I am looking for any rules of thumb or design guidelines for torsion boxes. I have reviewed several threads and articles regarding this topic, but most of them refer to specific size and spacing of grid material. Are they any general rules to relate width and thickness of grid materials with the appropriate spacing. I realize that larger grid material and smaller spacing will make the box stronger, but I am looking for guidelines to understand limitations. My application should be strong enough to support a couple sheets of 3/4″ plywood without deflecting, but I dont expect much more weight than that (and no point loads). Since I would also like for it to be lightweight, I am trying to keep the grid material smaller with greater spacing. Thanks.
Craig
Replies
Fine Wood working did an article many, many years ago on this. I believe 15+ years. Try searching for that I'd try to help more but all my magazines are packed away until our new house is built (6 months from now).
David Marks built one on his show Wood Works. Here's the link to the website. From this page click on the topics under "in this episode".
http://www.diynet.com/diy/shows_wwk/episode/0,2046,DIY_14350_26946,00.html
I saw David Marks build one over on DIY. I know he has a web site, might be useful. He built it for an assembly table.
Edited to Wayne, great minds....J/K
Edited 10/3/2003 9:08:08 AM ET by Dirt Stirrer
Edited 10/3/2003 9:09:41 AM ET by Dirt Stirrer
Craig,
I've used torison boxes in several projects and I'm always amazed at how strong they are. I've even used 1/4 inch ply over the grid and its been strong enough to stand on! Not sure there are many rules but it seems I could almost always go lighter than I first estimated. They are a great way to create a large flat surface thats light with little flex. Which brings up the major problem in the construction, it has to be clamped to a flat surface while the glue is setting. I have used hollow core doors for this (a torison box itself) but that can be a limiting size. I have tried, with little success, using large weights on top while on a flat (?) floor. I think torison boxes are a great building tool, just surprised more woodworkers don't use them.
Dennis
I took some time to search the FWW index, both printed and on-line. The printed one gave zero results. The on-line index gave quite a few. Unfortunately, the most likely possibility falls in a gap in my FWW collection -- Issue #95, pages 39-40, indicated as torsion-box construction for assembly table. Several of the other "hits" I checked were targeted for furniture construction. Here's the search page, and I got the best results when using only the word "torsion":
http://www.taunton.com/cgi-bin/artresult-fw.cgi
forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
The article in FWW years ago was by Ian Kirby. He's the first one I ever heard talk of torsion boxes. The David Marks thing on torsion box assembley tables looks good. Also Kim Carleton Graves book on tables uses honey comb for the interior, a pratice used by the aircraft industry for interior cabinets. I've seen some lame articles in Popular Woodworking or some such magazine where they talked of torsion panels but really butchered the whole concept. The Ian Kirby article is very good.
See also FWW back issues #123, p.76 ("building a torsion box" sidebar); and #151, pp. 76-81 ("Tackling Large Tabletops").
Bill, just an aside here. The paper-index (annual index) has no listing in it for "torsion." Weird.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
FG--Both of those articles were listed in the annual index updates that followed their publication.
By index updates I'm referring to the blue pages that appear each year near the back of the February issue of the magazine. The #123 torsion box mention is listed in the index update in issue #128, and the #151 mention is listed in the index update in issue #154.
Oh, I see. I was ####uming they were cumulative indexes. Silly me, but then that's what the on-line index is for, right?forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
FG--Those annual updates get added to the cumulative index of all of the issues of FWW ever published. The online index is just a digital version of the same thing.
The APA site has the information you want.
You can get the honeycomb material from Vacuum Pressing Systems (www.vacupress.com). That company is also a good source of info on how to do it.
Thanks for all the info. I have looked at several of the articles and the DIY link, which was very informative. I purchased my grid material (1/2" mdf) this afternoon, but I couldn't decide on the best skin material. I was hoping to find 1/4" mdf with laminate top (or something similar, but this was not available at the big orange or blue stores. The three options I considered were 1/4 clear ply, 1/4 hardboard (masonite) with 1 rough & one smooth side, or 1/4 hardboard with both sides smooth.
I am attempting to build an extended table (side & rear) for my portable tablesaw. I want it to be fairly lightweight, storable and portable, since I am currently working in a temporary space, and I would like to use this in the future at various sites. For this application, the hardboard was attractive, because of the smooth surface, but I'm concerned about the integrity of the glue joints with the rough or smooth surface facing the grid. I'm also not sure how much strength the hardboard will have. The plywood is obviously strong and will glue very well, but the surface isn't as smooth. I am currently leaning toward the plywood, but I thought I would post the question first. I could glue laminate to the plywood (and I might do that in the future), but this is already one of those "quick" projects that is becoming more complicated.
By the way, one reason I decided not to use honeycomb is due to the thickness. My table will be supported by folding legs (cafeteria table style), which are 3-4 inches lower than the top of the tablesaw. This is another reason I decided on the torsion box design - the grid depth will be defined to compensate for the difference in heights. Thanks again for all the input.
Craig
Masonite will sag over time, and doesn't glue well. The glue will stick to the surface, but the material will delaminate because there isn't much strength internal to the product.
With the price of plywood so high, it wouldn't cost much more to go with veneer plywood, which would have a nice surface.
How about a sheet of 1/4" mdf or ultra-lite and fut a coat or two of lacquer or urethane/varnish?
Unfortunately none of the stores I checked carry 1/4" mdf. 1/2" was the thinnest I could find. Is 1/4" common in other areas or is it only at a few stores?
Criag
My local Lowe's has the 1/4" MDF. You don't find it with the standard stock there. They have it hidden with the 1/4" and 3/8" interior paneling. Slid under a shelf waiting for someone to discover it. You might try looking around that section at your local "blue box".
I picked up from Kirby's idea a while back and have built several work-benchs that use a torsion box design. I agree with Wayne that you might use 3/4" veneer ply. I have used the birch top and bottom coating it with 3 layers poly. HD has been running specials on maple ply in my area for several months. $30 a sheet.
Good luck...
sarge..jt
I believe that home depot carries it, or at least they did when I lived in So. Calif.
When you build your box make sure that all the pieces are run full lenght and width of the top. If the web is 2" thick, then set your datto blade to come up 1" and knotch 1/2 way through each one. This forms what we call a "1/2 cross lap". Make sure tyou glue the snot out of the top and bottom skins. Also make sure that this whole assembly process is done on a extremely FLAT suface. If the assembly table is warpe, bowed or twisted, so will the fininished product
About 18 years ago, I built torsion boxes that would hold a full lift of 3/4" plywood for a shop that I worked in. The torson boxes measured about 8-10" X 5' X 9'x. They didn't sag at all
Craig --
Don't look for 1/4" MDF. Instead look for "tempered hardboard". As far as I can tell, it is the same stuff as MDF.
Whoa there, Jamie! Hardboard and MDF are different products entirely. 1/4" MDF is made, but plywood has way more tensile strength than MDF or Masonite (a brand of hardboard) and it glues better than tempered hardboard any day. Tensile strength and gluing are critical for torsion box skins.
Buglehead --
Whoa there, yourself. I mentioned two products, MDF and tempered hardboard. You mentioned those, plus you mentioned plywood. Yes, plywood is very different from the other two. But that isn't what the poster asked for. He wanted 1/4" MDF. Thin MDF and tempered hardboard seem darn similar to me.
Jamie
Hi Jamie,
Yes, I did mention plywood because it is superior to the other two for a torsion box skin. MDF is different from hardboard in several ways. Hardboard is laid up in strata like sheets of felt which are then pressed with heat. MDF lacks this layered approach, having more 3-dimensional homogeneity. When you glue something to the face of hardboard and peel it off, it tears off a skin of the hardboard because the face is a lot stronger than the bond between the layers. Hardboard is also more dense than MDF. The tempered stuff has been treated with resin (I think it is a resin) to resist water absorbtion. However, it still can be glued with Titebond.
By the way, MDF can work quite well as a core material for a torsion box, and tempered masonite makes a useful wear surface due to its hardness. I hope this helps. I did not have time when I posted before, so I did not get into any details then.
Regards, Bill
MDF and hardboard are two completely different animals. Hardboard is composed of wood fibers held together only by the natural adhesives in the wood, after being compressed using heat. MDF has binders (resins) added to the wood fibers.
The difference between tempered hardboard and "plain" hardboard is the addition of a special oil which provides "extra water resistance, surface hardness, rigidity, bending and tensile strength." [Phillips Plywood website]
I'm sure one of our engineering Knotheads has a chart somewhere that would tell us the difference in relative strengths between the two. forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Forestgirl --
Thanks for educating me about the difference between MDF and hardboard. They indeed are different animals. A little surfing yields modulus of elasticity numbers for MDF in the range of 350,000-450,000 psi, while hardboard is around a tenth of that. That is, hardboard elongates a lot more than MDF when you pull on it, so an MDF-based torsion box would be much stiffer --- just as you said.
Jamie
Articles in FWW were whereI first was exposed to torsion box construction and I have made quite a few over the last coulple of years. I have some pictures posted below and will give you some of my thoughts. First off I do a few things the way of others but I have departed on others. I have tried to make a torsion box fail for about 8 years now and to my knowledge not one of the has. One thing I do use is a vacuum to make my final presses of the skins. I feel this is the best and easiest way to create a strong stable TB and I am so pleased with the end result I just keep finding ways to intergrate TB's in to my work
That said I will post the picts I have and let you decide
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