Chapter 6 Datums and Datum Feature References 173
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surface is identifi ed as datum feature B. The sec-
ondary datum plane must be perpendicular to the
primary datum plane and its location established
by a datum simulator (a plane) that makes contact
with the two highest points on the datum feature.
Only two points are needed because the second-
ary plane is already known to be perpendicular to
primary plane A.
The locations of the two highest points are
unknown, so the workpiece is pushed against a
simulator, such as an angle block or perpendicular
plate. Three point contact with the primary plane
is maintained. The workpiece will stop when con-
tact is made with the two high points on the sec-
ondary simulator, thus constraining two degrees
of freedom that were not already constrained by
the primary datum. The location of the high points
cannot be predicted. Therefore, the simulator must
be at least as large as the datum feature.
It is possible that more than two points will
contact the plate. Contact with more than two
points is acceptable. Contact with fewer than two
points is not acceptable. Contacting only one point
would permit the workpiece to rotate (rock); there-
fore, the required rotational degree of freedom for
the workpiece would not be constrained. If surface
form variation results in only one point making
contact, or should the two points be close together,
then steps must be taken to stabilize the part.
Three rotational and two translational degrees
of freedom are constrained when the primary and
secondary datum planes are established using
fl at datum features. The workpiece is still free to
translate (slide) along one axis. A tertiary datum
is needed to constrain the one remaining transla-
tional degree of freedom of the workpiece.
Tertiary Datum
The third datum feature reference in a fea-
ture control frame is to the tertiary datum. See
Figure 6-21. The tertiary datum feature in the
given fi gure is used to establish the location of
the third plane of the datum reference frame. The
location and orientation of the primary and sec-
ondary planes are already established, and the ter-
tiary plane must be perpendicular to both of them.
A position tolerance referencing datum C as
the tertiary datum is shown on the given part. The
right end of the part has a perpendicularity tolerance
related to primary datum A and secondary datum
B, and that surface is identifi ed as datum feature C.
The tertiary datum plane must be perpendic-
ular to the primary and secondary planes. The ter-
tiary datum plane is located by one high point on the
surface. The tertiary datum plane is perpendicular
to the primary and secondary planes and its loca-
tion relative to the datum feature is established by
moving a planar datum simulator to make contact
with the highest point on the datum feature. Only
one point is needed because tertiary plane C only
needs to constrain one degree of freedom, the
only remaining translation.
Tertiary
datum
Primary datum plane A
Secondary datum
plane B
Two point contact
One point
contact
Three point contact
Perpendicular
plate
Perpendicular plate
Workpiece
Surface plate
Tertiary datum
feature reference
included
plane C
Goodheart-Willcox Publisher
Figure 6-21. The tertiary datum plane is perpendicular
to the primary and secondary planes, and is located by
the tertiary datum feature on the part.