-walled parts in milling based on process damping with relative transfer functions

Chatter usually occurs in cutting of thin-walled workpiece due to poor structural stiffness, which results in poor surface quality and damaged tool. Aiming at process damping caused by interference between a tool flank face and a machined surface of thin-walled part, the dynamic model and critical condition of stability are proposed by the relative transfer functions, when both the tool structure and the machined workpiece have similar dynamic behaviors in this paper. Using the frequency method to solve the stability of the cutting chatter, it can be seen that the process damping can significantly improve the stability of the low speed region. Moreover, the stability domain is different and more exact than the one that derives from the simple superposition of the tool and the workpiece lobe diagrams. The correctness of the model is validated by experiments. These conclusions provide a theoretical foundation and reference for the milling mechanism research.     

Chatter stability prediction in milling using time-varying uncertainties

The chatter stability in milling severely affects productivity and quality of machining. Tool wear causes both the cutting coefficient and the process damping coefficient, but also other parameters to change with cutting time. This variation greatly reduces the accuracy of chatter prediction using conventional methods. To solve this problem, we consider the cutting coefficients of the milling system to be both random and time-varying variables and we use the gamma process to predict cutting coefficients for different cutting times. In this paper, a time-varying reliability analysis is introduced to predict chatter stability and chatter reliability in milling. The relationship between stability and reliability is investigated for given depths and spindle speeds in the milling process. We also study the time-varying chatter stability and time-varying chatter reliability methods theoretically and with experiments. The results of this study show that the proposed method can be used to predict chatter with high accuracy for different cutting times.     

Three-dimensional process stability prediction of thin-walled workpiece in milling operation

High-speed machining of thin-walled workpiece is widely used in aerospace industry. To optimize the machining parameters in milling operations, the related process stability is required to be predicted. Compared to the existing two-dimensional (2D) milling stability model, a more completed three-dimensional (3D) regenerative process stability prediction model of thin-walled workpiece is presented based on the newly developed dynamic model. The efficiency and accuracy of the regenerative milling stability can be improved in the presented 3D model. The analysis procedure of the stability of flexible dynamic milling is developed in details. The 3D stability lobes are calculated according to the full discretization method and direct integration scheme. To verify the accuracy of presented 3D stability model, the thin-walled workpiece milling sound pressure signal and surface quality are determined in experiments.     

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.     

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.     

A coupled dynamics,multiple degree of freedom process damping model,Part 2: Milling

Self-excited vibration, or chatter, is an important consideration in machining operations due to its direct influence on part quality, tool life, and machining cost. At low machining speeds, a phenomenon referred to as process damping enables stable cutting at higher depths of cut than predicted with traditional analytical models. This paper describes an analytical stability model for milling operations which includes a process damping force that depends on the surface normal velocity, depth of cut, cutting speed, and an empirical process damping coefficient. Model validation is completed using time domain simulation and milling experiments. The results indicate that the multiple degree of freedom model is able to predict the stability boundary using a single process damping coefficient.     

Milling Force Model for Aviation Aluminum Alloy:Academic Insight and Perspective Analysis

Aluminum alloy is the main structural material of aircraft,launch vehicle,spaceship,and space station and is pro-cessed by milling.However,tool wear and vibration are the bottlenecks in the milling process of aviation aluminum alloy.The machining accuracy and surface quality of aluminum alloy milling depend on the cutting parameters,material mechanical properties,machine tools,and other parameters.In particular,milling force is the crucial factor to determine material removal and workpiece surface integrity.However,establishing the prediction model of milling force is important and difficult because milling force is the result of multiparameter coupling of process system.The research progress of cutting force model is reviewed from three modeling methods:empirical model,finite element simulation,and instantaneous milling force model.The problems of cutting force modeling are also determined.In view of these problems,the future work direction is proposed in the following four aspects:(1)high-speed milling is adopted for the thin-walled structure of large aviation with large cutting depth,which easily produces high residual stress.The residual stress should be analyzed under this particular condition.(2)Multiple factors(e.g.,eccentric swing milling parameters,lubrication conditions,tools,tool and workpiece deformation,and size effect)should be consid-ered comprehensively when modeling instantaneous milling forces,especially for micro milling and complex surface machining.(3)The database of milling force model,including the corresponding workpiece materials,working condi-tion,cutting tools(geometric figures and coatings),and other parameters,should be established.(4)The effect of chatter on the prediction accuracy of milling force cannot be ignored in thin-walled workpiece milling.(5)The cutting force of aviation aluminum alloy milling under the condition of minimum quantity lubrication(mql)and nanofluid mql should be predicted.     

多硬度拼接淬硬钢铣削动力学分析

颤振是金属切削加工过程中由于刀具和工件之间相互作用所产生的一种强烈的自激振动现象,会导致切削力幅值增加且发生剧烈波动,进而降低工件表面质量和刀具使用寿命。针对此问题,基于铣削过程稳定性预测分析方法建立多硬度拼接工件的动态铣削系统,对多硬度拼接模具铣削过程稳定性进行深入研究,实现了对拼接模具铣削加工过程颤振稳定域的仿真,进而研究了模态参数对稳定性叶瓣图形状的影响。最后通过时域分析、表面形貌和刀具磨损的研究,综合验证了稳定性预测曲线的精度。研究结果为多硬度拼接模具铣削加工提供理论基础,并设置合理的加工参数来实现金属最大切除率,为大型汽车覆盖件模具铣削加工提供理论依据及技术指导。     

过程阻尼效应在钛合金铣削加工中的应用

对过程阻尼效应在钛合金铣削加工中的应用进了行研究。利用隐式龙格库塔法,计算典型钛合金材料铣削加工中干涉产生的侵入面积以及阻力,建立考虑过程阻尼效应的非线性铣削动力学模型,并基于此模型设计减振后角抑制颤振。计算与试验结果对比分析表明,所建模型能够较为准确地预测稳定性极限,过程阻尼效应可使低速区极限切深显著增加,而减振后角可使过程阻尼效应增强,进一步拓展稳定区域。     

高速铣削时颤振的诊断和稳定加工区域的预报

给出一种通过测量加工过程中的噪声来诊断高速铣削时颤振的方法.先测量环境噪声,然后测量加工噪声.理论分析和试验结果表明,如果加工噪声的主谐振频率接近其中一个环境噪声主谐振频率或者是齿频的整数倍,那么系统无颤振,否则有颤振.建立系统结构和铣削过程动力学特征参数的数学模型.根据测出的颤振频率,通过所建模型可解出系统的固有频率、阻尼比和过程参数,并计算出稳定极限曲线.试验证明,该方法能较好地预报高速铣削时的稳定加工区域.     

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.     

INFLUENCE OF MATERIAL REMOVAL ON THE DYNAMIC BEHAVIOR OF THIN-WALLED STRUCTURES IN PERIPHERAL MILLING

Machining is a material removal process that alters the dynamic properties during machining operations. The peripheral milling of a thin-walled structure generates vibration of the workpiece and this influences the quality of the machined surface. A reduction of tool life and spindle life can also be experienced when machining is subjected to vibration. In this paper, the linearized stability lobes theory allows us to determine critical and optimal cutting conditions for which vibration is not apparent in the milling of thin-walled workpieces. The evolution of the mechanical parameters of the cutting tool, machine tool and workpiece during the milling operation are not taken into account. The critical and optimal cutting conditions depend on dynamic properties of the workpiece. It is illustrated how the stability lobes theory is used to evaluate the variation of the dynamic properties of the thin-walled workpiece. We use both modal measurement and finite element method to establish a 3D representation of stability lobes. The 3D representation allows us to identify spindle speed values at which the variation of spindle speed is initiated to improve the surface finish of the workpiece.     

INFLUENCE OF MATERIAL REMOVAL ON THE DYNAMIC BEHAVIOR OF THIN-WALLED STRUCTURES IN PERIPHERAL MILLING

Machining is a material removal process that alters the dynamic properties during machining operations. The peripheral milling of a thin-walled structure generates vibration of the workpiece and this influences the quality of the machined surface. A reduction of tool life and spindle life can also be experienced when machining is subjected to vibration. In this paper, the linearized stability lobes theory allows us to determine critical and optimal cutting conditions for which vibration is not apparent in the milling of thin-walled workpieces. The evolution of the mechanical parameters of the cutting tool, machine tool and workpiece during the milling operation are not taken into account. The critical and optimal cutting conditions depend on dynamic properties of the workpiece. It is illustrated how the stability lobes theory is used to evaluate the variation of the dynamic properties of the thin-walled workpiece. We use both modal measurement and finite element method to establish a 3D representation of stability lobes. The 3D representation allows us to identify spindle speed values at which the variation of spindle speed is initiated to improve the surface finish of the workpiece.     

基于MATLAB的铣削加工颤振稳定域仿真算法及实现

针对国内数控铣削加工工艺参数选择存在的问题,基于动态铣削力建模和颤振稳定域分析计算,以MATLAB为开发工具,实现了铣削加工颤振稳定域仿真算法.通过模态锤击实验获得的频响函数,仿真出了整个加工系统的颤振稳定域图形,为进行铣削加工切削参数选择和优化提供了理论依据.验证实验证实了仿真算法的有效性和准确性,仿真方法在工厂得到了实际应用并取得了良好的应用效果.     

Vibration analysis in milling titanium alloy based on signal processing of cutting force

In metal cutting processes, the chatter may cause fast wear of tools and poor surface quality of the processed parts; it can happen on different cutting parameters, but how do we identify the chatter and how do we select suitable cutting parameters to avoid chatter at high material removal rate (MRR). In this paper, the signal processing methods such as time domain, frequency domain, and time–frequency domain analysis are introduced. The signals of cutting force that were collected in milling titanium alloy Ti–6Al–4V at variable cutting speeds varied from 80 to 360 m/min; signal analysis methods such as time domain, frequency domain, and time–frequency domain were put forward. Further analysis results reveal that the chatter occur when cutting speeds are 240 and 360 m/min, when the maximum value of cutting forces increase by 61.9–66.8%, the standard deviation increase by 84.1–86.1%, and the surface roughness increase by 34.2–40.5% compared with that of at 80 m/min. Detail signal d2 is employed to monitor cutting stability state from the result of wavelet analysis.     

Effects of curvature characteristics of sculptured surface on chatter stability for die milling

In this paper, a predictive model of milling stability, which considers the surface curvature and the lead angle of the milling cutter, is proposed for milling of the panel die with sculptured surface. Firstly, influences of the radius of curvature and lead angle of milling cutter on uncut chip thickness are analyzed by exploring the curvature characteristics of sculptured surface. Subsequently, influence of the curvature characteristics of sculptured surface on chatter stability lobes in milling process is investigated by utilizing the full-discretization method. This analysis implies that the milling stability decreases with the increase in the radius of the die surface’s curvature; the milling stability decreases with the increase in lead angle. The experimental results demonstrate that the prediction of chatter stability lobes could match well with the measured results on the change trend, where the maximum relative percent error is less than 11.1 % in stable milling. It is further validated that this method can effectively predict the chatter stability lobes of the domain in ball end milling of sculptured surface of die.     

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.     

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.     

Modelling of segmentation-driven vibration in machining

Excessive vibration, such as chatter, is a common problem in machining processes. Meanwhile, numerous hard, brittle metals have been shown to form segmented chips, also known as sawtooth chips, during machining. In the literature, a cyclic cutting force has been demonstrated where segmented chips are formed, with the force cycle corresponding to the formation of segments. Segmented chip formation has been shown to be linked to high vibration levels in turning and milling processes. Additionally, it has been proposed that the amplitude of chatter vibrations can be limited by interference between the tool flank and wavy workpiece surface, a phenomenon known as tool-flank process damping. In this contribution, a model is proposed to predict the amplitude of forced vibration arising due to the formation of segmented chips during turning. The amplitude of vibration was calculated as a function of cutting parameters. It was demonstrated that the model can be extended to account for the effect of tool-flank process damping. For validation, titanium Ti6Al4V alloy was turned using a flexible toolholder, with surface speed ranging from 10 to 160 m/min, feed rate from 0.1 to 0.7 mm/rev and width of cut from 0.35 to 4 mm. In the experimental validation, 25 of 68 test cuts exhibited high-amplitude vibration. In 16 of these cases, the main cause was concluded to be chip segmentation, which can be predicted by the model. The model is thus considered of practical value to machinists.