Tail Vise installation - binding

[wojo2]

I am having an issue with the installation of the tail vise.  Like others, the nut starts out moving easily then binds as it gets closer to the end cap.  

 

I drilled the hole for the screw on a drillpress, so I am confident that the hole is true.

 

I have used a set of small levels on across the rails, and the rails seem to be level across the gap.

 

I have also used a bullet level on the plate as the nut travels, and it seems to remain steady.

 

I have tried the following:

 

Nut with NO rails,  moves the length of the screw no problem.  

 

Ran nut on one rail at a time again no problem.  Did both sides, one rail at a time.  

 

Two rails loose fitting, problem.  Binds about 75% towards the end cap.  

 

Any help appreciated.

 

Earl