quarter sawn oak and plywood glue up

[Brian Knop]

I want to glue some quarter sawn white oak to some plywood to make some four sided post... Question; how thick can one go without having to worry about the wood trying to move and split?

[Jamie Buxton]

I'm thinking you're short on the quartered oak, so you're going to stretch it by laminating thin layers of it to plywood to form the posts. Instead of laminating it to plywood, you could use a solid-lumber substrate. Use poplar or pine or what have you for the substrate. Run the grain direction of the oak the same as the substrate. There's a couple nice things about this approach. First, the substrate and the veneer grow and shrink in exactly the same way, so that removes your orginal question. Second, the laminated plank can be handled just like solid lumber. In particular you can edge-joint it for tighter joints than you can plywood.

[Joe Jensen]

If you still want to use plywood as a substraight, I've used 1/8" with no issues. I've read one can go up to 1/4". I personally would not go that thick, but I can resaw on the bandsaw and get 1/16-1/8" easily.

[Howard Acheson]

In general, 1/8" thick is about as thick you could go without problems. I've seen 1/4" both fail and work.

[Paul Atkins]

"First, the substrate and the veneer grow and shrink in exactly the same way" Yes, but not at the same rate if they are different species.

[Jamie Buxton]

Yes, but not at the same rate if they are different species.

That's technically true, but of little practical consequence. A substrate that moves almost like the veneer stresses the veneer much less than plywood, which doesn't move at all.

[Paul Atkins]

I was thinking of a thicker veneer that might result in cupping if glued to poplar or pine. Obviously, it would be small, but glued to oak substrate would be less cupping.

[Jamie Buxton]

I was thinking of a thicker veneer that might result in cupping if glued to poplar or pine. Obviously, it would be small, but glued to oak substrate would be less cupping.

Here are expansion rates of a few wood species, quoted from Bruce Hoadley's book Understanding Wood. The rates are for shrinkage from green lumber to oven-dry, which is a much bigger moisture variation than completed furniture will see, but the numbers make the point. Sr is the shrinkage in the radial direction, and St is shrinkage in the tangential direction.

Red oak. St=8.9%. Sr=4.2%.
Sugar Pine. St=5.6%. Sr=2.9%.
Maple. St=8.2%. Sr=4.0%
Walnut. St=7.8%. Sr=5.5%.
Birch. St=9.2%. Sr= 7.2%

In any given species, the spread between tangential and radial shrinkages is larger than the difference between similar (for instance tangential) shrinkage in different species. I take that to mean that laminating two species doesn't stress the composite very much at all. Normal grain direction changes in unlaminated lumber give larger shrinkage variations. If one wanted to be very careful about laminations, it would be more important to match the grain directions than to match the species.