If tolerances are not actually specified on a drawing, certain assumptions can be made regarding the anticipated accuracy by applying the following principles: For dimensions ending in a fraction of an inch, such as 1/8, 1/16, 1/32, or 1/64, the required accuracy will be to the nearest 1/64 inch. Figure 7.3 demonstrates typical symbols used in lieu of or in conjunction with notes to state the geometric characteristics being toleranced. Surfaces being toleranced have geometrical characteristics such as roundness or perpendicularity to another surface. Limit dimensioning method (source: Blueprint Reading and Sketching, Navedtra 14040).ħ.2 illustrates a typical method used to show tolerances for holes and shafts. Click here for terms of use.įigure 7.1 Three methods of indicating tolerances: A. FigureĬopyright © 2009 by The McGraw-Hill Companies, Inc. The limit-dimensioning method, where the maximum and minimum measurements are both stated. The bilateral method, where the dimension figure shows the variation in either direction that is acceptable and C. The unilateral method, which is used when variation from the design size is permissible in one direction only B. Tolerances may be shown on drawings in a number of different ways. Limits are the maximum and/or minimum values prescribed for a specific dimension, while tolerance represents the total amount by which a specific dimension may vary. Tolerance is shown on a drawing as ± (plus or minus) a certain amount, either as a fraction or decimal. A clear understanding of tolerance and allowance can go a long way toward preventing small but potentially critical errors. Work must therefore be implemented within the limits of accuracy specified on the drawing. Working to absolute or exact basic dimensions is impractical and unnecessary in most instances therefore, the designer calculates in addition to the basic dimensions an allowable variation. It is basically defined as the difference between the upper and lower limits. Tolerance represents the total amount a dimension may vary. Although today's cad packages make the production of industrial drawings much easier, it is still imperative to follow industry standards and conventions. One of the first steps in learning to read machine drawings is to become familiar with key terms, symbols, and conventions in general use in the industry. Some of the basic mechanisms usually found in detail and assembly drawings of machines are also presented. 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. 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.
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