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You Can Use a Slot as a Primary Datum Feature - If it Is Large Enough
(In accordance with Y14.5M-1994 standard)

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A recent question I received asked how to identify a slot width as a primary datum feature. I drew the illustration below to indicate the proper method. Keep in mind that a slot may be thought of as containing two features of size-a length and a width. Rarely though is a slot large enough to serve as a "good" primary datum feature. We use datum features to establish a datum reference frame for the purpose of immobilizing the part. All parts in space have six degrees of freedom (DOF): 3 translations and 3 rotations. These DOF are described and labeled in the Y14.5.1 standard and are illustrated in the lower figure. Datum precedence requires that each datum feature in turn arrest or take away all of the DOF it is capable of. When used as a primary datum feature, a plane arrests 3 DOF, a cylinder 4 DOF, a cone 5 DOF and a complex mathematically defined surface could arrest all 6 DOF. In this example the datum is the center plane of the slot width. As a plane, it has to arrest 3 DOF: u, v and z. Given the size of the slot relative the the size of the part, I would question if the designer really feels the slot is "sufficient in size to permit its intended use" as a primary datum feature. That is why on most parts I review, slots are used as the secondary ot tertiary datum feature.

Back to Tips Tip added Mar 2004