Flat stable Worksurface

I need to make a flat tabletop worksurface 40" x 96". It will only be supported 3 spots over the 96" length at 6", 48" and 90" from either end. I want to come back and measure it years from now and its still flat. The other criteria is I only have 4" of clearance below the top. Flatness should be what is praticle with an 8' straight edge and feeler gauges. Ideally a maple top that I could put an mdf wear surface on would be great except, I haven't had any luck finding one that size, and the cost of one smaller plus shipping is pretty hefty, $400+. It won't be supporting lots of weight, just the usual woodworking worktop. Ideas?
Phil
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torsion box
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Which ones? 0.001?? 0.01? How flat?

DJM built an assembly table flat "enough" for woodworking purposes using a torsion box approach with MDF skin.
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I would be quite satisfied with .01.
patrick conroy wrote:

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How about this:
http://www.diynet.com/diy/shows_wwk/episode/0,2046,DIY_14350_26946,00.html
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I'm thinking granite, but 40 x96 x1.25 = 24 sq ft x $50 (est) (approx) $1200.
You can probably find a better price.
It'll still be flat in 20 years tho.
Lou
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I am using my Dad's workbench. It is a 36" x 72" solid wood door, 1 1/2" thick from Weyerhauser. This door is 60 years old and still dead flat across the entire surface. It is supported by a 3x3 post on each corner - no support in the middle. This thing is so heavy my brother and I can barely carry it.
Bob

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Torsion Box:
(2) sheets of 1/4" plywood (1) sheets of 3/4" birch plywood (2) sheets of 1/2" mdf glue Staples and gun.
Make the frame out of strips of 3/4" birch ripped to 3", with interior seperator strips "about" 6"-8" apart.
Skin it over on both sides with 1/4" ply using glue and staples. Top it off with 1/2" mdf.
Very flat and very strong.
How flat depends on you.
Phil wrote:

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I had considered building a torsion box, the problem to me is getting 90" of straight surface for the supporting members. 4' or less no sweat...
Pat Barber wrote:

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