Unsupported lead screws/ball screws/cantilever questions

Been looking at printer designs for a while now. I keep noticing many seem to leave the lead screws unsupported at the opposite end to the motor.

What is the reasoning behind this? I've read this is because many arent straight, but would it not be better to have a small machined end sat in a bearing? Like a ball screw.

Which leads me to ask, is cost the only reason we don't see ball screws used more often in printer builds? If they were to be used, would it be better for these to be supported with the BK/BF type bearing mounts or would this be considered "over constrained"?

With a 3 screw/motor design, assuming we want to use linear rails, how many rails would be best for a larger bed? 500mm+. 3, one per screw, or with the one sided screw would spacing 2 be better? Is either configuration more susceptible to binding? Could buying a "linear stage" for each screw make alignment a bit easier?

Something like this, but longer:

[www.ebay.co.uk]

E3d are using a cantilever setup for the tool changer unit, does anyone have any experience with this? Does it wobble?

Edited 2 time(s). Last edit at 12/29/2019 11:21AM by mission.

Cost is the main reason we don't see them used much, but they also aren't necessarily a better choice anyway. Leadscroews (trapezoidal or ballscrew) are used mostly just for the z axis, where the lower backlash of a ballscrew is a moot point, and the fact they are sometimes back drivable is a disadvantage.

And yes, one end is often left free to avoid over constraining. If you fix the leadscrew at both ends, and it is not perfectly straight (most aren't), the bit in the middle will wobble as it rotates. Basically you're constraining it at 3 points, which is more than a straight line needs, and the only way for these three constraints to remain aligned regardless of the rotation of the leadscrew, or how far along the leadscrew the nut is, would be for the leadscrew to be perfectly straight and all three constraints be perfectly in line.

In general you want to avoid over constraining things, because then your constraints can end up fighting each other and causing warp/twist etc. This becomes less true for larger/longer/heavier things which can droop and twist under their own weight if only supported by the minimum number of constraints. That doesn't tend to apply for desktop machines though.