Recalling basic Geometry, three points are required to define a
plane (-A-), two additional points to define a perpendicular, secondary (-B-),
plane to the primary and one additional point to define a tertiary plane perpendicular
to the first two (-C-). As castings are
never perfectly flat, straight or square, this Geometric fundamental is often
used by Casting Designers to establish a datum structure from which to obtain a
consistent measurement of cast parts.
Moreover, Datum Points allow a casting to be machined from the same
datum structure.
The selection of datum surfaces, and assignment of the datum point
(also called Tool Points) locations, is a critical consideration for good
casting design, so here are a few helpful tips:
Generally the longest & largest surface of a part is designated
as the primary (-A-) datum surface. The
second largest perpendicular surface would then be then designated as the
Secondary (-B-) and the Tertiary (-C-) a surface perpendicular to the first
two. For example envision a Shoe Box,
with the floor of the Box being Primary Surface the longest side as being
Secondary and the short end as being Tertiary.
Although most castings are not as straightforward as a Shoe Box, the
same logic applies.
Once the datum surfaces are determined, Datum Point locations, with
three points on the Primary, two points on the secondary and one on the
tertiary need to be designated on each of the datum planes. By placing the part against these six datum
points, generally a Tool Point Fixture, a casting then can be measured from the
theoretical planes using either a height gage or CMM probe.
For additional information about our recommendations for Datum and
Datum Points, please see our next blog posting.
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