机械毕业设计（论文）外文文献翻译-精镗中的摩擦阻尼器.doc

中北大学信息商务学院 外文翻译精镗中的摩擦阻尼器学生姓名： 学号： 12020143X01 系 别： 机械工程系专 业： 机械设计制造及其自动化指导教师： 职称： 副教授 2016年6月2日Stabilization of high frequency chatter vibration in fine boring by friction damperAbstract Machining performance such as that of the boring process is often limited by chatter vibration at the tool-work piece interface. Among various sources of chatter, regenerative chatter in cutting systems is found to be the most detrimental. It limits cutting depth (as a result, productivity), adversely affects surface finish and causes premature tool failure. The new damper is characterized by simple structure that consists of an additional mass attached to the main vibrating structure with small piece of permanent magnet. The principle is straightforward in which Coulomb and viscous frictions dissipate vibration energy at the interface between the damper and main vibrating structure. The damper needs no tuning, and is effective at high frequency. The paper first introduces a typical design of the friction damper with experimental proof by cutting tests of its effectiveness in eliminating the high frequency chatter in fine boring, and assuring normal tool life of the cutting edge. Theoretical and experimental analyses are introduced for understanding the fundamental principle and characteristics of the new damper. The new damper is effective for boring tools, which vibrate at frequency more than 5,000Hz.Keywords: High frequency chatter; Friction damper; Fine boring.1. Introduction Chatter in metal cutting process, in general, is the result of both forced and self-excited vibrations. Forced vibration is due to the unbalance of rotating members, such as unbalanced driving system, a servo instability, or impacts from a multi-tooth cutter. In practice, the forced vibration sources can be traced by comparing the frequency of chatter with the frequency of the possible force functions. Corresponding measures can then be taken to reduce/eliminate such vibration sources. Self-excited vibration consists of two types, namely primary (or non-regenerative type) and regenerative type. The primary/non-regenerative type of self-excited vibration occurs when theses is no interaction between the vibratory motion of the system and the adulatory surface produced in the revolution of the work piece, such as that in threading. Hence if is inherently related to the dynamics of the cutting process. While the regenerative type of self-excited vibration is due to the interaction of the cutting force and the work piece surface undulations produced by previous tool passes. The regenerative type of self-excited vibration is found to be the most detrimental phenomena in most machining process.Effective chatter prevention during cutting operations may be achieved by increasing the damping capacity of cutting tool system. Damping capacity is generated through (i) micro-slip at certain interfaces included in the tool system, (ii) slip at the grain boundary within a vibrating body by material damping (internal friction), (iii) friction at an interface between the main vibrating body and the damper structure . Studies on various kind of damper to prevent chatter vibration, and to improve stability of boring tools or other cutting operation have been carried out by many researchers.Practical types of damper have been conventionally either dynamic or impact damper . Dynamic damper consists of additional spring-mass sub-system, and needs tuning of natural frequency of the sub-system to match that of the main structure. The dynamic damper is usually designed to include energy dissipation by either sliding or internal friction of the spring material. Impact damper consists of one or more of free moving bodies, and the principle mechanism is to dissipate energy by the impact of free moving body with the main structure. Impact damper needs certain velocity to effectively function, thus cannot be applied to suppress vibration at low frequency. A hybrid design of dynamic and impact dampers has been reported recently, and found to be effective to suppress the low frequency vibration .In the present study, the damper is required to be effective at frequencies as high as 10,000Hz, and it should be designed within size limitation of the boring tool to accommodate space for seating the tool insert, chip pocket and the damper itself. It is also preferable that the damper needs no tuning. The damper proposed in the present study consists of a piece of mass attached to the main structure by permanent magnet.The objective of the present study is to analyze the effectiveness and characteristics of the proposed damper in preventing chatter vibration that occurs at high frequency.To achieve the objective, cutting tests have been conducted in boring operation analogues to the one having high frequency chatter problem in the plant, as well as theoretical and experimental analyses of energy dissipation of the proposed damper.2. System modelMachining systems, in general, can be modeled as one-dimensional distributed structures with various boundary conditions. For a non-rotating boring process, the work piece is much stiffer than the boring bar itself. And typically boring bars are much stiffer in torsion than in bending. Hence it can be modeled as a cantilevered rod in bending. Using Euler-Bernoullis bending model for one-dimensional uniform distributed structure, the equation of motion is as follows: (1) where x is the distance along the beam, t the time, f(x,t) the external force, E the Youngs modulus, I the cross-sectional area moment of inertia, the mass density and A the area of cross section of the boring bar.The solution to the classical partial differential equation is usually found using Eigen function expansions. The response can be expressed as a sum of an infinite number of modal components as (2)Where and (i=1,2, ) are the mode shapes and modal coordinates of the system , respectively. The mode shapes are in general, mass-normalized such that (3) where L is the length of the structure andthe Dirac delta function. The equations of motion can be written in terms of the model coordinates as (4) where is the i th emodal force given by (5)It should be pointed out that in practice. Only a finite number of modes are excited, as a result, the number of modal components is in general n instead of .In the case of one dominating mode, n=1.Now considering the dynamic interaction of the cutting force and the work piece surface undulations produced by previous tool passes during the cutting process, Eq.(1) then becomes 1 (6a)Where is cutting stiffness determined by work piece material and tool geometry, B the depth of cut and T the tooth passing period, is the so-called overlap factor, which accounts for the overlapping of successive cuts. The value of varies between 0 and 1. Considering the worst-case scenario (where=1), then the above equation becomes (6b)The corresponding equations of motion in terms of the modal coordinates, by following the same procedures as that of Eqs.(1)-(4),are (7)Where b= is the cutting depth-related dimensionless parameter and the natural frequency of the I th mode.Eq.(7) describes an undammed structure. Since no purely undammed structures exist in reality, assuming viscous damping in the structure, the equation of motion is then rewritten as (8)Usually is a small positive number between 0 and 1, with most common values of 0.05. .3.Boring tools tested and the proposed damper structureThe boring tool under study that originally had a problem of high frequency chatter consists of a 13mm diameter and 20mm long cantilevered steel bar integral with a base flange. A small diameter hole, 5.5 mm, is prepared at the end of the bar to accommodate the damper mass of which diameter may be 5mm or less. The position of the hole is selected in radial orientation om1, because the high frequency vibration due to X-Y looping has been known to occur dominantly in the orientation om2 as depicted When the tool is rotated in boring operation, the damper is pushed to the wall of the hole by the centrifugal force, but is free to move in the orientation of the vibration om2. A cap is provided to protect the damper from chips removed during the operation.The effectiveness of the damper has been tested for the tool as shown in the figure, as well as other boring tools that have been prepared for comparison.One of the comparison tools has the same diameter 13mm, but extended 10mm beyond the cutting edge, and generates chatter vibration at about 5,000Hz. Other comparisons are 16mm diameter cantilever type boring tools, designed with greater length (L) to diameter (D) ratios that exhibit chatter at lower frequencies.Basic structure of the new friction damper is the combination of a mass and permanent magnet, which anchors the mass to the main structure on a flat surface parallel to the direction of vibration. The magnet may be either integral or separated with the mass. A third member, a spacer, may be inserted between the permanent magnet and the main structure whose purpose is to control magnitude of magnetic force. Effectiveness of the friction dampers in suppressing high frequency chatter has been evaluated.4. Method of experimentTo validate the effectiveness of the damper in view of controlling the chatter, and to assure normal tool wear and surface roughness generated, cutting tests have been performed with the 13mm diameter boring tool rotated as it is in production site. In this case,the boring tool is mounted on the main spindle of a horizontal machining center via a setting head whose function is to adjust the radial position of the tool tip for automatic control of the hole diameter in production.Ring type work pieces have been prepared whose inner surface is to be machined by the rotating boring tool. Rings are made of SCM420H alloy steel, hardened to 313 to 332 Brinnell hardness with 25mm outer diameter, 14.720.05mm inner diameter, and 15mm length. A milling chuck clamps the ring on a specially designed fixture with sufficient stiffness.The standard condition for cutting test is set to 130m/min cutting speed, 0.03mm/rev feed rate, 0.14mm depth of cut, and using no cutting fluid. A new cutting edge is prepared for each set of cutting tests in which work pieces are continuously machined. The cutting test is repeated twice for each boring tool system with and without the damper. Tool insert material used for the boring tool is TiC Cermet non-coated, with axial rake angle -5, radial rake angle -15, and nose radius 0.4mm.For measuring vibration of the 16mm diameter tool, another setup was prepared with the tool held stationary, and used to machine outer surface of the rotating ring work piece. In this setup, the tool is clamped by a milling chuck staged on a baseplate on the machine table of vertical machining center. The ring work piece is mounted and rotated by the machine spindle.5. Analysis of friction damper mechanism5.1 Theoretical analysisDuring the development of chatter, once the vibration reaches certain threshold amplitude, the damper will start sliding, therefore introducing friction at the interface between the damper mass and the main structure. The friction dissipates the vibration energy, and prevents the chatter from growing beyond the threshold amplitude.5.2 Experimental analysisIn order to ascertain validity of the two theoretical models assuming Coulomb and viscous friction respectively, vibration of a main structure model has been monitored with and without the damper mass attached, and excited externally by an electro-dynamic exciter. a cantilevered steel beam 16mm diameter, having similar cutting edge design with the original boring tool and 170mm length, has been used as the main structure whose second order bending mode was excited around 5,700Hz frequency. The vibration at the end of the beam is detected by micro size accelerometer pickup. A damper with an integral magnet is attached on top of the main structure via cleaned and dried interface as well as oiled interface.Random excitation is first applied to identify the natural frequency of the main structure. Then sinusoidal excitation is applied at variable amplitude f of the input dynamic force F at frequency Z finely tuned around the natural frequency identified by random excitation. At the same time, amplitude x of response vibration X of the main structure, and the phase difference between input dynamic force and response vibration , are measured by the FFT Analyzer.Amount of energy supplied Es per vibration cycle by the sinusoidal excitation is computed from the measured f,as follows:vibration amplitude x of the main structure is reduced when the damper is attached on either dried or oiled interface. When the damper is used, the amplitude x exhibits a stagnant step during the excitation force increment from 0.3 to 0.6N.6. Conclusion In this paper, a control from wave point of view designed to absorb vibrational energy in a broad frequency range is applied in the control of regenerated chatter in non-rotating boring cutting process, Though it was found that the first mode dominates the response, the rest of the modes more or less contribute to the chatter of the system. Furthermore, when designing a controller based on the consideration of only single dominating mode, it is likely to induce control spillover(that is , the control force designed to control the first mode will adversely excite some or all the rest of the modes of the system), as observed in the experimental results of Boring .In this study, with the controller designed based on a broad frequency range, damping is added to all the modes in the frequency range of interest, but also greatly reduces the chatter caused by not only the dominating first mode but also that by the rest of the modes.To control chatter vibration occuring at frequencies as high as 10,000Hz, as previously reported in fine boring operation, performance of a new damper mechanism utilizing friction between a damper mass and the main vibrating structure has been evaluated by cutting and excitation experiments.The new damper consists of a piece of mass attached to the main structure by permanent magnet. It has been confirmed by the present study that both Coulomb and viscous frictions are occurring at the sliding interface. Due to the Coulomb friction, there occurs threshold amplitude where the mass starts sliding with respect to the main structure and dissipates a certain amount of vibration energy, which is approximately in linear proportion to the vibration amplitude. When the energy dissipation at this condition is sufficient, the vibration is suppressed to this threshold amplitude.The damper has been found to be more effective for tools that generate chatter vibration at higher frequencies. From the physical size limit of the damper mass for attachment to the main structure, friction damper is practical for tools which vibrate at frequencies higher than 5,000Hz.Due to simple structural design and no need of tuning, the proposed damper is a viable solution for the high frequency chatter vibration of continuous cutting operations such as fine boring.AcknowledgementsThis research was supported by NT-Engineering Inc. that has provided materials including workpieces, some of the boring tools and instruments by care of Mr. Y. Komai, Executive Director of Engineering,and Mr. M. Nakagawa.8814精镗中的摩擦阻尼器摘要：镗削的加工性能经常受加工中振动的影响。在各种振动来源中,再生颤振是最不利的。它不仅限制了切削深度，对表面质量也有不利的影响，同时也会损害工具寿命。尽管加工系统是一种分布式的系统，通用的控制器是根据一个简化后的单自由度切削过程模型来设计的。这是因为大部分切削过程只存在着一个主导模式。然而，简化后就会出现一些问题。首先，因为系统本身是分布式系统，理论上它是由无数个振动模型组成，当控制器仅仅控制主要的模型，被用来控制主导模型的能量会激起原本静止的机构的振动，即引起所谓的溢流问题。第二、单自由度控制器设计的成功依赖于有效的精确的模型参数（如质量当量，刚度，阻尼），但不幸的是获取这些参数非常困难。新阻尼器结构简单，它由一个联接在主振动结构上的附加质量与一小块永久磁铁构成。其原理是简单的，利用库仑力和粘性摩擦将振动能量消散在阻尼器和主振动结构的接口之间。阻尼器对高频也有效，因此无需调谐，本文首先介绍了一种在精镗中消除高频颤振的摩擦阻尼器的典型设计，其有效性由切削试验得以证明，并保证刀尖的正常寿命。对这种新型阻尼器基本原理的理解在理论和实验分析中得以介绍。在镗削过程中这种新型阻尼器能够有效的防止超过5000赫兹的颤振。关键词 ： 高频振动 , 摩擦阻尼器 , 精镗1、 引言 金属切削中的振动总体上是受迫振动和自激振动引起的。受迫振动是由回转件的失衡引起的，比如失衡的驱动系统，伺服不稳定或者多齿零件的撞击。受迫振动可以认为是由振动频率和受迫力频率对比引起的，但相应的措施可以被用来减小或消除这些振动来源。 自激振动包括两部分：基本类型（不可再生类型）和再生类型。不可再生式自激振动出现在回转件的波动表面对系统的振动没有相影响的时候，如车螺纹。因此它只和切削过程中受到的力有关。再生式自激振动是因为工具通过时，系统的振动和回转件的波动表面相互作用产生的，再生式自激振动对加工过程的影响最为不利。 先前有研究报告称精镗中出现超过10000赫兹的高频颤振。这种频率首先发现于留在切削表面的振纹上，然后在切削实验中直接使用激光位移计测量得到进一步的证实。从镗刀的自然弯曲振动以及自我激发的切削过程中的动力学再生效果、内调制虚部的影响和x-y方向的循环发现了这种颤振。本研究的目标是防止这种颤振振动的发生。预防切削颤振的有效措施可能是通过提高刀具系统的阻尼能力。阻尼能力是通过以下方面产生的：（1）包含在刀具系统接口处的某些微量滑动；（2）在晶界滑移内部振动引起的阻尼损耗（内耗）；（3）在主振动结构和振动阻尼器接口处的摩擦。许多研究人员对不同类型的用以防止颤振振动，并提高镗刀或其他切削操作稳定性的阻尼器进行了研究。该阻尼器已不是传统阻尼器的动态特性或冲击特性了，动态阻尼器包括额外的弹簧质量子系统，通过调节系统的固有频率，使之与主体结构相匹配。一般动态阻尼器设计包括任意方向的滑动或内部摩擦耗能的弹性材料。弹性阻尼器由一个或多个的自由移动机构组成，其原理是利用自由移动体撞击主体结构来耗散颤振能量。阻尼器受一定的速度影响才能有效的发挥其功能，因此不能适用于抑制低频振动。近来有报道一种动力与摩擦混合阻尼器，并发现它能有效地抑制低频振动。本文中所设计的阻尼器必须能有效地抑制高达10000赫兹的高频率颤振，而且它的设计受到镗刀本身的工作空间及其自身大小的限制。它最完美的地方就是不需要调整。该阻尼器在本研究提出一个大规模隶属永磁结构的概念。本研究的目的是为了分析抑制高频振颤阻尼器的有效性及其阻尼特性。为了实现这一目标，已进行一个类似于抑制精镗中高频颤振的切削试验以及理论和实验的能源阻尼耗能分析。2、镗刀测试和阻尼器结构的构想根据研究，在精镗中原本有一个高频颤振问题，镗刀本身包括一个直径分别为13毫米和20毫米的长悬臂杆和法兰。在杆的一端有一直径为5.5毫米的小孔，以适应5毫米或孔径更小的阻尼器。该孔的位置选择在径向方向，因为我们已经知道高频振动在X-Y方向循环。当镗刀空转时，阻尼器被孔壁的离心力推动但可以再径向方向自由移动。上限用以保护运行中的阻尼器。该阻尼器的有效性已经通过了检测并准备和其他镗刀做比较。用作比较的工具之一具有相同直径的长悬臂杆即直径为13毫米，但其延伸超出了前沿10毫米并产生约5000赫兹的颤振振动。其他与之比较是16毫米直径悬臂式镗刀，将以更大的长径比产生较低频率的颤振振动。新型摩擦阻尼器的基本结构是一个附加质量和永久磁铁的组合，其中质量平面平行于主结构的振动方向。磁铁可以是不可分割的或者是可分割的