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Welding techniques have become so versatile that it is difficult nowadays to define“welding”.Formerly welding was“the joining of metals by fusion,”that is,by melting,but this definition will no longer do.Even though fusion methods are still the most common,they are not always used.Welding was next defined as the“joining of metals by heat,”but this is no longer a proper definition either.Not only can metals be welded,but also many of the plastic can.Furthermore,several welding methods do not require heat.Every machinist is familiar with heatless welding.Cold pressure welding is a proper production welding method under some circumstances.Besides pressure welding,we can weld with sound and even with light from the famous laser.Faced with a diversity of welding methods that increase year by year,we must here adopt the following definition of welding:“Welding is the joining of metals and plastics by methods that do not employ fastening devices.”
There is no uniform method of naming welding processes.Many processes are named according to the heat source or shielding method,but certain specialized processes are named after the type of joint produced.Examples are spot and butt welding.An overall classification can not take account of this because the same type of joint may be produced by a variety of processes.Spot welding may be done by electric resistance,arc,or electron-beam processes and butt welding by resistance,flash or any of a number of other methods.
There are a number of methods of joining metal articles together,depending on the type of metal and the strength of the joint which is required.
Soldering is the process of joining two metals by a third metal to be applied in the molten state.Solder consists of tin and lead,while bismuth and cadmium are often included to lower the melting point.One of the most important operations in soldering is that of cleaning the surface to be joined,this may be done by some acid cleaner.Soldering gives a satisfactory joint for light articles of steel,copper or brass,but the strength of a soldered joint is rather less than a joint which is brazed,riveted or welded.These methods of joining metal are normally adopted for strong permanent joints.
The simplest method of welding two pieces of metal together is known as pressure welding.The ends metal are heated to a white heat—for iron,the welding temperature should be about 1300℃—in a flame [1] .At this temperature the metal becomes plastic.The ends are then pressed or hammered together,and the joint is smoothed off.Care must be taken to ensure that the surfaces are thoroughly clean first,for dirt will weaken the weld.Moreover,the heating of iron or steel to a high temperature causes oxidation,and a film of oxide is formed on the heated surfaces.For this reason,a flux is applied to heated metal.At welding heat,the flux melts,and the oxide particles are dissolved in it together with any other impurities which may be present.The metal surfaces are pressed together,and the flux is squeezed out from the center of the weld.A number of different types of weld may be used,but for fairly thick bars of metal,a V-shaped weld should normally be employed.It is rather stronger than the ordinary butt weld.
The heat for fusion welding is generated in several ways,depending on the sort of metal which is being welded,and on its shape.An extremely hot flame can be produced from an oxyacetylene torch.
Generation of heat by an electric arc is one of the most efficient methods.Approximately 50% of the energy is liberated in the form of heat.The electric arc welding process makes use of the heat produced by the electric arc to fusion welded metallic pieces.This is one of the most widely used welding process,mainly because of the ease of use and high production rates that can be achieved economically.An arc is generated between two conductors of electricity,cathode and anode,when they are touched to establish the flow of current and then separated by small distance.An arc is sustained electric discharge through the ionized gas column called plasma between the two electrodes.It is generally believed that electrons liberated from the cathode move towards the anode and are accelerated in their movement.When they strike the anode at high velocity,large amount of heat is generated.
The endeavor of the welder is always to obtain a joint which is as strong as the welded metal itself and at the same time,the joint is as homogeneous as possible.To this end,the complete exclusion of oxygen and other gases to the detriment of the weld quality is very essential[ 2 ].In manual metal welding,the use of stick electrodes does this job to some extent but not fully.In inert gas shielded arc welding process,a high pressure inert gas is flowing around the weld electrode while welding would physically displace all the atmospheric gases around the weld metal to fully protect it.
Resistance welding process is a fusion welding process where both heat and pressure are applied on the joint but no filler metal or flux is added.The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the join and hence,the name resistance welding.In resistance welding,a low voltage(typically 1 V)with a very high current(typically 15 000 A)is passed through the joint for a very short time(typically 0.25 s).This high amperage heats the joint,due to the contact resistance at the joint and melts it.The pressure on the joint is continuously maintained and the metal is fused together under this pressure.
[1]The ends metal are heated to a white heat—for iron,the welding temperature should be about 1 300℃—in a flame.
火焰将金属的端部加热到白热状态——铁的焊接温度应在1 300℃左右。（本句中white heat的状语in a flame被插入的说明语隔开了，也可另外表述为:The ends metal are heated to a white heat in a flame.The welding temperature for iron should be about 1 300℃.）
[2]To this end,the complete exclusion of oxygen and other gases to the detriment of the weld quality is very essential.
为此，完全排除有害于焊接质量的氧和其他气体非常重要。（句中To this end=For this purpose,介词短语to the detriment of...是oxygen and other gases的后置定语，意为“有害于……”）
焊接技术的应用如此广泛，以至于现在都很难定义“焊接”了。原先，焊接被定义为“通过熔融连接金属”，也就是说，通过熔化，但这种定义已不再适用。尽管熔融法仍然是最普通的，却并非总被采用。焊接随后被定义为“借助热量将金属连接”，现在这个定义也不完善。不仅金属可以焊接，很多塑料也可以，很多焊接方法并不需要热量。每个机械工人都对无热焊接很熟悉。冷压焊接在某些情况下是很合适的生产焊接方法。除压力焊接外，我们还可以用声波甚至激光来焊接。面对历年创造的多样化焊接方法，我们必须在这里采用下述焊接定义：“焊接是金属和塑料不采用紧固件的连接法。”
命名焊接没有一致的方法。许多焊接过程根据热源和保护方法来命名，某些专门的焊接过程按照形成的连接形式来命名，例如，点焊和对焊。总体的分类不可能考虑这一点，因为同样的连接可以通过许多焊接过程产生。点焊可借助电阻焊、电弧焊或电子束焊来完成，而对焊可以借助电阻焊、闪光焊（火花对焊）或其他多种方法中的一种来完成。
根据金属的种类和焊接所需的强度，可有几种将金属件连接到一起的方法。
软钎焊是通过施加第三种熔化金属来连接两种金属的方法。焊接材料有锡和铅，同时加入铋和镉来降低熔化点。软钎焊极重要的一项操作是对焊接表面的清洁，这可用某种酸来完成。软钎焊对轻型钢、紫铜或黄铜工件的焊接效果不错，但软钎焊的强度比硬钎焊、铆接或（常规）焊接低很多。这些连接金属的方法通常用于高强度的永久连接。
将两块金属焊接在一起的最简单的方法为压焊。火焰将金属的端部加热到白热状态——铁的焊接温度应在1 300℃左右。在这个温度下，金属成为塑性状态。然后将端部挤压或用锤敲打在一起，再将接头修平。应当注意确保先将焊接表面彻底清洁，否则污物会降低焊接强度。尤其是铁和钢加热到很高温度时会引起氧化，并在加热表面形成一层氧化膜。因此，要在加热表面施加焊剂。在焊接热作用下，焊剂融化，氧化物颗粒和任何其他可能存在的杂质熔解在焊剂里。金属表面被压在一起，焊剂被挤出焊接中心。可采用几种不同的焊缝，但对相当厚的金属条，一般使用V型焊缝，它比通常的平焊缝强得多。
按照被焊接金属的不同种类和形状，熔融焊接的热量可由多种方式产生。极热的火焰可由氧乙炔炬产生。
电弧生热是最高效的方法之一。大约50%能量以热能的形式释放。电弧焊利用电弧产生的热量来熔化被焊接的金属件。这是最广泛采用的焊接方法之一，主要是因为它易用且经济、高产。两个导电体，阴极和阳极接触会产生电流，然后分开微小距离，则在两极之间产生一道电弧。电弧是通过两极间称为等离子体的电离气柱的持续的放电产生的。一般认为从阴极释放的电子向阳极移动并在运动中加速。当它们高速撞击阳极时，就产生了大量的热。
焊接者的不懈努力总是要获得和被焊金属本身一样强度的连接，并且同时连接处要尽可能均匀。为此，完全排除有害于焊接质量的氧和其他气体非常重要。在手工电弧焊中，采用棒状电极可一定程度上完成这一任务。采用惰性气体保护焊过程中，焊接时一股高压的惰性气体从焊接电极周围流过，实际上排除了焊接金属周围的全部气体，从而起到保护作用。
电阻焊是一种在连接处同时施加热和压力的熔融焊接，但不采用填料金属或焊剂。熔化连接所需的热量通过连接的电阻效应获得，因而取名电阻焊。在电阻焊中，一股低电压（典型1伏）但很强的电流（典型15 000安）以很短的时间流经连接处（典型0.25秒）。由于连接处的接触电阻，使这股强电流加热并熔化连接处。连接处的压力持续作用，使金属在压力作用下熔融在一起。
In addition to being familiar with the hardware components,to be an effective engineer one should also have some hands-on experience with the machine tools and machining technique that one day will be used to fabricate the products that you design [1] .In this section,we discuss cutting with drill presses,band saws,lathes,grinders and mills,which are some of the tools available to perform standard machining operations in prototyping and production shops.Each of these tools is based on the principle of removing unwanted material from a workpiece through the cutting action of sharpened bits or blades.
A drill press is used to bore holes into a workpiece.A drill bit is held in the rotating chuck,and as a machinist turns the pilot wheel,the bit is lowered into the surface of the workpiece.The bit removes material in small chips and creates a hole in the workpiece.As should be the case,whenever metal is machined,the point where the bit cuts into the workpiece is lubricated.The oil reduces friction,and it also helps to remove heat from the cutting region.For safety reasons,vises and clamps securely hold the workpiece as the hole is bored in order to prevent material from shifting unexpectedly.
A band saw makes rough cuts through metal or plastic stock.The blade is a long,continuous loop having sharp teeth on one edge,and it rides on the drive and idle wheels.A variable-speed motor enables the operator to adjust the blade’s speed depending on the type and thickness of material that will be cut.A tilting table is used for support.The machinist feeds the workpiece into the blade and guides it by hand,to make straight or slightly curved cuts.When the blade becomes dull and needs to be replaced,or if it should break,the band saw’s internal blade grinder and welder are used to clean up the blade’s ends,connect them,and form a loop[ 2 ].
Grinding machine is a random point cutting tool which use abrasives in the shape of a wheel,bonded to a belt,a stick or simply suspended in liquid.On a cylindrical grinding machine the grinding wheel rotates between 5,500 and 6,500 r/min,while the work rotates between 60 and 125 r/min.The depth of cut is controlled by moving the wheel head.Coolants are provided to reduce heat distortion and to remove chips and abrasive dust.The grinding process is very important in production work for several reasons:
(1)It is the most common method for cutting hardened tool steel or other heat treated steel.
(2)It can provide surface finish to 0.5μm without high cost.
(3)The grinding operation can assure accurate dimensions in a relatively short time.
(4)Tiny and thin parts can only be finished by this method.
A lathe holds the workpiece and rotates it,and the sharpened edge of a tool cuts and removes material.Some applications of a lathe include the production of shafts,and thread cutting.The chuck on headstock hold one end of a workpiece,and the drive mechanism spins it rapidly about its centerline.The tailstock can be used to provide support to the otherwise free end of a long workpiece.As a cutting tool fed against the bar stock and moved along its length,the diameter of the work piece is reduced to a desired dimension.Shoulders that will locate bearings on a shaft,grooves for holding retaining clips,and sharp changes in diameter of a stepped shaft can each be made in this manner.
Planing is a relatively simple cutting operation which flat surfaces,as well as various cross sections with grooves and notches,are produced along the length of the workpiece.Planing is usually done on large workpieces—as large as 25 m×15 m.In a planer,the workpiece is mounted on a table that travels along a straight path.A horizontal cross-rail,which can be moved vertically,is equipped with one or more tool heads.The cutting tools are attached to the heads,and machining is done along a straight path.Because of the reciprocating motion of the workpiece,elapsed non cutting time during the return stroke is significant.
A mill is versatile machine tool which is useful for machining the rough surfaces of a workpiece flat and smooth and for shaping them with slots,and holes.The workpiece is held by a vise on an adjustable table so that the part can be accurately moved in three directions(in the plane of the table and perpendicular to it)to locate the workpiece beneath the cutting bit.In a typical application,a piece of metal plate would be cut to approximate shape with a band saw,and the mill would be used to machine the surfaces and edges smooth,square,and to the final dimensions.
A machine component known as a lead screw is often used in the mill’s internal mechanisms for positioning the work table beneath the spindle.The lead screw converts rotation of a shaft,produced either by a hand wheel or an electric motor,into the straight-line motion of the work table.As the lead screw turns and engages the nut(which is not allowed to rotate),the nut moves along the screw with each rotation by an amount equal to the distance between threads.
By using electric motors to drive lead screws and position the workpiece or cutting bit,mills and other machine tools can be controlled by computers.In that manner,a shop operation can be automated to achieve high precision or to complete a repetitive task on a large number of parts.A computer numerically controlled(CNC)mill performs the same types of operations as a conventional mill,but instead of being manually operated,it is programmed either through entry on a keypad or by downloading machining instructions that are created by computer aided engineering software.CNC machine tools offer the potential to seamlessly produce physical hardware directly from a computer-generated drawing.With the ability to quickly reprogram machine tools,even a small general-purpose shop can produce a variety of high-quality machine components.
The fabrication techniques that an engineer selects for a certain product will depend,in part,on the time and expense of setting up the tooling.Some devices(for instance,air-conditioner compressors,microprocessors,hydraulic valves,and tires) are mass-produced,implying a process that is based on widespread mechanical automation.Mass production might be what you first imagine to take place in a large factory,and the manufacture of automobile engines is a prime example.The assembly line in that case comprises custom tools,fixtures,and specialized processes that are capable of producing only certain types of engines for certain vehicles.The assembly line will be set up so that only a small number of operations need to be performed in any one work area before the engine moves on to the next stage.Because engines will be produced at a relatively high rate,it is cost-effective for a company to allocate a large amount of factory floor space and many expensive machine tools to the production line,each of which might only drill a few holes or make a few welds[ 3 ].Aside from hardware that is produced through mass manufacturing,other products are unique(for instance,the Hubble space telescope) or made in relatively small quantities(such as commercial jetliners),the best production method for a given product will ultimately depend on the quantity to be produced,the allowable cost,and the level of part-to-part variability that is acceptable.
[1]In addition to being familiar with...to fabricate the products that you design.
除熟悉机械零件外，作为一个实际的工程师还要具有一些机床和机加工技术的第一手实践经验，也许有一天这些经验将用于制造你所设计的产品。（这个长句主语是one,In addition to短语是动词的方式状语,to be an effective engineer是目的状语。hands-on experience是指某人对某事做过或运用过的经验，而不是仅仅读过或学到过的经验）
[2]When the blade becomes dull and needs to...and form a loop.
当锯条变钝或万一断裂需要更换时，锯床的内部磨齿机和焊机可用来清洁锯条的端部，将它连接成环状。（本句使用了并列主语grinder and welder和并列谓语clean up,connect,form。如果具备专业素养，可以看出其结构本应是分列的，即用grinder来clean up,用welder来connect。两种工具联合作业，使saw重新form a loop。为使语句不累赘，在意思不混淆的情况下采用了并列结构）
[3]Because engines will be produced at a...of which might only drill a few holes or make a few welds.
因为机器生产高效，成本效益高，所以一家公司可能会投入大量的厂房，为生产线购置昂贵的机械工具，也许这些机器仅需钻少数几个孔，或焊接几个地方。（组合词cost-effective意为“成本高效的”。本句it作形式主语,for短语中的machine tools有一个很长的从句做补充说明，结构上把它放在后边）
除熟悉机械零件外，作为一名工程师还要具有一些机床和机加工技术的第一手实践经验，也许有一天这些经验将用于制造你所设计的产品。本处我们讨论用钻床、带锯、车床、磨床和铣床切削，这些机床是样机加工和生产车间进行标准机械加工的可选用工具。每一种机床都是基于通过刀具切削作用去除工件多余材料的原理。
钻床在工件上钻孔。钻头被夹持在旋转的卡盘上，当机械工人转动手轮，钻头降低进入工件。钻头除去变成碎屑的材料，并在工件上打一个孔。正如实际情况那样，任何时候加工金属，在刀头切入工件的点上都要润滑。油液减少了摩擦力，也有助于从切削区带走热量。为了安全起见，钻孔时夹具牢牢地夹住工件，以防材料意外移位。
带锯粗糙地切穿金属和塑料坯。锯条是边缘开了铣齿的连续长环，它在主动轮和从动轮间运行。高速电机使操作者可根据材料的种类和切削的厚度调节锯条的速度。一个可倾斜的工作台用来支承。机械工人将工件进给到锯条上并用手调节切成直线或略带弧形。当锯条变钝或万一断裂需要更换时，锯床的内部磨齿机和焊机可用来清洁锯条的端部，将它连接成环状。
磨床是一种采用轮状、黏结在带上、棒状或仅仅悬浮在液体中的研磨材料进行随机点切削的机床。在圆柱磨床上，砂轮以5 500至6 500转/分的速度旋转，而工件以60至125转/分的速度旋转。切削深度通过移动砂轮头来控制。冷却剂被用来降低热翘曲并将切屑和磨料粉带走。磨削加工非常重要，原因如下：
1.是切削硬质工具钢或其他热处理过的钢材的最普通的方法。
2.无须高成本即可提供至0.5微米的表面精度。
3.磨削加工可以在相对较短的时间内保证精确的尺寸。
4.细小和薄的零件只能用这种方法精加工。
车床夹持工件并使它旋转，而锋利的刀具则切除材料。车床的应用包括轴的加工和螺纹切削。床头箱上的卡盘夹住工件的一端，传动机构使之快速地绕中心线旋转。尾架可用来为长工件另一自由端提供支承。当刀具对着工件进给并沿长度方向移动时，工件的直径被减小到需要的尺寸。安装轴承的轴肩，容纳止推环的槽，阶梯轴的直径突变部位都可用这种方法制造。
刨削是相对简单的一种切削作业，用这种方法可沿工件的纵向加工出平面以及各种带沟作槽的截面。刨削一般用于大工件——大至25米×15米。在刨床上，工件被安装在直线移动的工作台上。可垂直移动的横向水平导轨上装备有一个或多个刀头。刀具安装在刀头上，切削沿纵向完成。由于工件的往复运动，返程花去的非切削时间很多。
铣床用于加工扁平工件的粗糙表面，在工件上打槽、孔，是一种很有用的多功能机床。工件由可调工作台上的台钳夹持，以便能够沿3个方向精确运动（工作台平面内和垂直于工作台方向）来定位刀具下的工件。在典型的应用中，一块钢板可用锯床锯成大致的形状，用铣床来加工表面和边缘，使之平整、方正，并达到最终尺寸。
通常在铣床的内部机构中常用一个称为丝杠的部件来将工件定位在主轴下。丝杠将手轮或电动机产生的轴向旋转运动转化为工作台的直线运动。当丝杠转动并结合螺母（不允许转动）时，螺母沿丝杠移动，丝杠每转一圈，螺母移动一个螺距。
在借助电动机来驱动丝杠并定位工件或刀具的情况下，铣床或其他机床可实现计算机控制。按这种方式，车间的作业可实现自动化，以达到高精度或在大量零件上完成重复的任务。尽管一台计算机数字控制(CNC)机床做的工作与普通铣床一样，但它不是手动操纵，而是由键盘输入的指令操控或由下载的工程软件生成的机器指令进行操纵的。数控机床提供从计算机图纸到硬件“无缝”生产的潜力。由于机床快速再编程的能力，甚至一个小型通用车间可生产大量不同的高质量机器零件。
工程师为某一特定产品选用的加工工艺将部分取决于调整加工工具的时间和成本。某些装置（例如，空调压缩机、微处理器、液压阀和轮胎）是批量生产的，这意味着一个基于广泛采用的机械自动化过程。批量生产可能是你最先想象到发生在大工厂的情况，汽车发动机生产就是一个典型的例子。这种情况下的装配线包括专用工具、夹具和仅能生产某种车上特定型号发动机的专门化工艺。装配线的建立使得任一个工位在发动机移动到下一工位前只需完成少量的操作。因为机器生产高效，成本效益高，所以一家公司可能会投入大量的厂房，为生产线购置昂贵的机械工具，也许这些机器仅需钻少数几个孔，或焊接几个地方。除了批量生产的硬件，其他产品无论是单件加工（例如，哈勃天文望远镜）还是少量生产（例如，商用喷气式客机），对某一给定产品的最佳生产方法将最终取决于生产的数量、成本开支范围和可接受的零件与零件之间的差异级别。
