Hi. Is there any reason folks build torsion boxes the way they do? I get the perimeter frame, but why all the internal pieces? It would be far simpler, and probably more rigid, to fill the internal space with foam sheets. This is how many aircraft wings are constructed (using as little as 3mm plywood skins top and bottom). These wings bear thousands off kilograms when pulling high G forces.
My plan is to build the frame, top and bottom out of 6mm plywood, and bond in 100mm foam sheet. Light, extremely rigid and easy to construct.
That’s a very interesting idea. I built a few R/C airplane wings similar to that, way back in the day. The skin was balsa, overlaid with fiberglass, and those wings were darned strong.
I built my 1F torsion box the conventional way, from 18 mm BB ply, following “stressed skin structures” design guidlines from the building-construction industry. It performs very well, but certainly may not be optimal.
If there are any mechanical or structural engineers out there with finite-element analysis software and skills, they could probably give you fairly accurate performance projections. Either way, I look forward the your experimental results.
I personally would want something a little beefier than 6 mm ply to hold the machine-mounting fasteners. I’d at least double it up at the mounting hardpoints.
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I fully agree about the hard points.
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Superb! But using foam panels is both cheaper and easier to build. The foam/ aluminium honeycombs / wooden batons internally serve one purpose only. And that is to keep the upper and lower panels apart. Even pasting ordinary A4 paper on either side of a foam sheet turns the wobbly foam into a remarkably stiff panel.
Squaring and flattening of the resulting plywood/foam sandwich can then be accomplished on your cnc. No need for the overly complex process in the above post )as fascinating as it is).
My torsion box will probably end up as a 6mm plywood box with foam internals. To this I will bond an 18mm plywood top, and an MDF spoilboard
In fact, thinking about this, there is little to be gained by having solid ply for the top, bottom and sides. Turning the flat ply into a honeycomb would have the same torsional stiffness at half the weight.
A Cracking idea. Two people could hardly lift my torsion box when I built it using 1 inch mdf top and bottom and batons (i wont do that again).
Things i would consider: allowance for fixing (mentioned above) and bonding the layers.
A thought: sometimes the mass of the unit may be helpful when milling with a lump of a spindle. But that mass doesnt have to be The table, it could be detachable from the table.
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Torsion boxes depend on the internal grids for carrying loads and resists bending. While foam adds significant rigidity, it is far less effective in carrying the loads. For light and evenly distributed loads, simple foam with plywood skin works well.
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Sorry to disagree. The internals of a torsion box have one job - to keep the outer panels apart. As the box tries to “bend” the outer panel stretches, and the inner panel compresses. The internal structure simply keeps these two panels from collapsing onto themselves. This is why airplane wings can be built with foam internals and 1.5mm wing panels. Even wing spars have spar caps kept apart by a single sheet of 2mm ply. All the stress in the skins.
Hey, interesting concept. On a 4’ wide machine you will get deflection. You want the top to be as rigid as possible. My thoughts on the torsion box are they are unnecessary for the application of cnc top. I built essentially a 2x4 “wall” studded at 12in o.c. And sheeted the top with 1in mdf. This was very affordable and extremely solid. Also very easy. My base is custom made from 4x4s and steel and bolted to the floor. Not sure what size machine you are building for but you want the weight and rigidity. Also for sure wherever bolting to needs to be at least 3/4” + and not 6mm but it sounds like you already knew that. Good luck on the build, just my 2cents!