Unit 1  Metals
金属
    1 The use of metals has always been a key factor in the development of the social systems of man. Of the roughly 100 basic elements of which all matter is composed, about half are classified as metals. The distinction between a metal and a nonmetal is not always clear-cut. The most basic definition centers around the type of bonding existing between the atoms of the element, and around the characteristics of certain of the electrons associated with these atoms. In a more practical way, however, a metal can be defined as an element which has a particular package of properties.
    2 Metals are crystalline when in the solid state and, with few exceptions (e.g. mercury), are solid at ambient temperatures. They are good conductors of heat and electricity and are opaque to light. They usually have a comparatively high density. Many metals are ductile-that is, their shape can be changed permanently by the application of a force without breaking. The forces required to cause this deformation and those required to break or fracture a metal are comparatively high, although, the fracture forces is not nearly as high as would be expected from simple consideration of the forces required to tear apart the atoms of the metal.
    3 One of the more significant of these characteristics from our point of view is that of crystallinity. A crystalline solid is one in which the constituent atoms are located in a regular three-dimensional array as if they were located at the corners of the squares of a three-dimensional chessboard. The spacing of the atoms in the array is of the same order as the size of the atoms, the actual spacing being a characteristic of the particular metal. The directions of the axes of the array define the orientation of the crystal in space. The metals commonly used in engineering practice are composed of a large number of such crystals, called grains. In the most general case, the crystals of the various grains are randomly oriented in space. The grains are everywhere in intimate contact with one another and joined together on an atomic scale. The region at which they join is known as a grain boundary.
    4 An absolutely pure metal (i.e. one composed of only one type of atom) has never been produced. Engineers would not be particularly interested in such a metal even if it were to be produced, because it would be soft and weak. The metals used commercially inevitably contain small amounts of one or more foreign elements, either metallic or nonmetallic. These foreign elements may be detrimental, they may be beneficial, or they may have no influence at all on a particular property. If disadvantageous, the foreign elements tend to be known as impurities. If advantageous, they tend to be known as alloying elements. Alloying elements are commonly added deliberately in substantial amounts in engineering materials. The result is known as an alloy.
    5 The distinction between the descriptors “metal” and “alloy” is not clear-cut. The term “metal” may be used to encompass both a commercially pure metal and its alloys. Perhaps it can be said that the more deliberately an alloying addition has been made and the larger the amount of the addition, the more likely it is that the product will specifically be called an alloy. In any event, the chemical composition of a metal or an alloy must be known and controlled within certain limits if consistent performance is to be achieved in service. Thus chemical composition has to be taken into account when developing an understanding of the factors which determine the properties of metals and their alloys.
    6 Of the 50 or so metallic elements, only a few are produced and used in large quantities in engineering practice. The most important by far is iron, on which are based the ubiquitous steels and cast irons (basically alloys of iron and carbon). They account for about 98% by weight of all metals produced. Next in importance for structural uses (that is, for structures that are expected to carry loads) are aluminum, copper, nickel, and titanium. Aluminum accounts for about 0.8% by weight of all metals produced, and copper about 0.7%, leaving only 0.5% for all other metals. As might be expected, the remainders are all used in rather special applications. For example, nickel alloys are used principally in corrosion-and heat-resistant applications, while titanium is used extensively in the aerospace industry because its alloys have good combinations of high strength and low density. Both nickel and titanium are used in high-cost, high-quality applications, and, indeed, it is their high cost that tends to restrict their application.
    7 We cannot discuss these more esoteric properties here. Suffice it to say that a whole complex of properties in addition to structural strength is required of an alloy before it will be accepted into, and survive in, engineering practice. It may, for example, have to be strong and yet have reasonable corrosion resistance; it may have to be able to be fabricated by a particular process such as deep drawing, machining, or welding; it may have to be readily recyclable; and its cost and availability may be of critical importance.
        在人类社会的发展中，金属的应用起着关键性的作用。构成物质的大约100种基本元素中，大约有一半为金属。金属和非金属之间的区别不是特别明显。最基本的定义集中在元素原子间存在的连接形式和与这些原子相关联的电子的某些特性。然而，在实际应用中，可以将具有某些特性集合金属定义为某种元素。
        除了少数例外金属在常温下是固态的。它们是热和电的良导体，不透光。它们往往具有较高的密度。许多金属具有延展性，也就是说，在不被破坏的情况下它们的形状在外力的作用下可以发生变化。引起永久变形所需的力和最终使金属断裂所需的力相当大，尽管发生断裂所需的力远没有像所预期的撕开金属原子所需的力那么大。
    从我们的观点来看，在所有的特性中结晶性是最重要的。结晶体是这样一种结构，组成它的原子定位在规则的三维排列中，仿佛位于三维棋盘的方格的角上。原子间距随着原子大小呈规律性变化，原子间距是金属的一种特性。三维排列的轴线决定了晶体在空间中的方向。在工程实践中应用的金属由大量的晶体组成，这些晶体称之为晶粒。在大多数情况下，晶粒在空间中是自由排列的。在原子范围内，晶粒之间相互接触紧密结合。晶粒之间连接区域被称为晶界。
    绝对纯净的金属从来也没有被生产出来过。即使绝对纯净的金属可以生产出来，工程师们对它们也并不会特别感兴趣，因为它们很柔软、脆弱。实际应用中的金属往往都包含着一定数量的一种或多种外来金属或非金属元素，这些外来元素可能是有害的也可能是有益的或者它们对某种特定的属性没有影响。如果是有害的，这些外来元素被认为是杂质。如果是有益的，它们被认为是合金元素。在工程材料中往往被特意地加入一定数量的合金元素。得到的物质被叫做合金。
    金属和合金区别不大。金属这个词可以包括工业用纯金属和它的合金。也许可以这样说，合金元素越故意的被添加，被添加的合金元素的量越大，那么生产出来的产品越倾向于被称之为合金。不管怎样，如果想使一种金属或合金在使用中表现出稳定一致的特性，在其中添加何种化学成分，它的量多大都应该在控制范围之内。因此，当想了解决定金属和合金性质的因素时，应充分考虑它们的化学组成。
    在50种左右的金属元素里，工程实践中只有少数金属被大量生产和使用。到目前为止最重要的是铁，以它为基础构成了处处可见的钢和铸铁。（主要由铁和碳构成的合金）它们的重量占所有生产出来的金属重量的98%。在结构应用（也就是说，可以承受载荷的结构）中居于其次位置的是铝、铜、镍和钛。在所有的金属产量中，铝占0.8％，铜占0.7％，剩下的占0.5％。剩下的金属用于相对特殊的用途。例如，镍合金主要用于抗磨损和耐高温的用途，由于钛合金具有高强度和低密度的综合特性，钛被广泛应用于航空工业中。镍合钛有高成本和高质量的使用特性，事实上，它们高的成本限制了它们的应用。
    我们不能在这里讨论这些深奥的特性。在合金材料被采用和应用于工程实际之前，掌握其结构强度和它的综合性质就够了。举例来说，它可以强度很高，并且有好的耐磨性；它可以被例如拉伸加工，机械加工，或焊接等特殊工艺来加工出来；它可以被循环利用；它的成本和实用性是首要的。
Unit 2 Selection of Construction Materials
工程材料的选择
1 There is not a great difference between “this” steel and “that” steel; all are very similar in mechanical properties. Selection must be made on factors such as hardenability, price, and availability, and not with the idea that “this” steel can do something no other can do because it contains 2 percent instead of 1 percent of a certain alloying element, or because it has a mysterious(神秘的，不可思议的) name. A tremendous range of properties is available in any steel after heat treatment; this is particularly true of alloy steels.
在钢之间没有太大的区别；所有的钢在机械性能方面都是近似的。它们的选取标准是诸如脆硬性，价格，和可用性等。不仅仅是因为这种钢含有2％的合金元素另一种钢含有1％而使前者具有了后者没有的某些能力，或者是某种钢具有神奇的名字。经过热处理后，任何一种钢都具有大范围的特性；这种性质同样在合金钢中存在。
Considerations in fabrication(制造)
2 The properties of the final part (hardness, strength, and machinability), rather than properties required by forging, govern the selection of material. The properties required for forging have very little relation to the final properties of the material; therefore, not much can be done to improve its forgeability. Higher-carbon steel is difficult to forge. Large grain size is best if subsequent heat treatment will refine the grain size.
关于加工的考虑
最后零件的特性（硬度、强度和可加工性）而不是锻造特性决定了材料的选择。可锻性与材料的最后特性联系不大；因此，提高金属的可锻造性价值不大。高碳钢很难锻造。如果在随后的热处理过程进行细化，大尺寸晶粒是最好的。
3 Low-carbon, nickel-chromium(铬) steels are just about as plastic at high temperature under a single 520-ft·lb(1 ft·lb=1.35582J) blow as plain steels of similar carbon content. Nickel decreases forgeability of medium-carbon steels, but has little effect on low-carbon steels. Chromium seems to harden steel at forging temperatures, but vanadium(钒) has no discernible(可辨别的) effect; neither has the method of manufacture any effect on high-carbon steel.
在高温下低碳，镍铬合金钢在受到520-ft·lb的冲击下表现出与相同碳含量普通钢几乎同样的塑性。镍减少了中碳钢的可锻性，但对低碳钢影响不大。铬在锻造温度下时使钢硬化，但钒没有明显的效果；两种加工方法对高碳钢没有影响。
Formability
4 The cold-formability of steel is a function(功能) of its tensile strength combined with ductility. The tensile strength and yield point must not be high or too much work will be required in bending（弯曲）; likewise（同样地）, the steel must have sufficient（充足的） ductility to flow to the required shape without cracking. The force required depends on the yield point, because deformation starts in the plastic range above the yield point of steel. Work-hardening also occurs here, progressively（日益增多地） stiffening（使变硬） the metal and causing difficulty, particularly（独特的，显著的） in the low-carbon steels.
成形
钢的冷成形是它的拉伸强度和延展性相结合的结果。拉伸强度和屈服点不能太高否则在发生弯曲时需要做很多工作；与之相类似，钢应该有高延展性，使其在没有断裂的情况下成形。加工力的大小取决于屈服点，因为钢在屈服点之上才开始变形。与此同时，加工硬化也同时发生，金属变得越来越硬，增加加工难度，尤其在低碳钢中容易发生。
5 It is quite interesting in this connection（关于这一点，就此而论） to discover that deep draws can sometimes be made in one rapid operation that could not possibly be done leisurely（缓慢地，从容不迫地） in two or three. If a draw is half made and then stopped, it may be necessary to anneal（退火） before proceeding, that is（换句话说）, if the piece is given time to work-harden. This may not be a scientific statement, but it is actually what seems to happen.
在这方面，相当有趣的是你将发现有时可通过一次快速加载完成大拉伸，但以缓慢的方式两三次加载却不能实现。如果拉伸进行了一半就停止了，那么在再加工之前应先退火，也就是说工件是否有时间进行加工硬化。这不是一种科学的叙述方法，但确实是发生了。
Internal stresses
6 Cold forming is done above the yield point in the work-hardening range, so internal stresses can be built up easily. Evidence of this is the springback（回弹） as the work leaves the forming operation and the warpage（翘曲，扭曲） in any（任何一种） subsequent heat treatment. Even a simple washer might, by virtue of(依靠) the internal stresses resulting from punching（冲压） and then flattening（整平）, warp（弯曲） severely（严格地，激烈地） during heat treating. （virtue n.德行, 美德, 贞操, 优点, 功效, 效力, 英勇 believed in the virtue of prayer.相信祈祷的力量
内应力
在高于屈服点的加工硬化区进行冷加工很容易产生内应力。例如工件停止成型加工后会发生回弹，在随后的热处理后，工件会发生翘曲。即使是一个简单的垫圈，由于打孔和随后的平整加工中产生内应力，也会在热处理中呈现严重的翘曲。
7 When doubt exists as to（关于） whether internal stresses will cause warpage, a piece can be checked by heating it to about 1100 and then letting it cool. If there are internal stresses, the piece is likely to（可能） deform. Pieces that will warp severely while being heated have been seen, yet （然而）the heat-treater was expected to put them through and bring them out better than they were in the first place.
当是否内应力会引起翘曲的怀疑存在时，可以通过将工件加工至1100然后进行冷却来验证。如果存在内应力，工件会发生变形。经过热处理的工件像我们看到的那样会发生严重的翘曲，但是我们仍然希望工件被扔到热处理炉中被处理，这样好过它存在内应力的状态。
Welding
8 The maximum carbon content of plain carbon steel safe for welding without preheating or subsequent heat treatment is 0.3%. higher-carbon steel is welded every day, but only with proper preheating. There are two important factors: the amount of heats that is put in ; the rate at which it is removed.
焊接
不需要预热或之后进行热处理就能安全焊接的最高碳含量为0.3％。高碳钢通过合适的预热通常也可焊接。有两点值得注意：吸收热量的多少；移除速度。
9 Welding at a slower rate puts in more heat and heats a large volume of metal, so the cooling rate due to loss of heat to the base metal is decreased(减少). A preheat will do the same thing. For example, sae4150 steel, preheated to 600 or 800, can be welded readily（容易地）. When the flame or arc is taken away from the weld, the cooling rate is not so great, owing to the higher temperature of the surrounding metal and slower cooling results. Even the most rapid air-hardening（风硬钢） steels are weldable if preheated and welded at a slow rate.
低速焊接带来了更多的热量，这对金属的大量体积进行了加热，所以冷却速度降低。预热可以取得与之相当的效果。例如当  被预热至  或  时可以很好的焊接。由于周围金属的较高温度，当焊接弧移开焊接点后，冷却速度不会太快，产生了低速冷却的结果。即使是冷作硬化速度最快的金属也可以通过预热和慢速焊接达到良好的焊接效果。
Machinability
10 Machinability（机械加工性能） means several things. To production men it generally means being able to remove metal at the fastest rate, leave the best possible finish, and obtain the longest possible tool life. Machinability applies to（应用于） the tool-work（工具，零件） combination.
可加工性
可加工性意味着几件事情。对于加工者来说，它意味着可以快速的移除金属，取得最好的加工效果，得到最长的刀具寿命。可加工性是刀具和零件的结合。
11 It is not determined by hardness（硬度） alone, but by the toughness（韧性）, microstructure, chemical composition（成分）, and tendency（倾向） of a metal to harden under cold work. In the misleading expression “too hard to machine”, the word “hard” is usually meant to be synonymous（同义的） with “difficult”. Many times a material is actually too soft to machine readily. Softness and toughness may cause the metal to tear（撕裂） and flow ahead of the cutting tool rather than cut cleanly. Metal that are inherently（天性地，固有地） soft and tough are sometimes alloyed to improve their machinability at some sacrifice（牺牲） in ductility. Examples are use of lead（铅） in brass（黄铜） and of sulfur（硫磺） in steel.
加工性不仅仅只由硬度决定，它还由韧性，微观结构，化学成分和在冷加工下金属所呈现的硬化特性所决定。在容易混淆的表示“难加工”中，“hard”与“difficult”同义。许多时候，因为材料过软而难于稳定加工。材料柔软性和韧性能够产生金属撕裂，使金属在完成切削前流动至刀具前端。柔软的金属往往会被加入合金从而牺牲它的延展性来提高加工性能。如黄铜中加入铅钢中加入硫磺。
12 Machinability is a term used to indicate the relative（比较的） ease（不费力） with which a material can be machined by sharp cutting tools in operations such as turning（车）, drilling（钻）, milling（铣）, broaching（拉削）, and reaming（铰）.
机械加工性能是在指对工件材料使用刀具进行诸如车、钻、铣、拉削、铰加工时的难易程度。
    13 In the machining of metal, the metal being cut, the cutting tool, the coolant, the process and type of machine tool（机床）, and the cutting conditions all influence the results. By changing any one of these factors, different results will be obtained. The criterion（标准） upon which the ratings（等级） listed are based(等级评定的标准) is the relative volume of various（不同种） materials that may be removed by turning under fixed conditions to produce an arbitrary（任意的） fixed amount of tool wear.
在对金属进行加工时，被切削的金属，切削刀具，冷却液，使用的机床的种类，切削条件均影响着切削效果。改变任何一种均会产生不同的切削效果。切削效果评定的准则是：车削时在固定的切削条件下产生一定量的刀具磨损时，被加工试件相应的材料去除量。
Unit 3 Mechanical Properties of Materials
1 The material properties can be classified into three major headings: (i) Physical, (ii) Chemical, (iii) Mechanical.
Physical properties
2 Density or specific gravity, moisture content, etc., can be classified under this category.
Chemical properties
3 Many chemical properties come under this category. These include acidity or alkalinity, reactivity and corrosion. The most important of these is corrosion which can be explained in layman’s terms as the resistance of the material to decay while in continuous use in a particular atmosphere.
Mechanical properties
4 Mechanical properties include the strength properties like tensile, compression, shear, torsion, impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of cross-section of the specimen.
5 This is a curve plotted between the stress along the Y-axis(ordinate) and the strain along the X-axis(abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappear s. For many materials this occurs up to a certain value of the stress called the elastic limit. This is depicted by the straight line relationship and a small deviation thereafter, in the stress-strain curve.
6 Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality. In this region, the metal obeys Hooke’s law, which states that the stress is proportional to strain in the elastic range of loading (the material completely regains its original dimensions after the load is removed). In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the elastic limit. This may be attributed to the time-lag in the regaining of the original dimensions of the material. This effect is very frequently noticed in some no-ferrous metals.
7 While iron and nickel exhibit clear ranges of elasticity, copper, zinc（锌）, tin（锡）, etc, are found to be imperfectly elastic even at relatively（相当地） low values of stresses. Actually the elastic limit is distinguishable（可区分的） from the proportionality（比例性） limit more clearly depending upon the sensitivity（灵敏性） of the measuring instrument.
8 When the load is increased beyond the elastic limit, plastic deformation starts. Simultaneously the specimen gets work-hardened. A point is reached when（这时） the deformation starts to occur more rapidly than the increasing load. This point is called the yield point Q. the metal which was resisting the load till then, starts to deform somewhat rapidly, i. e. , yield. The yield stress is called yield limit.
9 The elongation of the specimen continues form Q to S and then to T. the stess-strain relation in this plastic flow period is indicated by the portion QRST of the curve. At T the specimen breaks, and this load is called the breaking load. The value of the maximum load S divided by the original cross-sectional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength.
10 Logically speaking, once the elastic limit is exceeded, the metal should starts to yield, and finally break, without any increase in the value of stress. But the curve records an increased stress even after the elastic limit is exceeded. Two reasons can be given for this behavior:
1. the strain hardening of the material;
2. the diminishing cross-sectional area of the specimen, suffered on account of the plastic deformation.
11 The more plastic deformation the metal undergoes, the harder it becomes, due to work-hardening. The more the metal gets elongated the more its diameter(and hence, cross-sectional area) is decreased. This continues until the point S is reached.
12 After S, the rate at which the redution in area takes place, exceeds the rate at which the stress increases. Strain becomes so high that the redution in area begins to produce a localized effect at some point. This is called neching.
13 Reduction in cross-sectional area takes place very rapidly; so rapidly that the load value actully drops. This is indicated by ST. Failure occurs at this point T.
14 Then percentage elongation and reduction in area indicate the ductility or plasticity of the material:
Where and are the original and the final length of the specimen; and are the original and the final cross-sectional area.
    材料特性主要分为三类：(i)物理特性，(ii)化学特性，(iii)力学性能。
物理特性
    密度或特定的重力，湿度等都属于此范畴。
化学特性
    许多化学特性都归入到这个范畴。其中包括酸性或碱性，活性和耐腐蚀性。而在这其中最重要的是耐腐蚀性，通俗的解释是材料在特定大气中长期使用时，抵抗腐蚀的能力。
力学特性
    力学特性包括诸如拉伸，压缩，剪切，扭转，冲击，疲劳和蠕变等强度特性。一种材料的拉伸强度由试件承载的最大载荷除以试件的横截面积得到。
    如图所示为在拉伸试验中沿着X轴（横轴）的应变和沿着Y轴（纵轴）的应力之间的关系曲线。材料在加载时，随着载荷大小的变化，尺寸会发生改变。当卸载时，变形消失。对于许多材料来说，上述情况发生的应力极限值称为弹性极限。在应力-应变曲线中，直线关系和随后的小小的弯曲描述了上述的加载和卸载。
    在弹性范围内，应力应变成比例的应力极限值称为比例极限。在这个区域中，金属服从胡克定律—阐述了在加载的弹性范围内，应力和应变成比例关系（材料在卸载后，能够完全回复它原来的尺寸）。在曲线的实际绘制中，比例极限值要稍微比弹性极限值低。这可能是由于材料回复原尺寸需要的时间延迟。这种现象在一些有金属中常常出现。
    铁和镍存在明显的弹性范围，铜，锌，锡等即使在相当低的应力值下弹性也表现得不是很充分。实际上依靠测试仪器的精确性可以使比例极限和弹性极限区分得更清晰。
    当在弹性极限之上增大载荷时会产生塑性变形。同时，试件发生加工硬化。到达某点后变形的速度快于载荷增加的速度。这一点叫做屈服极限点。一开始一直在抵抗载荷的金属在这一点后开始迅速地发生形变，也就是，屈服。屈服应力叫做屈服极限。
    试件从Q到S在到T不断地延长。在这个塑性流动期间的应力-应变关系表示为曲线上的QRST段。在T点试件断裂，此时的载荷称为断裂载荷。最大荷载值S除以试件的横截面积为金属的最大拉伸强度或简单地称为拉伸强度。
    逻辑上来说，一旦超过弹性极限，金属应该就会屈服直至最后断裂，在应力值上应该没有增加。但是实际的曲线却记录了在超过弹性极限后的增加了的应力。这种现象的发生可能有两种原因：
1. 材料的应变强化；
2. 由于塑性变形引起的试件横截面积的缩小。
    （For Christ was God, and suffered on account of us, being himself the Father, that he might be able to save us.因为基督就是上帝，为我们受苦，他自己身为父，好叫祂也能救赎我们。）
    由于加工硬化，金属在发生塑性变形时会变得越来越硬。金属拉伸越长它的直径越小。这种现象一直持续到曲线上的S点。
    超过S点后，面积减少的速度超过了应力增加的速度。应变变得很大，面积的减少在某些点产生了局部效应。叫做颈缩。
    横截面积减小的速度非常快；以至于实际上载荷降低。
    伸长率和面积减少率表示了材料的延展性。
   
        Unit5  Design of machine and machine elements
                机械设计制造及其自动化就业机器和机器零件的设计
Machine design机器设计
  1 Machine design is the art of planning or devising new or improved machines to accomplish specific purposes. In general, a machine will consist of a combination of several different mechanical elements properly designed and arranged to work together, as a whole. During the initial planning of a machine, fundamental decisions must be made concerning loading, type of kinematic elements to be used, and correct utilization of the properties of engineering materials. Economic considerations are usually of prime importance when the design of new machinery is undertaken. In general, the lowest over-all costs are designed. Consideration should be given not only to the cost of design, manufacture the necessary safety features and be of pleasing external appearance. The objective is to produce a machine which is not only sufficiently rugged to function properly for a reasonable life, but is at the same time cheap enough to be economically feasible.
    机器设计为了特定的目的而发明或改进机器的一种艺术。一般来讲，机器时有多种不同的合理设计并有序装配在一起的部件构成的，在最初的机器设计阶段，必须基本明确负载、元件的运动情况、工程材料的合理使用性能。负责新机器的设计最初的最重要的是经济性考虑。一般来说，选择总成本最低的设计方案，不仅要考虑设计、制造、销售、安装的成本。还要考虑服务的费用，机械要保证必要的安全性能和美观的外形。制造机器的目标不仅要追求保证只用功能的合理寿命，还要保证足够便宜以同时保证其经济的可行性。
  2 The engineer in charge of the design of a machine should not only have adequate technical training, but must be a man of sound judgment and wide experience, qualities which are usually acquired only after considerable time has been spent in actual professional work.
    负责设计机器的工程师，不仅要经过专业的培训，而且必须是一个准确判断而又有丰富经验的人，具有一种有足够时间从事专门的实际工作的素质。 
Design of machine elements机器零件的设计
3 The principles of design are, of course, universal. The same theory or equations  may be applied to a very small part, as in an instrument, or, to a larger but similar part used in a piece of heavy equipment. In no ease, however, should mathematical calculations be looked upon as absolute and final. They are all subject to the accuracy of the various assumptions, which must necessarily be made in engineering work. Sometimes only a portion of the total number of parts in a machine are designed on the basis of analytic calculations. The form and size of the remaining parts are designed on the basis of analytic calculations. On the other hand, if the machine is very expensive, or if weight is a factor, as in airplanes, design computations may then be made for almost all the parts.