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Flat Plywood For A Table Saw Sled - A Myth?

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Forum topic by breaknrn posted 11-08-2011 09:06 AM 5771 views 0 times favorited 29 replies Add to Favorites Watch
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breaknrn

39 posts in 2749 days


11-08-2011 09:06 AM

Topic tags/keywords: jig question tablesaw

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

-- breaknrn


29 replies so far

View 1yeldud1's profile

1yeldud1

298 posts in 1795 days


#1 posted 11-08-2011 12:27 PM

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

411 posts in 1900 days


#2 posted 11-08-2011 01:11 PM

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.

-- Altruism is, ultimately, self-serving

View NiteWalker's profile

NiteWalker

2710 posts in 1330 days


#3 posted 11-08-2011 01:59 PM

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

-- He who dies with the most tools... dies with the emptiest wallet.

View tfenne's profile

tfenne

6 posts in 1176 days


#4 posted 11-08-2011 02:07 PM

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.

View Gene Howe's profile

Gene Howe

6058 posts in 2181 days


#5 posted 11-08-2011 02:40 PM

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

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

View Sawkerf's profile

Sawkerf

1730 posts in 1821 days


#6 posted 11-08-2011 04:23 PM

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.

-- Adversity doesn't build character...................it reveals it.

View David's profile

David

13 posts in 1207 days


#7 posted 11-08-2011 04:42 PM

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.

View Rick  Dennington's profile

Rick Dennington

3613 posts in 1947 days


#8 posted 11-08-2011 04:47 PM

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…..

-- " I started with nothing, and I've still got most of it left".......

View Betsy's profile

Betsy

2914 posts in 2649 days


#9 posted 11-08-2011 08:09 PM

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.

-- Like a bad penny, I keep coming back!

View Viking's profile

Viking

857 posts in 1948 days


#10 posted 11-08-2011 08:15 PM

Jason;

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

-- Rick Gustafson - Lost Creek Ranch - Colorado County, Texas

View tfenne's profile

tfenne

6 posts in 1176 days


#11 posted 11-08-2011 08:32 PM

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

View dusty2's profile

dusty2

319 posts in 2182 days


#12 posted 11-08-2011 09:53 PM

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.

-- Making Sawdust Safely

View GregD's profile

GregD

637 posts in 1889 days


#13 posted 11-08-2011 10:09 PM

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”.

-- Greg D.

View breaknrn's profile

breaknrn

39 posts in 2749 days


#14 posted 11-08-2011 11:33 PM

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.

-- breaknrn

View Earlextech's profile

Earlextech

1024 posts in 1443 days


#15 posted 11-09-2011 12:39 AM

Baltic birch 3/4”.

-- Sam Hamory - The project is never finished until its "finished"!

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