Here are some pictures of the jig I built for routing a compound radius on fretboards (12" radius at the nut end and 20" radius at the soundhole end). The original designer of this type of jig is Mario Proulx (of Proulx Guitars fame) who has a brief description and some photos of it on his website. My version of this jig occupies the left hand end of a mobile work unit that has a Williams/Fleischman type binding routing jig on the right hand end. The work unit has a set of 10 drawers of different heights that I use for some of my luthier tools and supplies.
The jig works by virtue of a stainless steel shaft that runs in four spherical bearings, one pair on the sides of the work bench and the other pair on the sides of a swinging router guide. The shaft is set at an angle to the work surface such that the radius of the swing is exactly 12" at the nut end of the work piece and exactly 20" at the soundhole end of the work piece. The shaft forces the router guide to swing in a compound arc, tracing the surface of a section of a cone.
The swinging router guide subscribes exactly the right compound arc, per the calculations! It has plenty of free swing to route the widest fretboard I'm ever likely to want (over 4" wide!). The swinging router guide binds on the sides of the work unit at its extremes and requires a little judicious shaping to allow it to swing fully. Some aluminum angle was added to set the router path straight and stiffen the top plate. I added a couple of locking knobs (using modified straight box stays) front and back to hold the swinging platform steady during routing. It's incredibly smooth in operation and works extremely well.
I have heard from many luthiers who want to make a version of this jig themselves but have trouble sourcing some of the parts. I started a thread on the OLF that shows my version plus several other worthy jigs that achieve the same goal here.
Here are the sources for the parts I used:
The bearings are from The Big Bearing Store (part number SBSFL205-16 at $10.50 each currently) and the shaft is from Mc Master Carr product number 89785K173 ($48.01 currently). You can get alumin(i)um angle for the router guides from your local metal supplier. The work unit is built using plain old 3/4" A/C birch plywood which is glued and screwed using pocket-hole joinery courtesy of Kreg. The sturdy castors are from Woodcraft - product #141050 ($12.95 each currently).
Please feel free to contact me with any further questions.