Hello
i want to make torsion box and i want to know which material is better for the core, wood or plywood 3/4in. thick.
thanks
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Replies
Can you provide us with the dimensions of the torsion box? Generally I would recommend quartersawn solid wood for the web and I would use the same wood for the sides, not necessarily quartersawn.
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 8/15/2009 10:29 pm ET by KiddervilleAcres
Thanks,
I want to make this box torsion for use it as bench-pressing veneer. I think a 2'' thick total and MDF as sides of 4 ft x 2 ft and i want to know what is better for the core wood or plywood?
thanks very much
I have made two worktable torsion boxes for veneering. I set my platen and bag on these boxes which are set on sawhorses. Both are entirely made of plywood: 4'x8'x3" thick. They are strong, flat, and light enough to handle without killing myself. The skins are 1/4" ply.
herber,
You said, "bench-pressing veneer". Do you mean a veneer press? Will it use a vacuum or clamping type of pressure?
4' x 2' size would be very large for a screw down clamping type of system I would think. In any case I would use quarter sawn solid hardwood for both the grid inside and the sides of the torsion box. Do you plan on laminating the top with a formica like material?
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
" I would use quarter sawn solid hardwood for both the grid inside and the sides of the torsion box."Why do you think hardwood, qtr. sawn no less, is necessary?
Well for one thing I'm not sure what type of veneer press he's trying to build. I suppose either way one would want significant structural integrity and stability.
In my mind quartersawn hardwood fits the bill and it is easier to mill.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Bob..
As Jammersix? The next post after yours stated..
Gluing the edges of plywood is generally not a good strategy.
And QS wood?
Not fighting anybody but I wonder if the 'web' REALLY needs to be anything but a TRUE cut 'stick' as to it's thickness!
Yes, You are BOTH Correct.
However, if the sticks have a true thickness surface to glue to what difference does it make? PLENTY of gluing surfaces that make up for the 'not so perfect'. Ok, just me.
I have edge glued plenty of reasonable quality Ply without glue failures.
I had my say and I'll hide in the corner and put my flack jacket on and eye protection. No ear plugs. I want to hear the flack....
I could be wrong?
Edited 8/22/2009 2:55 am by WillGeorge
Will,
Gluing the edges of plywood is generally not a good strategy.
Not sure I understand what he meant by that. We do that very thing when edge banding p-wood and have seen it recommended as a sizing and/or sealant for end grain.
As to cost considerations I don't see where this is a factor. As has been mentioned, cardboard webbing can be used and q-sawn lumber (pine) is readily available.
Recalling one of the clamp tests it suggested that pressure expands from the caul surface but it only showed it in one direction. I'm wondering how the thickness of the webbing material affects distribution of pressure, i.e. do thicker web frame members spread pressure farther in the other direction?
Assuming the answer is yes, therefore would this also allow for a thicker torsion box by using wider web frame material? I suppose at some point q-sawn material could get mighty expensive but the same would also hold true for p-wood methinks.
Many questions.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Re plywood, although end-grain-to-long-grain joints are generally not as strong as long-grain-to-long-grain joints, I'd think that consideration would be overcome by the surface quantity within a torsion box. Even voids would become a minor consideration, I'd think.As to web material dimensions, and their relation to the thickness of the torsion box, I'm guessing that there is some ideal ratio involved, but I'm not sure what that might be. One might, for example, use 1/8" or 3/16" x 1" webbing for a thin torsion box/panel, but go thicker (1/4", 5/16", 1/2" ?) as the depth (and, the anticipated load) increases. At some point, choice of materials might also be affected by the anticipated load (e.g. how much weight can be placed on the torsion box/panel before it fails), but isn't that the benefit of the torsion box concept? (The ability of relatively light-weight construction to support surprising loads.)
The thickness and composition of the skins needs more attention as does thickness of the core , the correlation is paramount here .
As far as using QS lumber or plywood or other lumbers for the core :
some feel the qs is more stable or stronger in some way in this application for those folks I ask you to show us on a tape measure what the difference will be . Do you think it will expand and contract at a different rate ?
How do you feel about creating an airtight hollow cavity without an air vent ?
Just want to throw this out there :
From my experience we run the risk of having the torsion box or hollow core door for that matter blow out from internal pressure unless we plan a air vent slot. I have not heard this mentioned yet and while many are concerned over which is the best , they all may blow out .
regards dusty
I'd agree that all of the components need some sort of harmonious balance. I put air vents with reeds in all of mine, so they play a tune when the atmospheric pressure changes. ;-)
Hey dusty man,
Next time ye make one put a baloon on it. Let us know when it bursts, ok?
I just had to say that. <G>
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
" From my experience we run the risk of having the torsion box or hollow core door for that matter blow out from internal pressure unless we plan a air vent slot. I have not heard this mentioned yet and while many are concerned over which is the best , they all may blow out ."Really? You'll have to try to build a door that is airtight. Even if you did, an internal door/box would only suffer at most 0.5psi difference from atmosphere. With core members every half a foot, you would only have 1.5 pounds of pressure per linear inch of core. Even cardboard should support that. How do you have a "blowout"? External hollow doors would be under up to 2psi pressure difference (in extreme climates like Winterpeg or AZ) if airtight. But once you install the hardware, they are not airtight.
I've seen hollow core cabinet doors with thin skins on each side fail at a glue line of the skin usually near a corner .External hollow core are very different then a hollow core cabinet door.
Thanks for the data on the internal pressure stuff .
Maybe the off gases and fumes from the various cements and finishes and glues contributes to this condition not so much the core material itself ?
How the skin is fastened or bonded varies .
Maybe it also depends on the skins used as well , an 1/8" door skin for a hollow core cabinet door seems less resilient then say 1/2" skin on a torsion box .
regards dusty , has run a few sets of hollow core doors
http://www.amazon.com/Woodworkers-Essential-Facts-Formulas-Short-Cuts/dp/1892836211/ref=sr_1_1?ie=UTF8&s=books&qid=1250511020&sr=8-1
I would say that most of these posts are idle speculation on torsion box making. Ian Kirby introduced it to woodworkers in FWW #32 in 82. For the grid spacing and skin strength info see the book referenced above by Ken Horner. If people don't know the answer why do they guess. Quartersawn grids, vented cores and issues of plywood grids...
In a torsion box you only need to glue the skin to the grid. There is no need to do halflaps to the grid material. Make the long pieces of the grid and the short grid pieces only need to be stapled to the long pieces to hold the grid together until the skin is applied. Folks are making this discussion long and filled with unecessary and useless info.
Folks are making this discussion long and filled with unecessary and useless info.
I sort of agree but I am just a old tank recovery mechanic. Not that much wood support needed if reasonable sized logs.. Tying it all together counts!
"Folks are making this discussion long and filled with unecessary and useless info."But, isn't that what forums are for? ;-)On the other hand, I don't recall the election where Kirby and Horner were appointed as the sole authorities on the matter. ;-)
Have you seen the articles by Kirby and Horners book? All talk on the web is just specualtion with no documentation! Most is taking a simple concept and making it more complicated unnecessarily.
If it wasn't for Kirby no one would even know about torsion boxes. You missed the election...
Edited 8/25/2009 8:28 am ET by RickL
"All talk on the web is just specualtion with no documentation! Most is taking a simple concept and making it more complicated unnecessarily. "Why do you and JohnWW keep trapsing around this forum spoiling the entertainment by dropping reasonable and practical advice. If you give them the answer so fast, I won't get to watch them flop around like fish on the dock. You just put your foot on one at a time; don't throw them all back in at once. Now you might as well tell them that heavy cardboard will work better than quartersawn hardwood. (I don't need a book for that one.)I need something comical to read to fill the void until the next Canadian election... 50 messages about torsion boxes with no mention of statics -- beat that! Andy
"...50 messages about torsion boxes with no mention of statics -- beat that!"You've just slighted everyone who's contributed to this discussion regardless of their input. And you've hinted that you might actually have a grasp of the physics involved in a torsion box. Well please divulge this. I, for one, would love to read it.
"You've just slighted everyone who's contributed to this discussion regardless of their input."I don't think I did. Not exactly. Many others 'slighted' each other. I just took a shotgun approach. There were some good suggestions. Just making the point that there has been a lengthy discussion on the topic without much deep thought. But if you want me to pick some fun examples..."Generally I would recommend quartersawn solid wood for the web and I would use the same wood for the sides"Um, did he consider expansion? That would rip the table appart. QS wood moves, albeit less."Gluing the edges of plywood is generally not a good strategy"Not a good strategy as the carcass of standalone furniture where the edges will undergo shear or torsional forces. Perfectly ok inside a torsion box where this is not at all the case."For a work table type of box,I would suggest Baltic/Russian/Finnish ply for core and banding (up to 5'x 5'), MDF for surfaces.
I've never gotten Baltic in any size larger than 5x5."BB ply comes 4x8. I just bought a few sheets. MDF kind of defeats the purpose of a torsion box; one of its benefits is less weight through better design. Why use MDF, which boasts smoothness, when the design of the torsion box makes any plywood flat. Flat is what is desired here, not smoothness. Syntax matters here, so I hope this makes as much sense to someone other than me. "just like my wrinkly skin could use replacing"Ewwww! "Does that mean your the same age as Joan Rivers?"eeewWWW! I'm just old enough to be repulsed by that (image)."Given that, even OSB would work"Not very well. It is cheap, flat, and heavy. Defeats the weight gain of the torsion box. Made of chips that would shear with a load on its edge."I believe I built mine of 3/4" thick,unfinished maple veneered, D-fir plywood." "I cringe knowing a "crafts person" would even consider wasting good material where it isn't needed."Where do I start? These two statements do not belong to the same author!"Torsion boxes gain their resistance to collapse from the distance between the two skins and the surface area of glue, and by changing the direction of force.":) Really? If you want to test any comparison, be it length versus height when comparing wood strength, or who will survive a crash between two different vehicles, try making the dimensions extreme. It will help visualize it. The compressive strength of a torsion box is not proportional to its height. It will gain strength from the height to a certain point and then that factor will become negligible. The height of the box changes the width of each core piece and how much shearing load it will take on its edge. After a certain point, it won't help much. They also do not gain compressive strength from the glue surface. Imagine a torsion box with really thin core pieces with elaborate joints that increased the glue surface several times. No benefit. The force does not change direction. Any force created that is not parallel to the load will just have to have an opposite component which would mean forces in three directions acting on the table(?)"For a super-strong torsion box, one might even consider making mini I-beams for the web."The I-beam IS the same idea as the torsion box, but in one dimension. Not necessary to put the top and bottom on a 'beam' if it is glued to the top and bottom skins of a torsion table."I put air vents with reeds in all of mine, so they play a tune when the atmospheric pressure changes."How about a whoopi cussion? Yes I'm a guy, and yes, I knew this was a joke."I would say that most of these posts are idle speculation on torsion box making."The party pooper. I agree, but still killing my evening entertainment. ;) "Not that much wood support needed if reasonable sized logs."What does this mean? Anyone have a Will-to-english translator? Am I not old enough?"comical to read to fill the void until the next Canadian election"What idiot said that?!"And you've hinted that you might actually have a grasp of the physics involved in a torsion box. Well please divulge this. I, for one, would love to read it."I sense sarcasm, but I'll give you the benefit of the doubt. To be honest, being a little out of practise, it would take me a while to dredge up the math that would describe this topic. For our purposes, it does not require it. I already gave away that cardboard would suffice, if the web is tight enough. PSeverin and Zolton were the voices of reason in this thread and did not get any attention. The attention went to quartersawn wood. Why not veneer it or apply french polish shellac to the faces before hiding them in the box? ;) The idea behind the torsion box is to use the web material in its strongest direction. That is parallel to its face. Get this material running in both axes and you won't have to worry about edge-glue strength or shear (causing bending) strength (of core pieces). The strongest axis of the web material running in one direction will prevent the skin from moving in that direction. Run two, perpendicular directions (or even better, three as with honeycomb), will prevent the skin moving any x-y direction. The z (crush) strength comes from the load forces being supported through the width of the core pieces (parallel to the face). Even thin stock is quite strong if you could place a load on its edge and direct the force of the load straight through the middle of it. Any lateral force at the same time, and the material would be overstressed with shear force (as the main load force will no longer be along the material), especially at the edge joint. The perpendicular core pieces do not allow that. They support each other. Now imagine the whole load on the table being spread (somewhat evenly) over a larger area by the skin and allowing several core pieces to shoulder it. For this reason, it makes sense to use a thicker skin and thinner core pieces. Several, light-weight core pieces can provide the compressive strength to prevent the skin from deforming (it sees shear forces in the z direction). The skin will take forces perpendicular to its face, so it needs to be thicker. Try an experiment. Compare the shear strength of a 1/4" piece of ply with the compressive strength on its edge. Put one end in a vise. How much force does it take on the face at the other end to deform it? How much MORE force does it take on the edge to deform it? A LOT. Now imagine a lattice of several of these edges inside a table bounded by plywood, top and bottom. The closer the parallel core pieces to each other, the less shear strength the skin has to have. Of course, it also matters what the load is. If you apply a large, point load to a torsion box with a thin skin, the skin will shear between the core pieces. I demonstrated that on my closet door with my foot one time after it pinched my finger. Those doors were turned into a homemade panel saw. ;) I plan on putting the other pair to use as an assembly table (further skinned with BB ply) when I have more shop time.Enough rambling. Get the book Rick pointed out. My guess is that it will pinpoint the best balance between thickness of skin, thickness of core pieces, and desired load, etc.Andy
My link for Horners book has pretty much the whole chapter on torsion boxes. Books can be the cheapest tool in the long run. If it keeps you from wasting time and materials it's a bargain. Most of the info posted by well meaning folks is just speculation and a lot of it is not necessarily wrong but adds unnecessary work and time. The whole idea of the torsion box is it's speed and simplicity to make as well as being able to make physically sizable boxes that are light and strong. Consider the cost and weight of a piece of 16/4 walnut and the torsion box is a pretty good dea.
Quarter sawn is totally unnecessary for the grid.
The grid can be made of wood, plywood or MDF.
The grid only needs the parts to be stapled together. No need for half laps or any kind of gluing to the grid. The sandwiched skins glued to the grid will do the work.
Kirby laid the foundation for torsion boxes and Horner provide the technical details so one can build one to the desired specs. What else do you need? Sure honeycomb cardboard or plastic honeycomb is an option.
Thanks for those links I git a way down the book but couldn't make it to page 208.I'll have to look for the book or order it. It looks like a good resource and from what I read well written. Yes good info is always welcome.I was googling torsion boxes and saw a link to a CNC site and this one fellow was talking about triangles and there inherient stability. I think they were talking about aircraft applications. I also saw a video in the Woodwhisper web site about making a torsion box.
Triangles - look at Buckminster Fuller's Geo domes. One massive torsion assembly, all built on triangle shaped Cartesian panel units. Or circles - Modern submarines and WW 1 aircraft fuselage.
Don't forget old farm screen doors :)
I have access to 5x5 Balitic birch and 4x8 for years. 1/4 3/8 1/2 . It is one store only!! A COOP in Saskatchewan.I have used some ProBoard 1/4 inch underlay for flooring. Most is quite flat and no voids, and some even has a nice grain, 3 ply birch. Great for cabinet backs paint or stain.Getting back to the torsion box thing. I have made a few vacum tables for pressing orthotics and doing orthopedic shoe work. First one collapsed at about 25 mmhg.
Next one was scrap wood for the outer rails, 1/4 inch masoinite with holes for the top and a web of 1/4 masointe with holes as webs. needed the air flow. 1'2 inch ply bottom. all edges sealed for leakage.Made a frame to hold a thin piece of rubber to "pull down the plastics"Nice and stable, no "sickening crack" when you have a plaster cast snap in half.
RE: "BB ply comes 4x8" in response to my "I've never gotten Baltic in any size larger than 5x5"
It's great that you can get it that size. The top supplier and importer of BB in North America (The Cross Pacific Group, Inc.) states that 4x8 has only recently become available and is relatively new. Perhaps you haven't used it long enough to know the max size of true BB has been 60"x60" for at least the last 35 years. I'm glad it's changed.
Yes, I was aware of that. I still have the remains of my last purchase of 5x5 sheets. Perhaps you would not know to ask for 4x8 sheets if I had not mentioned them? I'm sure there are people lurking that were interested. Both form factors are useful.Andy
"PSeverin and Zolton were the voices of reason in this thread and did not get any attention. "
I was one of the voices of reason?! I'm blushing! Thank you for noticing...
I think we're overcomplicating this stuff.
ZoltonIf you see a possum running around in here, kill it. It's not a pet. - Jackie Moon
I appreciate the link to the book but just can't order it. Can you share some knowledge about it? I have never heard of Kirby or Horner but I have heard about torsion boxes back in the 70's.Forums are for idea exchange. I don't live near a well stocked library and can't afford every book mentioned on the forums.I get to see peoples work from all over the world, If I had to wait for a publisher to decide what they want to print and then print and distribute it. the idea/concept would not make it to as many as these forums. Think Lataxe's garden bench. Will George talked about a Bombe demuline table. So I started scratching some designs for one. Should I not expand in this as I don't have a designer to say my design is OK? or have to run a table design by a structural engineer to make sure a vase with flowers won't lead to a catastrophic failure?"Waiting for the next Canadian Election" Now I got a good laugh over this one and the poster is not an official, licensed jester appointed by the Canadian Government.
http://books.google.com/books?id=5xYBcTalTWQC&printsec=frontcover&dq=ken+horner#
http://books.google.com/books?id=5xYBcTalTWQC&printsec=frontcover&dq=ken+horner#v=onepage&q=&f=false
Go to page 208.
Edited 8/25/2009 2:33 pm ET by RickL
Gluing the edges of plywood is generally not a good strategy.
Herber,
For a finish box like a flush wall unit, I would use F&S for banding, Baltic for core and 1/8th F&S veneer for surfaces.
For a work table type of box,I would suggest Baltic/Russian/Finnish ply for core and banding (up to 5'x 5'), MDF for surfaces.
I've never gotten Baltic in any size larger than 5x5.
Have also seen core and top in hardboard (Masonite) 1/4, 1/8th. Wasn't preeety but reeeel flat and stable.
FWIW,
If it matters, Masonite is a Green(er) product as no formaldehyde based resins for glue (plywood) are used.
You can also use a honeycomb core. web search will give you many sources - paper, plastic, cardboard, all very strong,rigid and flat/square in many different thicknesses. Good for "in" sandwich on large conference tables.
I prefer Baltic.
Boiler
herber , the size could make a difference but in general either will work but plywood unlike wood will not move because the moisture content is so low.It may be easier to use a flat core not real wood depending on the application.
I have used 1/2" and 3/4" ply with great success many times, I think using solid would be a waste.
About 15 or so years ago FWW did an article on a large torsion box. I built the box pretty much as the article was written and have used it as an 4' x 12' assembly platform. Pretty dead on flat and sturdy but way too heavy. Tehn again I seldom need to move it.
http://www.amazon.com/Woodworkers-Essential-Facts-Formulas-Short-Cuts/dp/1892836211/ref=sr_1_1?ie=UTF8&s=books&qid=1250511020&sr=8-1
Get Ken Horner's book. There is a great definitive chapter on torsion box making that will answer all your questions and more.
You can use Wood, MDF, Plywood for the cores and skins. Depends on what you want to do with it. For lightweight use pine for the core. For a heavy assembley table use MDH.
The book will tell you all about grid spacing, why to use what material for skins. Forums will just give you undocumented opinions without a fact or reference to back up their claims. Ken's books are great simple reference books on many topics. It's the best on torsion boxes. Also torsion boxes are very fast and simple to make with butt joints and staples for the grid. Half laps are just more work and totally unnecessary for torsion boxes.
Ian Kirby, who first published the torsion box info in FWW years ago, designed a workbench which would double as a veneer press with clamps and cauls used MDF as the core and if memory serves me right it was about 4" thick total. He used a seperate torson box to go above the veneer panel. Quite simple.
I think 2" might be a bit thin but in MDF it would be very heavy. No reason you can't use solid wood.
Edited 8/17/2009 1:32 pm ET by RickL
Edited 8/17/2009 1:45 pm ET by RickL
I built this tortion box assembly bench in about 1990 of 3/4" plywood. The original plastic laminate surface is easy to keep clean of glue, but it needs replacing.
Gary W
gwwoodworking.com
Gary,You can't get away that easily, I think we need an explanation for that sorely tortured spot in an otherwise virginal box! Enquiring minds and all that.BB
Edit: PS. nice wall layout.
Edited 8/17/2009 8:45 pm by boilerbay
You know, that happened such a long time ago, and I don't remember how the chipped away spot got there. We clean dried Titebond off with a razor sharp 6" taping knife, and I think I may have made it when I ran the blade against the edge of a small hole from an errant drill bit. You know, "mistakes beget mistakes." But the 32" by 96" table is still dead flat! It just needs a new skin, just like my wrinkly skin could use replacing. Gary W
gwwoodworking.com
Gary,"my wrinkly skin could use replacing"Mine too.John
Wait a minute now, you're from somewhere near the Oregon coast, so your skin doesn't absorb nearly as much sun as mine. Just don't use a sharp taping knife to clean it.Gary W
gwwoodworking.com
Think of "pruning"after a long bath.
This close to such a big pond and it all goes to wrinkles.
Wrinkly skin is easily fixed by a "second growth" process. This is where every time you see a wrinkle starting you just eat a little more and it fills in, sorta like filler. Trust me on this.
Eating more to fill in the wrinkles only works until one's heart gives out.n ;-)Personally, I'm at the "Bondo" stage. Just don't ask me to smile.
Does that mean your the same age as Joan Rivers? She has more Bondo than a 53 chevy.
I'm with others here in questioning your need too use quartersawn lumber. This to me would be to expensive and completely unnecessary.Not that you couldn't use it! I just can't think of any good reason to do so, unless you have a lot of free quartersawn lumber about. Plywood wood be ideal, especially Baltic Birch or Apple Ply, which is the American equivalent of Baltic Birch and can be purchased in 4 x 8 sheets.You can't buy this stuff at HD or Lowes. You have to go to a cabinet makers lumber supply yard.There's usually one in every larger city. A good quality plywood would work very well and not be nearly as pricey as the BB or AP and you could probably get that at HD. You could also get 1/4" melamine skins for the top and bottom so glue won't stick to it. That would be far less expensive than gluing up plastic laminate, but if I were making one today I would staple 1/4" Masonite on top and bottom and then glue plastic laminate over that to make it smooth and glue resistant as I can usually get my laminate for free.
Edited 8/19/2009 12:16 am ET by brownman
Edited 8/19/2009 12:16 am ET by brownman
Edited 8/19/2009 12:17 am ET by brownman
Edited 8/19/2009 12:18 am ET by brownman
This to me would be to expensive and completely unnecessary.
How so? I've never had a problem selecting q-sawn stock from almost any pile of boards. If one examines the end grain on boards, q-sawn stock is relatively easy to find.
Granted, a log sawn specifically for quarter sawn materiall will be more expensive.
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Yep, there are at least a couple of QS boards in every flat-sawn log, and a few more that are almost QS. Just takes digging through the pile, assuming the dealer will let you do that.Personally, I like the idea of using QS strips for the interior web, so that surface variations (skin ripples) are minimized - at least in theory. In practice, though, I'd assume that if the finish cuts and assembly are done in fairly short order, there wouldn't be too much expansion variations after the web is "sealed" inside. Assuming, of course, that the entire torsion box is sealed.
assuming the dealer will let you do that.
I just saw this.. I had to respond.
If they will not let you.. Go someplace else! OK, so you have to put the pile back for the next person looking. AND not damage the 'goods'...
I use quarter-sawn material for appearance and when it's necessary for its stability, but I can't understand it's application when it'll be hidden within a torsion box between two surface panels, which will likely be plywood. We're talking here about a strong, dead-flat workshop table or vaccuum press form. What's the point of using quarter-sawn wood instead of a dimensionally stable, high-grade plywood within a torsion box? Why not save the quarter-sawn wood for when solid stock is truly called for?
Gary W
gwwoodworking.com
Edited 8/19/2009 1:05 pm by GaryW
Another method of making a torsion box is to use either honeycomb material or simply insulation board for the core. This is a very fast and strong alternative to building a core from 1x material.
I'm on both sides of this as I can see Bob's point and if it's handy and cheap, why not. But I think since it's hidden -- Baltic etc, is a lot cheaper if you're going to go out and buy it and I think even more stable than QS. In addition, a lot of the stability is added by the additional qualities of the construction itself. Given that, even OSB would work. Heavy, smelly, crappy and oozing formaldehydes but it would work...
Not a pretty picture though.Which brings us to the question of "if only you know, is it still important as long as it's flat"?The bring home the bacon side says yes and the craftsman side cringes.You all sure it's not Friday?Boiler
I believe I built mine of 3/4" thick,unfinished maple veneered, D-fir plywood. I'd probably still use that to keep it a bit lighter than if made with either Baltic birch plywood or Appleply. Any one of those probably would be more stable than ANY solid stock material, regardless of how it's sawn from the log.
The "bring home the bacon" and the craftsman sides of me are aligned. Why would a craftsman insist on using beautiful quarter-sawn lumber within a torsion box? And that'd be my decision even if I had a thousand b.f. of that lumber in my shop's storage racks. I cringe knowing a "crafts person" would even consider wasting good material where it isn't needed.Gary W
gwwoodworking.com
Torsion boxes do not gain their strength from length of span. If that were the case, cardboard would never be used.Torsion boxes gain their resistance to collapse from the distance between the two skins and the surface area of glue, and by changing the direction of force.
Now watch, I will have said THAT wrong, too...
All this discussion of torsion boxes leads me to ask why don't people use either solid or hollow core doors for this type of construction. A hollow core door blank is, in fact, a torsion box, skinned with a thin plywood. The core is made of a stiff cardboard honeycomb. If you wanted to beef it up by laminating another skin to it, that would make it all the more sturdy.
Personally, unless there was a pressing need to build something larger than a standard 3-0, 6-8 door blank, that's the way I'd go. You can pick up hollow core doors at big box stores for 35 bucks or so. It's hard to beat that price for a flat, stable surface. Why waste lumber and time when this technology is available so inexpensively?
My primary workbench has a top built of a 1 3/4 thick, 3-0 solid core door covered with 1/2 inch Baltic birch plywood. It never needs flattening and is as sturdy and heavy as one built of solid lumber. Because someone had cut the hinge gains into this particular door incorrectly, I was able to buy it for about 20 bucks in the cast-off bin. If the top ever needs replacing (which it hasn't yet in over ten years of use), I can simply strip off the ply and slap a new sheet down.
I like simple. I like cheap even better...
Zolton If you see a possum running around in here, kill it. It's not a pet. - Jackie Moon
Just avoid demolishing a torsion box after building it. Otherwise, you'll be indicted for ex-torsion by the woodworking police. ;-)
Ralph, I know a lot of guys won't appreciate your puns. But I do..ZoltonIf you see a possum running around in here, kill it. It's not a pet. - Jackie Moon
I appreciate puns at least as much as I appreciate mimes.
It should be noted that not all quarter-sawn lumber is "beautiful". QS, after all, is just a matter of grain orientation. Some can be scruffy, or even roughy, but for the interior of a torsion box, should at least be toughy. ;-)It's the crafty crafts-person who selects materials well-suited to the engineering requirements of the object being crafted, I'd say. ;-)
I give, Ralph. I think this is one of those discussions where each of us has made a decision, and I don't wish to argue any more for the use of plywood.
I do agree with Jammersix that the box needs to be thicker to carry a heavier load. The thickness of the torsion box, needs to be directly proportional to the weight placed on it, and the span between the supports the box rests on.
As far as a flat form for vaccuum pressing, I use a sheet of 1" thick melamine in which I sawed a grid of 1/8" deep grooves on one side - the side on which I place the parts to be pressed. That sheet goes into my vaccuum bag, and all rests on my torsion box when pressing expensive veneers onto a substrate. I can say that that all have measurably dead flat.Gary W
gwwoodworking.com
No argument intended. Just exploring possibilities.For a super-strong torsion box, one might even consider making mini I-beams for the web. Heck, a fella might even use plywood for that. ;-)
Plus ply wood is much more stable,especially since your looking for flat here. Also considering where you live ,don't you have a lot of moisture that can effect wood movement? Just seems like ply would be your best choice. I'd even use MDF or melamine covered particle board if I wasn't concerned about weight. Besides sometimes when you cut real lumber, quarter sawn or not, you get tension that's released causing the wood to twist or bend so it's not straight. In making a torsion box you want all your materials to be straight, flat and true , which is much easier to accomplish with ply or manufactured sheet stock. It's mostly a matter of common sense and careful preparation.
It's a structural question of how strong does it have to be and even cardboard might be more than enough. It just has to keep from being crushed...but then so does the surface between web members. If you're going for light weight use 1/4" plywood grid in a 2 or 3" grid with light surface skin. If weight is not the issue why not just laminate a couple of pieces of plywood together?I used to use this system to make model bases out of cardboard with the chip board faces being held apart by 1 or 2" cardboard strips and cross members. When you think about it there is an efficiency in a grid where thin members are prevented from rolling by the abutting piece. Think of floor joist and bridging. They were amazingly strong and light.As a structural system that sets out to use material more efficiently the problem is what is enough to keep the surfaces parallel and the top compression side skin from buckling. The spacing of the grid members is really about reducing the size of the area under compression (the shorter the span the greater resistance to buckling) and holding the two faces to work together so to compress one you have to stretch the other. It is always slightly hard to understand how the forces are really being handled in one these structures...speaking for me only. It depends upon how much of a load you want it to carry.
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