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Lathe and TurningThe Lathe and Its ConstructionA 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)Toolroom 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. Following is a description of each of the main elements of an engine lathe, which is shown in Fig.11.1.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 guideways 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 guideways (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 workpiece. 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 handwheel 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 cuttingtool) 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 include 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 workpiece 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 workpiece.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 shanksFig.11.2 indicates an arrangement of this latter type, which 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 workpiece 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 workpiece 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 workpiece 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 workpiece 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 workpiece. In most facing operations, the workpiece 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 workpiece 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 workpiece.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 workpiece must be slightly smaller than that of the tool, and the workpiece 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 workpieces that are required to have small taper angles (less than 8). The workpiece 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 workpieces, 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 workpieces with a small cone angle, i.e., 8through 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 workpiece. 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 workpiece), 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 workpiece=rpm of the workpiece/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 workpiece can either be held in the chuck or mounted between the two lathe centers for relatively long workpieces. 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 filelike surfaces against the rotating workpiece to cause plastic deformation of the workpiece 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 FeedThe 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 workpiece in 1 minute. The relationship between the surface speed and rpm can be given by the following equation: SFM=DNWhereD=the diameter of the workpiece in feetN=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/(D) 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 workpiece.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.车床和车削 车床及车床是机床用于生产革命和平面边缘表面为主。 根据他们的目的，施工，可同时安装工具的数量，自动化程度，车床，或者更准确地说，车床型机床可分为如下： （1）发动机车床 （2）工具间车床 （3）转塔车床 （4）立式车床镗床 （5）自动车床 （6）专用车床 对该项车床型机床多样性尽管如此，他们都与尊重的建设和运作原则的共同特点。这些功能可以最好的说明，考虑常用的代表类型，发动机车床。以下是对一个普通车床，这是Fig.11.1所示的主要内容每个描述。 车床的床。 车床的床身是主框架，涉及两个垂直支撑横梁。它通常是采用灰铸铁或球墨铸铁潮湿的振动，并通过铸造制成。它有导轨，使运输方便纵向滑动。该车床的床的高度要适当，使技术人员做他或她的工作轻松，舒适。 启闭。 主轴箱固定在床身的左侧，包括主轴的轴线平行于导轨（床的滑动面）。主轴驱动，通过变速箱，这是在车头内。齿轮箱的功能是提供一个不同的主轴转速号码（一般为6到18的速度）。一些现代的车床主轴无级变速有速度，雇用摩擦，电气，液压驱动或headstocks。 主轴始终是空心的，一大肠杆菌，它已通过纵向延伸洞。酒吧股票可以通过该孔如果美联储连续生产的通过。另外，那个洞有一个圆锥表面，使安装一个普通车床的中心。主轴外表面有螺纹，可让卡盘，一个面盘，或类似的安装。 尾座。 尾座大会基本上由三部分组成，其基数较低，中间的一部分，羽毛笔。较低的基数为铸造，可以幻灯片上沿导轨床身，而且它的夹紧装置，以便在任何需要的位置锁定整个尾座，根据工件的长短而定。中间部分是一个可移动横，以便在与车头车尾的轴对齐的铸造。第三部分，羽毛，是一种坚硬的钢管，可以纵向移动并根据需要出中间的一部分。这是通过一个手轮和螺丝钉，围绕着它固定在套筒螺母从事使用。在开放的羽毛边孔是锥形，使车床中心或类似麻花钻镗杆或其他工具的安装。鹅毛笔可锁定在任何道路沿线的旅游点通过一个夹紧装置的手段。 马车。 马车的主要功能是安装的切削工具，产生纵向和/或跨饲料。它实际上是一个H形块，在车头和车尾之间的车床床身滑动而受到V型导轨，床型为指导。马车可以移动手动或机械由停机坪，要么饲料杆或丝杆手段。 螺纹切削时，电源提供给停机坪变速箱由丝杆。在所有其他车削操作，它是饲料棒驱动车。丝杆螺母通过半，这是固定在一定的驱动后轮对去杠杆apron.When，半坚果钳制在一起，并参与到作为一个单一的丝杠螺母旋转，这是美联储，连同与马车，沿着床。当杠杆脱开，半坚果被释放了，马车stops.On另一方面，当进料杆使用，它提供通过蜗轮权力到停机坪。后者是挂钩的饲料杆和沿饲料棒，它有一个键槽延伸至涵盖其整个length.A现代车床通常有一个快速更换变速箱位于下启闭，并通过从主轴驱动停机坪旅行火车的齿轮。它是连接这两个饲料杆和丝杆，使饲料选择，只需适当的杠杆移动方便，迅速等。在快速变化的变速箱是采用纯车削，线切割面对和行动。由于该变速箱是联系在一起的主轴，停机坪的距离（和cuttingtool）每次主轴革命旅行是可以控制的，是被称为饲料。 车床切削工具 的形状和车刀几何取决于他们的目的，是employed.Turning工具可分为两大类，即外部和内部的刀具切削工具分类。这两个组别，分别包括以下类型的工具： 车削工具。 车刀可以是整理或粗车刀。粗车刀有小鼻子半径和受聘时作出大量削减。另一方面，整理工具有更大的鼻子半径，并获得通过使轻微切深度，具有良好的表面光洁度最终所需的尺寸使用。粗车工具可以右手或左手的类型，这取决于饲料的方向。他们可以有直，弯曲，或抵销柄。 面对工具。 面对工具受聘于加工平面面临侧面或端面的操作。有加工左手侧表面，为右手边面工具的工具。产生这些侧面通过饲料的交叉使用，违反了车削，在那里通常的纵向进给用。 截止工具。 截止工具，有时也称为分型工具，服务分开成几部分和/或机器外部环形沟槽工件。 螺纹切削工具。 螺纹切削工具要么三角形，方形或梯形切割边缘，而在该线截面所需而定。此外，这些工具中，飞机的角度必须始终是相同的线程的形式的。螺纹切削工具有外螺纹切削直柄和弯柄型切割时的内部线程。 表格工具。 表格工具，特别是生产采取一定的形式，这是对面的高速钢刀具的加工workpiece.An所需的形状通常是在一个单件的形式提出的边缘，违背硬质合金或陶瓷，这是在做形式的提示。后者是焊接或机械固定在钢shanks.Fig.11.2表明后者的一个类型，其中包括硬质合金尖，断屑，垫，夹紧螺钉（带洗衣机和一个螺母）的安排，小腿。顾名思义，该芯片的断路器功能就是要打破长期芯片每一个现在，然后，从而防止了很长的彩带形成扭曲可能导致加工过程中出现的问题。在硬质合金（或陶瓷提示）可以有不同的形状，经加工操作的，他们是被雇用而定。的提示可以是固体或通孔与中央，取决于钎焊或机械夹紧用于安装在柄端就业。 车床操作 在下面的部分，我们讨论各种机械加工，可在一个传统的发动机lathe.It进行的操作，必须牢记，但是，现代的电脑数控车床有更多的功能，可以做其他操作，如轮廓，因为，例子。以下是普通车床的操作。 圆柱转动。 圆柱转动是最简单，最常见的车床操作。一个单一的工件整圈产生一个圆，圆心轴的车床汪洋大海;这项议案，然后复制作为工具的轴向进给运动的结果无数次。由此产生的加工痕迹，因此，有一个螺