Falcate Classical Guitar Build Blog:
January 15th 2015
I am just starting a project to build a Classical guitar with carbon fiber reinforced falcate bracing. The design of this guitar relies heavily on the guitar design concepts and the build procedures contained in the Gore\Gilet Contemporary Acoustic Design and Build books. This guitar is not being built for anyone so I do not have a time constraint nor fear of failure. I have always been a relatively lazy design engineer relying mostly on my understanding of relationships from a known point rather than starting with a clean sheet. Before starting I have read through posts on the Australian/New Zealand Luthiers Forum on the same topic [url]http://www.anzlf.com/posting.php?mode=reply&f=33&t=3819[/url] and a few other build threads and have reread chapter 4 in the design book. So here are my thoughts. I may be way off base but I thought I would share them. Sorry about references to specific topics in the books, but that is what this blog will be about.
For a start I have built a Medium Sized Gore Falcate SS, its size especially the length of the body nearly matches a Hauser 37 mold and bending pattern I have. The Hauser has a narrower lower bout but still the SS falcate bracing pattern fits. Both are 12 fret to the body and the scale length are not that far off so the bridge placement is close as well. This helps as I took the time to build bending and layup molds for the braces.
From the Design book I do have some information that I can use. Looking at a couple of graphs in section 4.4 The Flexural rigidity of the Gore falcate medium sized guitar looks to be about 40 Nm^2 and the Falcate Classical is about 20. This nicely matches the 50% difference in string load between a steel string and a classical guitar.
Another bit of information I happily accept for this project is using the vibrational stiffness value of 60 for a traditional classical guitar in equation 4.5-7 for plate thickness. This resulted in a 20% reduction in the top thickness when compared to a top I would build for a SS. I will use the target size from this calculation.
Assuming (I read it somewhere in the book) that the top plate is around 70% of the mass but the bracing is about 80% so the stiffness I can start making a few estimates. The 20% reduction in plate thickness while a huge change in plate stiffness (50%) results in closer to a 10% loss of stiffness to a braced top. The carbon fiber complicates doing a real Flexural rigidity calculation but given that going from SS to Classical I would be removing a half mm from a 9.5mm stack of brace and top I could see the relative loss of stiffness moving toward 15% I still have a way to go to get the Flexural rigidity halved. Also while I achieved a 10-15% reduction in stiffness with the thinner top the relative loss of mass to the top mass is about 14% (20% *.7).
My target top frequency of 190 is a bit higher than the 180 target for the SS I made, but taking into account the difference in the effect of a steel string bridge vs a classical bridge I will for this exercise ignore this difference. Given mode frequency is proportional to SQRT(k/m) where k= stiffness and m = mass), it seems that I should try to keep the k/m ratio the same. Based on my above assumption and the fact that I will remove the bridge plate and the bridge will weigh a little less, I could make the top a bit less stiff to maintain the ratio, but really within the margin of error of my estimates I could be there with just the difference is the thinner plate vs the difference in the mass of the top. I am less trusting of this comparison as I feel that the different body size and volume plus the effect of the different string load could make the comparison between the SS and the Classical more apples to oranges, still each proportional to SQRT(k/m) but from a different base frequency.
Looking at figures 4.4-9 and 4.4-12 showing the Flexural rigidity of the two guitars I am comparing , I really feel I need to lower the falcate brace hights relative to the SS a bit and just see where the top frequency ends up on this guitar. I think for a first try I will reduce the main falcate braces from 7 mm to 6 mm close to a 40% loss of stiffness in the braces. I think I may be still high (stiff) using my rough assumptions for relative flexural rigidity. But I think I will go with this size and see where I end up.
January 26th 2015
I had a week out of the shop with a bad case of sciatica, scary I usually have no back pain. The pain is going away so I can stand and do a bit of work in the shop.
I usually use wood for my rosettes on all of steel string guitars. I have been buying pre made classical guitar rosettes. Although I plan to make a classical mosaic rosette at some point for this guitar I thought I would try a hybrid. I had a nicely made classical rosette that included a mosaic tiled purfling ring inside and out. I inlaid the rosette into my top and routed out the center for an Australian Blackwood ring. All and ll it came out OK.
I decided to use some Austrialian Blackwood bindings as well with a BWB purfling. Keep with a classical style design I used the purfling for the back strip and will use it for the end strip. I will go for the mitered purfling look on the back and sides. Given that the binding strip was thinner than the back I glued up the back and inlaid the strip into the center of the back. I used a scalpel to first score the outside lines of the channel. I have in the past made my channels just a bit too wide in places. With the channels scored deep I routed to close to the line, but for the most part the thin strip left came out while I was routing. It was the cleanest channel I have made. I also got to try out my new old #8 when jointing the plates.
I used a technique from the Gore\Gilet build book to shape a bunch of back braces. I usually shy from big spinning router bits. I have to say using my table saw with a good blade to get a bunch of dimensioned braces, and then routing the shape on my router table sure when a lot quicker than hand shaping the braces.
I have a board with a rabbet the same size as the brace with two screws that I took care to place into the center of the rabbet. I screw the dimensioned brace stock in the jig route one side, unscrew the brace, flip it and screw it in using the same holes. I can then route the other side. In the pictures you can see that I add a block to my jig to make it easier to keep from tipping also my hand could stay away.
This guitar is planned to have an active back, so the back is thinner than I normally have it to reduce mass, to increase the stiffness the back has a 10' radius, more than the 15' seen on the Hauser plans and it will have the radial braced lower bout in the Gore Medium size guitar plans.
On this guitar I get to reuse a bunch of the jigs I made for my Gore steel string like this 1.5' 10' radius stick I made. It was great for putting the 10' radius on the bottom of the braces
After laying out the braces on the back and fitting them into the back reinforcement strip I used my vacuum box to glue down the braces on a 10' radius board. I go back and forth between my vacuum box and the go bar deck. With the shaped braces it is a lot easier to use the vacuum box
With the pre-shaped braces I only needed to takes the ends down to 2mm and the radial braces tapered to nothing.
Next up will be to be to build the rim.
January 27th 2015
Well I got a bit more work done, before my physical therapist suggested I cut down the time that I work in the shop for another week or so.
I have build a few guitars using the Hauser 37 shape. This is my first cutaway and the first classical with a 10' radius on the back. Before making the sides I needed to make two side templates.
To make the side profile I placed a strip of poster board in each of half of the mold. I placed the mold on the 10' radius dish and traced the radius on to the strip. I used a scalpel to cut on the radius line.
On a strip of Plexiglas I measured down from the top line the heel and the neck heights in their respected positions relative to the strip and taped down the strip such that the radius line when from the heel line to the neck line
I did this for the none cutaway side and cut out both templates on the band saw and used the templates to profile the sides.
I used my drum sander to take the sides down to 2 mm .080" and then sprayed them with super soft II. I also taped up pairs of Australian Blackwood purfling to purfling and spray them with the super soft as well. I took forever, checking and rechecking, doubting myself and checking again to assure myself that I was bending the sides for a right handed guitar. Also checking to make sure that the heel was at the heel and neck side was at the top of the upper bout. After a few hours they felt dry. For the first time I wrapped each side with one pair of purfling in aluminum (I was going to say tin foil put that would date me) and bent them in my fine John Hall bending machine using a pair of John Hall bending molds.
I think I got this trick from a John Hall utube.
Tomorrow I will sneak into the shop and fabricate the head block for a bolt on bolt off neck and start profiling the rims.
Febuary 5th 2015
Continuing to build components before starting the assembly of the guitar I have had time to fabricate the end blocks and bend the a bunch of mahogany to make the laminated linings for the top side of the rims.
The neck block for a bolt on bolt off neck takes a bit of work to support a mortise for the heel of the neck and a second mortise to support a tenon under the fret board extension. Not a big deal, the main part of the task was to find a large enough block of mahogany that allowed me to make blocks with the grain running the right way. Luckily I had a large bock intended for a classical Spanish heel that I was able to cut to make two fretboard extension support block and still have a large enough block for the heel. Once the heel bock and
fretboard extension block were cut to dimension I glued them together.
As this is the first time building this style of classical guitar I used the mold to make a template for the fret board extension block profiled for the cutaway. My cutaways have an absolutely flush joint between the cutaway side and the neck, so much care must be taken to assure that the side will be in the correct location relative to the center line. (basically half the width of the fret board at the 12th fret.
But BIG whoops! cut the cutaway profile on the wrong side. Luckily I had enough wood and made two extension blocks
Cut the heel block off and cleaned it for reuse, glued the new block on and finally profiled a cut the correct side
You can sort of see how the extension works and how the side meets the neck in the following photo.
I made a handful of tail blocks to have some on hand. I make the same side for all of my guitars. I used 3/4 birch plywood. To make I cut the wing to dimension on my band saw and used my luthiers friend as a drum sander to cut the curve into the wings. Then just a bit of cleanup on my belt sander.
To make the laminated binds I used a couple of orphaned mahogany sides that I had purchased in a deal for bunch of orphaned sides. The Gore\Gilet build book suggested using three ply each ply about 2.2 mm. I decided four 1.8mm plys my make bending the cut away easier. I just used my Blues Creek bender to bend the non cutaway. The curves are gentle enough that I can stack them together. I hand bent each of the cutaway so that they would nest properly. The curves on the outside ply sure get small. In any case it gave me a bit of bending practice.
I just ripped the bend side for the non cutaway into four strips. I stored them on the bending molds that I have used.
I profiled the two end blocks to the curve of the top and bottom and started to assemble the rims. It takes a bunch of carefully placed clamps to get the heel block in.
I used the rims them self to profile the bottom of the tail block.
A few clamps to glue the block to the rims.
I used the outside mold as the form to laminate and glue on the cutaway top linings. I built a very accurate clamping caul for the cutaway section. When I saw how well that clamped I added some strips to force the apex in a bit more. To give me a bit of open time so I was not in a panic during clamping I used fish glue.
I used a couple small clamps to square the strips together as I worked at clamping the strips to themselves and the rims.
I used a bunch of clamps to really assure that the linings were properly laminated.
I will let this set until tomorrow to give the fish glue the time it needs.