1、The Lathe and Its ConstructionA 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 cl
2、assified as follows: (1) Engine lathes (2) Toolroom lathes (3) Turret lathes (4) Vertical turning and boring mills (5) Automatic lathes (6) Special-purpose lathesIn spite of that diversity of lathe-type machine tools, they all have all have common features with respect to construction and principle
3、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 be
4、am 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 comfo
5、rtably.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 provid
6、e 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
7、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 .Tallstock . The tailstock assembly consists basicall
8、y 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 i
9、ntermediate 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 th
10、rough 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 a
11、ctually 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, p
12、ower 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
13、 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 l
14、atter 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
15、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
16、 tool) travels for each revolution of the spindle can be controlled and is referred to as the feed.Lathe Cutting ToolsThe 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
17、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 sligh
18、t 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-h
19、and-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 workpie
20、ce 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 threa
21、d 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 too
22、ls 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.11.2 indicates an arrangement of this latter type, which includes the carbide tip , the chip breaker ,
23、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
24、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 Operati
25、onsIn 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 exampl
26、e . 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 r
27、esult 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 au
28、tomatically, 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 t
29、han 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 th
30、e 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
31、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
32、,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 perf
33、ormed 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 .
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