Position Tolerance with One Datum

[[Al...]]

Hello all, 

I am confused about the effect of a position control with only one datum reference. In the attached example, Datum B has a positional control only referencing Datum A while another hole of the same size has three Datum references. 

I am trying to create a CMM program to inspect both of these positional tolerances, however, the position of Datum B is highly variable and has poor repeatability. What is the positional tolerance on Datum B actually controlling? Does it make sense that the CMM program would struggle to repeat this measurement reliably?

Any thoughts/ input is appreciated. Thanks!

GD&T_Question.pdf

[[Jo...]]

Datums consume all possible degrees of freedom.  In this case datum A is only consuming Rotation in X and Y. Translation in X and Y and Z are not constrained (This is effectively the same as perpendicularity) 

Since there are no basics, Wherever that hole lies *is* your datum B. It can lie anywhere on the surface as long as its perpendicular to the datum A hole. 

 

[[Ch...]]

Joe is correct.  You need to remember that position callouts control position AND orientation.  It seems the designer intends to control the orientation of both cylinders to each other.