DDM:
The rack and pinion I bought at
McMaster:
1. X-Axis Drive Shaft- 1/2" OD, 72", Shaft for Pinion Gears, McMaster - 1346K23
2. X-axis Pinion Gear-16 tooth, 1/2" Shaft, McMaster - 6325k22
3. X-Axis Gear Rack-4', 3/4"x3/4", 12 Pitch, Actually 49" Long, McMaster - 6295K142
I cut the shaft to size, about 54", and that leaves me some extra shafting for a pulley step-down shaft I need elsewhere. You can use any old pillow-block bearings for the shaft.
Add a pulley on your motor, a pulley on your shaft, a belt to transfer power and your done.
The racks will be mounted on the inside of the top T-slot rails, and the shaft with pinions will be on the back of the gantry. I'll post some pictures by the middle of next week...I'm still drilling holes in the racks tonight.
P.S. The aluminum T-slots only cost $625 shipped. I used surplus stuff. Sometimes you can find a good deal here and there.