166 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
The datum feature symbol is optional when
targets are shown. In the given fi gure, the datum
feature symbol is shown on an extension line from
the surface even though targets are also shown.
Showing the datum feature symbol on a large fea-
ture where targets are far apart may result in mis-
interpretation, thus causing the entire feature to be
used instead of the targets. Typically, the datum
feature symbol should not be shown in combina-
tion with targets.
Points
Target points on a surface indicate the loca-
tions where tooling or processing equipment must
attempt to make point contact with the workpiece
surface. Surface variations or tooling inaccuracy
may result in contact location being imperfect. See
Figure 6-14. Target points may be used to establish
all the datums required to complete a datum refer-
ence frame.
Each target point must have a defi ned location
and be labeled. Points forming a single datum refer-
ence frame should be dimensioned relative to one
another whenever possible. In the given example,
the dimensions that are provided show the relation-
ship between the targets. This allows the targets to
be located without the part. A tool or setup can be
established without the part being present.
Dimensions used to locate targets may be
basic or toleranced dimensions. If target locating
dimensions are basic, the ASME Y14.5 standard
has in the past indicated that standard toolmaker
tolerances apply to the target locations. Some peo-
ple consider this guideline to be vague because
toolmaking standards vary from one company to
another.
Although not defi ned by the current standard,
it is possible to defi ne the allowable location tol-
erance for the target point positions. If and when
this is done, a note shall be placed on the drawing
to explain what the requirement means. A note is
needed because no standard defi nes the meaning
of a tolerance applied to a datum target.
Near side
Far side
Dashed
Solid
Optional
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Figure 6-13. A dashed leader from a datum target
symbol indicates the target is on the far side of the
feature. Dashed leaders are not used when tolerancing
solid models.
Workpiece
Tool
Tooling posts contact
datum targets
Datum
feature A
Datum target
C1 is a point
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Figure 6-14. Target points may be used to establish
all the datums for a part. Target points are commonly
picked up by spherical-ended tool posts.