Torsion box moveable work surface

[Ted Sander]

Brand new guy here.
I've been primarily a model builder for many, many years, but have also done several larger "real" wood working projects. The big constraint has been space. I usually have to do the work out in the garage, preferably either with the door open or in the driveway for dust control. Being Minnesota, that limits me to about 6 mos. of the year. "Job site" type table saw on folding stand, miter saw, mid-size router (no table). When needed, my "bench" for those projects has been a partial sheet of ply on saw horses. Smallish basement room for the modeling workshop, with a 12" bandsaw, benchtop drill press. Currently in there is a 40"x88" bench ('tis as big as would fit in the space!) consisting of 2x4's cleated to a wall, with the long rails out into the middle of the room with legs at the ends. The top is two pieces of 1/2" ply. I've contemplated converting it to a torsion box style top for a long time, but never got around to it - a dead flat surface would be a huge boon to the modeling.
A broken pipe in my exterior wall and the associated insurance/contractor brouhaha has got us seriously thinking about replacing a very old basement wet bar, and on the other side of the wall, upgrading the laundry room.

Which means cabinets.
That would be new, but it seems within my abilities.

But now it is time to get serious about a bench.
I find the current indoor bench size just about ideal for my general uses. So am thinking about making the torsion box the same dimensions. I would remove the current ply tops, and just use the existing frame to support the box, but not fasten it in place. When the need arises, I would then like to lug it upstairs to the garage for the once-in-a-while bigger projects, supporting it on saw horses.
I've seen the Wood Whisperer videos, and seen most of the posts here about building one. While MDF sounds easiest/cheapest, the overall weight seems to be the killer - it could be up to 200-250 lbs! I also saw the videos for the "Paulk" portable bench. While along the lines of what I'd want, it doesn't seem to be a "true" torsion box - meaning it may not be as dead flat and rigid as it might otherwise be. So, I'm contemplating a hybrid - 1/2" birch ply for everything, following mostly the WW ideas - with lightening holes in the interior braces.
The other stumbling block is the base to build it on - the video assumes I've got access to a jointer/planer to run 2x4's through (which are used with saw horses, levels and winding sticks to serve as a flat base to build on top of)...which I don't.
So, the priorities:
Very rigid and flat. Will stay that way, even when well supported only near the ends for long periods (meaning my current bench rails sag a bit currently!)
"luggable" by one, or perhaps two (if I can talk my wife into helping).

Comments, links, tips, and suggestions for the design and construction are all appreciated!

Ted

[Jamie Buxton]

My extra table is a torsion box which can sit on sawhorses. The box hangs on a wall when it is not in use. The box is 72 by 42 by 4 1/4. The skins are quarter inch for ply. The spacer grid is made from eighth-inch plywood — door skins — on six inch centers. The box weighs 32 pounds. If I put the horses at the ends of the box, and a straightedge running the length of the box, I cannot observe deflection of the box when I stand on it. I’m sure I could see 20 thousandths deflection, and I think I could see 10 thou. That is, you can have stiffness and light weight.

[Ted Sander]

My extra table is a torsion box which can sit on sawhorses. The box hangs on a wall when it is not in use. The box is 72 by 42 by 4 1/4. The skins are quarter inch for ply. The spacer grid is made from eighth-inch plywood — door skins — on six inch centers. The box weighs 32 pounds. If I put the horses at the ends of the box, and a straightedge running the length of the box, I cannot observe deflection of the box when I stand on it. I’m sure I could see 20 thousandths deflection, and I think I could see 10 thou. That is, you can have stiffness and light weight.

Wow, that's perfect! Any tips on getting the spacers fit together, glued and fastened to the skins? I could try brad nailing the skins to the spacers...but that's a narrow target to hit.

[Kevin Jenness]

I used one for many years with a somewhat more substantial structure than Jamie's, but still light enough for one person to horse around, maybe 70-80 lbs. 5/16" x 3 1/2" basswood core on 4" centers with a 3/4" rim. 1/4" luan skins covered with p-lam. Clearly from Jamie's example thinner grid elements and larger spacing can be strong enough to resist overall deflection, but clamping in the field can bend the skins between the core pieces. A small test section with your proposed scantlings will give you an idea.

Waxed p-lam is easy to keep clean, but for minimum weight just finish the ply. I supported my 48"x96" top on light framed boxes 20" x 28" x 40" which allowed for different working heights, 32" for general work and 24" for installing hardware in assembled base cabinets.

The easiest way to fit the grid together is to use half-lap joints like an egg crate. The core grid need not be fastend to the rim.The first one I did had stringers and short spacers stapled together but that is excessively laborious. The important thing is to have a flat surface to assemble your new top on. I built my first top on one that my friend had glued up on a welder's cast iron jig table, and mine spawned several others. If your existing top is flat enough use that, shimming as necessary. If you have to set up a framed base, sub out the milling to a shop with a decent jointer or sliding tablesaw. Numerous crowned 2x4 cauls and clamps will be handy, and epoxy will be more friendly to low clamping force (though I have done several with yellow glue). If you lack clamps, use weights with an mdf caul sheet to help distribute the force. A few brads through the skins into the rim will keep things in place.

[Jamie Buxton]

Wow, that's perfect! Any tips on getting the spacers fit together, glued and fastened to the skins? I could try brad nailing the skins to the spacers...but that's a narrow target to hit.

The spacer grid is built from strips of plywood that run the full length or full width of the box. At each intersection, there is a kinda-half-lap. I say kinda because the "joint" isn't a joint. It is just tablesaw cuts which provide clearance so the strips can run past each other. A tablesaw kerf is 1/8" thick, and eighth-inch plywood is actually more like .1", so one cut is what you need. I gang-cut the clearance cuts by wrapping the strips together with saran wrap. You wrap the saran wrap anyplace, and cut through it with the saw. The grid is glued to the face skin without nails or anything -- just glue.