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譯文題目： The Lathe and Its Operations
車床及其操作
學生姓名： 學 號：
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20xx年 2月 27日
英語原文：
The Lathe and Its Operations
The Lathe and Its Construction
A lathe is a machine tool used primarily for producing surfaces of revolution and flat edges. Based on their purpose, construction, number of tools that can simultaneously be mounted, and degree of automation, lathes-or, more accurately, lathe-type machine tools can be classified as follows:
(1)Engine lathes
(2)Tool room lathes
(3)Turret lathes
(4)Vertical turning and boring mills
(5)Automatic lathes
(6)Special-purpose lathes
In spite of that diversity of lathe-type machine tools, they all have common features with respect to construction and principle of operation. These features can best be illustrated by considering the commonly used representative type, the engine lathe.
Lathe bed. The lathe bed is the main frame, involving a horizontal beam on two vertical supports. It is usually made of grey or nodular cast iron to damp vibrations and is made by casting. It has guide ways to allow the carriage to slide easily lengthwise. The height of the lathe bed should be appropriate to enable the technician to do his or her job easily and comfortably.
Headstock. The headstock is fixed at the left hand side of the lathe bed and includes the spindle whose axis is parallel to the guide ways (the slide surface of the bed). The spindle is driven through the gearbox, which is housed within the headstock. The function of the gearbox is to provide a number of different spindle speeds (usually 6 up to 18 speeds). Some modern lathes have headstocks with infinitely variable spindle speeds, which employ frictional, electrical, or hydraulic drives. The spindle is always hollow, i. e., it has a through hole extending lengthwise. Bar stocks can be fed through that hole if continuous production is adopted. Also, that hole has a tapered surface to allow mounting a plain lathe center. The outer surface of the spindle is threaded to allow mounting of a chuck, a face plate, or the like.
Tailstock. The tailstock assembly consists basically of three parts, its lower base, an intermediate part, and the quill. The lower base is a casting that can slide on the lathe bed along the guideways, and it has a clamping device to enable locking the entire tailstock at any desired location, depending upon the length of the work piece. The intermediate part is a casting that can be moved transversely to enable alignment of the axis of the tailstock with that of the headstock. The third part, the quill, is a hardened steel tube, which can be moved longitudinally in and out of the intermediate part as required. This is achieved through the use of a hand wheel and a screw, around which a nut fixed to the quill is engaged. The hole in the open side of the quill is tapered to enable mounting of lathe centers or other tools like twist drills or boring bars. The quill can be locked at any point along its travel path by means of a clamping device.
The carriage. The main function of the carriage is mounting of the cutting tools and generating longitudinal and/or cross feeds. It is actually an H-shaped block that slides on the lathe bed between the headstock and tailstock while being guided by the V-shaped guideways of the bed. The carriage can be moved either manually or mechanically by means of the apron and either the feed rod or the lead screw. When cutting screw threads, power is provided to the gearbox of the apron by the lead screw. In all other turning operations, it is the feed rod that drives the carriage. The lead screw goes through a pair of half nuts, which are fixed to the rear of the apron. When actuating a certain lever, the half nuts are clamped together and engage with the rotating lead screw as a single nut, which is fed, together with the carriage, along the bed. When the lever is disengaged, the half nuts are released and the carriage stops. On the other hand, when the feed rod is used, it supplies power to the apron through a worm gear. The latter is keyed to the feed rod and travels with the apron along the feed rod, which has a keyway extending to cover its whole length. A modern lathe usually has a quick-change gearbox located under the headstock and driven from the spindle through a train of gears. It is connected to both the feed rod and the lead screw and enables selecting a variety of feeds easily and rapidly by simply shifting the appropriate levers. The quick-change gearbox is employed in plain turning, facing and thread cutting operations. Since that gearbox is linked to the spindle, the distance that the apron (and the cutting tool) travels for each revolution of the spindle can be controlled and is referred to as the feed.
Lathe Cutting Tools
The shape and geometry of the lathe tools depend upon the purpose for which they are employed. Turning tools can be classified into two main groups, namely, external cutting tools and internal cutting tools. Each of these two groups includes the following types of tools:
Turning tools. Turning tools can be either finishing or rough turning tools. Rough turning tools have small nose radii and are employed when deep cuts are made. On the other hand, finishing tools have larger nose radii and are used for obtaining the final required dimensions with good surface finish by making slight depths of cut. Rough turning tools can be right-hand or left-hand types, depending upon the direction of feed. They can have straight, bent, or offset shanks.
Facing tools. Facing tools are employed in facing operations for machining plane side or end surfaces. There are tools for machining left-hand-side surfaces and tools for right-hand-side surfaces. Those side surfaces are generated through the use of the cross feed, contrary to turning operations, where the usual longitudinal feed is used.
Cutoff tools. Cutoff tools, which are sometimes called parting tools, serve to separate the work piece into parts and/or machine external annular grooves.
Thread-cutting tools. Thread-cutting tools have either triangular, square, or trapezoidal cutting edges, depending upon the cross section of the desired thread. Also, the plane angles of these tools must always be identical to those of the thread forms. Thread-cutting tools have straight shanks for external thread cutting and are of the bent-shank type when cutting internal threads.
Form tools. Form tools have edges especially manufactured to take a certain form, which is opposite to the desired shape of the machined work piece. An HSS tool is usually made in the form of a single piece, contrary to cemented carbides or ceramic, which are made in the form of tips. The latter are brazed or mechanically fastened to steel shanks．This latter type includes the carbide tip, the chip breaker, the pad, the clamping screw (with a washer and a nut), and the shank. As the name suggests, the function of the chip breaker is to break long chips every now and then, thus preventing the formation of very long twisted ribbons that may cause problems during the machining operation. The carbide tips (or ceramic tips) can have different shapes, depending upon the machining operations for which they are to be employed. The tips can either be solid or with a central through hole, depending on whether brazing or mechanical clamping is employed for mounting the tip on the shank.
Lathe Operations
In the following section, we discuss the various machining operations that can be performed on a conventional engine lathe. It must be borne in mind, however, that modern computerized numerically controlled lathes have more capabilities and can do other operations, such as contouring, for example. Following are conventional lathe operations.
Cylindrical turning. Cylindrical turning is the simplest and the most common of all lathe operations. A single full turn of the work piece generates a circle whose center falls on the lathe axis; this motion is then reproduced numerous times as a result of the axial feed motion of the tool. The resulting machining marks are, therefore, a helix having a very small pitch, which is equal to the feed. Consequently, the machined surface is always cylindrical. The axial feed is provided by the carriage or the compound rest, either manually or automatically, whereas the depth of cut is controlled by the cross slide. In roughing cuts, it is recommended that large depths of cuts (up to 0.25in. or 6mm, depending upon the work piece material) and smaller feeds would be used. On the other hand, very fine feeds, smaller depths of cut (less than 0.05in, or 0.4mm), and high cutting speeds are preferred for finishing cuts.
Facing. The result of a facing operation is a flat surface that is either the whole end surface of the work piece or an annular intermediate surface like a shoulder. During a facing operation, feed is provided by the cross slide, whereas the depth of cut is controlled by the carriage or compound rest. Facing can be carried out either from the periphery inward or from the center of the work piece outward. It is obvious that the machining marks in both cases take the form of a spiral. Usually, it is preferred to clamp the carriage during a facing operation, since the cutting force tends to push the tool (and, of course, the whole carriage) away from the work piece. In most facing operations, the work piece is held in a chuck or on a face plate.
Groove cutting. In cut-off and groove-cutting operations, only cross feed of the tool is employed. The cut-off and grooving tools, which were previously discussed, are employed.
Boring and internal turning. Boring and internal turning are performed on the internal surfaces by a boring bar or suitable internal cutting tools. If the initial work piece is solid, a drilling operation must be performed first. The drilling tool is held in the tailstock, and the latter is then fed against the work piece.
Taper turning. Taper turning is achieved by driving the tool in a direction that is not parallel to the lathe axis but inclined to it with an angle that is equal to the desired angle of the taper. Following are the different methods used in taper-turning practice:
(1) Rotating the disc of the compound rest with an angle equal to half the apex angle of the cone. Feed is manually provided by cranking the handle of the compound rest. This method is recommended for taper turning of external and internal surfaces when the taper angle is relatively large.
(2) Employing special form tools for external, very short, conical surfaces. The width of the work piece must be slightly smaller than that of the tool, and the work piece is usually held in a chuck or clamped on a face plate. In this case, only the cross feed is used during the machining process and the carriage is clamped to the machine bed.
(3) Offsetting the tailstock center. This method is employed for external taper turning of long work pieces that are required to have small taper angles (less than 8°). The work piece is mounted between the two centers; then the tailstock center is shifted a distance S in the direction normal to the lathe axis.
(4) Using the taper-turning attachment. This method is used for turning very long work pieces, when the length is larger than the whole stroke of the compound rest. The procedure followed in such cases involves complete disengagement of the cross slide from the carriage, which is then guided by the taper-turning attachment.
During this process, the automatic axial feed can be used as usual. This method is recommended for very long work pieces with a small cone angle, i.e., 8°through 10°.
Thread cutting. When performing thread cutting, the axial feed must be kept at a constant rate, which is dependent upon the rotational speed (rpm) of the work piece. The relationship between both is determined primarily by the desired pitch of the thread to be cut. As previously mentioned, the axial feed is automatically generated when cutting a thread by means of the lead screw, which drives the carriage. When the lead screw rotates a single revolution, the carriage travels a distance equal to the pitch of the lead screw.
Consequently, if the rotational speed of the lead screw is equal to that of the spindle (i.e., that of the work piece), the pitch of the resulting cut thread is exactly equal to that of the lead screw. The pitch of the resulting thread being cut therefore always depends upon the ratio of the rotational speeds of the lead screw and the spindle: Pitch of the lead screw/ Desired pitch of work piece=rpm of the work piece/rpm of lead screw=spindle-to-carriage gearing ratio. This equation is useful in determining the kinematic linkage between the lathe spindle and the lead screw and enables proper selection of the gear train between them. In thread cutting operations, the work piece can either be held in the chuck or mounted between the two lathe centers for relatively long work pieces. The form of the tool used must exactly coincide with the profile of the thread to be cut, i.e., triangular tools must be used for triangular threads, and so on.
Knurling. Knurling is mainly a forming operation in which no chips are produced. It involves pressing two hardened rolls with rough file like surfaces against the rotating work piece to cause plastic deformation of the work piece metal. Knurling is carried out to produce rough, cylindrical (or conical) surfaces, which are usually used as handles. Sometimes, surfaces are knurled just for the sake of decoration; there are different types of patterns of knurls from which to choose.
Cutting Speeds and Feed
The cutting speed, which is usually given in surface feet per minute (SFM), is the number of feet traveled in the circumferential direction by a given point on the surface (being cut) of the work piece in 1 minute.
The relationship between the surface speed and rpm can be given by the following equation: SFM=πDN
Where
D=the diameter of the work piece in feet
N=the rpm
The surface cutting speed is dependant primarily upon the material being machined as well as the material of the cutting tool and can be obtained from handbooks, information provided by cutting tool manufacturers, and the like.
Generally, the SFM is taken as 100 when machining cold-rolled or mild steel, as 50 when machining tougher metals, and as 200 when machining softer materials. For aluminum, the SFM is usually taken as 400 or above. There are also other variables that affect the optimal value of the surface cutting speed.
These include the tool geometry, the type of lubricant or coolant, the feed, and the depth of cut. As soon as the cutting speed is decided upon, the rotational speed (rpm) of the spindle can be obtained as follows:
N=SFM/(πD)
The selection of a suitable feed depends upon many factors, such as the required surface finish, the depth of cut, and the geometry of the tool used. Finer feeds produce better surface finish, whereas higher feeds reduce the machining time during which the tool is in direct contact with the work piece.
Therefore, it is generally recommended to use high feeds for roughing operations and finer feeds for finishing operations. Again, recommended values for feeds, which can be taken as guidelines, are found in handbooks and in information booklets provided by cutting tool manufacturers.
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譯文：
車床及其操作
車床及其結(jié)構(gòu)
車床是主要用于生成旋轉(zhuǎn)表面和平整邊緣的機床。根據(jù)它們的使用目的、結(jié)構(gòu)、能同時被安裝刀具的數(shù)量和自動化的程度，車床—或更確切地說是車床類的機床，可以被分成以下幾類：
(1)普通車床
(2)萬能車床
(3)轉(zhuǎn)塔車床
(4)立式車床
(5)自動車床
(6)特殊車床
雖然車床類的機床多種多樣，但它們在結(jié)構(gòu)和操作原理上具有共同特性。這些特性可以通過普通車床這一最常用的代表性類型來最好地說明。
車床床身：車床床身是包含了在兩個垂直支柱上水平橫梁的主骨架。為減振它一般由灰鑄鐵或球墨鑄鐵鑄造而成。它上面有能讓大拖板輕易縱向滑動的導軌。車床床身的高度應適當以讓技師容易而舒適地工作。
主軸箱：主軸箱固定在車床床身的左側(cè)，它包括軸線平行于導軌的主軸。主軸通過裝在主軸箱內(nèi)的齒輪箱驅(qū)動。齒輪箱的功能是給主軸提供若干不同的速度(通常是6到18速)。有些現(xiàn)代車床具有采用摩擦、電力或液壓驅(qū)動的無級調(diào)速主軸箱。主軸往往是中空的，即縱向有一通孔。如果采取連續(xù)生產(chǎn)，棒料能通過此孔進給。同時，此孔為錐形表面可以安裝普通車床頂尖。主軸外表面是螺紋可以安裝卡盤、花盤或類似的裝置。
尾架：尾架總成基本包括三部分，底座、尾架體和套筒軸。底座是能在車床床身上沿導軌滑動的鑄件，它有一定位裝置能讓整個尾架根據(jù)工件長度鎖定在任何需要位置。尾架體為一能橫向運動的鑄件，它可以調(diào)整尾架軸線與主軸箱軸線成一直線。第三部分，套筒軸是一淬硬鋼管，它能根據(jù)需要在尾架體中縱向進出移動。這通過使用手輪和螺桿來達到，與螺桿嚙合的是一固接在套筒軸上的螺母。套筒軸開口端的孔是錐形的，能安裝車床頂尖或諸如麻花鉆和鏜桿之類的工具。套筒軸通過定位裝置能沿著它的移動路徑被鎖定在任何點。
大拖板：大拖板的主要功能是安裝刀具和產(chǎn)生縱向和/或橫向進給。它實際上是一由車床床身V形導軌引導的、能在車床床身主軸箱和尾架之間滑動的H形滑塊。大拖板能手動或者通過溜板箱和光桿(進給桿)或絲桿(引導螺桿)機動。
在切削螺旋時，動力通過絲桿提供給溜板箱上的齒輪箱。在其余車削作業(yè)中，都由光桿驅(qū)動大拖板。絲桿穿過一對固定在溜板箱后部的剖分螺母。當開動特定操作桿時，剖分螺母夾在一起作為單個螺母與旋轉(zhuǎn)的絲桿嚙合，并帶動拖板沿著床身提供進給。當操作桿脫離時，剖分螺母釋放同時大拖板停止運動。另一方面，當使用光桿時則通過蝸輪給溜板箱提供動力。 蝸輪用鍵連接在光桿上，并與溜板箱一起沿光桿運動，光桿全長范圍開有鍵槽?，F(xiàn)代車床一般在主軸箱下裝備快速變換齒輪箱，通過一系列齒輪由主軸驅(qū)動。它與光桿和絲桿連接，能容易并快速地通過簡單轉(zhuǎn)換適當?shù)牟僮鳁U選擇各種進給?？焖僮儞Q齒輪箱可用于普通車削、端面切削和螺旋切削作業(yè)中。由于這種齒輪箱與主軸相連，主軸每轉(zhuǎn)一圈溜板箱(和切削刀具)運動的距離能被控制，這距離就可以被認為是進給。
車床切削刀具
車床刀具的形狀和幾何參數(shù)取決于它們的使用目的。車削刀具可以分為兩個主要組別，即外部切削刀具和內(nèi)部切削刀具。這兩組中的每一組都包括以下類型刀具：
車削刀具：車削刀具可以是精車刀具或粗車刀具。粗車刀具刀尖半徑較小，用于深切削。而精車刀具刀尖半徑較大，用于通過微量進刀深度來獲得具有較好表面光潔度的最終所需尺寸。粗車刀具按其進給方向可以是右手型的或是左手型的。它們可以有直的、彎的或偏置的刀桿。
端面刀具：端面刀具用在端面作業(yè)中加工平板側(cè)面或端部表面，也有加工左右側(cè)表面之分。與一般采用縱向進給的車削作業(yè)相反，那些側(cè)表面通過采用橫向進給產(chǎn)生。
切斷刀具：切斷刀具，有時也稱為分割刀具，用于將工件分割成若干部分和/或加工外部環(huán)形槽。
螺紋切削刀具：螺紋切削刀具根據(jù)所需螺紋的橫截面，有三角形的、矩形的或梯形的切削刃。同時，這些刀具的平面角必須始終與螺紋形狀的平面角保持一致。車外螺紋的螺紋切削刀具為直刀桿，而車內(nèi)螺紋的螺紋切削刀具則是彎刀桿。
成形刀具：成形刀具有專門制成特定形狀的刀刃，這種刀刃形狀與被加工工件所需外形正好相反。高速鋼刀具通常以單件形式制造，而硬質(zhì)合金或陶瓷刀具則以刀尖形式制造。后者用銅焊或機械方法固定于鋼質(zhì)刀桿上。機械式固定布置方式，它包括了硬質(zhì)合金刀尖、斷屑槽、襯墊、卡裝螺桿(帶有墊圈和螺母)及刀桿。顧名思義，斷屑槽的功能就是不時地折斷長切屑，以防形成很長的可能會在機加工操作中引起問題的纏繞切屑條。硬質(zhì)合金刀尖(或陶瓷刀尖)根據(jù)采用它們的機加工操作，可以有不同的形狀。根據(jù)將刀尖裝配在刀桿上是通過用銅焊還是機械卡裝，刀尖可以是實心的或是帶有中心通孔的。
車床操作
在下面這節(jié)中，要討論的是能在傳統(tǒng)普通車床上進行的各種機加工作業(yè)。然而，必須記住現(xiàn)代計算機數(shù)控車床具有更多的功能并且可以進行其它操作，例如仿型。下面是傳統(tǒng)車床的操作。
圓柱面車削：圓柱面車削是所有車床操作中最簡單也是最普通的。工件旋轉(zhuǎn)一整圈產(chǎn)生一個圓心落在車床主軸上的圓；由于刀具的軸向進給運動這種動作重復許多次。所以，由此產(chǎn)生的機加工痕跡是一條具有很小節(jié)距的螺旋線，該節(jié)距等于進給。因此機加工表面始終是圓柱形的。軸向進給通過大拖板或復式刀架手動或自動提供，然而切削深度則由橫向滑板控制。粗車中，推薦使用較大切削深度(根據(jù)工件材料可達0.25英寸或6毫米)和較小進給。另一方面，精車則最好采用很小的進給、較小的切削深度(小于0.05英寸或0.4毫米)和較高的切削速度。
端面車削：端面車削操作的結(jié)果是將工件整個端部表面或者像軸肩之類的中間環(huán)形表面加工平整。在端面車削操作中，進給由橫向滑板提供，而切削深度則通過大拖板或復式刀架控制。端面車削既可以從外表面向內(nèi)切削也可以從工件中心往外切削。很明顯在這兩種情況下機加工痕跡都是螺線形式。通常在端面車削作業(yè)時習慣于采用夾住大拖板，這是因為切削力傾向于將刀具(當然包括整個大拖板)推離工件。在大多數(shù)端面車削作業(yè)中，工件被支撐在卡盤或花盤上。
開槽：在切斷和開槽操作中，刀具只有橫向進給。要采用前面已經(jīng)討論過的切斷和開槽刀具。
鏜孔和內(nèi)部車削：鏜孔和內(nèi)部車削通過鏜桿或合適的內(nèi)部切削刀具在內(nèi)表面進行。如果初始工件是實心的，則必須首先進行鉆孔作業(yè)。鉆孔刀具安裝在尾架上，然后對著工件進給。
錐面車削：錐面車削通過沿著與車床主軸不平行而傾斜成一個等于錐面所需角度的方向進刀來實現(xiàn)。下面是在實際錐面車削中采用的不同方法：
(1) 將復式刀架盤旋轉(zhuǎn)一個等于圓錐體頂角一半的角度。通過搖動復式刀架操縱柄手動提供進給。當錐角相對較大時切削外錐面和內(nèi)錐面推薦使用這種方法。
(2) 對很短的外錐面采用特殊的成型刀具。工件的寬度必須略小于刀具的寬度，并且工件通常由卡盤支撐或夾緊在花盤上。在這種情況下，機加工作業(yè)時只有橫向進給而大拖板則夾緊在床身上。
(3)偏移尾架頂尖。對需要較小錐角(小于8°) 的較長工件外錐面車削采用這種方法。工件安裝于兩頂尖之間；然后將尾架頂尖朝垂直于車床主軸方向移動一距離S。
(4) 采用錐面車削附加裝置。這種方法用于車削很長的工件，其長度大于復式刀架的整個行程。在這種場合下要遵循的步驟是將橫向滑板完全脫離大拖板，然后通過錐面車削附加裝置進行引導。
在此作業(yè)中，能照常使用自動軸向進給。對具有較小錐角(即8°到10°)的很長工件推薦采用這種方法。
螺紋切削：在螺紋切削作業(yè)時，軸向進給必須保持恒定速率，這取決于工件的轉(zhuǎn)速(rpm)。兩者之間的關系基本上由被切削螺紋所需的節(jié)距決定。如前所述，當依靠驅(qū)動大拖板的絲桿切削螺紋時軸向進給是自動產(chǎn)生的。絲桿旋轉(zhuǎn)一圈，大拖板就行進等于絲桿節(jié)距的一段距離。
因此如果絲桿的旋轉(zhuǎn)速度等于心軸的轉(zhuǎn)速(即工件的轉(zhuǎn)速)，生成切削螺紋的節(jié)距就正好等于絲桿的節(jié)距。所以被切削生成螺紋的節(jié)距總是取決于絲桿和心軸的轉(zhuǎn)速比：絲桿的節(jié)距/工件所需節(jié)距=工件轉(zhuǎn)速/絲桿轉(zhuǎn)速=心軸到大拖板的傳動比。這公式在決定車床心軸和絲桿之間的運動學關系時很有用，并且提供了正確挑選它們之間輪系的方法。在螺紋切削作業(yè)中，工件既能支撐于卡盤中，對相對較長的工件也能安裝在兩個車床頂尖之間。使用的刀具外形必須正好與要切削螺紋的輪廓一致，即三角形刀具必須用于三角形螺紋等等。
滾花：滾花主要是一種不產(chǎn)生切屑的成型操作。它使用兩個帶有粗銼式表面的淬火滾輪壓在旋轉(zhuǎn)的工件上使工件金屬產(chǎn)生塑性變形。滾花用于生成粗糙的圓柱(或圓錐)面，通常用來作手柄。有時表面滾花只為裝飾之故；有不同的滾花圖案類型可供選擇。
切削速度和進給
切削速度，通常用每分鐘表面英尺給出，就是一分鐘內(nèi)工件(被切削)表面給定點在圓周方向上行進的英尺數(shù)。
表面速度與轉(zhuǎn)速之間的關系可以用下式給出：
SFM=πDN
式中
D=用英尺表示的工件直徑
N=轉(zhuǎn)速
表面切削速度主要由被切削材料和切削刀具材料決定，可以從手冊、切削刀具生產(chǎn)商提供的資料及類似的東西上查取。
一般而言，SFM當機加工冷軋或低碳鋼時取100，機加工較堅韌的金屬時取50，而機加工較軟材料時取200。對鋁而言，SFM通?？扇?00以上。也還存在其它一些變量影響表面切削速度的最佳值。
其中包括刀具形狀、潤滑劑或冷卻液的類型、進給和切削深度。切削速度一旦確定，心軸轉(zhuǎn)速(rpm)就能按下式得到：
N=SFM/(πD)
合適進給的選擇取決于許多因素，例如所需表面光潔度、切削深度和所用刀具的幾何形狀。進給越小生成的光潔度越好，而在刀具與工件直接接觸時進給越大則可以減少機加工時間。
所以對粗車一般推薦使用較大進給，而精車則用較小進給。再者，作為指導方針的進給推薦值可以從手冊和切削刀具生產(chǎn)商提供的資料小冊子上找到。