[gio666]

It certainly reduces the problem, but does not cure it.

To the opening poster: have a very good look at the costs. In a dual screw you need the two screws and two motors. However in a single screw you have the cost of the beam at the bottom joining, a moving cable system if you implement, often wider bearing spacing is required. My point is that these little things often add up, probably not to as much as dual screws but you won't save a full screw and motor cost.

With centre screw there is a big sacrifice in rigidity. You can't support your table in the middle.

Perhaps I sound too negative on single screw, but I've built a machine with round supported rails with a centre screw, with a gantry width of 2 foot (cutting width 400mm) and it's just not good enough. Racking leads to inaccurate cuts, and the table sags in the centre. So I like to help others avoid my mistakes / issues.

Your machine was raking because on using supported rails, not from single ballscrew. Supported rails have no supported at all if any forces are toward the opening in the bearing. All that's there is plastic. If mounted on the side, it will rack left and right. If mounted on top , gantry will move back and forth. I use single ballscrew on everything and not had problem with racking.
Here is single screw machine cutting 540ipm at 1 inch pass .

540 ipm - YouTube