Milling machine spindle analysis using FEM and non-contact spindle excitation and response measurement

In this paper a method for analysing lateral vibrations in a milling machine spindle is presented including finite-element modelling (FEM), magnetic excitation and inductive displacement measurements of the spindle response. The measurements can be conducted repeatedly without compromising safety procedures regarding human interaction with rotating high speed spindles. The measurements were analysed and compared with the FEM simulations which incorporated a spindle speed sensitive bearing stiffness, a separate mass and stiffness radius and a stiffness radius sensitive shear deformation factor. The effect of the gyroscopic moment and the speed dependent bearing stiffness on the system dynamics were studied for different spindle speeds. Simulated mode shapes were experimentally verified by a scanning laser doppler vibrometer. With increased spindle speed, a substantial change of the eigenfrequencies of the bearing-related eigenmodes was detected both in the simulations and in the measurements. The centrifugal force that acted on the bearing balls resulted in a softening of the bearing stiffness. This softening was shown to be more influential on the system dynamics than the gyroscopic moment of the rotor. The study performed indicates that predictions of high speed milling stability based on 0 rpm tap-test can be inadequate.     

Investigating chatter vibration in deep drilling, including process damping and the gyroscopic effect

This paper investigates chatter vibration occurring in drills for deep hole machining. A model is developed that considers process damping occurring due to the interference between the drilling flank surface and the workpiece surface. Furthermore, the gyroscopic effect due to the rotation of the tool is included. Borders of stability, which indicate critical radial widths of cut (RWOC) at each spindle speed, are obtained analytically from the eigenvalues of a frequency domain equation. Results at this stage show good agreement with the experimental data. In addition, a numerical method is used to simulate the tool path at different RWOC and spindle speeds. Numerical simulation agreed with analytical results. The hole form produced by the tool tip is investigated at different speeds and RWOC with respect to borders of stability. These investigations show that spindle speed selection is an important manner in industrial practice. That is, even below borders of stability, where chatter does not occur, spindle speed selection affects roundness, concentricity, and surface roughness of the hole.     

An integrated thermo-mechanical-dynamic model to characterize motorized machine tool spindles during very high speed rotation

High speed machining (HSM) is a promising technology for drastically increasing productivity and reducing production costs. Development of high-speed spindle technology is strategically critical to the implementation of HSM. Compared to conventional spindles, motorized spindles are equipped with built-in motors for better power transmission and balancing to achieve high-speed operation. However, the built-in motor introduces a great amount of heat into the spindle system as well as additional mass to the spindle shaft, thus complicating its thermo-mechanical-dynamic behaviors. This paper presents an integrated model with experimental validation and sensitivity analysis for studying various thermo-mechanical-dynamic spindle behaviors at high speeds. Specifically, the following effects are investigated: the bearing preload effects on bearing stiffness, and subsequently on overall spindle dynamics; high-speed rotational effects, including centrifugal forces and gyroscopic moments on the spindle shaft and, subsequently, on overall spindle dynamics; and the spindle dynamics on the cutting point receptance. The proposed integrated model is a useful tool for differentiating quantitatively different effects on the spindle behaviors. The results show that a motorized spindle softens at high speeds mainly due to the centrifugal effect on the spindle shaft.     

Virtual Design and Optimization of Machine Tool Spindles

An integrated digital model of spindle, tool holder, tool and cutting process is presented. The spindle is modeled using an in-house developed Finite Element system. The preload on the bearings and the influence of gyroscopic and centrifugal forces from all rotating parts due to speed are considered. The bearing stiffness, mode shapes, Frequency Response Function at any point on the spindle can be predicted. The static and dynamic deflections along the spindle shaft as well as contact forces on the bearings can be predicted with simulated cutting forces before physically building and testing the spindles. The spacing of the bearings are optimized to achieve either maximum dynamics stiffness or maximum chatter free depth of cut at the desired speed region for a given cutter geometry and work-piece material. It is possible to add constraints to model mounting of the spindle on the machine tool, as well as defining local springs and damping elements at any nodal point on the spindle. The model is verified experimentally.     

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.     

Model-based chatter stability prediction for high-speed spindles

The prediction of stable cutting regions is a critical requirement for high-speed milling operations. These predictions are generally made using frequency response measurements of the tool/holder/spindle set, obtained from a non-rotating spindle. However, significant changes in system dynamics occur during high-speed rotation. In this paper, a dynamic model of a high-speed spindle-bearing system is elaborated on the basis of rotor dynamics predictions, readjusted with respect to experimental modal identification. Variations in dynamic behaviour according to speed range are then investigated and determined with accuracy. Dedicated experiments are carried out in order to confirm model results. By integrating the proposed speed-dependant transfer function into the chatter vibration stability approach of Budak–Altintas [S. Tobias, W. Fishwick, Theory of regenerative machine tool chatter, The Engineer February (1958)] a dynamic stability lobes diagram is predicted. The proposed method enables a new stability lobes diagram to be established that takes into account the effect of spindle speed on dynamic behaviour. Significant variations are observed and allow the accurate prediction of cutting conditions. Finally, experiments are performed in order to validate chatter boundary predictions in practice. The proposed modelling approach can also be used to qualify a spindle design in a given machining process and can easily be extended to other types of spindle.     

五轴机床刀尖点频响特性及切削稳定域位置的演变

五轴机床在航空航天领域大型曲面零件加工中应用极其广泛,其高速、高精度和大材料去除率的加工特点对加工稳定性提出了很高要求,影响五轴机床铣削稳定性的主要问题是切削颤振。通过对一台转摆头五轴机床进行动力学性能测试,对比分析主轴系统在不同位置和转摆头在不同姿态下刀尖点的频响特性,得出刀尖点频响特性随位置的演变规律。以频域法构建切削稳定性叶瓣图,得到极限切深的位置演变规律。研究结果为五轴机床切削参数的优化提供了参考。     

A comparative study on the spindle system equipped with synchronous and induction servo motors for heavy duty milling with highly stable torque control

In order to create a higher torque spindle system for productive milling operations, rotational speed stability against the torque disturbance has been studied with respect to the spindle mechanical design parameters, actuator types and spindle control algorithms. The study showed a remarkable difference in the spindle rotational speed stability against torque disturbance between a spindle system equipped with an induction servo motor and a permanent magnet synchronous servo motor. The results of this study have been obtained by theoretical analysis, numerical simulation and physical experiments, and the experimental study showed that the hybrid actuation spindle achieves longer tool life.     

Machining stability in high-speed drilling—Part 1: Modeling vibration stability in bending

In this work, dynamic models for chatter in drilling are developed that deal with the transverse vibration due to bending, and the axial vibration due to torsion. In the first part, a dynamic model is developed to obtain the limit of stability for the bending vibration mode. The equations of motion are formulated based on a lumped representation of the drill, and the gyroscopic effect due to the rotation of the tool is included. It is shown that, including this gyroscopic effect has a profound effect on the resulting stability lobes, especially at very high speeds; it makes the lobes wider but at the same time lowers the minimum stability boundary. In the second part of this work, a time domain simulation model is developed that combines both bending and torsion modes. This model is verified [1] using experimental cutting tests.     

机床主轴的PLC直流伺服系统

利用可编程控制器(PLC)控制系统的高可靠性、编程及维护方便、体积小的特点，将其应用于环流可逆调速系统中，研制出基于PLC的机床主轴直流伺服系统，使机床主轴的伺服控制更加简单、可靠和稳定。     

基于模态解耦-状态反馈控制的磁悬浮铣削系统稳定性研究

铣削加工效率和加工精度的提高,必然要求铣床加工速度的提高。随着磁悬浮技术的发展,高速磁悬浮电主轴已成为铣床的关键部件。然而磁悬浮电主轴转子系统在高速运行时存在较强的陀螺耦合效应,其过强的耦合效应使得高速铣削的稳定性区域变窄,甚至使铣削系统失稳。为此,提出基于PD控制器的模态解耦-状态反馈的控制方法。通过该方法对转子系统的平动模态和转动模态之间的耦合以及2个方向上转动模态之间的耦合进行解耦,独立调节各个模态控制器的比例和阻尼系数来降低陀螺耦合效应对磁悬浮铣削系统稳定性区域的影响,从而增大铣削的稳定性区域。通过仿真研究基于PD控制器的模态解耦-状态反馈控制前后陀螺耦合效应对铣削稳定性区域的影响。结果表明:通过基于PD控制器的模态解耦-状态反馈控制技术,不仅能够降低陀螺效应对铣削稳定型的影响,也能够改善铣削的稳定性区域。     

开关磁阻调速式车床主轴及其数控方法

介绍了开关磁阻调速式车床主轴的结构、机理及其数控方法.在分析目前车床主轴驱动情况的基础上,提出了用开关磁阻调速驱动车床主轴的机理,并进行了驱动系统结构设计和控制系统设计,同时设计了其数控方法.利用开关磁阻数字无级调速代替齿轮调速,用电机的制动反转功能代替离合器、制动器和机械换向机构.应用结果证明,开关磁阻调速式车床主轴具有无级调速范围宽、稳速精度高、加速减速时间短等优点,是一种简单易行、节能高效的车床主轴驱动方法,在机床领域有广阔的应用前景.     

Suppressing vibration modes of spindle-holder-tool assembly through FRF modification for enhanced chatter stability

Modifying dynamic response of a machine tool is of great importance for chatter mitigation. Tool tip frequency response function (FRF) can be suppressed by capitalizing on the absorber effect due to dynamic interactions among vibration modes of spindle, holder and tool. In this paper, a practical method is presented to modify the system’s FRF by selecting proper dimensions for assembly component without extensive testing. Robustness of the method is demonstrated through simulation and test results. Milling stability tests were also conducted where significant improvements in chatter free Material Removal Rate (MRR) is achieved.     

A study of dynamic stresses in micro-drills under high-speed machining

In this paper, a dynamic model of micro-drill-spindle system is developed using the Timoshenko beam element from the rotor dynamics to study dynamic stresses of micro-drills. The model includes effects of eccentricity of the spindle-clamp-drill system, the axial drilling force, the system rotational inertia, the gyroscopic moment, and bearings of micro-hole drilling machines on bending deformation of micro-drills during machining. After the model is verified using the published work, effects of the clamped length of micro-drills, the bearing stiffness and damping, the spindle speed, the system eccentricity, and the axial drilling force on dynamic stresses of micro-drills are analyzed using the model.     

PVA基粘弹性磁性磨具的实验研究及加工参数优化

目的检验新研制的PVA基粘弹性磁性磨具的表面光整加工性能,掌握配比参数、加工条件等因素对加工效果的影响规律,并对加工参数进行优化以达到最佳加工效果。方法以6061铝合金管外圆表面为光整加工实验对象,通过先导实验首先确定出影响加工效果的主要因素及其参数范围,而后基于响应曲面法实验,对主轴转速、两相质量比、磨粒尺寸及加工时长等因素与工件表面粗糙度下降率(%?Ra)之间的关系进行了探究分析。结果最后通过对实验结果进行方差分析,建立了PVA基粘弹性磁性磨具加工铝合金管外表面的%?Ra预测模型,并对影响参数进行了优化设计,得到在最佳实验条件下(加工时间46 min、两相质量比1.45、主轴转速635r/min、磨粒尺寸65目),工件表面粗糙度下降率为92.5%,最低表面粗糙度为59 nm,显著改善了加工效果。结论作为一种新型光整加工介质PVA基粘弹性磁性磨具,其具有良好的自适应性及流动性,能达到较好的光整加工效果。影响%?Ra的单因素显著性从强到弱依次为:加工时长、主轴转速、磨粒尺寸、两相质量比。交互作用显著的因子为两相质量比+主轴转速、加工时长+主轴转速、两相质量比+磨粒尺寸。在主轴转速、加工时长取高水平,两相质量比取中等水平,磨粒尺寸取低或高水平时,能得到较好的表面加工效果。     

Process stabilization with an adaptronic spindle system

For economical reasons it is necessary to reduce the machining time and to increase the process automation. This leads to the need for fast machine tools with high process stability in order to enhance the material removal rate. However, the machine often does not limit the process stability but the tool because of its compliance. This paper presents a new possibility of expanding the stable process range of long and slender end mills with an adaptronic spindle system. The system is able to position the spindle dynamically in the range of microns with three piezo actuators. In order to disturb the regenerative effect, which leads to an instable process, the chip thickness is modulated by a dynamic spindle actuation. This is realized by a superposition of vibrations of the tool in feed direction. In milling tests the degree of stabilization is verified for different superpositions. Hence, the stable process range could be improved for spindle speeds up to 5,000?rpm.     

双气缸自动松拉刀装置电主轴的结构设计及分析

为提高机床加工中心的工作效率,设计了双气缸自动松拉刀装置电主轴,相对于单气缸或油缸的自动松拉刀电主轴系统,该设计更安全可靠且无污染。介绍了双气缸自动松拉刀系统电主轴的基本结构及工作原理,通过ANSYS Workbench分析软件对双气缸电主轴进行有限元仿真分析,并进行试验测试。结果表明:该电主轴松刀力变大,主轴静态刚度满足要求,工作转速远低于其一阶临界转速,前轴承振动速度有效值小于0.6 mm/s、温升小于20℃,证明该电主轴设计合理。     

Application of active disturbance rejection control to variable spindle speed noncircular turning process

In application of variable spindle speed machining to noncircular turning process, the tracking control of the fast tool servo yields a periodic sampling rate in the real-time domain. However, in the view of the angle domain, the sampling rate is constant. Moreover, the reference acceleration signal is easily available. This implies that it is advantageous to design the control system from the angle domain perspective, but the plant for a linear time-invariant system will become a periodically time-varying system. In this paper, the active disturbance rejection control strategy is applied to actively estimate the time-varying dynamics and other disturbances and compensate for them in the control law. The acceleration feed-forward strategy is employed to achieve better tracking performance. The stability analysis based on the lifting technique is proposed. Experimental machining results demonstrate the tracking performance of the proposed design, as well as the ability to increase machining stability using variable spindle speed machining.     

Development of the active balancing device for high-speed spindle system using influence coefficients

A high-speed spindle can be very sensitive to rotating mass unbalance which has harmful effect on many types of rotating machinery. Therefore, the balancing procedure is certainly needed to reduce vibration in all high-speed rotating systems. In this study, an active balancing program using influence coefficient method and an active balancing device of an electro-magnetic type with both simple and reliable structures were applied to the developed high-speed spindle system. A gain scheduling control using influence coefficients of the reference model was proved to be effective in balancing the spindle system although its characteristics were changed. The stability of reference influence coefficients was verified by experiments with frequency response functions. The active balancing experiment of the manufactured spindle system using an active balancing program and device was also performed efficiently during the operation. As a result, controlled unbalance responses after balancing work were below the vibration limit at all rotating speed ranges including critical speeds.     

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.