Coupled interaction of dynamic responses of tool and workpiece in thin wall milling

Chatter free thin wall machining requires knowledge of the dynamics of a machine-tool system and workpiece either for designing damping solutions or for modelling impact dynamics. Previous studies on thin wall milling mostly focussed on stability studies. However studies on the interaction between the tool and workpiece responses in thin wall machining are scarce in the literature. In this work, the coupled dynamic response of tool and workpiece is presented both for an open (thin wall straight cantilever) and for closed (thin wall ring type casing) geometry structures. Experiments were carried out for different tool overhangs and depths of cut and the machining vibration signal was analysed in time–frequency domain to study the interaction, i.e. coupling, of tool–workpiece dynamic response at various cutting tooth engagement/idle times. The findings from this study highlight the importance of tool's frequency, particularly torsional and first bending modes, in impact dynamics of thin wall milling. Moreover, the differences in dynamic response interaction between a cutting tool and thin wall plate and a cylinder are identified. While the analysis of the open geometry structure showed the presence of tool and workpiece responses for any depth of cut, results on closed geometry structure exhibited a complete dominance of tool mode at higher depths of cut. These findings are of critical importance in understanding the impact dynamics in thin wall milling and also of effectiveness of passive damping solutions.     

Optimization of multiple tuned mass dampers to suppress machine tool chatter

Chatter is more detrimental to machining due to its instability than forced vibrations. This paper presents design and optimal tuning of multiple tuned mass dampers (TMDs) to increase chatter resistance of machine tool structures. Chatter free critical depth of cut of a machine is inversely proportional to the negative real part of frequency response function (FRF) at the tool–workpiece interface. Instead of targeting reduction of magnitude, the negative real part of FRF of the machine is reduced by designing single and multiple TMD systems. The TMDs are designed to have equal masses, and their damping and stiffness values are optimized to improve chatter resistance using minimax numerical optimization algorithm. It is shown that multiple TMDs need more accurate tuning of stiffness and natural frequency of each TMD, but are more robust to uncertainties in damping and input dynamic parameters in comparison with single TMD applications. The proposed tuned damper design and optimization strategy is experimentally illustrated to increase chatter free depth of cuts.     

A new two-dimensional ultrasonic assisted grinding (2D-UAG) method and its fundamental performance in monocrystal silicon machining

A new two-dimensional (i.e., elliptical) ultrasonic assisted grinding (UAG) technique is proposed to achieve high material removal rate and high surface quality in the machining of hard and brittle materials such as monocrystal silicon. In this method, the workpiece attached on an elliptical ultrasonic vibrator is ground with a resin bond diamond grinding wheel under the presence of elliptical ultrasonic vibration. The elliptical ultrasonic vibrator is produced by bonding a piezoelectric ceramic device (PZT) on a metal elastic body (stainless steel, SUS304) and its detailed structure/dimensions are determined by FEM (finite element method) analysis. When two alternating current voltages with a phase difference are applied to the PZT at the same frequency that is close to the resonant frequency of the longitudinal mode and bending mode of vibrator, two ultrasonic vibrations are generated simultaneously, and the synthesis of the vibrations results in an elliptical motion on the end face of the vibrator. If the phase difference is set at 0° or 180°, two kinds of one-dimensional ultrasonic vibrations, i.e., axial or vertical vibrations, can be obtained. Grinding experiments are carried out involving monocrystal silicon to confirm the performance of the proposed elliptical UAG. In addition, grinding experiments under the presence of the axial and vertical ultrasonic vibrations and the absence of ultrasonic vibrations, i.e., conventional grinding, are also carried out for comparison. The obtained results show that: (1) compared with conventional grinding, the axial ultrasonic vibration results in greatest improvement in the work surface quality and a slight reduction in the grinding forces; (2) under the vertical ultrasonic vibration, the grinding forces are decreased significantly but the surface roughness is increased slightly; (3) elliptical ultrasonic vibration leads to the significant reduction of both the surface roughness and grinding forces. These indicate that the high efficiency and high-quality grinding of monocrystal silicon can be performed with the proposed two-dimensional ultrasonic assisted grinding technique.     

抑制钢轨波磨的梯形轨道结构的多参数拟合与优化

目的研究地铁车辆结构一系悬挂及轨道结构支承刚度和阻尼对梯形轨道小半径曲线钢轨波磨的影响规律,并提出抑制相应区段钢轨波磨的多参数拟合优化方法。方法基于轮轨系统的摩擦自激振动理论,构建梯形轨道小半径曲线段上带有一系悬挂轮轨系统的有限元模型。然后通过复特征值分析,研究地铁梯形轨道小半径曲线段轮轨系统的摩擦自激振动特性。最后开展地铁车辆结构一系悬挂及轨道结构支承刚度和阻尼的参数化分析,采用最小二乘法,对影响不稳定振动的关键参数进行多参数拟合。根据该拟合方程,采用遗传算法进行关键参数的优化匹配。结果在曲线半径为350 m的梯形轨道上,轮轨间因存在饱和蠕滑力,引起摩擦自激振动,在439.02 Hz时,产生了不稳定振动,从而导致了钢轨波磨的产生。影响轮轨间不稳定振动的关键因素为扣件的垂向刚度、横纵向刚度、垂向阻尼和减振材料的垂向刚度。结论当扣件的垂向刚度为36 MN/m、扣件的横纵向刚度1.68 MN/m、扣件的垂向阻尼为2.9 kNs/m、减振材料垂向刚度为5.57 MN/m时,可有效抑制梯形轨道小半径曲线段上钢轨波磨的发生。     

嵌入式共固化阻尼复合材料十字型加筋板的自由振动

为大幅度地提高嵌入式共固化复合材料阻尼结构固有频率,优化其刚度,以嵌入式共固化阻尼复合材料薄板为研究对象,利用有限元的方法对比分析了该结构薄板及十字加筋薄板固有振动特性,并得到了筋材薄壁宽度和高度对结构固有频率的影响规律。结果表明：加强筋大大提高了薄板结构的固有频率和刚度,加筋薄板结构的各阶固有频率随着加强筋高度增大而增加,其中一阶固有频率增加幅度最大。增加加强筋宽度可以提高薄板结构的各阶固有频率,随着加强筋宽度的增加,固有频率的增加幅度减小。     

An experimental investigation of circular saw vibration via a thin plate model

This paper deals with important practical problems related to vibration reduction in a circular saw used for wood cut-offs. An investigation is made of the static and dynamic characteristics of thin plate structures as a model of a circular saw, to obtain fundamental knowledge to improve circular saw performance. The circular saw is represented experimentally by a thin rectangular plate clamped by two shims. The bending rigidity and damping capacity of a structure are measured for six different clamping methods. The results are evaluated by a coefficient, which is introduced through Merritt's regenerative chatter vibration theory. The most effective thin plate structure is found to improve vibration-proof capacity. This structure is easily applicable to an actual circular saw.     

基于二维激光扫描的复合材料层合板热振试验装置研发与应用

研发了一套基于二维激光扫描测试装置的复合材料层合板热振试验装置。利用PLC控制器、触摸屏、滑台型导轨、步进电机、激振器、功率放大器、激光测振仪、加热及温控装置、振动控制仪及数据采集分析装置等,建立了满足层合板高温测振需求的硬件系统。基于PLC梯形图语言编写了二维激光扫描测试装置的控制软件,并详细介绍了手动、自动控制模块对应的操作界面与测控优势。以HF10碳纤维/树脂基层合板试件为例,对该装置进行了研究。结果表明:该装置可显著提高热环境下层合板结构模态振型的测试效率,且有利于研究不同温度对结构振动特性的影响。研究发现热环境对层合板固有频率和阻尼性能有着复杂影响,且随着温度的不断升高,模态阻尼比呈现减速增大的趋势。     

Vibration suppression device using permanent-electromagnet and MRF shear damper

A vibration isolation and vibration suppression device is presented. The device consists of an electromagnetic actuator and a magneto rheological fluid (MRF) rotary shear damper. In the device, the MRF damper supports a vibrating body. Since the body is connected to the base with large rigidity using the MR damper, vibrations are small when an exciting force acts on the body directly. When base vibrations occur, the electromagnetic actuator under the disturbance force cancellation control isolates vibrations, where the shear force of the damper is controlled to be small. This device can control both vibrations due to exciting force acting on the body and base vibrations without using a heavy vibration table. To validate the system, experimental tests have been performed, and it is clarified that this device works well. Especially, the transmissibility in low frequency region can be suppressed within one.     

Gear tooth stiffness reduction measurement using modal analysis and its use in wear fault severity assessment of spur gears

Due to excessive service load, inappropriate operating conditions or simply end of life fatigue, damage can occur in gears. When a fault, either distributed or localised, is incurred by gears, the stiffness and consequently vibration characteristics of the damaged tooth will change. A possible non-destructive technique for damage detection and severity assessment can be derived from vibration analysis. This paper presents the use of vibration analysis in the detection, quantification, and advancement monitoring of damage incurred by spur gear teeth. The stiffness of a single spur gear tooth is analysed theoretically, and due to the difficulties in measuring the gear tooth stiffness, an experimental procedure based on the modal analysis is developed to assess the severity of the gear tooth damage. A pair of spur gears was tested under accelerated wear conditions, and conventional time and frequency domain techniques are applied to the gear vibrations to indicate the presence and progression of the wear. The developed modal stiffness assessment technique is then used to quantify the resulting wear damage to the spur gear teeth.     

超声椭圆振动切削技术及装置应用研究现状及进展

首先,阐述了超声椭圆振动切削技术、原理及其装置特点,着重论述了装置各构成部分的特点及其主要应用类型与材料。其次,分析了国内外学者在双激励、单激励方式下实现纵弯、纵扭、双弯曲和弯扭等复合超声振动装置结构设计及其优化方面的研究成果与进展,并且比较了超声椭圆振动切削与传统加工在加工性能、工件表面质量和加工精度等方面的加工优势与适用范围,以及不同超声振动装置在各切削加工中所能获得的加工效果。最后,对超声椭圆振动切削装置的发展趋势进行了总结和展望,指出该装置发展将朝着结合能场的方向发展。     

船用惯性导航设备缓冲系统设计

针对船用惯性导航设备的无转角抗冲击要求,基于无谐振峰减振器,设计一种三维平移缓冲平台,由于减振器水平方向阻尼不足,在水平方向增加了横向和纵向阻尼器。利用缓冲基本原理,建立三维平移缓冲平台动力学模型。在此基础上,根据冲击设计条件利用ADAMS仿真软件计算出最优隔振器刚度和阻尼参数,最后对缓冲平台样机进行振动试验、垂向及横向冲击试验。结果表明：三维平移缓冲平台振动传递率小于3,冲击传递率小于40%。设计的三维平移缓冲平台为惯性导航设备抗冲击提供了一种有效的应用方案。     

薄壁盘类零件超声振动车削实验研究

薄壁盘是典型的弱刚度轻量化零件,其壁厚很薄(最薄处仅0.3 mm),型面面积较大,故刚性较差。在切削过程中,切削力导致的型面变形是薄壁盘类零件加工的主要难题,传统加工方法很难得到高精度和高质量的产品。利用超声振动切削技术尝试进行弱刚度薄壁盘加工,在自行搭建的实验平台上对超声振动车削和普通车削加工薄壁盘进行对比实验。结果表明:与普通车削相比,超声振动车削可显著减小切削力和切削变形,大幅提高加工质量,同时具有断屑特性,是加工薄壁盘类零件的有效方法。     

带有减振底座的双向液压振动试验系统研制

为解决液压振动试验系统需要建设减振地基所带来的建设周期长、占地面积大、成本高等问题,提出并研制了带有减振功能底座的双向液压振动试验系统。介绍该液压振动试验系统的结构组成和工作原理,并详细阐述减振底座、伺服作动器及翻转装置等核心部件结构组成;依据技术指标要求和JJG 638-2015液压式振动试验系统国家计量检定规程,对液压振动试验系统进行0.5～200 Hz/4g正弦扫频测试,对系统的台面加速度波形失真度、台面加速度均匀度和台面横向振动比等核心性能指标进行了测试。测试结果表明:液压振动试验系统的台面加速度波形失真度、台面加速度均匀度和台面横向振动比等核心性能指标均满足JJG 638-2015液压式振动试验系统国家计量检定规程要求。     

Cu-Al-Ni-SMA-Based High-Damping Composites

Recently, absorption of vibration energy by mechanical damping has attracted much attention in several fields such as vibration reduction in aircraft and automotive industries, nanoscale vibration isolations in high-precision electronics, building protection in civil engineering, etc. Typically, the most used high-damping materials are based on polymers due to their viscoelastic behavior. However, polymeric materials usually show a low elastic modulus and are not stable at relatively low temperatures (≈323 K). Therefore, alternative materials for damping applications are needed. In particular, shape memory alloys (SMAs), which intrinsically present high-damping capacity thanks to the dissipative hysteretic movement of interfaces under external stresses, are very good candidates for high-damping applications. A completely new approach was applied to produce high-damping composites with relatively high stiffness. Cu-Al-Ni shape memory alloy powders were embedded with metallic matrices of pure In, a In-10wt.%Sn alloy and In-Sn eutectic alloy. The production methodology is described. The composite microstructures and damping properties were characterized. A good particle distribution of the Cu-Al-Ni particles in the matrices was observed. The composites exhibit very high damping capacities in relatively wide temperature ranges. The methodology introduced provides versatility to control the temperature of maximum damping by adjusting the shape memory alloy composition.     

Improvement of viscoelastic damping in nickel aluminum bronze by indium-tin

Reduction of the vibration noise from submarine propellers is of interest in naval operations. Such an objective can be achieved via the use of materials with the ability to dissipate energy of vibration by means of heat, i.e. high damping materials. An additional problem is that the extreme hydrostatic pressure environment of a submarine requires the chosen material to exhibit considerably high stiffness. Most materials demonstrate a compromise between the two properties, i.e. stiffness and damping. This paper aims to discuss research into high stiffness and high damping materials conducted using a dynamic mechanical analyzer (DMA) under variations of testing temperature, frequency, and strain amplitude. Alloys of nickel aluminum bronze and indium tin are the subjects of this study. Defect damping represents a large portion of the overall damping properties of the nickel aluminum bronze while increasing indium content is shown to boost the damping properties of the indium tin alloy. The study then continues with the development of a new material that combines both indium alloying and defects introduction into the nickel aluminum bronze alloy. The new alloy is observed to have high damping, as measured in its high tan δ, with minimum reduction of the stiffness |E*|.     

Low vibration control system in turning

This paper presents a system developed for on-line vibration control on a CNC lathe. The on-line vibration control system is composed of two subsystems, namely, the vibration monitoring subsystem, and the vibration reduction subsystem. A simple chatter criterion was adopted in the vibration monitoring subsystem in order to detect the occurrence of chatter as soon as possible. The vibration reduction subsystem is then triggered to suppress vibrations when chatter was detected. It was shown that chatter can be detected by comparing the standard deviation of signal collected on-line to an off-line experimentally determined value. The vibration reduction subsystem provides possible ways to reduce vibrations by adjusting cutting parameters including cutting speed and feed rate. Series of experimental tests were conducted to verify the feasibility of the proposed system. It was shown that the proposed system can generally detect the occurrence of chatter and reduce vibrations effectively when chatter occur.     

Design and implementation of tuned viscoelastic dampers for vibration control in milling

Passive means of vibration attenuation have been employed successfully and efficiently in machining systems such as turning and milling. Traditional approach to controlling vibration in a milling system is to develop control mechanisms for cutting tools or machine spindles. However, due to the nature of milling operations where the cutting tools rotate at high speed, the passive vibration control methods find very limited application with the traditional approach. In order to utilise the potential of the passive vibration control methodology in milling applications, the milling operation should be viewed as a system comprising an elastic structure and operation parameters. Dynamics of this closed-loop system should improve with improvement in dynamics of any of the system components, especially within the elastic structure that comprises the cutting tool, the machine tool, the workholding system and the workpiece. Although the level of improvement will vary depending on which component of the elastic chain is targeted for this purpose. This paper presents the development and testing of tuned viscoelastic dampers (TVDs) for vibration control through their application on a workpiece in milling operations. This work targets workpiece held on a palletised workholding system for the control of unwanted vibration and thus deviates from the traditional approach where cutting tool and/or machine spindles are targeted for vibration control strategies. Palletised workholding systems, due to their compact design, offer an opportunity to design passive damping mechanisms that are easier to implement in the case of a milling system. The TVD developed through this research is based on a commercially available viscoelastic damping polymer. Advantage of such materials is their high damping performance over a wide range of excitation frequencies. The TVD design process has used a unique combination of analytical modelling with experimental FRF data. Modal impact testing showed that the application of the TVD reduced the amplitude of vibration acceleration by 20 dB for the target mode. Since the target mode corresponded to torsional vibration, the TVD was effective in two planar coordinates, i.e. X and Y. In addition, the TVD also significantly reduced the amplitude of a vibration mode far from the mode it was designed for. The system has been tested experimentally to demonstrate significant reduction in vibration amplitudes during a milling process. The milling tests with different combinations of cutting parameters show that multi-TVD approach is always valid regardless of the parameters being used. The only requirement for TVDs to function effectively is that the natural frequency of the system, for which the TVDs are designed, is excited during the milling process.     

基于并联机构多自由度减振平台仿真与测试研究

为了解决许多重大工程领域中的多自由度减振问题,本文针对橡胶具有多自由度弹性阻尼减振特性,但容易老化,失去弹性的特点,采用并联机构作为组合弹性阻尼减振装置主体结构,在机构原动件处辅以弹性阻尼装置,运用反向自适应原理,模拟橡胶实现减振;同时按照给定减振要求,用ADAMS软件建立系统模型和仿真,并制作了试验样机.将仿真和试验结果进行分析比较,结果表明动态模拟与试验结果一致,从而说明了基于并联机构组合弹性阻尼减振装置设计理论的正确性和可行性,可推广解决其它多自由度减振问题.     

轻质多孔材料与结构研究的最新进展

大力推进材料和装备的轻量化、减量化是实现节能减排、加快建设节约型社会的关键措施,是新世纪工程科技的发展方向。大至海洋平台、大飞机机身和动车组车体,再到日常生活中的车辆,乃至小电子散热器件等,轻量化和多功能化均成为其发展中重要一环。围绕相关特殊工况应用条件下的轻质材料与结构的设计和研究面临一系列挑战：质量轻、力学强度高、散热性能好、动力学性能和隔振、隔声性能可调等多功能要求,因此如何在现有的材料和结构基础上进一步减轻重量并获得更优良的综合性能是材料科学、固体力学、传热、声学、优化设计等诸多领域工作者面临的共同挑战。基于本课题组近5年来围绕“超轻多孔结构创新构型的多功能化基础研究”国家基础研究计划项目所开展的一系列工作,综述了国内超轻多孔材料与结构最新发展水平的研究成果,总结了具有特定或多功能化应用的这类新型轻质多孔材料多学科交叉研究的进展,包括材料制备,力学、热学和声学特性,以及无损检测及优化设计等。