Chapter 6 Datums and Datum Feature References 165
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placing a datum feature symbol on the centerline
on the drawing. The cylinder must be identifi ed
as the datum feature because the cylinder will be
used in production and inspection. The centerline
cannot be used because it is not a physical feature.
As an example, a lathe chuck clamps on a physical
feature such as a cylinder; it does not clamp on the
centerline. Holding a cylinder in a chuck or collet
locates the axis of the cylinder, and thereby locates
the datum axis. The centerline is not identifi ed as
the datum feature because it cannot be physically
contacted.
Targets
It is not always practical to identify an entire
feature as a datum feature. A very large feature,
such as the outside diameter of a rocket section,
is not practical for use as a datum feature. Fea-
tures subject to distortion, such as castings and
weldments, are also poor features to use in their
entirety. Any time features have areas on them
that are known to vary signifi cantly, it is usually
poor practice to utilize the entire feature to estab-
lish a datum. Other examples include surfaces
with draft angles or parting lines on them.
Targets may be used when the entire feature is
not used as a datum feature. See Figure 6-12. The
types of targets that may be used are points, lines,
and areas. Target labels include a letter and a num-
ber. The letter identifi es the plane or axis. Each tar-
get used to establish a single plane or axis is given
a number, and the targets for each datum letter are
numbered independent of others. If datum plane
D is established by three target points, they are
labeled D1, D2, and D3.
A leader is extended from the target identifi -
cation symbol to the target. See Figure 6-13. The
leader does not terminate with an arrowhead and
it extends radially from the symbol. A horizontal
segment is sometimes used in industry but is not
standardized. A solid line is used for the leader
when the target is on the near side of the feature. A
dashed line is used for the leader when the target is
on the far side of a drawing view. In CAD models,
the leader is always solid. If the target is indicated
as movable, as explained later in this chapter, the
leader extends from the end of the triangular seg-
ment of the symbol.
Or
Tool makes contact with
datum feature surfaces
Workpiece
Tool
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Figure 6-10. Application of a datum feature symbol
on either an extension line or a surface has the same
meaning.
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Figure 6-11. Datum feature symbols are applied to
physical features and not to centerlines.
Point
Line
Area
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Figure 6-12. There are three datum target types: point,
line, and area.
Pro Tip
Datum Target Symbol Leader
You are allowed to identify a datum target
symbol on the far side by using a dashed
leader in a 2D drawing, but it is clearer when
views are selected that allow you to use solid
leader lines. It is not necessary to create an ad-
ditional view just to avoid the dashed leader
lines.