Torsion Boxes

[Mike OMelia]

I have a need for two torsion boxes, 2'x4' and hopefully only 3" or so thick. The use is in the guitar assembly area where the two boxes will be separated by about 3 feet. Flexible rods (go-bars) are used to press the back or top to the sides (with glue of course). Just using layers of plywood for the two surfaces is normally inadequate (the flex and bow, releasing previously installed bars!) PITA.

I would like to make this as light and inflexible as possible. Typical forces after bars are installed is 200-300 pounds of force! So if I can build this light in weight, I want VERY little deflection under that kind of load.

BTW, each "go-bar" (1/4" diameter carbon fiber rods, used in the kite flying hobby) exerts a maximum af about 7 pounds force. I can use as many as 30-40 to secure a back or top to a guitar side.

Yes I have heard of Ian Kirby. No access to his stuff. I was thinking 1/2" plywood skins and 2" tall spacers, no-lap method using crown stables, brads and lots of tightbond glue. Can I build it lighter? (thinner skins)? My approach ok?

Mike

[Jamie Buxton]

Here's what you could do: 1/2" ply "front" face (the one that the go bars bear on), 1/8" ply "eggcrate" for the spacer grid, 1/4" ply for the back face.

Virtually all of the stiffness of the torsion box comes from the skins, not from the spacer grid. So using thinner materials for the grid reduces weight without reducing stiffness.

To make the grid with thin material, make each slat go from edge to edge of the face. Slot it at each intersection with the crossing slots. The slots are just clearance, not real joints. (I wrap the slots together with saran wrap and gang-slot them on the table saw.) When you slot all the slats together, you'll have a floppy eggcrate sort of thing. Floppy is okay. The grid's only purpose is to space the two skins apart from each other.

[Rich Engelhardt]

Have you considered a hollow core door or two?

They are amazingly strong.

I have 7 old ones right now - I just replaced all the old builder's grade doors in the house with 6 panel oak ones.
I'm trying to think of some good use to put them to - other than making a big storage cabinet out of them.
An assy table is high on my list of uses.

[Mike OMelia]

Thanks for the assembly idea Jamie!

Rich, I have thought about the doors... I would have to cut one down to size... would that be ok?

Mike

[Mike OMelia]

Wow, Lowes carries solid slab Luan doors (hollow core) for $24.

Flatness is not a goal. Stiffness is. You think if I cut one of these in half I'd have a good system? Minimal flex under 300 pounds or so.

Mike

[Rich Engelhardt]

Mike,
I use hollow core doors a lot on rehabs. There's always a couple of doors that are going to be replaced, so, the old ones are the first thing down.
I use them all through the rehab as a tool station for a miter saw, router table - whatever - assy table, drafting table, lunch counter, seat - you name it.

I've also had to cut a few of them down to size and rip a section of material to fit in for support.
It's 100% doable to fit in a section of a cut down hollow core door.
You'll need a digital caliper and a good fence on the table saw and some trial and error - but - it can be done.

Rather than Lowes or HD, check to see if there's a Restore (Habitat for Humanity store) in your area or check CL for the "free stuff".


Another idea would be a modification of the Shop Notes portable work surface.
http://www.shopnotes.com/issues/98/

Down near the bottom.
It uses I beam construction and a 1/2" top. In the magazine (Issue 98) they have a picture of three or four guys standing on it.
I made one using a pegboard skin and added some cross braces underneath that fits across a couple of saw horses.
Mine weighs all of about 10 pounds & is strong enough for me (250#) to walk on.

[Jim O'Dell]

I'd think the solid core door would be fine. Being that thick (1 3/8") it should be plenty strong. Jim.

[Mike OMelia]

Thanks Rich, Given that Lowes is 3 miles from me, I think I will just go get one. Seems a reasonable and inexpensive experiment. Building my own torsion box will be more expensive. I think I will coat the sides exposed to the pressure points of the flexible rods with something to add high pressure point protection. So, the interior of these doors is just a cardboard honeycomb?

Jim, the solid core door would be too heavy, expensive, and a waste for my application. At least, that is how I see it...

Mike

[Darl Bundren]

It might be that if you cut a hollow core door, you may decrease its structural inegrity. In the past when I've removed doors, the surfaces sometimes get really flexy once the outer frame is cut.

[Alan Schaffter]

As Jamie said the flex strength comes from the web- it goes up by the cube so taller is better. That said, most hollow core doors have very thin skins and short webs made from cardboard. I would be concerned about local deformation. Remember a door has almost no surface load- the reason they use a torsion box structure is so they can make it cheap and light, and so it will stay flat. Also, as Darl pointed out, if you cut the door more that just trimming a little off you will need to clean the web back from the exposed edge and glue in a new solid edge piece.

It is worth a try.

If it doesn't work, try making one from 1/4" or 3/16" hardboard. I made the one below for an article on torsion boxes that was published last year in the Dec/Jan Issue of American Woodworker. It is incredibly easy to build with a web made using "halved joints" (half-laps cut edgewise) which are easily cut on the tablesaw using a box-joint-like jig (second photo). The 8' long X 18" wide torsion box in the first photo was made with 3/16" hardboard (the cheap fuzzy-backed stuff) and yellow glue. In the photo it is holding over 350 lbs at mid span, yet deflecting less than 1/2" there! It is incredibly light- less than 30 lbs.