Bowed stack?

JSquare · Post by **JSquare** » Thu Feb 28, 2013 9:50 am

Hey gang-

So I've got a few cores under my belt now. I buy 9 1x4s from the lumber yard, glue them up using titebond3 and about 8 pipe clamps, with the pipe clamps laying flat on the floor. Next, I resaw using a bandsaw, plane and join, and then marry two halves to make a snowboard. I get two cores this way.

One thing is getting me though. I've noticed that the stacks don't seem to come out straight. They take a bow or smiley shape (i guess frowny shape?).
Picture looking at the layers of the stack and noticing that they are all bowed from the top one to the bottom one.

Anyone run into this before? It can be off as much as 3/16"

heretic · Post by **heretic** » Thu Feb 28, 2013 8:30 pm

perhaps the wood is still wet? try stacking and drying before you cut/glue/clamp[/quote]

prospectsnow · Post by **prospectsnow** » Thu Feb 28, 2013 10:30 pm

Could be a couple things.

We ripped 1x4s into strips with a tablesaw for a while. Some, but not all bowed after a few days. I thought it might have been because the cuts weren't totally straight.

Now all my strips go through a CNC shaper and use a large clamp press. They come out totally flawless. They still bowe.

Pretty sure the properties in the wood change in the temp, air humidity, or fibers stretch differently after getting cut.

I usually just lay it toward the bottom of my stack of cores to let press flat again. I have pressed a bowed cores, it didn't affect quality.

Richuk · Post by **Richuk** » Fri Mar 01, 2013 3:22 am

I think Head Monkey has a section on storing wood - always worth a read.

If the underlying quality of the core is important, then you might want to start to look at issues such as 'dimensional stability of wood' and what the 'moisture content' of the wood you are using is, at the time of pressing.

It could relate to what part of the stack the wood originally came from when kiln dried, to heat generated while cutting/planning and or the grain interacting with the glue. I imagine SD is a good source for this detail? Personally, I tend to try and match the wood I'm using. Weight (just by feel is one indicator) as it the tone of the wood, when the end is bounced on the floor (drop it about an inch).

JSquare · Post by **JSquare** » Fri Mar 01, 2013 11:15 am

Ahh yes. This all would make sense. (someone needs to make a face-palm smiley)

Yo prospect-- My stack is bowing planer to the initial board face. Yours sounds as if it's bowing the other direction....question is... are you turning the grain when you rip and glue up?

To illustrate, i glue up 2 halves to make a symmetric board. When I do this, there are gaps along the seam. I can joint them out, but there is an obvious bow. Right now, i just clamp it and force them into shape.

skidesmond · Post by **skidesmond** » Mon Mar 04, 2013 4:07 am

Bowing can be causing by a couple of things, which Prospectsnow mentioned, moisture content and alternate the grain when gluing up. Those 2 things can minimize bowing.

I found that joining wood the edge can cause bowing to a certain degree. The joiner flattens the wood edge. You may think it's giving you a 90 degree edges but it doesn't unless you have an expensive parallelogram joiner. It will be close to 90 but not exact, the more passes you make the more pronounced the effect. Took me a long time to figure this out. Just for kicks take a scrap piece of wood and run the edge over the joiner 10 times, then put a square along each opposing edged. You'll probably see a less than 90 degree angle. If it's square then it's one less thing causing the bowing.

I usually take 1 or 2 very light passes to clean any saw marks from the table saw.

Huck Pitueee · Post by **Huck Pitueee** » Tue Mar 05, 2013 1:28 pm

Could be your jointer is slightly off 90 deg. Or. Do you put half of your clamps under the glueup and half over to equalize clamp forces? If the wood is still wet the inside of the plank is wetter than the outside so when you split them they're likely to smile.

JSquare · Post by **JSquare** » Tue Mar 12, 2013 7:17 pm

Whew. I'm sorry guys. I thought I did a good job describing this, but something went astray somewhere...

My bow is in my stack of 1 x 4s , than I re saw into 4 snowboard core halves. It shows up after clamping, but becomes obvious when i lay two halves side by side on a flat surface and the edges are bowed. The fix is to joint a bunch of material off, or just clamp the piss out of it...which has been my path to date.

The bow is is planer to the planks. It has nothing to do with the jointer... but i learned a lot here about some other problems I'll likely face sooner or later, so I appreciate all the input. I'll differ to happy/head monkey's wood storage advice and assume moisture is playing a role here.

Head Monkey · Post by **Head Monkey** » Sat Mar 16, 2013 11:55 am

JSquare wrote:My bow is in my stack of 1 x 4s , than I re saw into 4 snowboard core halves. It shows up after clamping, but becomes obvious when i lay two halves side by side on a flat surface and the edges are bowed. The fix is to joint a bunch of material off, or just clamp the piss out of it...which has been my path to date.

Yea, I get this too sometimes. Depends on the stack. It's awfully hard to clamp a stack that's 7-8 feet long with mulitple clamps and have it come out perfectly straight. It's also hard to plane something that long and heavy flat afterwards, too. Honestly, for me if the bow is small I just clamp it. If it seems too big to clamp I go ahead and clean it up with a jointer, clamping the two halves together and doing them together.

Now, I'm assuming the total gap between your halves is still pretty small... on the order of 1/8" or less. If you've got a big bow and you're having to plane significant material to get them to join then I'd look at changing something about the way you clamp the stacks. Perhaps a long straight edge would help you see the bow as you're clamping, and allow you to bend the stack closer to straight as you go. Clamp from the center outwards, of course, and I think you'll find the stack is fairly flexible when you just have the center clamped.

tufty · Post by **tufty** » Sat Mar 16, 2013 1:40 pm

Head Monkey wrote:Perhaps a long straight edge would help you see the bow as you're clamping, and allow you to bend the stack closer to straight as you go. Clamp from the center outwards, of course, and I think you'll find the stack is fairly flexible when you just have the center clamped.

I clamp direct to a builder's straightedge, centre outwards as head monkey suggests.

PTTR · Post by **PTTR** » Sun Mar 17, 2013 10:15 am

This is how I do it now. I used to do the "glue up a stack and resaw" but think this is a lot simpler.

http://youtu.be/0DWnojBzc8E

vinman · Post by **vinman** » Sun Mar 17, 2013 1:00 pm

I like the idea of a piece of 1/2" steel tubing to create a nice flat surface to clamp against. to prevent bowing. I rip all of my hardwood 17x19mm and reglue in 4 way wood clamps. I can lay up a pair of cores in one glue up.I see some small bowing on occasion, less that 4mm usually. This usually does not effect my core at all since I'm cutting away those outside stringers anyway to shape the cores down before attaching the sidewalls.

Why clamp from the center out? Does this somehow distribute the forces better?

JSquare · Post by **JSquare** » Mon Mar 25, 2013 2:22 pm

Great last few posts. And yes...why must we clamp from the center out?

JSquare · Post by **JSquare** » Mon Mar 25, 2013 2:25 pm

PTTR wrote:This is how I do it now. I used to do the "glue up a stack and resaw" but think this is a lot simpler.

http://youtu.be/0DWnojBzc8E

Holy smokes what a dialed method. The time lapse turbo speed makes it seem even faster.

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