Thanks David,

I have attached my URDF/xacro file. But in the meantime, I think I have 
sorted out my confusion.  It appears that the FK calculation is affected 
only by the joint positions, not the link lengths or origins.  This 
probably should have been obvious, but I was thinking I could change the 
length of a link (e.g. the hand), and get_fk would show a change in the 
pose of its children (e.g. the finger link).  But now I realize that if 
you alter the length or origin of a link, you would want to alter the 
joint location of the child link accordingly.  Otherwise, the output 
from get_fk does not change.

So I think I have everything working now.

--patrick