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譯文題目： 車床和車削
學生姓名：
?！ I(yè)：
所在學院：
指導教師：
職 稱：
2016年 3 月3 日
外文原文：
Lathe and Turning
The Lathe and Its Construction
A lathe is a machine tool used primarily for producing surfaces of revolution 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 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 supporis. 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 jib 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 silde 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 througth that hole if continous production is adopted . A lso , 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 guidewayes , 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 parte is a casting that can be moved transversely to enable alignment of the axis of the 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 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 o 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 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 wrom gear . The latter is keyed to 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 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 tow main groups,namely,external cutting tools and internal cutting tools , Each of these groups include the following types of tools:
Turning tools. Turing tools can be either finishing or rough turning tools . Rough turning tools have small nose radii and are used for obtaining the final required dimensions with good surface finish by marking slight depth 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 annual grooves.
Thread-cutting tools. Thread-cutting tools have either triangular, square, or tranpezoidal 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 tools is usually made in the form of a single piece ,contrary to cemented carbides or ceramic , which are made in the form of tipes. The latter are brazed or mechanically fastened to steel shanks. Fig.1indicates 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 operations . 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.
Fig.1
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 capabiblities and do other operations ,such as contouring , for example . Following are conventional lathe operations.
Cylindrical turning . Cylindrical turning is the the simplest and the most common of all lathe operations . A single full turn of the workpiece generate 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 depths of cuts is controlled by the cross slide . In roughing cuts , it is recommended that large depths of cuts (up to 0.25 in. or 6 mm, depending upon the workpiece material) and smaller feeds would be used. On the other hand , very fine feeds, smaller depth of cut (less than 0.05in. , or 0.4 mm) , 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 in ward or from the center of the workpiece outward . It is obvious that the machining marks in both cases tack 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 are performed on the internal surfaces by a boring bar or suitable internal workpiece is solid, a drilling operation must be performed first . The drilling tool is held in the tailstock, and latter is then fed against the workpiece.
Taper turning . Taper turning is achieved by driving the tool in a direction that is not paralled 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 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 . I n 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 esternal tamper turning of long workpiece that are required to have small tamper 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 workpoece , 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 recommend for very long workpiece 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 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 rotates a single revolutional 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 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 scew and the spindle :
= = spindle-to-carriage gearing ratio
This equation is usefully 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 workpiece . The form of the tool used must exactly coincide with the profile 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 prodyced . Tt 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 concile )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 Feeds
The cutting speed , which is usually given in surface feet per minute (SFM), is the number of feet traveled in circumferential direction by a given point on the surface (being cut ) of the workpiece in one minute . The relationship between the surface speed and rpm can be given by the following equation :
SMF=πDN
Where
D= the diameter of the workpiece in feet
N=the rpm
The surface cutting speed is dependent primarily upon the machined as well as the material of the cutting and can be obtained from handbooks , information provided by cutting tool manufacturera , 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 sofer materials . For aluminum ,the SFMis 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 sped is decided upon , the rotational speed (rpm) of the spindle can be obtained as follows :
N =
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, recommend values for feeds , which can be taken as guidelines , are found in handbooks and information booklets provided by cutting tool manufacturers.
Here I want to introduce the drilling:
Drilling involves producing through or blind holes in a workpiece by forcing a tool , which rotates around its axis , against the workpiece .Consequently , the range of cutting from that axis of rotation is equal to the radius of the required hole .In practice , two symmetrical cutting edges that rotate about the same axis are employed .
Drilling operations can be carried out by using either hand drills or drilling machines . The latter differ in size and construction . nevertheless , the tool always rotates around its axis while the workpiece is kept firmly fixed . this is contrary to drilling on a lathe .
Cutting Tool for Drilling Operations
In drilling operations , a cylindrical rotary-end cutting , called a drill , is employed . The drill can have either one or more cutting edges and corresponding flutes , which can be straight or helical . the function of the flutes is to provide outlet passages for the chips generated during the drilling operation and to allow lubricants and coolants to reach the cutting edges and the surface being machined . Following is a survey of the commonly used drills.
Twist drill . The twist drill is the most common type of drill .It has two cutting edges and two helical flutes that continue over the length of the drill body , The drill also consist of a neck and a shake that can be either straight or tapered .In the latter case , the shank is fitted by the wedge action into the tapered socket of the spindle and has a tang , which goes into a slot in the spindle socket ,thus acting as a solid means for transmitting rotation . On the other hand , straight –shank drills are held in a drill chuck that is , in turn , fitted into the spindle socket in the same way as tapered shank drills.
The two cutting edges are referred to as the lips , and are connected together by a wedge , which is a chisel-like edge . The twist drill also has two margins , which enable proper guidance and locating of the drill while it is in operation . The tool point angle (TPA) is formed by the lips and is chosen based on the properties of the material to be cut . The usual TAP for commercial drills is 118 , which is appropriate for drilling low-carbon steels and cast irons . For harder and tougher metals , such as hardened steel , brasss and bronze , larger TPAs (130 OR 140 ) give better performance . The helix angle of the flutes of the commonly used twist drills ranges between 24 and 30 . When drilling copper or soft plastics , higher values for the helix angle are recommended (between 35 and 45).
Twist drills are usually made of high speed steel ,although carbide tipped drills are also available . The size of twist drills used in industrial range from 0.01 up to 3.25 in . (i.e.0.25 up to 80 mm ) .
Core drills . A core drill consists of the chamfer , body , neck ,and shank . This type of drill may be have either three or four flutes and an equal number of margins , which ensure superior guidance , thus resulting in high machining accuracy . It can also be seen in Fig 12.2 that a core drill has flat end . The chamfer can have three or four cutting edges or lips , and the lip angle may vary between 90 and 120 . Core drills are employed for enlarging previously made holes and not for originating holes . This type of drill is characterized by greater productivity , high machining accuracy , and superior quality of the drilled surfaces .
Gun drills . Gun drills are used for drilling deep holes . All gun drills are straight fluted , and each has a single cutting edge . A hole in the body acts as a conduit to transmit coolant under considerable pressure to the tip of the drill .
There are two kinds of gun drills , namely , the center cut gun drill used for drilling blind holes and the trepanning drill . The latter has a cylindrical groove at its center , thus generating a solid core , which guides the tool as it proceeds during the drilling operation.
Spade drills . Spade drills are used for drilling large holes of 3.5 in .(90 mm ) or more . Their design results in a marked saving in cost of the tool as well as a tangible reduction in its weight , which facilitates its handling . moreover , this type of drill is easy to be ground .[13]
中文譯文：
車床和車削
車床及它的結構
車床是一個主要用來生產(chǎn)旋轉表面和端面的機床。 基于他們的目的，結構，能同時裝夾刀具的數(shù)量，自動化的程度，車床， 或更正確的說， 車床類型的機床依下列各項被分類為:
(1) 普通車床
(2) 刀剖車床
(3) 六角轉塔車床
(4) 立式的車削和鏜銑機床
(5) 自動化車床
(6) 專用車床
盡管車床類型機床的多種多樣，他們結構和工作的原則都有很大程度上的相似性。通過具有代表性的普通車床這些特征能最好地被說明。
床身 車床的床身是主要的