Working with Plywood


I made part of my box for the in-wall stereo cabinet last night.
First problem is that the plywood was bowed. These started out as 4'x4' 3/4" plywood when I brought it in the house. I set them on the basement floor leaning up against the wall. Eventhough the cement floor is sealed, I am assuming the wood absorbed moisture from it? Because it was not bowed when I brought it into the basement, but its bowed now :( Anyway, The show must go on... The sides are 4'x1'6", the top and bottom are 1'5.5"x1'6". The sides are still bowed. I think I will glue a stationary shelf in the middle to remove the bowing.
Second question is, the cut at the lumberyard (1-4x8 into 2-4x4)was kind of off since someone appearantly hit the guys saw with their vehicle. How do I square this edge? (wouldnt fit into subaru without a cut)
third question is how does one cut 1/4" off a 2'x4' piece? When I cut the two sides, one was about 1/4" shorter than the other. Table saw didn't seem a good strategy since the fence cant get that close. I have an old B&D circular saw my dad re-found in the basement of a house he was selling. But teh motor housing is so big, It wouldnt clear the clamps I tried to use to clamp a straight piece of wood to the panel to cut that 1/4" off. I could have used the saber saw, but this end will be receiving the glue and I was scared the edge would be nasty!? I ended up just cutting another side piece on the tablesaw and it came out better.
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Mon, Feb 6, 2006, 1:16pm snipped-for-privacy@ThisOneIsFake.com (dnoyeB) doth admit: <snip>I set them on the basement floor leaning up against the wall. Eventhough the cement floor is sealed, I am assuming the wood absorbed moisture from it? Because it was not bowed when I brought it into the basement, but its bowed now :( <snip>
Simple. You leaned it. It bowed. If you'd laid it flat, it wouldn't have bowed.
JOAT Don't believe everything you think.
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J T wrote:

Does wood have to be so picky. I thought i was being nice bringing it in our of the cold.
I just have this urge to get to work. Yet everytime I do I end up realizing I should have finished reading the whole chapter. Thanks for the tip, next time Ill lay it flat.
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On Mon, 06 Feb 2006 14:55:01 -0500, dnoyeB wrote:

I've learned a lot of things about woodworking, by diving in and doing it. One of the most important things I learned is patience. A little of that goes a long way, and you can never have too much of it.
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I disagree. It's 3/4" plywood that's been leaning for a few days. There's no way it should bow in that time simply from gravity.
I have pieces of plywood that have leaned against walls in my shop for months and months and they're still straight as an arrow. On the other hand, I've had pieces that I laid flat on a concrete floor that cupped up in just a day or two. I've also had pieces that I kept in my humid basement that curled up like a potato chip when one side was exposed to the sun for a few days.
This is a moisture issue.
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Josh wrote:

Which way did it cup? I suspect the moisture takeup is from the air, the board being a cooler temperature by conduction from the concrete.
The direction of the cup would tell.
er
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Yep, the concrete floor is colder - higher RH - and with less movement. The corners come up from the floor.
Outdoors in the sun, same-o, only it's loss from lower RH in the heat that makes the down side seem like it's gaining.
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Enoch Root wrote:

It cupped in a sagging direction, along the grain. The grain was going vertically as the wood was standing long. But the bow was so small, perhaps 1/2" in a 4 foot board, and they were practically vertical that I doubt if it was really weight, but could have been a combination of factors. The cement floor is sealed with waterproofing too.
Its true that the concerete is likely a few degrees colder than the air but the bow was even along the whole board.
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I still think it's because the outward side of the plywood dried more quickly than the side facing the wall. That would cause the outside to shrink faster than the inside.
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Do you have a router? That would be your best bet. Use a flush cutting router bit with the factory end of one of the other pieces of plywood as a template.
Chuck
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Woodchuck34 wrote:

I was looking at them today. My brother has a router and Ill try to borrow that from him. That idea about using another sheet of plywood with the factory end is genious.
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You can wick moisture through concrete. I doubt this is the problem though. The plywood was probably wetish to begin with and is warping as it dries.

That could fix it. You may want to screw through the outside panels into the center shelf to add some strength. Even if you dado the shelf into the uprights, the glue joint won't be that strong. If it were me, I'd spend $30 and pick up another sheet of plywood at home depot.

Most of us make panel cutting sleds that ride in the miter slot. Baring that, I'd pick up a framing square.

Don't do what you're describing, it's dangerous. The way to cut 1/4" off is to run the large piece between the fence and blade, with the 1/4" piece to the left of the blade. Trapping the 1/4" piece between the blade and fence is a recepe for kickback. Doing it right will make you account for the blade kerf correctly also.
brian
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brianlanning wrote:

It was stored outside so that is likely true. Any way to avoid this?

I keep forgetting im going to put a 1/2" backing on this box. Ill use one clamp to bring in the bow, then glue the square back on. That should square it up. This is getting clamp crazy though. Last night I had planned on using 2 clamps on the top and 2 clamps on the bottom piect, but I ended up using 4 clamps to glue in the top, and ill have to glue in the bottom tonight since 2 clamps didnt cover the 18" too well.
If I get another sheet of plywood im scared it will just do the same thing. perhaps I should shop at indoor lumber yards during the winter months? Or when I bring it in, sit a bunch of weights on it for a week?

I had the thought, but that would be one HUGE sled. I really need to make a full table for this so I can get a fence out that far. Or stop making bad cuts...

OK, so I should always put the larger piece against the fence? Or just never put small pieces against the fence? I better read the rest of the cutting sections in my books.
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Don't store it outside? :-) Wood is like a sponge. It will aclimate to the relative humidity in the environment. And as it dries, it will warp and check. Plywood would just warp.

I have about $800 just in clamps, and I would use more.

At places like home depot, they're indoors, stored flat, and stored in large stacks which helps to keep them flat. They'll also be aclimated to the indoor environment. I buy 3/4" 4x8 sheets of chinese birch plywood and I never have warping problems. The thinner stuff warps sometimes.

Woodworking stores have this special clamp. It's made of aluminum and comes in various lenghs. The one I have is maybe 6' long. It has a black clamping mechanism built in. You could use a framing square to line up a clamp like this, the clamp it in place. Then you could use a circular saw with the shoe riding along the edge of the clamp. If you have a really straight board or something, you could clamp it in place and use it with teh circular saw the same way.

You want to avoid trapping things between the fence and the blade. As the piece you're cutting gets thinner, the ganger goes up a lot. If you're ripping a 2" piece into two 1" pieces, it's not so bad. But if you're trying to cut less than an inch off a larger panel, it's better to have the smaller piece falling off and the larger piece against the fence.
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wrote:

When the box is assembled won't this take out the bow? If not glue some 1x2" cleats on the side that won't show.

A hand plane or jointer will square the edge.

If I understand correctly, put the larger piece against the fence so that the 1/4" is trimmed off on the other side. The side put against the fence MUST be straight and square. Also, set up a featherboard. Use strong (1/16" more) cuts and smooth with a plane or jointer.
Instead of clamps you can use a larger straight edge that won't hit the motor housing or consider using double-sided carpet tape or hot glue instead of the clamps.
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