This was the most differcult part of the conversion, because I dived in without really planning it. I first made a half-arsed attempt at direct drive off the existing lead screw, figureing it would be good enough to test the water. Lame effort I made it so that it would be near impossible to assemble.
I decided to can the existing cross slide drive components. The leadscrew is too short, the anti-back lash system on the nut doomed to wear quickly, the thrust bearing non-existant, the stand off too short limiting travel.
V2 was basically what you see, except the bearings were OD12 ID8 Width 4 ball bearings designed for RC car wheels, and were preloaded by a nut which could no longer be adjusted after the epoxy which joined the shaft to the leadscrew, bonded the nut. Of course backlash immediately rendered that set up unusable.
V3 builds on that experience by usings more robust bearings (ISO 608 - roller blade/skate board bearings), with an easily accessible adjustment. The 2:1 gear reduction with HTD belt was choosen to allow a little misalignment, to gear down the rather steep pitched lead screw and to fold the motor back under the stand-off. It would have been better to use a ball screw, but I couldn't find any where the nut would fit the slot, even 6mm ball nuts are 22mm round.
I would have liked to use ball screws, but the slot in the saddle is only 20mm wide by 18mm high, which was too small for the ball nuts I could find. There didn't seem much point in using a ball screw on Z when an acme thread was used on X, seeing as Z rearely reverses during a cut on a lathe.
The pullys were choosen to fit inside a peice of rectangular steel tube I had picked up at the scrap dealers. The belt calculations were done to give the minimum offset, consistant with NEMA 23 motor. The best choice was 201mm long, giving a 45mm offset.
The Motor is a new Vexta PK268-2.0A. With 1.35Nm (187 Ozin) this motor seems well matched to FrankenSieg's cross slide.