Truss style Gantry

[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