Truss style Gantry

[Musht]

>>Any discussion of structures failing is hardly relevant, since in a CNC machine we are aiming to obtain a certain level of stiffness, not just aiming to ensure the material does not fail.

Used an example of failure as an extreme example of how lighting trusses are only stiff in certain directions and almost without strength in others. It is made in a certain way to exploit weight savings for very specific reasons.

It may not be obvious that lighting truss has standard outside diameter chords , similar to aluminium scaffold pole, but the wall thickness is much thinner, the strength being compensated for by the solid rod welded in a triangular form between chords.

Not expecting catastrophic failure of a moving gantry on a router to be considered a high risk.

As said it may not be immediately apparent that trusses secret is generally thinner that would think walls on main tubes, like a staging system also linked to.

But then have already said all this.........

[m.marino]

The only issue I can see is torsional flex under load might become an issue. I have seen That be a with this type of structure under loads that where not along their axis (which the Z plate and the resistance to cutting will not be at all times (other times it will be). As far as weight moving up to a 2:1 reduction in the pulley system you are going to using should more then take care of any mass issues. Also depending on what screws you use that can become a non issue rather quickly. Truss designs are fun to work with as I have helped build more then a few and done properly they can be extremely strong.

Michael

[D.C.]

The model at the moment is just a rough, I will join up the supports it was just pain in the butt trying to rotate things in to odd angles in sketchup. :)
I was going to epoxy to avoid any welding distortions, the rear struts I will weld onto plates and then epoxy all the rear strut plates on to the beams in one go.
My maths still isn't good enough to confidently sit down and work it out, so thanks for that!
I'm almost positive I won't upgrade to guides, if I upgrade this machine it will be to extend the x-axis to cater for 8x4 wood sheets and I can't really see anything I'll be doing at size needing the accuracy of guides.
If resonance does become an issue I always have the option of filling the beams with vermicrete or something similar, a poster over on practicalmachinist reckons truck driveshafts just use thick cardboard tubes inside to dampen vibrations.
The epoxy path is starting to get expensive, £120 quid just for a flatish surface which would mean I would have to ditch the shiny leadshine drives. I think I'm going to have to see if I can find some where local with a decent sized surface table and use the cheap stuff & clingfilm method...

[mocha]

DC, you'll love this one then! :-)

MOAT

[D.C.]

DC, you'll love this one then! :-)

MOAT

Pass me the tissue paper, I may be some time...

[Ricardoco]

Hi DC, ive been reading this thread with interest and i was wondering, what is the maximum tool size you will be using and what forces would be required to break it, Im not sure but isnt there a relationship with that information and the gantry stiffness required? I would be interested to know..

Rick

[routercnc]

Hi D.C.

If it helps answer your first questions about the suitability of this design for a cnc router gantry, then my answer would also be yes. Trying to avoid getting to technical:

Although your individual box sections are slightly smaller than a typical size used for a 'single piece gantry', they are located along way from the 'neutral axis' (region of ZERO stress which for your shape is the exact centre of the triangular shape). This makes it much stiffer than the 'single gantry' designs under all loading conditions because the 'distance of material away' factor in stiffness is to the power 4 for torsion and to the power 3 for bending. Doubling the distance away from the neutral axis makes the structure 2^3 or 2x2x2 or 8 times stiffer in bending.

There are 3 loading conditions you need to consider:

X axis cutting loads put bending into the frame are resisted by both front beams in compression and the rear lower beam in tension. Because the beams are spread well apart (and linked by the connection pieces) this is what mainly makes it very stiff. Cutting loads in the other X direction reverse the compression and tension.

X axis cutting loads also put torsion loads into the frame because the tool cutting forces are offset from the centre of the beam. This tries to bend the beams which in themselves are not that stiff, but are prevented from bending by the bracing connection pieces. A structure under torsion has maximum stress at a 45 degree angle to the central axis, therefore angling your bracing pieces at about 45 degrees will be optimal.

Z axis cutting/plunging loads are resisted by both bottom beams in compression and the top beam in tension. Weight of the spindle is resisted with these forces reversed. Again wide beam spacing is the dominant factor to give good stiffness.

So the only thing 'wrong' with this idea is the work involved.

All simply supported beams (of any shape) will suddenly fail when they get to the yield stress, but the loads are massive. You are at the other end of the graph so don't worry about that.

[D.C.]

Hi DC, ive been reading this thread with interest and i was wondering, what is the maximum tool size you will be using and what forces would be required to break it, Im not sure but isnt there a relationship with that information and the gantry stiffness required? I would be interested to know..

Rick

To be honest I can not answer your questions, I'm approaching things from the exact opposite direction.

I don't have any calculations for minimum acceptable tolerances, I'm just trying to make the best that I can knowing the skills and budget that I have. This is a hobby cnc, more one off multirole artsy kind off stuff and not industrial high tolerance required scenario.

Despite all the fussing about the arrangement of bits of steel, I will initially be using MDF plates for many parts of the machine, horrible I know but I can cut the alu parts I require and destroy the MDF in the process of doing so.

For what I want to do I need to 4 axis woodwork, I need to cut acrylic, I need to do PCB work and any alu past upgrading the machine is strictly artistic so the tolerances are not very high, as long as a curve is smooth enough for the human eye, I'm a happy bunny, most of that smoothness can be achieved by a good walnut finish, exterme accuracy not required.

Despite that it just is not in my nature not to study, investigate and attempt to the best job possible, if I can make a better machine than I need I would be disappointed in myself to settle for anything less.

[Ricardoco]

To be honest I can not answer your questions, I'm approaching things from the exact opposite direction.

I don't have any calculations for minimum acceptable tolerances, I'm just trying to make the best that I can knowing the skills and budget that I have. This is a hobby cnc, more one off multirole artsy kind off stuff and not industrial high tolerance required scenario.

Despite all the fussing about the arrangement of bits of steel, I will initially be using MDF plates for many parts of the machine, horrible I know but I can cut the alu parts I require and destroy the MDF in the process of doing so.

For what I want to do I need to 4 axis woodwork, I need to cut acrylic, I need to do PCB work and any alu past upgrading the machine is strictly artistic so the tolerances are not very high, as long as a curve is smooth enough for the human eye, I'm a happy bunny, most of that smoothness can be achieved by a good walnut finish, exterme accuracy not required.

Despite that it just is not in my nature not to study, investigate and attempt to the best job possible, if I can make a better machine than I need I would be disappointed in myself to settle for anything less.

Hey DC thanks for Answering anyhow, i guess i will have to wait till someone else knows the answer, i do look forward to seeing your machine come together as ive not seen one like this before, and to be fair MDF machines are a lot more usefull than may be first imagined, when i had Mine it surprised me.

Rick