torsion box question

[Pat Meeuwissen]

I have just finished watching the saw whisperers you tube video about building one of these for an assembly table. I also read several pdf's that say it does not really matter what the center material is as long as it is level and consistent. To come to my question, if this is the case could I not then offset the interior divisions like carpenters do when they install fire blocking? I've attached a really rough quick drawing of what I'm thinking, would do this to be able to nail from either side fully.
My other thought was why not half lap the joints, is there a reason not to do this? I would think that a much larger glue surface could be created this way. Thanks for any help. I have ordered more essential wood working from the library, since this had a bunch of great info about creating these types of box structures. Pat

[Jamie Buxton]

Yes, you can offset the spacer material with no impact on the torsion box stiffness. And yes, you can make the spacer grid with half-lap joints. In fact, you can make the half-lap "joints" completely sloppy -- with the two crossing ribs not touching each other at all.

[Joe Chritz]

Jamie hit it on this one. Remember the glue surface on the top and bottom of the center material is what gives the strength (along with the characteristics of the skins) and not the half lap, etc.

Just start flat and end flat and all is good.

I have two of them and they are very handy indeed.

Joe

[Pat Meeuwissen]

Joe, do you have a link to your build of the ones you built? Pat

[Erik Christensen]

I setup my dado blade for width of the spacers, clamp them together on the sliding table and cut all x then y at once - spacing does not have to be dead on - you just need the right number of slots for the number of right angle pieces - make the length < 1/4" or so short - they do not have to butt up to the outside edge pieces

get the 2 top & bottom panels sized, cut your outside ribs accurately - here is how I assemble it with no musss nor fuss

I use a pin nailer to assemble the outside ribs in a square over one of the panels - just nail them to each other - put your internal ribs in with your half-laps - just run a big old bead of yellow glue on the top of each rib - put the other panel on top and pin each corner - have a helper assist you in flipping it over - take the other panel off - run a bead a glue on the rib tops - pin the other panel back - pop into a vacuum bag and let it rip for 30 min

you are done

[Pat Meeuwissen]

Jamie, wouldn't you want to keep a little tension on your halflaps to allow the glue to grab? I understand the concept of the glue on the top and bottom being key, but would the crossing points not also add some stability? Pat

[Jamie Buxton]

....I setup my dado blade for width of the spacers, clamp them together....

That's pretty much how I do it too. However, I get nervous about putting clamps on the stack of ribs and running it through the saw. Somehow big hunks of metal near table saw blades seems like a problem waiting to happen. Fortunately, there's a really easy alternative: saran wrap. There's saran wrap stuff used for packaging. (For example http://www.staples.com/Staples-Extended-Core-Bundling-Stretch-Film/product_SS1007931?cmArea=SC1:CG16:DP1808:CL141878) I wrap the stack of ribs with the stuff. It keeps the ribs in a stack, and I can even cut right through the wrap with the blade. No scariness, and easy to use.

[Jamie Buxton]

Jamie, wouldn't you want to keep a little tension on your halflaps to allow the glue to grab? I understand the concept of the glue on the top and bottom being key, but would the crossing points not also add some stability? Pat

It isn't necessary. I view the half-laps as clearance cuts, not real joints. I don't expect the crossing ribs to touch, and I don't put glue in there.

[Tom Veatch]

Pat, what you've shown is perfectly acceptable in a thick skin structure that's subject to unidirectional loading well below ultimate. The internal members do little more than stabilize the skins and resist shear. Their contribution to bending and torsional stiffness is negligible compared to the skins. Virtually all bending and torsion loads are carried by the skins.

[Pat Meeuwissen]

Tom, I really appreciate the input but could you translate that into somethin' a good ol' boy can understand?:D
As one more point of information this will be installed over a couple of sawhorses with no other support, would this make any difference in the way this is built and material used? Pat

[rick carpenter]

Pat,

What are they for exactly that you want/need the big open area? That may give the skins some up and down motion that you don't want. I built a torsion box for my mobile cms. Five foot by 18 inches, 1x3 poplar and baltic birch ply, glued and screwed with no lap joints. I put in spacers where the mounting bolts went through so I could tighten down the base securely without warping the skins in that area. It's supposed to be detachable, but with the motor off and a handle on the side, the box/base is easily transportable.

[Tony Bilello]

The direction of the solid pieces usually run the long direction which is opposite from your sketch. I dont know if this is an error or modified for your particular use.
When I have made large torsion boxes in the past, I dado'd the bottom sheet of MDF to help line up the long strips. The spacers were cut shorter in height so that all of the tops lined up. By doing this I could line up my pieces with glue under them, set them in place and and then add the top without anything sliding. Then brad nail the top and slowly slide the whole thing off my workbench brad nailing the underside as I went along. This allowed me to make the entire torsion box top by myself all in one sitting.

[Joe Chritz]

Tom, I really appreciate the input but could you translate that into somethin' a good ol' boy can understand?:D
As one more point of information this will be installed over a couple of sawhorses with no other support, would this make any difference in the way this is built and material used? Pat

When the box tries to sag it has to compress the bottom skin and stretch the top skin. That is why they are so stiff and the webs really don't add much in the way other than grab the skins and keep them apart.

Joe

[Steven DeMars]

This month's issue of SHOP NOTES, Volume 19, Issue 112 has this very topic very well covered . . .

Steve:)