# Flat Plywood For A Table Saw Sled - A Myth?



## breaknrn

Hello Fellow Lumberjocks,

Has anyone tried to build a table saw cross cut sled with plywood and discovered that it wasn't truly flat? The end result is an inaccurate cut. I've tried going back to the store to find the ones I think are the most flat, and can't seem to find one. I really don't want to buy another piece only to take it home and find out that the new piece isn't truly flat either. I've used cabinet grade and baltic birch all to no avail. I'm wondering if it is because the pieces of plywood weren't always stored horizontally.

I've contemplated buying the dubby sled or the rockler one, but both are a bit pricey.

I'm currently trying to weigh it down with some paint cans and hoping that it will go flat. I've had the weights on top of the jig for a day now and it's still crowned. All the commercial jigs are made of melamine, so I'm also contemplating buying a cheap cabinet and cutting it up. I've also experimented with MDF but it doesn't hold up well to screws or the day to day use in my shop. Anyone have any suggestions?

Thanks in advance!

Jason


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## 1yeldud1

I used "MDF" but i keep the sled waxed and I hang it on the wall when Im not using it - I have noticed that when I ocassionaly "drop" it the corners dont fair too well. I was wondering what about the cut out piece from a sink install into a kitchen counter - is that material flat ???


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## bunkie

Let me guess, the plywood came from one of the two big home improvement stores, right?

Give Baltic or Finnish birch plywood a try. You can get it from Rockler or Woodcraft. It's much more dimensionally stable than the imported junk sold by the big guys. Alternately you can use MDF as 1yeldud1 suggested, but it makes for a heavy sled.


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## NiteWalker

+1 on baltic birch plywood. I get mine from rockler.


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## tfenne

I made my sled from 3/4" ply from the orange big-box store. They carry Columbia PureBond plywood which is american made and doesn't have any added formaldehyde in it. It worked absolutely fine for me.

My sled was constructed by cutting a 4' by 2 1/2' piece for the bottom and then screwing two 30' metal miter bars to it. I then cut four 4' by 5" pieces, glued two together for the front and another two for the back of the sled, and then attached with glue and pocket hole screws. I can't swear that the plywood was totally flat to begin with, but with the bars and front/back attached it's flat in it's final configuration and has plenty of support to keep it that way.


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## Gene01

+1 again, on the Baltic Birch.
2nd choice would be MDF….2nd only due to it's weight. It's definitely flat!


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## Sawkerf

Flat plywood isn't a myth, but you probably won't find it in a big box store. I make my sleds from cabinet grade ply (more than five plys) and can't remember having any warping.


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## davidiv

I've always used 1/2" baltic birch with no complaints - A couple of months back I tried 1/2" mdf for a new sled and have been happy with the weight and accuracy. I do keep it well waxed and try not to drop it. Last year one of the mags (fine woodworking?) featured a huge MDF sled. It looked wonderful until I thought about lifting it on and off the saw.


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## Rick Dennington

Try this: I've done it lots of times on 4'x 8' sheets, and it'll work for your sled, also.

Remove the runners for the miter slots….Lay the sled down on a flat table, and soak the plywood with water on both sides. Don't get it soggy….just soak it really good, and then put heavy weights on it, like a few cinder blocks, or the paint cans you mentioned (just make sure they are full or close). Also, you can use some cauls to keep it flat while drying….Let it dry for a couple of days, and it should be flat. Put the runners back on. Then add the front and rear braces across the width of the sled with screws…...no glue….It'll work…..


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## Betsy

Most of my sleds are cabinet grade ply - but I've also got a few made from 1/2" MDF. I prefer the ply because they are easier to pick up (weight wise).

Big Box ply is probably not going to give you anywhere near flat - at least in my experience.


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## Viking

Jason;

You might try 1/2" MDF with a layer of laminate on both sides.


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## tfenne

cr1: Check it out here . They use a soy-based adhesive instead of the usual UF one.


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## dusty2

When I want baltic birch plywood, I look here first. I seldom have to look any further.

However, occasionally I need to go to another outlet - never a box store. If my first choice fails me I go here.

Never a box store for lumber or plywood.


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## GregD

I have a sheet of 1/2" baltic birch that I got for jigs and it is flat-ish but not so flat. The odds of getting a flat piece are much better when you buy a better quality product, but it seems to me even the good stuff isn't always perfect.

IMHO, MDF is not flat so much as it is flexible and won't hold a warp as well as plywood, which is stiffer. I'm currently using 3/4" melamine from HD for my sleds and jigs. Works great. I recommend a bit of finish on any exposed MDF surfaces of the jig/sled to make them less moisture sensitive.

If ya got money to burn the Incra Miter 5000 works really well although I think they overstate the cross cut capacity by about 2".


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## breaknrn

Thanks Everyone (especially Rick).

I spent some time searching on Google and found several articles from eHow, Woodweb and Finehomebuilding that says that one of the reasons why plywood warps is because of uneven moisture content in the plies. The side which has less moisture (likely the one facing up or exposed to air) will tend to shrink, causing it to contract and thus cup in this direction. The solution according to all the articles is to use a spray mister or a sponge to wet the side which is cupping towards, and then let it dry out in the sun or in a warm room. Apparently, laying it out on grass (inside of the curve facing down) in the sun also does the same thing because moisture will be absorbed from the grass (wetting this side), while the sun dries out the moisture on the outer side.

As I've said, I see the problem in baltic birch as well as the cabinet grade stuff from HD and Lowes. Also, if you read the reviews on Rockler, it sounds like getting a flat plywood board is hit or miss (probably due to the moisture problem). I'm going to try this and let people know how it goes.


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## Earlextech

Baltic birch 3/4".


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## longgone

I also vote for the baltic birch. I use the 3/4 and buy 60"x60" sheets at the lumberyard for $40.


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## Finn

I use MDF with no problems.


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## woodymays

I have to confess that I buy birch plywood from Menards. I would have to drive a ways to get to a regular lumber yard. Carters is close by, but I dislike how the wood is stored. I have built a sled for my saw and have no issues at all. I not saying Mernards is the best, but you have to be selective in what you buy.


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## casual1carpenter

I look to GregD's statement, "MDF is not flat so much as it is flexible and won't hold a warp as well as plywood, which is stiffer." I also understand that the Express is 0.540" (1/2" MDF, 0.020" laminate top & bottom) with of course the fancy accurate metal parts, along with slider strips. If the saw table is flat I would consider the sled would follow suit if it is not stiffened out of the flat plane. Myself, I use 3/4" melamine also, it's relatively cheep and suits the job. On my poor excuse for a table saw I often make up a jig, cut the jig so as to give a slot to help with desired kerf placement and lineup and act as a zero clearance for the blade, kind of like an old miter box.


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## waho6o9

MDF with UHMW runners from Rockler, easy peasy.


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## madts

I have had very good luck with MDO ( medium density overlay) plywood. This stuff is great for all cabinets that is to be painted.


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## MrRon

When I want baltic birch plywood, I look here first. I seldom have to look any further.

However, occasionally I need to go to another outlet - never a box store. If my first choice fails me I go here.

Never a box store for lumber or plywood.

Dusty2. I contacted Hood in Mobile, Al, but they only sell wholesale. I know there are great materials available, but I can't find any retailer who carries quality plywoods and woods. Sure I can buy from Woodcraft or Woodworkers, but if you need a 4×8 sheet, shipping costs become prohibitive. Any advise on how to obtain quality materials on a local level?


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## DIYaholic

Ask a local cabinet maker. Either purchase from them or piggy back on their order.


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## Patches

Baltic Birch, definitley. I use it almost exculusively. I buy it from a Doit Best lumber yard and have been for 12 years now. Ask and you might be surprised what they will get or have on hand. 1/2" Baltic Birch is $25 for a 5' x 5' sheet and it is delivered for $10 with a sheet of 1/8" or 1/4 packing, free Jig material. I usually buy 3 or 4 at a time.


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## Tomj

I have been wanting build a sled for my table saw for some time now but the miter tracks are a weird t-track size from Craftsman not to mention I have not been able to find one non-warped piece of plywood from my local big box store, I can't find any non-warped MDF either and I have gone through the whole stack 1/2" mdf and 3/4" the 3/4" has some close to flat but still. I just going to have to look elsewhere as in a local lumberyard i haven't checked out yet. Good luck with your sled.


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## dubsaloon

Seal the material no matter the kind you choose. I live in the south and the variations of moisture can make sheets of MDF or Plywood into potato chip looking thingies. Seal them when they are flat and avoid the woulda, coulda, shoulda later.


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## crashn

If the piece is only marginally cupped, use hardwood fences front and back, which should/could bring it back to flat.


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## BritBoxmaker

Birch ply is usually pretty good but MDF is more reliably flat.


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## sandhill

Try HDO its denser then MDO and can often be re-used if needed you can put a coat of wax on both sides.


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