Allowing expansion and contraction of wood
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Hello everyone,
There is something I don’t quite understand, even after watching dozens of Paul Sellers videos.
When we use screws joining two pieces of wood, we have to keep in mind that it is necessary to leave space for expansion and contraction to occur (I’m remembering the previous project, for example, where the drawer runner was screwed to the side of the side cupboard and one of the screw holes was wider than the screw itself).
However, when the pieces are glued together (imagine the top of that same side cupboard dovetailed with the sides), how will there be expansion and contraction without cracking the wood?
Or for example, the rail of a table glued to the legs with mortise and tenon: there is no room for expansion and contraction of the rail.Thanks.
I too have wondered about this at one point. Below is all speculation:
I imagine it’s because the tenons are small and if they contract, it might not be enough to crack, either because the wood itself can sustain it, or the glue is a little bit flexible and can move a tiny bit.
As an aside, another related curiosity:
How do guitar backs not always crack? I’ve seen them crack below 30% humidity, but why is everything else pretty much ok? The back is very wide and very thin, and is glued to braces running across the grain. Logic tells me this is a terrible idea and it should always crack a lot given enough time… But it doesn’t nearly that much, apparently.
Does wood really only crack like that under extreme conditions, and otherwise it’s fine?
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