机械专业英语董建国第16章

It seems you are very busy now.
Yes, a little. What are you doing? I am measuring a part. Are all sizes of the part correct?
Unit 16
Mr. Yang:No, I find the hole is bigger than the maximum limit
specifications that indicate the dimensions of a part. These
dimensions control the size of a part, its features, and/or their location on the part or relative to other parts. Through this, parts are able to be interchanged and mated to each other to form complete assemblies. The purpose of this unit is to introduce the basic terminology of tolerances and fits.
(4) Tolerances for press fits cannot become very critical.
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READING MATERIAL
Fits
When two parts are interchanged in an assembly, tolerance
becomes vitally important. We consider the following example
[nEJ5teIF(E)n]
[5nBmIn(E)l] [WIE5retIk(E)l] [baI5lAtErEl] [ju:nI5lAtEr(E)l]
n. 符号，标志
adj. 名义上的，名义上的 adj. 理论的 adj. 双向的 adj. 单向的
(be) relative to
相对于，关于
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Unit 16
Unit 16
TEXT
READING MATERIAL
Unit 16 TEXT
Tolerances
Although standardized measurement is essential to modern industry, perhaps even more important are the design
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Fig. 16-1 Tolerance notations
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LEARN TO SPEAK
Mr. Wang:
Mr. Yang:
Good morning, Mr. Yang.
Good morning, Mr. Wang.
Mr. Wang:
Mr. Yang: Mr. Wang: Mr. Yang: Mr. Wang:
(as shown in Fig.16-2). The shaft must fit the bearing and be able to turn freely. The diameter of the shaft is specified as 1.000±0.001. This means that the maximum limit of the shaft is 1.001 and the minimum is 0.999. The tolerance is then 0.002 and
bilateral.
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The maximum limit of the bearing bore is also 1.001 and the minimum limit is 0.999. The tolerance is once again 0.002. Will the shaft made by one machine shop fit the bearing made by another machine shop using the tolerances specified? If the shaft is turned to the maximum limit of 1.001 and the bearing is bored to its minimum limit of 0.999, both parts would be within acceptable tolerance, but would not fit to each other since the shaft is 0.002 larger than the bearing. However, if the bearing bore was specified in limit form or unilateral tolerance of 1.002±0.002, the parts would fit as intended. Even if the shaft was turned to the high limit of 1.001, it would still fit the bearing even though the bore was machined to the low limit of 1.002. Although machinist is not usually concerned with establishing tolerance and limit specifications, you can easily see how fit problems can be created by overlapping tolerances discussed in this example.
(2) 理论尺寸
(3) 极限偏差 (4) The maximum and minimum sizes of a size
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3．Point to the following sentences is true or false:
(1) since it is impossible to machine a part to an exact size ,
Mr. Yang: Because it is virtually impossible to manufacture a part without error and the designer also must consider the dimensions of mating parts to keep the precision of fit, so the limited sizes of a part must be given. Mr. Wang:Oh, I see. I am very glad to know this. Thanks. Mr. Yang: You’re welcome.
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NEW WORDS AND EXPRESSIONS
specification [spesIfI5keIF(E)n]
interchange mate assembly [5IntEtFeIndV] [meIt] [E5semblI]
n. 规定，技术要求，规范
vt. (指两人等)交换 n. 配对物 v. 配合，配成对 n. 总成，组件
total amount by which a part dimension may vary. Tolerances
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on drawings are often indicated by specifying a limit, or by plus and minus notations(as shown in Fig.16-1).With plus and minus tolerance, when the tolerance is both above and below the nominal (true theoretical)size, it is said to be bilateral (two sides). When the tolerance is indicated all on one side of nominal, it is said to be unilateral (one side).
of the size.
Mr. Wang:Can it be done over again? Mr. Yang: No. It has become a waste product.
Mr. Wang:Why does the designer give a tolerance of a part.
a designer do not most specify an acceptable range of sizes that will still permit the part to fit and function as intended. (2) When the tolerance in indicated all on one side of nominal，it is said to be bilateral (two sides ). (3) Many times a machinist is concerned with press or clearance fits.
terminology
plus minus
[t\:mI5nBlEdVI]
[plQs] பைடு நூலகம்5maInEs]
n. 术语，专门名词
adj. 正的，略大的，正量 adj. 负的，减去的
n. 负量，负号
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notation
nominal theoretical bilateral unilateral
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EXERCISES
1．Discuss the following questions in English: (1) What is tolerance？ (2) Why is tolerance important in manufacturing? (3) How do you indicate the tolerances on drawings? 2．Use English for replacing Chinese or use Chinese for replacing English. (1) 零件的尺寸公差
NOTES
The maximum and minimum sizes in part dimensions that are acceptable are limits, between which the actual part dimension must fall. 零件尺寸可接受的极大值和极小值称为极限尺寸，零件 的实际尺寸必须在这两个极限尺寸之间。 该句的基本句型是“主语+动词+表语”。 介词短语“in part dimensions”和定语从句“that are acceptable”均作定语修饰主语“The maximum and minimum sizes”。 定语从句“between which…must fall”用于修饰主句中 的表语“limits”。在该定语从句中“which”作介词 “between”的宾语。
minimum sizes in part dimensions that are acceptable are limits,
between which the actual part dimension must fall. The difference
between the maximum and minimum limits is tolerance, or the
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Since it is impossible to machine a part to an exact size, a
designer must specify an acceptable range of sizes that will still
permit the part to fit and function as intended. The maximum and