Unit FOUR
PartⅠPassage
Dimensional Control
tower
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.1
2.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 n. 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 the days, we expect to be able to purcha a replacement for a broken part, and fo
r it to function correctly without the need for scraping and other fitting operations.
When one part can be ud “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, becau dimensional errors may not be revealed until some time has elapd, and often many miles from the place where the machining was done.
6.Tolerance and limits of size
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,3 to be able to proclaim it to be perfect ( becau the measuring instruments are subject to errors), it is necessary to indicate the maximum errors permitted
7.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 incread 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 ud.
1.Unilateral limits. The are usually ud 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 u up the whole of the tolerance before the job is rejected.6
11. 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. The 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 datu
m 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, (100±0.02) mm.
New Words and Phras
英语作文例文
富贵不能淫翻译dimensional a.尺度的,尺寸的
mating n.配合函授成绩查询
accompanying
bench n.钳工台
fitter n.装配工,钳工
祝酒辞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.明确规定背英语,评述,确切说明
elap v.过去,消逝 了不起的盖兹比
tolerance n.sincere容忍,宽容,opp公差,容许偏差
