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Why Datum Shift Is NOT a Bonus
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

I have created many Tips on datum shift and yet I receive numerous emails all asking the same question, "Can I treat the M or L modifier on a datum feature reference as a bonus on the toleranced feature?"  The answer is "NO!"

The drawing is of the Cap for the assembly shown.

Notice that the MMB modifier has been applied to the datum feature B reference.  In layman terms this means that the smaller datum feature B is produced the more the part may be shifted around in order to bring the four holes into their position tolerance zones.  This is not the same as using this shift as an additional bonus tolerance on the holes.  Treating it like a bonus would allow one of the holes to shift more to the left while a different hole can shift more in the opposite direction as the size of the datum feature decreases.  So, treating it like a bonus is saying that at assembly, you can shift the part one way to get some of the screws in.  Then take those screws out so you can shift the part in a different direction in order to assemble the other screws.

The shift allowed at the datum feature can only be in one direction in order to bring all of the holes into their position tolerance as is shown. The graphic illustrates a part that is shifted up and to the right in order to assemble all four screws.  Be sure to view the video of this Tip for a more in- depth explanation.

Back to Tips Tip added Feb 2013