Falcate Braced Classical Guitar - Finished!

[John Parchem]

I am just starting to work on the bolt on bolt off neck for a classical design. I am going to follow as closely as I can the method laid out in the Contemporary Acoustic Guitar Build book. At least after reading this section in the book three times, I find that this process is one of the more complex areas to understand the geometry. The complexity arise do to an inherent conflict in the design task: A bolt on, bolt off neck with bolts in the fret board extension area requires that the top and fret board board are co-planer (like a SS). The conflict is that the top of a classical guitar fret board should be pitched forward such that an imaginary straight edge on the plane of the neck should end up 2 mm under the top at the saddle location. That is the neck should be pitched forward, about as much as a neck is pitched back on a SS.

Classical guitar builders working on a solera in the Spanish style often deal with this in one of 2 ways: have no neck angle and make the fret board into a wedge thick on the nut side. If the FB is sloped such that it loses 1 mm by the 12 fret then the above described 2 mm would be achieved at the saddle location. I did this on my first classical and it would be a compatible solution to the bolt on bolt off neck. (The wedge shape of the FP is noticeable though) The second option is to pitch the neck forward such that the nut would be 2 mm above the plane of the top. In this case the neck and top are not co-planer so a reverse wedge needs to be sanded into fret board extension so that the FB will fit properly. This is how I built my second and third classical guitar. Not compatable with the bolt on bolt off neck.

Moving away from the solera or workboard to using a mold, alternatives like an elevated fret boards are often used. The design from the book is more along those lines. Simply put the top of the neck is made co-planer to the top and a suitable sized wedge glued between the neck and the fret board and is used to change the FB angle. If a the wood matches and there is a good glue line it should appear as part of the neck. The wedge would be 3-4 mm tall on the nut end sloping to zero at the bottom of the fret board.

While this sound simple, it creates a few complications that would need to be dealt with. The obvious one is that without modification the neck will feel 4 mm thicker than it should at the nut, so the neck taper will need to be modified thinner on the nut side. Removing 4 mm from the neck would put a truss rod closer to the back of the neck than desired. Also the nut slot would appear to be 9-10 mm deep (half of the neck width) which would appear weird. It is these complication that make this procedure tricky.

The nut depth is solved by extending the wedge past the nut end of the fret board and planing it to the slope of the head stock, In effect it becomes a part of the head stock and the neck and the head stock veneer is glued on to it. if you draw this out you will find that the fret board has to be placed a bit farther away from the original angle break where the head stock starts.

If one is using a truss rod, then with the wedge glued to the bottom of the fret board; the truss channel is routed with a slope in reverse of the wedge and a reverse matching channel needs to be routed in the wedge. So at the nut end part of the truss rod is in the neck part of it is in the wedge. Thus the truss rod will be at or close to the bottom of the fret board the whole length.

As I said I am just starting this work and if you follow the blog you will see the above work happen as the guitar progresses. At this point I have cut a glued the scarf joint to form the 15 degree angle I am using.

[John Parchem]

I am sort of stuck again with two long epoxy cure waits so you get another post.

Bit by bit I am finding there are little things between clean craftsmanship and my usual work. On the falcate brace design the falcate braces pass through a rebate in the upper transverse brace. On my previous two falcate guitar my work was not as tight as it could have been. The problem had been that I was a bit sloppy marking where the rebates should be. My brilliant light bulb moment was to take the 1.4 second I needed to clamp the brace as shown before I attempted to mark the 4 corners of the rebate.



With the brace stable it was really easy to take the time to make really clean tight marks on the brace.



Not shown but I measure the height of each brace on the top and marked the height on the transverse brace. Then it was easy to put the brace in my parrot vice with the jaws right on the marked depth line. I used a razor saw to both cut the out line and some inside cuts to make clearing the channel with a chisel easy.

Much better this time





This brace was clamped flat



I had some extra epoxy after gluing down the transverse brace so I used the excess to glue the heel block on to my neck blank.



I also went ahead and completed the top profile on the rims by first using the radius dish to mark a 32 ft radius on the rims. I planed the neck side and the bottom side close to my line and used the 32' sanding dish to clean up the profile.

Because I want the top with very little angle I then flattened the upper bout mostly with a plane. I think a straight edge on the top following the neck block extension will be 1 maybe 2 mm above the top. I will use the wedge I talked about in the last post reverse that slope.

[ken cierp]

So what is the carbon fiber actually supposed to be doing? What's the point/purpose of it especially on the straight braces?

[John Parchem]

So what is the carbon fiber actually supposed to be doing? What's the point/purpose of it especially on the straight braces?

The carbon fiber is both under and over the brace. It makes the braces significantly stiffer while adding little mass. So I can get a lighter top for a given stiffness than an all spruce brace. Another feature is that the CF will significantly reduce cold creep of any of the braces under tension. Probably is overkill on the sound hole braces.

[John Link]

The CF, top and bottom, amounts to forming an I-beam, one of the strongest type of structures around. I understand how this extra strength can be the occasion for making the top thinner. But what troubles me about T. Gore's use of CF is that it seems like it makes each brace difficult to adjust. I wonder how much difference carving off the sides of the brace to weaken the top makes, given the intrinsic strength of the I-beam configuration. You can't change the distance between the two strips, which renders the basic mechanism of the I-beam non-adjustable.

So John, is it the case where you need to get all the parts sized and adjusted the first time out, before glue up? Gore discusses ways to measure and predict a good outcome with both the top and back. But what if the apriori measurements do not seem to be working and the horse is too far out of the barn to reasonably call it back? Does selective sanding down of the braced top make much difference? The geometry of the brace layout looks like it too contributes to overall rigidity, even if the CF were not used, at the same time pushing more of the string energy into moving the top as a whole, which might favor bass.

This is a very interesting project because it uses a bracing scheme that is known to serve the relatively high energy steel stringed instruments well, but will need to function with a much lower energy budget when used with the less tense nylon strings. I'm looking forward to some You Tube video of this instrument.

[ken cierp]

When the issue of energy usage and transfer comes up -- I go back to a conversation I had with Bob Taylor regarding an 800 series guitar I was not real happy about -- he instantly said "you must have the wrong tension strings on it -- That guitar was intended to have such and such total tension." He suggested a set and sure enough the guitar came to life. From that day on -- my first suggested adjustment on a poor performing guitar is to increase the total string tension -- even a couple of different strings (a custom set) can make a big difference. My point is you may not to carve on a guitar to make it sound better.

[John Parchem]

Yes, there is no adjusting the top braces on these guitars I have as Ken suggested changed the tension son one or more of the strings when I was unhappy with the sound, I have never trusted going in and carving braces.

With this design I can lower the top resonance a bit by adding side mass (I have mounts) I also have a place to carve on the lower back brace that would allow me to lower that resonance. These are for fine tuning.

This whole system is designed to achieve some constancy, once a bunch of targets are set to calibrate a design. The book has a way to calculate the stiffness of the top as a system of braces, as this guitar was so close to my steel string brace wise, I thinned the top more than a SS and lowered the braces 1 mm. I think this about halved the stiffness of the top. This matches the 2 to 1 difference in tension SS to nylon. We will see where this ends up. If it is off I end up with data to tweak my design for the next one. Knowing what the knobs are and being able to measure the results will make that iterative process easier.

I think this will be an OK guitar as I believe that I can hit my tap tuning targets. I have the first tap results at the end of this post.

I just closed it today.

While waiting for the previous epoxy to cure I did end up radiusing the top rims to 32'. I then flatten the upper bout as the to of a classical really should be nearly square to the the rims on the neck side. I did most of the profile with a plane. The rims were already level, so I did not want to go totally with the sanding dish. It is really easy to go heavy on the neck side and round back the top rims near the neck joint. I place the rims top down in my dish and used a pencil against the dish to mark out the profile. I did clean it up of the radius dish. A lot less sanding so less time to make errors.



One last epoxy layup step is to apply a couple of CF rows to the transfer brace.




Not shown but I used l pencil mill grinder to route out the linings to accept the top and used a bunch of clamps to glue the top down. I did not use a caul on the top, Just a bunch of Cam clamps and violin clamps.





I routed off the overhang and prepared everything to glue the back on.
I vary carefully aligned the back to the rims to make sure the center strip matched the center of the neck and and the end graft. With the back clamped on I used a scalpel to mark where the braces need to be inlet into the kerfed linings.





I used the 32' radius dish as a gluing caul when I glued the back on, Keeping the mold, the spreader clamp and the using the dish as caul helped me make sure the sides were aligned when I glued the back on.









The resonance frequencies will change as the guitar is finished with a bridge and strung but I tapped it to see what it is doing at this point. It looks sort of wild to me. I see a clear peak at about 87Hz T(1,1)1 the top is at about 187Hz T(1,1)2 and the back was 238Hz T (1,1)3. It looks like the cross dipole was at about 257Hz. The taps do sound good.

It will be interesting to see the tap after I get a neck and bridge on and string it up.