Making Torsion Boxes for Pressing Veneer

[Mike Henderson]

Earlier I posted some pictures of the torsion boxes I use for pressing veneer - see the end of this thread (http://www.sawmillcreek.org/showthread.php?t=94118). Dewey asked me to discuss how to build the torsion boxes so here's my attempt to explain that.

First, let me discuss the theory of torsion boxes. A torsion box is essentially a bunch of I-beams running next to each other, and perpendicular to each other. An I-beam gets it's strength from the flanges (same as the skins of a torsion box). The separators of the flanges is known as the web, and its only purpose is to keep the two flanges apart.

If we look at an I-beam supporting a load in the middle of the beam (with both ends fixed), the forces on the flanges are compressive on the top and tension on the bottom. If the top flange can compress, the beam will deflect downward. If the bottom flange can stretch, the beam will deflect downward. The depth of the web affects the strength of the beam. If the web is narrow (meaning not much separation between the flanges), a small amount of compression and expansion of the flanges will give more deflection of the beam, than if the web is deeper for the same amount of expansion and compression of the flanges. Of course, for any beam under load, some deflection does take place but it's not much in a well designed beam.

You can think of a torsion box as a bunch of I-beams laid side by side with the flanges hooked together. Such a structure would be very strong and would not deflect very much. But in a torsion box, we not only lay I-beams side by side, but we construct I-beams that run perpendicular to the original I-beams. Such a structure can be very stiff.

So now, let's talk practicality in torsion boxes. We want a box that can press our veneer "evenly" across the work which means the box cannot deflect very much at all. We can achieve that goal by using very thick skins and very deep webs, but we need to be able to pick up and handle the boxes so they can't be too heavy. Maybe a structural engineer will chime in and give some recommendations, but in the meanwhile I'll tell you what material and dimensions I used - and the boxes work well for me.

The small torsion boxes I built before I knew very much about torsion boxes and I built them out of 3/4" Baltic birch plywood - both the skins and the webs. Bad decision!! 3/4" is more than necessary for the skins and the webs do not at all need to be that thick. The boxes are very heavy - much heavier than they should be. The webs are 6" high.

For the large torsion boxes, I used 1/2" Baltic birch plywood for the skins and 1/4" tempered hardboard for the webs. My large boxes are about four times the size of my small boxes and plenty strong enough. The webs are 6" high, so the total box is 7" high with the skins. They weigh about 42 pounds each, light enough to handle by yourself.

But let me post this and put pictures in my next entry.

[Mike Henderson]

When I built my large torsion boxes, I was not planning on doing a tutorial so I only have a few pictures of the construction. I hope they're enough for you to understand.

First, decide how big you want the torsion box to be. Then calculate the number of web pieces you need. I spaced my webs about 6" apart. The length of the web pieces is 1" less than the size of the skins (for 1/2" plywood). Plywood pieces are used on the periphery of the box to protect the web pieces which are more fragile.

When you cut the web pieces and the plywood periphery pieces, they MUST be done with one setting of the table saw fence so that they are all exactly the same size. In this picture, you see a long periphery piece, a short periphery piece, a short web piece and a long web piece (the box is rectangular). If you make a mistake and don't cut enough pieces(and you moved the fence), what you have to do is set your fence just a bit narrower, and re-cut all your old pieces and cut the new pieces. You must assure that they are all exactly the same width.
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I cut the web pieces half way through so that I could interconnect them. If I remember right, I did that on my table saw using a sled. I just cut half way and to a width that equaled the thickness of the tempered hardboard (so they'd slide together snugly). You can't see it here, but I drew a line down the middle of the web pieces so when I cut the slots, I just cut a bit past that line. A bit too much is not a problem but a bit to little is a serious problem because the top of the webs will not be flat when assembled.
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The next picture shows the interconnected web pieces. You can see the line down the center of some of the web pieces - this was used to know how far to cut the slots.
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And this picture shows the web pieces with the periphery pieces.
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Finally, I have a picture of the making of the handles. Just drill a couple of holes, use a jig saw to cut out the waste between them, then use a round over bit to round the inside of the holes.
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Glue up is a chore. You need a slow glue because it takes a long time to apply the glue and assemble everything. I used some slow epoxy - I think it was an hour open time.

You must be very careful to apply glue so that the webs are glued very well to the skins. I've heard that you don't have to glue the joints between the webs, but I did. I figured it wouldn't hurt and might help.

I laid the web structure on one skin and marked the location of the webs on the skin. The connection between the skins and the webs is where the strength comes from so glue them well. If the skins can move relative to the webs, the box will not be stiff. Then I applied epoxy at those locations, then laid the web structure on top of that skin (I applied glue to the joints between the web pieces earlier). Next, I put the periphery pieces on, using glue on the top and bottom, and used a nail gun to hold them together. Then I applied glue to the top of the webs (and the periphery pieces) and put the top skin on. The whole thing must be glued in one job - if the glue sets in any place before you clamp, it's likely the box will not be flat. You CANNOT for example, glue the web pieces together then glue the top and bottom on. Note: you do not have to glue the web pieces to the periphery pieces - it won't add any strength.

I clamped it on top of my table saw so I had a known flat surface to glue against, and used cauls across the top.

When you glue up the second box, clamp it against the first box so the two boxes match perfectly. Mark on the side of your boxes how the two boxes go together so you always press the same way.

[Mike Henderson]

When you press with torsion boxes, you like to have some material that "gives" on the faces so that if you have irregularities in the veneer, it all gets pressed. I put cork on the pressing faces of my torsion boxes but duck cloth (sail cloth) works well also.

The cost for the boxes was two sheets of 1/2" Baltic birch and one or two sheets (I don't remember what I had to use) of 1/4" tempered hardboard. So if the Baltic birch was $50 a sheet (I don't remember what I paid) and the tempered hardboard was $25 a sheet, the cost for the torsion boxes was about $150 (I'm pretty sure it was less than that), plus the epoxy glue. I used the small sheets of Baltic Birch but I don't remember what size that is.

I hope I covered everything. Please ask questions about anything I didn't cover.

Mike

[Dewey Torres]

Mike,
Awesome post! Thanks.

[Leigh Betsch]

Thanks Mike. I plan to veneer the top of the cherry dresser I started 2 nights ago and really didn't want to spring for a vacuum press. The torsion box concept is just what I need!!

[Alan Schaffter]

Mike, a lot of nice work!

A couple of comments- I think you may be overbuilding for your application (veneer press).

First, the best way to ensure even pressure is use a vacuum press instead of a dead weight or clamp press. With a vacuum press the pressure is the same at every point. Plus with vacuum you only need a flat table that will support the the weight of the panels being veneered. It doesn't need to resist clamping pressure!

Your discussion of the stresses as far as skins is partially correct, but they only need to be thick enough so they won't deflect between webs when under irregular or point surface loads. As you said, the stress in a torsion box is in the plane (along the surface) of the skin not perpendicular to it. The top is under compression and the bottom under tension. The web maintains the constant distance between them and by being well bonded to the skin helps the skin resist buckling. But while deep webs add considerably to strength (the cube* of the depth), increasing skin thickness does little unless your skin material is really weak. * Increasing the web thickness by 25% yields almost 100% increase in strength, doubling it yields an eight fold improvement!

Not sure how much pressure you are applying with your dual torsion box clamping press, but with a vacuum press you could get higher pressure more evenly distributed, you would only need one torsion box and, you could make the whole thing from 1/4" tempered hardboard. What!?!?! Yup!! It would be crushed by the vacuum pressure!!! Nope!!! If the bag is sealed to the top skin and small vent holes are drilled in the bottom skin in the center of each web void, then atmospheric pressure will be the same on the top and bottom of the piece being veneered.

A simple jig (fence and registration peg) like a giant box joint jig makes quick and easy work of cutting the halved joints in the web. With thicker webs like the 1/2" mdf I cut in this pic I used a dado blade but you can do the same with thinner web material as long as you have a blade the makes the proper size kerf (or even a blade that makes a narrower kerf).

http://www.ncwoodworker.net/pp/data/500/medium/P4290038.JPG

Here is an 8' long X 14" wide x 3" thick torsion box I made from a single sheet of very thin 3/16" big box store hardboard (fuzzy on one side) and standard WW glue, no epoxy!. In the pic the torsion box is supported at each end and loaded with over 300# of bricks (two rows deep)! Deflection was only 1/2" and it showed no signs of failure. I decided not to load it to failure. Closer support would have likely resulted in minimal deflection.

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

http://www.ncwoodworker.net/pp/data/1421/medium/P1080032.JPG

http://www.ncwoodworker.net/pp/data/1421/medium/P1080030.JPG

You don't need to do the glue up all at once! I glue my web as it sits on both skins which sit on top of a assembly platform. I glue up web, then one skin, then the other. I just protect the skin that is under the web from glue drips with plastic food wrap. I weight down completed sections of the web as I go. After the web is done I glue on one skin then when that is dry the other skin. For my heavier torsion boxes I glue and brad nail the skin to the web, for the little hardboard one pictured I just used glue and weights. Here is one I am gluing up that way. I ran out of regular hardboard so used some peg board for the laterals. It looks like I have applied glue to the web and am getting ready to attach the second skin.

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

[Leigh Betsch]

I'm think about giving the veneer torsion box set up a try. I think if I build the boxes right I can use them as in and out feed tables for by BS when not in use as a veneer press. So my question is does someone have a recommended box thickness and materials for a 3' x 5' platen? Recommended materials? I'm thinking I would laminate the entire thing with Formica and then use a cork pad when veneering.

[Mike Henderson]

Alan - I'm sure I'm overbuilding my torsion boxes. My problem is I have no way to "design" them. I don't know what mathematics to use, or have the data (strength of materials) to input to the mathematics (if I had that).

I also don't know what specifications to use. For example, how much deflection should I allow? When the boxes deflect, a dome in created in the middle of the box. How much of a dome can I allow before I don't get good pressing in the center of my work?

So I overbuild. I am reducing the size of my materials as I build new boxes (note I went from 3/4" skins to 1/2" skins), but the only way I have to find out if I went too far is to use the boxes to press veneer and see if I encounter problems. Of course that means I lose my veneer work, and the boxes, if I go too far.

I'd really like to see someone who has the software tools provide some recommendations for different size torsion boxes.

Mike

[The reason I recommend gluing the box all in one step is that I'm afraid I'll make a mistake in gluing the web together. If I leave one web high (say it slips and I don't notice it) the skin will not contact the webs around that "high" web piece, which will affect the strength of the box. By gluing all at one time, I eliminate that problem.]

[Mike Henderson]

I'm think about giving the veneer torsion box set up a try. I think if I build the boxes right I can use them as in and out feed tables for by BS when not in use as a veneer press. So my question is does someone have a recommended box thickness and materials for a 3' x 5' platen? Recommended materials? I'm thinking I would laminate the entire thing with Formica and then use a cork pad when veneering.
I'm going to make some suggestions but recognize that I'm just commenting off the top of my head.

Based on Alan's posting, I'd suggest 1/4" MDF for the skins and the webs (you probably can't get anything thinner for the webs). Strength will be increased by the width of the webs so I'd recommend at least 4" and if you can go to 5" it'll be stronger. Because the skins might deflect between the webs, you might space the webs closer - say 5" instead of the 6" I used.

One issue might be the clamping - if you were to put a clamp in the middle of a cell, it might break the skin. To avoid that, lay a 2/4 on top (and under the bottom box) between the box and your clamps. That 2x4 will spread the clamping force over the periphery of the box.

Cork is hard to find and somewhat expensive. You can use thick cloth and it will work fine. When you're ready to glue up the veneer, lay down the cloth, then the plastic, then your work - then do the opposite on the top (plastic, cloth, then top torsion box). You'll still need plastic even if you laminate the boxes (unless you don't use cork or cloth). You need to keep the cork or cloth from getting glued to the work (glue leaks through veneer sometimes).

And let us know how it all works for you.

Let's see what suggestions Alan has for you.

Mike

[Alan Schaffter]

All good. valid points.

Without knowing the properties of the materials you can't compute anything and I doubt the the manufacturers of the stuff we typically use even know or publish the data we would need. I was frankly surprised by the strength of my hardboard bridge. (I didn't want to load it to failure, because it is part of an article I am writing for a WW mag)

If you have a good flat platform, the one step gluing certainly reduces chances of misaligned web intersections. Builders need to be sure they use SLOW setting epoxy!!! I doubt slow setting Titebond would be slow enough. But you can see from my demo, yellow glue is quite adequate otherwise.

The next time you make a torsion box, try my jig method for cutting the halved joints. Even if your initial setup is a bit off, you still get a perfectly aligned web and all it does is affect the size of the last cell which has no real effect. You just need to be careful that the slots are not so tight they splay the notch or even snap the grid- happened on my first table. It really works great!

I still contend, that a small torsion box (just for the flat surface) and a vacuum bag press is the way to go. There are some nice shopmade rigs to do that. One woodworker even sells a kit. You can use a pump (like a Gast) or just a compressor driven venturi for the vacuum.


Alan - I'm sure I'm overbuilding my torsion boxes. My problem is I have no way to "design" them. I don't know what mathematics to use, or have the data (strength of materials) to input to the mathematics (if I had that).

I also don't know what specifications to use. For example, how much deflection should I allow? When the boxes deflect, a dome in created in the middle of the box. How much of a dome can I allow before I don't get good pressing in the center of my work?

So I overbuild. I am reducing the size of my materials as I build new boxes (note I went from 3/4" skins to 1/2" skins), but the only way I have to find out if I went too far is to use the boxes to press veneer and see if I encounter problems. Of course that means I lose my veneer work, and the boxes, if I go too far.

I'd really like to see someone who has the software tools provide some recommendations for different size torsion boxes.

Mike

[The reason I recommend gluing the box all in one step is that I'm afraid I'll make a mistake in gluing the web together. If I leave one web high (say it slips and I don't notice it) the skin will not contact the webs around that "high" web piece, which will affect the strength of the box. By gluing all at one time, I eliminate that problem.]

[Ben Martin]

I design guarding systems for landfill compactors and we use a "torsion box" design, but it only includes one outside skin though (for weight reasons). The outside (on the bottom of the machine) is 1/2" material and the inner ribs are 1/4" thick material but of higher strength. These are designed to take half of the machines weight, in the neighborhood of a 65,000# load. The beer-crate/torsion box design is very very strong and lightweight.

[Alan Schaffter]

As I said in the earlier post, you need to be careful of point loading, i.e. clamps without pads to distribute the load. That is why I made my assembly table with 1/2" MDF skins. I would think that would be easy to avoid clamping veneer if you use pads on the clamps. It should not be a problem with the veneered piece (since it is a pad itself) unless you are veneering very thin ply.

As I responed to Mike, you would only need a lightweight (thin skins, thin web, shallow, 2" deep web) torsion box (for flatness) if you vacuum bag the veneer. Also, if you cover the top skin with laminate like I have done to protect my assembly table from glue, etc, it actually adds to the strength of the top skin. It would make a great outfeed table. That is what I use for my table saw extension and outfeed tables. Also, with a vacuum bag, you don't need two torsion boxes, nor cork, etc. I used to vacuum bag composite aircraft parts (layers of fiberglass to rigid foam) when I was building my fiberglass airplane. We covered the tops of the wet fiberglass-on-foam parts with a tight weave, but porous, dacron (it was called peel ply) that epoxy could not bond to. Then we would cover that with an absorbant material before putting the bag on top of everything. Since it was a vacuum bag layup the pressure was still equal everywhere despite the soft absorbant top layer. The pressure would force excess resin out of the glass where it could be absorbed. This yielded a strong, but light assembly.


I'm going to make some suggestions but recognize that I'm just commenting off the top of my head.

Based on Alan's posting, I'd suggest 1/4" MDF for the skins and the webs (you probably can't get anything thinner for the webs). Strength will be increased by the width of the webs so I'd recommend at least 4" and if you can go to 5" it'll be stronger. Because the skins might deflect between the webs, you might space the webs closer - say 5" instead of the 6" I used.

One issue might be the clamping - if you were to put a clamp in the middle of a cell, it might break the skin. To avoid that, lay a 2/4 on top (and under the bottom box) between the box and your clamps. That 2x4 will spread the clamping force over the periphery of the box.

Cork is hard to find and somewhat expensive. You can use thick cloth and it will work fine. When you're ready to glue up the veneer, lay down the cloth, then the plastic, then your work - then do the opposite on the top (plastic, cloth, then top torsion box). You'll still need plastic even if you laminate the boxes (unless you don't use cork or cloth). You need to keep the cork or cloth from getting glued to the work (glue leaks through veneer sometimes).

And let us know how it all works for you.

Let's see what suggestions Alan has for you.

Mike

[Mike Henderson]

Just a comment for discussion. I've wondered why they're called "torsion boxes'. If you look at an I-beam it's not very stiff in torsion. But when you build a box, the I-beams cross at a right angle providing strength in torsion as well as bending. Maybe that's why they're called "torsion boxes".

Mike

[Leigh Betsch]

1/4 mdf would be just about right in terms of weight to move around. I do recognize that a vacuum system would be better than clamps and calls but for this first veneer project I'm trying to avoid the cost of a vacuum pump and bag. Maybe I'll look into the venturi but that still leaves the bag cost. I read an article in a WW mag about hide glue and hammer veneering. I imagine that a lot of veneering has been done that way in the past.
So at this point I plan to make a couple of 1/4 MDF torsion boxes and give it a try. And if the veneer application doesn't work out as well as I want I will always have a couple of in/out feed tables for my BS.
Thanks for the info guys.

[Tony Joyce]

Cork is hard to find and somewhat expensive. You can use thick cloth and it will work fine. When you're ready to glue up the veneer, lay down the cloth, then the plastic, then your work - then do the opposite on the top (plastic, cloth, then top torsion box). You'll still need plastic even if you laminate the boxes (unless you don't use cork or cloth). You need to keep the cork or cloth from getting glued to the work (glue leaks through veneer sometimes).


Mike

Mike,
It has been suggested to me that 1/32" - 1/16" packaging foam is a good alternative to cork or cloth for veneer clamping. I have some 3/32" I'll be trying soon. Wax paper is a good barrier paper, I use poly coated kraft paper, because I can get wider rolls.

Tony Joyce

[Alan Schaffter]

1/4 mdf would be just about right in terms of weight to move around. I do recognize that a vacuum system would be better than clamps and calls but for this first veneer project I'm trying to avoid the cost of a vacuum pump and bag. Maybe I'll look into the venturi but that still leaves the bag cost. I read an article in a WW mag about hide glue and hammer veneering. I imagine that a lot of veneering has been done that way in the past.
So at this point I plan to make a couple of 1/4 MDF torsion boxes and give it a try. And if the veneer application doesn't work out as well as I want I will always have a couple of in/out feed tables for my BS.
Thanks for the info guys.

Here is something from WoodWeb about vacuum bagging (http://www.woodweb.com/knowledge_base/Vacuum_Clamping.html). Here is Joe Woodworker's site (http://www.joewoodworker.com/veneering/welcome.htm)- the last word in do-it-yourself vacuum bagging.

Some folks successfully use heavy poly sheeting or thin clear vinyl that you can find at fabric outlets. They seal it to the table with double stick tape, Mortite window seal, clay, etc. You can find inexpensive hand vacuum pumps like this brake bleeder (http://www.sciencefirst.com/vw_prdct_mdl.asp?prdct_mdl_cd=78-610)that will fit the bill I believe- they pull 500 mm (20") HG.

http://www.sciencefirst.com/pctr/78-610.jpg

The disadvantages of this pump are you need to do a lot of pumping and if you have any leaks, must come back a few times and pump more until your glue is dry. They typically cost $20 - $30.

Rockler sells a hand pump vacuum bagging kit (http://www.rockler.com/product.cfm?Offerings_ID=11315&TabSelect=Details) for a little over $50.

http://images.rockler.com/rockler/images/28777-02-200.jpg

[Leigh Betsch]

Hey,:rolleyes: I've got some heavy plastic curtain from an industrial clean room. I've been saving it to make a spray booth but maybe I have enough to make my own bag. Don't know about that hand pump though, looks like a a lot of work.

[Rick Lizek]

[QUOTE=Mike Henderson;1022677]Alan - I'm sure I'm overbuilding my torsion boxes. My problem is I have no way to "design" them. I don't know what mathematics to use, or have the data (strength of materials) to input to the mathematics (if I had that).

You are definitely over building them. No need to do all that notching and such. Torsion boxes are simple and fast to build. Ian Kirby in FWW over twenty years ago laid out the basic foundations for torsion boxes for woodworkers. Ken Horner's 2nd book has the technical info and details you are asking for. Make life easier... get the book.