Stability-based spindle speed control during flexible workpiece high-speed milling

High-speed machining (HSM) is a technology used to increase productivity and reduce production costs. The prediction of stable cutting regions represents an important issue for the machining process, which may otherwise give rise to spindle, cutter and part damage. In this paper, the dynamic interaction of a spindle-tool set and a thin-walled workpiece is analysed by a finite element approach for the purpose of stability prediction.The gyroscopic moment of the spindle rotor and the speed-dependent bearing stiffness are taken into account in the spindle-tool set finite element model and induce speed-dependent dynamic behaviour. A dedicated thin-walled workpiece is designed whose dynamic behaviour interacts with the spindle-tool set. During the machining of this flexible workpiece, chatter vibration occurs at some stages of machining, depending on the cutting conditions and also on the tool position along the machined thin wall.By coupling the dynamic behaviour of the machine and the workpiece, respectively, dependent on the spindle speed and the relative position of both the systems, an accurate stability lobes diagram is elaborated.Finally, the proposed approach indicates that spindle speed regulation is a necessary constraint to guarantee optimum stability during machining of thin-walled structures.     

Chatter milling modeling of active magnetic bearing spindle in high-speed domain

A new dynamical modeling of Active Magnetic Bearing Spindle (AMBS) to identify machining stability of High Speed Milling (HSM) is presented. This original modeling includes all the minimum required parameters for stability analysis of AMBS machining. The stability diagram generated with this new model is compared to classical stability lobes theory. Thus, behavior's specificities are highlighted, especially the major importance of forced vibrations for AMBS. Then a sensitivity study shows impacts of several parameters of the controller. For example, gain adjustment shows improvements on stability. Side milling ramp test is used to quickly evaluate the stability. Finally, the simulation results are then validated by HSM cutting tests on a 5 axis machining center with AMBS.     

高速铣削及其加工策略探讨

介绍高速铣削技术的工艺特点及其应用,并就高速铣削的粗加工、半精加工和精加工策略进行探讨.     

Investigation of spindle bearing preload on dynamics and stability limit in milling

Many spindle designs offer automatic, speed-dependent preload adjustments to improve the bearing service life. This can result in spindle speed-dependent dynamic properties at the tool tip and errors in process stability predictions. In order to improve stability prediction accuracy for a representative tool and tool holder assembly, the tool tip frequency response functions are measured for different bearing preload values. Using stability models, stability limits are then predicted. Effects of bearing preload on the stability limits are demonstrated via simulations and cutting tests.     

A new on-line spindle speed regulation strategy for chatter control

A new on-line control method to suppress regenerative chatter vibration during the machining process by regulating spindle speed is proposed. The dynamic cutting force signal collected from a dynamometer is passed through a low pass filter, and then digitized. The fast Fourier transform is carried out to obtain the corresponding power spectrum. The chatter frequency is identified when the intensity at a certain frequency other than the spindle speed and tooth passing frequency exceeds a critical value. Based on the identified chatter frequency, a new spindle speed is computed by applying the principle of keeping the phase between the present and previous undulations to 90°. The new speed command is executed while the cutting proceeds. It is found from simulation that the chatter vibration can be suppressed by this approach in the shortest time. This method is also verified by experiments through actual cutting of various materials by a computer numerically controlled milling machine. The main feature of this approach is that the feed of the machine tool does not need to be halted during the change of spindle speed. Hence, tool wear can be reduced. Furthermore, no system identification of the machine tool structure is needed, and therefore it has great potential in actual applications.     

Optimal spindle speed determination for vibration reduction during ball-end milling of flexible details

In the paper a method of optimal spindle speed determination for vibration reduction during ball-end milling of flexible details is proposed. In order to reduce vibration level, an original procedure of the spindle speed optimisation, based on the Liao–Young criterion [1], is suggested. As the result, an optimal, constant spindle speed value is determined. For this purpose, non-stationary computational model of machining process is defined. As a result of modelling, a hybrid system is described. This model consists of following subsystems, i.e. stationary model of one-side-supported flexible workpiece (modal subsystem), non-stationary discrete model of ball-end mill (structural subsystem) and conventional contact point between tool and workpiece (connective subsystem). The method requires identification of some natural frequencies of stationary modal subsystem. To determine them, appropriate modal experiments have to be performed on the machine tool, just before machining. Examples of vibration surveillance during cutting process on two high speed milling machines Mikron VCP 600 and Alcera Gambin 120CR are illustrated.     

Prediction of chatter stability in high-speed finishing end milling considering multi-mode dynamics

The strong demand for increasing productivity and workpiece quality in high-speed milling make the machine-tool system has to operate close to the limit of its dynamic stability. This requires that the chatter stability is predicted accurately to determine the optimal milling parameters. An analytical stability prediction method was proposed with multi-degree-of-freedom (MDOF) system modal analysis. This paper describes the development of this new method which allows considering the effects of multi-mode dynamics of system, higher excited frequency (i.e. tooth passing frequency) and wider spindle speed range on stability limits in high-speed milling, and these to help in selection of milling parameters for a maximum material removal rates (MRR) in real operations without chatter. Some tests were carried out to demonstrate the quality of this method used in real machining. Final, the main influencing factors of stability limits in high-speed milling were analyzed.     

A new analytical–experimental method for the identification of stability lobes in high-speed milling

Increased working speeds and accelerations of high-speed machining produce excitation of oscillations and cause dynamic problems. These problems affect the tool life (tool wear and tool failure), produce shoddy end surface, reduce productivity, produces scrap parts and affect the environment. A chatter’s analytical prediction method was combined with experimental multidegree-of-freedom systems modal analysis to achieve the objective of generating a new method to obtain the stability lobes information. This paper describes the development of this new method which obtains the stability information for some vibration modes that can be used to graph the stability lobes for high-speed milling, and these to help in the selection of parameters for chatter free operations. Some tests were carried out to demonstrate the quality of this method and the accomplishment of the proposed goals.     

An advanced FEA based force induced error compensation strategy in milling

The study introduces a multi-level machining error compensation approach focused on force-induced errors in machining of thin-wall structures. The prediction algorithm takes into account the deflection of the part in different points of the tool path. The machining conditions are modified at each step when the cutting force and deflection achieve a local equilibrium. The machining errors are predicted using a theoretical flexible force-deflection model. The error compensation is based on optimising the tool path taking into account the predicted milling error. The error compensation scheme is simulated using NC simulation package and is experimentally verified.     

变齿距铣刀高速微铣削的动态特性及稳定性研究

在高速微铣削加工过程中,提高生产效率和零件质量的需求日益强烈,这使得机床一直在系统的动态稳定性极限附近工作,而机床颤振的存在是限制微铣削加工生产率的主要障碍.基于颤振稳定性的准确预测,能采取一些措施来提高动态稳定性极限,例如通过改变铣刀结构.提出了数值分析与铣削实验相结合的方法,采用变齿距微铣刀来研究铣削加工的动态特性和稳定性.另外提出了采用时域仿真的输出力来表征加工稳定性的新方法,采用变齿距铣刀可以非常有效地提高某些速度范围的颤振稳定性.对于选定刀具的加工,这种方法可以用于加工优化,或在设计阶段预测刀具新型结构的性能.     

Prediction of spindle dynamics in milling by sub-structure coupling

The Stability of machining process depends on the dynamics of the machine tool, among other things. However, the dynamics of the machine tool changes when the tool is changed. To avoid the need for repeating the measurements, sub-structuring analysis may be used to couple the tool and spindle frequency response functions. A major difficulty in this approach is the determination of joint stiffness and damping between the two sub-structures. In particular, the measurement of rotational responses (RDOFs) at joints is a difficult task. In this research, a simple joint model that accounts for RDOFs is proposed. It is shown that this model avoids RDOF measurement while taking into account the bending modes. An optimization method based on genetic algorithm is employed to find parameters of the joint model. Receptance coupling analysis is used to couple the machine tool and tool FRFs obtained from experiment and FE, respectively. The RC based response obtained in this way is compared with experimental FRF which shows good agreement and confirms that the joint model has been successful in predicting the tool bending modes.     

Fuzzy control of spindle power in end milling processes

This paper reports a fuzzy control system for power regulation in end milling processes. This control system is capable of adjusting both feedrate and spindle speed simultaneously. Experiments have been carried out using both steel and aluminum workpieces of various cutting geometries. Different tools (HSS and carbide tools of different diameters and different number of teeth) have been used for aluminum workpieces. Both full immersion slotting and partial immersion cutting were tested. Our test results show that the system was in sensitive to workpiece and tool changes and cutting power was well regulated around the target levels for various types of variations in depth of cut. Our test results also show that as compared to single parameter (feedrate) adjustment, further savings in machining time can be achieved by adjusting both feedrate and spindle speed.     

Surface shape prediction in high speed milling

Chatter is one of the main causes affecting the surface quality of machined parts. Since it occurs when a machining process is unstable, it is important to select process parameters that promote stability. However, this condition alone is not sufficient to ensure good surface shape because the dynamics of the machining system as a whole also affects the resulting surface. Thus, even in the absence of chatter, significant movement of the tool or the part may occur causing surface defects. This effect is significant in high speed milling. In order to select optimal machining parameters, a computationally efficient simulator has been developed based on a novel machined surface generation model capable of accurate and reliable prediction of the surface shape. The programme written in MATLAB^® predicts the surface shape from which surface finish, waviness, form and position parameters can be determined. A test bench consisting of a relatively low frequency second order dynamic mode is used to hold a test sample. The results obtained on a machining centre show that the predictions favourably compare with experimental results.     

Stability of milling processes with continuous spindle speed variation: Analysis in the frequency and time domains, and experimental correlation

Up to now, the theory for analysis of continuous spindle speed variation in milling processes was developed for sinusoidal variation only, and for average tooth passing frequency an exact multiple of speed variation frequency. This paper presents the general theory for analysis in the frequency domain and for any speed variation strategy. Results are compared with those obtained by semidiscretization and time integration, as well as with those obtained by experiments. The discrepancies of the results obtained by the different approaches are discussed, and the analysis of the evolution of the stability along the speed variation period is proposed.     

Analysis of thermal errors in a high-speed micro-milling spindle

Thermally induced errors account for the majority of fabrication accuracy loss in an uncompensated machine tool. This issue is particularly relevant in the micro-machining arena due to the comparable size of thermal errors and the characteristic dimensions of the parts under fabrication. A spindle of a micro-milling machine tool is one of the main sources of thermal errors. Other sources of thermal errors include drive elements like linear motors and bearings, the machining process itself and external thermal influences such as variation in ambient temperature. The basic strategy for alleviating the magnitude of these thermal errors can be achieved by thermal desensitization, control and compensation within the machine tool.This paper describes a spindle growth compensation scheme that aims towards reducing its thermally-induced machining errors. The implementation of this scheme is simple in nature and it can be easily and quickly executed in an industrial environment with minimal investment of manpower and component modifications.Initially a finite element analysis (FEA) is conducted on the spindle assembly. This FEA correlates the temperature rise, due to heating from the spindle bearings and the motor, to the resulting structural deformation. Additionally, the structural deformation of the spindle along with temperature change at its various critical points is experimentally obtained by a system of thermocouples and capacitance gages.The experimental values of the temperature changes and the structural deformation of the spindle qualitatively agree well with the results obtained by FEA. Consequently, a thermal displacement model of the high-speed micro-milling spindle is formulated from the previously obtained experimental results that effectively predict the spindle displacement under varying spindle speeds. The implementation of this model in the machine tool under investigation is expected to reduce its thermally induced spindle displacement by 80%, from 6 microns to less than 1 micron in a randomly generated test with varying spindle speeds.     

An efficient linear approximation of acceleration method for milling stability prediction

Chatter stability predictions catch much attention during machining operations in modern automotive and aerospace industry. This paper presents a novel time domain semi-analytical method for milling stability prediction based on linear acceleration approximation. Firstly, the milling dynamics considering the regenerative effect is presented as a linear time-delay system with periodic coefficients. The second step is to equally discretize the time duration of the forced vibration of the tooth passing period into small intervals where acceleration of the flexible cutter is approximated by linearly interpolating between the two boundary values, while the free vibration is analytically solved. Then, recursive formulas with constant recursive matrices are found for the presentation of relations between initial and final cutter motions (including position, velocity and acceleration) of each small time interval. Employing the method of weighted residuals over each time interval, discrete maps are constructed which relate motions of a period to the corresponding values one period earlier. Finally, the eigenvalues of the transition matrix are used to determine stability based on Floquet theory. By using the benchmark examples in literatures, the convergence and computational time of the proposed method are compared with those of the semi-discretization methods (SDMs), full-discretization method (FDM) and numerical integration method (NIM). The results verify the validity of the proposed approach, and the presented method is proved to be computationally highly efficient.     

Geometrical deviations versus smoothness in 5-axis high-speed flank milling

The paper deals with the Generation of Optimized 5-aXis Flank milling trajectories. Within the context of 5-axis High-Speed Machining, oscillatory trajectories may penalize process efficiency. The control of the trajectory smoothness is as essential as the control of geometrical deviations. For this purpose the Geo5XF method based on the surface representation of the tool trajectory has been developed. In flank milling, this surface, also called the Machining Surface (MS), is the ruled surface locus of the tool axes defining the trajectory. Based on a first positioning, the method aims at globally minimizing geometrical deviations between the envelope surface of the tool movement and the designed surface by deforming the MS while preserving trajectory smoothness. The energy of deformation of the MS is used as an indicator of the smoothness. Hence, in most cases, results obtained using Geo5XF show that minimum energy tool paths lead to minimal machining time. As geometrical deviations are not minimized for minimum energy tool paths, a compromise must be reached to find the best solution.     

The effects of spindle vibration on surface generation in ultra-precision raster milling

Spindle vibration has a significant influence on surface quality of ultra-precision-machined components. However, relatively few studies on the particular spindle vibration under the excitation of intermittent cutting forces in ultra-precision raster milling (UPRM) have been reported. In this study, a specialized model for an aerostatic bearing spindle under the impulsive excitation from intermittent cutting forces of UPRM is developed and its derived mathematical solutions reveal that the spindle vibration is impulsive response. The theoretical and experimental results signify that the impulsive spindle vibration produces inhomogeneous scallops forming ribbon-stripe patterns and irregular patterns like run-out on a surface of UPRM. The potential benefits for UPRM are the theoretical supports for optimization and prediction of surface generation through the optimal selection of spindle speed.     

Mechanism of minimum quantity lubrication in high-speed milling of hardened steel

The rapid wear rate of cutting tools due to high cutting temperature is a critical problem to be solved in high-speed machining (HSM) of hardened steels. Near-dry machining such as minimum quantity lubrication (MQL) is regarded as one of the solutions to this difficulty. However, the function of MQL in HSM is still uncertain so far which prevents MQL from widely being utilized in the machining of hardened steels. In this paper, the mechanism of MQL in HSM of hardened steel is investigated more comprehensively. Comparing with dry cutting, the tool performance can be enhanced by MQL under all cutting speeds in this study. It is found that MQL can provide extra oxygen to promote the formation of a protective oxide layer in between the chip–tool interface. This layer is basically quaternary compound oxides of Fe, Mn, Si, and Al, and is proved to act as diffusion barriers effectively. Hence, the strength and wear resistance of a cutting tool can be retained which leads to a significant improvement of tool life. It is found that there exists an optimal cutting speed at which a stable protective oxide layer can be formed. When cutting speed is lower than this speed, there is less oxide layer and the improvement of tool life is less apparent. As the cutting speed is far beyond the optimal value, the protective layer is absent and the thermal cracks are apt to occur at the cutting edge due to large fluctuation of temperature. Resultantly, application of MQL is inappropriate in the extreme high-speed cutting condition irrespective of its little increase in tool life. Based on this study, it is concluded that the tool life can be effectively improved by MQL in HSM of NAK80 hardened steels when cutting parameters are chosen properly.     

高速铣削加工表面位置误差的频域分析

在高速铣削加工过程中,提高轴向切削深度和主轴转速可以获得较高的材料去除率,然而限制轴向切削深度提高的一个因素是加工颤振.高速铣削系统动态失稳可能导致加工零件的表面几何精度偏差.分析高速铣削的表面位置误差对表征切削过程、刀具寿命估算和加工优化都起着重要作用.因此,在不考虑再生颤振影响的前提下,提出了一种数值分析和加工实验相结合的方法来研究表面位置误差.首先,构建了高速铣削加工过程模型,然后建立了动态铣削力模型,并推导了表面位置误差的分析方法.通过数值分析和铣削实验相结合,得到了高速铣削加工的稳定性叶瓣图.接下来,研究了逆铣削加工过程的表面位置误差,并详细分析了主轴转速和轴向切削位置对表面位置误差的影响规律.最后,把稳定性叶瓣和表面位置误差数据组合在同一个图里得到了高速铣削加工的综合分析图.借助综合分析图,能预测表面位置误差和优化高速铣削的工艺条件.