机械外文翻译中英文

精品文档 1欢迎下载 翻译 翻译 英文原文 DefinitionsDefinitions andand TerminologyTerminology ofof VibrationVibration vibrationvibration All matter solid liquid and gaseous is capable of vibration e g vibration of gases occurs in tail ducts of jet engines causing troublesome noise and sometimes fatigue cracks in the metal Vibration in liquids is almost always longitudinal and can cause large forces because of the low compressibility of liquids e g popes conveying water can be subjected to high inertia forces or water hammer when a valve or tap is suddenly closed Excitation forces caused say by changes in flow of fluids or out of balance rotating or reciprocating parts can often be reduced by attention to design and manufacturing details Atypical machine has many moving parts each of which is a potential source of vibration or shock excitation Designers face the problem of compromising between an acceptable amount of vibration and noise and costs involved in reducing excitation The mechanical vibrations dealt with are either excited by steady harmonic forces i e obeying sine and cosine laws in cases of forced vibrations or after an initial disturbance by no external force apart from gravitational force called weight i e in cases of natural or free vibrations Harmonic vibrations are said to be simple if there is only one frequency as represented diagrammatically by a sine or cosine wave of displacement against time Vibration of a body or material is periodic change in position or 精品文档 2欢迎下载 displacement from a static equilibrium position Associated with vibration are the interrelated physical quantities of acceleration velocity and displacement e g an unbalanced force causes acceleration a F m in a system which by resisting induces vibration as a response We shall see that vibratory or oscillatory motion may be classified broadly as a transient b continuing or steady state and c random TransientTransient VibrationsVibrations die away and are usually associated with irregular disturbances e g shock or impact forces rolling loads over bridges cars driven over pot holes i e forces which do not repeat at regular intervals Although transients are temporary components of vibrational motion they can cause large amplitudes initially and consequent high stress but in many cases they are of short duration and can be ignored leaving only steady state vibrations to be considered Steady StateSteady State VibrationsVibrations are often associated with the continuous operation of machinery and although periodic are not necessarily harmonic or sinusoidal Since vibrations require energy to produce them they reduce the efficiency of machines and mechanisms because of dissipation of energy e g by friction and consequent heat transfer to surroundings sound waves and noise stress waves through frames and foundations etc Thus steady state vibrations always require a continuous energy input to maintain them RandomRandom VibrationVibration is the term used for vibration which is not periodic i e has no made clear several of which are probably known to science students already Period Period Cycle Cycle FrequencyFrequency andand AmplitudeAmplitude A steady state mechanical vibration is the motion of a system repeated after an interval of time known as the period The motion completed in any one 精品文档 3欢迎下载 period of time is called a cycle The number of cycles per unit of time is called the frequency The maximum displacement of any part of the system from its static equilibrium position is the amplitude of the vibration of that part the total travel being twice the amplitude Thus amplitude is not synonymous with displacement but is the maximum value of the displacement from the static equilibrium position NaturalNatural andand ForcedForced VibrationVibration A natural vibration occurs without any external force except gravity and normally arises when an elastic system is displaced from a position of stable equilibrium and released i e natural vibration occurs under the action of restoring forces inherent in an elastic system and natural frequency is a property of he system A forced vibration takes place under the excitation of an external force or externally applied oscillatory disturbance which is usually a function of time e g in unbalanced rotating parts imperfections in manufacture of gears and drives The frequency of forced vibration is that of the exciting or impressed force i e the forcing frequency is an arbitrary quantity independent of the natural frequency of the system ResonanceResonance Resonance describes the condition of maximum amplitude It occurs when the frequency of an impressed force coincides with or is near to a natural frequency of the system In this critical condition dangerously large amplitudes and stresses may occur in mechanical systems but electrically radio and television receivers are designed to respond to resonant frequencies The calculation or estimation of natural frequencies is therefore of great importance in all types of vibrating and oscillating systems When resonance occurs in rotating shafts and spindles the speed of 精品文档 4欢迎下载 rotation is known as the critical speed Hence the prediction and correction or avoidance3 of a resonant condition in mechanisms is of vital importance since in the absence of damping or other amplitude limiting devices resonance is the condition at which a system gives an infinite response to a finite excitation DampingDamping Damping is the dissipation of energy from a vibrating system and thus prevents excessive response It is observed that a natural vibration diminishes in amplitude with time and hence eventually ceases owing to some restraining or damping influence Thus if a vibration is to be sustained the energy dissipated by damping must be replaced from an external source The dissipation is related in some way to the relative motion between the components or elements of the system and is caused by frictional resistance of some sort e g in structures internal friction in material and external friction caused by air or fluid resistance called viscous damping if the drag force is assumed proportional to the relative velocity between moving parts One device assumed to give viscous damping is the dashpot which is a loosely fitting piston in a cylinder so that fluid can flow from one side of the piston to the other through the annular clearance space A dashpot cannot store energy but can only dissipate it Basic Machining Operations and Machine Tools BasicBasic MachiningMachining OperationsOperations Machine tools have evolved from the early foot powered lathes of the Egyptians and John Wilkinson s boring mill They are designed to provide rigid support for both the workpiece and the cutting tool and can precisely control their relative positions and the velocity of 精品文档 5欢迎下载 the tool with respect to the workpiece Basically in metal cutting a sharpened wedge shaped tool removes a rather narrow strip of metal from the surface of a ductile workpiece in the form of a severely deformed chip The chip is a waste product that is comsiderably shorter than the workpiece from which it came but woth a corresponding increase in thickness of the uncut chip The geometrical shape of the machine surface depedns on the shape of the tool and its path during the machinig operation Most machining operations produce parts of differing geometry If a rough cylindrical workpiece revolves about a central axis and the tool penetrates beneath its surface and travels parallel to the center of rotation a surface of revolution is producedand the operation is called turning If a hollow tube is machined on the inside in a similar manner the operation is called boring Producing an external conical surface of uniformly varying diameter is called taper turning If the tool point travels in a path of varying radius a contoured surface like that of a bowling pin a can be produced or if the piece is short enough and the support is sufficiently rigid a contoured surface could be produced by feeding a shaped tool normal to the axis of rotation Short tapered or cylindrical surfaces could also be contour formed Flat or plane surfaces are frequently required The can be generated by adial turning or facing in which the tool point moves normal to the axis of rotation In other cases it is more convenient to hold the workpiece steady and reciprocate the tool across it in a series of straight line cuts with a crosswise feed increment before each cutting stroke This operation is called planing and is carried out on a shaper For larger pieces it is easier to keep the tool stationary and draw the workpiece under it as in planing The tool is 精品文档 6欢迎下载 fed at each reciprocation Contoured surfaces can be produced by using shaped tools Multiple edged tools can also be used Drilling uses a twin edged fluted tool for holes with depths up to 5 10times the drill diameter Whether the dril turns or the workpiece rotates relative motion between the cutting edge and the workpiece is the important factor In milling operations a rotary cutter with a number of cutting edges engages the workpiecem which moves slowly with respect to the cutter Plane or contoured surfaces may be produced depending on the geometry of the cutter and the type of feed Horizontal or vertical axes of rotation ma be used and the feed of the workpiece may be in any of the three coordinate directions BasicBasic MachineMachine ToolsTools Machine tools are used to produce a part of a specified geometrical shape and precise size by removing metal from a ductile materila in the form of chips The latter are a waste product and vary from long continuous ribbons of a ductile material such as steel which are undesirable from a disposal point of view to easily handled well broken chips resulting from cast iron Machine tools perform five basic metal removal processes turning planing drilling milling and frinding All other metal removal processes are modifications of these five basic processes For example boring is internal turning reaming tapping and counterboring modify drilled holes and are related to drilling hobbing and gear cutting are fundamentally milling operations hack sawong and broaching are a form of planing and honing lapping superfinishing polishing and buffing are avariants of grinding or abrasive removal operations Therefore there are only four types of basic machine tools which use cutting tools of specific controllable feometry 1 lathes 2 planers 精品文档 7欢迎下载 3 drilling machines and 4 milling machines The frinding process forms chips but the geometry of the barasive grain is uncontrollable The amount and rate of material removed by the various machining processes may be large as in heavy truning operations or extremely small as in lapping or superfinishing operations where only the high spots of a surface are removed A machine tool performs three major functions 1 it rigidly supports the workpiece or its holder and the cutting tool 2 it provedes relative motion between the workpiece and the cutting tools 3 it provides a range of feeds and speeds usually ranging from 4 to 32 choices in each case SpeedSpeed andand FeedsFeeds inin MachiningMachining Speeds feeds and depth of cut are the three major variables for economical machining Other variables are the work and tool materials coolant and geometry of the cutting tool The rate of metal removal and power required for machining depend upon these variables The depth of cut feed and cutting speed are machine settings that must be established in any metal cutting operation They all affect the forces the power and the rate of metal removal They can be defined by comparing them to the needle and record of a phonograph The cutting speed is represented by the velocity of the record surface relative to the needle in the tone arm at any instant Feed is represented by the advance the needle radially inward per revolution or is the difference in position between two adjacent grooves TurningTurning onon LatheLathe CentersCenters The basic operations performed on an engine lathe are illustrated in Fig Those operations performed on extemal surfaces with a single 精品文档 8欢迎下载 point cutting tool are called turning Except for drilling reaming and tapping the operations on intermal surfaces are also performed by a single point cutting tool All machining operations including turning and boring can be classified as roughing finishing or semi finishing The objective of a roughing ooperation is to remove the bulk of the material sa repidly and as efficiently as possible while leaving a small amount of material on the work piece for the finishing operation Finishing operations are performed to btain the final size shape and surface finish on the workpiece Sometimes a semi finishing operation will precede the finishing operation to leave a small predetermined and uniform amount of stoxd on the work piece to be removed by the finishing operation Generally longer workpieces are turned while supported on one or two lathe centers Cone shaped holes called center holes which fit the lathe centers are drilled in the ends of the workpiece usually along the axis of the cylindrical part The end of the workpiece adjacent to the tailstock is always supported by a tailstock center while the end near the headstock may be supported by a headstock cener or held in a chuck The headstock end of the workpiece may be held in a four jar chuck or in a collet type chuck This method holds the workpiece firmly and transfers the power to the workpiece smoothly the additional support to the workpiece priovided by the chuck lessens the tendency for chatter to occur when cutting Precise results can be obtained with this method if care is taken to hold the workpiece accurately in the chuck Very precise results can be obtained by supporting the workpiece between two centers A lathe dog is clamped to the workpiece together they are driven by a driver p ate mounted on the spindle 精品文档 9欢迎下载 nose One end of the workpiece is machined then the workpiece can be turned around in the lathe to machine the other end The center holes in the workpiece serve as precise locating surfaces as well as bearing surfaces to carry the weight of the workpiece and to resist the xutting forces After the workpiece has been removed from the lathe for any reason the center holes will accurately align the workpiece back in the lathe or in another lathe or in a cylindrical grinding machine The workpiece must never be held at the headstock end by both a chuck and a lathe center While at first thought this seems like a quick method of aligning the workpiece in the chuck this must not be done because it is not possible to press evenly with the jaws against the workpiece while it is also supported by the center The alignment provided by the center will not be maintained and the pressure of the jaws may damage the center hole the lathe center and prehaps even the lathe spindle Compensatng or floating jaw chucks used almost exclusively on high production work provice an exception to the statements made above These chucks are really work drivers and cannot be used for the same purpose as ordinary three or four jaw chucks While very large diameter workpieces are sometimes mounted on two centers they are preferably held at the headstock end by faceplate jaes to obtain the smooth power transmission moreover large lathe dogs that are adequate to transmit the power not generally available although they can be maed as a special Faceplate jaws are like chuck jaws except that thet are mounted on a faceplate which has less overhang from the spindle bearings than a large chuck would have BoringBoring The boring operation is generally performed in two steps namely 精品文档 10欢迎下载 rough boring and finish boring The objective of the rough boring operation is to remove the excess metal rapidly and efficiently and the objective of the finish boring operation is to obtain the desired size surface finish and location of the hole The size of the hole is obtained by using the trial cut procedure The diameter of the hole can be measured with inside calipers and outside micrometer calipers Basic Measuring Insteruments or inside micrometer calipers can be used to measure the diameter directly Cored holes and drilled holes are sometimes eccentric wwith respect to the rotation of the lathe When the boring tool enters the work the boring bar will take a deeper cut on one side of the hole than on the other and will deflect more when taking this deeper cut with the result that the bored hole will not be concentric with the rotation of the work This effect is corrected by taking several cuts through the hole using a shallow depth of cut Each succeeding shallow cut causes the resulting hole to be more concentric than it was with the previous cut Before the final finish cut is taken the hole should be concentric with the rotation of the work in order to make certain that the finished hole will be accurately located Shoulders grooves contours tapers and threads are bored inside of holes Internal grooves are cut using a tool that is similar to an external grooving tool The procedure for boring internal shoulders is very similar to the procedure for turning shoulders large shoulders are faced with the boring tool positioned with the nose leading and using the cross slide to feed the tool Internal contours can be machined using a tracing attachment on a lathe The tracing attachment is mounted on the cross slide and the stylus follows the outline of the master profile plate This causes the cutting tool to move in a path corresponding to the profile of the 精品文档 11欢迎下载 master profile plate Thus the profile on the master profile plate is reproduced inside the bore The master profile plate is accurately mounted on a special slide which can be precisely adjusted in two dirctions in two directionsm in order to align the cutting tool in the correct relationship to the work This lathe has a cam lick type of spindl