On that machine there is a bracket that holds a NEMA 34 stepper motor vertically. They used to sell an optional CNC drive set which included a ball screw which was driven by a pulley which was driven by another pulley on the stepper. The ball screw atteched to the bracket around the quill and drove it directly. You would pull the center knob of the drill press function out so the quil would travel freely. Click image for larger version. 

Name:	Z-ball screw assembly Shoptask and Bridgemill.jpg 
Views:	1 
Size:	63.7 KB 
ID:	512080