timing belt instead of lead screw

[Jonathan]

Belts... advantage is less power required to achieve higher acceleration and speed (since kinetic energy of moving belt and pulleys is much less than a long rotating screw). For a long axis it should be a lot cheaper since you don't need as high torque stepper motor (unless you use a rotating ballnut like I did, then you can still use the small stepper, but that's completely irrelevant for an only 48" axis).

Disadvantage of belts is, as 2e0poz said, they stretch. Any belt will stretch, it's just some are more stiff than others. Different tension members, cross sections/pulleys all make a differenc. To get reasonable resolution (and thus accuracy) from a belt it is apparent you need double reduction to get the right ratio from stepper to belt. That's more pulleys (expense) and more belts to stretch! Due to the stiffness I doubt you would be able to cut aluminium with belts, whereas you could with a ballscrew or another type of screw.

That does however make belts great for something like a laser cutter or rapid prototyper where there are no cutting forces and the accuracy is not so critical.