Torsion box madness??

[Jason White]

Why all the craziness over torsion boxes? Everybody here is always talking about painstakingly crafting their "perfectly flat" tops for their assembly tables, etc.

I'm sure they're great, but good grief! I use a solid core door on a 2x4 workbench and that thing is certainly flat enough for woodworking. If I had a "real" woodworking bench, I'm sure I could get it flat enough using winding sticks and a router or bench plane.

What am I missing here?

Jason

[Craig McDaniel]

Well, if you're happy with the door/sawhorses, I'd have to say you're not missing anything.

In my shop, I like having an assembly table that's apart from my workbench. That way, if I need to adjust a part, I can take it to the bench to work without being crowded for space.

As long as I'm making a separate assembly table, I'd like it to be flat so that the table itself can be the reference surface. I'd also like it to be very stiff so that if I working with a heavy piece, it doesn't cause the table to flex and change the reference. I'd also like for it to be light enough that I can move it and store it away when I'm not using it.

That's where the torsion box comes in. I've got a 5'x4' assembly table that's flat within 1/32" in any direction that I've measured it. With me standing in the middle of it (220+ lbs), it doesn't change the flatness that I've been able to tell. It's also light enough that I can take it off of the sawhorses and put it up against the wall when I'm not using it.

All of that said, if you're getting acceptable performance with your door, stick with it. To be honest, I never tried a door as an assembly table and so I don't have a basis for comparison.

Craig

[Mike Henderson]

I agree with Craig. A torsion box gives you light weight, stiff and flat. If you don't need that, you probably don't need a torsion box.

Mike

[Jeff Dege]

I understand using torsion boxes when you need stiff, flat, and light. But I've seen people use 3/4" ply for their torsion boxes,, both for skins and webbing, and that I simply didn't understand. Three sheets of 3/4" ply laminated together would have been lighter, thinner, and just as stiff.

[Sean Nagle]

As long as I'm making a separate assembly table, I'd like it to be flat so that the table itself can be the reference surface. I'd also like it to be very stiff so that if I working with a heavy piece, it doesn't cause the table to flex and change the reference. I'd also like for it to be light enough that I can move it and store it away when I'm not using it.

Having been struck with "Torsion Box Madness" myself, I have plans for a movable 4'x3' assembly table on stout sawhorses. I'm still considering whether to build the torsion box table top or simply to use a 36"x80" hollow core door slab cut to size. I've checked them out at The Borg and they look pretty flat, and they sure are cheap. The plan is to apply high pressure laminate to either shop-made or store-bought table top.

Has anyone made use of a hollow core door for an assembly table and has it been flat enough?

[Mac McQuinn]

I understand using torsion boxes when you need stiff, flat, and light. But I've seen people use 3/4" ply for their torsion boxes,, both for skins and webbing, and that I simply didn't understand. Three sheets of 3/4" ply laminated together would have been lighter, thinner, and just as stiff.


With all due respect, I feel 3 sheets of ply glues together would just be basically a 2-1/4" thick piece of plywood and with the method ply is made it would flex. I built my Torsion box bench with 3/4" MDO and 2x4 webbing and it is stiff even with 200+lbs sitting on it. Assembly is glued & screwed. I ran my 2x material through planer 1st to get all pieces to uniform height. I needed a bench I could disassemble and carry up basement steps to garage if need be. My bench comes apart in 4 pieces to do this and goes back together in 10 minutes. The leg system is Torsion design also.

Mac

[Mike Henderson]

You'd have to do some research on beams and what makes them stiff. An I beam, for example, could be made as a solid rectangle of steel and it would be as stiff as an I beam of the same outside dimensions (maybe stiffer) but it would be much heavier and would use a lot more material.

A torsion box is similar. You could make a "box" out of solid material, in the same dimensions as you'd make your torsion box, but it would be a lot heavier and would use a lot more material.

A torsion box, or an I beam, works because material is only put where it's needed for strength. The material that doesn't contribute much to the strength is left out. Basically, it substitutes knowledge for mass.

Mike

[Jason White]

Yes and yes on the hollow core door.

In fact, they're basically constructed like torsion boxes with a type of webbing/grid inside. I use one as a sacrificial cutting surface for sizing plywood with my circular saw. When it falls apart, I go find another one at the home center for about $30.

Jason


Having been struck with "Torsion Box Madness" myself, I have plans for a movable 4'x3' assembly table on stout sawhorses. I'm still considering whether to build the torsion box table top or simply to use a 36"x80" hollow core door slab cut to size. I've checked them out at The Borg and they look pretty flat, and they sure are cheap. The plan is to apply high pressure laminate to either shop-made or store-bought table top.

Has anyone made use of a hollow core door for an assembly table and has it been flat enough?

[Matt Armstrong]

I built a 7x5.5' torsion box as an outfeed table and assembly table. Having a flat outfeed surface is nice because the piece doesn't lift away from the front OR back of the blade and I also have a reference surface to build stuff on. Torsion boxes are not necessary for many applications but they are a small sacrifice of time and effort to gain a better shop-made tool, ultimately. My argument would be, "Why not a torsion box?"

It's also a good practice for the beginning woodworker with glue-up and assembly.

[Chris Friesen]

Why all the craziness over torsion boxes? I'm sure they're great, but good grief! I use a solid core door on a 2x4 workbench and that thing is certainly flat enough for woodworking. If I had a "real" woodworking bench, I'm sure I could get it flat enough using winding sticks and a router or bench plane.

A torsion box allows you to get the same stiffness as solid wood construction with less weight (but more thickness). This is useful for something that needs to be portable. Torsion boxes are generally made of sheet goods, which means that they're less susceptible to warp if the humidity changes significantly on a regular basis.

The downside is that generally you can't flatten them after construction, hence the attention to detail during the construction phase.

As for "flat enough for woodworking", if you use hand planes a lot then it's helpful to have a working surface that is _really_ flat. If you've got a hollow under the workpiece, it's going to flex while you're planing the top, resulting in a surface that is out of true. With power tools this is less of an issue.

[Chris Friesen]

But I've seen people use 3/4" ply for their torsion boxes,, both for skins and webbing, and that I simply didn't understand. Three sheets of 3/4" ply laminated together would have been lighter, thinner, and just as stiff.

Depends how thick they made the torsion box. The stiffness of a structural member goes up with the cube of the thickness. Make it twice as thick and it's eight times as strong. With a torsion box it's really easy to make it thick (and therefore stiff) without adding much weight since the interior is mostly air.

That said, 3/4" material is overkill for a torsion box. I suspect that many people just happen to have it handy so that's what they use.

[Alan Schaffter]

Check out my articles in the latest issue of American Woodworker Magazine. I talk about all these issues. Plus show how to make adjustable legs for the torsion box top! :)

http://www.ncwoodworker.net/pp/data/1421/medium/IMG_2543.jpg

If you don't need a durable surface- one that you can pound on or clamp on, here is one I made with 3/16" hardboard and yellow glue. It is 8' long, 18" wide and weighs less than 30#. In the first pic it is supporting over 300# of bricks concentrated at mid-span. I measured about 1/2" of deflection. I never loaded it to failure, but believe it could probably hold 400#!

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

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

[Scott Hildenbrand]

I dropped my plans to do one a while ago... Great info, btw, that everyone shared.. But it fell from my todo list..

I'm going with some hollow core doors for my outfeed table, hinge style off the back of the table saw. Works for me and will be light.. :)

Maybe later.. But for right now I've redirected onto more important things.

[glenn bradley]

Like the endless quest on some tool setups. If you are getting satisfactory results from your door and saw horses, your good. It is nice to have at least one dead-flat surface in the shop to use as a reference. Mine is my tablesaw and extensions.

When I had a contractor saw with stamped wings I was planning on making a torsion box just to have a reliable reference surface available. I just stalled long enough to not require one anymore. My bench and worktable are adequately flat for assembly (thanks in large part to building them on my tablesaw surfaces ;-)

[Alan Schaffter]

I dropped my plans to do one a while ago... Great info, btw, that everyone shared.. But it fell from my todo list..

I'm going with some hollow core doors for my outfeed table, hinge style off the back of the table saw. Works for me and will be light.. :)

Maybe later.. But for right now I've redirected onto more important things.

If you just need an outfeed table they will work great, nice and light, easy to fold up and down. My outfeed and right extension tables are 1/2 torsion boxes- web, top skin, but no bottom skin- works just fine. I did cover them with laminate for durability and to ensure wood slides easily.

http://www.ncwoodworker.net/pp/data/500/medium/P5260045.JPG

http://www.ncwoodworker.net/pp/data/500/medium/P9250001.JPG

[Scott Hildenbrand]

My thoughts are that my work will never be that straight anyway, so what does it matter! ;)

Seriously though... Some people striving for machines quality is nuts.. Wood moves at the drop of a hat, or barometric pressure. I'd seen people raising a fuss over their table saw being .005 out in the center.. That's nuts.

Aw well... To each their own..

Alan... I'd seen your work on the other forum.. Absofreakinlutly great stuff.. Bookmarked and gone back to several times over..

IF I ever have a shop space that large some day, I'll be doing a table saw setup like that.. With dual saws.. ;)

[Alan Schaffter]

My thoughts are that my work will never be that straight anyway, so what does it matter! ;)

Seriously though... Some people striving for machines quality is nuts.. Wood moves at the drop of a hat, or barometric pressure. I'd seen people raising a fuss over their table saw being .005 out in the center.. That's nuts.

Aw well... To each their own..

Alan... I'd seen your work on the other forum.. Absofreakinlutly great stuff.. Bookmarked and gone back to several times over..

IF I ever have a shop space that large some day, I'll be doing a table saw setup like that.. With dual saws.. ;)

Thanks. I have given some thought to a dual saw setup too- one for ripping, one where my cross-cut sled would reside.

[Jeff Dege]

With all due respect, I feel 3 sheets of ply glues together would just be basically a 2-1/4" thick piece of plywood and with the method ply is made it would flex. I built my Torsion box bench with 3/4" MDO and 2x4 webbing and it is stiff even with 200+lbs sitting on it.
Figuring a 24" wide bench, that spans 48" between supports, the Sagulator (http://www.woodbin.com/calcs/sagulator.htm), says the sag under a 200lb center load just 0.004" per foot.

In other words, three sheets of ply is plenty stiff. But it's heavy. A torsion box can achieve the same stiffness at far less weight.

You can build something that's stiff, either way - building a torsion box, or just layering solid materials. The thing is, torsion boxes take a lot more work than just layering materials. I understand why that extra effort would be worth it, when you can't afford the weight of solid materials. But I don't understand why people make that extra effort when keeping the weight down isn't important.

Ken Horner's "More Woodworker's Essential Facts, Formulas, & Short-Cuts" has a chapter on calculating sag and weight for torsion boxes. It's worth a read.

[John Coloccia]

Which is easier to keep straight? A 3/4" sheet of plywood that's trying to warp, or a 2" sheet plywood that's trying to warp? What about if you have 12" of wood? Sure, it won't sag but good luck keeping it straight should it decide to move a little. A torsion box is more than just making it lighter. Every section of wood you remove is a section of wood that can't warp. You can also orient the various pieces of wood so that expansion and warping have minimal effect on the rest of the structure. It's an optimization problem and you have to figure more than just the raw stiffness of the resulting part.

[Alan Schaffter]

Jeff and John, well said. A torsion box has definite applications, but is not for others.

I have a torsion box assembly table in my shop. I move it around some so it can't be too heavy, I clamp on it some, but rarely pound on it. I want it flat as a reference- answer: torsion box.

I also have a woodworking bench, which has some of the same attributes, except I clamp tightly, I sometimes pound hard. I don't need it to move and I want it to be rock solid and stable- answer: woodworking bench with a 3" thick hard maple top

[Glen Butler]

Can someone point me in the right direction for torsion box plans, materials, or just general construction technics? Most of it seems pretty straight forward, but I imagine their are some tricks for wood orientation and joinery that would help with stiffness.

[Scott Hildenbrand]

Where everyone seems to be getting the idea for one these days.. He does it HEAVY.. But not saying it's a bad thing..
http://thewoodwhisperer.com/episode-18-assembly-table-torsion-box/

Down and dirty discussion on the subject..
http://www.woodweb.com/knowledge_base/Fine_Points_of_Torsion_Boxes.html

Yet another heavy design.. But if you're never moving it and using it for assembly.. What ever works for ya. Crappy PDF formatting, but good pictures.
http://home.pacbell.net/paulcomi//Spectaculartrim/Woodworking/assemblytable.PDF


EDIT: This is a good PDF on the subject that covers the engineering perspective of load and stress. Well worth the read.
http://www.bayareawoodworkers.org/jun02/torsionbox1.pdf


There's plenty more info out there.. Not hard to find. My thoughts anymore is to keep it as light as possible. Unless you're smacking it with a hammer, 1/4" skin will suffice.

[Alan Schaffter]

A great source of torsion box info and instructions how to build one are in the current (Dec/Jan 2010) issue #145 of American Woodworker Magazine- on the stands now. It shows a simpler and quicker method to make the web than either the Woodwhisperer or his source David Marks. A second article in the same issue shows how to attach some neat adjustable legs to it. Here is a link to a video of the legs in operation (http://videos.americanwoodworker.com/video/AWW-Adjustable-Height-Assembl-2).

[Scott Hildenbrand]

I much prefer the long sections for the inner web which have slots cut into them to lock together. Way simpler and stronger method of doing things.

I don't recall seeing the metal bracing on that table before? Regardless, spiffy leg system for sure. Guessing you needed the bracing to prevent racking when raising the table?

[Alan Schaffter]

The braces are purely optional, not needed for racking as the legs are firmly anchored in the table. They are only needed if you want an extra stable table or are doing some heavy-duty work and placing side loads on it.

[Jeff Dege]

I'll repeat my cite to Ken Horner's "More Woodworker's Essential Facts, Formulas, & Short-Cuts". Not so much for how to build one, but so you can figure out how stiff, and how heavy, any particular design might be.

[Alan Schaffter]

I'll repeat my cite to Ken Horner's "More Woodworker's Essential Facts, Formulas, & Short-Cuts". Not so much for how to build one, but so you can figure out how stiff, and how heavy, any particular design might be.

I've got it and read it. It provides a good understanding of torsion boxes. He uses the equations to illustrate through comparative analysis- why one is stronger than another and to comparatively analyze the deflection between ones built differently. But frankly his equations are not good for much else. They won't allow you to design and build to spec. He gives "Rules of Thumb" without providing the source. To give you a good idea of the complexity of the issue, scan through a few pages of "Design and Fabrication of Plywood Stressed-Skin Panels (http://gp.com/BUILD/DocumentViewer.aspx?repository=BP&elementid=3815)," by the Engineered Wood Association (stressed-skin panels are torsion boxes). One thing it says in the "Design" section is that "stressed skin panels are designed by a "cut and try" method. A trial section must first be assumed then checked for its ability to do the job intended." The "Timber Designers Manual" also has similar info.

[Al Willits]

I built mine off the David Marks design, 3/4" is probably heavier than most of us need, so if I built another one, it'd be 1/2".

I also have a solid core door on top of a metal workbench I have, that gets the heavier treatment.

Like said, if your doing fine without one, pass it by, but I find I like the portability and space my table gives me.

Just nice to have a flat tabletop to work off of that's large enough for most projects.

Al