8 String Falcate Braced Nylon String Crossover Blog:


January 28th 2018

An up and coming guitarist\musician that I know through one of my guitar instructors commissioned me to build an 8 string nylon crossover. I worked out a design based on one of the nearly classical sized guitar body shapes. The guitar will be a venetian cutaway with a 14 fret body join. I am going to use a 660 mm scale to push the saddle down toward the center of the lower bout as I usually build this with a 12 fret body join. The guitar will be falcate braced with a live back.

The back and sides are build from some wild Bocote I bought on the OLF. The top is an amazing piece of sitka spruce I have. It tap rings clearly for over 6 seconds. The plate would start ringing just running a finger on it.

These plates once sanded to remove the saw cuts 2.6 mm were close to on the edge of being too thin. I tried to use tap tones to get the target thickness but was having trouble reading coherent resonances. So I measured the long gran and cross grain young's modulus using a deflection jig. Using those values I came up with a good target thickness. In both cases I used equations from the Gore\Gilet books.

I joined the plates and decided to stabilize them. A wash coat of naptha showed areas in the figure where the naptha could seep through. I did several applications of thin CA until I thought that the figured areas were filled and stable.

I use tap tones to determine the young's modulus of the top wood and it target thickness.

I joined the two top plates

I pull some less funky bocote from my stash to use for the sides. I had some that was a good color match and I had less fear of bending it.

The Normal side bent with no issues so I decided to attack the cutaway side.

I thinned the upper bout just past the waist to the end to .070" . I also sprayed the side with super soft 2 and let it sit overnight.

I bent is a side bender with a cutaway attachment. I am always careful to hold the cutaway press up. I once had it fall and snap a bunch of bindings I was going to bend. Also while the wood is heating I am warming the underside of the cutaway. I really want to be able to make the tight bends.

No loud cracks!

Amazingly a clean bend :)

So I glued on a tail block and started to make braces and the heel block.

March 5th 2018

While waiting for some bindings on the classical guitar I have made a bit of progress on the 8 string.

I decided on a pretty simple rosette using a rope purfling and a wood ring made from a cut off from the back. To try to clean up the edges a bit I used an LMI Rosette circle cutter to mark out the inside and the outside on the rosette. I also scored the sound hole as well.

To install the rosette I first route channels for the inside an outside bindings. In this case the purfling strip and a black fiber strip was exactly 1/8". So I cut the outside channel on the top. Keeping the same router setup made a channel on my wood ring blank and then I make the side channel on the top and also transfer the router without changing the settings to the wood blank.

The fit was so tight I decided to install the rope purfling and fiber strip and then just wick in thin CA. To avoid disaster I carefully sealed the channels with [url][/url] Seal-Lac. I would have used shellac, but the seal-lac was out on the counter.

Not shown is I bent the rope purfling on a hot bending iron to close to the circle shape. I worked my way around using a 1/4" steel pin to push in the binding. The fiber is so thin, I would guide it into the channel and then push in the rope purfling.

I also installed the inside ring and leveled them to the top. The ring was sized when I cut the channels in the top, So I run it upside down through the drum sander until the ring drops out. Looks like a good fit.

With the router sill set for the correct depth I routed out the inside of the rosette. I just start in the middle of the channel and work my way to the purflings until the bit just kisses the inside fiber ring and cleans it.

Then I just dropped in the ring and flooded the rosette with thin CA.

It came out OK.

I also braced the back using back stock that I made earlier this year. It is just dimensioned brace stock that I run through a 22.5 degree router bit to make a house shape.

First I needed to install the back strip and prepare it for the braces. I will let the pictures do the talking.

I use a 10' radius for the back. As it is pretty tight I mark the radius on the braces, get very close with a plane and clean them up on a radiused sanding jig I made.

I used a gobar deck with the back on a radius dish to glue on the braces. Of note the I made the gluing cauls for the shaped braces by gluing two small cutoffs of the brace stock together at their base. Perfect fit!

A second glue up for the radial braces (I only made so many cauls)

My handy brace end carving template sized just right for the brace ends allow me to use a chisel to safely care all of the braces in short order.

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