I am almost done with the book build Cnc machine, but when I run the motors from mach 3, the 1/2" coupling comes out of the middle spider (the joiner which connects 1/4 and 1/2" coupling. How do I pre

The lead screw must be held in position axially. In other words, you don't want your lead screw to move back and forth at all. This is the only way the coupling hub that is attached to the end of the lead screw could be moving away from its coupling assembly. Depending on the lead screw that you are using, the combination of bearings and collars/nut keep the lead screw in place.