Acoustic Guitar Construction

I am making my third flamenco. My hope is to get the build quality of my second and the sound of my first flamenco. Luckily I performed all of my plate analysis, so I can hope to built a top with close to the same resonances. Generally I use a plate tapping method to determine the young's modulus of the wood. The frequency that a rectangular plate vibrates correlates to the stiffness of the wood. On this guitar I did use that method to determine the thickness of a sitka top that I have. The back presented a challenge as the plate was not square. So I squared a portion and use deflection to determine the stiffness of the wood. I am starting a new flamenco blanca guitar with spanish cypress back and sides. The Gore plate thicknessing method using tap tones as described The Book requires evenly thicknessed rectangular plates. Often times with nice back and side sets the plates are non rectangular taking advantage of the fact that the upper bout is narrower than the lower bout. My spanish cypress set for a new flamenco guitar has trapezoidal plates that were too small to square up. So I went ahead and did the work required to allow me to get my thickness targets using deflection measurements. I thought some might find some value seeing how I did it. Also if I mucked anything up I will be sure to get corrections.

For this process I needed two good representative rectangular samples of the back plates for a long grain stiffness value and a cross grain stiffness measurement. As you could see in the first photo. I was able to create a square section on one of the plates. Also after taking a long grain deflection I will have enough length to cut off a rectangular section of the same plate.



Previously I have built a deflection jig that allows me to take deflection measurements on a variety of span lengths. So first off I measured the long grain deflection under a measured load.



Next I cut off a 50 mm or so rectangular section off of the plate, set up the deflection jig and measured the cross grain deflection





The last additional bit of information I needed was the density of the wood. To determine the density of the wood I needed the mass of the cross grain sample.



The last step was to modify my previously spreadsheet for calculating target thickness using tap frequencies to use deflection based young's modulus calculations. Luckily I had a handy spreadsheet from the Modal Tuning with Trevor Gore class I took at Robbie's place to calculate the long and cross grain young's modulus for the plate. The book also has the information needed to make these calculations. I substituted the deflection based El and Ec calculations for the frequency calculations in my original spreadsheet and set the Elc to an average value. This term has little effect in the overall calculation. With all the required inputs I got what looks like a good thickness target for the plate. This target is very close to my frequency based target from my first flamenco guitar using spanish cypress. So it seems like the method worked well.



Just for reference here is the spreadsheet for my tap baised thickness calculation for the top plates on this guitar.



With the properties of the tone wood I was in a position to joint both plates.

Next up is the neck. I am going to build this guitar with a traditional spanish neck joint. With this build style I will attach the neck to the top very early in the build process. I used my newly made jig to cut a 15° scarf joint. I cleaned up the surface of cut marks and joined the two pieces.





Next I thicknessed the headstock to 17 mm measuring from the back side. You can see that I extended the line I made with my marking gauge so that I can watch to make sure I keep the planing square.









Next up I mark where I want the nut relative to the scarf joint. I mark out a line to plane the surface flat to about 40 mm past the 12 fret. I will ultimately cut the remainder off and use it for the stack heel.





Ready for the stacked heel

Next up is the neck. I am going to build this guitar with a traditional spanish neck joint. With this build style I will attach the neck to the top very early in the build process. I used my newly made Jig to cut a 15° scarf joint. I cleaned up the surface of cut marks and joined the two pieces.





Next I thicknessed the headstock to 17 mm measuring from the back side. You can see that I extended the line I made with my marking gauge so that I can watch to make sure I keep the planing square.









Next up I mark where I want the nut relative to the scarf joint. I mark out a line to plane the surface flat to about 40 mm past the 12 fret. I will ultimately cut the remainder off and use it for the stack heel.





Ready for the stacked heel

Last post today, I am waiting for the headstock veneer glue up to cure.

When gluing up the stack heel I need to arrange the blocks I cut so I can cut out the spanish heel, note the template in the first photo.



Not shown, I glued up the blocks with the end grain zig zagging. Not what I would have done by default, but spanish flamenco players think that it shows that an experience builder made the guitar.



Once I worked out the alignment glued the whole stack to the neck.

I was in my office and I ran into the workshop worried that I glued the stack to the wrong side of the neck. But I was ok





I will use a Brazilian rosewood headstock veneer and a black red black stack of veneers.



Before gluing down the veneers I used the neck and headstock as a guild to trim the headstock veneer.





Just waiting for the glue to cure.

I mean to update this log soon. I made a neck carving presentation for the Seattle Luthiers Group from this build so I thought that I would share it here.

https://seattleluthiers.org/wp-content/ ... ving-2.pdf

I got a little bit behind posting but the guitar has progress. Just today I found the value of my build blogs, I am preparing a few guitars for the GAL this year and I was able to go back and see how I finished my osage orange classical. I was playing it yesterday and it really is a great sounding guitar. I plan to post an update soon to show its transformation of color with time. It was a traditional French Polish, so I can clean up the finish for a like new look.

This post will cover some of the material in my Seattle Luthiers Group presentation but I am including it for completeness of this blog.

The heel has to be taken to close to a final profile to move forward in a traditional spanish heel design. That is the inside portion of the heel and the profile for the intersection of the heel to sides. The order of some of the next steps can vary as they are not dependent on each other. I start by cutting the heel slots.

I am using my heel slot jig. This is the second time using it. This time I placed and index pin in the neck so that I could perfectly align the slots for each side.





The blade angle is set for 3°, that angle works out well with the curve of the top of the rims. I have seen recommendations for up to 5°.





Then I cut the rough profile of the spanish foot. There are no issues cutting the large radius I use on the bottom with my 1/4" blade



I need a bunch of relief cuts for the tighter curve.



I laid out the dimensions for the inside spanish slipper using plans from Paco Chorobo and used a chisel (also played with my fancy tenon plane) to cut the angle. It is important to use a backing board on the face of the fret board. Otherwise there is a 100% chance that it will blow out.









I cleaned it up with a sanding block and applied a few wipes of shellac to seal the end grain. Note how each block was reversed showing a zig zag grain pattern. I was told that spanish builders consider this good wood craft workmanship. Laminations are suppose to be made this way for stability. I suspect there are no issue lining up the grain, but I want the spanish builders to think well of me.



Neck up I drilled pilot peg holes, mounted a template and roughed out the head stock. I used the pilot holes to tape a more detailed paper template on to the head stock. Many people use a template router bit and a template and route the headstock shape. I have blown out about there of them over the years and finally just switched to the band saw planes and chisels.





Before using the neck I roughed out the heel profile where the heel meet the rims of the guitar. I laid out both the fretboard taper and the heel cap. Using abacking board on the fretboard taper line (I give my self a few mm extra, I first cut the heel to a straight angle using a chisel and my fancy rabbet plane









The other side



Adding a symmetrical curve to the tail of the heel takes a bit of care. There are a couple of ways to go about. One start dishing out wood from the center working toward the end cap. I like to end the curve about 10 mm from the heel cap so it transitions to the same width. With one side finished one could use a profile gauge to match the curve on the opposite side.

I however took a profile from a neck I liked and carved to that profile on each side.









Also before moving on with this neck I taper the neck pretty close to the final fretboard taper. I make a fretboard and taper it to its final dimensions and use it as a template for the neck.







I clean the bandsaw cut up with spokeshave



Also while the neck is detached from the body I plane the final thickness taper into the neck. I find life so much easier if I get a straight plane of the neck. With the center line thickness set I just need to profile the curve on each side. It is really important for neck feel for the centerline of the neck to be straight. A plane or planer is the easiest way to get a long flat surface.

I use a safe-t planer. To get the taper I use a sized shim at the nut. my taper goes from 14 mm 1st fret to 18 mm 9th fret. So I just used a 4 mm shim at the nut to start. I plane to just before the heal block extension and to right before the scarf joint near the nut location.





As I got close to my final dimensions checked and found that I needed more off of the tail for the proper dimension so I taped a 1 mm shim on the tail and did the final planing.





Normally I would start assembling the guitar at this point in the process. I rough carved the neck early as I planned a live demonstration of rough carving a neck profile.

To carve a neck profile I start by drawing out full size cross sections of the neck tapered and thicknessed neck blank



Into the cross sections I draw the profile that I want the neck to be. I have a template of a neck that I like so I used that to draw in the profile.



On the profiles I draw a tangent line to the profile that takes off most of the excess wood. The points where the line crosses the outline of the cross sections are the points that are transferred to the neck blank at the first and 9th fret on each side of the center line. When laying out the line for the 9th fret I like the distance to be the same from the fret board face of the neck. That is my preference, other option might be making the 9th fret tangent line at the same angle as the first fret.









I carve out the facets on the neck blank line to line with first a spokeshave and then either a chisel or a rasp to continue the facet where the spokeshave will not reach.



I also drew out a couple of 3" radius arcs for the start of the headstock transition and roughed them out with a gouge and a chisel.



Still continuing the facet ...



I carry the facet part way into the heel to help make sure I am maintaining a smooth transition.






At this point one can go back to the cross section diagrams and draw in a set of secondary of facets to cut. I pretty much just eyeball them knocking off the corners of the facets and then the corners of the newly made facets.

Here I am somewhat rough carved. I did not take a picture at the time but here is where the rough carving ended up.

With the neck ready foe assembly on my solera I turned my attention to the top. Luckily I have a stack of consecutive sets of the sitka I am using as I goofed up a set with a router mishap. I did not tighten my depth stop and happily routed out the top at the inside of the rosette. While I will admit the screw up I will not post pictures.

Instead of a fancy hand made rosette I routed out a channel for a store bought rosette and installed it cleanly. Pretend that the early pictures of me joining the top are for this top.





I do not have pictures but as I need to start working on the inside of the top I thicknessed the top to around 2.1 mm. My final target is 2 mm.

Use top off cuts to make a sound hole donut and install in on the inside of the top.















Dang if I did not have another router mishap right at the end of cutting out the sound hole. I thought I was not going to cut trough but I did and the top sipped a bit into the router. Luckily the fret board will cover it.





While I am doing so well with the router, decided to cut the ledge in the neck for the top to sit on. I use the fretboard face of the neck to guide the router on heel block portion of the neck. The ledge is cut so that the top will be level with the fretboard face of the neck. I used the top to set the depth of the router. The way my router fence is installed it is level with the bottom of the router. I extended it out a bit so I would have an easier time routing the ledge with the router cantilevered over the fret board. I actually managed to cleanly route the ledge so the top fit perfectly.





I sized and laid out the fan and transverse braces. I used a go bar deck to glue on the fans and the upper transverse brace. Leaving the lower transverse brace just make it easier to profile the braces and clean up.





Finally the lower transfer brace.



While that brace was curing I bend the sides. You can see that I am getting braver and profiling the sides close to the final dimensions.



I first use the mold center line to mark and trim one side and then the trimmed side itself to mark and trim the second side.



I trim the heel and tail side for a perfect fit.



I cleaned up what will be the back with a radius dish. The back profile is cut in the sides this is just a quick cleanup. So at this point I have the components to start assembling the guitar. At some point I will profile and brace the back.