Torsion Box -How thin can you go?

[Mr Eddie]

Today I finally tore off the old bench top (crappy particle board and hard board). I am going to make a torsion box top to replace it. I would like a solid wood top but my back problems just won't allow me to lift the glued up sections and to assemble a complete top. Anyway, the torsion box will work out fine and I will be happy to have a flat surface to work off of. I am going to use the old base as it is still in good shape. My concern is I don't want to make the torsion box too thick and cause the working height of the bench to be too high. Apparently, I am getting shorter as I get older. I am 5' 7". How high is your workbench? If you have made a torsion box to use as a bench top or assembly table how tall was the inner grid between the layers of ply? I am thinking of using a short grid maybe 2-2 1/2" so the overall height of my bench will be around 36". Do you see any problems using a shorter grid? I know most I have read about or seen used a 4" grid.

Lonnie

[BillN]

From what I understand the height of the box is what primarily gives it strength. Mine torsion box top is 4" with an overall bench height of 36" including casters. I don't know how much difference the thinner top would make.

I wish I had attached a few strategically placed blocks of wood under the top to allow for dogs.

[MichaelMouse]

The grid is for rigidity, the covers for break resistance. My bench is the 35" that most tablesaws and such use, and I'm a fairly short 31" inseam. Makes it perhaps a tad high for planing with power. If you support it under, it's the same as being part of the "box." My assembly over-the-tablesaw tabletop is a 1 1/4 thick 10" lattice under a piece of 1/4 ply. No bottom ply. It doesn't deform much over 5' when unsupported. When in place, not at all.

If you're a numbers guy, take a look here. http://www.woodbin.com/calcs/sagulator/

However, if you're planning dogs and such for your bench, might want to consider double thickness of 3/4 ply, as I have. Otherwise, make solid sections inside your lattice to accommodate.

[fredhargis]

There was an article in WWJ some years ago where a self proclaimed expert at everything (Ian Kirby was the name I think) demonstrated the "proper" way to make torsion boxes, it was declared by the magazine that he was a "master of torsion boxes". Anyway, I had always made mine (a grand total of 2) with a 4" web, but Kirby was building floating shelves, and making the web less than 2"; and using 1/4" material for the web (and the skins). He then laid the shelf on a pair of blocks and stood on it to demonstrate their strength. My point is, I think using a smaller web may well work, but I would make sure the top most piece is something that's thick enough to withstand what's happening, like hammer blows and so on. At 5'7" I made my workbench top 36" high, but that was more because my TS is also 36" than anything else.

[jteneyck]

Kirby was essentially correct. A torsion box gets it's bending strength from the height of the webs. On of formulae for bending strength is:

Stress = (3 x Load x Length) / (Width x Height^2)

That means that the stress increases inversely proportional with the square of the height of the webs. So a web that's 4" high will have 4x the load capacity of one 2" high (16 vs 4). No saying I remember correctly but I think the deflection is related to the ^3 power of the height, too. So if you want it to be really rigid thicker is better.

But if you look at a lot of workbench designs you'll see that they are often a solid top about 2" thick surrounded by a much deeper frame. Mine is made like that, after a design by Frank Klauz. The perimeter frame carries some of the load, and the the two massive legs underneath the top support it at the about the third points.

All said and done, if you make the torsion box 2 - 3" thick, surround it with a perimeter maybe 4" high, and set it on a legs joined by stretchers, it's not going to flex. People use hollow core doors all the time for a workbench top. But I would make the top and bottom with 1/2" plywood because you want to be able to beat on it without worrying about knocking a hole in it.

Sorry to be so long winded.

John

[EatenByLimestone]

Laminated 2x4s are just under 4" high. You can laminate them on the bench top so you are only picking 1 up at a time.

My bench was made for handtool use and is knuckle height. If I remember right it's around 29". I'm just under 6 ft tall.

[Robert Adams]

My table saw and outfeed table are around 38" tall as stock height involves too much bending over to use it which isn't comfortable or safe. My torsion box table it taller than that. Sure is nice not to have to bend over constantly when working in the shop and I'm only 6', used to be taller though as my spine has shrunk since hihhschool...

Not sure how thin you can go with a torsion box but think about this. Hollow core doors are 1 5/8 and they are torsion boxes and quite strong considering the interior webbing is cardboard now.

[Herb G]

OK, I'll bite. What is a torsion box?
Anyone got some pics of one?

I don't know what it is or what it's used for is why I ask.

[Mr Eddie]

Kirby,
A torsion box is just a sandwich - two outer skins of plywood with a grid work of squares between them. Very strong and if done correctly very flat. They are nice to have for assembly work. Do a google and you'll see what we're talking about.

Lonnie

[Mr Eddie]

I should have mentioned that I'm going to use 3/4" ply and double up on top so I can drill some dog holes and have some "meat" to pound on. I'll put a 1/4" hardboard over the top of everything and then frame it with hardwood. As many of you have pointed out, taller is stronger but I'm feeling better about a short grid with the mention of solid core doors. It looks like most of you have bench heights around 35-36" except Matt. Matt, I don't know how you do it with a 29" bench and being so tall. If I was your height I'd build this with a 4" grid and never think twice about it.

Lonnie