Design stage of 4030(ish) for wood and plastic

[JAZZCNC]

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.

Yes makes no difference and don't rely on the rails to provide strength.

Can you use nuts instead of threading?

Yes if you can access the nuts but threading is easy and less messing around. Why not thread.?

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

Not exactly sure what your meaning but brackets are mostly used to provide strengthening rather than hold together.

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?

Yes if going thru 2 sides then you'll need crush tubes. If bolting and you don't want to have crush tubes but can't access the material, IE in middle of long length, then just drill a larger access hole on the outer wall for a socket to pass thru. Bosch Holes saws are good for this and easily cut steel.

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).

Again not sure were or what your meaning with the access from side.? . . But if it's the bed supports then NO. If anything you want another one not taking away.!! Don't compromise the machine for sake extra £20 it's the foundation and any weakness here will pass to the finished result. IE Shitty finish.!

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)?

Drop the 120x60 and use Steel box, Often box section comes in 7.5mtr lengths so I'd source a local steel supplier and ask the price you may find it's not much more for full length than 3mtr off the net. Chances are they will even cut it up for you.?
This will then give you enough for gantry and to beef the frame up more, the steel for the whole machine will cost less than the price of just the 120x60.

Now regards previous post and you saying going to use Eco cast then don't bother has it's not required if your using SBR rails. The rails them self will have more error than the plate so it's waste of money. Only really need Eco cast if using Profiled rails so Just buy 5 series cut plate and it will be more than good enough. In any case you only really needed Eco cast for the important areas like bearing plates, Z axis etc Gantry sides and other things like general plates etc could all come from cheaper rolled plate lengths.