You
can Use a Slot as a Primary Datum Feature-If it is Large
Enough
A recent question I received asked
how to identify a slot width as a primary datum feature. I
drew the illustration below to indicate the proper method.
Keep in mind that a slot may be thought of as containing two
features of size-a length and a width. Rarely though is a slot
large enough to serve as a "good" primary datum feature. We
use datum features to establish a datum reference frame for
the purpose of immobilizing the part. All parts in space have
six degrees of freedom (DOF): 3 translations and 3 rotations.
These DOF are described and labeled in the Y14.5.1 standard
and are illustrated in the lower figure. Datum precedence
requires that each datum feature in turn arrest or take away
all of the DOF it is capable of. When used as a primary datum
feature, a plane arrests 3 DOF, a cylinder 4 DOF, a cone 5 DOF
and a complex mathematically defined surface could arrest all
6 DOF. In this example the datum is the center plane of the
slot width. As a plane, it has to arrest 3 DOF: u, v and z.
Given the size of the slot relative the the size of the part,
I would question if the designer really feels the slot is
"sufficient in size to permit its intended use" as a primary
datum feature. That is why on most parts I review, slots are
used as the secondary ot tertiary datum feature.
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