Steel Framed Router

[JAZZCNC]

Jonathans suggestion is a good one not just for strength but also it protects the ballscrew from flying debrie.
If you must go with 1 screw done the middle then I suggest you make the cross piece that the ballnut fastens to wide as possible. you need strong bracing and width to counteract racking with a single screw.
Something like the pics below will handle just about most jobs at this width with a single screw. A 16mm 10mm pitch would work nicely.

Edit: The sketchs are rough quick knock up's and I would add some more corner braces to the cross piece but it should give you an idea of what I mean.

It probably looks a bit extreme but honestly it's not and the extra work and slightly more expense would be worth every minute and penny spent belive me.
Dont under estimate the forces's even with steppers.!! . . The forces can be extreme when cutting Ali and hard materials and the torque produced by steppers and ballscrews can twist and mangle a gantry built like this in the blink of an eye when the stick stuff it's the fan. :surprised: