机械设计制造专业毕业外文翻译振动辅助攻丝装置的基本问题

1、河 北 科 技 大 学毕业设计(论文)外文资料翻译学 院： 机械电子工程学院 专 业： 机械设计制造及自动化学院 姓 名： 付海军 学 号： z060501623 外文出处： journal of materials processing technology 附 件： 1.外文资料翻译译文；2.外文原文。 指导教师评语： 签名： 年 月 日附件1：外文资料翻译译文振动辅助攻丝装置的基本问题摘要虽然现在攻丝已被广泛用于螺纹加工中，但是它通常是一个耗时的工艺过程并且会造成自动化生产线的延迟。本次研究的主要内容是振动辅助攻丝装置的基本问题。它主要涉及两个机构，用来减小振动辅助攻丝过程中的扭矩，例如

2、，减少摩擦扭矩以及在增强应变率的条件下材料性能的变化。本次研究通过在有无振动辅助的不同条件下的攻丝试验，来对振动在减小攻丝扭矩方面的影响进行理论分析。在攻丝试验中，共用了三种不同型号的丝锥以及两种不同波形的振动。该理论研究结果不仅仅适用于振动辅助攻丝，而且同样适用于其他的加工过程。1 引言在现代工业所涉及到的所有机械加工中，攻丝是最耗时的，因此它也通常是提高生产线生产率的瓶颈。另一方面，随着国防和民用工业方面的现代制造业的迅速发展，诸如汽车和飞机的发展，对大批量，高质量和高精度的螺纹孔，尤其是小直径大深度（例如m3.5毫米，深度10-25毫米）的孔提出了更高的要求。遗憾的是现在的攻丝技术不能完

3、全满足这一要求，这就对效率高成本低的攻丝技术提出了更高的要求。攻丝技术广泛应用于内螺纹的大批量加工中。在切削过程中要经常会遇到丝锥突然断裂的问题，因此必须得取出并维修攻丝部件。经调查，小于m6的丝锥在盲孔攻丝中更容易损坏，因此就产生了提高这种小规格丝锥寿命的需要。而且，在制造业中，诸如钛等高性能合金得到了广泛的使用。但是这种材料的弹性模量通常很低，因此它们难以加工。当对这种材料进行攻丝时，由于加工表面大量的弹性恢复，丝锥的提前断裂是很常见的问题。该弹性恢复所引起的摩擦扭矩是在普通碳钢上攻丝扭矩的7倍【1-2】，摩擦扭矩的增加导致了丝锥的断裂。在加工钛合金以及其他材料时应用振动辅助攻丝装置，不仅

4、可以减小摩擦扭矩，而且可以提高攻丝效率，减小总攻丝扭矩【1-5】。我们已经得到了诸如摩擦扭矩大幅度减小，丝锥寿命的显著提高以及螺纹精度提高等满意的结果。并且制造了一种振动辅助装置，该装置包含了一个步进电动机，以此在攻丝中提供频率为24khz，振幅为400微米的振动。实践证明，使用这种辅助振动攻丝装置，攻丝扭矩减小了三分之二，而丝锥寿命得到了显著的提高。库马比和塔里巴纳【6】开发了一款能提供频率为46-167khz振动的测试系统，来对铝合金，铜和钢铁进行攻丝。据报道，攻丝扭矩与常规扭矩相比下降了四分之一至三分之二。索图马【7】发现通过引进频率20khz和振幅15微米的振动可以提高丝锥的寿命（每把

5、丝锥能加工10000件工件）。帕蒂尔【8】等为攻丝工艺设计和开发了一种可控供振设备。他们还开展了各种调查来研究丝锥型号，工件材料，振幅及阵频不同的工艺条件在加工中对攻丝扭矩以及推力的影响，以期找到最适的条件来提高丝锥的寿命。铃木【4】等人用超声波振动辅助铝合金材料的攻丝过程，发现切屑厚度从250微米降到了100微米并且攻丝扭矩减小了20倍。因为扭矩和切屑厚度的减小，螺纹加工表面的精度得到了显著的提高。2 理论背景由于振动的应用，降低了工件材料的表面粗糙度以及摩擦，这对攻丝扭矩的降低，螺纹质量精度的提升以及丝锥寿命的延长有着直接的影响。因为摩擦有助于扭矩的改变，因此可以通过使用振动辅助装置降低摩

6、擦以及已加工螺纹的弹性恢复来降低攻丝扭矩的减小。2.1 摩擦力下的振动图1所示为一摩擦副模型，刚体1置于刚体2上。一静态正常载荷w作用于刚体1上且在平面滑动过程中始终保持垂直方向。两刚体间的相对滑动速度是v。负载随纵向振动的变化而变化。如果该载荷定义为正常动载荷wd，纵向振动可表示为:那么正常动载荷可用下式给出【9】:其中a是振幅，c是和摩擦副材料有关的常数。然而，当w<ca时，wd的值为负。这意味着两刚体能够定期的分离。因此，当w<ca时，wd用下式计算（见图2）：t1和 t2表示的是接触的起始和结束时间，对于给定的时间t，t1和 t2分别满足以下公式：摩擦力ft可用动摩擦系数和

7、动载荷ft和动载荷wd用下式表示：因此，平均摩擦力可以表示为：在微滑移区的微位移能够提高摩擦系数，这一点已经被鲍登和塔博尔10所证明。基于此，摩擦系数与相对位移可用下式表示：其中c1是一个常量。相对位移是：考虑到调查条件的影响，以及公式（9）（10）系数给出如下：t3是相对位移达到u1所用的时间：将公式（3）（10）（11）代入公式（8）并且考虑t1,t2和t3的影响，摩擦力f可表示为：公式（13）很复杂，但是它指出了平均摩擦力与振幅和频率之间的关系。如果其中的参数给定的话，以上的关系就可以用计算机来模仿了。图3就是用所给参数模拟的结果。可以看出，在所选振幅与频率下，应用振动时的平均摩擦扭矩总

8、是低于没有采用振动的情况。如果振频率在4khz到35khz之间，随振动频率的增加，摩擦力减小。当振频小于4khz或大于35khz时，摩擦力几乎保持不变。存在最佳的振幅使摩擦力最小。当振幅大于2微米时，摩擦力增加，而当振幅小于2微米时摩擦力减小。最佳的振幅在2微米左右。在辅助振动攻丝过程中，振动幅度和频率对摩擦力的影响是非常重要的。2.2 摩擦力减小的攻丝扭矩攻丝时，弹性变形能产生显著的弹性恢复，产生很大的摩擦力矩。为了减少攻丝时的弹性恢复，必须进行基于弹性和塑性理论的分析【2】。根据研究的结果，攻丝的最大的扭转力矩用以下公式表示：ts是工件的剪切屈服应力。r是丝锥半径，b是切削宽度，n是丝锥与

9、已加工表面的摩擦次数，k是一常数，l0是接触线程，lt是每次向前的切削距离，lg是每次切削向后的距离。这个公式表明，在攻丝中，当其他条件不变时，可以通过减小向前的切削线程和增加向后的长度来减小摩擦力矩。2.3 应变率效应当材料承受高速变形时如承受很大的应变率，其反映会有很大的差别。一般而言，金属韧性kic随应变率的增大而减小。图4所示半镇静钢断裂韧性随应变率的变化情况。应变率提高一个数量级，断裂韧性大约下降百分之十，这和金属的低温脆断很相似。另一方面，当应变率增加超过了大约log=10，钢的脆断韧性将会急剧上升。这主要是因为在材料高应变率产生热量的作用下发生了硬化。在这种情况下，可以在变形区形

10、成近绝热条件【4】。此外，如图5所示，金属的断裂应变在很大程度上取决于应变率。当金属的应变率为log=3时，可以在0.45的应变下发生断裂，而相同的金属如果应变率为log=4时可以在更低的应变0.25时发生断裂。应变率的增加可以进一步减小断裂应变，然而，应变速率提高到log>6时是不利的。因为存在最优应变率，所以可以得到最低的断裂韧性和最小的断裂应变。换句话说，我们可以用最佳的应变率来切除材料。如果金属以一个合适的应变率变形，那么它的断裂韧性可以减小。断裂韧性的下降意味着金属加工中所需更少的能量。如果振幅和频率选择合适的话，振动辅助攻丝能在切削区产生最佳的应变率，因此在材料去除过程中需要

11、更少的能量以及更小的扭矩。3.讨论攻丝扭矩源于工件对丝锥的抗力。它包括两个主要的组成部分：切削阻力和摩擦阻力。前者会因不同的应变率而变化很大，而后者会因材料的低弹性模量而变大。攻丝中，弹性变形可能产生很大的弹性恢复，从而增大攻丝扭矩。另外，摩擦是一种弹性现象，它可以在不去除材料的情况下显著提高攻丝扭矩。丝锥与工件接触后期所引起的摩擦力的增加而产生的扭矩是初期扭矩的好几倍。攻丝扭矩的增加通常会造成丝锥的折断，这在小深孔攻丝中是经常遇到的现象。振动可以减小丝锥和工件之间的摩擦，减轻工件材料的弹性恢复，因为振动不仅可以改变接触条件，而且还能反复压紧工件。在具有适当的频率和振幅的振动的作用下，摩擦力大

12、大的降低了，最终减小了摩擦力矩。然而，当振幅太大或是太小的时候，该作用就不明显了。例如，当施加了极小振幅的振动，丝锥和工件间的的接触条件没有得到足够的改变，振动在减小攻丝扭矩方面的作用就微乎其微了。与此相反，当使用了振幅极大的振动，工件材料可能会发生过度的变形，从而引起扭矩的增大。试验结果证明了振动的振幅过大或是过小对减小攻丝扭矩都是不利的。而且，振动频率的选择应该遵循机械系统的动态特性。此外，振动频率的选择应当有效的抑制弹性恢复和减少摩擦。众所周知，当材料承受不同的应变率的变形时，它们的特性例如断裂强度，断裂应变等也会变的各不相同。一般而言，高应变率是减小攻丝扭矩的首选。这完全是因为在高的应

13、变率下，金属会更加的脆弱，使得在机械加工时减小阻力，从而降低攻丝扭矩。此外，金属断裂应力相应的降低，这又意味着更小的加工阻力和更低的攻丝扭矩。遗憾的是，现有的处理材料应变率的方法在改变材料性能方面的影响是有限的。利用振动来减小机械加工阻力的方法还需要进行深入的研究。4 结语本文主要研究的是辅助振动攻丝中的一些基本的问题。并且对振动在减小攻丝扭矩方面的作用通过多种试验进行了理论分析。基于此，可以总结以下几点：(1) 对于已给定频率的振动，存在一个使攻丝扭矩最小的振动频率。另一方面，对于已给定振幅的振动，存在一个使攻丝扭矩最小的振幅。(2) 丝锥规格越小，攻丝扭矩减小的越多，反之亦然。(3) 攻丝

14、中使用振动总是可已达到减小扭矩的目的。(4) 振动的波形对扭矩的减小也有影响。在减小扭矩方面，方波比正弦波的效果更好。参考文献1 钛合金振动攻丝的研究，机械工业出版社，30(1)(1994)18-222 钛合金振动攻丝的参数优化，航空宇航技术研究所，13（10）（1992）b571 - b5733 光加藤，微观摩擦机制的超声波驱动，19964 铃木，米宇野，对天然橡胶振动攻丝的研究，19945 塔博尔，摩擦和固体润滑第一卷，牛津大学，19506 金属力学性能，机械工业出版社，1979附件2：外文原文（复印件）fundamental aspects in vibration-assisted t

15、appingabstractalthough tapping has been widely used for thread fabrication, it is often a time-consuming process causing a delay on an automated production line. this study investigates the fundamental aspects in vibration-assisted tapping. it addresses two major mechanisms that are responsible for

16、torque reduction during vibration-assisted tapping, i.e., friction torque reduction and material property change at an enhanced strain rate. theoretical analyses are conducted to account for the effect of vibration on tapping torque reduction, and verified by the tapping experiments with and without

17、 vibration assistance. taps of three different sizes and vibrations of two different waveforms are used in the tapping experiments. the theoretical results are applicable to vibration-assisted tapping and other machining processes.1. introductionamong all the machining processes practiced in modern

18、industry, threading is one of the most time-consuming and, therefore, is often the bottleneck causing a reduced productivity of a production line. on the other hand, with the rapid development of modern manufacturing technologies for both defense and civil industries, such as aircraft and automotive

19、, the demand for large quantity, high quality and high accuracy threaded holes, especially for those with a small diameter and large depth (e.g.,m3.5mm x 0.6mm with 10-25mm depth), has increased. unfortunately, current threading techniques cannot fully satisfy this demand, which creates a need for a

20、n efficient and inexpensive threading technology.tapping process is most widely used for the production of internal threads on a mass scale. problems are frequently encountered by the sudden breakage of taps during cutting that necessitates the salvaging of the components being tapped. in particular

21、, it is observed that small taps (below m6) fail more frequently during blind hole tapping. a need, therefore, exists to devise ways for enhancing the life of such small taps. furthermore, in the manufacturing industry, high performance materials such as titanium alloys are widely used. this type of

22、 materials usually has a low modulus of elasticity, which makes them very difficult to machine. when tapping such type of materials, premature breakage of taps often becomes a problem due to the large amount of elastic recovery of the machined surface. the elastic recovery creates a friction torque

23、seven times larger as compared to tapping plain carbon steels1,2.the increased friction torque can thus result in the breakage of a tap. vibration-assisted tapping has been applied to titanium alloys and other materials to reduce the friction torque and thus increase the tapping efficiency and reduc

24、e the overall tapping torque1-5. promising results have been obtained, such as considerable reduction in friction torque, large increase in tap lifetime, and great improvement in thread accuracies.zhang and chen 2built a vibration-assisted tapping apparatus consisting of a stepping motor to apply vi

25、bration during tapping at a frequency of 24 khz and an amplitude of 400mm.it turned out that with the vibration-assisted tapping, tapping torque was reduced by two-thirds, whereas the tap lifetime increased. kumabe and tachibana6developed a test setup to introduce vibration of frequencies 46-167 khz

26、 for tapping aluminium, brass and steel. it was reported that the tapping torque was reduced from 1/4 to 2/3 of the conventional tapping torque. saotoma et al.7obtained an increase in tap life(up to 10,000 pieces per tool)by introducing vibration of frequency 20 khz and amplitude 15 mm. patil et al.

27、8designed and developed an attachment to generate vibration in a controlled manner for tapping. investigations were carried out to study the influence of different process conditions, such as tap size, workpiece material, vibration amplitude and frequency, on tapping torque and thrust during machini

28、ng. optimum conditions were found to enhance the tap life. suzuki et al.4used ultrasonic vibration to assist the tapping process on pure aluminium, and found that chip thickness was reduced from 250 to 100 mm and tapping torque by 20 times; while the cutting ratio was tripled. because of the reduced

29、 torque and chip thickness, the surface integrity and accuracies of machined threads were significantly improved.based on the past research results, it is evident that vibration-assisted tapping can result in a significant torque reduction. consequently, productivity can be greatly enhanced, tool li

30、fetime increased and thread quality improved. however, only little information is found on what mechanisms cause tapping torque reduction and how they work. this study investigates the fundamental aspects of tapping torque reduction. it provides theoretical analyses on two major mechanisms for tappi

31、ng torque reduction, i.e., friction reduction and material property change due to vibration. a tapping experiment is also conducted to verify the theoretical analyses.2. theoretical backgroundthe toughness reduction of workpiece material and the friction reduction due to vibration application are th

32、e two major mechanisms that directly contribute to the reduction of tapping torque, the improvement of thread quality and accuracies, and the lifetime of a tap. as friction contributes to the built-up of tapping torque, a reduction in tapping torque can be realised by reducing friction and/or the el

33、astic recovery of machined threads through the application of vibration assistance.2.1 friction force under vibrationfig.1 shows a friction couple model in which body 1 slides against body 2.a static normal load w acts on body 1 that is vibrated in vertical direction during sliding motion.the relati

34、ve motion velocity between the two bodies is v. the contact load changes corresponding to the vertical vibration. if this contact load is defined as the dynamic normal load wd and the vertical vibration is expressed as:then the dynamic normal load wd is given as follows9:where a is the vibration amp

35、litude, the angular frequency, and c a constant related to the friction couple material.however, in the case of w<ca, the value of wd becomes negative. this means that the two bodies can separate periodically. therefore, when w<ca, the dynamic normal load wd is given as follows(see fig.2):wher

36、e t1 and t2 represent the starting and finishing time of contact, respectively, for a given periodic time t, t1 and t2 satisfy the following equations:friction force ft is expressed by the product of dynamic friction coefficient ft and dynamic normal load wd as follows:therefore, the average frictio

37、n force can be expressed as:it has been shown by bowden and tabor 10 that coefficient of friction increases with micro-displacement in the micro-slip region. based on this relationship, the relationship between coefficient ft and relative displacement ut can be simplified as:where c1 is a constant v

38、alue for one friction couple. the relative displacement is:with the consideration of the conditions in this investigation, and eqs.(9)and(10),the coefficient is given as follows:where t3 is the time when the relative displacement attains the value of u1.by introducing eqs.(3),(10)and(11)into eq.(8)

39、and considering t1,t2 and t3, friction force f is described aseq.(13)is complicated, but it describes the relationship between average friction force and vibration amplitude and frequency. if the parameters in eq.(13)are given, the above relationship can be simulated using a computer.fig.3 shows the

40、 simulation results with the simulation parameters shown in the figure. it is seen that with the application of vibration, the average friction torque is always smaller than 2.5 n, i.e., the average friction torque without vibration under the chosen vibration frequencies and amplitudes. the friction

41、 force decreases as vibration frequency increases if vibration frequency is between about 4 and 35 khz. the friction force remains almost constant if vibration frequency is smaller than 4 khz or larger than 35 khz. there is optimum amplitude that makes the friction force the smallest. the friction f

42、orce increases with amplitude when amplitude is larger than 2 mm whereas it decreases if amplitude is smaller than 2 mm. the optimum amplitude is around 2 mm. the effect of vibration frequency and amplitude on friction force is an important mechanism in torque reduction during a vibration-assisted t

43、apping process.2.2 tapping torque under friction reductionelastic deformation can produce a significant amount of elastic recovery during tapping that can cause the tapped threads to rub against the tap and, therefore, generates a large friction torque. in order to suppress the elastic recovery duri

44、ng tapping, a theoretical analysis has been proposed based on the theories of elasticity and plasticity2. according to the analysis, the maximum friction torque in tapping can be expressed by the following equation:where ts is the shear yield stress of workpiece, r the radius of tap, b the cutting e

45、dge width, n the number of times the flank face of a tap rubbing against the machined threads, k a constant,l0 the contact length between the flank face and tapped threads, t the forward cutting distance of each stroke and lg the backward length of each stroke. this equation shows that a reduced fri

46、ction torque in tapping can be obtained by decreasing forward cutting distance lt and increasing backward length lg of each stroke of vibration, if other conditions remain unchanged.2.3 strain rate effectthe behavior of a material can be different if the material is subjected to deformation at a hig

47、h speed, i.e., a high strain rate. in general, the fracture toughness of a metal decreases with an increasing strain rate.fig.4 shows the variation of fracture toughness kic of a semi-killed steel with strain rate. it is noted that as strain rate increases by an order of magnitude, fracture toughnes

48、s kic decreases by approximately 10%, an effect similar to the low temperature brittleness of a metal. on the other hand, as strain rate further increases over approximately log=10, the fracture toughness of the steel increases rapidly. this is mainly because the material is toughened by the heat du

49、e to the high strain rate of the material. in this situation, a nearly adiabatic condition can be generated in the deformation zone. likewise, the strain to fracture of a metal is heavily dependent on strain rate, as shown in fig.5. a metal can fracture under a strain of 0.45 if it is deformed at a

50、strain rate of log=3, whereas the same metal may fracture under a much lower strain of 0.25 if it is subjected to a strain rate of log=4. a further increase in strain rate will further reduce the strain to fracture, however, it is not advantageous to increase strain rate to a level of log>6.since

51、 from the existing literature, there exists optimum strain rate at which the lowest fracture toughness and the minimum strain to fracture can be expected. in other words, material removal can be accomplished at the optimum strain rate.the fracture toughness of a metal can decrease if the metal is de

52、formed at an appropriate strain rate. a reduced fracture toughness means less energy requirement in machining the metal. less energy, and thus less torque should be needed to accomplish material removal since vibration-assisted tapping can generate optimum strain rate in the cutting zone if vibratio

53、n amplitude and frequency are properly chosen.3 discussiontapping torque results from resistance of the workpiece to the tap. it comprises two major portions, cutting resistance and frictional resistance. the former can be very different if a metal is tapped under different strain rates, while the l

54、atter can be large if a material of a low modulus of elasticity is tapped. elastic deformation can generate a significant amount of elastic recovery during tapping that causes the tapped threads to rub against the tap and therefore, contributes to the increment of tapping torque. on the other hand,

55、since rubbing is an elastic phenomenon, it can significantly increase tapping torque without producing material removal. the tapping torque due to rubbing increases with the increase of tap engagement in workpiece, and can be several times more when at the end than at the beginning of tap engagement

56、. the increased tapping torque often vcauses a tap to break, a phenomenon frequently encountered in tapping small and deep holes.the function of vibration is to reduce friction between the tap and workpiece and mitigate elastic recovery of workpiece material because vibration alters the contact cond

57、itions between the tap and workpiece, and presses the workpiece repeatedly. with the application of appropriate frequency and amplitude of vibration, friction can be greatly reduced, which results in a reduced friction torque. however, the effect of vibration on friction reduction may not be obvious

58、 if vibration amplitude is too large or too small. for example, when vibration of very small amplitude is applied, the contact conditions between the tap and workpiece may not be altered enough so that vibration provides little effect on tapping torque reduction. in contrast, when vibration of very large amplitude is use