164 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
Datum feature symbols are applied to actual
features because of the clear defi nition of require-
ments that this provides. As previously explained,
they are not applied to centerlines on a drawing
or in a model because of the confusion that can be
caused. Another example of an ambiguous (and
wrong) application of a datum feature symbol is
to place it on the centerline of a counterbored hole.
If the counterbore is not perfectly centered on the
hole, then the two features will have separate axes.
Such placement does not indicate which feature,
the hole or counterbore, establishes the datum.
A datum target symbol is used to identify
datum targets. See Figure 6-9. The datum target
symbol is a circle with a horizontal line across the
center of it. The bottom half of the circle is used to
identify the datum target. The top half is left empty
except when specifying the size of a datum target
area. A size shown in the top half may be specifi ed
as a diameter, square, or possibly a rectangle if the
proportions of the rectangle are adequately clear.
Flat Surfaces (Planes)
Flat surfaces are commonly used as datum
features. See Figure 6-10. A datum feature symbol
placed on an extension line from a fl at surface iden-
tifi es that surface as a datum feature. The datum
feature symbol may also be attached directly to
the surface.
A fl at surface identifi ed as a datum feature
establishes a datum plane. If the surface of the pro-
duced part is not perfect, then the high points on
the surface establish the location of the theoretical
datum. The theoretical datum may be simulated
by placing the part on a very accurate surface, such
as a surface plate. A tooling surface or surface plate
is normally considered accurate enough to be used
as a datum simulator. The datum plane may also
be established mathematically by collection data
for points on the surface and calculating a plane
that passes through the high points. Software for
measurement machines typically provides means
for doing this.
Cylindrical Features (Axes)
Cylindrical features, such as shafts and
holes, are commonly used as datum features. See
Figure 6-11. A datum feature symbol placed in line
with the diameter dimension for a cylinder defi nes
that cylinder as a datum feature. Attachment to
the horizontal segment of a leader that points to
the hole is another method that may be used.
When a cylinder is used as a datum feature,
the cylindrical feature locates a datum axis. This
theoretical datum axis is not to be identifi ed by
Goodheart-Willcox Publisher
Figure 6-8. A datum feature symbol may be placed on
an extension line, placed in line with a dimension line,
combined with a feature control frame, or attached to
a feature.
Goodheart-Willcox Publisher
Figure 6-9. The leader on a datum target symbol does not include an arrowhead.