Made some otherwise nice chess boards, not end grain, so I followed the advise here somewhere to glue them on a piece of plywood to avoid breaks on the glued end grain parts of the pieces.
Now 2 of them are bent after some weeks:
The other ones are ok.
Now this is not a CNC issue, but more an issue about good carpentry.
However: What can be done to avoid such a flaw?
The squares need to be flipped so that the grain alternates.
See:
_ Understanding Wood: A Craftman’s Guide To Wood Technology_ by R. Bruce Hoadley
(be sure to get the updated edition)
The main issue that causes this is moisture content and the relative difference between the glued up side vs the side that is not. I think this is magnified due to the face grain glue up. You just added moisture to the entire surface, which adds up to quit allot of surface area forces and the plywood is now conforming to the wood itself. Why some do it and some don’t could be the amount of glue used or the initial moisture content of the wood and plywood.
If face grain glue ups are preferred then reduce the thickness of the wood itself when gluing up with plywood. Much more than .25” thick and your risk increases to have the same outcome. When I glue up projects like this .125” is my max thickness and then I just edge band with solid wood.
This also uses less wood for each board and allows for more total boards to be made for the same board feet of lumber.
Wood does not expand and contract equally in all directions. The radial and tangential expansions are roughly 4% and 8% respectively but the longitudinal expansion is close to zero. So, when fabricating as you are, one axis of a square is subject to far more expansion than the axis perpendicular to it. It appears you have run the longitudinal grain of the black squares perpendicular to that of the white squares which exacerbates these issues. Be sure the wood is dry and get it sealed ASAP.
I prefer an end grain surface for my chess boards, all of which have no backing. I prefer the look and it reduces the expansion issues mentioned above. I usually surround the board with edging. Here’s one I made circa 1970 something:
