Question to GD&T gurus - help! stuck on minimum wall thickness problem..

Hi everyone,

I've been studying J.Meadows' "Geometric Dimensioning and Tolerancing. Applications, Analysis & Measurement" and really stuck on minimum wall thickness problem (Pages 318-319) where a Displacement concept was introduced.
You see, he counted .400 as movement of D @LMC at the bottom of P.318 and then again at the top of P.319, naming it Displacement (Datum feature shift allowed).
The question is - why Two times?
And the second question regarding this problem - why didn't he consider datum shift of B as it's referenced at MMC in datum D feature control frame?
Thanks in advance guys - I'm really lost here and need help!





Sorry, just realized the picture's got resized enroute, but there's no attachment option available

 

Hi Aleksey721,
I think I understand what is trying to be determined in this problem but I cant read all the numbers on the drawing so it is difficult to verify or contend the author's calculations.
Perhaps scanning in just the drawing portion to see it larger scale would help.

 

minimum wall thickness problem