Ok. I would like to invite all those who are making a "torsion box" router and all mechanical engineers here to participate in this.
I have seen that many people are building torsion boxes. Very neat and excellent execution (esp. if you have a cnc router to begin with).
What struck me the instant I saw the first torsion box was "what a waste".
I shall explain:
After having read some books on space-frame design (from the racing car chassis design neck o' the woods) I often find myself looking at various structures around me, like buildings, bridges, cars, whatnot, looking for triangulation, looking for monocoque, spotting mistakes, appreciating well exectuted designs etc etc.
And I really think I can spot a valid space frame when I see one.
To elaborate on the "space frame" malarky so the uninitiated don't accuse me of talking gibberish, basically a space frame is a structure designed to connect 2 or more points in 3d space such that the connection is ultimately as stiff as possible. Does that make sense?
Now, the best way of connecting 2 points in space AB is with a straight line. The best way of connecting 3 points in space ABC is using a triangle. I don't need to elaborate on this further as since you are allready here it means you are literate, if you are literate then you definitely understand what a triangle looks like.
Turns out that connecting 4 points in space ABCD is also best done with a triangle. Not one but multiple triangles. Well 3 really. ABC, BCD, CDB, done.
Remember that we are creating for stiffness. You can do it with stainless steel rods. Well you can do it with matchsticks equally well. It'll be just as stiff.
So our torsion box, once we have "triangulated" it looks something like picture 1! Ok, I've left out a few diagonal links there which would probably confuse some people, but you get the general idea. Triangle, triangle, triangle...box of triangles.
Now, turns out that you can actually fill in the void between sides of our triangle with material and it won't hurt the performance of the spaceframe one bit, in fact it'll better it somewhat. And so we get to the infamous "monocoque". And now our torsion box looks something like picture 2.
Reminds you of a box, doesn't it? In fact it IS a box. Hidden from view are only 4 internal braces (those missing from picture 1 above) going diagonally to connect the opposite edges of our box in effect triangulating those as well.
So here's my tip:
next time you want to make a torsion box you may only need to cut the "skin" for it and those 4 diagonal braces for the innards. And you will be left with equally functional torsion box and a heap of MDF to use for something OTHER than making millions of internal braces for it...
Very interesting. I credit you for coming up with a lot of thought provoking ideas. I also just recently learned about the great dimensional force achieved with triangles. I never thought of it for a torsion box though. I can see where the sides and end would probably be just as strong, but the top and bottom I don't understand. I guess the skin's would evenly distribute the loads? I can see where maybe an X brace pattern, or really two triangles I guess would be better. With MDF, I think you'd want to cut those triangles out of once piece, which would cause a lot of waste. So I can see where the design might be better, but might have unnoticeable benefits over a torsion box wood-duty router. Since I would want to keep the triangles as one piece, and that would cause waste, I would probably just go with the standard torsion box anyway. But your post did get me thinking! Plus, I learned a few new words. (Well, I'll learn them after I look them up in the dictionary.)
Actually you don't have to make them out of a single piece of MDF. You can use off-cut strips of wood (mdf, ply, whatever) to construct those triangles.
But, as you can see in the 2nd picture you don't have to have triangles at all but just skins so it's even easier! They (skins) can be considered triangles with filled-in voids. So you just have to make a box and cross-connect the opposing edges INSIDE the box with just a few cross-braces.
Also people sometimes tend to go overboard with triangles. Lots of times you can reduce the number of unneeded cross-braces greatly. If someone wants an example I could "put something on paper"
I have here a picture of a car (well, it has 4 wheels, an engine and a steering wheel) created almost in its entirety out of bicycle spokes (you know, the wire spokes). Just goes to show that you don't need big-a** rods or fat bars for triangles. You just have to connect them properly and apply pre-load to every joint. You also have to make sure that there are AT LEAST 3 incoming rods/bars/pipes/matchsticks on EVERY joint. That's if you want your spaceframe to do it's job properly at a minimum weight/waste penalty!
Posix; you are correct in your analysis and even your diagonal braces are mostly redundant. If you have ever seen pictures taken inside box beam bridges, which have to be stiff in all directions, they don't use such cross braces and the same is true of ship's hulls although there you do get the complication that the surfaces are not flat.
But what you are overlooking is buckling of an unsupported skin on an empty torsion box. The torsion load puts different regions of the skin in compression and tension and while the tension is easy to handle with a thin(nish) skin the compression is a different thing. Internal bracing may not contribute anything to the torsional stiffness but it will stabilize the skin against buckling.
Also you have to take into account how and where things are going to be attached to a torsion box. You may need to incorporate braces or cross struts at attachment points in various places and these are doing nothing for the stiffness but are transmitting a load across a thin skin which will flex under loads applied perpendicular to its surface.
If you're saying for a given size and load a Box (Monocoque or exterior shell construction) would be adequate for a given requirement then that's probably true, it depends on the circumstance. A torsion box could be overkill. A diagonal braced torsion box would be stronger than a simple Box and might be a suitable solution but still less strong than a full torsion box. It's also true that neither a Box or diagonal torsion box would be better than a full braced torsion box in many of the machines being built here. A builder needs to select what is best for his requirement.
A space frame, a box monocoque and a diagonal torsion box and full torsion box are distinctly different structures and have different properties. A Box will twist for example, as will a space frame unless specifically braced for the load. Given similar materials and dimensions a Box is not as stiff in torsion (twisting) as a Torsion box (hence its name) and the more bracing a torsion box has the stiffer it becomes. A Diagonal Torsion box resists twist because of its internal bracing across the points, more material in the cross section and because of the transfer of some of the load to the skins. It but will still twist but less than a Box. A full braced torsion box adds more material and transfers more load to the skins and will twist less than a diagonal torsion box. Depending on the loads the skin of a Box or torsion box might carry no load and simply be dead weight.
A Box would not stronger under simple load than a Torsion box of the same material in the same size. Taking it to the extreme neither are stronger in load or torsion than a solid of similar size of the same material or the same structure in a stronger material. What is used should depend on the materials and loads and the builders choice and understanding of the loads the part must support, and the construction methods open to him. So I agree there's probably torsion boxes used when simpler structures would suffice, and similarly other parts where simple mdf sheets or boxes should be replaced by torsion boxes or stronger materials.
Ah! And I bet you thought I was going to give up, right!
Well, being a stubborn ba*tard that I am, and with a "here's one I made earlier" adagio I present my version of the "torsion box".
I also invite all you fine people to come up with something torsionally stiffer than this structure. Yes I have left out a few links and there are some superfluous ones, this can be tidied up but my eyes started to hurt so I called it a day with this.
Prizes to be won are, in order of importance:
1) a peck on the cheek and an apologetic admission that I was wrong
2) admission that I was wrong. forget about pecks, do better next time.
3) a secret nod of approval in your general direction
In case, however, that I win this contest then I get to choose my prize after I have given it some careful thought. Hmmm...what do I want...? Let's see....
P.S. I could be persuaded to provide a "skinned" version of this structure if you want.
And yes geof I get you regards STRENGTH of skin, but aren't these torsion boxes meant to be resistant to torsion rather than buckling under impact? I mean, I mostly see them as a gantry and support for Y/X-axis rails. No impact resistance there, just torsional stiffnes required.
Of course there will be additional "skin" on the X-axis to support the piece being machined but that's a different story. Let's just concentrate on the job of a "torsion box" for the moment and see how that can be done more efficiently, with far less wasted material.
The pictures show a space frame not a torsion box. A box is a six-sided object with an internal cavity that does not have any communication to the exterior. A structure into which someone should be pushed before it is sealed up!
I adopt the principle that it is better to expect that you will want to stick things on here there and everywhere and allow for it. If provision is there but you do not use it you are much less frustrated than if you need it but omitted it. When making a machine you can predict that you will likely need attachment points for linear rails or something and if you are expecting them to get support by being attached to the box the box surface had better not flex.
Last edited by Geof; 01-31-2006 at 05:01 PM.
Reason: To respond to Posix's query friendly like this time.
For the most efficient boxlike space occupying structure, if efficiency is measured in minimum amount of material for maximum stiffness in all directions, just look at a typical hollow core door. They are not hollow; the periphery is a slim piece of wood and the interior is filled with a cardboard honeycomb, large cardboard corrugations or foamed polystyrene. They are difficult to beat for lightweight stiffness but cannot take concentrated loads on the flat surfaces.
Yup, I'm into sandwich core construction as well and was about to suggest the best way of making a gantry would be some aluminium honeycomb and cf skins but then that would cost an arm, leg, a pair of kidneys, left lung and possibly a bucket-load of other internal organs that I'm not familiar with...
Anyway, another piccie of the "skinned" box with a see-through side panel, so you don't say I'm cheating.