Time domain coupled simulation of machine tool dynamics and cutting forces considering the influences of nonlinear friction characteristics and process damping

A higher machining ability is always required for NC machine tools to achieve higher productivity. The self-oscillated vibration called “chatter” is a well-known and significant problem that increases the metal removal rate. The generation process of the chatter vibration can be described as a relationship between cutting force and machine tool dynamics. The characteristics of machine tool feed drives are influenced by the nonlinear friction characteristics of the linear guides. Hence, the nonlinear friction characteristics are expected to affect the machining ability of machines. The influence of the contact between the cutting edge and the workpiece (i.e., process damping) on to the machining ability has also been investigated. This study tries to clarify the influence of the nonlinear friction characteristics of linear guides and ball screws and process damping onto milling operations. A vertical-type machining center is modeled by a multi-body dynamics model with nonlinear friction models. The influence of process damping onto the machine tool dynamics is modeled as stiffness and damping between the tool and the workpiece based on the evaluated frequency response during the milling operation. A time domain-coupled simulation approach between the machine tool behavior and the cutting forces is performed by using the machine tool dynamics model. The simulation results confirm that the nonlinear frictions influence the cutting forces with an effect to suppress the chatter vibration. Furthermore, the influence of process damping can be evaluated by the proposed measurement method and estimated by a time domain simulation.     

Force prediction and stability analysis of plunge milling of systems with rigid and flexible workpiece

Time domain simulation model is developed to study the dynamics of plunge milling process for system with rigid and flexible workpiece. The model predicts the cutting forces, system vibration as a function of workpiece and tool dynamics, tool setting errors, and tool kinematics and geometry. A horizontal approach is used to compute the chip area to consider the contribution of the main and side edge in the cutting zone and to deal with any geometric shape of the insert. The dynamic chip area is evaluated based on the interaction of the insert main and side cutting edges with the workpiece geometry determined by the pilot hole and surface left by the previous insert. For the case of system with a flexible workpiece, the workpiece dynamics, as well as its variation in the axial direction with respect to hole location, is considered in the simulation. Cutting tests with single and double inserts were carried out to validate the simulation model and predicted stability lobe for both systems with rigid and flexible workpiece and to check the correctness of the cutting coefficient model. Good agreement was found between the measured and the predicted cutting forces and vibration signals and power spectra. This indicates the ability of the model to accurately predict cutting forces, system vibration, and process stability for process planning prior to machining. The results show dominance of workpiece dynamics in the axial direction for systems with flexible workpiece due to its flexibility as compared to the tool axial rigidity. On the other hand, chatter behavior was found to occur due to tool lateral modes for case of rigid workpiece.     

Stability of milling with non-uniform pitch and variable helix Tools

We study mechanical vibrations in milling with non-uniform pitch and variable helix tools. The process is modeled by a periodic delay differential equation with distributed delay, which takes into account, for example, the nonlinear cutting force behavior, the effect of runout, and the exact delay distribution due to the unequally spaced flutes. We present a new method for the identification of the chatter stability lobes from the linearized system that is based on the multifrequency solution. We give detailed remarks on the truncation of the resulting infinite dimensional matrices and the efficient numerical implementation of the method. Cutting tests for steel milling with a customary end mill with non-uniform pitch and variable helix angle and a conventional end mill with uniform pitch and constant helix angle are performed. The numerical and experimental results coincide well. They reveal a significant increase of the limiting depth of cut for the variable helix tool compared to the conventional tool. Moreover, we show that in contrast to conventional tools, for non-uniform pitch and variable helix tools, an exact model with time-varying coefficients, nonlinear cutting force behavior, and runout is necessary for an accurate prediction of the stability lobes.     

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.     

Milling force vibration analysis in high-speed-milling titanium alloy using variable pitch angle mill

Chatter may cause fast wear of tools and poor surface quality of the workpieces at high cutting speed and it will happen on different process parameters; how do we select the suitable cutting speed to suppress the chatter? In this paper, a signal analysis method for milling force and acceleration is adopted to identify chatter, which can obtain the results not only in frequency of chatter but also in the contribution for milling force at different frequencies. Through the milling experiment, the machining vibration behaviors of milling Ti–6Al–4V with variable pitch end mill were investigated. Milling force and acceleration signals obtained from experiment were analyzed and compared at stable and unstable milling processes. The experimental results show that when the chatter occurs, milling forces were found to increase dramatically by 61.9–66.8% compared with that of at stable cutting; machining surface quality became poor and machined surface roughness increases by 34.2–40.5% compared with that of at stable cutting.     

Hybrid model- and signal-based chatter detection in the milling process

Chatter causes machining instability and reduces productivity in the metal cutting process. It has negative effects on the surface finish, dimensional accuracy, tool life and machine life. Chatter identification is therefore necessary to control, prevent, or eliminate chatter and to determine the stable machining condition. Previous studies of chatter detection used either model-based or signal-based methods, and each of them has its drawback. Model-based methods use cutting dynamics to develop stability lobe diagram to predict the occurrence of chatter, but the off-line stability estimation couldn’t detect chatter in real time. Signal-based methods apply mostly Fourier analysis to the cutting or vibration signals to identify chatter, but they are heuristic methods and do not consider the cutting dynamics. In this study, the model-based and signal-based chatter detection methods were thoroughly investigated. As a result, a hybrid model- and signal-based chatter detection method was proposed. By analyzing the residual between the force measurement and the output of the cutting force model, milling chatter could be detected and identified efficiently during the milling process.

     

Modeling dynamics and stability of variable pitch and helix milling tools for development of a design method to maximize chatter stability

Chatter is one of the major limitations in milling operations causing poor quality and reduced productivity. Stability diagrams can be used to identify deep stable pockets which usually occur at high spindle speeds. However, the required high cutting speeds may not be applied in some cases due to machinability or machine tool limitations. It is known that variable pitch and helix tools help suppressing chatter even at low cutting speeds. These tools may offer high productivity if they are properly designed. The literature on variable geometry milling tools is mainly limited to modelling and simulation whereas for industrial applications design guidelines are needed for selection of variation pattern and amount which is the focus of this paper. Dynamics and stability of variable pitch and helix tools are modelled and solved in frequency domain as well as using Semi-Discretization Method employing multiple delays. A practical but accurate design method is presented for selection of the best variation combination to maximize chatter free material removal rate without using time consuming computer simulations. Increased stability with the tools designed using the proposed method is demonstrated by several examples which are verified experimentally.     

插铣过程中稳定域研究现状

切削系统稳定性是插铣技术加工领域中的重要研究内容,插铣切削加工中很多优点的体现必须以无振动稳定切削的实现为前提。稳定域预测的准确与否直接关系到能否有效抑制切削颤振,而切削颤振直接影响工件的表面粗糙度和尺寸精度,刀具磨损和破损以及加工效率等。本文通过求解稳定域的一般步骤,综述稳定域研究现状。通过列举稳定性边界的判别的不同算法,分析和对比各种算法的优缺点,并总结插铣切削过程中稳定性研究存在的问题。     

Analysis of regenerative chatter suppression with adding the ultrasonic elliptical vibration on the cutting tool

A method is proposed to suppress regenerative chatter in turning operation, in which the ultrasonic elliptical vibration is added on the cutting tool. It results in the fact that the cutting tool is separated periodically from the chip and the workpiece, and the direction of the frictional force between the rake face of the cutting tool and the chip is reversed in each cycle of the ultrasonic elliptical vibration. The experimental investigations show that the regenerative chatter occurring in ordinary turning operation can be suppressed effectively by applying the ultrasonic elliptical vibration on the cutting tool. In order to clearify the reason of the regenerative chatter suppression, theoretical analysis and computer simulation are performed on turning with ultrasonic vibration. There is a good agreement among the experimental investigations, theoretical analysis and the computer simulation.     

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.     

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.     

Extraction of 5-axis milling conditions from CAM data for process simulation

5-axis milling is widely used in machining of parts with free-form surfaces and complex geometries. Although in general 5-axis milling increases the process capability, it also brings additional challenges due to complex process geometry and mechanics. In milling, cutting forces, tool deflections, and chatter vibrations may reduce part quality and productivity. By use of process simulations, the undesired results can be identified and overcome, and part quality and productivity can be increased. However, machining conditions and geometry, especially the tool–work engagement limits, are needed in process models which are used in these simulations. Due to the complexity of the process geometry and continuous variation of tool–work engagement, this information is not readily available for a complete 5-axis milling cycle. In this study, an analytical method is presented for the identification of these parameters from computer-aided manufacturing data. In this procedure, depths of cut, lead, and tilt angles, which determine the tool–workpiece engagement boundaries, are directly obtained the cutter location file analytically in a very fast manner. The proposed simulation approach is demonstrated on machining of parts with relatively complex geometries.     

Analysis of tool wear effect on chatter stability in turning

This study establishes an analytical basis for the prediction of chatter stability in the turning process in the presence of wear flat on the tool flank. The components contributing to the forcing function in the machine vibration dynamics are analyzed in the context of cutting force, contact force and Coriolis force. In this way, the effects of the displaced workpiece volume at the wear flat as well as the workpiece rotation in conjunction with its radial compliance can be incorporated in describing the motion of the vibration system. Laplace domain analysis provides the analytical solution for the limits of stability in terms of the machine characteristics, structural stiffness, cutting stiffness, specific contact force, cutting conditions and cutter geometry. Stability plots are presented to relate stiffness ratio to cutting velocity in the determination of chatter stability. Machining experiments at various cutting conditions were conducted to identify the characteristic parameters involved in the vibration system and to verify the analytical stability limits. The extent of tool wear effect and Coriolis effect on the stability of machining is discussed.     

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.     

Theoretical modelling of cutting forces in helical end milling with cutter runout

A theoretical cutting force model for helical end milling with cutter runout is developed using a predictive machining theory, which predicts cutting forces from the input data of workpiece material properties, tool geometry and cutting conditions. In the model, a helical end milling cutter is discretized into a number of slices along the cutter axis to account for the helix angle effect. The cutting action for a tooth segment in the first slice is modelled as oblique cutting with end cutting edge effect and tool nose radius effect, whereas the cutting actions of other slices are modelled as oblique cutting without end cutting edge effect and tool nose radius effect. The influence of cutter runout on chip load is considered based on the true tooth trajectories. The total cutting force is the sum of the forces at all the cutting slices of the cutter. The model is verified with experimental milling tests.     

基于颤振预报和预测的稳定性控制理论及方法研究

针对由颤振预测控制策略指导的稳定切削控制方法实时动态调整能力差的缺点,综合颤振预报、预测理论,进行了稳定性在线寻优控制理论及方法的研究。基于刀具—工件系统铣削再生颤振动力学模型,研究了考虑系统结构参数(刀具、工件装夹刚度)和加工参数(切削加工参数优化选择)的切削稳定性评价方法,提出了以扩大稳定性区域和稳定最大材料切除率为控制目标的机床稳定性控制方法。形成了以"预报—控制—效果评估—再控制"为步骤的在线监测、智能诊断和实时控制的集成一体化策略。设计并实施了稳定性控制理论验证实验,获得了与理论分析一致的结论。     

Active Damping of Milling Vibration Using Operational Amplifier Circuit

The problem of chatter vibration is associated with adverse consequences that often lead to tool impairment and poor surface finished in a workpiece, and thus, controlling or suppressing chatter vibrations is of great significance to improve machining quality. In this paper, a workpiece and an actuator dynamics are considered in modeling and controller design. A proportional-integral controller (PI) is presented to control and actively damp the chatter vibration of a workpiece in the milling process. The controller is chosen on the basis of its highly stable output and a smaller amount of steady-state error. The controller is realized using analog operational amplifier circuit. The work has contributed to planning a novel approach that addresses the problem of chatter vibration in spite of technical hitches in modeling and controller design. The method can also lead to considerable reduction in vibrations and can be beneficial in industries in term of cost reduction and energy saving. The application of this method is verified using active damping device actuator (ADD) in the milling of steel.     

Development of an automatic arc welding system using SMAW process

In end milling of pockets, variable radial depth of cut is generally encountered as the end mill enters and exits the corner, which has a significant influence on the cutting forces and further affects the contour accuracy of the milled pockets. This paper proposes an approach for predicting the cutting forces in end milling of pockets. A mathematical model is presented to describe the geometric relationship between an end mill and the corner profile. The milling process of corners is discretized into a series of steady-state cutting processes, each with different radial depth of cut determined by the instantaneous position of the end mill relative to the workpiece. For the cutting force prediction, an analytical model of cutting forces for the steady-state machining conditions is introduced for each segmented process with given radial depth of cut. The predicted cutting forces can be calculated in terms of tool/workpiece geometry, cutting parameters and workpiece material properties, as well as the relative position of the tool to workpiece. Experiments of pocket milling are conducted for the verification of the proposed method.     

Design for variable pitch end mills with high milling stability

The strong demand for increasing productivity and workpiece quality in milling process makes the machine?Ctool system operate close to the limit of its dynamic stability. Besides predicting accurately chatter stability, it is required that some optimizations should be conducted, e.g., cutter structure, for improving the dynamic stability limits. Milling cutters with variable pitch angles can be very effective in improving stability against chatter for certain speed ranges, which will be predicted by the model presented here. The present paper deals with the design of structural geometry of variable pitch end mills in detail. Based on the analysis of tooth engagement factor, which is expressed and extended in the paper, an approach is proposed to design variable pitch end mill with high milling stability. The certain speed ranges with high milling stability are given. An example of the design of variable pitch end mills is illustrated to demonstrate the validity of this method.     

Method for early detection of the regenerative instability in turning

Nowadays, approaches in chatter detection and control are based on chatter prediction, by using a machining system dynamic model, or on chatter detection by different techniques, but after chatter onset. They are not efficient because the models are complicated and specific (in the first case) respectively because of chatter unwanted consequences occurrence (in the second case). This paper presents a method for early detection of the process regenerative instability state (as a specific process current dynamical state), based on cutting force monitoring. Using the cutting force records, the process current dynamical state is assessed. Appropriate cutting force signal features are defined, based on signal statistic processing, signal chaotic modeling or signal harmonic analysis, and used on this purpose. The process dynamical state evolution is modeled aiming the features values prediction. Two types of models were used in this purpose: linear and neural. The instability regenerative mechanism is identified by using either dedicated features or input variable selection. The method was conceived and experimentally implemented in the case of turning process. The results show the method reliability and the possibility of using it in developing an intelligent system for stability control.