I see a fair number of guitars with bridges that are lifting at the back, often left that way a long time under string tension. Usually the bridge will have taken on a smile-shaped warp, most noticeable at the ends of the wings. Sometimes the warp is aggravated by the top of the bridge drying out when it's heated for removal. If the warp isn't too bad you can always hope it'll flatten out with enough clamping during glue-up — but it's no fun to watch the bridge corners start creeping up after the clamps come off.
I've used various blocks of wood and C-clamps over the years to flatten warped bridges, and it worked okay. I thought it would be nice to have a tool that was a little easier to use.
After I remove an (unfinished) bridge, I lay a piece of wet paper towel, folded several times, over the top of the bridge to rehydrate the wood. (I've also heard of putting the bridge in a pot of boiling water, but I've never tried that.) After about a day, I put the bridge in the Bridge Wing Flattener using the clamp radius that best matches the top of the bridge wing, and tighten the clamp knobs. I put small pieces of wet sponge next to the clamp edges to help relax the wood, then remove them after a few hours so the wings dry while still in the clamp.
If the bridge started with a severe warp, you may end up with the wings flat but still have a slight warp under the bridge-pin area. If that's the case, I use a scraper to true the bottom flat. Doug Proper's bridge cradle is a great tool for that operation. If you just need to sand the bottom of the bridge a bit, the Bridge Gripper may be useful.
I recommend printing the parts on edge, as shown in the photos, to get a smooth radius on the clamps and the bridge caul. Printing the parts flat leaves pronounced steps in the radiused surfaces that you don't want — particularly where the clamps contact the tops of the bridge wings.
I printed mine with Voxel PETG and Elegoo PLA. Both worked well, but I increased the caul-thickness parameter from 10 mm to 15 mm when printing in PLA. I included the Fusion 360 source file so parameters can be changed for 7 mm hardware if needed. The knob is an .stl — you can scale it up a few percent in your slicer, or just heat up the 7 mm nuts and do a press fit.
