a.General. A clear understanding of tolerance and allowance will help to avoid making small, but potentially dangerous errors. These terms may seem closely related but each has a very precise meaning and application. Tolerance, for example, is defined as the allowable deviation from a standard size.

b.Working to the absolute or exact basic dimension is impractical and unnecessary in most instances; therefore, the designer calculates, in addition to the basic dimensions, an allowable variation. The amount of variation, or limit of error permissible, is indicated on the drawing as plus or minus (±) a given amount, such as +0.005 or ±1/64. The difference between the allowable minimum and the allowable maximum dimension is tolerance (figure 1 ).For example,

Basic dimension  = 4
Long limit       = 4 1/64
Short limit       = 3 63/64
Tolerance       = 1/32

c.When tolerances are not actually specified on a drawing, fairly concrete assumptions can be made concerning the accuracy expected, using the following principles. For dimensions that end in a fraction of an inch, such as 1/8, 1/16, 1/32, or 1/64, consider the expected accuracy to be to the nearest 1/64 of an inch. When the dimension is given in decimal form, the following applies:

d.If a dimension is given as 3.000 inches, the accuracy expected is ±0.0005 of an inch; or if the dimension given is 3.00 inches, the accuracy expected is ±0.005 of an inch. The ±0.0005 is called in shop terms, “plus or minus five ten­thousandths of an inch.” The ±0.005 is called “plus or minus five thousandths of an inch.”

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