New Machine Build- My second CNC router out of Alluminum (bigger)

[ger21]

Originally Posted by jimmyaz PS: I don't think I will drive both side, since it only about 26inches in width, one in the middle should do it... One big one. 3/4inches screw. J

Size of the screw won't make it any stiffer. Clamp one side down so it can't move, and push on the other side. You might then change your mind back to two screws.

[LeeWay]

Your design screams for two screws. I can tell you for a fact that it will rack badly with a single screw. I was able to use a single screw on mine, but it is made of heavy steel and I built in adjustments to tighten the gantry up. I was able to load the gantry up tight enough to get it squre and eliminate racking.
There is nothing on your machine sides that will take that kind of pressure. Those rails will flex inward just as easily as down and up.
Two screws is really the only way to go. It will cost a little more, but you could run them both with one larger motor using a belt. That would likely be less than buying an extra motor and drive.