Datums Are Mutually Perpendicular - Even When the Datum
Features Are Not.
It isn't always possible or
practical to select datum features that are mutually
perpendicular to one another when establishing a datum
reference frame. Notice that datum feature C is not nominally
perpendicular to datum feature B. The datum feature simulator
for C would be made at 35° to the datum feature simulator for
B (shown here in red). The actual datum planes (shown in
blue), which comprise the datum reference framework, would
however be mutually perpendicular to one another as is
illustrated in the last figure. The deviation of the hole from
the 55mm BASIC location would be measured from the third datum
plane-not from the sharp point on the actual part.