This design opts to have the fretboard remain static while the Dremel (or router) performs all the movement. This certainly isn’t the be-all-end-all of compound radius jigs – some other options are posted below. If you prefer to do things manually, you may appreciate this guide for creating radiused sanding blocks!
One of the many methods of compound-radius routing
Your measurements don’t have to be exactly as they are in the instructions here because you’ll want to adjust lengths and heights of pieces so it can fit your needs. The design is simple enough to be improved upon – you’ll find the main idea of having the desired radii attached at either end of the board for a router or sander to trace is the basis for many other jigs that seek to achieve the same thing.
You’ll need to use the compound-radius calculator to you build a jig that functions exactly as you need it to. I’ve included an image that shows the specifications of an Ibanez SZ-320 being entered into the calculator so you can see how it’s properly used – the jig is also built to exact specifications in CAD for this particular fretboard.
You’ll also need to know how to use a circular routing jig to cut the radii.
Keep it Simple!
I decided to make a compound-radius routing device that would rotate the fretboard while the router would move up and down the length. Before long, I had designed a monstrosity – a functional one, but way over-engineered. I recommend using methods that don’t try to rotate both radii on the same axis.
Some Other Options
I’m including a couple videos of other peoples’ compound radius jigs for sanders and routers – hopefully you’ll find one that works for you. I’m particularly fond of the one that utilizes a belt sander.