I have some glued-up hardwood raised panels that are 3/4-in. thick and 12-in. wide that will fit between rabbeted posts. All of this will be mounted flush against a sheet of 3/4-in. plywood. My choices are to account for expansion by proper spacing of the adjacent posts on each side of the panels, or I can glue the back of the wide panels to the plywood backing board (which I’ve never done before). What can you tell me about how the wide hardwood panels will behave, in terms of seasonal expansion and contraction forces, if glued to the 3/4-in. plywood sheet? And what is the likely effect on the plywood backing board? – Phil Zeiss
Rob Johnstone: My advice would be to allow the panels to float between the posts so they can expand and contract. If that is not practical, my next suggestion is to glue the panels to the plywood, but just with a bead of glue down the center of the panel. That way, the panels can expand and contract from the center out or in as they react to the changes in seasonal humidity. To be frank, you could probably get away with gluing the panels to the 3/4-in. plywood completely as there is enough stiffness in the plywood to resist wood movement. But the danger is that the face of the solid-wood panel that is not glued to the plywood would expand, and the back of the panel would be secured — which would cause the panel to cup across the grain. It might flex the plywood or even break the glue bond or delaminate a layer of the plywood. As a friend of mine likes to say, the only two things in life that are certain are death and wood movement.
Chris Marshall: I agree with Rob, Phil. Don’t tempt fate by completely gluing those panels to the plywood substrate. If there’s a lot of wiggle room between the rails, and you’re worried that the panels might slip out of position and be noticeably misaligned, then a little glue down the center is all you’d need to hold them in place without restricting their cross-grain movement. But, if it were me, I’d just make these frames snug enough to allow for some wood movement but not so tall or wide as to give the panels the ability to slip out of position to a significant degree.
Tim Inman: Remember these basic lessons: Lesson 1: WOOD MOVES. Lesson 2: You cannot stop wood from moving. Lesson 3: Forget lessons 1 and 2, and you are doomed for failure.
I would strongly recommend you NOT glue the panels to plywood, and instead allow for proper expansion and contraction via loose fitting between stiles and rails. Wood moves almost entirely across the grain. Very little movement is along the length of the grain. Wood moves mostly because of moisture exchanges, but it also moves due to temperature changes. I have seen many times when wooden panels were glued tight to backing boards. It is always amazing to me to see how the sturdy wooden backings are actually warped and distorted by the movement of the panels glued to them, or the panels have split and torn themselves apart. On the other hand, it is amazing to find really old furniture with floating panels that are strong and tight after decades and decades, provided they are allowed to do what wood does naturally: move.