MMC vs MMB

[[Cl...]]

I know the definition MMC and what it does. I think I know what MMB is. The datum is allowed to shift (move) withing it's envelope of tolerance. Is this correct?

[[Ti...]]

Yes, basically you are correct.

This example should help.  You have a car rotor below. The 5 holes are located from the center hole(Datum A) in a dashed diameter basic callout on the print. And they should be all separated from each other by a 72 deg basic callout on the print.

 

Now imagine you are slipping this rotor over a center boss with 5 bolts coming out. The larger you make the rotor Datum A the more you can jiggle the rotor around until it slips over the 5 bolts. It is in your best interest to make this rotor Datum A as far from MMC as possible so you can move the rotor around more until it fits. If you are measuring these bolt holes on the rotor on a CMM or something know that you need to measure all the holes as a patten when using datum shift b/c you only get one Datum shift not 5 individual datum shifts to make the location of the 5 holes as good as possible...

 

 

[[Cl...]]

Thanks!

[[Je...]]

Tim,

Thank you for providing one of the most direct and understandable explanations of MMB that I have encountered.  Great job. 👌

[[To...]]

This video also does a good job explaining datum shift.   Thank you Don Day.

 

Edited November 19