First Build-Steel Gantry CNC - Comments on design wanted

[JAZZCNC]

As far as the Z plate/s its very simple . Steel is 3 times stronger than aluminum, 3 times cheaper and it takes 3 times the time to make it as you have to paint it.

Lets compare Apples with Apples here.!!. . . . It's not 3 times cheaper if you buy high grade Cast Ground Steel tooling plate.!!

If builder doesn't have the abilty to machine steel flat then they are in for a world of pain when it comes to accurecy.!

Weight should be a Concern but only in certain cutting conditions. If you require high positional feeds with high velocity direction changes then 90Kg is getting close to stressing steppers. Cutting material like aluminium won't be a problem has the feed rates are low but cutting materials like Mdf etc do require higher feedrates to cut correctly so if your cutting shapes that have high velocity direction changes this is where heavy gantry will show up.
That doesn't mean it will cause you problems and you can't cut these materials but it will mean cutting at lower feeds than could be achieved with motors better suited to the weight. It also lowers tool life and finish quality if your cutting lower than optimum feeds.!

It's like Mr "N" says " Every action as equal and opposite re-action" So heavy means slower or more power.! Cut below optimum means Wear tools quicker.!

Regards the Aluminium Bed then I wouldn't bother if i'm honest. Even when cutting aluminium for anything other than real accurate work (or when requireing flood coolant) then you'll use some kind of Spoil board under the material, often Mdf or Ply, so it's wasted money really.
I wouldn't even go to the trouble of building a torsion box if the frame is strong, which it is.? Some form of stable material like high grade ply wood for a base is all that's needed. Then on top of this have sacrificial board that you surface flat. Even if you go to the trouble of making a torsion box you'll still use a sacrificial board which needs surfacing so you gain nothing over just normal piece of Ply.

Put Matrix of threaded inserts into the lower ply and you have hold down system.