DATUM TARGET PATCH

We have seen many errors that customers have made by confusing this for any type of part.Īs always the print needs to specify with basic dimensions where each datum is relative to each other.

GD&T effectively uses the highest 3 points of contact in that plane to establish the datum and not the average plane that a 3-2-1 traditional alignment will use. If your primary datum is a plane for example, then a 3-2-1 alignment will try to align to the average plane which is not how GD&T is applied. Note if your casting datum reference frame is not strictly datum target points, then be careful what alignment you use. 3 points determine the primary datum, 2 points determine the secondary and 1 point determines the tertiary datum. If only datum target points are used, then something like a 3-2-1 arrangement happens. The principles are the same for casting datums as they are for machined datums, you must constrain all 6 degrees of freedom of the part and in a specified order. Sometimes they can make a prudent modification to one of the datum areas and dramatically bring the part into specification. Our casting customers will often ask for both alignments (casting datum alignment from the print and a best fit alignment) to understand where deviations are occurring in their parts.

All other datum alignments necessarily deviate from that and look worse. When reviewing laser scanning data compared to a CAD model (scan to CAD model comparison).The best alignment would be a best fit alignment. Those castings are complex automotive and aerospace parts that sometimes don’t have a flat surface on them.įigure 2: Prints can sometimes look like this due to all the datum references. Datum target points are commonly used in the castings that we see in our accredited dimensional inspection business.

DATUM TARGET PATCH

The targets can be an odd shaped patch area on a part, circular area or single points on the part surface. Since castings with complex surfaces don’t lend themselves to the typical datum features made from planes or cylinders, datum targets are typically used. Datums should be made from physical features on a part (not centerlines or ‘off part’ points). They are typically designated as X-Y-Z or Z-Y-X.įigure 1: Typical section from a drawing showing casting datums.īecause most prints will not have enough datums to even approach the 24th letter of the alphabet (X), the last 3 are used to designate casting datums. This where casting datums come into play. But what about raw castings? Generally the functional part datum reference frame(s) have not yet been cut into the parts yet. There can be so many that they look like an alphabet soup. The fact that there are multiple sets of datum’s is not an issue and is put in place for functional design reasons. We have all seen prints with many datum reference frames on them.