(b) The dimensional difference between the inside and the outside

(c) A part that has large inside and outside diameters and a

relatively thin wall thickness will split if installed with an excessive

interference allowance.

When there are no blueprints or other dimensional references available, all

of these variables must be considered before a fit is selected.

3.

Tolerances

a.

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 on the following page). 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: