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A Hole’s Primary Datum Feature Does Not Have to Be Perpendicular to the Hole
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

Where do these myths come from? A lot of folks think that the primary datum for a position tolerance on a hole must be the surface that is perpendicular to that hole. That might be helpful for the shop but may have nothing to do with function. The result might be a part that is easy to machine that does not function. The majority of the time datum features should be selected based on the functional requirements of the part.

In this assembly the Support Bracket mounts against the Base Plate. This mounting surface should be the primary datum feature. The hole receiving the Pivot Pin is positioned relative to the datum plane established by datum feature A. This will assure that the part functions according to the design intent.

A few things to keep in mind when a Designer or Design Engineer selects datum features based on how they think the part will be produced:

Back to Tips Tip added Apr 2008