机电英语文本4.doc

Unit FOURPart Passage Dimensional Control 1.In the early days of engineering, the mating of parts was achieved by machining one part as nearly as possible to the required size, machining the mating part nearly to size, and then completing its machining, continually offering the other part to it, until the desired relationship was obtained.12.If it was inconvenient to offer one part to the other part during machining, the final work was done at the bench by a fitter, who scraped the mating parts until the desired fit was obtained, the fitter therefore being a “fitter” in the literal sense. It was obvious that the two parts would have to remain together, and in the event of one having to be replaced, the fitting would have to be done all over again. 3.In these days, we expect to be able to purchase a replacement for a broken part, and for it to function correctly without the need for scraping and other fitting operations. When one part can be used “off the shelf” to replace another of the same dimension and material specification, the part is said to be interchangeable.2 4.A system of interchangeability usually lowers the production costs, as there is no need for an expensive “fiddling” operation, and it also benefits the customers in the event of the need to replace worn parts. It also, however, demands that the dimensions of mating parts be specified, and that dimensional variations, due to machine and operator shortcomings, be taken into account. 5. Some form of inspection must be introduced to ensure that the manufacture is controlled; this is particularly important, because dimensional errors may not be revealed until some time has elapsed, and often many miles from the place where the machining was done. 6.Tolerance and limits of sizeSince it is accepted that it is virtually impossible to manufacture a part without error, or in the rare event of a part being without error,3 to be able to proclaim it to be perfect ( because the measuring instruments are subject to errors), it is necessary to indicate the maximum errors permitted7.The draughtsman must indicate the largest and smallest sizes that can be permitted without the part functioning incorrectly. The extreme dimensions are called the limits of size, and the difference between them is called the tolerance. 8.The magnitude of the tolerance depends upon the type of operation involved, the skill of the machinist, the accuracy of the machine, and the size of the part. For a given grade of tolerance, the actual tolerance must be increased with size. The tolerance should be as large as possible, to keep the cost to a minimum.4 9.The method of indicating, on a drawing, the permitted tolerance depends mainly upon the type of operation involved, but local preference must also be taken into account.5The following examples will illustrate some of the methods used. 1.Unilateral limits. These are usually used when the distance between two faces, or the diameter of a hole or shaft is specified.10.For example, when a diameter is being ground, the machinist would prefer to aim at the largest size permitted, so that, in the event of his reaching a diameter that is just a little larger than the maximum size permitted, he can take another cut, knowing that he can use up the whole of the tolerance before the job is rejected.611. A draughtsman might dimension a nominal 75-0.012 mm diameter shaft as D75.Similarly, a nominal 75mm hole might be dimensioned as D75-0.012, the same reasoning applies as for shafts. 2.Bilateral limits. These are usually applied when, for example, the position of a hole is specified. 12.The machine operator may position the hole nearer the datum or further from the datum than intended, and as the operator is in no position to change the situation when the hole has been started, he must aim between the limits of position, so that the maximum error can be made without causing the part to be rejected.7 The center distance between two holes would therefore be specified as, for example, (1000.02) mm.New Words and Phrasesdimensional a.尺度的，尺寸的mating n.配合bench n.钳工台fitter n.装配工，钳工scrape n.v.刮削literal a.完全按照原文的，照字面本意的fit v.n.配合specification. n.规格，规格说明，具体说明，详述interchangeable ad.可互换的，可交替的system n.系统，组合装置fiddle v.用小提琴演奏，胡乱摆弄specify v.明确规定，评述，确切说明elapse v.过去，消逝tolerance n.容忍，宽容，公差，容许偏差virtually ad.事实上，实际上proclaim v.宣告，声明，显示draughtsman n.绘图员machinist n.机械工，机械师preference n.偏爱，优先选择unilateral a.单方面的，单方面作出的，单边的bilateral a.有两边的，双边的datum n.论据，资料dimensional control尺寸控制nominal diameter公称直径unilateral limits单边极限bilateral limits双边极限International Organization for Standardize(ISO)国际标准化组织Notes1.In the early days of engineering, the mating of parts was achieved by machining one part as nearly as possible to the required size, machining the mating part nearly to size, and then completing its machining, continually offering the other part to it, until the desired relationship was obtained. .在早期的工程(问题)中,获得配合零件的方法是,首先尽可能把一个零件加工到所需尺寸,然后将与它配合的零件加工到接近所需尺寸,再不断将这两个零件试配,进一步加工直至获得所需配合的尺寸此句中,by后面跟了四个并列成分:machining one part., machining the mating part.,completing.及offering.可翻译成“先然后再进一步”2.When one part can be used “off the shelf” to replace another of the same dimension and material specification, the part is said to be interchangeable. 当一个零件从货架上拿来就可以替换同样材料规格的另一个零件时，就说这个零件是可以互换的。 3.Since it is accepted that it is virtually impossible to manufacture a part without error, or in the rare event of a part being without error,大家知道，事实上零件不可能毫无误差地制造出来，或者说没有误差的零件是非常罕见的，4.For a given grade of tolerance, the actual tolerance must be increased with size. The tolerance should be as large as possible, to keep the cost to a minimum对于给定级别的公差,实际公差须随着尺寸的增大而增大为使加工成本最小,公差应尽可能取大5.The method of indicating, on a drawing, the permitted tolerance depends mainly upon the type of operation involved, but local preference must also be taken into account. 许用公差主要取决于所用操作的类型,但也必须考虑本国的优先级6.when a diameter is being ground, the machinist would prefer to aim at the largest size permitted, so that, in the event of his reaching a diameter that is just a little larger than the maximum size permitted, he can take another cut, knowing that he can use up the whole of the tolerance before the job is rejected. 当直径要圆整时，机械工更愿意向最大允许尺寸圆整。这样，当他加工到所得直径尺寸略大于最大允许尺寸时，还可以在整个公差范围内再切一次，而知道不会因此使产品报废。这里ground是grind 的过去式,译为“磨,削,圆整”prefer to:更喜欢,更愿意;aim at:朝着 7.The machine operator may position the hole nearer the datum or further from the datum than intended, and as the operator is in no position to change the situation when the hole has been started, he must aim between the limits of position, so that the maximum error can be made without causing the part to be rejected. 机床操作人员可能将孔的位置定得较近或远离所需数据,而且,孔的加工一旦开始,操作人员便不可能改变孔的位置,他必须在尺寸位置限度内加工,以便在最大误差时不会使零件成为废品本句为并列复合句and前面的分句是简单句,后面的分句是复合句as引导原因状语从句,其中包含一个when引导的时间状语从句,he must aim是第二个分句的主干,so that 引导目的状语从句 Exercises. Answer the following questions according to the passage.1. What is the way of parts mating in the early days of engineering?2. What is the advantage of interchangeability system?3. What is the disadvantage of interchangeability system?4. What does the magnitude of the tolerance depend upon？5. What should the tolerance be when we want to keep the cost to a minimum?. Translate the following phrases.1. 双边极限 2. 间隙配合 3. 过盈配合 4. 基轴制 5. 基孔制 6. tolerance and limits of size 7. a system of interchangeability8. dimensional errors 9. the maximum errors permitted 10. unilateral limits . Answer the following questions according to the passage1. 显然,两个配合件应该总在一起(工作),当其中任意一个需要替换时,所有的适配刮削 工作又要从头重做2. 具有可互换性的配件体系不必进行高成本的刮削操作,从而可降低了生产的成本 当要换掉磨损的零件时,零件的可互换性对客户而言也是很有益处的3. 必须采取某种形式的检测方法来确保对加工的控制,这一点特别重要因为尺寸误 差有时可能要过一段时间才会发现,而此时却往往已远离加工的地方4. 这些尺寸的机制就称作极限尺寸,它们之间的差值称为公差5. 公差的大小依赖于所涉及的加工操作的类型机械工的技能机床的精确度以及零 件的尺寸6. 单边极限通常用在两个面之间的距离孔径或轴径被指定的情况下Part SpeakingProductsDialogue 1A: Nice to meet you, Mr. Yang. Im sorry to have kept you waiting.B: Nice to meet you too, Miss Carter. Heres my card.A: Thank you. Well, lets get down to business.B: I have our latest catalogue with me. Look, this is the model you asked me about. It is the best one available.A: This looks very interesting.B: We can offer you the same price for this one as last years model.A: Well, well have to do some comparing on our side.B: Im sure youll be pleased with our products.Dialogue 2A: Its good of you to see me, Miss Stacy.B: Feel free to call me Jane. I have heard a lot about you. I also thought you dealt with washing machines. Am I wrong?A: No. We have been dealing with washing machines for years. We arent completely satisfied with the quality however.B: We seldom get complaints on quality.A: Thats the reason I want to do business with you.B: Is there any particular item you are interested in?A: I think your model No.1326 B is perfect for our needs.B: We should have no problems supplying you.A: There are certain areas I think we should discuss.B: What sort of things would you like to discuss?A: There are several questions.B: Where should we start?A: Lets begin with the price.Dialogue 3A: Id like to take this opportunity to discuss with you our yearly requirements of your woolen sweaters.B: Yes, our woolen sweaters are in high demand in both domestic and international markets at this time of the year. May I know your requirements this time?A: If you dont mind, Id like to say a few words on the quality of your products.B: Well, was there anything wrong with the quality of the woolen sweaters you bought last year?A: There was no specific quality problem, but some customers complained that the sweaters were not as good as the ones they had bought before.B: Im shocked to hear that. Generally speaking, our products go through strict inspection procedures before warehousing and shipment.A: I dont know how. Anyway the complaints do exist.B: Oh. I feel terribly sorry. But I can assure you that our woolen sweaters are really up to the standard this time because they are