Wednesday, March 21, 2012

Five-course Mandolin, part 2

 I've tried several different ways of attaching necks to bodies.  I decided to go with a straight dovetail for this project owing to the low-profile design I have in mind for the heel. The original Gibson mandolins had a complicated curved pattern obviously machined on a shaper with custom knives. Though not quite beyond my skill level, the amount of time needed to cut it by hand seemed wasteful.

This is a jig for my router table originally made to cut straight spline joints for classical necks.  It works for dovetails too. There is a bearing on the shaft of the bit which rides against the walls of the slot

The neck sits at a five degree angle and so I had to make up a wedge to keep it registered. You can see that I will nibble away at the sides of the heel and gradually move the neck over. As it turned out, removing material made the neck progressively more prone to tipping so I left a generous amount of waste to be refined by hand.

 Cleanup was accomplished with a chisel and this handy sanding stick that has the 14 degree angle cut into it. I aim for a very slight taper, so the fit gets tighter as the neck is pressed home into the slot.

Like so. The neck actually stands proud of the block by about 1/2" (12mm). This will accommodate the thickness of the top and an added fingerboard support extension.

I'm experimenting with solid-surface countertop material for a headstock inlay. Mother of pearl and the pretty shells of various bi-valves are now extremely hard to move across borders due to the U.S.A passing a piece of legislation called the Lacey Act. This is such a complicated issue.  In short, there are a great many natural materials which are legal to own, but sometimes illegal to sell, modify or move across state lines or into the country.  This Corian blank is about .100" thick (2.5mm). I've surrounded it with some hardboard to provide a stable surface for my miniature router and I glued on a slip of paper with some guidelines.
A tiny little carbide burr will be used to cut nearly all the way through the material and then the outlines will be smoothed up by filing. There will be some wiggles and eccentricities because I am not a laser engraving machine! A cavity in the headstock will be routed to fit and black epoxy employed to hold things in place and fill the gaps.  
I don't often put my name of the headstock of instruments, but this is small and subtle and will be partly obscured by the strings.  As it turns out, the radius I used on the end of the headstock is at the outer limits of what one can bend wood binding to. I had a big pot of boiling water and a little form to pin it to but it fought me every step of the way.

Friday, March 9, 2012

Five Course Mandolin with Fanned Frets

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.