Gantry to bed height.

[JAZZCNC]

Ok well first off regards the rails then I'd split the difference and go with bottom rail on the front face, top rail on the top face. That way you get more gantry clearance but keep a better rail spacing than both on the front face.
Obviously this depends on how much gantry clearance you need and material thickness you want to work on. Remember to think about tool lengths, how you have drawn now then you may struggle with clearance when using larger drills on thicker materials. With these dimensions, your thickest material size will be 75mm which might seem fine until you want to drill a hole thru it with say a 10mm drill and find the drill length won't fit.

Second, the Z-axis is a little narrow at 160mm and I'd be 180mm as a minimum with 200mm being better. Basically the wider the bearing spacing the better, obviously you don't want to waste too much travel but your better spending a little more money on the extra rail, screw, etc than compromising the machine trying to squeeze the Z-axis to fit the rails.


Third, you could lose some of that tubing on the frame. If you are going to use diagonal braces, like on the sides then you don't need the vertical piece or even the horizontal piece. You could drop at least one of the 3 little vertical pieces also and just have two supports. You want to reduce the amount of welding that goes into the frame because each weld is a potential stress point that will at some point release the tension causing the frame to move.