Originally Posted by
pippin88
A centre screw works ok when cutting in the middle of the table, however when cutting out at one side (one end of the gantry) there is racking, where one end of the gantry gets pushed further along the bearings than the other. Basically if you push on the front of one end the gantry will move relative to the other. Wider spaced bearings will reduce this but not eliminate it and means a smaller cutting area.
Another problem is table rigidity, which is much harder to achieve with the centre screw as you need the beam underneath. You can easily name a much more rigid table if you don't need this.
I've built a centre screw machine - never again.
Other than the centre screw, I think your basic design principles are good - you've used thick gantry uprights (most people still use thin plate).
The other thing is, if you can at all afford it, is to use profile rails. I haven't bought from them but BST automation on aliexpress have very cheap hiwin rails, other cnczone members have recommended them.