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I have a customer who wants 1-1/4" thick cabinet boxes. My preference is to laminate 5/8" plywood, however it's not available. I could use 1/2" & 3/4" - but in a sense it's unbalanced. Is that a concern?
Does it have to be plywood? What about just veneering a 1 1/4" substrate?
I could balance 3/4" ply with 1/4".
I would love to try 3/4" lumber core ply laminated with 1/4" MDF and veneered - but was told it hasn't been seen in these parts for 30 years.
We use to glue up pre-finish plywood, Oak, Maple, Beech.... With adhesive and industrial glue depending on quantities. To make 1"-1/2 material we were gluing 2 sheets of 3/4 plywood ea and there was no warping. We also did 2", and 2-1/4.
All this was mostly done on a heath press industrial size machine.
I will suggest gluing the two 5/8 plywood to make the 1-1/4 material and see if it will warp. (it shouldn't)
I use 3/4" veneer core plywood and two 1/4" HDF crossbands on either side. That gives you a balanced construction and a good surface to laminate the veneer face/back onto. It's basically the same as "combination core", which is an up-grade from regular veneer core.
Don't go the lumber core route, there is a reason it's no longer in widespread use. It's an inferior product at a superior price.
can someone explain why gluing together two different thickness plys would lead to warping issues
It wouldn't necessarily lead to warping, but it could. Normal plywood construction calls for every other layer (ply) to be perpendicular (90 degrees) to the layers on either side of it and an odd number (3,5,7,9, ect) of layers, so that each outer face has the same orientation (long grain or short grain). If you take two seven ply panels and glue them together, you now have 14 ply plywood, not an odd number of plies, so that means that two of those plies have the grain direction running parallel instead of perpendicular. That's where your problem is likely to occur, the veneer will "pull" more across the grain than with the grain and in the case I stated above, you would have 6 crossgrain plies and 8 long grain plies. That's not a balanced construction and can lead to problems.
Does that help you?
John thanks for explaining. I assumed that layer orientation was the reason though I wasn't sure how much of "real" concern it was