# Cross cut sled flat ness



## scribble (May 17, 2012)

I am finally building my cross cut sled after some time away from the shop. I have glued up 2 peices of 3/4 Birch plywood before taking a few months off from the shop. I went check my fence blanks this morning to find the best one and found the best one to be .023 out of flatness in the middle. Wondering if there is anything I can do other than scrapping it and building another set of fences?


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## HokieKen (Apr 14, 2015)

Can you glue a piece of hardwood to the face and plane it flat? Only other suggestion would be to put a stiffener across the back, maybe a piece of steel angle to pull it back flat.


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## WoodNSawdust (Mar 7, 2015)

Sorry, I would scrap it. I have found that any inaccuracies in a jig will compound with the pieces cut. So, is the cost of another piece of plywood worth the frustration and problems with anything you build with the jig? This was a hard hard lesson for me to learn.


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## TomsChips (Feb 17, 2014)

> Sorry, I would scrap it. I have found that any inaccuracies in a jig will compound with the pieces cut. So, is the cost of another piece of plywood worth the frustration and problems with anything you build with the jig? This was a hard hard lesson for me to learn.
> 
> - WoodNSawdust


Very good advise!! I second this from likely similar experiences…


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## TomsChips (Feb 17, 2014)

I'm not sure what type of plywood you are using, but IF you are using the stuff from one of the box stores. The consistency (flatness/thickness) of even the stated "cabinet grade" ply I find at the three main box stores are generally not good enough for fixtures and jigs. I have found that the pre primed MDF stock as in for shelving is very constant and uniform in a single piece, not always from piece to piece however. This is just my experience …. take it for what its worth…

good luck


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## scribble (May 17, 2012)

MY first sled I made used 1/2 birch from WC and after making my blade cut through the 2 fences the left side lifted 1/8 scrapped that one also. I'm thinking of leaning towards using MDF for stuff like this from now on.


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## Tony1212 (Aug 26, 2013)

You don't mention if it's high (straight edge rocks) or low (gap under straight edge).

0.023" is just over 1/50" (somewhere between 1/43" and 1/44"). According to the Menards website (only place I could find this info - a pdf tech spec linked from the product page) their 3/4" Birch plywood has face veneer that is 1/36" thick. Assuming you go at it slowly, you could probably sand it flat without sanding through the veneer.


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## HokieKen (Apr 14, 2015)

> MY first sled I made used 1/2 birch from WC and after making my blade cut through the 2 fences the left side lifted 1/8 scrapped that one also. I m thinking of leaning towards using MDF for stuff like this from now on.
> 
> - scribble


I used MDF for the base of my crosscut sled and have used it for all of my tool tables I've built. It is very stable and so far has stayed flat. BUT you have to make sure it's flat when you get it. I have had pieces from big box stores that I got home to find weren't flat at all.

I use 3/4" mostly but I used 1/2" for my cc sled so I didn't loose cut capacity. The 1/2" works great, but it's also pretty flexible so if your sled's going to be really big, you may need something thicker for stability. For the fences on my sled, I used 3/4 ply for the front fence, which is just a stiffener on my sled, and some salvaged white oak for the rear fence. I have MDF fences on my DP and router table but I think going forward I'll use hardwood so that if/when they do go out of flat, I can just pull them off, joint the face and put them back on.


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## rwe2156 (May 7, 2014)

Screw it to a jointed piece of ply to bring perfectly straight the screw that do the sled.
You don't need a doubled up piece, either.

.023 isn't close enough for ww'ing?


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## scribble (May 17, 2012)

> Screw it to a jointed piece of ply to bring perfectly straight the screw that do the sled.
> You don t need a doubled up piece, either.
> 
> .023 isn t close enough for ww ing?
> ...


What????


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## rwe2156 (May 7, 2014)

Screw it to a jointed piece of wood at right angles and that will pull it flat.
Still not making any sense?


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## HokieKen (Apr 14, 2015)

> Screw it to a jointed piece of ply to bring perfectly straight the screw that do the sled.
> You don t need a doubled up piece, either.
> 
> .023 isn t close enough for ww ing?
> ...


rwe's saying to make an "L" shaped fence. If the bottom piece, which gets bolted to the sled, is jointed on the edge that attaches to the upright, when you screw the 2 together it will pull the upright back to flat. Assuming of course that the bottom piece is stiff enough that it doesn't flex.

.023" is a lot of daylight under a straight edge. I don't think I would have let it go either. But rwe's point is valid, it doesn't need to be dead flat. Which is a good thing 'cause I've never seen a piece of wood that was. Even if starts that way, it won't stay that way.


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## scribble (May 17, 2012)

ok so how much is exceptable for out of flatness. I understand there should be none. but I don't want to keep making scraps due to it not being "flat"


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## HokieKen (Apr 14, 2015)

> ok so how much is exceptable for out of flatness. I understand there should be none. but I don t want to keep making scraps due to it not being "flat"
> 
> - scribble


That's a question that can't be simply answered. I depends on a lot of factors. For instance, let's assume you have .023" cup, or concavity, right in the center of a 24" fence. If you're cutting a board and the end of the board seats right in the center of the cup then (math, math, math, math) your going to end up with a cut .1 deg out of square which is about .002" per inch of board. No big deal unless your cutting something 24" wide and really need it to be square. On the other hand, if your board spans the entire fence, it'll bridge the cup and there'll be no effect.

So, having done the math, I'd say you're probably just fine with .023" out of flat. The above case is worst-case-scenario for a 24" fence. If the fence is longer, it's a little less error. If the fence is shorter, it's a little more error.

In short, it's really subjective. It needs to be as flat as you need it to be. For me, I rely on a straight edge. If I can see more than just a hint of daylight, it's not flat enough. But that's mostly just the machinist in me coming out  Metal behaves much differently than wood though, so I'm probably overly-anal about it. rwe was probably right, you're good to go.

How are you attaching your fence to the sled? If your screwing it on (which I would recommend so you can adjust it in the future if needed), I'd say put it on there, square it up, and make a few test cuts with the board hitting at different points along the fence. You'll probably find that you're fine with it the way it is. If not, take the screws out and fix it or make a new one.

Good Luck! Make sure you let us know how it comes out and what you end up doing with it.


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## Pezking7p (Nov 17, 2013)

I had a similar issue after glueing up two pieces of plywood for my sled's fence. I set my power jointer to an extremely light cut and ran it across to flatten. Worked a real treat.


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## bearkatwood (Aug 19, 2015)

.023 out over how far a distance. If you said 24" I would say that it won't work for NASA, but it should work for woodworking. Now .023 out in 6" is another thing all together.


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## scribble (May 17, 2012)

.023 out in a 28" span cupped in the center. I followed the Wood-whispers build plans when building it.I think I am going to give it try with the five cut method and worst case is I have to build a new one.


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## AZWoody (Jan 18, 2015)

> .023 out in a 28" span cupped in the center. I followed the Wood-whispers build plans when building it.I think I am going to give it try with the five cut method and worst case is I have to build a new one.
> 
> - scribble


I'm not sure how using and kind of method to get square will work if the piece is cupped to begin with.


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## scribble (May 17, 2012)

> .023 out in a 28" span cupped in the center. I followed the Wood-whispers build plans when building it.I think I am going to give it try with the five cut method and worst case is I have to build a new one.
> 
> - scribble
> 
> ...


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## HokieKen (Apr 14, 2015)

> .023 out in a 28" span cupped in the center. I followed the Wood-whispers build plans when building it.I think I am going to give it try with the five cut method and worst case is I have to build a new one.
> 
> - scribble


Oh yeah. Over 28" you should be in good shape. The 5-cut method is a great way to square it up too. I'd do it with the largest scrap I had that is manageable on the sled. That way, you'll be kind of averaging out the error of the cup. You'll get the best result if the center of your 5-cut board hits at the center of the cup. So if the center of your fence is 8" from the blade, a 16" 5-cut board will give you the best results.


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## Pezking7p (Nov 17, 2013)

Don't use it at 23 thousandths off. You won't be happy. Plus it's 23 thou over 12" if I'm understanding you correctly.


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## scribble (May 17, 2012)

> Don t use it at 23 thousandths off. You won t be happy. Plus it s 23 thou over 12" if I m understanding you correctly.
> 
> - Pezking7p


Dan you are understanding correctly. I think at this point i'll try the slight shave on the jointer and if that doesn't do any good build another with hardwood.


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## scribble (May 17, 2012)

> Don t use it at 23 thousandths off. You won t be happy. Plus it s 23 thou over 12" if I m understanding you correctly.
> 
> - Pezking7p


Dan you are understanding correctly. I think at this point i'll try the slight shave on the jointer and if that doesn't do any good build another with hardwood.


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## rwe2156 (May 7, 2014)

> .023 out over how far a distance. If you said 24" I would say that it won t work for NASA, but it should work for woodworking. Now .023 out in 6" is another thing all together.
> 
> - bearkatwood


You won't be satisfied with it for the reason Hokie gave you.

You have a jointer? Man, your problems are over!


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## scribble (May 17, 2012)

> .023 out over how far a distance. If you said 24" I would say that it won t work for NASA, but it should work for woodworking. Now .023 out in 6" is another thing all together.
> 
> - bearkatwoodYou won t be satisfied with it for the reason Hokie gave you.
> 
> ...


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## scribble (May 17, 2012)

Well I was able to get the fence to the jointer before the noise curfew cut in but it looks like I was able to salvage the fence. I am now only getting .001 out of flatness.

i appreciate everyones help. hope to be able to get out to the shop and try to finish the sled still tonight


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## CecilD (Jan 6, 2009)

Has anyone used HDPE to build one or is it not a good option? Maybe Lexan would work also?


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## HokieKen (Apr 14, 2015)

> Has anyone used HDPE to build one or is it not a good option? Maybe Lexan would work also?
> 
> - CecilD


Either one of those should work well as long as it's thick enough to be rigid. I have only seen 1 made from lexan and never seen an hdpe sled. Probably because plywood and mdf are well suited materials and much more cost effective. A slab of 1/2" hdpe or lexan that size will set u back a pretty penny.


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## oldnovice (Mar 7, 2009)

*HDPE* would need to be very thick for a large sled.
I had a piece 8" square 1/4" thick that, in my opinion would not be stiff enough.

If I build a new sled I woul use micarta, either linen or canvas, very stable but, not inexpensive sled.
This material is similar to the circuit board material which is G10 FR4 Glass Epoxy and used because of its thermal characteristics. (250" x 24" x 36" CE Canvas/Phenolic Laminate Sheet at $112 without shipping)

I have used 1/4" PaperStone counter top material for other projects, very stable, very heavy, and a expensive.

Right now my sled is 1/4" peg board with a steel runner.
The holes in the peg board help with all kinds of setups!
When the peg board warps I get a new piece!


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## lateralus819 (Mar 24, 2013)

After watching William Ng' video, I used a S4S piece of 8/4 mahogany to glue my two fence blanks. Dead flat.


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