On 11/20/2013 6:05 AM, dadiOH wrote:
I used to make lots of them. Now, less because I have more tools but I
still do on occasion and I wonder why they work? We all know that wood
expands and contracts and lap joints have to pieces of wood doing that at
right angles to each other yet they remain nice and solid. How come?

Someone might say it is because each piece is narrow and doesn't move much.
That is true but I've often made fairly wide laps. For example, all our
toilet seats are wood, each made with four pieces about 3" wide lapped at
the corners.

Discussion is invited.



I make lap joints on every one of my back face frames, typically 6~8 on
every one of 20 in the last 2 years. The joints are any where from 3/4"
to 3.5" wide.

I use the regular water based wood glues which always have some degree
elasticity to them so the wood is able to move. Basically the same goes
for mortise and tenon and floating tenon joints where the grain is not
matched in direction. Wood moves but typically not enough to out
stretch the glue's ability to stretch until you get in to wide panels
like cabinet sides, solid wood door panels, and table tops.