Hi Reefy,

I've replied to your emails and much of what I'm going to say now is just repeated for other's who may be thinking to go large.!

Regards bolting or welding then bolting the mainframe together at this size won't help much other than making it easier to move if need it to go through small doors etc.
Bolting the top rail is a good idea because it allows you to adjust the rail surfaces onto the same plane, however, this is where it gets difficult because you need some way to accurately measure that each surface is on the same plane.?
Also, and here is where it gets even more difficult when it gets long.? The steel you use to mount the rails onto must be perfectly flat and straight or you are wasting your time with shimming etc.
If the rail surface is up and down or twisted along its length then you are screwed and you'll never build an accurate machine. I can tell you with 100% certainty that the box section you buy, no matter how thick, will not be flat and will be bent or twisted to some degree.

This is why people use the Epoxy method on large machines, it self levels to a reasonably accurate degree and removes the twisted or bent steel problem.
So in this case bolting the frame and top tube doesn't really matter so much because the epoxy levels everything away.

If you don't want to use epoxy then the next best option is to just have the top rail tubes machined flat, then you just have to align and shim the surfaces onto the same plane to deal with.!

Or the best option is to weld the whole frame up and send it away for the top tubes machining flat and on the same plane. This will cost you but will give you an accurate machine.

If you try to do this without either epoxy or machining surfaces flat and just rely on the steel tube surfaces then you'll end up with a poor and low accuracy machine. If you just want to cut 2D shapes, like signs or paneling, etc then it will be perfectly fine, but if you want to do engraving or work which requires higher accuracy then it will fail badly.

People confuse strength and accuracy, they think building strong with thick tubes, etc gives the machine accuracy and it doesn't, it just makes it strong with less vibration. The accuracy comes from the frame alignment, surfaces, and quality of rails,ball-screws, etc.
If the rail surfaces are out of whack then doesn't matter if the machine was cast from a solid block of titanium it will still give rubbish accuracy. Likewise ball-screw alignment and quality drastically affect performance and accuracy so again if the surfaces these mount onto are not perfectly flat and parallel to the rails then you'll have set up issues which affect performance.

I know you are confident to give this a try but still make sure you fully understand what's required to build an accurate machine at this size that will do all you need.!! . . . I'll warn you again it's very very very difficult at this size to build an accurate machine and very very very very very very very very easy to f$ck up.!!

My best advice is that if you are determined to carry on at this size then weld it up solid and send it away for machining. It will be money well spent.