steel tube build concept - 1500y,1000x,250z

[njhussey]

all good questions , ill attempt to bluff my way through
......

That's all I do

gantry - steel tube 3mm, drill and tap thread , or weld threaded rod to tube ?
x carriage - aluminium plate and angle bolted togeather, rails bolted through
z axis - maybe thicker plate 20mm ?

3mm is not thick enough to tap, unless you're using M4 bolts, you'll either need to use thicker section or glue a strip behind and drill and tap into that. Whilst steel Z axis can be done (see Boyan Silyavski's builds) it's far more common practice on here (not saying it's correct, but just what gets done on here as it works!) to use a 20mm plate for the Z axis, both for the back plate and the front.

i worried about this, maybe only weld below y rails and shim them out to put y rails on then above this bolt together so better just to have stiffer plate ? and forget sides

Welding the frame is fine. You can, with a bit of thought, weld sections that bolt together which then allows for shimming to get the frame square. You can also file/scrape/fill the steel to get it flat or use the epoxy method and either use metal putty to set the steel rail supports at the correct height or pour slow setting epoxy on the top of the rails (like I did) and this (should) then be a flat surface to put the rails onto. Have a good look through the build logs and see what others have done, this should give you some ideas and will show how others have overcome the issues they encountered.

3mm , lots of jigs and clamping

Again 3mm isn't much good for tapping holes into, it's too thin so you won't have enough thread engagement.

very good suggestion, my current machine has no issues like this as it is so sloppy no resonance builds up !
Can't comment in the plasma thing as no experience with them, I'm sure a dual machine could be built but as with anything that doesn't do just one thing, there will be compromises.
thanks for persevering with my somewhat undeveloped project ,
cheers
ben

Keep the changes coming, if you see my build from what I originally planned and drew to what I ended up with you'll see how all this questioning and re-drawing is good!!