Bow problem in a conference table

[vern]

I delivered a conference table to a client a couple of weeks ago. It is a 4 1/2' by 10' table. It is 3/4 pecan with an underlayment of 3/4 mdf, and a 3" wide 8/4 boarder of pecan. They left the table in a hot room stored horizontally (the grain also runs horizontally).
It sits on 2 large pedestal bases.
The problem is that the table has bowed across the 4 1/2' section from end to end. That is to say that the bow goes from short edge to short edge the whole length of the table. It bowed so badly that it actually popped the end borders off, and they were epoxied on.
We picked up the table and brought it back to the shop put it up on saw horses. I took a piece of 2x2 pecan and placed it across the top with a c clamp on each end and was only able to pull about 1/2 of the bow out. So I tried a peice of 1" wide 8/4 going across the 4 1/2' section and there was still about 1/4 - 3/8 gap under each end of the straight edge.
Question is does anyone have any suggestions on how to solve the bow in this table. Right now I am putting the table in the back storage area with clamps and cauls on it till I find a better idea.

[Iain]

Do we assume that there was no bracing between the supports?
Would steaming be an option, probably a big plastic bag and some weights to encourage it back to were it was.
Can the MDF substrate be removed?
This could be replaced later and the timber straightened.

[echnidna]

I tend to think it may be uneven moisture in the timber but the problem may also be to some extent influenced by the structural design and even the type of finish that was used.

Some pics of the table may help us give you the correct advice.

[Ianab]

I think you have a problem with the design itself.

The solid wood will be shrinking (it will shrink and expand seasonally) while the MDF is relatively stable. If the 2 are bonded together and one side shrinks or expands... it gotta warp.

I dont know how to fix the problem you have, but it would have been better to build the table with a solid wooden top that would move the same on both sides (You can possibly use a cheaper wood for the underside)... OR... make the top as thin Veneer over the MDF, a thin veneer will be stable on top of MDF or ply.

Sorry that doesn't help your current problem.. but hopefully it explains what went wrong.

Ian

[Glue and Screw]

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It’s the design. There is no where for the timber to do what the timber wants to do.<o></o>
If a remake is not an option I would suggest slotting the mdf sub top to remove its strength but keep its bulk (similar to kerfing)<o></o>
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Good luck

Cheers
Marty

[Skew ChiDAMN!!]

Apart from seperating the two layers and rebuilding, if possible, there's no elegant solution to what sounds to me like a design flaw.

IMHO the best "brute force" approach would be to remove the side strips and run the top over a tablesaw to run slots in the MDF parallel to the top grain. How far apart depends on the severity of the bow. I think anout 3/4 - 1" would probably do... and a thin-kerf blade would be best.

Care'd need to be taken when sawing, depending on which way the bow is you'll possibly encounter one of two problems... if the edges are bowed up, then the kerf may close on the blade as you near the end of the cut making kickback more likely. Or, if the edges re bowed down (so the edges hold the centre of the wood clear of the table) the kerft may spread a bit as you approach the end of the cut... this means the bow will flatten out a tad and the blade will cut deeper at the end! Hope you understand what I mean? :confused:

After slotting, use cauls to pull the top flat again and screw (not glue!) a couple of slats across to both restore some lateral integrity and hold it flat before reapplying the side sections. The idea of screwing is to allow the wood lateral movement. Ovalled screw holes may also be a good idea for just this reason.

Like I said, not elegant. But it should work.