Page 1 of 2 12 LastLast
Results 1 to 10 of 11

Thread: What is a torsion box?

  1. #1
    Join Date
    Nov 2006
    Location
    Houston, Texas
    Posts
    2,323

    What is a torsion box?

    Hi Folks ,
    Pardon my ignorance, and to some the simplicity of the answer, but what is a torsion box and what is it's purpose. It is sometimes mentioned here.
    Just looking for knowledge.
    Thanks,
    Shaz
    I am a registered voter and you can be too. We ( registered voters ) select the moderators for this forum by voting every six months for the people we want to watch over this family forum.
    Please join me. Register now.
    Shaz
    Here is how

  2. #2
    Join Date
    Sep 2007
    Location
    Shorewood, WI
    Posts
    97
    http://www.diynetwork.com/diy/ww_mat...278181,00.html

    A torsion box is a way of making a strong but light structure from two sheets of material with a grid between. The link leads to a detailed description of making one.

  3. #3
    Join Date
    Oct 2006
    Location
    ABQ NM
    Posts
    30,008
    Shaz, I'd wager you've built a torsion box in your woodworking career and didn't even know it. A standard hollow core door is a classic example of a torsion box, too.
    When the going gets weird, the weird turn pro. - Hunter S. Thompson
    When the weird get going, they start their own forum. - Vaughn McMillan

    workingwoods.com

  4. #4
    Join Date
    Nov 2006
    Location
    Villa Park, CA
    Posts
    1,407
    Shaz - a torsion box is a box made a certain way so that it's very stiff compared to it's weight. It's a bit hard to explain how to make one in words but let me try.

    The underlying principle of a torsion box is the I beam. An I beam has a top and bottom, called the flanges, and a center called the web. It is strong because the flanges resist tension and compression, and the web keeps them from moving relative to each other (and keeps them the same distance apart). If you had an I beam supported on both ends, and put a load in the middle, the top flange would be under compression while the bottom flange would be under tension. The further apart the two flanges (the deeper the web) the stronger the beam.

    Now, suppose you put a bunch of I-beams side by side and welded the flanges together. You'd have a flat surface that was very rigid along one axis.

    Now, suppose you had some magic machine that would go between the I beams and every foot or so it would weld a web from one I beam to the adjacent one. In essence, it would be creating I beams along the other axis of the surface. Now, that stucture will be very stiff along both axis.

    To do this in wood, you would cut some webs, maybe 4" wide out of maybe 1/4" plywood. You then arrange those webs in a square pattern interlaced (it'll look like a bunch of pigeon holes when you're finished.

    Now make an upper and lower surface from maybe 1/2" plywood. When you glue this together, it's very important that the webs are well glued to the top and bottom, and the pigeon hole structure is glued together.

    What you're doing is creating a bunch of I beams along both axis.

    Questions:

    1. How high should the web be? Answer: Depends on how strong you need the torsion box to be (and how big the box is). For many applications 4" webs will be fine.

    2. How thick do the webs have to be? Answer: Many people make them too thick and that adds weight to the box. 1/4" is plenty strong for most boxes.

    3. How far apart do the webs have to be? Answer: That's complex. Closer is stronger, but beyond that, it depends on the wood used for the flanges. If the webs are too far apart, you'll get depression of the top between the webs.

    4. How thick does the top and bottom have to be. Answer: See Q3, and thicker is stronger. But for many applications 1/2" plywood is sufficient. When in doubt, go with a thicker top.

    I hope I didn't forget too much and that this comes close to answering your question.

    The advantage of a torsion box is great strength and great rigidity with relatively light weight.

    Mike
    Ancora imparo
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #5
    Join Date
    Dec 2006
    Location
    Charlotte, NC
    Posts
    1,487
    Think honeycomb!

  6. #6
    Join Date
    Oct 2006
    Location
    Tokyo Japan
    Posts
    15,807
    Think Airplane Wing
    The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
    William Arthur Ward

  7. #7
    Join Date
    Nov 2006
    Location
    Oak Harbor Washington on Whidbey Island
    Posts
    3,134
    Here is one I built before I closed the top.
    Attached Thumbnails Attached Thumbnails torsion box 004.jpg  
    "Forget the flat stuff slap something on the spinny thing and lets go, we're burning daylight" Bart Leetch
    "If it ain't round you may be a knuckle dragger""Turners drag their nuckles too, they just do it at a higher RPM"Bart

  8. #8
    Join Date
    Feb 2007
    Location
    Catalunya
    Posts
    4,632
    Apart from all that has been said, the most important role of a torsion box on a woodworking shop is to provide a light but very stiff structure to give you a completely flat and level surface of reference to check and make your assemblies adn glue ups.

    Of course all this provided that you have built it perfectly flat. If you've built it twisted, it will be stiff but twisted, hence useless.

    Check at www.thewoodwhisperer.com for a tutorial on how to make one.
    Last edited by Toni Ciuraneta; 05-24-2008 at 08:00 AM. Reason: some after thoughts
    Best regards,
    Toni

    __________________________________________________ __________________________________________________ _________________
    web site:http://www.toniciuraneta.com
    I also dream of a shop with north light where my hands can be busy, my soul rest and my mind wander...

  9. #9
    Join Date
    Nov 2006
    Location
    Houston, Texas
    Posts
    2,323
    Quote Originally Posted by Alan Schwabacher View Post
    A torsion box is a way of making a strong but light structure from two sheets of material with a grid between.
    Hi Alan ,
    "Thank you for your input, I did go to that link and learned much. Shaz"
    Quote Originally Posted by Vaughn McMillan View Post
    Shaz, I'd wager you've built a torsion box in your woodworking career and didn't even know it. A standard hollow core door is a classic example of a torsion box, too.
    "Hi Vaughn , As you mention it, I have. The hollow core door is a good example! Thanks, Shaz "
    Quote Originally Posted by Mike Henderson View Post
    Shaz - a torsion box is a box made a certain way so that it's very stiff compared to it's weight. It's a bit hard to explain how to make one in words but let me try.

    The underlying principle of a torsion box is the I beam. An I beam has a top and bottom, called the flanges, and a center called the web. It is strong because the flanges resist tension and compression, and the web keeps them from moving relative to each other (and keeps them the same distance apart). If you had an I beam supported on both ends, and put a load in the middle, the top flange would be under compression while the bottom flange would be under tension. The further apart the two flanges (the deeper the web) the stronger the beam.
    "Hi Mike , This is very clear and interesting!..S "

    Now, suppose you put a bunch of I-beams side by side and welded the flanges together. You'd have a flat surface that was very rigid along one axis. "Very clear"

    Now, suppose you had some magic machine that would go between the I beams and every foot or so it would weld a web from one I beam to the adjacent one. In essence, it would be creating I beams along the other axis of the surface. Now, that stucture will be very stiff along both axis.
    "I can understand this."
    To do this in wood, you would cut some webs, maybe 4" wide out of maybe 1/4" plywood. You then arrange those webs in a square pattern interlaced (it'll look like a bunch of pigeon holes when you're finished.
    "Each piece gets a half notch to accept the perpendicular pieces, correct?"

    Now make an upper and lower surface from maybe 1/2" plywood. When you glue this together, it's very important that the webs are well glued to the top and bottom, and the pigeon hole structure is glued together.

    What you're doing is creating a bunch of I beams along both axis.
    "That is a sweet deal!"

    Questions:

    1. How high should the web be? Answer: Depends on how strong you need the torsion box to be (and how big the box is). For many applications 4" webs will be fine.

    2. How thick do the webs have to be? Answer: Many people make them too thick and that adds weight to the box. 1/4" is plenty strong for most boxes. "I guess some folks go thicker and cut the perpendicular pieces to serve as spacers that can be nailed rather than half notched.

    "What do you think the comparison would be between 1/4" webs, half notched verses a 3/4" web with long pieces one way and spacers cut in between, nailed and glued? Shaz "

    3. How far apart do the webs have to be? Answer: That's complex. Closer is stronger, but beyond that, it depends on the wood used for the flanges. If the webs are too far apart, you'll get depression of the top between the webs.

    4. How thick does the top and bottom have to be. Answer: See Q3, and thicker is stronger. But for many applications 1/2" plywood is sufficient. When in doubt, go with a thicker top.

    I hope I didn't forget too much and that this comes close to answering your question.
    "Well done, Thank you very much! Shaz"

    The advantage of a torsion box is great strength and great rigidity with relatively light weight.

    Mike
    Quote Originally Posted by Ed Nelson View Post
    Think honeycomb!
    "Thanks Ed, Good one! "
    Quote Originally Posted by Stuart Ablett View Post
    Think Airplane Wing
    "Not familiar with that construction but am guessing you are on target Stu! Thanks "
    Quote Originally Posted by Bart Leetch View Post
    Here is one I built before I closed the top.
    Thanks for that in process example Bart!
    Quote Originally Posted by Toni Ciuraneta View Post
    Apart from all that has been said, the most important role of a torsion box on a woodworking shop is to provide a light but very stiff structure to give you a completely flat and level surface of reference to check and make your assemblies adn glue ups.
    " I think you express the great value for a woodworker Tony! Thanks"

    Of course all this provided that you have built it perfectly flat. If you've built it twisted, it will be stiff but twisted, hence useless.

    Check at www.thewoodwhisperer.com for a tutorial on how to make one.
    Hi Folks ,
    I would like to thank all of you for your consideration, in posting your reply. They have each had their own merit and are appreciated.
    "And here I thought I could get a flat table by just adding more sheets of MDF! "
    Shaz
    I am a registered voter and you can be too. We ( registered voters ) select the moderators for this forum by voting every six months for the people we want to watch over this family forum.
    Please join me. Register now.
    Shaz
    Here is how

  10. #10
    Join Date
    Nov 2006
    Location
    Villa Park, CA
    Posts
    1,407
    Shaz - let me see if I can answer the questions you posed to me.

    The making of the webs is a tricky problem. The problem is that you want the "surface" of the webs to be flat (the top and bottom). This means that you really need to "lock down" the whole thing all at one time. If you make the webs by using long pieces in one direction with spacers between them, and try to nail or glue the spacers before putting the top and bottom on, you'll almost surely wind up with several spacers too high or too low. Then when you put the top and bottom on, you won't get a good glue joint between some of the webs and the top and bottom (which is the strength of the box), and/or the top and bottom won't be flat.

    Because of this problem many people do the "half notch" technique. The mating of the half notch is not critical - meaning if you cut the notches too deep (so that there will be a hole when the two are mated) - it doesn't matter much. It's like having an I-beam with a hole drilled in it. It doesn't affect the strength very much.

    But the width of the notch is critical. When you slide the two pieces together, they should be snug.

    Assembling a torsion box should be done all in one glue-up so you'll need a slow glue. Lay the grid on the bottom and mark where the webs fall. You'll put a bead of glue along these lines. Put glue on all the notches and assemble the web. Lay the bottom on the flattest surface you have - if it's a small box, use your table saw. Run a bead of glue along the lines on the bottom. Lay the web on the bottom. Now run a glue bead along the top of the webs. Put your top on and glue down, using cauls to clamp the middle. Using a flat surface when gluing is critical - the box will take the "shape" of the surface you use to glue against - if that surface is twisted, your torsion box will be twisted. Check the surface you use to glue against carefully to make sure that's the surface you want on your torsion box.

    Once you have a good torsion box, you can use it to make others. Use it as the flat bottom to glue up the others.

    I didn't specify earlier, but the best material to make a torsion box out of is plywood. It's strong and you can glue it together at the notches (there's not an end grain to face grain problem like you'd have with solid wood).

    Regarding using 3/4" plywood for the webs, one of the main reasons for making a torsion box is to reduce weight. Basically, you're building an engineered structure. 3/4" plywood for the webs will work but the resulting box will be HEAVY. The majority of the weight in a torsion box is in the webs, which is why you want to use the lightest material possible in there. You need to determine the strength you will need, and remember that most of the strength is in the top and bottom, not in the webs. I've been thinking of making my next box with 1/8" tempered hardboard for webs, just to reduce the weight.

    One extra thought - make sure you cut all your webs at one time with one setting of the table saw. If you screw up and have to cut more after you changed the table saw setting, set the saw cut a bit smaller and run ALL of the web pieces through again. They need to be exactly the same height.

    Mike
    Last edited by Mike Henderson; 05-25-2008 at 12:18 AM.
    Ancora imparo
    Go into the world and do well. But more importantly, go into the world and do good.

Similar Threads

  1. Torsion Beams - Short
    By glenn bradley in forum Jigs and Fixtures
    Replies: 1
    Last Post: 11-21-2013, 03:58 AM
  2. Torsion Beams - Assembly "Table"
    By glenn bradley in forum Jigs and Fixtures
    Replies: 18
    Last Post: 02-24-2013, 10:19 PM
  3. can i skin a torsion box table with only 1/4 inch masonite?
    By allen levine in forum General Woodworking Q&A
    Replies: 13
    Last Post: 10-13-2011, 11:50 PM
  4. Torsion box assembly table-what size?
    By Allen Bookout in forum Jigs and Fixtures
    Replies: 6
    Last Post: 04-17-2010, 10:26 PM
  5. Torsion box question. Now progress reports.
    By Jim O'Dell in forum Carpentry and Construction
    Replies: 48
    Last Post: 08-06-2007, 02:04 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •