I want to use two leadscrews on the Y and Z axes on my mill, so that I can put the rails further apart for more stability. The Y-rails would be around 36-48" apart (with the leadscrews being close to that), so the table would be fully-supported. On the Z, I would change the column to a bridge-style one, with one column / rail / leadscrew on each side of the table (about 24" apart from eachother). This would both allow for greater head / column rigidity, and also shrink the Mill's overall Y distance from as high as 48", down to about 29".

The belts would be used to turn the leadscrews, which will likely have a thread pitch of between 13 and 20 TPI. Everything that sits on the Y-rails (Y-slide, X-axis, and table) will probably weigh 100-150lbs, but speed is not at all a concern, so acceleration shouldnt play much of a part in it. I am only concerned about the forces on the belt to overcome the static friction from the weight it has to move (although, the slides are ball bearing). Like I said, the distance between leadscrews will be probably ~36". I dont know if Id mount the motor on one leadscew with a belt going frm there to the other, or if Id mount the motor in the middle and use belt(s) to drive each leadscrew from there.

The same questions and givens apply with the Z-axis, except the leadscrew would be ~25" apart. The head will probably weigh about 50-70lbs.

Ive been researching belt drives, and it seems they can be pretty accurate with routers, even over long distances, even when used to move the axes directly. I would be using them to turn leadscrews, so there would be a large reduction in required force there, which will help alleviate a potential stretching problem.

BTW, I would be doing this because then I could use one of the simple 4-axis CNC kits, and not have to worry about adding extra drives / motors / cables / programming. I am far more mechanically inclined than electronically inclined,
so the potential for extra work (although, not much - its basically either belt tensioners or an extra motor mount).

What do you guys think?