Still working on it Boyan, and thanks for all your help as well as Jonathan. I made the Inner diameter of the RBN to be 1.0787. That would be 2mm of space for the ball screw as you suggested. Then I made the outer diameter of the RBN to be 1.3720 in which is about .006 smaller for the 7207 to fit nicely. Still as you can see, at the HTD gear part, The wall is about .0421 in in thickness. I think since both sides of the gear are attached to 1.3720 it will be strong enough. What do you think. MutzyClick image for larger version. 

Name:	Front New.jpg 
Views:	890 
Size:	228.2 KB 
ID:	23481Click image for larger version. 

Name:	Inner wall of rotating ball nut.png 
Views:	879 
Size:	27.7 KB 
ID:	23482Click image for larger version. 

Name:	New B.png 
Views:	867 
Size:	21.5 KB 
ID:	23483