MDF torsion box..what fasteners?

[Jim O'Dell]

I'm going to build 2 10' long 14" wide 3" tall torsion boxes for my multistation tops out of 3/4" MDF. What should I use to fasten the pieces while the wood dries? I don't want to shoot anything through the top that I'll have trouble with when I go to put the laminate surface on. Don't cared so much about the bottom. Just toe nail some brads at an angle from the inside? Same from the outside bottom piece...shot in at an angle? I'' be able to use 2 other sheets of the same size MDF as weight for the curing time, and the first finished torsion box as weight for the second one. TIA!! Jim,

[Tyler Purcell]

Yellow glue and brads should get the job done just fine.

[David DeCristoforo]

If you are going to use MDF under PL, there is no need to worry too much. Use countersunk screws and glue (plenty of both) to attach the top. Then fill the screw holes with Bondo, sand flush and slap down your PL.

[Steve Aiken]

Have you already purchased the supplies? If not, I'm wondering if you are overbuilding it with 3/4" MDF skins. Check out a hollow core door -- a wonderful torsion box. It's made with cardboard strips inside and less than 1/8" skins. You can probably make this with 1/4" skins or 3/8" if you really want to beef it up. You could use 3/4" plywood or MDF for the webbing -- it's more than is required, but it will give you (a) lots of glue surface, and (b) lots of room to make sure the brads actually make contact with the webbing strips inside.

Make sure we get to see pics when you're all done.

Steve

[Jim O'Dell]

Thanks guys.
Yes Steve, a hollow core door is a torsion box, but you can deflect it just pushing in on the skin. Not enough there to be rigid, at least not as rigid as I want for what I'm doing. I thought at one time about using 1/2", and making 2 layers on top, one on the bottom, but then I came to my senses. Jim.

[Andy Haney]

FWIW, I constructed 3' x 5' TB, all from 1/2" MDF. The "web" was assembled @ 6" o.c. Then I glued a hardwood perimeter on. I did the whole this with glue, no hardware.

I found a flat(ish) spot on the concrete floor;
laid down one face;
assembled the "grid" on top of that surface (no glue);
glued the top surface over that assembly, held in place by sand tubesuntil glue dried;
flipped 'er over; and
glued the other face (the first one down).

Worked well for me.

Andy

[Mike Henderson]

That's going to be very heavy if you make the ribs from 3/4" MDF. 3/4" MDF would be good for the skin but you can make the ribs from much thinner stock and still get plenty of strength. I'd use 1/8" hardboard for the ribs and 3/4" MDF for the skins. You can use 3/4" MDF for the outside ribs where they might get hit.

The strength comes from the depth of the box, and how well you glue the ribs to the skins.

Good luck in whatever you choose.

Mike

[Rick Lizek]

Been making torsion boxes for years based on Ian Kirby's article in FWW from over 25 years ago. He pioneered the idea. He recently did a rewrite of the same article in American Woodworker.
You don't need to use 3/4" for the skins. The strength is in the height of the core materials. !/4" skins are plenty strong and the extra height of the core will be stronger in the long run. Staples are the best bet to hold the core together. The staples are plenty strong enough to keep the core together untill you glue the skins on. No need to use glue on the v\core pieces either. The glue will be plenty to adhere the skins to the core. I'm speaking from practical experience, not theory. The MDF core will be fine and 1/4" luan or hardboard will make a tremedously strong torsion box for your work tables.
http://www.amazon.com/Woodworkers-Es...3462799&sr=8-1
Ken horner's book and Ian's article will tell you everything you need to know about torsion boxes. Kirby has the foundation and Horner has the mathematical aspects covered to explain how it works.

[Phil Thien]

No need to use glue on the v\core pieces either. The glue will be plenty to adhere the skins to the core. I'm speaking from practical experience, not theory.

So you can assemble the core w/o using any glue, and just use the glue between the skins and the core? Am I getting that right?

Any comments on thickness of core material?

Maybe I should get that article.

[randy street]

Jim,

Here is one that David Marks did, 1/2" for the ribs (glue and brads) and 3/4" for the top & bottom.

http://www.diynetwork.com/diy/ww_mat...278182,00.html

Randy

[Rick Lizek]

So you can assemble the core w/o using any glue, and just use the glue between the skins and the core? Am I getting that right?
Yes...just glue for the skins is all you need. The great thing about torsion boxes is the strength and the speed to make them. Smaller grids in the core material also make it stonger. The other nice thing about making them this way is you can easily make round and odd curved shapes relatively fast. Folks that insist that one must notch the frame material are making a simple thing unnecessarily complicated. The staples are countersunk in the core material. One only needs to hold the core material in place until the glue sets with the gluing of the skins. Do both skins at the same time.

Any comments on thickness of core material?
!/4" skins are plenty strong. The strength of the core box is the height of the core material. 1/4" skins and 2-1/2" core material is stronger than 3/4" skins and 1-1/2" core material. I made a torsion box with 1/4" luan skins and 3/4" mdf core that got stored in a damp garage for 5 years and it is still as flat as the day I made it. If you need to screw legs or something to the torsion box add some built up blocking to key areas.

The torsion box is based on aircraft wing constuction.

Maybe I should get that article.

Get Kirby's article...FWW from 25 years ago so it might be in a reprint or someone will have it archived. The article in American Woodworker was from last month so it shoulf still be around. Get Ken Horner's book that I referenced in a previous post. Great book!

[Matt Benton]

I have so far made 2 torsion boxes, the last one just finished for my contractor saw and router/extension table.

I used (3) 2X4's lengthwise on end and a few pieces across for the ribs and 3/4 mdf for the top and bottom, screwed and glued. It is 6 feet long on 5" casters.

My saw now feels like a cabinet saw, even on the locked casters. Cheap, easy and quick.

[Jim O'Dell]

WOW!! Lots of information! Thanks!!!!! If I use 3/4", I can get both sides done with 2 sheets of MDF. If I use 1/2", then I'll need 3 sheets. But I'll look into that, especially with the strength coming from the height of the ribs, whick makes sense, since there would be less deflection on taller ones. I'll rethink this. Was going to go get MDF after work today, but just found out this am that the other guy in the office is not here today, and failed to notify me. In fact 2 days this week I get to work his late nights as well as mine. So with company here from out of town, I may bypass that for today and rethink what I'm going to use. The weight doesn't bother me.
New question: What would be best to use on the bracing? If I use the 1/2" skins, then my braces will be max 2" tall. Would 1/2" MDF or Plywood be best? Continued thanks for the information! Jim.

[Rick Lizek]

These are work tables right. I'd go with 1/4" skins with 2-1/2" MDF frames. I gather this is your first torsion box. If you use 4" or 5" grid spacing and 1/4" skins you will swear the the top is thicker than 1/4" if you rap it with your knuckles.
Don't forget this is aircraft technology. Think about a wing with dope covered canvas supporting thousands of pound stress in flight fighting gravity and heavy wind currents.

Find the article by Kirby and read Horners book. Why reinvent the torsion box? Sure Marks used 3/4" skins but for an assembley table it's overkill. The math proves the thick core along with the gluing of the skins to the frame is where the strength is from.

[Mike Henderson]

Just an added comment. I agree with an earlier poster who said that thin skins will provide plenty of strength for the torsion box. The problem is attaching tools to the box - if you use thin skins, you pretty much have to use bolts that go completely through the torsion box, and may have to use a backer board underneath to spread the pressure. If you use a thicker top skin, you can screw into it to hold your tools. But if you're not going to fasten tools to the torsion box, that won't make any difference.

The thing that will make the most difference in the strength of the box is the depth of the ribs - how thick the box is. To a large degree, the spacing between the ribs is chosen to limit the deflection of the skin between the ribs (the thickness of the skin affects this also).

I use torsion boxes for veneer pressing - where I have to handle the boxes to put one on top for pressing so the weight means a lot to me. An all MDF torsion box can be surprisingly heavy.

Mike