That’s the reason for having ballscrews: reduction of backlash.
For the opposite, one merely needs to look at the backlash on the Acme leadscrew on my lathe. 
Backlash there is measured in significant fractions of an inch.
I love it D Rob - I also planned on redesign my x y with ball screws as you have here, I think this and the servo motor idea you’ve been taking about will have great results!
So what would happen if the ballscrews were moved from the outsides to the inside? (other than cutting the number of ballscrews in half. (yeah, yeah, how do you drive it, what happens to packaging, where do you put the motors, etc, etc. It’s just a thought experiment)
Maintenance removed dust 
I thought you would like ball screws @Shauki? Rolling vs sliding, no backlash?
@Mike_Miller the motors would be mounted on the ball screws and have to move on at least 1 axis. A replicator style gantry would be best for that option.
There’s also an issue with the mass of the motor on one side…adding twist and mass…or a wierd fishingline/belt arrangement that keeps everything under tension.
ooh. Or have all three motors (X, Y, Extruder) on the SLED! Man, that’s kinky for kinky’s sake.
I’m slowly exposing you to American slang. putting three motors on the sled would be an awful solution (weight, inertia). But an interesting exercise.
@Shauki , I don’t have a lot to say. It’s already built, so I’m waiting to hear how things go. I don’t see an advantage to the ball screws, but there may be a goal for this machine that I’m not aware of. The mention of Hydra suggests his secret hot end may have 7 extruders, or maybe a 7 color mixing extruder. To move that mass, he might need the thrust the screws will provide. If that’s the case, the round rods look a little on the thin side.
The part that does worry me is the arrangement of bearings on the platform. I’d love to hear how rigid that is at the corners. It’s already overconstrained, so I’m thinking it may as well have 3 or 4 screws and no smooth rods. I think he’s confident that these are straight enough for parallel applications, so I think he missed an opportunity to take advantage of that.
Hmmm. Apparently, I did have a lot to say.
What @Dale_Dunn said. I’m on. printer hiatus due to home renovations, work commitments and a few furniture projects that need completing. Looks good @D_Rob . I’d look to covert it to an all metal bot and use it for milling. Seems like a waste to just use those screws for fdm.
@Dale_Dunn no the Hydra isn’t quite that elaborate. But it leans more to the removing of heads and two growing in it’s place. @Tim_Rastall has hit the nail on the head. I intend the UL-T-SLOT to be multipurpose. This is also why I am interested in high res optical rotary encoder servos. I want light 2.5D milling/routing capabilities as well as later using a laser eye protection paneling for an enclosure as well. The future of this machine will be milling, fdm,and laser etching (via piped laser or diode array) as of yet I am undecided.
@Dale_Dunn yes there are 4 smooth rods on the platform. And alignment is excellent. You may notice in the pictures that there are 3 individually mounted pieces for each side of the platform. This made alignment a little easier. And the upper rod/screw clamps are the same as the lower other than modifying the .stl to also screw into the plywood base that separates the electronics cabinet.So accurate spacing in cad was a breeze, and the accuracy of my mendel 90/i3 was sufficient to print them
@D_Rob Ah, like hydraraptor, but faster. Got it. I should think you’d be able to cut acrylic and plywood fairly well.
Alignment of the platform guides is not my concern, so much as the stability offered by that arrangement of bearings and screws. Is it sufficiently rigid at the corners? Now that I know you plan to cut with it eventually, I’m even more concerned.
You would pick up a LOT of rigidity if you enclosed it in sheetmetal, screwing it to the aluminum in 50mm increments.
That’s my plan, though I’m not sure if I’ll use metal or acrylic.
@Dale_Dunn this frame is very rigid. I believe Mitte than adequate for small milling jobs, like acrylic, plywood, PCB, block wood, and maybe more.