Calculation of Maximum Material Boundary

[kannanj21]

I have a quick question about the calculations of maximum material boundary. Please see the attached image.
If the D datum is called out as primary datum, we just add up the size tolerance, if it is called as secondary datum we
take the perpendicularity tolerance, and the third case we add the 0.4 positional tolerance.


I do not understand how is this calculated and based on what rule and reason behind this calculation.
It may be a naive question, but I really appreciate if someone clarify my question.
Thanks a lot for your time.

[Kelly_Bramble]

This is Figure 4-16 in ASME Y14.5-2009 standard.

The explanation is provided in paragraph 4.11.6.1 and the other referenced in the lower right hand corner of the figure.

In general, MMB is the smallest value that will contain the datum feature of size while respecting the datum feature precedence.

[kannanj21]

Thank you Kelly Bramble.
I appreciate your time in answering my question.