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Tip #11 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. In addition to controlling the primary datum feature, secondary and tertiary datum features must be controlled with respect to the higher order datums. Many people seem to miss the point that 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 must be geometrically controlled. In the 1994 standard this is not a strict requirement (as we showed in tip 16), but that changed in 2009 and 2018. In the 2009 standard, section 4.9, it states “Datum features shall be controlled directly by applying appropriate geometric tolerances or indirectly by dimensions such as the size of a primary datum feature of size.” In the 2018 standard, section 7.9, it is again stated that datum features “shall” be controlled and additionally that they “shall” be related to each other. In section 1.4.1, it is also made clear the word “shall” establishes a requirement. In spite of this, many of the figures in the Standard omit geometric tolerances on datums. This is done not because it is ok to leave datums uncontrolled, but to simplify the drawings in order to focus on the concepts being discussed. For most parts the following flow chart should be helpful.
Although this flowchart won’t work all of the time, it does work on most parts. One common case that presents a challenge for inspection is when the height of the secondary datum feature is very small, like the hole labeled datum feature B in the following figure. The same thing often occurs with thin parts, like sheet metal parts and circuit cards. The short length of the feature makes it difficult to reliably measure the perpendicularity of the axis. One solution is to add a note to the perpendicularity control that requires the inner boundary is inspected instead of measuring the angle of the axis. This ensures the part meets the functional requirements for fit. Such cases are good to review with inspection prior to releasing the drawing.
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.
This tip is in accordance with ASME Y14.5-2009 and ASME Y14.5-2018.
This tip was originally released in September 1998.