178 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
Rectangular Features of Size (Parallel
Surfaces)
Slots, rails, and tabs are often used as datum
features of size. See Figure 6-27. The given fi gure
shows a keyseat that is position toleranced relative
to datum A primary and datum B secondary. The
width of a keyseat is identifi ed as a datum feature
of size, datum feature C. Datum C is a plane that is
perpendicular to primary datum A, is centered on
datum B, and passes through the center of the key-
seat. The means of locating datum C is explained
in the following sections and is dependent on
the order of precedence, the applicable material
boundary modifi er, and whether or not the trans-
lation symbol is shown.
The datum feature symbol is placed in line
with the dimension. It is incorrect to show the
datum feature symbol on the centerline. Placement
of the datum feature symbol on an extension line
from one side of the keyseat would not have the
same meaning as its placement on the dimension.
Placement on an extension line would indicate a
single surface as a datum feature.
To identify a rectangular feature of size as
a datum feature, the datum feature symbol may
be in line with or on the dimension line. It may
also be attached to a feature control frame that is
applied to the feature.
Datum Simulation
In theory, a datum feature of size referenced
RMB is simulated by something called an actual
mating envelope. For a primary datum, the actual
mating envelope is free to fl oat (not constrained).
The actual mating envelope for a primary datum
feature is the perfect geometric shape that is col-
lapsed (contracted) around an external datum
feature or expanded within an internal datum
feature. For a secondary or tertiary datum feature
of size, the actual mating envelope is related (con-
strained) to applicable higher precedence datums.
When the datum references are applicable at
MMB or LMB, the theoretical simulation is similar
to the explanation for RMB, except the simulator
is fi xed in size to the appropriate material bound-
ary. More about the simulation methods will be
explained in following pages.
Figure 6-28 provides a conceptual look at a
practical way in which a datum axis may be estab-
lished from a shaft that is referenced as a primary
datum feature of size RMB. In this example, the
shaft is placed in a machine chuck to establish the
axis location. Of course, a machine chuck is not a
perfect simulator because it does not completely
enclose the shaft as does a theoretical actual mat-
ing envelope. Another option is to clamp the shaft
in a collet. The collet may contact more of the sur-
face, but it is not a perfect representation of the
actual mating envelope. In both examples, the axis
of rotation for the clamping device acts as the sim-
ulated datum axis. Each of the simulation methods
is intended to make suffi cient contact with the sur-
face of the workpiece to adequately establish the
datum axis.
Tolerance requirements on the workpiece
must be evaluated to determine the needed level
of accuracy for datum simulation equipment.
Although a lathe chuck or collet is not perfect,
their accuracy is suffi cient for many datum simu-
lation needs.
Datum Feature References in a
Feature Control Frame
Identifi cation of a datum feature has no impact
on fabrication requirements unless a reference
is made to the datum. Generally, datum feature
Goodheart-Willcox Publisher
Figure 6-27. A center plane is identifi ed as a datum
by placing the datum feature symbol in line with the
feature width dimension.
Datum feature
(physical entity)
(theoretical entity)
Datum axis
Chuck jaw (tool)
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Figure 6-28. A datum feature of size is simulated by a
tooling device that picks up the feature of size.