I use two very simple, shop made jigs to crown my fretboards to a 12″ radius. In the beginning I used a Stew-Mac 12″ radius block and lot of elbow grease. That was OK but the primary problem was keeping the radius block centered on the tapered fretboard. Quite a bit of material had to be removed and it was difficult to keep the fretboard from moving. As a result I resolved to try to figure out a way to do this tedious job quickly and precisely.
I came up with two simple jigs to do the job.
one
Two
This jig is designed to keep the fretboard secure and to center the Stew-Mac radiusing block on the fretboard as you sand. Again, it is a very simple jig. Just a piece of plywood with two thin strips of wood (less than the thickness of the fretboard) glued to the plywood at the same taper as your fretboards. You can use the fretboard as a pattern to glue these strips. Lastly, lay the Stew-Mac Radiusing block centered on the fretboard. Now glue a strip of wood on one side that will track the lay of the radiusing block. Once dry, it provides a straight edge which keeps the radiusing block centered on the fretboard. If you did step one correctly, a couple of minutes with the radiusing block by hand will give you a perfect radius with a minimal amount of work.
Note that this works with any radius you wish to use!