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Thread: Torsion box question

  1. #1
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    Torsion box question

    Pardon if this question has been asked before.

    I have watched a bunch of videos on YouTube recently about building a torsion box workbench top. Various kinds of tops, various materials. It seems like most of them are more-or-less slapped together with little consideration for the concept that a torsion box is meant to be FLAT. Dead-flat without sag, bow, warp or any other malaise which would throw off subsequent construction. It should be a reference surface. The concept is there, the capability to produce such an end product is available, but the method leaves me shaking my head and wondering how it's possible. I never see any straightedges, winding sticks, or even a good sightline.

    So the question - what does one do to make a torsion box flat? It seems to be an offshoot of the adage that you need a workbench to build a workbench; similarly you should have to use a flat reference surface to build a flat reference surface, no? I can't trust a concrete floor to be flat; I know for certain it isn't. I can't trust a 27" x 40" tablesaw top when my torsion box is four times that size and will sag just from it's own weight.

    Or, is it just not worth agonizing over and I should just hope for "flat enough" and be done with it?
    Jon Endres
    Killing Trees Since 1983

  2. #2
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    David Marks built a torsion box in one of his WoodWorks episodes and took the time to make sure his temporary work surface was flat. If I get a chance this evening, I'll try to figure out which episode it was.

    EDIT: The Google made it easy. Season 4, episode 409.
    Last edited by Charles Taylor; 06-14-2016 at 2:08 PM.
    Chuck Taylor

  3. #3
    I think the Wood Whisperer video also shows setting up a flat surface.
    Gerry

    JointCAM

  4. #4
    I set up a couple sawhorses and adjusted with winding sticks to make parallel, then add a few 2x4's and you have a flat surface to work off of.
    Put the top sheet on them upside down and proceed.

  5. #5
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    I have watched a bunch of videos on YouTube recently about building a torsion box workbench top. Various kinds of tops, various materials. It seems like most of them are more-or-less slapped together with little consideration for the concept that a torsion box is meant to be FLAT. Dead-flat without sag, bow, warp or any other malaise which would throw off subsequent construction.
    Seems you have been looking into unreliable places if they are not first and foremost going through the reference surface steps on which all else will be built. Delete those bookmarks, if any as they will only take you down a road to a poor result.

    The Wood Whisperer, American Woodworker and others have good examples. These examples include methods for establishing a flat reference to work from. Generally supports such as saw horses are topped with true, jointed ribs that can be evened up via winding sticks or long straight edges. True, if you do not have something that is long and true or are proficient with winding sticks you are ill prepared to begin.

    A long builder's level can help and comes in handy for other things. Winding sticks can be shop made and do a fine job of confirming or leading you to a flat platform (or row of ribs on saw horses) from which to begin. If this will just be for assembly, you're all set. If this will double as a work surface remember to add solid areas for dog holes or vice mounting options.
    "A hen is only an egg's way of making another egg".


    – Samuel Butler

  6. #6
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    You're right: you have to pay attention to make a flat torsion box. Here's how I've done it without a prior flat surface...

    The torsion box consists of two skins on the outside, and a spacer thing in the middle. The skins might be quarter-inch plywood. There are many ways to make a spacer grid, but I do it with plywood. Eighth-inch plywood works just fine. Rip the plywood into straight strips, perhaps 5-6 inches wide. Pay attention to make the strips not curved, and not wedge-shaped. When I'm ripping dozens of strips off a full sheet of plywood, I've had the cut line wander away from ideal. You're going to assemble the strips into a grid structure. Every strip of plywood is going to run from one end of the torsion box to the other, or one edge of the torsion box to the other. To get that to happen, cut slots half-way through each strip at each junction. The "joint" could be called a half-lap, although it isn't really a joint. It is just clearance so the strips can get past each other. I gang-cut the slots. I wrap bundle of strips with saran wrap so they all stay together, and then cut the slots, cutting through the saran wrap where necessary. The great thing about gang-cutting is that all the slots through all the strips are aligned, so the crossing strips can go right through. Assemble the strips into the grid. It will be quite floppy, and that's okay. Put one skin down on something flat-ish. Maybe that flat-ish thing is a sheet of 3/4 ply on several sawhorses, shimmed a bit if your shop floor is really out of whack. When I say flat-ish, it only needs to be flat enough that when you put the spacer grid down on it, you can get the grid to conform to the bottom skin without too much trouble. Maybe you need some weights at various places around the grid, or maybe just another sheet of 3/4 plywood. Mark where you need glue on the bottom skin, and pick up the spacer grid. Apply glue, put the grid back down, put on those weights, and wait for the glue to dry. After the glue dries, you'll have a structure with one skin plus the spacer grid. You'll find that it doesn't cup much, but it does twist very easily. You must remove any twist before you glue on the second skin. Stone masons have a trick for seeing twist: crossed strings. Run a string from one corner of the assembly to the opposite corner, and a crossing string from the other two corners. I screw one end of each string, and add a weight to the other end to keep it in tension. Near the assembly corners, add shims under the upper string. The shims are the thickness of the lower string. If the assembly is untwisted, the strings will just kiss where they cross. Shim an assembly corner until they kiss. Remove the strings and glue on the second skin. Done.

  7. #7
    I think this ^^ is over complicating the construction.
    The error of not having a dead flat surface upon which to begin the assembly will transmit though the entire project, and therefore requires the use of strings and tweaking at the end. Totally unnecessary.

    It is simply a matter of glue the strips on and brad nail the cross pieces (1/2 laps are not necessary).

    Allow the bottom and torsion assembly glue to dry, then turn over and apply top the same way.

    IMO MDF is a better material for the bottom and/or top.

    On mine, I used 1/2" MDF for bottom and 3/4 for the top.

    To Jon I would say check Marc Spagnolo's video on WoodWhisperer. Its very similar to the way I described above.
    Last edited by Robert Engel; 06-14-2016 at 6:49 PM.

  8. #8
    Quote Originally Posted by Robert Engel View Post
    I set up a couple sawhorses and adjusted with winding sticks to make parallel, then add a few 2x4's and you have a flat surface to work off of.
    Put the top sheet on them upside down and proceed.
    This is how I built the two torsion beams I have. I joined the 2x4's first, aligned them into a single plane and set the 3/4" MDF bottom on top to start construction.

    Start with a flat reference.

    My set make a portable workbench that was put into service 8 or 9 years ago. 3/4" MDF top and bottom, 3/4" plywood skins. No glue on mine, just screws. In and out of the truck, on and off jobsites and still flat.

  9. #9
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    Here's the Wood Whisperer video, fast forwarded to the part you should be interested in:


  10. #10
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    I was recalling the David Marks video also.
    NOW you tell me...

  11. #11
    I recently finished a torsion box work table primarily following Spagnolo's video. The reference surface is the key and does take some time to set up but well worth the effort. I got everything perfectly level then hot glued everything together and to the floor. I opted to use half lap joints which I think is easier than a pile of pieces to put together. First thing I built was the bottom of the table on my dead flat top - should have done this long ago.

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    Last edited by Harry Holzke; 06-15-2016 at 1:56 PM.

  12. #12
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    Thanks, guys, for all of the helpful advice. I watched the Wood Whisperer video on the torsion box construction and it includes the important information about setting up your reference surface. I have watched several of his videos but never that one.

    I've also watched several videos on the "Paulk" style workbench, which seems to be a modified torsion box - I like the concept because I have a relatively small space to work in and I like the idea of storage underneath the top. "Multi-tasking" as my wife would say. I now see how I can use the same construction techniques to build one of those type surfaces just as flat (or close, considering the materials).
    Jon Endres
    Killing Trees Since 1983

  13. #13
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    Quote Originally Posted by Jon Endres View Post
    Thanks, guys, for all of the helpful advice. I watched the Wood Whisperer video on the torsion box construction and it includes the important information about setting up your reference surface. I have watched several of his videos but never that one.

    I've also watched several videos on the "Paulk" style workbench, which seems to be a modified torsion box - I like the concept because I have a relatively small space to work in and I like the idea of storage underneath the top. "Multi-tasking" as my wife would say. I now see how I can use the same construction techniques to build one of those type surfaces just as flat (or close, considering the materials).

    While I think a torsion box would make a usable workbench top, it's not going to be very massive. Weight is a good thing in a workbench, especially for hand-tool work -- it absorbs the punishment and stays put. If hand tool working is not your thing, then it's not so critical. But if you want to do some planing, chiseling and/or sawing, you should consider the more traditional laminated top of three inches or more in thickness. Believe me, you'll appreciate that mass. Pick your wood well, and orient the grain consistently during glue-up, and you'll have a surface you can plane perfectly flat as many times as you need to over the life of the bench.

  14. #14
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    This is going to be more of an assembly table/outfeed table/light-duty workbench. Primarily power-tool use, and it will also be on casters so I can move it around my shop. I would like to build a heavy hand-tool bench sometime in the future, there's a stash of 8/4 white oak waiting for it.
    Jon Endres
    Killing Trees Since 1983

  15. #15
    I built a 4' X 8' torsion box top assembly table. I carefully welded up a square fame and then simply built the top on the frame. No big deal but a very sturdy and flat surface that is an essential tool in my shop. Great storage space underneath too.

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