Why would a 3"x4' strip of laminated baltic birch warp?


I glued up two three inch strips of baltic birch plywood intended for special-purpose fence. After attaching a 3/4" strip of ash to the top, the damn thing warped. Not severely but enough to annoy me. Any ideas why this would have happened?
And, if I leave a forty pound cement block resting on it overnight, is there any chance of "unwarping" it? Or is it a lost cause?
I thought the baltic ply was supposed to be extraordinarily stable but this is my first time building a jig with it.
Your insights are appreciated.
Regards,
--
Keith Hanlan
Ottawa, Canada
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snipped-for-privacy@hanlan.ca (Keith Hanlan) wrote in

Did you glue the ash to the baltic birch ply glueup, perhaps?
Was the glue waterbased?
Maybe the ash has different expansion characteristics than baltic birch ply, particularly since it still has 'grain'.
The advice to veneer both sides of a piece is so that the veneer won't warp the substrate.

If you set it down on edge in a well ventilated corner of the shop for a few days, it may settle back down. Then, perhaps, joint the face lightly, to establish 'flat'. Check again in a few more days. Then shellac everything lightly, wax the surfaces, and enjoy.

Compared to many other materials, it is.

And worth at least what you paid for them.

Patriarch
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On another forum, problems associated with veneering only one side of a piece have been discussed numerous times. It appears to be the most common cause of warping. JG
Patriarch wrote:

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Yes, the cross-section looks like:
<-- 1.5"-->
+---------+ --- | ash | ^ +---------+ | |. . |. . | | |. . |. . | | |. . |. . | | |. . |. . | 3" |. . |. . | | |. . |. . | | |. . |. . | | |. . |. . | | |. . |. . | v +---------+ ---

generic yellow PVA - I think that's a yes?

I guess that's all it takes. The ash had been well-dried but I glued it shortly after ripping the 1.5" strip from a 5" strip. That was probably my mistake. I should have let the strip relax after disturbing its equilibrium and then jointed it again.
My dumb mistake but that's how I learn...

I think that that advice only really applies to wide surfaces. I was just covering up the edge of the baltic ply.

That's not really an option for baltic birch plywood. The outer veneers are too thin.

That's a good idea. I think I'm going to need to finish my jigs from now on.
Thanks for your advice.
--
Keith Hanlan
Ottawa, Canada
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I had envisioned a three-layer sandwhich, with the ash on the face. Now that I re-read your post, I see that I read you incorrectly.

Sometimes, it's good to remember that it's wood, and not entirely controllable.

That advice was only partly serious. Shellac and wax on everything seems to be a regular habit of mine. ;-) Moisture absorbtion control is a good thing, however.

So what is the purpose of the ash?
BTW, Bridger's suggestions make a ton of sense to me. But then, they often do.
Patriarch
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Patriarch asked:

I wanted to put a 3/4 x 3/8 dado in the top for a t-slot extrusion. I though that the ash would be a better seat for it than plywood.
I also completely forgot that I now have a dado blade set for my table saw (I've only used it twice in the past six months) and didn't like the idea of forcing a router bit through that much plywood glue.
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Patriarch wrote:

AWWWWW.....
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Keith Hanlan wrote:

That's a yes.
Were the plywood strips cut the same way and glued symetrically?
E.g. if the plywood was A-B you'd want to glue A-side to A-side or (more likely) B-side to B-side and have parrallel grain on both sides of the glue joint.
If that is how you did the plywood then I concur that the ash must have crooked (crook is at right angles to bow)
--

FF


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Keith Hanlan wrote:

If it were me, I'd trash the existing and start over.
Call it tuition if you want to fell better about the loss<G>.
This time, I'd cut some rabbits in both the ply and the ash, then glue it up as an assembly.
Trust me, it won't warp.
Lew
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On Tue, 19 Jul 2005 20:50:40 -0400 (EDT), snipped-for-privacy@hanlan.ca (Keith Hanlan) wrote:

I build them from baltic birch, but I don't just stack them face to face and glue them up. I assemble them into hollow square tube box beams. way strong, way stable and lightweight for their size.
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That's a good idea. Having a four-inch wide beam would make it easier to place the second fence perpendicular to it. (What I am trying to do here is have one extra-long fence that attaches to my current mitre fence and then have a second extra-long fence that I can add to and remove from it at exactly 90 degrees. I want to do this so that I can cut mating oblique compound mitres at the "right" end of one board and the "left" end of the other *without* adjusting either the mitre or the blade. This would be easy with a dual-bevel compound mitre saw but I can't afford one right now.
The original reason for the ash strip, which I hadn't mentioned, was because I wanted to put a 3/8" x 3/4" dado along the top for a T-slot extrusion. I though that putting it into the edge of the plywood (even doubled to 1.5" thick) wouldn't be a good idea. I was probably wrong on this count. I also planned to put a vertical post into each of the fences so that I could run something rigid between the two posts to fix the two fences at 90 degrees. I think now that I'll use a square sheet of plywood that has brackets that hang over the top edge of each fence.
Time to sit back and think a bit harder about what I'm trying to do here... (persuade SWMBO that $400 for a nice mitre saw might be a good investment...)
Regards
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Keith Hanlan
Ottawa, Canada
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Keith Hanlan wrote:

bridger wrote:

Keith Hanlan wrote:

consider a miter sled. if you need complementary miters at other than 45 degrees, you could make the head swivel. I made mine from 2 triangles of 3/4" baltic birch ply (face laminated!) on a 1/4" melamine bed with baltic birch for the runners. 2 runners seems to work better for me than 1. the triangle of BB has a hole drilled in it to get the head of a bar clamp into to keep the workpiece from slipping around during the cut.
Keith Hanlan wrote:

no,you were probably right on this count.
I run one face of the box beam long to make a lip I can clamp stuff to.
I'm not a big fan of aluminum t slot extrusions. the inclusion of a high thermal coefficient of expansion but low humidity coefficient of expansion material like aluminum into a low thermal coefficient of expansion but high humidity coefficient of expansion material like wood makes me nervous. but then I live in Arizona.
mostly, though I don't like aluminum's tendency to gall.
Keith Hanlan wrote:

that's probably a good approach.
Keith Hanlan wrote:

my table saw with a miter sled way outperforms any miter saw I have ever used. the mass of the saw, beefier arbor, bigger motor belt drive, etc... all add up to a much better cut. I have _seen_ miter saws that will outperform my sled setup- but they cost more than my truck. heck the blades alone cost more than my miter saw. my bigger miter saw has deeper cut capacity than my table saw with a miter sled setup, but the cut quality is less than stellar.
consider investing in a miter trimmer instead.

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