George's 8x4 router-me-bob

[Jonathan]

Some will complain the Ballscrew is too far back from the Y axis carriage, which it is slightly, but because you are using profiled rails they will help absorb some of the effects.

I was about to complain... it's true that you'll probably get away with it because you're using profile rails, but its unwise to rely on one very strong component to compensate for suboptimal design elsewhere.

20mm for the side plates for a machine this size sounds a bit small. To compensate I would make the 4 triangular brackets extend all the way down the sides as that will really stiffen things up.

The alternative is to eliminate the gantry sides completely, like on mine (or Jazz's) machine:


(Ignore the rotary table madness and yes my Y-ballscrew is badly positioned)

It's close to the strongest format.

Consider carefully what the profile rails on the X-axis are mounted to. Since you have orientated them with the mounting faces pointing outwards (i.e. in the YZ plane) the distance between the rails is critical, so you must have minimum one of the box sections bolted down, not welded, so that it can be adjusted. If you go for the gantry format in the picture above the rails are both facing up which would make this no longer significant.
Same is true for the Y-axis. 120*80*6 box section is a good size, but by putting the rails on opposite faces you're relying on those faces of the box section being parallel. To the tolerances required for profile rails they wont be, so you'll probably end up needing to use shims and stuff to get it to run smoothly. With supported round rails the tolerances are lower, so you get away with it, and you want them in that configuration to even up the loading on each block. However for profile rails (with 4 rows of balls which are the only ones worth getting) the force rating is equal in all directions, so it wont make much difference if you mount them facing or in the same plane. Mounting them in the same plane will make life easier ... lends itself well to getting the Y-axis ballscrew in the correct position - right between the rails.