Truss Structures - in larger gantry router design

[JAZZCNC]

So, the upper limit of mass for a stiff, damped gantry to run on the optimum motors (Nema 23), from what I've read would be about 80kg - any more and it's 34 motors. Is that correct?

Well it's a ballpark figure but yes it's about right. Thou depending on the design/use if using Slaved motors on same axis it can be pushed higher.

Will look at dry sand in 2 horizontal 80x80x3 box section tubes, spaced say 300 centres (in Z) and truss brace (a triangular fashion, in cross section) with 30x3 equal angle, by as much as the X-spacing of the X linear brgs. The gantry, Z axis and spindle is the business end, so if I start there and work out, conceptually, that feels right to me.

Dry sand works very well for damping and yes the Z axis is the Key stone.! . . Make this weak and the whole thing falls down.