Your Source for GD&T Training and Materials

Back to GD&T Tips

Datum Features Must Be Referenced to Be Used.
Another Control of Datum Features Tip  

 

There continues to be the misconception that if a feature is labeled as a datum feature, its use is somehow implied. Take the drawing shown. The position tolerance on the hole references datum features A and B but not C. Therefore the hole must be perpendicular to the plane established by datum feature A and 20 mm 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 a centerline that is perpendicular to datum feature C. So, we know what the goal is. But, since datum feature C was not referenced in the position tolerance, there is no limit to how far the hole can be off the 90 angle. A couple of possible, acceptable parts are shown.


 

In order to avoid this, all you have to do is add datum feature C to the position tolerance.

  Back to GD&T Tips