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Stop Using Datum Shift as a Bonus!
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

I have created several Tips on this subject because it is one of the most confusing topics in GD&T.  Many folks wrongly treat datum shift as an additional bonus tolerance.

On this drawing, in the position tolerance feature control frames, datum feature B and C are modified with the MMC symbol.  This means there is an allowable shift of the datum feature relative to the datum as these features depart from their maximum material boundaries.  Although you do not have to hard gage this part, a sample hard gage helps explain the meaning.

As you can see from the gage, the hole in the gage that is establishing the axis of datum feature B is fixed at 12.3 mm.  So, if the actual datum feature on a part is less than 12.3 mm and pretty perpendicular, the part is allowed to shift around a bit in the gage.  This shift might allow the 12±0.2 mm boss to meet its position tolerance.  Some folks will use this amount of allowable shift as an additional bonus on the boss with the position tolerance.  One possible consequence of doing this would be to allow more perpendicularity variation in this feature since position also controls orientation.  In other words, out of spec parts might be accepted.

In the past a technique called paper gaging was used to evaluate inspection data and deal with this shift. Today, people tend to rely on their coordinate measuring machine or vision system software to deal with this even though the software is often not capable. Note, the old technique works with the inspection machines but people are not aware of the technique or they just don’t bother. At Tec-Ease, Inc. we can help with this issue.

Back to Tips Tip added Nov 2012