184 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
The symbol for datum feature B is attached
to the feature control frame that shows the
perpendicularity requirement. Showing the
datum feature symbol in this manner indicates
the same thing as if the datum feature sym-
bol were applied to the size dimension as in
Figure 6-26.
Four holes in Figure 6-39 have a position toler-
ance of .016″ diameter at MMC relative to primary
datum feature B at RMB and secondary datum
feature A. RMB is assumed on datum feature ref-
erence B, because no modifying symbol is shown.
Referencing datum feature B as primary at RMB
has a specifi c effect.
Datum axis B is located by contracting a cyl-
inder (an actual mating envelope) around datum
feature B. Because datum B is primary, the datum
axis locates two of the planes in the datum refer-
ence frame. Because datum B is referenced at RMB,
the datum axis must be located by a simulator that
makes the maximum possible contact with datum
feature B. This is typically simulated by a device
that clamps on the surface of the shaft. A machine
chuck (or collet) may be used to simulate the datum
axis. The jaws of the chuck are adjusted to make
contact with the shaft regardless of its size or form
(RMB). The datum axis will coincide with the axis
of the chuck when the shaft is clamped in place.
Hole locations on the shown part may be
inspected with the shaft clamped in a chuck, pro-
vided the secondary datum is also located. To locate
the secondary datum, the shaft would be moved into
the chuck far enough for secondary datum feature
A to make contact. Point contact with datum feature
A might be all that is possible because the shaft and
fl ange may not be perfectly perpendicular.
Changing the order of datum precedence or
the applicable material boundary modifi er would
establish a different requirement. It should be
noted that in this example datum feature B has
an orientation tolerance relative to datum feature
A, and the holes have a position tolerance that ref-
erences B primary and A secondary. In the given
example, datum A is used as a primary datum
feature for one tolerance and a secondary datum
feature for another tolerance. This is allowed, but
care must be taken to ensure the applicable MMB
may be calculated if datum feature references are
made at MMB.
Clamp on datum
feature B since it
is referenced RMB
Only one point
contact required
Datum axis B
coincides with the
axis of the simulator
Chuck jaw
Establishes two planes on axis B
Establishes the third plane
modifier indicates
how primary datum B
is simulated
Assumed RMB
Datum A
Goodheart-Willcox Publisher
Figure 6-39. A primary datum feature referenced at
RMB requires the datum to be simulated by contacting
the surface of the datum feature.
Pro Tip
Theoretical and Practical Application
The simulation method in the above example
is a practical application and not meant to be
theoretically perfect. To be theoretically per-
fect, the chuck would completely enclose all
of the datum feature B surface and the end of
the chuck would be a surface that could po-
tentially contact all points on the datum fea-
ture A surface.
It is important to realize that secondary datum
plane A is perpendicular to primary datum axis
B. This is true even if datum feature A is not per-
pendicular to the shaft. The datum planes must be
perpendicular because a datum reference frame is
always made of perpendicular planes that defi ne a
coordinate system.
Simulating datum axis B on the given part con-
strains two rotational degrees of freedom and con-
strains two translational degrees of freedom. Two
of the planes in the coordinate system intersect on
datum axis B, and the third plane is perpendicular