机械英语复习(1)

Vocabularies:课堂所学课文词汇表中的词汇Reading Comprehension:1 Machining has evolved from individual machines which, with the aid of man, performed individualprocesses to machines capable of performing many processes.In 1968, a NC machine was marketed which could automatically change tools so that many different processes could be done on one machine. Such a machine became known as a "machining center"--- a machine that can perform a variety of processes and change tools automatically while under programmable control.The study of machining centers begins with the history of numerical control (NC). NC is programmable automation in which certain functions of the machine tools are controlled by coded instructions.Computer and numerical control is used on a wide variety of machines. These range from single-spindle drilling machines, which often have only two-axis control, to machining centers, which can do drilling, boring, milling, tapping. and so forth with four-axis control. A machining center can automatically select and change as many as 32 preset tools. The table can move left/right or in/out and the spindle can move up/down or in/out, with positioning accuracy in the range of 0.0003 in. in 40 in. of travel. The machine has automatic tool change and automatic work transfer so that workpiece can be loaded/unloaded while the machining is in process.The concept of automatic tool changing has been extended to NCN lathes. The tools are held on a rotating tool magazine and a gantry-type tool changer is used to change the tools. Each magazing holds one type of cutting tool..The versatility is being increased by combining both rotary-work and rotary-tool operations-----turning and milling-----in a single machine. Tools are changed in six seconds or less. It is also common to provide two or more worktables, permitting work to be set up while machining is done on the workpiece in the machine, with tables being interchanged automatically. Consequently, the productivity of such machines can be very high, the chip-producing time often approaching 50% of the total.Two new trends are observed in the development of machining centers. One is the growing interest in smaller, more compact machining centers and the other is the emphasis on extended-shift or even unmanned operations. Modern machining centers have contributed significantly to improved productivity in many companies. They have eliminated the time lost in moving workpieces from machine to machine and the time needed for workpiece loading and unloading for separate operations.In addition, they have minimized the time lost in changing tools, carrying out gauging operations, and aligning workpieces on the machine.2 In selecting a material, the primary（首要的，首位的）concern of engineers is to match thematerial properties to the service requirements of the component. Knowing the conditions of load and environment under which the component must operate, engineers must then select an appropriate material, using tabulated（列表的）test data as the primary guide. They must know what properties they want to consider, how these are determined, and what restrictions or limitations should be placed on their application.One means of distinguishing（区别, 辨别）one material from another is by comparison ofphysical properties. These include such characteristics as density (weight); melting point; optical properties (such as transparency, opaqueness, or color); the thermal properties of specific heat, coefficient of thermal expansion（膨胀）, and thermal conductivity; electrical conductivity; and magnetic properties. In some cases, the physical properties may be of prime importance when one is selecting a material.Mechanical properties are those responses（响应, 反应）a material has to the application of mechanical forces. Of specific interest are: (1) deformation (strain) which occurs when a stress（应力）is applied, (2) hardness, and (3) toughness. These mechanical properties are determined by subjecting prepared specimens（样品, 试样）to standard laboratory tests designed to evaluate （评价,估计）the material's reaction to applied forces.The properties of materials arise from（起因于…，由…引起）their internal structures. If the engineer wants a specific set of properties, he must choose his materials appropriately so that they have suitable structures. Should the internal structure of a material be changed during processing of service, there are corresponding changes in properties.A material's thermal properties become apparent when energy, which is introduced by the inflow of heat, causes the atoms to vibrate（(使)振动, (使)摇摆）more vigorously（强劲，有力，活泼）and raises the temperature of the material. The results are (1) thermal expansion, and (2) thermal conductivity.Electrical properties are exhibited by materials which are subject to applied electric fields. The more common electrical properties include: (1) electrical resistivity and conductivity, and (2) the relative dielectric constant（介电常数）, which relates the accumulated charge across an insulator to the electric field. These properties receive additional significance each year, since engineers must continually develop new and more automated products.3Most of the mechanical operations are commonly performed on five basic machine tools:●The drill press.●The lathe.●The shaper or planer.●The milling machine.●The grinder.Drilling is performed with a rotating tool called a drill. Most drilling in metal is done with a twist drill. The machine used for drilling is called a drill press. Operations, such as reaming（扩孔，铰孔）and tapping（攻丝，攻罗纹）, are also classified as drilling. Reaming consists of removing a small amount of metal from a hole already drilled. Tapping is the process of cutting a thread insidea hole so that a cap screw or bolt may be threaded into it.The lathe is commonly called the father of the entire machine tool family. For turning operations, the lathe uses a single-point cutting tool which removes metal as it travels past the revolving workplace. Turning operations are required to make many different cylindrical（圆柱的）shapes, such as axes, gear blanks, pulleys（滑轮）, and threaded shafts. Boring operations are performed to enlarge, finish, and accurately locate（定位）holes.Milling removes metal with a revolving（旋转的）, multiple cutting edge（多刃的）tool called milling cutter（铣刀）. Milling cutters are made in many styles and sizes. Some have as few as two cutting edges and others have 30 or more. Milling can produce flat or angled（有角度的，倾斜的）surfaces, grooves（凹槽, 槽口）, slots（缝, 狭缝）, gear teeth, and other profiles（外形, 轮廓，型面）,depending on the shape of the cutters being used.Shaping and planing produce flat surfaces with a single-point cutting tool. In shaping, the cutting tool on a shaper moves back and forth while the work is fed automatically towards the tool.In planing, the workpiece is attached to a worktable that reciprocates（往复运动）past the cutting tool. The cutting tool is automatically fed into the workpiece a small amount on each stroke.Grinding makes use of abrasive particles to do the cutting. Grinding operations may be classified as precision（精密）or nonprecision, depending on the purpose. Precision grinding（精磨）is concerned with grinding to close（接近的,精密的）tolerances（公差, 容许量）and very smooth finish.Nonprecison grinding involves the removal of metal where accuracy（精确性）is not important.4 However simple, any machine is a combination of individual（单个的，个别的）components（元件,构件）generally referred to as machine elements. Thus, if a machine is completely dismantled, a collection of simple parts remains such as nuts, bolts, springs, gears, cams, and shafts ----the building blocks of all machinery. A machine element is, therefore, a single unit designed to performa specific function and capable of combining with other elements. Sometimes certain elements areassociated（联合，结合在一起）in pairs（成对地，成双地）, such as nuts and bolts or keys and shafts.In other instances, a group of elements is combined to form a subassembly, such as bearings, couplings, and clutches（离合器）.The most common example of a machine element is a gear, which, fundamentally, is a combination of the wheel（轮，轮子）and the lever（杆，杠杆）to from a toothed（带齿的）wheel. The rotation of this gear on a hub or shaft drives other gears which may rotate faster or slower, depending upon the number of teeth on the basic wheels.Other fundamental machine elements have evolved from wheel and levers. A wheel must have a shaft on which it may rotate. The wheel is fastened（扎牢、拴紧、固定于）to the shaft with a key, and the shaft is joined to other shafts with couplings. The shaft must rest in（搁在…上）bearings, may be started by a clutch or stopped with a brake. It may be turned by a pulley with a belt or a chain connecting it to a pulley on a second shaft. The supporting structure may be assembled with bolts or rivets or by welding. Proper application of these machine elements depends upon a knowledge of the force on the structure and the strength of the materials employed.The individual reliability（可靠性）of machine elements becomes the basis for estimating the overall life expectancy（期望值）of a complete machine.Many machine elements are thoroughly standardized. Testing and practical experience have established the most suitable dimensions for common structural and mechanical parts. Through standardization, uniformity（均匀性，一致性）of practice and resulting economies are obtained. Not all machine parts in use are standardized, however. In the automotive industry, only fasteners, bearings, bushings, chains, and belts are standardized. Crankshafts（曲轴）and connecting rods（连杆）are not standardized.Translate the underlined sentences of the following passages into Chinese: Lesson 1、6、7、11、14、16。