building cabinet doors with cope and stick. field is 3/8 plywood and they are painted. About half of them are twisting. How can I build these doors so they all stay flat
The larger the door, the more difficult it is to keep them from twisting. If you are doing a back panel style, where the plywood is attached to the back of the frame, then you'll probably see a bit more warpage than if the plywood floated within the frame. The biggest thing when assembling the frames is to work on a flat surface AND don't over tighten clamps. You only need enough pressure to hold the pieces together while the glue sets, nothing more. Over tightening will cause the joint to skew and introduce stresses into the wood that the glue will retain, causing warp. Keep the materials as flat to one another as possible during the gluing and you will have better luck. Also, on larger doors, a mid section crossbar will had more structural stability.