is any plywood FLAT these days?!

[Cdshakes]

seems like all the plywood at the big box stores are no where near flat. I'm trying to make a crosscut sled for my tablesaw, and the base is way uneven, and the glue up of the pieces to be the fences are out too... i'm trying to flatten it out, but i wonder if it's just going to get uneven again as summer humidity comes in again...
Any suggestions on how to flatten it?

[Splinter Puller]

Start sourcing better quality plywood from cabinet and casework supply shops. It will cost more but you get what you pay for.

[Robert Adams]

Little of it is flat. Even the good stuff from the plywood suppliers isn't flat or it is when you get it then as soon as it gets home it warps. Baltic birch though is pretty flat most of the time. The only stuff that starts flat and generally stays that way is mdf.

[Ohio Mike]

I had the same problem and so I used MDF for the base of the sled.


A bit heavier but very flat.


Mike

[Cdshakes]

Ohio Mike said:

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I had the same problem and so I used MDF for the base of the sled.


A bit heavier but very flat.


Mike

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i wonder if i did thinner mdf with a thin plywood on top (so the screws have something more to bite into...?

[Paul K. Murphy]

Quote: "is any plywood FLAT these days?!"
Probably not. The quality really isn't what it used to be.
Splinter Puller gives a good answer. He's right. I'll add my $0.02, though I think he's already covered it, don't buy panel products at "Big Boxes" with the expectation you're getting the good stuff. Don't.
Even the good stuff requires intervention at the bench these days. Unfortunately, we don't seem to be able to buy good quality raw materials anymore. We're not workmen buying useful commodities, we're doctors treating patients admitted to our facilities. Unfortunately, the first thing you've got to do before you build is to figure out a way to stabilize the patient.

[GNP]

I used Baltic birch from Rockler for mine. The quality has been dropping on it too.

[Steve N]

When you say is it flat, I wonder are you talking about the entire 4x8' sheet? It never really has been Big box or specialty supplier. If you are using a 4x8 piece it is never alone, it's always as a sheathing, or framed in sides of something. Usually in a project you aren't using the entire piece, and just like a length of solid wood with a twist, or bend, once you cut it into a shorter piece that becomes less so. SO the quick and dirty answer, and one you probably don't want to hear is that piece of 28" x 36" (probably max size for a sled) is pretty darn flat, and once you attach a solid back fence, and especially if you attach a front fence it will be really flat. If it is not, then I'd suggest that it is you at fault, and your piece of ply isn't square doing a 5 cut, and things become worse when you try to treat it as square you are torquing it into a wad. Wads aren't flat.

Instead of blaming the plywood, let's look at the problem. I'd back you up to your method of how the "sled" got to be cut, was your saw set up square. and did the resulting cuts come out with 4, 90* corners, with lengths equal to widths? It really is easy to make an error trying to do that.

Over dramatization




What you really are looking to have. Lengths are identical, as are the widths. Actually harder to make than one would imagine






I have cut a LOT of plywood, and will agree in the 4x8 size it has floppy edges that make it appear to be non flat. Cutting it down to a smaller size, and especially stiffened with solid wood across one or all 4 outer edges, well I've not seen non flat ply. I've seen non square cuts though.

I will not argue plywood has become cheap, and the plies are a lot thinner, but if you lay any of it in a dead flat surface, and frame it's perimeter, it will calm down.

Follow the instruction EXACTLY from this video, and see if your plywood doesn't behave nicely.



https://www.youtube.com/watch?v=UbG-n--LFgQ

[Howard Acheson]

If you use construction lumber avaiable at the Big Box you can expect it not to be flat fairly frequently. You are at the mercy of the relative humidity in the place the lumber is stored, A better choice would be MDF. It tends to be flatter. Be sure to store it in a manner that allows air to freely flow over the panels.

Keep in mind that plywood sheets sold in the big boxes are intended for construction, not furniture.

The best you can do is to carefully choose the the panels you want. Or, go to a wholesale lumber yard where they generally have better lumber. Stack and sticker your sheets and cut then over-size so you can trim to you final dimension.

[Steve N]

Howie 5 years ago I would agree, and possibly where you live it is still true. John Menard is opening stores all across the country and fortunately they are here. You can special order any wood or ply you can get at a specialty store. There are a fair number of specialty lumber yards around here, and because they no longer are selling the amount of wood they did just a few years ago their stock sits. I have always said the worst death plywood can suffer is sitting unsold. Between the moisture fluctuations you mention having people sorting it and restacking it causes damage. If it sits 1 year, it's toast. Because it;'s coming in from a warehouse that is turning it over at a great rate, and all of the better grade of ply I've gotten from Menards has been sent out with 2 free sheets of CDX to wrap the good stuff inside, so it comes to me as near virginal as any I have ever seen from Paxtons, and the like. Not to mention it's at a better price.

I might add since Menards arrival, that local Lowes, and HD also are doing much the same, however they don't have near the inventory available that Menards does. For my part it's nice try. they couldn't offer the low prices until forced to do so, and they didn't offer the service, so they are pretty much lost to me, unless they carry a brand I desire. HD = Behr paint is a case in point.