Truss Structures - in larger gantry router design

[CharlesJenkinson]

Strength and Stiffness is quite important but vibration dampening is more important IMO.
Chasing light weight for gantry is a mistake unless you have high velocity/accel and fast directional change requirements.

Mass really helps when it comes to cutting and often a heavy gantry while going slower can cut a job faster than light gantry going fast due to much deeper DOC(with correct spindle), it will also give a better quality finish.

Ok, thanks Jazz. I think this is F =ma; so if cutting Force stays constant, then for increased inertial mass, accelerations will be lower, ...or the converse, for tolerably higher accelerations (vibration) the cutting force may be higher than gantries with less inertial mass. 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?

I think I'm gonna put a couple of orthogonal layouts together of my initial gantry truss idea, and iterate on it, till the mass budget adds up. 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.

[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.