MASSIVE 2.5M x 4M BED - GANTRY DESIGN HELP NEEDED!!

[woodhouse]

Thanks boyan, you're being really constructive. I appreciate it.
It wasn't the actual tools deflection I was concerned about.
What I am trying to work out is obviously the gantry design.
Now I could go crazy and use a 500x500 square 10mm RHS and would be incredibly heavy but would do the job but waste a fortune trying to accommodate it.
To far the other way and my two 100x50s spaced apart 250mm would be much lighter but probably twist too much to be useable.
Add to the fact that on this build the rails will probably be spaced apart half the distance the spindle is from the bottom rail (eg 250mm apart rails, 500m from bottom rail to spindle)
Then the force on the bottom rail will be 3x that from the tool on the bottom rail and 2x in the opposite direction for the top.
Likewise any deflection in the gantry is magnified at the tool.

If I can work out the force produced by the tool then the gantry can be sized properly for them forces and not twist.
But that's the million dollar question

[Boyan Silyavski]

The key being here is how long the Z axis movement will be. That's how to start , then you will decide on the gantry box section. So how long the z axis movement down will be?