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View Full Version : Truss Structures - in larger gantry router design



CharlesJenkinson
10-02-2014, 11:13 AM
Has anyone used or incorporated any truss type structures into their designs before now - using steel? I'm meaning more than just a few, or the more obvious, gussets and braces. I know full on tubular trusses are not easy to make, but I was thinking of using angle iron welded to the outside of square box (flange inwards or outwards) as the brace member. I feel gantry stiffness (vs weight) is an area where this could be utilised, but the reason to look at using it may be qualitatively or quantitatively assessed, by which I mean not just static deflection under some X-Y-Z combined loading at a Z-point offset, but modal shape and frequency (vibration) performance, indicating which area of moving gantry type machine as a whole to look at first. The latter is probably more a real machine measurement exercise than modelling, due to complexity of including all the stiffnesses in a model, and the variability of excitation forces. Assuming that increased stiffness is the way to go, what are the areas that should be looked at first?

I think it would be useful to benchmark a gantry truss structure stiffness and weight against the typical shapes as in routercnc's spreadsheet.

P.S. eg. aluminium lighting trusses. Whilst theyare pretty stiff, to use as a basis for incorporating into a design, (I would think) wouldn't have the local material strength to cope with getting forces in and out where required, without careful reinforcement. But obviously, there's different sizes and spec's of off the shelf aluminum trusses around.

EddyCurrent
10-02-2014, 12:18 PM
Some ideas to ponder ?
http://www.mycncuk.com/forums/gantry-router-type-machine-building/6536-please-review-router-gantry-idea.html
http://www.mycncuk.com/forums/gantry-router-build-logs/6516-monster-fixed-gantry-router-aluminium-wood.html

JAZZCNC
10-02-2014, 07:40 PM
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.

CharlesJenkinson
10-02-2014, 08:22 PM
Some ideas to ponder ?
http://www.mycncuk.com/forums/gantry-router-type-machine-building/6536-please-review-router-gantry-idea.html
http://www.mycncuk.com/forums/gantry-router-build-logs/6516-monster-fixed-gantry-router-aluminium-wood.html
Thanks Eddy,
i like the big tube idea, I just don't think it's easy to terminate at the ends, with something to take the linear brg blocks and to get down onto the screws. Composite sounds good, but it's heading away from mass, and I hadn't appreciated the value of mass, though composite is good for damping.

CharlesJenkinson
10-02-2014, 08:43 PM
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
10-02-2014, 08:56 PM
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.