Thought I was gonna be OK but..

[Brian Penning]

...after gluing up the dozen pieces of cherry and letting the assembly sit for a few days I was happy to see the full length of this bench-to-be stayed flat.
BUT almost as soon as I started routing out a 1/2" from the top side of the bench it started to bow.
Now I have a 1/2" deflection in the center. Don't dare have straight legs in the corners now because I think it would only accent the bow in the seat. Gonna have to get creative with those legs all of a sudden. http://www.canadianwoodworking.com/forum/images/smilies/frown.gif

Not sure why this happened.

http://i47.photobucket.com/albums/f193/Bri68/Miscilaneous/IMG_3314.jpg?t=1207305541

[Sam Yerardi]

First, some questions:
Was it moving as you were working?
How long did the wood rest after dimensioning it but before glue-up?
Do you know the moisture content of the wood and what the humidity is in your shop?

It could be several things. One may be that the wood needed to stablize for some time prior to assembly & glue-up. Another might be that there is tension in the wood that allowed the wood to move when you started routing. Yet another is when you cut wood you expose fresh surfaces to its environment. That allows it to more easily take on or give up moisture. Another thing is this may be one of those case where the orientation of the grain really does make a difference. There has always been the old school thought of alternating boards for panel glue-ups, and there are as many people that adhere to that as there are that don't. Look to see if all of your grain is oriented the same way. If it is, then all of the boards are probably working in concert. It may be very difficult if not impossible to correct it without ripping the panel and starting over with alternating boards.

You could try wetting the wood on the concave side to see if it will bring the wood back. The problem with that approach obviously is introducing a lot of mositure to the wood. If this approach does work, then I would seal both sides of the wood with shellac to help equalize the moisture transfer on both sides of the boards.

[Rick Thom]

Hi Bri, I presume you are working on a variation of this prototype?
My guess is that the root cause involves moisture content, but any way you cut it, that is a fairly long span to keep flat over time without support. If there is enough thickness left, what about a pair of stretchers end-to-end in floating d/t's? Normally for sheer strength they would be deeper than wide, but in this case to keep the profile obscured perhaps have them laid on the flat and well in from the front and back edges, using something like a 5/4 x 3" quarter sawn and bevel the edges so they didn't detract from the contour of the seat.
I like the shapes you've come up with. Interpreting that into a practical design may be tricky.

[Cliff Rohrabacher]

moisten the other side with a mister.

There is a good possibility that you heated the wood while routing. This would have caused swelling but only along the side that you were working.

Moistening the flip side may reverse that.

You can also take an iron to it after moistening and by-eye level the whole thing out by heating it.

[Joe Chritz]

Any chance routing out the pocket relieved some stresses causing it to bow.

I did a cutting board recently with a couple pockets and after it hit the sink for a rinse down the first time it got about a 3/32 bow over 17". It also developed a crack right over a relief grove which I am thinking is do to the pocket being routed in.

Thats a big span and I think a stretcher is in order for sure.

Isn't it fun to make a nice glue up to come out tomorrow and see it smiling at you? I feel the pain.

Joe

[Brian Penning]

moisten the other side with a mister.

There is a good possibility that you heated the wood while routing. This would have caused swelling but only along the side that you were working.

Moistening the flip side may reverse that.

You can also take an iron to it after moistening and by-eye level the whole thing out by heating it.

Shouldn't I apply the water to the top side where I routed the wood out? This will make the top expand, no?

[Sam Yerardi]

Brian,

Typically you will mist the concave side. This should cause the wood to expand, moving the wood back out (if it will work).