金屬切削外文翻譯

1、<p>　　METAL CUTTING</p><p>　　The importance of machining processes can be emphasised by the fact that every product we use in our daily life has undergone this process either directly or indirectly.<

2、/p><p>　　(1) In USA, more than $100 billions are spent annually on machining and related operations.</p><p>　　(2) A large majority (above 80%) of all the machine tools used in the manufacturing ind

3、ustry have undergone metal cutting.</p><p>　　(3) An estimate showed that about 10 to 15% of all the metal produced in USA was converted into chips.</p><p>　　These facts show the importance of me

4、tal cutting in general manufacturing. It is therefore important to understand the metal cutting process in order to make the best use of it. A number of attempts have been made in understanding the metal cutting process

5、and using this knowledge to help improve manufacturing operations which involved metal cutting.</p><p>　　A typical cutting tool in simplified form is shown in Fig.7.1. The important features to be observed a

6、re follows.</p><p>　　1. Rake angle. It is the angle between the face of the tool called the rake face and the normal to the machining direction. Higher the rake angle, better is the cutting and less are the

7、 cutting forces, increasing the rake angle reduces the metal backup available at the tool rake face.This reduces the strength of the tool tip as well as the heat dissipation through the tool. Thus, there is a maximum lim

8、it to the rake angle and this is generally of the order of 15°for high speed steel tools cutti</p><p>　　2. Clearance angle. This is the angle between the machined surface and the underside of the tool c

9、alled the flank face. The clearance angle is provided such that the tool will not rub the machined surface thus spoiling the surface and increasing the cutting forces. A very large clearance angle reduces the strength of

10、 the tool tip, and hence normally an angle of the order of 5~6°is used.</p><p>　　The conditions which have an important influence on metal cutting are work material, cutting tool material, cutting too

11、l geometry, cutting speed, feed rate, depth of cut and cutting fluid used.</p><p>　　The cutting speed, v, is the speed with which the cutting tool moves through the work material. This is generally expres

12、sed in metres per second (ms-1).</p><p>　　Feed rate, f, may be defined as the small relative movement per cycle (per revolution or per stroke) of the cutting tool in a direction usually normal to the cuttin

13、g speed direction.</p><p>　　Depth of cut, d, is the normal distance between the unmachined surface and the machined surface.</p><p>　　Chip Formation</p><p>　　Metal cutting process i

14、s a very complex process. Fig.7.2 shows the basic material removal operation schematically.The metal in front of the tool rake face gets immediately compressed, first elastically and then plastically. This zone is tradit

15、ionally called shear zone in view of fact that the material in the final form would be removed by shear from the parent metal. The actual separation of the metal starts as a yielding or fracture, depending upon the cutti

16、ng conditions, starting from the cutt</p><p>　　Plastic deformation can be caused by yielding, in which case strained layers of material would get displaced over other layers along the slip-planes which coin

17、cide with the direction of maximum shear stress.</p><p>　　A chip is variable both in size and shape in actual manufacturing practice. Study of chips is one of the most important things in metal cutting. As

18、would be seen later, the mechanics of metal cutting are greatly dependent on the shape and size of the chips produced.</p><p>　　Chip formation in metal cutting could be broadly categorised into three types:

19、 (Fig.7.3)</p><p>　　(1) Discontinuous chip</p><p>　　(2) Continuous chip</p><p>　　(3) Continuous chip with BUE (Built up edge)</p><p>　　Discontinuous Chip. The segmented

20、 chip separates into short pieces, which may or may not adhere to each other. Severe distortion of the metal occurs adjacent to the face, resulting in a crack that runs ahead of the tool. Eventually, the shear stress acr

21、oss the chip becomes equal to the shear strength of the material, resulting in fracture and separation. With this type of chip, there is little relative movement of the chip along the tool face, Fig.7.3a.</p><

22、p>　　Continuous chip. The continuous chip is characterized by a general flow of the separated metal along the tool face. There may be some cracking of the chip, but in this case it usually does not extend far enough to

23、 cause fracture.This chip is formed at the higher cutting speeds when machining ductile materials. There is little tendency for the material to adhere to the tool. The continuous chip usually shows a good cutting ratio

24、and tends to produce the optimum surface finish, but it may becom</p><p>　　Continuous with a built-up edge. This chip shows the existence of a localized, highly deformed zone of material attached or “welde

25、d” on the tool face. Actually, analysis of photomicrographs shows that this built-up edge is held in place by the static friction force until it becomes so large that the external forces acting on it cause it to dislodge

26、, with some of it remaining on the machined surface and the rest passing off on the back side of the chip, Fig.7.3c.</p><p>　　Shear Zone</p><p>　　There are basically two schools of thought in th

27、e analysis of the metal removal process. One school of thought is that the deformation zone is very thin and planar as shown in Fig.7.4a. The other school thinks that the actual deformation zone is a thick one with a fan

28、 shape as shown in Fig.7.4b.</p><p>　　Though the first model (Fig.7.4a) is convenient from the point of analysis, physically it is impossible to exist. This is because for the transition from undeformed mat

29、erial to deform to take place along a thin plane, the acceleration across the plane has to be infinity.Similarly the stress gradient across the shear plane has to be very large to be practical. </p><p>　　In

30、the second model (Fig.7.4b) by making the shear zone over a region, the transitions in velocities and shear stresses could be realistically accounted for.</p><p>　　The angle made by the shear plane with t

31、he cutting speed vector, Φ is a very important parameter in metal cutting. Higher the shear angle better is the cutting performance. From a view of the Fig.7.4a, it can be observed that a higher rake angles give rise to

32、higher shear angles. Cutting Tool Materials</p><p>　　Various cutting tool materials have been used in the industry for different applications. A number of developments have occurred in the current century. A

33、 large variety of cutting tool materials has been developed to cater to the variety of materials used in these programmes. Before we discuss the properties of these materials, let us look at the important characteristics

34、 expected of a cutting tool material.</p><p>　　1. Higher hardness than that of the workpiece material being machined, so that it can penetrate into the work material.</p><p>　　2. Hot hardness, w

35、hich is the ability of the material to retain its hardness at elevated temperatures in view of the high temperatures existing in the cutting zone.</p><p>　　3. Wear resistance—The chip-tool and chip-work inte

36、rfaces are exposed to such severe conditions that adhesive and abrasion wear is very common. The cutting tool material should therefore have high abrasion resistance to improve the effective life of the tool.</p>

37、<p>　　4. Toughness—Even though the tool is hard, it should have enough toughness to withstand the impact loads that come in the beginning of cut or force fluctuations due to imperfections in the work material. This

38、requirement is going to be more useful for the interrupted cutting, e.g. milling.</p><p>　　5. Low friction—The coefficient of friction between the chip and tool should be low. This would allow for lower wear

39、 rates and better chip flow.</p><p>　　6. Thermal characteristics—Since a lot of heat is generated at the cutting zone, the tool material should have higher thermal conductivity to dissipate this heat in the

40、shortest time, otherwise the tool temperature would become high, reducing its life.</p><p>　　All these characteristics may not be found in a single tool material. Improved tool materials have been giving a b

41、etter cutting performance.</p><p>　　Surface Finish</p><p>　　Machining operations are utilized in view of the better surface finish that could be achieved by it compared to other manufacturing op

42、erations.Thus it is important to know what would be the effective surface finish that can be achieved in a machining operation. The surface finish in a given machining operation is a result of two factors:</p><

43、;p>　　(1) the ideal surface finish, which is a result of the geometry of the manufacturing process which can be determined by considering the geometry of the machining operation, and</p><p>　　(2) the nat

44、ural component, which is a result of a number of uncontrollable factors in machining, which is difficult to predict.</p><p>　　Ideal Surface Finish in Turning</p><p>　　The actual turning tool u

45、sed would have a nose radius in place of the sharp tool point, which modifies the surface geometry as shown in Fig.7.5a. If the feed rate is very small, as is normal in finish turning, the surface is produced purely by

46、 the nose radius alone as shown in Fig.7.5.</p><p>　　For the case in Fig.7.5, the surface roughness value is to be</p><p>　　Ra=8f2/(18R√3)</p><p>　　Where: Ra is the surface roughnes

47、s value</p><p>　　R is the nose radius</p><p>　　f is the feed rate</p><p>　　The above are essentially geometric factors and the values represent an ideal situation. The actual surfac

48、e finish obtained depends to a great extent upon a number of factors such as:</p><p>　　(1) the cutting process parameter, speed, feed and depth of cut</p><p>　　(2) the geometry of the cutting to

49、ol</p><p>　　(3) application of cutting fluid</p><p>　　(4) work and tool material characteristics</p><p>　　(5) rigidity of the machine tool and the consequent vibrations.</p>

50、<p>　　The major influence on surface finish is exerted by the feed rate and cutting speed. As the feed decreases, from the above equations, we can see that the roughness index decreases. Similarly as the cutting sp

51、eed increases, we have better surface finish. Thus while making a choice of cutting process parameters for finish, it is desirable to have high cutting speed and small feed rates.</p><p>　　Cutting Fluids <

52、;/p><p>　　The functions of cutting fluids (which are often erroneously called coolants) are:</p><p>　　To cool the tool and workpiece </p><p>　　To reduce the friction</p><p&g

53、t;　　To protect the work against rusting</p><p>　　To improve the surface finish</p><p>　　To prevent the formation of built-up edge</p><p>　　To wash away the chips from the cutting zo

54、ne</p><p>　　However, the prime function of a cutting fluid in a metal cutting operation is to control the total heat. This can be done by dissipating the heat generated as well as reducing it. The mechanisms

55、 by which a cutting fluid performs these functions are: cooling action and lubricating action.</p><p>　　Cooling action. Originally it was assumed that cutting fluid improves the cutting performance by its co

56、oling properties alone. That is why the name coolant was given to it.Since most of the tool wear mechanisms are thermally activated, cooling the chip tool interface helps in retaining the original properties of the tool

57、 and hence prolongs its life. However, a reduction in the temperature of the workpiece may under certain conditions increase the shear flow stress of the workpiece, thereby decr</p><p>　　Lubricating action.

58、The best improvement in cutting performance can be achieved by the lubricating action since this reduces the heat generated, thus reducing the energy input to the metal cutting operation. However, if the cutting fluid

59、is to be effective, it must reach the chip tool interface. But it is not easy to visualize how it is accomplished in the case of a continuous turning with a single point turning tool, specially when the chip-tool contact

60、 pressure is as high as 70 MPa. Merchant</p><p>　　金屬切削 </p><p>　　機(jī)加工過程的重要性可通過日常生活使用的每件產(chǎn)品都直接或間接經(jīng)歷這一過程的事實(shí)來強(qiáng)調(diào)。</p><p>　　(1)在美國，每年花在機(jī)加工及其相關(guān)作業(yè)上的費(fèi)用都多于千億美元。</p>

61、;<p>　　(2) 用于制造業(yè)的全部機(jī)床中的大多數(shù)(多于80%)都經(jīng)歷過金屬切削。</p><p>　　(3) 有估計顯示美國生產(chǎn)的所有金屬中約10到15%轉(zhuǎn)變成了切屑。 </p><p>　　這些事實(shí)說明了金屬切削在常規(guī)制造中的重要性。因此了解金屬切削過程以充分利用它是重要的。在了解金屬切削過程并運(yùn)用這些知識幫助改善與金屬切削有關(guān)的制造作業(yè)方面已經(jīng)做了許多努力。</

62、p><p>　　典型切削刀具的簡化形式如圖7.1所示。要注意的重要特征如下。</p><p>　　1.前角：它是被稱為前傾面的刀具面與垂直機(jī)加工方向的夾角。前角越大，則切削越好且切削力越小，增加前角可以減少刀具前傾面上產(chǎn)生的金屬阻塞。但這會和減少通過刀具散發(fā)的熱量一樣減少刀尖強(qiáng)度。因此前角有一最大限制，用高速鋼刀具切削低碳鋼通常為15°。前角取零度或負(fù)值也是可能的。</p>

63、;<p>　　2. 后角：這是機(jī)加工面與被稱為后側(cè)面的刀具底面夾角。后角使刀具不產(chǎn)生會損壞機(jī)加工面的摩擦和增加切削力。很大的后角會削弱刀尖的強(qiáng)度，因此一般采用5~6°的后角。</p><p>　　對金屬切削有重要影響的條件有工件材料、刀具材料、刀具幾何形狀、切削速度、進(jìn)給率、切削深度和所用的切削液。</p><p>　　切削速度v指切削刀具經(jīng)過工件材料的移動速度。通

64、常用米每秒 (ms-1)表示。</p><p>　　進(jìn)給率f可定義為每循環(huán)(每轉(zhuǎn)或每行程)切削刀具在通常為垂直于切削速度方向的次要相對運(yùn)動。</p><p>　　切削深度d是未加工面與已加工面之間的垂直距離。</p><p><b>　　切屑的形成</b></p><p>　　金屬切削過程是一個很復(fù)雜的過程。圖7.2用圖

65、的形式顯示了基本材料去除作業(yè)。在刀具前傾面前的金屬直接受到壓縮，首先彈性變形然后塑性變形。考慮到最終形狀中的材料是通過剪切從母體金屬去除的，此區(qū)域傳統(tǒng)上稱為剪切區(qū)。</p><p>　　金屬的實(shí)際分離始于屈服或斷裂(視切削條件而定)，從切削刀尖開始。然后變形金屬(稱為切屑)流過刀具(前傾)面。如果刀具前傾面與切屑(變形金屬)底面之間的摩擦相當(dāng)大，那么切屑進(jìn)一步變形，這也叫做二次變形?；^刀具前傾面的切屑被提升離開

66、刀具，切屑彎曲的結(jié)果被稱為切屑卷。</p><p>　　屈服能導(dǎo)致塑性變形，在這種情況下材料變形層沿著與最大剪應(yīng)力方向一致的滑移面被其它層所取代。</p><p>　　在實(shí)際加工過程中切屑的尺寸和形狀都是變化的。對切屑的研究是金屬切削最重要的事情之一。如同后面將要看到的那樣，金屬切削力學(xué)極大地依賴于所產(chǎn)生切屑的形狀和尺寸。</p><p>　　金屬切削中的切屑形成可

67、以寬泛地分成三個類型(圖7.3)：</p><p><b>　　(1)間斷切屑</b></p><p><b>　　(2)連續(xù)切屑</b></p><p>　　(3)帶切屑瘤的連續(xù)切屑</p><p>　　間斷切屑：分段的切屑分散成小碎片，既可能相互附著也可能不相互附著。在靠近切削面處發(fā)生金屬的劇烈

68、變形，導(dǎo)致在運(yùn)動刀具前方金屬層產(chǎn)生裂縫。最后，橫過切屑的剪切應(yīng)力與材料的剪切強(qiáng)度相等，造成斷裂和分離。生成這類切屑時，切屑沿刀具面幾乎沒有相對運(yùn)動，見圖7.3a。</p><p>　　連續(xù)切屑：連續(xù)的切屑一般具有分離金屬沿刀具面流動的特征。切屑可能有一些破裂，但在這種情況下切屑通常不會延長到足以引起斷裂。這種切屑形成于用較高切削速度機(jī)加工有延展性的材料時。材料幾乎沒有粘附刀具的傾向。連續(xù)切屑通常具有良好的切削率和

69、趨向于產(chǎn)生最適宜的表面光潔度，但可能成為操作的危險之源，見圖7.3b。</p><p>　　帶切屑瘤的連續(xù)切屑：這種切屑顯示了粘合或“焊接”在刀具面上材料局部高度變形區(qū)的存在。實(shí)際上，對顯微照片的分析顯示這種切屑瘤受到靜摩擦力抑制直至它變得大到作用在它上面的外力使其移動，一些留在機(jī)加工表面上而另一些延伸到切屑的背面，見圖7.3c。</p><p><b>　　剪切區(qū)</b&

70、gt;</p><p>　　在對金屬去除過程的分析中主要存在兩種思想學(xué)派。一種思想學(xué)派認(rèn)為變形區(qū)如圖7.4a所示那樣非常薄而平坦。另一學(xué)派則認(rèn)為真實(shí)變形區(qū)象圖7.4b所示那樣為一厚的帶有扇形的區(qū)域。</p><p>　　雖然第一種模型(圖7.4a)從分析的角度看是方便的，但實(shí)際上是不可能存在的。這是由于未變形的材料沿著剪切面發(fā)生變形，而且越過剪切面的加速度無窮大。同樣在實(shí)際運(yùn)用中越過剪切面

71、的應(yīng)力梯度必須很大才行。</p><p>　　在第二種模型(圖7.4b)中讓剪力區(qū)分布于一個范圍，速度和剪應(yīng)力的轉(zhuǎn)變能說明得更符合實(shí)際。</p><p>　　由剪切面和切削速度矢量形成的角度Φ在金屬切削中是一個十分重要的參數(shù)。剪切角越大，切削作業(yè)越好。從圖7.4a觀察，可以看到較大的前角能增大剪切角。</p><p><b>　　切削刀具材料</b&

72、gt;</p><p>　　在工業(yè)中為了不同的應(yīng)用可以使用各種各樣的切削刀具材料。在最近的百年里產(chǎn)生了許多進(jìn)展。多種切削刀具材料被開發(fā)出來以滿足這些方案中使用材料的多樣性。討論這些材料性能之前，先看一下作為切削刀具材料應(yīng)具備哪些重要特性。</p><p>　　1. 硬度要比被切削工件材料高，這樣它才能進(jìn)入工件材料。</p><p>　　2. 熱硬度，即材料由于存在于

73、切削區(qū)的高溫而升溫時仍能保持其硬度的能力。</p><p>　　3. 耐磨性—切屑-刀具與切屑-工件的接觸界面處于如此嚴(yán)酷的狀態(tài)，粘附和磨損是很普遍的。因此切削刀具材料應(yīng)具有高耐磨性以提高刀具的有效壽命。</p><p>　　4. 韌性—雖然刀具是堅(jiān)硬的，但也應(yīng)有足夠的韌性以經(jīng)受住沖擊載荷，這些載荷來自于切削的開始或由于工件材料的缺陷而產(chǎn)生的作用力波動。這個要求對如銑削之類的間斷切削更有用

74、。</p><p>　　5. 低摩擦系數(shù)—切屑與刀具間的摩擦系數(shù)應(yīng)當(dāng)較低。這會使磨損率較小及切屑流動更好。</p><p>　　6. 熱特性—因?yàn)榇罅康臒岙a(chǎn)生在切削區(qū)，刀具材料應(yīng)當(dāng)具有較高的熱傳導(dǎo)性以在最短的時間內(nèi)散發(fā)熱量，否則刀具溫度會升高，壽命會減少。</p><p>　　所有這些特性不可能存在于單一刀具材料中。改進(jìn)的刀具材料已經(jīng)被賦予較好的切削性能。</

75、p><p><b>　　表面光潔度</b></p><p>　　由于機(jī)加工能獲得比其它制造作業(yè)更好的表面光潔度，所以機(jī)加工作業(yè)具有實(shí)用價值。因而了解能在機(jī)加工作業(yè)中獲得怎樣的實(shí)際表面光潔度是重要的。給定機(jī)加工作業(yè)中的表面光潔度是兩個因素共同作用的結(jié)果：</p><p>　　(1)理想的表面光潔度，是通過考慮機(jī)加工作業(yè)的幾何體系所決定的制造工藝幾何學(xué)

76、的結(jié)果，和</p><p>　　(2)自然要素，即在機(jī)加工中一些難以預(yù)測的不可控因素作用的結(jié)果。</p><p><b>　　I</b></p><p>　　車削中的理想表面光潔度</p><p>　　實(shí)際使用的車削刀具有一個刀尖半徑取代鋒利刀尖，它將表面幾何形狀加工為如圖7.5a所示。如果進(jìn)給率很小，象精車中很正常的那

77、樣，工件表面則完全是由刀尖半徑單獨(dú)產(chǎn)生的，如圖7.5所示。</p><p>　　對圖7.5這種情況，表面粗糙度值為</p><p>　　Ra=8f2/(18R√3)</p><p>　　式中：Ra是表面粗糙度值</p><p><b>　　R是刀尖半徑</b></p><p><b>　

78、　f是進(jìn)給率</b></p><p>　　上述基本為幾何要素，其值代表了理想情況。而實(shí)際獲得的表面光潔度很大程度上還取決于下列一些因素：</p><p>　　(1)切削工藝參數(shù)、速度、進(jìn)給和切削深度</p><p>　　(2)切削刀具的幾何形狀</p><p><b>　　(3)切削液的運(yùn)用</b></

79、p><p>　　(4)工件和刀具的材料特性</p><p>　　(5)機(jī)床的剛度及其伴隨發(fā)生的振動</p><p>　　對表面光潔度產(chǎn)生主要影響的是進(jìn)給率和切削速度。從上述公式可以看到，隨著進(jìn)給的減少，粗糙度指標(biāo)會降低。同樣隨著切削速度的增大，能得到較好表面光潔度。因此在為光潔度而選擇切削工藝參數(shù)時，采用較高的切削速度和較小的進(jìn)給率是適當(dāng)?shù)摹?lt;/p>&l

80、t;p><b>　　切削液</b></p><p>　　切削液(經(jīng)常誤稱為冷卻液)的功能如下：</p><p><b>　　冷卻刀具和工件</b></p><p><b>　　減少摩擦</b></p><p><b>　　保護(hù)工件不生銹</b><

81、;/p><p><b>　　改善表面光潔度</b></p><p><b>　　防止切屑瘤的形成</b></p><p><b>　　從切削區(qū)沖掉切屑</b></p><p>　　然而，在金屬切削作業(yè)中切削液的主要功能是控制總熱量。這可通過既散發(fā)又減少所產(chǎn)生的熱量來達(dá)到。切削液實(shí)現(xiàn)

82、這些功能的機(jī)理是：冷卻作用和潤滑作用。</p><p>　　冷卻作用：最初設(shè)想切削液僅僅是通過冷卻特性來改善切削作業(yè)。這也是它曾被稱為冷卻液的原因。由于大多數(shù)刀具的磨損機(jī)理都是由熱引起的，冷卻切屑刀具接觸界面有助于保持刀具的原有特性，從而延長其使用壽命?？墒枪ぜ囟鹊慕档驮谔囟l件下會增加工件的剪切流動應(yīng)力，從而降低刀具壽命。通過一些研究已經(jīng)表明實(shí)際上冷卻只是改善切削作業(yè)的主要因素之一。</p>&

83、lt;p>　　. 潤滑作用：切削作業(yè)的最大改善可通過潤滑作用來達(dá)到，由于它減少了熱量的產(chǎn)生因而減少了金屬切削作業(yè)的能量輸入。可是，如果要使切削液起作用就必須讓它到達(dá)切屑刀具接觸界面。但如何在采用單尖刀具連續(xù)車削的場合尤其是切屑-刀具接觸壓力高達(dá)70MPa時實(shí)現(xiàn)并非易事。Merchant認(rèn)為：在切屑與刀具接觸界面上存在微小的粗粒，切削液通過這些表面的微小粗粒組成連鎖的網(wǎng)絡(luò)的毛細(xì)管被吸入到切屑與刀具的接觸界面上。</p