Easy to build torsion box and rails

Hi,
Just an idea. Build a torson box with a foam core. You would only need to match cut the top and bottom skins and sides. Glue together with foam in the middle. This will provide a very stiff torson box that is easy and cheap to build.

You could then match cut and drill the torsion box ends. The holes would be for the pipe rails.

For the pipe supports, you could cut a couple of full length pieces of MDF cut on a angle glued and screwed sandwiched between the pipe. See the attached drawing.

Larry

Excellent idea. Since you can buy sheets of styrofoam in varying thickness and up to 4' x 8' sheets it would save a lot of piecing small parts together to make the internal ribs. The box itself could be built in an hour or so. You could use 1/4" ply and 2" styrofoam for a small budget. The supports for the pipes would be a simple process and rather accurate placement if using a table saw to cut the V in the boards. This method would work well to build joe's dual rail router with minimal effort. I'll definately explore this further. Simple idea right there waiting for someone to see it. Thanks for the inspiration.

Check out this thread, from a while back. Similar idea, but not well received. In fact, it got down to the level of semantics.

I still think it's a good idea...

http://www.cnczone.com/forums/showthread.php?t=19020

-- Chuck Knight

The idea of using foam for shear material has been around for many years.

1000's of homebuilt aircraft use foam in their construction. One example is the Skypup. It has a cantilever wing design with thin fir spar caps with foam used as a shear web. Not one wing failure in 20+ years.

SM foam has allowable compression strength of 22 psi. A torsion box with a cross section of 24" X 3" (foam area) will give you a cross section of 72". If the torsion box had a load of 200 lbs, the foam would have a shear load of less then 3 psi.

The torsion box would help reduce vibration and noise. Yes, it is still a torsion box that uses sandwich construction.

The idea for this type of construction was for us that have very basic wood working skills or someone looking for quicker construction methods. It has the added bonus of costing less to build.

When building my first router, there were times after I had to cut a piece for the third time that I wanted to throw the whole thing in the garbage. I think this method of construction will help more machines get finished.

Larry

Quote:

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Originally Posted by lgroulx200

If the torsion box had a load of 200 lbs, the foam would have a shear load of less then 3 psi.

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That's assuming the load is equally distributed over the entire section, which it isn't going to be. Load is going to be concentrated at support points; on gantry sides the loci are going to be the attachments of the y member and the bearing seats, for instance.

Only case by case beam load calcs are going to decide if local failure is likely, just as with any other structural assembly under load.


Tiger

You would not need to do this.

Get a couple of pieces of 3/4" 2' X 2' MDF or ply. Place one piece of ply on level ground, lay on a piece of foam on, then the other piece of ply on top. Start jumping on it. Take the top piece of ply off and you will see that the foam is still intact.

Well that's pretty much a "duh" scenario, imo. Take a piece of 3/4 ply and prop it up at the edges on 2x4's and jump up and down on it and the ply all by itself will be undamaged, so it's saying nothing about the point load carrying capacity of the foam when it's added. All it's saying is the load carrying and distributing ability of 3/4 ply.

Do the same thing with 3/8 ply (which most of the builds seem to skin torsion boxes with) and watch the foam get mangled ;)

The major factor in strength and rigidity of a torsion box is the transmisibility of load to extreme fiber (far side). Unless your core structure, be it uniform or localised in ribs is able to handle without failure the maximum localised load generated by working stress at the peak load point of the entire structure, it will fail under load plain and simple.

Foam core is doable, I'm not saying it isn't. All I'm saying is that it's going to have to be either very beefy foam or the assembly is going to need engineering to avoid excessive localised peak loads. Or third option, you simply expect a lower level of strength than a ribbed box would produce with the same skin.


Tiger

you talk about an airplane wing, they are designed for flex, we do not want anyflex what so ever in out torsion boxes, foam will flex and allow the top and bottom skins to flex.

WhiteTiger pretty much has it..

Foam in composite panel construction like boats or aircraft are very strong, but the panel is usually curved and composites get their strength from 3d curvature. Flex and vibration is allowed and even a desirable attribute. The foam simply seperates the two skins forming the shearwebs and there is little compressive strength in the core required. Having said that cores up to several hundred lds per sqft are often needed in localised areas.

A flat panel with foam will put the foam mostly in shear rather than compression and will rip up the foam, will still flex and will require local reinforcement.

Andrew

Here is a couple of pictures I have just taken. I grabbed a couple pieces of scrape MDF and foam.

Quote:

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Originally Posted by lgroulx200

Here is a couple of pictures I have just taken. I grabbed a couple pieces of scrape MDF and foam.

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I really do not see what that test has to do with a torsion box, the foam coar will flex, and sping back, We do not want any movement what so ever in a torsion box, i can tell you when you drove upon it the thichness measurement changed. now make a torion box 36-48" long and try it.

I just wanted to show that the foam can handle the loads. If I built a 24" X 36" box and drove the car on it, there would be much less load that the cores would have to carry. The twisting loads are still carried by the torsion box skins.

All I wanted to do is share an idea that could help people, like myself, who are new to wood working build a strong, workable router.

I am going to build my next router using this method. I will keep you posted on my progress.

I hate to interject here but all this talk about trying to make a stiff torsion box out of foam laminated with what have you has me a bit perplexed. I will give you that plywood or MDF skinning isn't to bad but your don't need a core to make torsion boxes out of these materials.

Now if you consider doing the construciton with glass or other structural molding materials you really have to think long and hard about costs. You would be just as far ahead to buy square or box steel beams and a good mig or stick welder. At least when you are done you will have an additional asset. With just a bit of effort you will have a superior frame. That effort likely to be a lot less then the effort to mold up all of your beams out of synthetic materials.

Thanks
Dave

Thanks guys for your input.

On the idea of using foam, some of us will agree and some won't. That's what makes this forum so great. I have read so many great ideas from people willing to share.

On a side note, what do you think about the rail supports?

I would think that the skin would distribute the stresses enough to prevent damage to the foam core. On the other hand, greater mass is desirable, as it would prevent vibration/chatter. Right?

Hi,
Yes, mass is desired. But you could build a simple torsion box with a foam core very quickly, easily and with greater accuracy for many people just starting out. The machine would be up and running much faster and if desired, use the machine to cut out complicated parts for a replacement gantry. For most work, the foam filled gantry would be more then enough.

They are building a new ring road in Edmonton AB. They are laying down 4" of foam, the road bed and then the asphalt. The foam is used to stop the frost. If heavy trucks can drive on the road then it should be more then fine for our machines.

Larry

Quote:

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Originally Posted by lgroulx200

Hi,
Yes, mass is desired. But you could build a simple torsion box with a foam core very quickly, easily and with greater accuracy for many people just starting out. The machine would be up and running much faster and if desired, use the machine to cut out complicated parts for a replacement gantry. For most work, the foam filled gantry would be more then enough.

They are building a new ring road in Edmonton AB. They are laying down 4" of foam, the road bed and then the asphalt. The foam is used to stop the frost. If heavy trucks can drive on the road then it should be more then fine for our machines.

Larry

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The foam is on top of "terra firma." Also, just because trucks can drive over it doesn't mean it won't flex or compress. Ever see someone lay on a bed of nails, or even sandwiched between two beds of nails with people standing on top? It doesn't have much to do with the actual strength of the person's skin as much as the weight on top being distributed among thousands of nails.

Unless you have wood or another substrate as a frame member along with the foam, your only strength in your foam "sandwich" would come from the sheer strength of the glue bond between the foam and the wood. The top layer of the foam would probably delaminate first.

If you don't believe me, take two pieces of whatever skin you'll be using and the foam, and glue them together with a glue of choice. Then support each end leaving the center free, and stand (or drive) in the middle. If the pieces were long enough you probably could flex it with just hand pressure.

If you want to make a composite beam that is rigid and light, use a honeycomb core. Hint: Most other methods (wood torsion box, steel, aluminum) are cheaper and easier than trying to build a proper composite beam that is both light and rigid.

BTW, I build rockets with composite materials.