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Flatness of double-layer MDF vs. torsion box

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Forum topic by live4ever posted 11-07-2011 11:28 PM 3202 views 2 times favorited 5 replies Add to Favorites Watch
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live4ever

983 posts in 1734 days


11-07-2011 11:28 PM

Topic tags/keywords: assembly table torsion box shop question mdf flat

I’ve been building an assembly table that will hopefully be flatter than my workbench. I know sometimes we woodworkers have an obsession with “dead-flat” when it comes to assembly surfaces, which I don’t, but I do want to make sure the table provides a decently flat surface.

Originally I was planning on a torsion box top (a la WoodWhisperer’s assembly table) but recently started to realize some of the clamping options I want to build into the top would work better with a thicker top (e.g. double-layer MDF).

Would 2 layers of 3/4” MDF glued together be as reliably flat as a torsion box? Am I splitting hairs?

-- Optimists are usually disappointed. Pessimists are either right or pleasantly surprised. I tend to be a disappointed pessimist.


5 replies so far

View Lee Barker's profile

Lee Barker

2169 posts in 1575 days


#1 posted 11-08-2011 12:03 AM

In a given area, MDF can be impressively flat but a full sheet may not be so flat because of exposure or storage errors or mishandling.

If you had two identically cupped 3/4” sheets, and glued them together convex to convex, you might have all you are thinking you need.

The unflatness in a given sheet diminishes with thickness. Therefore, I’d suggest 1” piece of MDF if you really think that much mass is required.

I have been working off the same assembly table top of 3/4 particle board for about 17 years. There is some webbing underneath. It is screwed down from the top. Pretty practical all in all, and it’s never failed or broken or even talked back to me.

Kindly,

Lee

-- "...in his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"

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live4ever

983 posts in 1734 days


#2 posted 11-08-2011 12:40 AM

Thanks Lee…sounds like I’m splitting hairs. :)

-- Optimists are usually disappointed. Pessimists are either right or pleasantly surprised. I tend to be a disappointed pessimist.

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jusfine

2280 posts in 1650 days


#3 posted 11-08-2011 02:39 AM

I am not obsessed with dead flat either, although there are a few times when you need a perfectly flat surface.

I have not used MDF alone, but have used laminate on both sides of a single 3/4” mdf sheet for a great layout and assembly table. Glue comes right off and it stays very flat.

-- Randy "You are judged as much by the questions you ask as the answers you give..."

View Mike Morphis's profile

Mike Morphis

5 posts in 1121 days


#4 posted 11-09-2011 04:06 AM

I’ve always wanted to build a torsion box since I watched David Marks build one on Wood Works; however, I find myself using a solid slab door instead. I could make my project dead flat but to be honest, I haven’t found a perfectly flat service in my house or anyone elses! :o)

-- Mike... It's a big mistake to allow any woodworking tool to realize that you are in a hurry.

View GregD's profile

GregD

634 posts in 1860 days


#5 posted 11-09-2011 06:32 AM

What is a torsion box that doesn’t have the bottom skin? That is what I do. Simple torsion boxes are actually pretty easy to make with pocket hole screws, especially if you leave off the bottom skin. If you can rip straight and parallel you can get good results. But you do use up plenty of screws. I prefer 3/4” plywood because it is lighter and tougher, and the torsion box frame is sufficient to pull it flat. Extend the top 3” or so beyond the frame if you want to have easy clamping. Don’t use glue (and leave off the bottom skin) and it is easy to go back and shim the low spots if they annoy you (now or later).

-- Greg D.

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