Was up all night thinking about my requirements...i'd like to throw alu back in the mix.

Thinking of a steel base, welded or bolted. Can i still use sbr16 or would i have to go up to 20? I don't have the budget for anything better.

Few more questions...

Can you use nuts instead of threading?

Do you have to use brackets or can you bolt the box section directly (less holes to drill and tap)?

If you are bolting 2 pieces of section together, one on top of the other, do you bolt from outsides (i.e. the bolt running through 2 walls of each piece) or the inside (1 wall of each piece) and if through 2 of each then do you put some kind of spacers inside?

If I kept the base design the similar but moved to steel, can i get away with only 2 pieces running the width of the machine? This would allow me to keep to 3m of steel and also it would leave the option of working on slightly wider pieces if required (feeding my material in from the side rather than the end).

My gantry...do i need the 120x60 piece running behind the 20mm plate or can i streamline that a little (reduce either the plate or the extrusion)?

Also toyed with the idea of making the whole thing out of plate. Was looking at the lumenlab machine...would this suffer badly from sagging? (was explaining to the gf..a little bit of sag is ok and is inevitable)

p.s. I know i got the spindle mount the wrong way round in my pics...