All materials expand and contract due to heat, and in some cases moisture, and in some cases both. Each material expands and contracts at different rates. Different metals expand at different rates due to heat, as the metal warms up the molecules move apart and as it cools they contract. Wood primarily expands as it absorbs moisture and contracts as it dries out. Different woods expand and contract at different rates.
The amount of expansion in relation to the amount of heat or moisture is called the coefficient of expansion.
When we build objects we need to take this expansion and contraction in to account, we primarily think about wood movement but in some cases we must also be aware of the movement of metal. Wood glued directly to metal always fails eventually, because of the difference in expansion rates, it doesn’t matter what glue you use. Even a flexible glue like construction adhesive will eventually fail due to repeated expansion cycles, it’s like bending a piece of wire back and forth until it breaks. Even if there was a glue that would not fail the difference in expansion rates would cause the object to warp and deform. When laminating metal to wood one must allow for movement. We must also consider the effects of compression set. If you are not familiar with compression set it is explained here.
Below are several tools made of brass and wood.
Here is what happens when wood is glued directly to metal.
Notice the piece of brass that is proud of the wood.
The brass is dovetailed into the wood allowing for movement.
Mechanically attached to the wood.
Fastened in the center allowing movement