I have used these handy little jigs to find loose braces and repair cracks for acoustic guitars and ukulele's for years. Even if you are not repairing guitars, there will be the occasion when you need to preform a repair on a brand new instrument. We all do it-an unintended bump against an immovable object and there goes your back plate - a nice crack right down the line of the grain. That's where our crack alignment jig comes into play.
You can build this tool out of a used or orphaned tuning machine and a guitar string (high E preferably), and a small scrap of wood All you need to to is thread the string through the crack and secure a small reinforcement wafer on the underside of the plate. (Match the plate material) Do this about every 2 inches along the crack as well and force glue into the crack.
The second tool is really an inspection tool for finding loose braces, bridge plates, and reinforcement strips that may not be glued properly or the glue has dried out and the brace or reinforcement has let loose over time.
Run the probe along each side of each brace or reinforcement by accessing the braces through the soundhole of the instrument. If the probe will shove under a brace, it needs to be glued securely in place.
Plans show the assembled jig, along with complete dimensions and notes. Every part of the jig is carefully dimensioned and noted for clarity and ease of construction.