The correct
answer is "B" Datum A removes one translation and
two rotations from the mobility of the part. Datum axis
B removes the last two translational degrees of freedom
of the part relative to the datum reference framework.
Datum C is the axis of datum feature C. This axis is
perfectly oriented to the datum reference framework by
the basic dimensions. The only purpose of datum C is to
remove the last rotational degree of freedom of the part
relative to the datum reference framework. The figure
below illustrates the datum reference frame associated
with the part. This framework made up of three mutually
perpendicular planes could have been drawn at the angle
shown so that one of the planes passes through the axis
of datum feature C without changing the meaning. The
hole in the top of the part is shown perpendicular to
vertical and horizontal centerlines on the drawing.
These centerlines represent the datum reference frame
illustrated. There is an implied 90° basic angle between
the hole and one of the planes of the datum reference
frame. |