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Cross cut sled flat ness

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Forum topic by scribble posted 11-04-2015 01:35 PM 998 views 0 times favorited 29 replies Add to Favorites Watch
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scribble

113 posts in 1661 days


11-04-2015 01:35 PM

Topic tags/keywords: question

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?

-- If you can't read it Scribble wrote it!! “Experience is merely the name men gave to their mistakes.”


29 replies so far

View HokieKen's profile (online now)

HokieKen

1738 posts in 599 days


#1 posted 11-04-2015 01:42 PM

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.

-- Kenny, SW VA, Go Hokies!!!

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WoodNSawdust

1417 posts in 637 days


#2 posted 11-04-2015 03:30 PM

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.

-- "I love it when a plan comes together" John "Hannibal" Smith

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TomsChips

6 posts in 1020 days


#3 posted 11-04-2015 03:37 PM



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

6 posts in 1020 days


#4 posted 11-04-2015 03:43 PM

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

113 posts in 1661 days


#5 posted 11-04-2015 03:59 PM

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.

-- If you can't read it Scribble wrote it!! “Experience is merely the name men gave to their mistakes.”

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Tony1212

111 posts in 1195 days


#6 posted 11-04-2015 04:14 PM

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.

-- Tony, SW Chicago Suburbs

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HokieKen

1738 posts in 599 days


#7 posted 11-04-2015 04:42 PM



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.

-- Kenny, SW VA, Go Hokies!!!

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rwe2156

2190 posts in 941 days


#8 posted 11-04-2015 08:28 PM

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?

-- Everything is a prototype thats why its one of a kind!!

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scribble

113 posts in 1661 days


#9 posted 11-04-2015 10:32 PM



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?

- rwe2156

What????

-- If you can't read it Scribble wrote it!! “Experience is merely the name men gave to their mistakes.”

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rwe2156

2190 posts in 941 days


#10 posted 11-04-2015 10:46 PM

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

-- Everything is a prototype thats why its one of a kind!!

View HokieKen's profile (online now)

HokieKen

1738 posts in 599 days


#11 posted 11-05-2015 03:38 PM

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?

- rwe2156

What????

- scribble

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.

-- Kenny, SW VA, Go Hokies!!!

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scribble

113 posts in 1661 days


#12 posted 11-05-2015 04:04 PM

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”

-- If you can't read it Scribble wrote it!! “Experience is merely the name men gave to their mistakes.”

View HokieKen's profile (online now)

HokieKen

1738 posts in 599 days


#13 posted 11-05-2015 05:57 PM



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.

-- Kenny, SW VA, Go Hokies!!!

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Pezking7p

3097 posts in 1112 days


#14 posted 11-05-2015 06:34 PM

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.

-- -Dan

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bearkatwood

1194 posts in 472 days


#15 posted 11-05-2015 06:35 PM

.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.

-- Brian Noel

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