Baltic birch plywood with twist

[Mark Rainey]

I have read about the superiority of baltic birch plywood in general. I want to make some jigs ( bent laminations, shooting boards, etc ) and bought a couple of 3/4 inch 5 by 5 sheets. When working with it I notice my boards have some mild twist. Not real obvious, but enough to concern me, and I just don't feel comfortable using it. I thought the stuff would be dead flat, or at least close to that. Any thoughts?

[glenn bradley]

Surface area versus thickness may yield flex in any number of products. Sheet goods do not tend to hold themselves dead flat without support; ply does better than composites. If the material is arched when laying on a flat surface it has probably picked up some memory due to improper handling. Sheet goods should be stored flat.

All that aside, jigs normally consist of multiple parts and clamping the assembly to a known flat surface like your bench, assembly table or a machine table should yield a good result once the glue dries. I have a lot of jigs and they remain stable and consistent. I would forge ahead.

Ready to clamp.jpg . Shootingboard v2 (1).jpg . Taper Jig (15).jpg

[Mark Rainey]

Thanks for the wisdom Glenn!

[Larry Frank]

The place I buy my Baltic birch store the sheets flat. The sheets I get are very flat. How any plywood product is stored is critical.

I dislike the orange and blue big box stores because the plywood is stored in arms and has a wave.

[Andrew Seemann]

I find baltic birch, which is one of my favorite materials, to be a little prone to cupping/warping, with thinner thickness being the most prone. Even though there are lots of plys, birch is an unstable wood. While lots of cross laminated plys of an unstable wood yields a more stable piece, it is still a little bit unstable. I find the most stable plywood to be the USA made 3/4 birch veneer plywood. The core of that seems to be either fir or poplar, which is much more stable than birch. I tend to use that for jigs. As Glenn says, having right angles, braces, etc. does wonders for stability as well.

[Mark Rainey]

Thanks for the advice Larry & Andrew

[Ira Matheny]

I, too, have found 'Baltic Birch' ply to be unstable. So when I returned it to Aura Hardwoods in Modesto CA, I learned it was not BB, but a Chinese 'Knock-Off'.
I then insisted upon genuine BB. Paid only a couple dollars per sheet more than the knock-off price, but I got good, first quality stable ply. So now, I always insist upon REAL Baltic Birch!

[Phil Mueller]

I’ll second the way that it’s stored matters. My local Rockler stores it vertical and you have to sort through it to find reasonably flat pieces. My local Woodcraft stores it flat, and it’s as perfect as you can get. Also 1/4” and even 1/2” can be tricky. I think the stores shudder when I start looking, because I usually have every sheet pulled out (of course I do put it all back!).

[Ben Rivel]

The place I buy my Baltic birch store the sheets flat. The sheets I get are very flat. How any plywood product is stored is critical.

I dislike the orange and blue big box stores because the plywood is stored in arms and has a wave.

Same here.

[Jim Becker]

If my sheet goods supplier inadvertently sends a "non-flat" piece of material...and it's not inexpensive "shop grade" stuff...I note it and they replace it. Most of the non-fat stuff I've seen (including 5x5 baltic birch multi-ply) originates from smaller suppliers who carry it as a convenience and don't necessarily have proper flat storage setups for the material.