Torsion box using Foam.

[fRED mCnEILL]

I want to build a torsion box as a cutting table for cutting heavy cloth. The size would be 4 ft by 8 ft(sheet of plywood) and would sit on my saw or workbench.The object is to build a "table top" that is VERY light(so I can handle and store it away easily) so I was thinking of using 1/8 (or perhaps 1/4 in) hardboard and a 2 inch thick styrofoam sheet. Glue would be PL200(for styrofoam)
Rigidity is not paramount as there is VERY little weight being placed on it , just as long as it holds its shape.
Anyone have any suggestions on this. I was also thinking of applying to glue in strips so as to mimic a wood spacer. Appling glue to the WHOLE surface seems overkill.

Thanks

Fred

[ian maybury]

Foam makes an excellent sandwich, but it's advisable to glue the whole surface to hold everything flat, and to prevent the bond failing in shear if loaded heavily.

You could use polystyrene foam, but would probably be a lot better with one of the 'blue' polyurethane types - flatter and more to the point much stronger in compression. Less picky about glue types too.

The reason is that the task of the core is to prevent rippling, buckling or dinging of the skins, and that primarily involves compressive loading.

There are purpose made structural foams designed for use in sandwich mouldings which are stronger and probably flatter again, but last time I looked which was years ago they were expensive. A high end glassfibre and composites moulding supplies place could fill you in quickly on this if you wanted to take a look, but it'd probably be overkill unless you were keen to use very thin plywood skins...

ian

[Peter Quinn]

I ran into these doing some research for an upcoming project. Its an interesting site that might provide some food for thought for your idea.


http://www.superhoneycomb.com/product_mdf.html

[HANK METZ]

I had the same need for cutting ply sheets, but I built a frame that folds up for storage yet sits atop sawhorses when needed. Don't see why you can't toss a sheet of 1/4" hardboard on it to use as an aux. work surface

217188

Here it's collapsed scissors- style and hung on the wall out of the way, it is extremely light too. I have a few more detail shots of construction showing pivot points if you like.
217189

- Beachside Hank

[fRED mCnEILL]

Thanks, Hank.
That look very interesting.

Regards

Fred

[Mattias Puide]

I have made something similar to what you want to do. I have reported it here:
http://www.swedishwoodworking.com/articles/the-micro-workshop-part-2/

[Jim Matthews]

I have made something similar to what you want to do. I have reported it here:
http://www.swedishwoodworking.com/articles/the-micro-workshop-part-2/

Now that is clever.
Bra gjort! Tak so mucket.

[Alan Schaffter]

Based on your requirements, a tempered hardboard-foam-tempered hardboard sandwich torsion box should be fine- use the 3/16" - 1/4" hardboard with smooth top & bottom surfaces. Don't use closed cell low density Stryrofoam as it is too flexible and compressible. You need about 2" of a rigid, high density foam like the blue or yellow stuff. You can buy 2" thick rigid insulation foam in sheets from many building supply houses. It will likely come with a vinyl, foil or other surface so that must be taken into account when you select a glue or adhesive. You can also get the foam from homebuilt aircraft suppliers like Aircraft Spruce and Specialty (http://www.aircraftspruce.com/menus/cm/foam.html), or surfboard manufacturers. If the foam is rigid it will make ribs (shear web) unnecessary, since the foam serves this purpose. I recommend a solid edge so the edges aren't compressed. You need a good adhesive to bond the skins to the foam. Two coats of a good water-based laminated adhesive on the hardboard and foam surfaces should be fine. Don't use a polyurethane glue- it expands as it cures. Check any glue or adhesive first to ensure it doesn't soften or dissolve the foam.

[johnny means]

Check any glue or adhesive first to ensure it doesn't soften or dissolve the foam.

Reminds of a trick we would play on the new guys. Send them across the shop with a styrofoam cup to get a little thinner, then yell at them for making a mess.

[Ole Anderson]

With such a thin first layer, you are going to need to start with a flat surface to support it properly or your torsion box will not be flat. Something to think about. If it is just for a cutting surface as stated, this is not a big deal, but if you want to also use it for critical assembly and glue up work, it could be. David Marks (remember him?) has a nice video on torsion box construction, he is not using a foam core, but some of the same ideas for getting it flat will apply.

[Rich Engelhardt]

I made a 2' X 4' work surface out of some pegboard and some scraps I'd trimmed off of some T&G flooring.
Instead of using a torsion box, I made I-beams for the supporting structure.
It's based on a design from Shop Notes.
I'd guess the whole thing weighs about ten pounds.
It's strong enough to easily support my fat carcass (250 lbs) and then some.

For a 4' X 8', there's no reason you couldn't clamp two of them together.

[Dave Zellers]

You'd need to clamp 4 together to get 4 x 8.
;)

[Alan Schaffter]

With such a thin first layer, you are going to need to start with a flat surface to support it properly or your torsion box will not be flat. Something to think about. If it is just for a cutting surface as stated, this is not a big deal, but if you want to also use it for critical assembly and glue up work, it could be. David Marks (remember him?) has a nice video on torsion box construction, he is not using a foam core, but some of the same ideas for getting it flat will apply.

I found Mark's method (and the Wood Whisper's repeat) to be too time consuming. Regardless of your shear web material and skins it is a pain to make the shear web by cutting individual cross pieces (laterals), especially when you must trim and hand fit every piece in the last row. The method I showed in my American Woodworker article is more precise, quicker, and easier. Using an accessory fence on your miter gauge or sled with an indexing pin, you can quickly cut perfectly spaced notches in the lateral and longitudinal web pieces that make assembly quick and easy:

http://www.ncwoodworker.net/pp/data/1421/medium/P10100792.JPG

Cutaway view of the torsion box top on my adjustable height assembly table.

http://www.ncwoodworker.net/pp/data/1421/medium/Cutaway_Test-1.jpg

Unless you have a good, flat construction platform it will be hard to make a flat torsion box. Mark's method is as good as any and what I used- make a frame with all lumber trimmed to the same width, set it on saw horses anchored to the floor, use winding sticks to adjust until flat (doesn't need to be level), and secure everything with hot melt glue. For a torsion box made with thin hardboard skins, you might want to set a sheet of 3/4" MDF on the construction platform.

http://www.ncwoodworker.net/pp/data/1421/medium/P10100834.JPG

A torsion box made with thin masonite can be strong- I made a lightweight one (less than 30 lbs) that was 8' long by 18" wide, by 2.5" thick from a sheet of 3/16" tempered hardboard and some leftover 1/4" pegboard. I assembled it with yellow glue only. I applied glue to the edges of the web and am about to attach the second skin in the first pic. In the second pic the completed torsion box is supporting over 300 lbs at mid-span yet sagging less than 1/2"!

http://www.ncwoodworker.net/pp/data/1421/medium/P1040010.JPG

http://www.ncwoodworker.net/pp/data/1421/medium/P1080028.JPG

Fred doesn't need one this strong, but if he plans to use a rotary cutter, might want to consider a replaceable top skin protector.

[Rich Engelhardt]

You'd need to clamp 4 together to get 4 x 8.
;)
Oops!
Yes - it would take more than two.

[Dave Zellers]

Alan- what size is that large assembly table? And what did you use for the final surface?

[Jim Matthews]

It's strong enough to easily support my fat carcass (250 lbs) and then some.

Hey, I resemble that remark. The toughest fixture in the shop is my barstool.

[Alan Schaffter]

Alan- what size is that large assembly table? And what did you use for the final surface?

The article assembly table is 40" X 62" the larger one is 4' X 7'. The skin on the smaller is 1/2" MDF covered with plastic laminate (Formica). The skin on the larger one is 3/4" MDF covered with laminate. I if I had to do it again I'd use 1/2" for the top skin of that one too.