Advice on Mammoth CNC Router Build - Steel Framed - 2440mm x 1220mm cutting area!

[Jonathan]

expand on the disadvantages??

If the rails are on the bottom surface then the bearings, i.e. your supporting points, are closer to the tool/bed. The overhang is therefore reduced, so the stiffness is correspondingly higher. It does however make mounting them on epoxy more challenging - although you could conceivably mount the Y rails and X bearing blocks on the same epoxy leveled surface and get very good accuracy. I'd try drawing both orientations and see which looks the most promising.

Surly enough accuracy can be achieved when cutting and then wielding the frame that any small differences would be "levelled" out from doing an epoxy bed for the rails to then sit on?

I tend to agree - there becomes a point when an extra couple of mm of epoxy doesn't cost much compared to the added material/time required to get the frame more accurate. The epoxy is pretty strong/hard and the damping properties may be useful, so I wouldn't be too concerned about minimizing the thickness, within reason. If you look in my build log the epoxy is quite thick, but don't treat that build log as gospel - there's plenty of room for improvement.