Help unwarping board


I created an inlayed table top by gluing 1/4 stock to a 1/2 piece of plywood. The problem is that somewhere through the inlay process, the plywood warped on one side, and now I need to unwarp it.
The table top is 40"x20". Two corners attached to one of the 20" sides have curled upwards (towards the inlay). The curl is signifcant (about 1/2" upwards), and seems to be very strong (I can't bend it back with my hands).
What I was thinking of doing was to score the underside of the plywood with a circular saw about 3/8 of an inch at 1 inch intervals, and then forcing it straight with some clamps. I would then glue another sheet of 1/4" plywood to the bottom of the board once it's forced straight.
Will this work? Does anyone have any suggestions of additional things I could try, or tips on my proposed method? I've never done this before, and I don't want to mess up the inlay.
Thanks
John
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julvr wrote:

curling you experienced.
dave
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cause warping? I am about to try my first veneer project; wouldn't it be adequate to just varnish the opposite side?
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Plywood is a balanced set of veneers. Check the grain orientations in plywood. That is why there is always an odd number of plies.You are adding another single sided veneer with different properties and grain orientation. To help keep the piece from warping, you should do the same thing to both sides. If you veneer one side, you should veneer the other side with the same veneer or with something with similar properties. Otherwise, each side has different wood movement properties. Similarly, if you finish one side of a piece of plywood, you should put the same finish on the other side to even moisture gain/loss.
Preston

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Toller wrote:

dave
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Toller wrote:

either side of the core. Isn't plywood always an odd # of plys ?

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On Sun, 27 Nov 2005 00:01:16 GMT, snipped-for-privacy@spam.invalid (julvr) wrote:

My guess would be that the inlay has shrunk at a different rate than the plywood and has pulled the plywood up.

If what I suspect has happened is indeed the case the same thing could happen again but this time with the 1/4". The 1/4" isn't really going to add any strength.

The first thing I would try would be to clamp the top down on something flat to see if I'm going to ruin the inlay anyway when I straighten it. If the inlay survives that, could you put an apron around the bottom of the top to hold it flat?
Mike O.
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Was the 1/4in stock well dried?

That's a lot of bend for the size of the piece. Why don't you provide more information on the 1/4" stock that you used to make the inlay. Type of wood, length of pieces, width, etc. Type of glue? Glue application details? How long did it take for the plywood to curl? Was the plywood wet (been laying around outside or something)? What is the orientation of the inlay stock relative to the dimensions of the plywood?
I'm having a hard time believing that the 1/4" stock pulled the plywood like you're describing the movement. Not to say it couldn't, but I'm imagining a project that you just completed, using "normal" lumber pieces - stuff you'd get from the BORG. This just does not seem to be a simple case of putting a veneer on one side of a substrate and suffering problems with unequal moisture absorbtion.

If it is a moisture related problem, you can moisten the underside of the plywood and try to clamp it back to flat. I'd try that before I scored the piece.
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Whatever you do to the top you have to do to the bottem otherwise it will twist. Sorry. Dave

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I understand that but his project seems to have warped immediately and that is not characteristic of the type of thing you're speaking of. I could well be missing something here Dave, but I don't see any way his project could have absorbed enough moisture to have moved 1/2" ply 1/2" overnight. I suppose you could suggest that the glue did it, but I've never seen glue do that before and I've glued up a lot of wood.
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The finished product will have an apron, but it doesn't help me in the short term. The problem is if I try to use clamps to straigthen the edge, I start to hear cracking noises, which is the glue giving way. The other problem is that I need to straighten it before I run it through a thickness sander (I'm planning on taking 3/32" off the top layer), and I can't run it through with the apron on.
The wood I used on the top is mostly 1/4 oak. I glued several 1/4"x 2" strips together, so there are 10 pieces of oak glued together on the warped side. When Gluing the pieces, I did each peice individually, and I clamped the peice beside it, waited for the glue to dry and repeated the process. One of the pieces might have been a fraction to small, which might have pulled the top in...
As for other details, the glue I used was LePage Carpenters glue.
The plywood was cheap borg plywood, so it has a soft center, and had only one vaneered side (which I placed at the bottom -- I glued the oak to the unfinished side). I just checked and it has only four layers.
John
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This is weird. Even if movement due to moisture absorbtion were the cause, I would not expect this degree of movement from the wood you used. You just don't get 1/2" of shrinkage out of a 2" piece of oak. One piece being too small would likewise not produce the pull you've described. If you simply glued the individual pieces down, and did not have to clamp them aggressively to get a good piece to piece fit, then I don't understand the problem. Moisture problems as mentioned in other posts, which result from not veneering both sides of a substrate take time to show up, and I just can't see them being the culprit with a 1/4" piece of oak for an inlay. I don't know - maybe I'm wrong on this one.

I think what I would try is to construct a tent out of plastic and apply some moisture inside the tent, while all the time exerting clamp pressure on the assembly. I'd increase the clamp pressure over intervals until I could get it flat again. How long of an interval would be a process of experimentation. I think I'd let it absorb some moisture for a day or two and then try increasing clamp pressure every few hours and see how that goes.
This one is intriguing to me. Keep this thread alive, and post your results, regardless of the method(s) you choose.
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Fixed it!
What I did was I set my circular saw to a depth of 1/2", and scored the plywood everywhere there was a seem in the inlay above. I then built a steam-tent, steamed it up, and using some clamps, straigtened out the board over the course of an hour. Once straight, I let it dry for a few days, and then glued a piece of 1/4" handy pannel to the back, using lots of glue to fill the scores I made in the plywood. I then got it trimmed on a table saw to ensure the edges were straight.
It worked quite well, and it's perfectly straight except a 1/16" deflection in one corner, which will dissappear once it's run through a thickness sander (see my next post...)
I still don't know what caused the warp, but I'm glad it could be fixed. Once I attach the drop-down border it shouldn't be able to warp any more. For now, I'm keeping it clamped to a straight board just in case...
John
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