机械毕业设计（论文）外文文献翻译-精镗中的摩擦阻尼器(17页).doc

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1、-机械毕业设计（论文）外文文献翻译-精镗中的摩擦阻尼器-第 14 页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 ch

2、atter 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

3、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 vibrati

4、ng 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 th

5、e 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 b

6、oring.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 f

7、orced 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)

8、 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

9、 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 phenomen

10、a 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

11、 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 o

12、ut 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 designe

13、d 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 veloci

14、ty 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 freq

15、uencies 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

16、 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 o

17、peration 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

18、 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

19、 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

20、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,

21、 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 e

22、xcited, 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

23、 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 (

24、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) descri

25、bes 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 damp

26、er 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 diame

27、ter 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

28、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 bee

29、n 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 tha

30、t 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

31、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

32、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 machini

33、ng 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, har

34、dened 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, 0.14m

35、m 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 Ce

36、rmet 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 b

37、y 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

38、 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

39、 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

40、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 attac

41、hed 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 tun

42、ed 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 cycl

43、e 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 fro

44、m 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 r

45、est 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 som

46、e 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 onl

47、y 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 structur

48、e 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