PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 1
(b) The dimensional difference between the inside and the outside
diameter (wall thickness) also has an effect on the interference allowance.
(c) A part that has large inside and outside diameters and a
relatively thin wall thickness will split if installed with an excessive
When there are no blueprints or other dimensional references available, all
of these variables must be considered before a fit is selected.
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
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,
When tolerances are not actually specified on a drawing, fairly
concrete assumptions can be made concerning the accuracy expected, using the
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: