45
Single limits are common for radii, depths of holes,
length of threads and knurls, and other features that
require a set limit.
PLUS AND MINUS TOLERANCES
PLUS AND MINUS TOLERANCING is the
method of tolerancing that specifies a dimension’s
variation in a positive and negative direction. The
plus symbol (+) indicates the tolerance in the positive
direction while a minus symbol (–) represents the
tolerance in the negative direction. The plus and
minus tolerances are placed next to a basic size.
The plus tolerance is above the dimension line
while the minus tolerance is below the dimension
line. See Fig. 4-16. The tolerances can be unilateral
or bilateral.
UNILATERAL TOLERANCES
UNILATERAL TOLERANCE is a variance from a
dimension in only one direction, either plus (+) or
minus (–). See Fig. 4-17.
BILATERAL TOLERANCES
BILATERAL TOLERANCE is a variance from a
dimension in both directions: plus (+) and minus (–)
dimensions. The variances can be different values
or they can be the same value. See Fig. 4-18. When
the plus and minus variances are the same, the
dimension and tolerance will appear on one line
with the
±
symbol next to them. See Fig. 4-19. Also,
when calculating a plus and minus tolerance with
equal value, the tolerance value is doubled because
it represents the variance in both directions. For
example, if an equal bilateral tolerance is ±.005,
then the actual tolerance is .01 (.005
×
2).
SPECIFIED TOLERANCES
A SPECIFIED TOLERANCE is any tolerance
directly applied to a dimension. The unit of
measurement for a specified tolerance is the same
unit used for their related dimension. Tolerances
can be specified in fractional, decimal inch, angular,
and metric units.
FRACTIONAL TOLERANCES
Fractional dimensions require fractional
tolerances. Fractional tolerances appear on a
drawing as shown in Fig. 4-20.
DECIMAL INCH TOLERANCES
Tolerances for dimensions in decimal inches can
be expressed as limits or plus and minus tolerances.
See Fig. 4-21.
Fig. 4-16. Plus and minus tolerancing.
Fig. 4-17. Unilateral tolerances indicate the allowable variation of a basic size from one direction only, either + or –.
A—Fractional dimensions with unilateral tolerances. B—Decimal inch dimensions with unilateral tolerances.
C—Angular dimensions with unilateral tolerances.
BASIC SIZE
PLUS TOLERANCE
MINUS TOLERANCE
A B C
Dimensions and Tolerances