Chapter 6 Datums and Datum Feature References 167
Copyright Goodheart-Willcox Co., Inc.
The target point locations shown on a draw-
ing indicate where a production tool or the pro-
cessing equipment is to make contact to establish
the datum reference frame relative to the work-
piece. Contacting the datum target points locates
the datum planes. When tooling is used to locate
the workpiece, the tool must have a feature to pick
up each of the identifi ed targets.
Because the targets on the drawing, shown
in Figure 6-14, are points, point contact must be
made. Spherical-ended tool posts are typically
used because they result in point contact and are
not likely to damage the workpiece.
Standard spherical-ended tool posts and
spherical locators are commercially available and
should be used. Using standard tooling compo-
nents is typically less expensive than fabricating
special tooling parts.
The example in Figure 6-14 has six targets on
the workpiece. Six targets are adequate to com-
pletely locate and stabilize the workpiece. They
constrain all six degrees of freedom. Targets A1,
A2, and A3 establish a plane on which the work-
piece rests. This plane constrains z translation and
u and v rotation. Targets B1 and B2 constrain y
translation and w rotation of the workpiece while
on the datum A targets. Target C1 constrains x
translation of the workpiece—it stops the part
from sliding along the datum B targets.
The three point contact on the primary plane,
two point contact on the secondary plane, and
one point contact on the tertiary plane is standard
when using fl at surfaces to establish a datum refer-
ence frame and when the part is adequately rigid.
This 3, 2, 1 system is also used on irregular sur-
faces when it is adequate to fully constrain all six
degrees of freedom.
No datum feature symbol is required when a
datum feature is defi ned using datum targets. It
is allowable to apply the datum feature symbol in
addition to the targets. The presence of the datum
feature symbol does not indicate that the entire
feature is applicable when targets are shown. The
targets defi ne the contact locations that must be
used to establish the datum.
Lines
Target lines on a surface indicate the locations
where the datum target simulators are positioned.
The target lines are oriented at the basic dimen-
sions defi ned on the drawing so that line contact
may be made with the workpiece surface. See
Figure 6-15. The given example is identical to the
previous fi gure except that datum target C1 is now
a target line. The datum target line appears as a
phantom line in the right side view. The end view
of the line, as shown in the front view, looks like a
target point. Because the end view of a target line
appears the same as a target point, it is essential
to look at all views of a drawing before deciding if
the targets are points or lines.
Target lines, like target points, must be located
and labeled on the drawing. Target lines used to
form part of a datum reference frame should be
located relative to other datum targets when pos-
sible. In the given example, the provided dimen-
sions do show the relationship between the targets.
The tool used for this workpiece is similar to
the one in the previous fi gure, except for the fea-
ture that locates datum target C1. Datum target C1
requires a tooling feature that makes line contact.
Line contact is achieved in the shown tool by a
dowel pin. Contact between the side of the round
dowel pin and the fl at side of the workpiece results
in a line contact.
Line contact along the full length of the target
line is only required when the workpiece is per-
fect. Where the workpiece is not perfect, the dowel
Datum target
C1 changed
to a line
Vertical dowel
pin provides
line contact
along the edge
Datum
feature A
Goodheart-Willcox Publisher
Figure 6-15. A datum target line appears like a target
point when seen as an end view. Target lines are picked
up by tooling features that make line contact.