Shawn Trotter is a versatile local instrumentalist. I met him at a coffeehouse where he was displaying his dazzling acoustic fingerstyle guitar and mandolin chops. Soon after I found myself singing in a vocal ensemble with his partner Nikki and it was at one of our performances that he gave me a nod and mentioned that he'd been messing around with some Bach on mandolin and he was thinking about an instrument with an extra course. "Interesting", I think I replied. He had me at Bach, really. Play one of the cello suites on slide whistle and I'll sit in rapturous admiration. It seemed like a fun challenge and I had some nice old wood just waiting for something like this to come along.
"What about fanned frets?"
Shawn had done his homework. The low C strings would be pretty floppy, otherwise. I was aware of the concept of fanned frets. The Orpharion was a wire-strung, lute-like instrument from the 16th and 17th centuries that employed the multi-scale idea. Essentially, each string has its own scale length, thus the basses can be made a little longer to offset intonation and tension problems. More recently Ralph Novak patented a fanned fret system coupled with some innovative bridge designs for use in his electric guitars.
The idea is deceptively simple. Just plot out two scale lengths, decide on a common fret and join them together. Easier said than done. Accuracy is always of utmost importance when it comes to frets and not having them perpendicular to the center line adds some challenges! I decided to keep the treble scale close to what Shawn was familiar with on his Gibson F-9. After drawing a theoretical fretboard I came to the conclusion that an additional 1-5/8" (39mm) was as much as I felt comfortable adding to the bass. So it will be 13-7/8", sloping to 15-1/2". I have an expensive machinist's rule marked out in 64ths that I use to plot the scales. Using a magnifying lens I can split those increments.
I couldn't use my table saw to cut these slots. It's ancient and fine for parallel slots in a standard board but this called for hand work. I came up with a little scratch knife to make things easier. My fretting saw cuts a kerf .022" wide. I took a .023" feeler gauge and glued it into a handle made from ebony offcuts and ground the edge into a little hook. It's basically a tiny scraper that initiates a groove for the saw to sit in.
I used some double-sided tape to keep the board secure and overlap the end of my bench. There's a knack to setting up the bevel gauge. One has to get things in position, gently snug it up, move it off the board before cinching it tight and then test to make sure nothing moved. I was able to center the scratch tool right on my marks. This seems like an ideal system for people without mechanized fretting equipment - certainly more accurate than trying to hold the saw in place!
Don't be offended by the rippled surface from the jointer. I'll be sanding a 16" radius into the board and that will go away soon enough. The original A-5 had a flat board but a little curve just feels better.
Here's the other tricky bit. The board will be bound in white-black-white. I decided the best way to do this was to plane the board to its full dimension, cut off the area of the binding, glue on the purfling strip, radius the board, re-cut the slots through the purfling, then finally glue on the binding. This would take care of any chipout on the ends of the slots and reduce the amount of filthy ebony dust ground into the pores of my nice white holly binding.
I set my marking gauge very carefully and did a test-run before touching the board. I'll use a sharp plane to remove this excess material before continuing.