Cupped Plywood

[Mark Kristan]

I'm in the process of putting together a tool cabinet, I'm mixing Oak veneer (top and sides) with Baltic Birch (less-often visible interior parts). I thought one of the good things about plywood was that it doesn't generally bow/cup/warp the way natural wood does. I have limited time to spend on fun things like woodworking, at one time, so I decided to dimension the parts and cut some dadoes, with the intention of coming back several weeks later to do the gluing up. I am attaching the bottom of the (OV) sides to the ends of the (BB) base with a dado about an inch from the bottom of both sides. One one side the joint goes together reasonably well, but on the other, when I fit one end, the other end is out like 1-1.5", the board is cupped that badly. I can use clamping to get that end to about .25" of where it should be, but I wonder if that joint will likely hold up over the long run (planning to use glue and screws on those joints). That corner wouldn't exactly look pretty but it is a utilitarian piece - I just want to avoid buying a fresh sheet and starting over, if there is a realistic way to do so.

Mark

[Stephen Cherry]

I'd load it up and take it back to the store; plywood is not what it used to be, but that sounds pretty extreme.

[Steve Peterson]

Wood will always try to expand as the moisture level changes. Typical expansion rates from fully wet to fully dry may be about 8% in the direction of the growth rings and 4% across the growth rings. The length stays almost constant.

The wood will still try to expand and contract after it is glued into plywood. However, the long grain orientation prevents the cross grain from expanding like it wants to. This creates tension in the plywood and it may cup if one side has more moisture than the other side. Often cupping will occur if the plywood is laid flat on a table with one side exposed. One side changes its moisture level while the other one stays constant. You can try wetting the side that is bowed inwards to get it to expand back to being flat.

One more point: Baltic birch has many thin layers. This makes it naturally more stable than most other plywoods.

Steve

[Myk Rian]

You didn't say where you got the ply, so I'm going to assume a Big Box store.
It's said over and over on forums, don't buy their junk.

[Jeff Duncan]

As has been stated, plywood rarely stays flat. It's made that way, but it doesn't stay that way. Now there's a reasonable expectation of flatness for most quality ply. For instance if I bought a sheet at the local box store, i wouldn't count on it staying very flat. On the other hand if I got a sheet from my supplier that was 1"+ out of flat over say 48"....that's too much and it would go back. I don't know where the exact line is where it's "too much", but luckily I've rarely had it be a problem.

As for your specific application....I would need more info before offering decent advice. Details make the difference.....for instance if it's off 1"+ over an 8' length I would clamp it together screw and glue and move on. If it's off 1"+ over 30" of length I'd get a new piece. I don't think it's reasonable to pull shorter pieces that much and expect the cabinet to last.

good luck,
JeffD