Torsion Box build.. What did I do wrong?

[Greg Woloshyn]

I just finished building an MDF torsion box as designed by David Marks and followed the exact procedure for making one, but ended up with a top that is bowed a total of 3/16". What I did to make my flat surface to start with was build a series of MDF I beams, since I don't have a jointer. These were placed on a workbench, leveled and shimmed so they were all on the same plane. Then I laid a sheet of MDF on top and confirmed flatness. After ripping all my pieces for the gridwork at the same time and gluing everything up, I flipped it over and added my MDF skins.

It seemed fairly flat at that point, but when I transferred the box to the top of my cabinet it is now very warped. What did I do wrong?

Picture 069 (1000 x 750).jpgPicture 070 (1000 x 750).jpg

[Shawn Pixley]

I am reluctant to comment on this because the last torsion box post I commented upon, others were unhappy that my viewpoint didn't agree with theirs. But here it goes. I think the problem may lie in the I beams. By shimming you got a flat surface on the "top" but you don't know that the bottom plane is at all flat. If the I beams were not perfectly true and small inperfections were added with the webs/gridwork, you can end up as you did. This might or might not be the problem but would be my first choice given the information available.

[Greg Woloshyn]

I am reluctant to comment on this because the last torsion box post I commented upon, others were unhappy that my viewpoint didn't agree with theirs. But here it goes. I think the problem may lie in the I beams. By shimming you got a flat surface on the "top" but you don't know that the bottom plane is at all flat. If the I beams were not perfectly true and small inperfections were added with the webs/gridwork, you can end up as you did. This might or might not be the problem but would be my first choice given the information available.

I think your right. My problem does lie in the I-beams because I checked them and they turned out to be bowed at the ends. I thought for sure this type of setup would be strong enough to resist any movement but I guess I was wrong.

At this point I'm probably going to scrap the entire torsion box and start over. In the videos I saw jointed 2x4's were used, but don't those tend to move as well? Or will I have to work fast and hope for the best?

[Jim Matthews]

Please elaborate on the construction.

How far apart are each of the "ribs"?
In the David Marks design, he mentions that they're just far enough apart to
fit his pin nailer; in passing he stated that the smaller each box section is, the stiffer the torsion box.

If the box is drooping or sagging - that sounds like the culprit.
If the box is rigid, but out of plane - you should be able to skim on a final surface to get back to flat.

I'm thinking that a skim coat of Durham's Rock Hard with a screed made out of an aluminum level would serve.

http://waterputty.com/

[Andrew Gold]

I've made torsion boxes in the past with plywood, never tried mdf or solid sawn lumber. I would think mdf would run about as easily as plywood, no real reason why not at least.

I would have fear the "I-beam" glueup is a step that would introduce error, even a little bit could skew the final steps. I've always ripped thin strips of 3/4" ply to a bit wider than I want the final torsion box to be, face glued two of them together (with cauls and the like), then ripped the glued strips to finished width and assembled the box. Using a table saw you can easily ensure the last step before the final assembly you have even strips, very little chance for error...

[Art Mann]

I'm quite surprised that the I beam construction didn't work. Were they not stiff enough to start with or was it an issue of constructing them to be flat and straight. Did you glue the "I" together or just count on fasteners? The reason I am asking is I have constructed beams of similar dimensions to yours out of 3/4" plywood and used them to stretch between two saw horses that spanned about 6 feet. They supported a step ladder on a piece of 3/4" plywood while painting high interior walls. They were stiffer and lighter than 2 X 4s would have been. I got the idea and plans out of a woodworking magazine.

The same principle of dynamics that makes a torsion box so stiff is also what makes an I beam stiff. I don't see how using 2X4s to support the construction would be any better unless you have some way to make them flat when on edge.

Edit: Andrew, it was my understanding that Greg used the I beams to support the construction of the torsion box and not as a part of the box itself. If that is not the case, then I can see how that would complicate assembly and might introduce error as you say.

[Greg Woloshyn]

Lots of good information here. My torsion box grid was made 6 inches apart, with staggering sections so I could fit my nailer in, glued as well.

The I beams I made were NOT glued, just brad nailed. This could be why my beams shifted out of flat when I setup all the weight of the MDF on top of them. As you see in the picture, I used 2 additional beams underneath the longitudinal beams to start with - these remained flat. These two beams were spaced about 5-6 feet apart, with nothing to support the main beams in the middle.

I guess now I will either use the putty method as mentioned above, or start over with a more rigid setup. Possibly ripping plywood strips and face gluing together.

[Jim Matthews]

I'm disappointed for you.

I thought your preparation was well thought out, and can't see
anything I would have done differently, myself.

I'm contemplating one for my garage space and follow your thread closely.

[Pat Barry]

I just finished building an MDF torsion box as designed by David Marks and followed the exact procedure for making one, but ended up with a top that is bowed a total of 3/16". What I did to make my flat surface to start with was build a series of MDF I beams, since I don't have a jointer. These were placed on a workbench, leveled and shimmed so they were all on the same plane. Then I laid a sheet of MDF on top and confirmed flatness. After ripping all my pieces for the gridwork at the same time and gluing everything up, I flipped it over and added my MDF skins.

It seemed fairly flat at that point, but when I transferred the box to the top of my cabinet it is now very warped. What did I do wrong?

Picture 069 (1000 x 750).jpgPicture 070 (1000 x 750).jpg

I think we are assuming that the warp is along the the length of the torsion box and not across it. Could you confirm that this assumption is correct?

I like your I beam. That whole process seems a great way to go as you described it. Of course, I am also assuming that you clamped the torsion box assembly to these I beams as you built it up to keep the flat I beam reference. What this doesn't do however, is to give you the reference of flatness side to side, I could see the result being a twist or cup laterally rather than longitudinal. In order to minimize that element, it seems lateral I beams are in order as well.

[Bradley Gray]

I think the flex is from the brad nailed flanges. The image in the OP does not show any cross members in the grid. How were they installed in relation to the flanges?
I have only built torsion boxes from plywood and did not use flanges. I think the skins become the flanges in this type of assembly.

[Greg Woloshyn]

I think we are assuming that the warp is along the the length of the torsion box and not across it. Could you confirm that this assumption is correct?

I like your I beam. That whole process seems a great way to go as you described it. Of course, I am also assuming that you clamped the torsion box assembly to these I beams as you built it up to keep the flat I beam reference. What this doesn't do however, is to give you the reference of flatness side to side, I could see the result being a twist or cup laterally rather than longitudinal. In order to minimize that element, it seems lateral I beams are in order as well.

My side to side flatness was obtained by using 2 I-beams underneath the longitudinals as shown in the picture. I believe that since I didn't glue the flanges but just brad nailed them, they flexed under the weight because in the middle, there is no lateral I-beam to support it.

I did not clamp anything when I assembled the torsion box. The first skin of MDF was placed on top of the I-beams, then pushing down each piece of the grid and gluing/nailing them in place.

How about I build more lateral I-beams underneath the main ones to better support the load, and glue and nail the beams together this time.

[J.R. Rutter]

The I beams I made were NOT glued, just brad nailed. This could be why my beams shifted out of flat when I setup all the weight of the MDF on top of them. As you see in the picture, I used 2 additional beams underneath the longitudinal beams to start with - these remained flat. These two beams were spaced about 5-6 feet apart, with nothing to support the main beams in the middle.

How about I build more lateral I-beams underneath the main ones to better support the load, and glue and nail the beams together this time.

That's the ticket!

[Greg Woloshyn]

A disadvantage to the I-beam design would be any deviation from an exact 90 degree angle when the flanges are put on will cause trouble with side to side flatness. Maybe I should rip and face glue strips of plywood instead.

I decided to go with I-beams because of strength, and also to keep them standing straight.

[Greg Hines, MD]

I would agree. For making I-beams, I was always taught to dado the ends, so the beam fits into it for lateral structure, and lots of wood glue, in addition to screws. However, the members crossing your bench seem to be a big part of this problem, and are likely the cause of your sagging. One of the things that Marks did in his video (as I recall) was to use sawhorses that spanned the width of his build up. I think that is where your problem lies.

Doc

[scott vroom]

For what its worth, here's The Wood Whisperer's torsion box video.

http://www.thewoodwhisperer.com/vide...e-torsion-box/