Do I need a thick skin???

[Charlie Barnes]

Good Evening All,

I have yet another torsion box question, but I'll keep it simple. The total thickness I have is 3". My original idea was to use ribs 1 1/2" thick with a 3/4" plywood skin on each side (3/4 + 1 1/2 + 3/4 = 3). But I'm wondering if it would be better to use thicker ribs with thinner skins like 1/4"? Maybe something like (1/4 + 2 1/2 + 1/4 = 3). Thoughts???

Charlie

[Alan Schaffter]

Depending on your application, you may be over-building the torsion box. Torsion boxes get their strength from the strength (not stiffness) and spacing of the skins. Up to a limit, being of same material, a thicker torsion box will be stronger (cube of the thickness). Here's the deal with torsion boxes-

Skins:
Can be thin and flexible- think airplane wing aluminum, but . . . they should resist stretching along their surface. For many table applications 1/4" tempered hardboard will work just fine and be sufficiently strong. The torsion box shown below is 3" thick, 16" wide, and 8' long. It was made totally (skins and ribs) from 3/16 and 1/4" tempered hardboard and yellow glue- in the second photo it is holding over 300# of bricks in the middle and sagging less than 1/2" supported only at the ends!!!
Thin skins can't handle point loads very well like hard hammering and clamping. You need to consider how you will use the torsion box. In most shop bench applications 1/2" MDF is thick and strong enough with rib spacing of 6" - 8".


Ribs (called shear web);
Don't need to be thick. Their sole purpose is to keep the skins separated.
As in my torsion box, ribs can be made from hardboard as well. They just need to bond well to the skins.
Need to resist edge compression (parallel to the surface)
For my torsion box assembly tables, for commonality of materials, and the attributes listed, I typically use 1/2" thick by 2" tall MDF ribs and 1/2" MDF skins. MDF is flat and unlike most plywood, stays that way, is smooth, dimensionally stable, glues well, and is relatively inexpensive.
3" total thickness is probably overkill, but that depends on what you will use the torsion box for

I wrote an article on this for American Woodworker a couple of years ago with all this info. There are equations for computing all this stuff, but they are useless due to many variables like the materials, glue, assembly technique, etc., etc. we use. Most suppliers of torsion beams only use the equations for the initial design. Then they do extensive testing to failure of many samples, before selecting the final design.

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

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

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

http://www.ncwoodworker.net/pp/data/1421/medium/145_AssemTable_spread1.jpg

[Scott Vigder]

http://www.sawmillcreek.org/showthread.php?45036-Torsion-Table-David-Marks-Designed-Pics-O-Plenty

I used 1/2" and 3/4" MDF on my table. It is still the go-to table in the shop.

[Jim Matthews]

I love that picture of a all the bricks piled up in the middle of the span.

I guess thick enough that it will hold a screw to anchor things, if needed?
Thicker adds weight, too thin and it's likely to suffer damage from regular use.

My mentor has a table he built with 1x1 stock and 1/4" skins twelve years ago.
It looks like a mile of bad road, but it's dead flat to this day.

jim
wpt, ma

[Alan Schaffter]

A few more comments about construction. I used David Marks' method for building the construction platform- jointed and ripped 2 X 4 ladder, MDF sheets on top and winding sticks to get it flat- remember it doesn't need to be level, just flat. But I find the method he (and the Wood Whisperer) used to make the grid slow, painful, and potentially error prone; and not any stronger.

If you have a dado blade take some time to set it to the precise thickness of your MDF and make full length ribs with halved joints on your tablesaw using a simple indexing jig. They all fit together perfectly, quickly, without the laborious trimming and fitting the last row. And it comes out square with straight sides. You only need to glue the ribs together, brad nails are not needed.

Construction platform- checking flatness with winding sticks. Once flat, everything was secured with hot melt glue, including the saw horses which were screwed and/or glued to the floor. I wanted it to be flat and stay that way!

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

A simple indexing jig makes quick and easy work of cutting perfect ribs. Use an aux fence with a wooden peg (by my left hand in the photo) to cut perfect rib spacing. Obviously you don't want the last cell to be 2" when all the rest are 6", but forget the higher math figuring out precise even spacing- pick a nice number that is close and don't worry if the last cell is 5 3/4" wide instead of 6" wide like all the rest- it just doesn't matter, all ribs will be the same and it will still be square with straight sides if you cut all ribs to the same length!! You can use the same fence setup for the longitudinal and lateral ribs for square cells like I do, or reset the fence/peg position for a different longitudinal (or lateral) spacing. Make sure your dado is not too tight- the MDF can bend or snap during assembly. You want a close not a tight fit.

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

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Spreading glue prior to attaching the skin. Leg pocket reinforcing is at lower left.

http://www.ncwoodworker.net/pp/data/1421/medium/P10101181.JPG

[Jeff Monson]

Nice writeup Alan. That looks like a nice assembly table, how sturdy is it??

[Sam Murdoch]

Yes thanks Alan. This one has been bookmarked!

[Alan Schaffter]

Nice writeup Alan. That looks like a nice assembly table, how sturdy is it??

Thanks. I still have my original table that is 4 years+ old - still sturdy, strong and flat. I made the one in the photos in 2009 specifically for the article, and sold it to a furniture maker once I was sure my editor didn't need any more photos. I haven't received any complaints. You can see it in the Dec 2009/Jan 2010 issue of American Woodworker. There is also a video of the assembly table and its adjustable legs (http://videos.americanwoodworker.com/video/AWW-Adjustable-Height-Assembl-2p://) in action on the AWW site.

[Charlie Barnes]

Alan (and others) - thanks for the responses. I should have been a little more forthcoming in my original question regarding the application. I'm building a loft/platform bed for my youngest son. It will have a mattress, but no box spring to support it. I want to minimize the total height taken up by the mattress and support structure, so I was thinking about some sort of torsion box assembly to use in this situation.

I don't need the support structure to be flat as much as I need it to be strong. But I still think a torsion box is the answer in this case. It will be supported on all four sides at the edges, so I'm just thinking about having the top skin overhang the sides so it rests on the support structure inside the headboard/footboard and rails. I think the top skin will need to be 3/4" just for this purpose. But I might be able to make the bottom skin thinner and make the gussets thicker.

Are there any issues with having the skins of unequal thicknesses?

Charlie

[johnny means]

What size is the mattress. I build cgildrens furniture for a living and never use anything more than a 3/4" board for twins, for full and queen I add 3 2x6 slats.

[Jeremy Brant]

This thread has a lot of great info on torsion box construction and tables. For your application, I hate to say it, but you'll achieve what you're looking for by doing a google search for bunkie boards and buying one prebuilt at a mattress store (most of them will have something available). It's a very basic box spring that's constructed to be thin for child safety in a loft/bunk bed, and covered with fabric for the purposes of protecting the mattress. I built my son's bunk bed, but I paid the $40-50 for the bunkie board when I picked up a mattress for it.

[Jerome Hanby]

I think the only time you really need a thick skin is if you post something positive about using tung oil as a finish <g>. For a bed frame, I think I'd use thinner skins because the mattress and/or box spring will already be spreading out the load, so punch through shouldn't be a worry and the resulting box will be lighter. Seems like one always ends up moving a bed around and lighter will be a blessing.

[Alan Schaffter]

Been out of town at a WW show- skins do not need to be the same thickness.

As to the Bunkie Board- I had them on by kids bunk beds. They are a bit cheesy and not all that well built but outlasted my son and daughter. But at today's lumber prices may be cheaper than a sheet of 1/2" ply which is probably all you need. You can also combine slats and ply or tempered hardboard.

If you want to build a torsion box as a project, for your application I would use 3/16" or 1/4" tempered hardboard for the web and skins. A 1-1/2" tall web should be fine.


If you really want to get fancy- make a strong but very light torsion box from 3/16" tempered hardboard skins and honeycomb web. Here is an article about Tricel honeycomb (http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cts=1330824418345&ved=0CCYQFjAA&url=http%3A%2F%2Fwww.woodworkersjournal.com%2Fuplo ads%2Fimports%2FPublicationsArticle%2Fian_kirby_to rsion_boxes.pdf&ei=3MRST5j1IIj40gGsnsDCDQ&usg=AFQjCNGnEqbv0MtFKnpT-XBhqgeWIfjm8A&sig2=yu6Tu2E4hCgpHAtJ9QpdNA) by Ian Kirby (noted woodworker and torsion box guru).

http://www.finewoodworking.com/assets/uploads/posts/31618/HONEY_sqm.jpg