I also recommend that all Datum Points be co-planer with
each other. The basic philosophy of
using Datum Points is to establish an arbitrary “Zero Plane” from which to
measure. As there is allowable variation
between the Zero Plane and other cast features, it is a poor practice to
designate features that are not a part of the Zero Plane as also being at
“Zero”. Any dimensional variation from
nominal, between non-coplanar datum points will skew the measurements of an
entire part. I recommend, that if a
configuration does not readily offer coplanar datum points, then coplanar
features should be added to the configuration such as raised / recessed pads or
lugs.
I also recommend that a part should balance on the Primary
Datum Points without clamping. Although
not always possible, when a part will balance on the Datum Points it helps to
prevent any miss-positioning of the casting on the Datum points.
The final point I’d make is to encourage the centering of
the datums on the part configuration.
Casting variation is cumulative, so the longer the dimension being
measured the greater the amount of tolerance should be allowed. Especially when living in the Model Based
Definition world, Engineers are being compelled to allow profile tolerances
that accommodate the most distant features.
Centering the three datums effectively shortens the longest dimensions
of the part and lessens that amount of tolerance needed to accommodate the
configuration.
I want you to avoid selecting theoretical centerlines as
datums as they increase both part and tooling expense while decreasing the
accuracy of the part measurements. Datum
points need to reside on hard features on the Zero Plane. If there are no hard features, we recommend
that you either create them or shift the datum to an adjacent surface.
For additional information about our recommendations for
Datum and Datum Points, please see our blog posts:
