機(jī)械英語方面文章.doc

Grinding is a manufacturing process that involves the removal of metal by employing a rotating abrasive wheel. The latter simulates a milling cutter with an extremely large number of miniature cutting edges. Generally, grinding is considered to be a finishing process that is usually used for obtaining high-dimensional accuracy and better surface finish. Grinding can be performed on flat, cylindrical, or even internal surfaces by employing specialized machine tools, which are referred to as grinding machines. Obviously, grinding machines differ in construction as well as capabilities, and the type to be employed is determined mainly by the geometrical shape and nature of the surface to be ground, e.g. , cylindrical surfaces are ground on cylindrical grinding machines. Surface grinding. As the name surface grinding suggests, this operation involves grinding of flat or plane surfaces. Fig.8.1 indicates the two possible variations, either a horizontal or vertical machine spindle. In the first case (horizontal spindle), the machine usually has a planer-type reciprocating table on which the workpiece is held. However, grinding machines with vertical spindles can have either a planer type table like that of the horizontal-spindle machine or a rotating worktable. Also, the grinding action in this case is achieved by the end face of the grinding wheel, contrary to the case of horizontal-spindle machines, where the workpiece ground by the periphery of the grinding wheel. Fig.8.1a and b also indicate the equations to be used for estimating the different parameters of the grinding operation, such as the machining time and the rate of metal removal. During the surface-grinding operations, heavy workpieces are either held in fixtures or clamped on the machine table by scrap clamps and the like, whereas smaller workpieces are usually held by chucks.Cylindrical grinding. In cylindrical grinding, the workpiece is held between centers during grinding operation, and the wheel rotation is the source and cause for the rotary cutting motion, as shown if Fig.8.2. In fact, cylindrical grinding can be carried out by employing any of the following methods:(1) The transverse method, in which both the grinding both the grinding wheel and the workpiece rotate and longitudinal linear feed is applied to enable grinding of the whole length. The depth of cut is adjusted by the cross feed if the grinding wheel into the workpiece.(2) The plunge-cut method, in which grinding is achieved through the cross feed of the grinding wheel and no axial feed is applied. As you can see, this method can be applied only when the surface to be ground is shorter than the width of the grinding wheel used.(3) The full-depth method, which is similar to the transverse method except that the grinding allowance is removed in a single pass. This method is usually recommended when grinding short rigid shafts.Internal grinding. Internal grinding is employed for grinding relatively short holes, as shown in Fig.8.3. The workpiece is held in a chuck or a special fixture. Both the grinding wheel and the workpiece rotate during the operation and feed is applied in the longitudinal direction. Any desired depth of cut can be obtained by the cross feed of the grinding wheel. A variation from this type is planetary internal grinding, which is recommend for heavy workpieces that cannot be held in chucks. In that case, the grinding wheel not only spins around its own axis but also rotates around the centerline of the hole that is being ground.Centerless grinding. Centerless grinding involves passing a cylindrical workpiece, which is supported by a rest blade, between two wheels, i.e. the grinding wheel and the regulating of feed wheel. The grinding wheel does the actual grinding, while the regulating wheel is responsible for rotating the workpiece as well as generating the longitudinal feed. This is possible because of the frictional characteristics of that wheel, which is usually made of rubber-bonded abrasive. As can be seen in Fig.8.4, the axis of the regulating wheel is tilted at a slight angle with the axis of the grinding wheel. Consequently, the peripheral velocity of the regulating wheel can be resolved into two components, namely, workpiece rotational speed and longitudinal feed. These can be given by the following equation: Vworkpiece=Vregulating wheel*cos Axial feed = Vregulating wheel * c * sinWhere c is a constant coefficient to account for the slip between the workpiece and the regulating wheel (c=0.940.98).The velocity of the regulating wheel is controllable and is used to achieve any desired rotational speed of the workpiece. The angle is usually taking from 10 to 50 and the larger the angle, the larger the longitudinal feed would be. When is taken as 00, i.e., the two axes of the grinding and regulating wheels are parallel, there is no longitudinal feed of the workpiece.Grinding wheelsGrinding wheels are composed of abrasive grains having similar size and a binder. The actual grinding process is performed by the abrasive grains. Pores between the grains within the binder enable the grains to act as separate single-point cutting tools. These pores also provide space for the generated chips, thus preventing the wheel from clogging. In addition, pores assist the easy flow of coolants to enable efficient and prompt removal of the heat generated during the grinding process.Grinding wheels are identified based on their shape and size, kind of abrasive, grain size, binder, grade (hardness), and structure.Shape and size of grinding wheels. Grinding wheels differ in shape and size, depending upon the purpose for which they are to be used. Various shapes are shown in Fig.8.5 and include the following types:(1) Straight wheels used for surface, cylindrical, internal, and centerless grinding.(2) Beveled-face or tapered wheels used for grinding threads, gear teeth, and the like.(3) Straight recessed wheels for cylindrical grinding and facing.(4) Abrasive disks for cutoff and slotting operations.(5) Cylinders, straight cups, and flaring cups are used for surface grinding with the end face of the wheel.The main dimensions of a grinding wheel are the outside diameter D, the bore diameter d, and the height H. These dimensions vary widely, depending upon the grinding process for which the wheel is to be used.Kind of abrasive. Grinding wheels can be made of natural abrasives such as quartz, emery, and corundum or of industrially prepared chemical compounds such as aluminum oxide or silicon carbide (known as carborundum). Generally, silicon carbide grinding wheels are used when grinding low-tensile-strength materials like cast iron, whereas aluminum oxide wheels are employed for grinding high-strength metals such as alloy steel, hardened steel, and the like.Grain size of abrasive used. As you may expect, the grain size of the abrasive particles of the wheel plays a fundamental role in determining the quality of ground surface obtained. The finer the grains, the smoother the ground surface is. Therefore, course-grained grinding wheels are used for roughing operations, whereas fine-grained wheels are employed in final operations.The grade of the bond. The grade of the bond is actually an indication of the resistance of the bond to pulling off the abrasive grains from the grinding wheel. Generally, wheels having hard grades are used for grinding soft materials and vice versa（反之亦然）. If hard-grade wheel were to be used for grinding a hard material, the dull grains would not be pulled off from the bond quickly enough, thus impeding（妨礙） the self-dressing process of the surface of the wheel and finally resulting in clogging （堵塞）of the wheel and burns on the ground surface. In fact, the cutting properties of all grinding wheels must be restored periodically by dressing with a cemented carbide （硬質(zhì)合金）roller or a diamond tool to give the wheel the exact desired shape and remove all worn abrasive grains.Structure. Structure refers to the amount of void space between the abrasive grains. When grinding softer metals, larger void spaces are needed to facilitate the flow of the removed chips.The binder. Abrasive particles are bonded together in many different ways. These include bond, silicate, rubber, resinoid, shellac, and oxychloride. Nevertheless, the bond is the most commonly used one.In fact, the standard marking system is employed for distinguishing grinding wheels, by providing all the preceding parameters in specific sequence.機(jī)械工程專業(yè)英語文章：Lapping and Polishing專業(yè)英語、行業(yè)英語 2010-07-08 16:01:57 閱讀28 評(píng)論0 字號(hào)：大中小訂閱 Lapping Lapping is a finishing process operation used on flat and cylindrical surfaces. The lap, shown in Fig.9.1a, is usually made of cast iron, copper, leather, or cloth. The abrasive particles are embedded in the lap, or they may be carried through slurry. Depending on the hardness of the workpiece, lapping pressures range from 7 kPa to 140kPa.Lapping has two main functions. Firstly, it produces a superior surface finish with all machining marks being removed from the surface. Secondly, it is used as a method of obtaining very close fits（過盈配合、密配合） between mating parts（配件） such as pistons and cylinders.The lapped workpiece surface may look smooth but it is actually filled with microscopic peaks, valleys, scratches and pits（凹點(diǎn)）. Few surfaces are perfectly flat. Lapping minimizes the surface irregularities, thereby increasing the available contact area. The drawing in Fig.9.1a shows two surfaces. The upper one is how a surface might look before lapping and the lower one after lapping. Lapping removes the microscopic mountain tops and produces relatively flat plateaus. Entire microscopic mountain ranges may need to be ground down in order to increase the available contact area.Production lapping on flat or cylindrical pieces is done on machines such as those shown in Fig.9.1b and 9.1c. Lapping is also down on curved surfaces, such as spherical objects and glass lenses, using specially shaped laps.PolishingPolishing is a process that produces a smooth, lustrous surface finish. Two basic mechanisms（機(jī)械機(jī)構(gòu)）are involved in the polishing process: (a) fine-scale （精密標(biāo)度）abrasive removal, and (b) softening and smearing （污點(diǎn)）of surface layers by frictional heating during polishing.ElectropolishingElectropolishing is an electrochemical process similar to, but the reverse of, electroplating（電鍍）. The electropolishing process smoothes and streamlines（把什么做成流線型） the microscopic surface of a metal object. Mirror-like finishes can be obtained on metal surfaces by electropolishing.In electropolishing, the metal is removed ion（離子） by ion from the surface of the metal object being polished. Electrochemistry and the fundamental principles of electrolysis (Faradays Law) replace traditional mechanical finishing techniques. In basic terms, the object to be electropolished is immersed （陷于）in an electrolyte （電解質(zhì)）and subjected to a direct electrical current. The object is maintained anodic陽極, with the cathodic陰極 connection being made to a nearby metal conductor.Smoothness of the metal surface is one of the primary and most advantageous effects of electropolishing. During the process, a film薄膜 of varying thickness covers the surfaces of the metal. This film is thickest over micro depressions and thinnest over micro projections. Electrical resistance is at a minimum wherever the film is thinnest, resulting in the greatest rate of metallic dissolution. Electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro-depressions or “valleys.” Stock is removed as metallic salt. Metal removal under certain circumstances is controllable and can be held to 0.0001 to 0.0025 mm.Chemical Mechanical PolishingChemical mechanical polishing is becoming an increasingly important step in the fabrication制造 of multi-level integrated circuits. Chemical mechanical polishing refers to polishing by abundant slurry泥漿 that interacts both chemically and mechanically with the surface being polished. During the chemical mechanical polishing process, a rotating wafer晶片 is pressed face down onto a rotating, resilient有彈性的 polishing pad while polishing slurry containing abrasive particles and chemical reagents化學(xué)試劑 flows in between the wafer and the pad. The combined action of polishing pad, abrasive particles and chemical reagents result in material removal and polishing of the wafer surface. Chemical mechanical polishing creates flat, damage-free on a variety of brittle materials脆性材料 and it is used extensively on silicon wafers in the manufacture of integrated circuits.Chemical mechanical polishing is a complicated multiphase process. It mainly includes the following two dynamics. First, the active component in polishing slurry reacts with the atoms of the wafer, and the process is chemical reaction step with oxidation-reductive reaction氧化還原反應(yīng). The second step is the process of desorption解吸附作用, that is to say, the resultants gradually separate from the wafer surface and new surface is exposed to polishing slurry. If chemical reactive rate is smaller, the total removal rate of the water is also small; furthermore, the surface degree of finish is not good. On the contrary, even if chemical reaction is very rapid, but desorption is very slow, the total removal rate is not good. Because resultants cannot separate from the wafer surface, the active component in the polishing slurry cannot expose and react with the atoms on the new surface, which holds up chemical reaction. The balance and compositive effects of two steps decide the total removal rate and its surface degree of finish.機(jī)械工程專業(yè)英語文章：Surface Engineering專業(yè)英語、行業(yè)英語 2010-07-08 16:03:31 閱讀25 評(píng)論0 字號(hào)：大中小訂閱 The process of surface engineering, or surface treatments, tailor the surfaces of engineering materials to: (1) control friction and wear, (2) improve corrosion resistance, (3) change physical property, e.g., conductivity, resistivity, and reflection, (4) alter dimension, (5) vary appearance, e.g., color and roughness, (6) reduce cost. Common surface treatments can be divided into two major categories: treatments that cover the surfaces and treatments that alter the surfaces. Cover the surfacesThe treatments that cover the surfaces include organic coatings and inorganic coatings. The inorganic coatings perform electroplatings, conversion coatings, thermal sprayings, hot dippings, furnace fusings, or coat thin films, glass, ceramics on the surfaces of the materials.Electroplating is an electrochemical process by which the metal is deposited on a substrate by passing a current through the bath.Usually there is an anode (positively charged electrode), which is the source of the material to be deposited; the electrochemistry which is the medium through which metal ions are exchanged and transferred to the substrate to be coated; and a cathode (negatively charged electrode) which is the substrate to be coated.Plating is done in a plating bath which is usually a non-metallic tank (usually plastic). The tank is filled with electrolyte which has the metal, to be plated, in ionic form.The anode is connected to the positive terminal of the power supply. The anode is usually the metal to be plated (assuming that the metal will corrode in the electrolyte). For ease of operation, the metal is in the form of nuggets and placed in an inert metal basket made out non-corroding metal (such as titanium or stainless steel).The cathode is the workpiece, the substrate to be plated. This is connected to the negative terminal of the power supply. The power supply is well regulated to minimize ripples as well to deliver a steady predictable current, under varying loads such as those in plating tanks.As the current is applied, positive metal ions from the solution are attracted to the negatively charged cathode and deposit on tire cathode. As a replenishment for these deposited ions, the metal from the anode is dissolved and goes into the solution and balances the ionic potential.Thermal spraying process. Thermal sprayed metal coatings are deposition of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties.Thermal spray is a generic term for a broad class of related processes in which molten droplets of metals, ceramics, glasses, and /or polymers are sprayed onto a surface to produce a coating, to form a free standing near-net-shape, or to create an engineered material with unique properties.In principle, any material with a stable molten phase can be thermally sprayed, and a wide range of pure and composite materials are routinely sprayed for both research and industrial applications. Deposition rates are very high in comparison to alternative coating technologies. Deposit thickness of 0.1 to 1 mm is common, and thickness greater than 1 cm can be achieved with some materials.The process for application of thermal spray metal is relatively simple and consists of the follow stages.(1) Melting the metal at the gun.(2) Spraying the liquid metal onto the prepared substrate by means of compressed air.(3) Molten particles are projected onto the cleaned substrate.There are two main types of wire application available today namely arc spray and gas spray.ARCA pair of wires are electrically energized so that an arc is struck across the tips when brought together through a pistol. Compressed air is blown across the arc to atomise and propel the autofed metal wire particles onto the prepared workpiece.GASIn combustion flame spraying the continuously moving wire is passed through a pistol, melted by a co