Chapter 2 Dimensioning and Tolerancing Symbology 35
Copyright Goodheart-Willcox Co., Inc.
Application to Surfaces
Datum feature identifi cation may be accom-
plished in a number of ways. In a CAD model, the
datum feature symbol may be attached to the sur-
face using a leader that terminates on the surface
with a dot instead of an arrow. It is also a common
practice to attach the datum feature triangle to the
surface. This is best done with the triangle lying
on a plane that is perpendicular to the surface.
Also, in a CAD model, the datum feature triangle
may be attached to a feature control frame that is
applied to the surface.
In orthographic drawings, the datum feature
triangle may be placed on an extension line from
the surface. The side of the extension line to which
the symbol is applied is not important. Another
means for applying the symbol is to attach the
triangle directly to the surface with a leader. The
triangle may also be attached to a feature control
frame that is applied to the feature.
Datum Target Symbol
Surface variation on a produced part or part size
can make it impractical to use an entire feature to
establish a datum. Datum targets may be identi-
fi ed on a part to require specifi c locations on the
datum features to be used for establishing datum
locations. See Figure 2-25. Datum target symbols are
used to identify each datum target. These symbols
are not drawn at the target location, but are con-
nected to target locations with a leader. No arrow-
head is drawn on the datum target symbol leader.
The current datum target symbol, imple-
mented in 1982, is a circle with a horizontal line
drawn across it. The datum identifying letter and
a number are shown in the lower half of the circle.
The letter identifi es the datum. The number is the
target number for that particular datum. Targets
for each datum are numbered beginning with 1.
If datum target areas are used, the target size
may be shown in the top half of the circle. Unless
single-digit decimals are being used, the size value
will not fi t in the symbol. If two- or three-place
decimals are being used, the size may be placed
adjacent to the symbol with a leader extending
from the size value to the top half of the symbol.
A movable datum target is indicated with
a datum target symbol that includes two tangent
lines forming a 60° angle and centered on the hori-
zontal line through the symbol. The symbol may
point to the right or left with the leader extending
from the point.
Datum Targets
Datum target symbols may be attached to any
of three datum target types. The target type shown
on the part depends on many factors, including
the design function and the characteristics of the
part. The proper application of targets is explained
in the chapter on datums.
Datum target points are indicated with two
perpendicular lines, and the target point symbol
is oriented so the lines of the symbol do not coin-
cide with the object lines of the feature. See Fig-
ure 2-26. The size of the symbol prevents it from
being mistaken for a letter X. Datum target lines
are indicated with a phantom line except when the
target appears as an end view of the line. An end
view of a target line is drawn with the same sym-
bol as a datum target point. Datum target areas
are outlined with a phantom line and fi lled with
cross hatching. When a datum target area is small,
it may be represented by a datum target point
Application to surfaces
2X
X
60°
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Figure 2-24. Datum feature symbols may be associated
with the feature in any of several ways.
60°
Or
Ø3.5X
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Figure 2-25. Datum targets must each be identifi ed
with a datum letter and number.