Chapter 10 Dimensioning
269
acceptable dimension allowed and the low limit
is the smallest acceptable dimension allowed.
The difference between the high limit and the
low limit is the tolerance.
When specifying a limit tolerance, the high
limit appears above the low limit. If the limits
are displayed next to each other, the low limit
appears fi rst, followed by the high limit.
Plus-and-Minus Tolerancing
The plus-and-minus tolerancing method
specifi es the permissible variation of a dimen-
sion in the positive and negative directions. A
plus symbol (+) is used to indicate variation
in the positive direction. A minus symbol (–)
indicates variation in the negative direction.
The plus-and-minus tolerances are placed next
to the basic size, with the plus tolerance above
the minus tolerance.
A unilateral tolerance allows for variation
in only one direction. A bilateral tolerance
allows for variation in both directions. A
bilateral tolerance can have equal or unequal
values. If the values are equal, a plus-and-
minus symbol (±) is used. See Figure 10-39. The
total tolerance for an equal bilateral tolerance
is double the expressed value. For example,
the total tolerance expressed by ±.003″ is not
.003″. The tolerance is actually .003″ + .003″,
or .006″.
dimension. Tolerances permit allowances in
size from the specifi ed dimension that occur
during manufacturing.
Tolerances are added to the basic size, the
theoretically exact size represented by the
dimension. Tolerances can be applied to all
feature dimensions, including size and location
dimensions. The unit of measure for a tolerance
is the same as the unit of measure for the
related dimension.
There are two major methods for applying
tolerances directly to the dimensions: limit
tolerancing and plus-and-minus tolerancing.
Limit Tolerancing
Limit tolerancing uses limits to defi ne the
allowable sizes of a dimension. Limits are the
absolute maximum and minimum sizes allowed,
Figure  10-38. The high limit is the largest
High limit
Low limit
Goodheart-Willcox Publisher
Figure 10-38. Limit tolerancing uses limits to define
the maximum and minimum acceptable values for a
dimension.
Unilateral Equal Bilateral Unequal Bilateral
Goodheart-Willcox Publisher
Figure 10-39. Plus-and-minus tolerancing defines the basic size of a dimension and the allowable variation in the
positive and negative direction. The unilateral, equal bilateral, and unequal bilateral tolerances shown all permit
the same variance: .006″.
Copyright Goodheart-Willcox Co., Inc.