How Flat is Flat?

[jackellis]

I've discovered that plywood will warp (surprise surprise)! Bad enough that it happened with some really cheap stuff. I also have some pieces that look like they're good quality stock and even they aren't as dead flat as I'd like or as I'd hoped.

So, how flat is flat? I've got a piece of 1" thick ply (actually 31/32nds) that I'd like to use as a fence for a crosscut sled. It has about a 1/32 crown across its 11" width but it appears to be flat along the length. Other pieces appear to rock a tiny bit when tested on a flat surface (saw table).

Just how flat should I expect a sheet of plywood to be?

[jonmulzer]

On a crosscut sled, I would not think it would matter. It will be screwed to other pieces of wood and you can use them to help true it up by design. And 1/32nd really should not matter. Can you give us some idea of your design so perhaps we can help with this?

[drumpriest]

I went through a bunch of ply before finding a couple of strips of 3/4 baltic that I laminated together to create a nice straight flat fence. The bed of my sled is MDF, and that stuff is pretty reliable for flatness. I am going to disagree with Jonmulzer, to me the fence of the cross cut sled is mission critical, I would not accept a 1/32" discrepancy over 11", if it's the face side of the fence.

[jackellis]

The length of the plywood is flat as far as I can tell, so it should make a fine fence. The crown along the width won't be a problem so long as it's uniform along the length.

I guess what I'm really trying to find out is what I should reasonably expect from "flat" plywood? No sense knocking myself out if none of it is perfectly flat, and worth paying more if I can find stuff that *is* flat.

[jonmulzer]

I went through a bunch of ply before finding a couple of strips of 3/4 baltic that I laminated together to create a nice straight flat fence. The bed of my sled is MDF, and that stuff is pretty reliable for flatness. I am going to disagree with Jonmulzer, to me the fence of the cross cut sled is mission critical, I would not accept a 1/32" discrepancy over 11", if it's the face side of the fence.

He said it was "flat" along its length. Which to me says it is straight. Who cares if it has a 1/32" crown if you are going to screw it down to a piece of plywood or mdf anyway? Once you do that, it is "flat" regardless.

Drumpriest is right though, and so was I. I just posted before my morning coffee and that made me very vague. The key point to the fence is that it is straight after it is screwed down. If the face side is dead true, don't worry about a little crown. I am sure screws can handle pulling a piece of plywood 1/32".

We might need to know more to give you a definitive answer though. What kind of design are you using? Is it a very simple design with just a runner attached to a plywood base and a simple screwed down fence or something more exotic?

[newood2]

I guess what I'm really trying to find out is what I should reasonably expect from "flat" plywood? No sense knocking myself out if none of it is perfectly flat, and worth paying more if I can find stuff that *is* flat.

I think you are on the right track of answering the question. I'm no scientist, but I think flatness is a subjective or relative term. In other words, it's determine by what's acceptable by the user and the application, especially in the context of woodwork -perhaps absolute flatness is reguired in certain areas of science. For example, Baltic birch ply supposedly flatter sitting in the shop than regular "birch" ply, but it's more expensive for certain uses. So I use the regular for most cabinets, taking into account that certain warps will be straightened out when glued-up. It would be nice if plywood was "flat" as MDF but that's wishful thinking for now.
Just my 2cents.
Howie

[jackellis]

to answer Jon's question, my intention is to build a pretty traditional sled that uses both miter slots and straddles the blade so I can cut large panels more accurately than I'm able to now. Ideally no more than 1/64" error in two feet. Nothing terribly complex but even dust at the point where the base and fence meet can introduce that much error.

More generally, though, I'm trying to get a sense for what more experienced folks run across. A while back I noted that I'd discovered to my horror that a number of the drawers, face frames and carcases on cabinets in our home were not as square as I'd thought. Some of the responses were on the order of, "leave well enough alone", which suggests good looking work is not necessarily going to be perfect work.

After all, this is wood, not metal.

[JimD]

For my fences, I laminated two pieces of 3/4. I got most of the pieces out of some yellow pine plywood that stayed straight, even when glued together (which raises the mositure of the surface the glue is on). On what I used as the back fence of the smaller sled, I had to use a piece of chinese plywood. It wasn't too bad before glueing but was pretty crooked when glued. I used it for the back fence and pulled some of the crook out of it while screwing it to the mdf base.

If I had some baltic birch I would have used that. I've never had an issue with it warping badly. 1/32 isn't bad to me.

Jim

[pecker]

I don't know how flat your fence needs to be in order to be "acceptable". But I know if I found my stock rocking on the fence every time I used it I'd be wishing it was flatter.

For my crosscut sled fence, I used a piece of 2" x 2" angle aluminum...it's straight to within a few thousandths of an inch. Then I attached several layers of the flattest plywood I had lying around to the face of the aluminum as the sacrificial area that the blade would hit first.

Even still, the thing is not as flat as the original piece of aluminum was, but I can crosscut a 12" panel and it's out of square by just a few thousandths of an inch...which is good enough for my purposes.

[SARGE..g-47]

If the ply is the Chinese stuff, I would add glue two ( or the 1" and a piece of 3/4) together.. screw it and add 3 coats of poly. If you want it flat.. go to a granite dealer and have them machine a piece of polished granite to use. But even then they can only gauranee .001.

I have a router fence I made 6 years ago from laminated baltic birch. I looked at router fences recently at the Atlanta WW Show, thinking it had to have moved. Found some nice ones, but when I threw a machinist straight edge on my fence and table made of MDF laminated.. it was as flat as the day I made it.

1/32 isn't diddly squat with wood. If it moves to well beyond latter.. that could be diddly.. but your results will alert you and you simply adjust on the fly.