I just finished building an MDF torsion box as designed by David Marks and followed the exact procedure for making one, but ended up with a top that is bowed a total of 3/16". What I did to make my flat surface to start with was build a series of MDF I beams, since I don't have a jointer. These were placed on a workbench, leveled and shimmed so they were all on the same plane. Then I laid a sheet of MDF on top and confirmed flatness. After ripping all my pieces for the gridwork at the same time and gluing everything up, I flipped it over and added my MDF skins.
It seemed fairly flat at that point, but when I transferred the box to the top of my cabinet it is now very warped. What did I do wrong?
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