Torsion Box materials

[Doug Mason]

In my quest for a dead-flat surface for my assembly table, I recently made my first torsion box using pine for the inner core and outer core; well, the pine moved a little after surfacing/etc, and I ended up using a bit of clamping pressure during glue-up of the mdf top/bottom (big mistake!!). Needless to say, it wasn't dead-flat. I'm going to begin my second attempt, and will use mdf for both the outer skin/and inner core. Of course the other option is decent quality plywood; but it seems to me mdf is the better choice? Does anyone have a reason for favoring one material over the other?

[Jim Thiel]

In my quest ... for my assembly table...will use mdf for both the outer skin/and inner core...Does anyone have a reason for favoring one material over the other?


Is this table a knock-down or always up version? If I was looking for a table to do glue-ups, I'd rather it be lighter, so when done it is easier to get out of the way, like overhead or something.

If you do go the overhead route, fyi, make sure the legs are on the up side instead of the down side, and if they are on the downside make very sure that one of those legs isn't positioned directly above the drill press. If that leg is on the downside and right above the drill press, just walk away. It's amazing how funny a doctor can find something while sewing ones scalp back together.

Jim

[Jason Morgan]

When I made my torsion box, I used 3/4 MDF for the whole thing. It was a beast at 4' square. The piggest problem I found was getting the glue up flat. You are only as good as your reference surface, but the assembly process can help you there.

This is the way I did it.

1. make a reference surface that is as flat as you are happy with. I used my bottom skin of MDF shimmed on sawhorses ala David Marks

2. Attach my gridwork one piece at a time to the bottom skin. This was glued and tacked with brads. The trick here is to make each grid piece as flat as possible.

3. when the glue dries, lay out and flatten your other(top) skin.

4. apply glue to and invert your bottom assembly on to the top. For the 4'x4'x4.5" table I was making, the weight of the bottom assembly was enough. If you were using lighter materials then some light clamping pressure might be needed, but that is the problem, unless the pressure is very uniform, youwill introduce waves into the top.

this worked for me. My slab has about 1/32 bow across its diagonal of >4'

[glenn bradley]

I have not made one but most folks seem to use MDF for the grid and the surfaces. 1/2" seems to be the happy medium between too thin (chair legs might poke through) and too heavy (3/4" MDF = 90lbs a sheet).

[Al Willits]

I did the Marks version.
Made mine out of 3/4" MDF top and bottom and 1/2" for the grid, came out 4'x5' and ended up screwing handles to each end so it could be moved.
I also have it sitting on a table that has wheels so I can move it around.

Not only heavy but a bit awkward to deal with with out the handles, or wheels.
Not sure I'd want it any smaller though.
I didn't do any clamping, just used Brad nails and glue.

Al

[Howard Rosenberg]

- 1/4" MDF for the skins
- 3/4" MDF for the grids
- 4" spacing
- never had a prob
- never poked through the 1/4" skin

HTH
Howard

[Philip DiPaolo]

hi --
we're about to build a couple of torsion boxes for our shop, so i did some research. i'm sure there are many other links out there, but these both seem like very thorough resources:

this first one, (referenced above) by david marks, has about 4 pages, so be sure to click through all. well written and photographed --

http://www.diynetwork.com/diy/ww_materials_products/article/0,2049,DIY_14442_2278181,00.html

the second is by jamie buxton, from the bay area woodworking association site. it's a pdf file with good info and illustrations -- esp for how to level the structure during construction.

http://www.geocities.com/bawanewsletter/techtalks.html

hope these are helpful, and good luck.

philip

[brian gallagher]

Has anyone ever tried using a thick 2-4 inch hard foam sheet instead of the plywood grid? in the torsion table.

seems like that stuff must be pretty uniform
and it would be lighter than 3/4" mdf strips every 4".

you could reinforce it every 12" or so with plywood but I guess it would need a pretty heavy hit to dent it more than a few thous when it had a skin on both sides.


brian

[Jeff Patrick]

Brian,
I've made a number of torsion boxes using blue styrofoam as the core. I use polyurethane glue spred with a roller. The outer skin is usually plywood but mdf would be ok although heavier and not as durable. I've pressed all of these in my vacuum press. I believe that the foam is only available up to 2" so if you want a thicker box then you'll have to use at least two layers of foam. In this situation it is best to insert a layer of thin ply between the foam layers. If you are making a panel that is larger than 2'x8' you may want to use two layers of foam and stagger the joints. Again, use a sheet of thin ply between foam layers. The foam torsion boxes are very strong and very light for their size.

My original torsion box (4" thick) that I use under my vac press has a 3/4" plywood core on 4" centers as I recall. The outer skins are 1/4" fir ply. This is very strong.

[Philip DiPaolo]

Has anyone ever tried using a thick 2-4 inch hard foam sheet instead of the plywood grid? in the torsion table.


AFAIU the principle, a torsion box can be made with any material that creates a stiff core. surfboards use foam for example, and door cores often use cardboard. (i know you can buy cardboard "filler" in sheets about 1" thick. if i can find the link for where to buy it, i'll post it, but i'm sure you can google it.)

from what i understand, thickness and stiffness of the overall grid is more important than the weight of the grid material, so i'd think if you could cut the sheets consistently to create a uniform, stiff grid, you'd probably be ok.

on the other hand, i think that if an assembly table were too light it might be less stable. might be worth a small-scale test first to get the grid spacing, skin and thickness relationships right.

thanks for the idea -- you got me thinking!

[Mike Henderson]

This is a question for those who might know more about "strength of materials".

I see manufactured beams made from pretty thin plywood as the main element, with a piece of wood on top and bottom to keep the plywood straight. When making a torsion box, why couldn't you use fairly thin material in the center, as long as it was well glued to the top and bottom to keep it straight and in place (and maybe set in grooves). Seems like a quarter inch piece of hardboard would make good center stock, as long as it was maybe four to six inches wide.

The reason I ask is that the one problem I have with large torsion boxes is that they're heavy. I use small torsion boxes for gluing up veneer flat panels. I'd like to build a couple of bigger torsion boxes, but I want to keep the weight down enough that I can handle them.

Mike

[Eric Wong]

Is this table a knock-down or always up version?

Come on Jim, I know you wanted to say it..."Is this table a knock-down or knocked-up version?"

Anyway, The beauty of a torsion box is that it provides tremendous stiffness in comparison to the strength of the internal materials. So I think that MDF for the box core is overkill and would just add weight. 1/2" plywood should work great, I would think.

[John Schreiber]

Thinking about using foam made me realize that a SIP (structural insulated panel) is basically a torsion box.

I also found on-line that you can buy Styrofoam or equivalent up to 4" thick. I'm sure it would be hard to source it, but it is made.

I've never glued wood to styrofoam, so I'm not sure if there is sufficient sheer strength, but it seems that you could make a very stiff and light structure with 4" foam and two layers of 1/2" plywood.