Although the top of an acoustic guitar may appear to be flat, it's not. It is actually domed with a very long radius (in my case, 28 feet). In order to be able to glue up the the top to the braces, and hold that curve, everything is sanded to the proper radius using, and then glued into place using, a radiused dish.
I first thought the 28ft radius would be easy to draw... just get a piece of string 28ft. long and tie a pencil to it, and draw the arc. No such luck. Too inconsistent and the string stretched. So, I drew it up in AutoCad and then printed a 3 ft. long section of it in lifesize. I glued it to a piece of wood for a template, bandsawed, and sanded to hit the line perfectly. Then I used that pattern and template routed 2 guides out of scrap poplar (one of those guides will later be used as a convex template for shaping the internal braces of the guitar).
Then I threw together a quick jig for my router to follow those guides, and made the biggest mess of my life.
Here's the jig. I drilled a center hole in the bottom of the 24" diameter mdf circles and a corresponding hole in the top of my cheapo mdf bench and used a metal dowel to make the dish pivot.
Here's the finished dish with a jointed board on top so you can see just how subtle the radius is.
It then got a quick sanding with the ROS, a light coat of shellac to seal the big fuzzy surface, and will subsequently get 80 grit sandpaper applied to the whole dish.