Radius dish making
One of the first tools to make is the radius dish. I hope this explanation makes it easier for you.
Layout the desired radius on two 30" x 3/4" boards and cut out the radius on the band saw. Sand these boards for a nice uniform curve.
Make 24" diameter dish blank with 1/4" hole in center. This hole will be the pivot point for the dish making fixture.
Make a block for router base to guide the router between the radius boards. Set depth on 1/2" router bit so the it just cuts the outside edge. Cut out radius dish with router, spinning dish the width of the router bit after each pass. Take your time. This will generate massive amounts of saw dust. Be sure to use ear, eye and lung protection! When done, sand smooth.
After that, you have a spherical radius dish ready for sandpaper. I like using 60 - 80 grit paper. The dish is useful for shaping braces, bridge plates, and most importantly, in go-bar decks. I make two dishes, one at 15' and the other at 28' radius.
In the picture below, I show the dish with a board that I am adding a radius to. I will add some sandpaper to it. This new tool will be used for adding the radius to the bottom of my bridges. Because I use the dish for final shaping of my braces and in the go bar deck to make tops, this will ensure that the bridge will match the top of my guitars.
In the section above, one thing that many struggle with is how to draw a large radius. You can try a 28 foot string, but it will stretch and make an uneven arc. I use a CAD print out that is very accurate. If you don't have access to CAD and a large format printer, you should try the method below. Boat builders use to make large arches without using gigantic compasses.
It is really easy to do and quite accurate. You do not need a center point. Take a look at the drawings below. You need to know the rise and the run. The rise is simpy the depth from the to of the arc to the bottom of the arc. With basic geometry you can determine that distance. The run is 1/2 the length of the arc.
Now that you have that down, let's take a practical guitar building example -
For a 24 inch radius dish with a 25 foot radius, the run would be 12" and the rise would be 0.240".
With the rise less than 1/4 inch, you'll need to made the jig taller to make it stiff enough. I would add another inch or 2 to the bottom of the jig while maintaining the shape of the upper portion.
Drive nails into the the template at the center and the ends of the arch. The nails will serve as the pins in the illustration below.
Position the jig with the hypotenuse against the end nail, the notch for the pencil next to the end nail and the flat section against the center nail. (This is why the flat section needed to be longer than the run/radius.)
Hold a pencil in the notch and advance the fig while bearing against the nails.
Flip the jig to complete the other half of the arc.