GD&T - Datum Features Must Be Referenced to Be Used
Sometimes, we receive from our customers industrial drawings where the datum features are not always correctly indicated. Datum features are essential for the designing and manufacturing of checking fixtures, and if they are not taken into account correctly, the mounting of the part on the fixture may be disastrous. To explain you the importance of datum features and how to set them correctly on an industrial plan, we have found this video from GD&T, presented by Don Day, who already provided us with very useful information in a previous article (GD&T - Datum Features Have Axes). You will find bellow the written transcription of what is said in this video.
“There continues to be the misconception that when fake features are labelled as datum features, their use is somehow implied.
Take the drawing shown. The position tolerance for the hole references datum features A and B, but not C. Therefore, the hole must be perpendicular to the plan established by datum features A at 20mm from the axis established by datum feature B. Datum feature C has nothing to do with the position of the hole.
Yes, the hole is shown on the centre line and is perpendicular to datum feature C, so we know what the goal is. But since the datum feature C was not referenced in the position tolerance, there is no limit to how far the hole can be off in a 90º angle. In order to avoid this, all you have to do is add datum feature C to the position tolerance.”
