Chapter 6 Datums and Datum Feature References 163
Copyright Goodheart-Willcox Co., Inc.
Degrees of Freedom
Datums, according to their form and the spec-
ifi ed order of precedence, are used to constrain
the degrees of freedom for a part. Some tolerances
require constraining a limited number of the
degrees of freedom and others require constrain-
ing all six degrees of freedom. Adequate datums
must be used to constrain the appropriate degrees
of freedom.
There are three translational and three rota-
tional degrees of freedom. Translational degrees of
freedom are along the X, Y, and Z axes. Rotational
degrees of freedom are identifi ed as u, v, and w,
and they exist around the X, Y, and Z axes. The u
rotation is around the X axis, v rotation is around
the Y axis, and w rotation is around the Z axis. See
Figure 6-4.
The relationships of the X, Y, and Z axes are
easily remembered using the thumb and two fi n-
gers on the right hand. If the thumb and index fi n-
ger are held at 90°
to one another while pressed
against a sheet of paper, the thumb pointing to
the right will represent the X axis, the index fi nger
pointing to the top of the paper will represent the
Y axis, and the middle fi nger pointing up from the
paper will represent the Z axis. See Figure 6-7.
The positive direction of rotation around an
axis is determined by pointing the right thumb in
the direction of the positive axis and wrapping the
fi ngers around the axis. The fi ngers wrap in the
direction of the positive rotation.
Datum Identifi cation
Datum features must be identifi ed through
the application of datum feature symbols or datum
targets. Although standards prior to 1982 permit-
ted datums to be implied, it is no longer acceptable.
Even when working to the 1966 or 1973 standard,
it is not advisable to use implied datums because
of the possible misunderstandings that can occur.
Symbols
A datum feature symbol is a square or rect-
angle connected by a short line to a triangle. The
datum feature symbol is used to identify sur-
faces and features of size as datum features. See
Figure 6-8. The datum feature symbol typically
has a square containing one letter to identify a
datum feature. The square may be replaced with
a rectangle where two letters are used to iden-
tify the datum feature. The square or rectangle is
connected by a short leader line to a triangle. The
triangle is attached to a part feature, an extension
line from a feature, a dimension line, a leader line,
or a feature control frame in a manner that indi-
cates the particular feature on the part that is to
serve as a datum feature. Datum feature symbols
are only applied to physical features.
Measurement origin undefined
Datum plane B
Datum plane C
Measurement origin well defined
with datums
without datums
Goodheart-Willcox Publisher
Figure 6-6. Datum references provide a known origin
for measurements.
X
Y
Z
Goodheart-Willcox Publisher
Figure 6-7. The directions of the X, Y, and Z axes may
be remembered by placing the fi ngers of your right
hand in the position shown.