Casting Datums and Datum Points – Optimizing their selection

Let us further consider the selection of the datum surfaces.  If we visualize a Shoe Box as an example, we may choose either the inner, outer or a combination of each to serve as datum surfaces.  My recommendation is that Exterior surfaces should be selected in preference to interior.  Consider our Shoe Box and the physical volume that a Tool Point Fixture, built for internal Datum Points, will consume.  The presence of the Tool Point Fixture, within the box interior, will constrain our ability to inspect the interior of the casting.  A better situation exists when the Datum Points are located on the casting exterior and where the Tool Point Fixture will not impede the instruments needed to measure part features.

I have several thoughts to help select the best locations for the three Primary Datum Points.  First, it is important that the Datum Points be spread to the extents of the surface as distant from each other as possible.  Remembering the castings a never perfectly flat, square or perpendicular, spreading the Datum Points helps to minimize the multiplying effect that surface variations can impart to measurements.  Let’s consider our Shoe Box again.  The walls and floor of most Shoe Boxes are distorted, bowed outward, or “oil canned”.   If a Datum Point were placed onto this distortion you can n appreciate the potential affect this “bow” might have, on measurements of the box, especially if the Datum Points are in close proximity to each other.  When, however, the datum points are spread to the corners or edges of the Shoe Box, the wall & floor distortion will have little effect on the measurements of the Box. 

This also brings us to a second recommendation, to locate the Datum Points onto “Stable” areas of the casting.  In the previous example the “bow” in the base of the Shoe Box that exists because the floor is free to flex.  However, areas of the floor, that are adjacent to the corners and edge of the box are constrained by the perpendicular wall from flexing.  Areas of the Shoe Box, or any part, less susceptible to distortion are considered “Stable” areas.  When the Datum Points are located on Stable areas, a part it is easier to measure and also to identify, and correct, areas with distortion. 

For additional information about our recommendations for Datum and Datum Points, please see our blog posts concerning: Casting - Datum & Datum Point Basics

Casting - Datum & Datum Point Basics

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.