Thinnest material for torsion box?


Space limitations doesn't permit me to build a large FLAT assembly table. What I use now is my portable outfeed table (actually a heavy metal cabinet on casters with leveling legs), but it isn't quite wide enough to assemble many of my projects. I'm considering building a removable top (torsion box) that can be placed on top to increase it's size AND be as flat as possible. When not in use it will be stored against a wall. I can't leave the box on top of the cabinet to do double duty as an outfeed table because the cabinet is too high for a torsion box to be level with the TS.
Will 1/4" MDF or 1/4" hardboard be thick enough to maintain a rigid structure? I want it as thin as possible so the height of table isn't increased any more than necessary when the torsion box is in place, so what should be a minimum height of the grid? I'm guessing that if the grid is too short it won't provide good rigidity?? The torsion box will be 4'x6' and will be supported by a base table of 26"x45" so there will be 48-26"/2 inches of overhang on the width (each side) and 72-45'/2.5" overhang in length on each end. The heaviest object I'd expect to be built on the table would be under 100 lbs.
Requirements: light as possible since it's movable, and thin as possible yet sturdy.
Dave
Dave
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says...

I used 1/4" oak plywood for the skins and 1x2s for the grid. This was for a router table to replace a table saw extension wing. No problem after several years. But I do have two edges supported by the rip fence rails. OTOH, there's a hole in the middle for the router which weakens the structure. No sag yet.
--
BNSF = Build Now, Seep Forever

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I've used a salvaged hollow core door for such purposes. At about 34"x 80"ish, it's been adequate for that.
Cheap, too.
Patriarch
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Good tip, Pat. Alas, I gotta go with a 48 width for the assy table.
Dave
Patriarch wrote:

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[snipped for brevity]

I am often faced with situation like this: thin and strong. I the solid surface business, the unsupported overhang for a breakfast bar is limited to 6". To go beyond that, I am either looking at brackets (NOT elegant) or another type of support: Torsion box. I have built and used many of the following type: 1/4" MDF skins and 1" wide x3/4" maple for the ribs. I glue the whole thing together with WEST. I have also used PB and MDF for the ribs..needless to say, accuracy is paramount.
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I built full size doors for a 6' high x 4' cabinet using 1x 3 members and 1/8" hard board for the skins. works fine with no sags so far. You will have different requirements for an horizontal table.     mahalo,     jo4hn
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David,
I built one using 3/4"x2-1/4" pine and 1/8" hardboard for a top. Overall size is 4' x 6'. The top is screwed down so it's replaceable if it gets damaged. I had some 1x6's laying around so I ripped them down - hence the odd ball size (2-1/4"). It's nice and sturdy and has held everything I've thrown at it weight wise but I think the most I've had on it was about 100lbs.
When not needed, it's up against a wall. The top was waxed with several coats of Johnson's paste wax so glue and paints don't stick.
Bob S.

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David wrote:

Built a model railway layout board that way, 3/16 ply faces with 3/4 x 1+1/2 inner frame in a square/diamond/square/diamond etc style, put it upsidedown on trestles and it makes a table for all sorts of uses, lives on one edge in the conservatory. As long as you point loadings aren't too high 1/4 sheeting should be fine.
Niel.
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Dave, to meet your requirements, you really need engineering help, which I am NOT qualified to give. But some ideas:
1) How flat is your cabinet/outfeed table? If it provides good support, you need very little stiffness in this part of your structure. Shims on the underside of your table could take care of any twist or other unevenness.
2) Make a thin torsion box the size of the cabinet, surrounded by a thicker one that will overhang the cabinet.
3) Look at a plane's wing design, optimized for strength and weight, and use some of their concepts, such as lightening holes in the web and small stiffeners to give local rigidity to a thin skin. Doing that right is well beyond my knowledge base, but I'd guess that making the surrounding torsion box 4" thick with interior pieces of 1/8" ply filled with 3" holes and attached to each other and the top and bottom with 1/4"x1/4" glue block strips would be both stiffer and lighter than a 2" thick box with 1/2" thick interior pieces.
--
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I went across town to my primary wood supplier AFTER they said by phone they had 1/4" hardboard. I get there and NO, then don't even CARRY it! aaargh! Another retail clerk blows it--thank you very much!
I go to the next supplier about 4 blocks from the first one. yes, they have 1/4" hardboard, but it's $22 a sheet. No thanks. I'll try HD near home.
I go back home and drop off what I DID find in the way of wood (in case some miscreant decides he needs the walnut more than I do) and then drive to my local HD for some 1/4" hardboard, which, last time I bought some, was available at there. Walk 14 miles to the back of the store to get it. hmmm.... 1/8"...3/16". no 1/4". Look at the smaller sizes like 2x4 and 4x4. Yup. 1/4". go find a clerk to get the phone number of the next HD about 1.5 miles away. Call them. "Nope" NONE of the HD's carry 1/4" hardboard in 4x8 size because "it gets broken". Gee, that hasn't stopped them from carrying 1.6 million OTHER items that are broken or missing parts!!! Screw it; I get three pieces of the 4x4 stuff for under $9 each and call it a day.
I live in a city of nearly a million people and all I wanted was a piece of 1/4" thick hardboard 4'x8'. This isn't the only common item I've had difficulty with. Does anyone share my pain?? Are you often amazed at the lack of availability of common stuff in YOUR town?
Dave
David wrote:

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You can always try an old fashioned lumber yard.
There are not as many of them as there used to be. But in a town of that size, there should be something.
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Lee, 2 of the suppliers were lumber yards. One didn't carry it and the other charged more than I thought it should cost so I passed on it.
Dave
Lee Michaels wrote:

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They charge that because they know that no one else has got it.

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Not to mention, it "breaks" so easily they lose quite a bit....
All the running would easily have paid for the expensive stuff.

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David,
1/8" hardboard will work just fine as I stated in my post.
Bob S.
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