7

Understanding

Industrial Blueprints

7.1 GENERAL.

Industrial drawings may often necessitate more description and detail than some other types of working

drawings, mainly because of the close tolerances and finished surface requirements. In this chapter we

will cover some of the more common terms and symbols that the blueprint reader must be familiar with

in order to read machine drawings. Some of the basic mechanisms usually found in detail and assem-

bly drawings of machines are also presented.

One of the first steps in learning to read machine drawings is to become familiar with key terms, sym-

bols, and conventions in general use in the industry. Although today’s CAD packages make the produc-

tion of industrial drawings much easier, it is still imperative to follow industry standards and conventions.

Tolerance.

Tolerance represents the total amount a dimension may vary. It is basically defined as the difference be-

tween the upper and lower limits. Working to absolute or exact basic dimensions is impractical and un-

necessary in most instances; therefore, the designer calculates in addition to the basic dimensions an

allowable variation. Work must therefore be implemented within the limits of accuracy specified on the

drawing. A clear understanding of tolerance and allowance can go a long way toward preventing small

but potentially critical errors.

Tolerance is shown on a drawing as ± (plus or minus) a certain amount, either as a fraction or dec-

imal. Limits are the maximum and/or minimum values prescribed for a specific dimension, while toler-

ance represents the total amount by which a specific dimension may vary. Tolerances may be shown on

drawings in a number of different ways. Figure 7.1 shows three examples: A. The unilateral method,

which is used when variation from the design size is permissible in one direction only; B. The bilateral

method, where the dimension figure shows the variation in either direction that is acceptable; and C. The

limit-dimensioning method, where the maximum and minimum measurements are both stated. Figure

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