Should I orient the cupped sides toward each other when laminating boards?

Question

I'm making a replacement press plate for a small apple press using some edge glued oak boards from a hardware store. The board was flat as purchased, and after acclimating to my shop conditions remained flat until I cut out the required circles. As I cut I noticed them bending away from the board, revealing pre-existing stress in the wood.

My intention is to glue these together into one thick plate, with the grain orthogonal to each other. The press metal pressure spreaders will be orthogonal to the grain of the top piece. I'll be using titebond ii glue with deck screws.

Given the use case of these parts and the cupping present, how should I orient the faces?

Cupped sides both down, up, together, or apart?

Answer 1

Honestly if the glue-up is done right I don't think this is going to matter much, or at all.

Following on from the same principle as sprung-jointed edges (where a very slight hollow is deliberately planed on board edges) you might like to have the cup in between. Obviously in this case you need to take pains to have sufficient clamp pressure in the centre to ensure that the two boards come into intimate contact — they must press firmly together.

So alternatively, prefer to have the boards where they cup away from each other as it's far easier to apply generous clamp pressure around the perimeter than in the centre (use a dozen clamps if needed).

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In case it needs to be said, the mating surfaces should be freshly worked wood. Obviously any piece that has been handled a lot might have minor surface contamination, but material from the wood itself can rise to the surface and interfere with a proper glue bond forming.

So as a rule you don't want to glue any wood surface that was worked some time previously. Even a surface some hours old can glue less well than one worked just a few minutes prior to glue application. So lightly scrape or sand the wood just prior to the glue-up.