Torsion box question

[chuckknigh]

The new fad in machine design seems to be torsion boxes. But, they're being made tediously by hand, or some of the fancier ones are using CNC routers to cut the ribs. Pretty good idea, as it reduces the mass (and momentum) while providing an extremely stiff final product.

But, they can be hard to build well. So, let's approach this from a different direction. I don't want to tediously hand cut all those ribs...what can I do?

A torsion box, by definition, is a construct comprised of a honeycomb of "space filling" ribs which hold skins in position while resisting compression. The skins operate primarily in tension.

Following this through to its logical end, we need something which acts well in compression, as the core. Styrofoam! It's easily available, cheap, and a snap to cut.

Skins -- thin plywood would work -- heck, paper would probably work, but I'd prefer something with more integrity for the outside of a CNC machine. Formica might be a good choice, as it's very strong, very tough, and designed to attach with contact adhesive.

Extremely strong structures are built using fiberglass over foam construction, and similarly strong structures with concrete, wood, and other coatings -- all over foam cores.

Has anyone built a router using this type of construction? It seems like it'd be a natural for homebuilders who want something strong and cheap.

Comments?

-- Chuck Knight

[mechsiah]

Its funny you mention it- while working on my second version (CNC is a gateway hobby) I considered using "gatorfoam" which uses the torsion principle sandwiching some "super" foam between two sheets of plastic. The stuff is tough, but when I priced it out I was paying more.

Someone far wiser about materials may be able to comment on the styrofoam, but I like the idea of an easier torsion box. It took me a while to put mine together from mdf...

[keyne]

Styrofoam does act well on compression? It compresses easily.. i think this would be a poor choice of material.
Foam core structures usually don't get their strength from the core. The foam is just used as a mold in most cases.
Anyway, weight is good.
However, i think alot of folks really overcomplicate torsion box building. Most use lap joints in the structure, which are hard to cut (accurately) manually.
the boards that make up the structure don't really need to be interlocking or even connected at all. (please correct me if i'm wrong). All they need to do is connect the skins.
Ripping boards to same width isn't that hard with a tablesaw. You could also use a reference board (alu straightedge?) and a copy bit on a router to make all same width boards easily.

[posix]

sandwich construction isn't torsion box. it uses the principle that the stiffness of a cross section increases with the distance between skins. it's only there to keep the skins apart.
to put simply what's stiffer a small diameter pipe or a large diameter pipe? large diameter pipe of course. and why? because of the cross-sectional area.

why are cans so stiff if you try to compress them (without cheating and squashing it slightly to one side)? because they are so big in cross-seciton even though their walls are so thin.

[chuckknigh]

Very well, then...sandwich construction.

Terminology aside, would this approach work? With reasonably strong skins (thin ply or sheet metal is enormously strong) and a sufficiently thick section, it seems a simple and very home-buildable approach.

-- Chuck Knight

[vacpress]

try it.

[WhiteTiger]

The problem with styrofoam is that it is both highly compressible (being an expanded material) and inelastic. First time you have a serious inward displacement of your skin the styro is crushed and the skin at that point is thereafter just flapping in the breeze unsupported. Might as well just make a hollow box to start with


Tiger

[dansutula]

SIPS aka (stressed skin panels) are sandwich panels used in the construction industry. They are typically EPS (styrofoam) or rigid polyurethane foam sandwiched between two or OSB (oriented strand board) sheets. For more info check out www.sips.org. Depending on your size and anticipated loads, for a router table you would probably want to use a thick 6+" SIP, and perhaps laminate formica or bond-on a thin steel or aluminum facings to provide some additional structure and moisture resistance. You would also cut/route out foam along the edges of the panel and glue and screw in edge boards. And then there is the DIY foam-in-place option in which you could make a hybrid torsion box / SIP with whatever internal structure you desire.

Now, for the "out-of-the-box" DIY Honeycomb panel using low-cost, readily available, precision formed, lightweight, corrosion resistant, aluminum alloy components for the honeycomb cells. Start saving you beer and soda cans! Simply stated..Bond the cans to the bottom skin (or better yet have someone route shallow positioning rings directly into the skin), make a little dam around the bottom skin and then flood it with thin layer of epoxy. Flip the panel over and bond on the other skin. And to maximize strength you could also bond the cans along their contacting edges. For a big panel larger diameter (ie coffee cans) might be more manageable.

I'll be using a more proven traditional construction methods in building my router table, but I thought I would share these thoughts.

[spalm]

Chuck,

I really don’t think cutting all those MDF/PLY ribs is really something to avoid. With a couple of woodworking techniques you can bang them out quite fast. First thing is to rip all the ribs to the right height. Table saw makes this a breeze. If you don’t have one, then over size them by a half inch. Rough cut them with any saw, or have the hardware store do it. Now use a router with a straight edge to route both sides so they are all to the right height. Once you get one cut, you can use a pattern bit to bang out the rest. Use the same technique to cross cut them to the appropriate length (oversize cut, and route to length, use a pattern bit with the first one to cut the rest).

I do like this discussion on looking for different ideas, but I thought I would throw in my thoughts.

Steve

[Madclicker]

I can't see making a router table, especially a cnc table till you have a decent table saw, planer, jointer, band saw, and maybe even a compound miter saw.

[spalm]

Steve,

Those are all great, but there are other ways of doing stuff was my point. A lot of people don’t have a table saw (or room for one).

This is a little off thread, but my heaviest used tools for CNC making (and heck, all my woodworking) are the table saw, router table, and drill press.

Steve

[WhiteTiger]

If it's alternatives, then how about the simple old craft paper glueup? 1 or 2 inch wide roll of heavy paper, unroll and spread a band of glue edge to edge at regular intervals then top with another layer of the same paper. Repeat, staggering glue bands as many times as you need, then press and let the glue dry.

You get a honeycomb of heavy paper when you stretch it out. Spread a layer of glue on each skin, arrange the honeycomb between them and weight/clamp and let the glue cure.

Just time and materials. No sawing, jointing, etc.


Tiger