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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 should 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 should be geometrically controlled. The 1994 standard, in section 4.3.3, states "Consideration shall be given to controlling the desired accuracy of the datum features by applying appropriate geometric tolerances." In spite of this, nearly all of the figures in the Standard omit these geometric tolerances. 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 feature, datum 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 to be 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 Y14.5M-1994.
This tip was originally released in September 1998.