畢業(yè)設計_英語論文翻譯.doc

裝訂線畢業(yè)設計（論文）報告紙LATHES The basic machines that are designed primarily to do turning, facing and boring are called lathes. Very little turning is done on other types of machine tools, and nine can do it with equal facility. Because lathe can do boring, facing, drilling, and reaming in addition to turning, their versatility permits several operations to be performed with a single setup of the workpiece. This accounts for the fact that lathes of various types are more widely used in manufacturing than any other machine tool. Lathes in various forms have existed for more than two thousand years. Modem lathes date from about 1797, when Henry Maudsley developed one with a leadscrew. It provided controlled, mechanical feed of the tool. This ingenious Englishman also developed a change-gear system that could connect the motions of the spindle and leadscrew and thus enable threads to be cut. Lathe Construction. The essential components of a lathe are depicted in the block diagram of Fig.15-1.These are the bed, headstock assembly, tailstock assembly, carriage assembly, quick-change gear box, and the leadscrew and feed rod. The bed is the backbone of a lathe. It is usually made of well-normalized or aged gray or nodular cast iron and provides a heavy, rigid frame on which all the other basic components are mounted. Two sets of parallel, longitudinal ways, inner and outer, are contained on the bed, usually on the upper side. Some makers use an inverted V-shape for all four ways, whereas others utilize one inverted V and one flat way in one or both sets. Because several other components are mounted and/or move on the ways they must be made with precision to assure accuracy of alignment. Similarly, proper precaution should be taken in operating a lathe to assure that the ways are not damaged. Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed. The ways on most modern lathes are surface hardened to offer greater resistance to wear and abrasion. The headstock is mounted in a fixed position on the inner ways at one end of the lathe bed. It provides a powered means of rotating the work at various speeds. It consists, essentially, of a hollow spindle, mounted in accurate bearings, and a set of transmission gears-similar to a truck transmission-through which the spindle can be rotated at a number of speeds. Most lathes provide from eight to eighteen speeds, usually in a geometric ratio, and on modern lathes all the speeds can be obtained merely by moving from two to four levers. An increasing trend is to provide a continuously variable speed range through electrical or mechanical drives. Because the accuracy of a lathe is greatly dependent on the spindle, it is of heavy construction and mounted in heavy bearings, usually preloaded tapered roller or ball types. A longitudinal hole extends through the spindle so that long bar stock can be fed through it. The size of this hole is an important size dimension of a lathe because it determines the maximum size of bar stock that can be machined when the material must be fed through the spindle. The inner end of the spindle protrudes from the gear box and contains a means for mounting various types of chucks, face plates, and dog plates on it. Whereas small lathes often employ a threaded section to which the chucks are screwed, most large lathes utilize either cam-lock or key-drive taper noses. These provide a large-diameter taper that assures the accurate alignment of the chuck, and a mechanism that permits the chuck or face plate to be locked or unlocked in position without the necessity of having to rotate these heavy attachments. Power is supplied to the spindle by means of an electric motor through a V-belt or silent-chain drive. Most modern lathes have motors of from 5 to 15 horsepower to provide adequate power for carbide and ceramic tools at their high cutting speeds. The tailstock assembly consists, essentially, of three parts. A lower casting fits on the inner ways of the bed and can slide longitudinally thereon, with a means for clamping the entire assembly in any desired location. An upper casting fits on the lower one and can be moved transversely upon it on some type of keyed ways. This transverse motion permits aligning the tailstock and headstock spindles and provides a method of turning tapers. The third major component of the assembly is the tailstock quill. This is a hollow steel cylinder, usually about 2 to 3 inches in diameter, that can be moved several inches longitudinally in and out of the upper casting by means of a handwheel and screw. The open end of the quill hole terminates in a Morse taper in which a lathe center, or various tools such as drills, can be held. A graduated scale, several inches in length, usually is engraved on the outside of the quill to aid in controlling its motion in and out of the upper casting. A locking device permits clamping the quill in any desired position. The carriage assembly provides the means for mounting and moving cutting tools. The carriage is a relatively flat H-shaped casting that rests and moves on the outer set of ways on the bed. The transverse bar of the carriage contains ways on which the cross slide is mounted and can be moved by means of a feed screw that is controlled by a small handwheel and a graduated dial. Through the cross slide a means is provided for moving the lathe tool in the direction normal to the axis of rotation of the work. On most lathes the tool post actually is mounted on a compound rest. This consists of a base, which is mounted on the cross slide so that it can be pivoted about a vertical axis, and an upper casting. The upper casting is mounted on ways on this base so that it can be moved back and forth and controlled by means of a short lead screw operated by a handwheel and a calibrated dial. Manual and powered motion for the carriage, and powered motion for the cross slide, is provided by mechanisms within the apron, attached to the front of the carriage. Manual movement of the carriage along the bed is effected by turning a handwheel on the front of the apron, which is geared to a pinion on the back side. This pinion engages a rack that is attached beneath the upper front edge of the bed in an inverted position. To impart powered movement to the carriage and cross slide, a rotating feed rod is provided. The feed rod, which contains a keyway throughout most of its length, passes through the two reversing bevel pinions and is keyed to them. Either pinion can be brought into mesh with a mating bevel gear by means of the reversing lever on the front of the apron and thus provide “forward” or “reverse” power to the carriage. Suitable clutches connect either the rack pinion or the cross-slide screw to provide longitudinal motion of the carriage or transverse motion of cross slide. For cutting threads, a second means of longitudinal drive is provided by a lead screw. Whereas motion of the carriage when driven by the feed-rod mechanism takes place through a friction clutch in which slippage is possible, motion through the lead screw is by a direct, mechanical connection between the apron and the lead screw. This is achieved by a split nut. By means of a clamping lever on the front of the apron, the split nut can be closed around the lead screw. With the split nut closed, the carriage is moved along the lead screw by direct drive without possibility of slippage. Modern lathes have a quick-change gear box. The input end of this gear box is driven from the lathe spindle by means of suitable gearing. The output end of the gear box is connected to the feed rod and lead screw. Thus, through this gear train, leading from the spindle to the quick-change gear box, thence to the lead screw and feed rod, and then to the carriage, the cutting tool can be made to move a specific distance, either longitudinally or transversely, for each revolution of the spindle. A typical lathe provides, through the feed rod, forty-eight feeds ranging from 0.002 inch to 0.118 inch per revolution of the spindle, and, through the lead screw, leads for cutting forty-eight different threads from 1.5 to 92 per inch. On some older and some cheaper lathes, one or two gears in the gear train between the spindle and the change gear box must be changed in order to obtain a full range of threads and feeds.CUTTING TOOL Shape of cutting tools, particularly the angles, and tool material are very important factors. The purpose of this unit is to introduce the cutting tool geometry and tool materials.Cutting Tool Geometry Angles determine greatly not only tool life but finish quality as well. General principles upon which cutting tool angles are based do not depend on the particular tool. Basically, grinding wheel are being designed. Since, however, the lathe (turning) tool, depicted in Fig.14-1, might be easiest to visualize, its geometry is discussed. Tool features have been identified by many names. The technical literature is full of confusing terminology. Thus in the attempt to clear up existing disorganized conceptions and nomenclature, the American Society of Mechanical Engineers published ASA Standard B5-22-1950. what follows is based on it. A single-point tool is a cutting tool having one face and one continuous cutting edge. Tool angles identified in Fig. 14-2 are as follows: (1) Back-rake angle, (2) Side-rake angle, (3) End-relief angle (4) End-relief angle (5) Side-relief angle (6) End-cutting-edge angle, (7) Side-cutting-edge angle, (8) Nose angle, (9) Nose radius. Tool angle 1, on front view, is the back-rake angle. It is the angle between the tool face and a line parallel to the base of the shank in a longitudinal plane perpendicular to the tool base. Then this angle is downward from front to rear of the cutting edge, the rake id positive; when upward from front to back, the rake is negative. This angle is most significant in the machining process, because it directly affects the cutting force, finish, and tool life. The side-rake angle, numbered 2, measures the slope of the face in a cross plane perpendicular to the tool base. It, also, is an important angle, because it directs chip flow to the side of the tool post and permits the tool to feed more easily into the work. The end-relief angle is measured between a line perpendicular to the base and the end flank immediately below the end cutting edge; it is numbered 3 in the figure. It provides clearance between work and tool so that its cut surface can flow by with minimum rubbing against the tool. To save time, a portion of the end flank of the tool may sometimes be left unground, having been previously forged to size. In such case, this end-clearance angle, numbered 4, measured to the end flank surface below the ground portion, would be larger than the relief angle. Often the end cutting edge is oblique to the flank. The relief angle is then best measured in a plane normal to the end cutting edge perpendicular to the base of the tool. This clearance permits the tool to advance more smoothly into the work. The side-relief angle, indicated as 5, is measured between the side flank, just below the cutting edge, and a line through the cutting edge perpendicular to the base of the tool. This clearance permits the tool to advance more smoothly into the work. Angle 6 is the end-cutting-edge angle measured between the end cutting edge and a line perpendicular to the side of the tool shank. This angle prevents rubbing of the cut surface and permits longer tool life. The side-cutting-edge angle, numbered 7, is the angle between the side cutting edge and the side of the tool shank. The true length of cut is along this edge. Thus the angle determines the distribution of the cutting force. The greater the angle, the longer the tool life; but the possibility of chatter increases. A compromise must, as usual, be reached. The nose angle, number 8, is the angle between the two component cutting edges. If the corner is rounded off, the arc size is defined by the nose radius 9. the radius size influences finish and chatter.Cutting Tool Materials A large number of cutting tool materials have been developed to meet the demands of high metal-removal rates. The most important of these materials and their influence on cutter design, are described below. High Carbon Steel. Historically, high carbon steel was the earliest cutting material used industrially, but it has now been almost entirely superseded since it starts to temper at about 220 and this irreversible softening process continues as temperature increases. Cutting speeds with carbon steel tools are therefore limited to about 0.15m/s (30ft/min) when cutting mild steel, and even at these speeds a copious supply of coolant is required. High-speed Steel. To overcome the low cutting speed restriction imposed by plain carbon steels, a range of alloy steels, known as high-speed steels, began to be introduced during the early years of this century. The chemical composition of these steels varies greatly, but they basically contain about 0.7% carbon and 4% chromium, with addition of tungsten, vanadium, molybdenum and cobalt in varying percentages. They maintain their hardness at temperatures up to about 600, but soften rapidly at cutting speeds in excess of 1.8m/s (350ft/min), and many cannot successfully cut mild steel faster than 0.75m/s (150ft/min). Sintered Carbides. Carbide cutting tools, which were developed in Germany in the late 1920s, usually consist of tungsten carbide or mixtures of tungsten carbide and titanium or tantalum carbide in powder form, sintered in a matrix of cobalt or nickel. Because of the comparatively high cost of this tool material and its low rupture strength, it is normally produced in the form of tips which are either brazed to a steel shank or mechanically clamped in a specially designed holder. Mechanically clamped tool tips are frequently made as throwaway inserts. When all the cutting edges have been used the inserts are discarded, ad regrinding would cost more than a new tip. The high hardness of carbide tools at elevated temperatures enables them to be used at much faster cutting speeds than high-speed steel (of 3-4m/s（600-800ft/min）when cutting mild steel). They are manufactured in several grades, enabling them to be used for most machining applications. Their earlier brittleness has been largely overcome by the introduction of tougher grades, which are frequently used for interrupted cuts including many arduous face-milling operations. Recently, improvements have been claimed by using tungsten carbide tools coated with titanium carbide or titanium nitride (about 0.0005mm coating thickness). These tools are more resistant to wear than conventional tungsten carbide tools, and the reduction in interface friction using titanium nitride results in a reduction in cutting forces and in tool temperatures. Hence, higher metal removal rates are possible without detriment to tool life or alternatively longer tool lives could be achieved at unchanged metal removal rates. The uses of other forms of coating with aluminum oxide and polycrystalline cubic boron nitride are still in an experimental stage, but it is likely that they will have important applications when machining cast iron, hardened steels and high melting point alloys. Ceramics. The so-called ceramic group of cutting tools represents the most recent development in cutting tool materials. They consist mainly of sintered oxides, usually aluminum oxide, and are almost invariably in the form of clamped tips. Because of the comparative cheapness of ceramic tips and the difficulty of grinding them without causing thermal cracking, they are made as throw-away inserts. Ceramic tools are a post-war introduction and are mot yet in general factory use. Their most likely application is in cutting metal at very high speeds, beyond the limits possible with carbide tools. Cramics resist the formation of a built-up edge and in consequence produce good surface finishes. Since the present generation of machine tools is designed with only sufficient power to exploit carbide tooling, it is likely that, for the time being, ceramics will be restricted to high-speed finish machining where is sufficient power available for the light cuts taken. The extreme brittleness of ceramic tools has largely limited their use to continuous cuts, although their use in milling is now possible. As they are poorer conductors of heat than carbides, temperatures at the rake face are higher than in carbide tools, although the friction force is usually lower. To strengthen the cutting edge, and consequently improve the life of the ceramic tool, a small chamfer or radius is often stoned on the cutting edge, although this increases the power consumption. Diamonds. For producing very fine finishes of 0.05-0.08um（2-3um) on non-ferrous metals such as copper and aluminum, diamond tools are often used. The diamond is brazed to a steel shank. Diamond turning and boring are essentially finishing operations, as the forces imposed by any but the smallest cuts cause the diamond to fracture or be torn from its mounting. Under suitable conditions diamonds have exceptionally long cutting lives. Synthetic polycrystalline diamonds are now available as mechanically clamped cutting tips. Due to their high cost they have very limited applications, but are sometimes used for machining abrasive aluminum-silicon alloys, fused silica and reinforced plastics. The random orientation of their crystals gives them improved impact resistance, making them suitable for interrupted cutting.車床用于車