表面切削速度主要由被切削材料和切削刀具材料决定，可以从手册、切削刀具生产商提供的资料及类似的东西上查取。 一般而言，SFM当机加工冷轧或低碳钢时取100，机加工较坚韧的金属时取50，而机加工较软材料时取200。对铝而言，SFM通常可取400以上。也还存在其它一些变量影响表面切削速度的最佳值。 其中包括刀具形状、润滑剂或冷却液的类型、进给和切削深度。切削速度一旦确定，心轴转速(rpm)就能按下式得到：
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:
(4)Vertical turning and boring mills
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 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 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 shanks．Fig.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.
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., 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. 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.
=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 file like 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 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 workpiece in 1 minute. The relationship between the surface speed and rpm can be given by the following equation:
Where D=the diameter of the workpiece in feet
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:
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.