|When a flat surface is
used as a primary datum, three considerations should be made
to assure reproducible measurement and a functional
1. Consider a flatness control on the datum
feature. The primary datum is often a mounting surface that
may be clamped during assembly. Clamping may cause strain in
the part. Strain causes stress and other critical features to
vary in position or orientation. The flatter the datum
feature, the less distortion of the part.
|INSPECT THE PART WITH DATUM FEATURE
MOUNTED AGAINST A FLAT SURFACE USING
BOLTS TORQUED TO 9-15
2. If the part is prone to distort
and a tight flatness control is impractical, consider a
constraint note. A constraint note is usually intended to
describe the condition of the part after assembly.
3. If a tight
flatness control or constraint note does not fit your
situation, datum targets should probably be specified.
Since three points determine a plane, three targets
should be used on a primary datum feature that is
establishing a plane. If four or more targets are used,
it is advisable to also include a constraint