Your Source for GD&T Training and Materials

Back to GD&T Tips

Datum - Patterns of Features

   Patterns of Features Used to Establish Datums Are Tricky

The Y14.5 standard tells us we can use a pattern of features to establish a datum or datums (section 4.5.8). Unfortunately, the explanation that follows in the Standard only covers a pattern of holes at MMC. There is no support for using a pattern of features of size RFS. Where two parallel holes are used as a pattern to help establish a datum reference frame, it is pretty certain that two datum planes will result - one plane through the two holes and a second plane midway between the holes. Once more than two holes are used as a pattern to establish a datum, the resulting datum reference frame is unclear. This month's Tip is to avoid using a pattern of more than two holes to establish datums when referenced RFS. 
On this drawing the pattern of three holes is identified as the datum feature(s). There is no support for how to establish this datum reference frame Regardless of Feature Size when inspecting the profile and other position controls.

The next two drawings illustrate ways to use two of the holes to help clearly establish a datum reference frame.

Back to GD&T Tips