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Tie Your Datum Features Together - But Not Too Much
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

The April 1998 Tip pointed out the importance of applying flatness to a primary datum feature that is a plane surface if datum targets or a constraint note are not used. The secondary and tertiary datum features should be related back to the primary. There is a difference between a datum, that is theoretical, and a datum feature, that is an actual feature on a part. These datum features should be geometrically controlled. Although the Y14.5 standard in section 4.3.3 the Standard states "Consideration shall be given to controlling the desired accuracy of the datum features by applying appropriate geometric tolerances. ", nearly all of the figures omit these geometric tolerances. For most parts the following flow chart may be used.

Although this flowchart won’t work all of the time, it does work on most parts. On parts such as sheet metal where a secondary datum feature is usually very short the part will probably always meet the control but because it is so short, it will be nearly impossible to collect data to prove that the process is capable. Also, depending on the shape of the part, the tertiary datum feature may require a location control (position or profile) relative to the primary and secondary datums rather than perpendicularity.

Although this flowchart won’t work all of the time, it does work on most parts. An exception might be on parts where the dimension of the secondary datum feature that is perpendicular to the primary datum is small. It may be impractical to collect data to prove that the process is capable of holding the perpendicularity (see figure). In such cases the process capability for perpendicularity is usually sufficient. A perpendicularity control is probably not necessary and would only cause unnecessary inspection time.

Depending on the shape of the part, the tertiary datum feature may require a location control (position or profile) relative to the primary and secondary datums, rather than perpendicularity. Ask yourself if you need to know where this feature is relative to the primary and secondary datum features.

Back to Tips Tip added Sep 1998