1st Acoustic Guitar Happy-Fun Time #4: Rosette Design, Joining the Soundboard, and Performax 22/44 Fun

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Blog entry by Cosmicsniper posted 10-16-2012 05:12 PM 7552 reads 0 times favorited 9 comments Add to Favorites Watch
« Part 3: The Mold and the Bending Machine Part 4 of 1st Acoustic Guitar Happy-Fun Time series no next part

I’m swamped with two other concurrent projects at the moment, but I have found time to contemplate a Design for the rosette, as well as play around a little with my new eBay bargain, the Performax 22/44 drum sander.

Rosette Theory

The idea of the rosette is derived less from artistic design opportunities, which it certainly is, but more by the structural need to reinforce the sound hole in such thin, soft woods like spruce, cedar, redwood, or whatever stock is normally chosen for the guitar top. Doing so can help prevent cracks from developing at the sound hole edge and continuing across the face of the soundboard. Although some guitars will have minimal rosette designs, many will still provide reinforcement on the back side of the soundboard, to add back some thickness.

That said, I like modern art and opportunities for non-traditional expression. And because I wanted my first guitar to be a skill-builder for me, I decided against purchasing a complete rosette online in favor of designing my own. I knew I wanted to use some fancy, exotic woods pieced together to form the rosette ring, but I also wanted to be a little more creative than just a circular, symmetric inlay. I also wanted to do something that does not confine the rosette only to the sound hole area, but rather to expand the design, perhaps, up the fingerboard a little bit.

So, what to do?

The Math Guy

I teach high school math. This year, I teach mostly Geometry. When I think of a circle, I think of radii and diameters and circumferences and area formulas. I also think of Pi, which is the irrational number that is the ratio of a circle’s circumference to its diameter, which is 3.1415926538 and so on and so on. But can I make a rosette connection with Pi? Hmm.

Google search.

I came up with this…

That’s a crop circle, and on the surface it looks really cool, but its also a representation of Pi. I guess aliens love Pi because, you know, the universal language of math, and all that. Here it is a different way…

So, I thought I could do something similar, only beginning around the edges of the sound hole rather than at a circle’s center. After a couple of quick renderings, here is what I came up with…

For now, ignore the notes I’ve made about the shape of the soundhole and the concept of a soundhole “ledge,” which might be the dumbest idea to ever come across luthiery. But note that the soundhole will be slightly less than 4” to begin with, which allows for more room for “Pi” while making sure the circle doesn’t get too big. This smaller soundhole will certainly affect the guitar’s “voicing,” but I’ll cross that bridge, no doubt recklessly, when I come to that part of the build.

The design divides the circle into ten 36 degree slices representing our number system from 0 to 9. The rings start out at the starting line (a black mark) and spirals around counterclockwise, much like the crop circle does. A new material accounts for each digit of Pi, or, in the case of the first digit (3), no material at all. An abalone dot will represent the decimal and some herringbone will even give a hint to the direction of rotation. Borders will be in thin maple dyed red and/or black (not sure yet). Regardless, it will match the guitar’s standard purfling. You will notice as well that Pi is represented up to 3.1415926, whereas an ellipsis continues the sequence up the fingerboard and into theoretical infinity. Material choices are the aforementioned abalone and herringbone, with possibly some purpleheart, some red gum burl I have handy, walnut to tie in the back and sides, and some end grain black palm (wait until you get a load of that).

Materials might change as I do this, and I might create some negative spaces during the process because I want to avoid it being too busy, but I think you might expect the result to look something a Raymond Kraut design seen here…

Of course, when I actually do this and I ruin four or five soundboards, I’ll probably end up just buying a pre-made rosette on eBay. :)

But I have added a nice set of spiral down cut inlay bits from Stewart-McDonald to go with my new DeWalt DW611 and I’ve made a wooden prototype of a circle cutter jig that I’ll probably convert to acrylic. So, I’m hoping that I can make the precise routs for my inlay pieces. More on that soon.

Joining the Soundboard

Remember that European spruce soundboard? Well, now it’s time to get it properly jointed and joined together.

Here are a couple of soundboard halves resting on the shooting board, which I did together using my Woodriver #5. The light one is the spruce and the darker one is redwood (which I’ll use on another guitar down the road).

For me, the shooting board and jack plane makes for quick work of the jointed edges. And it produces some cool curls to boot!

You know you have a good joint by using the “light test.” If you hold the halves up to bright light and you can still see the light through the seam, then you’ve got more shooting to do!

Using regular old Titebond 1, some sandwich boards, non-stick wax paper, thick rubberbands, and Dr. Pepper, you have a easy method of doing the glue-up…

It doesn’t take a lot of clamping pressure. Once dried, here’s the soundboard ready for final thicknessing.

A couple of month ago, I dreaded the next step of trying to thickness the boards, particularly the figured walnut I will be using for the sides. But as you can now see, I’ve got a better method now…

Performax 22/44 Fun

I picked up that sander I won on eBay. It was good of my cousin, Brady, to pick it up for me down in Austin and I enjoyed spending a day with him making the exchange. Brady has the start of an awesome shop, with a hand-built workbench that I admire and some tooling for making some banjos and knifes…including forging the blades himself.

Although the sander is 15 years old or so, it’s amazing how well it performs. As with many of our machines, there really isn’t a whole lot that can go wrong. For $405, this was a Craigslist deal of gloat worthy proportions.

The drum is a little off, however, shaving about 5 thou lower on the drum end, but I actually used that setting to push through my Euro Spruce soundboard. By running it through both ways, it actually makes the edges a little thinner than the center, which I’ve heard will help loosen the guitar up a little when new. For me, it will probably prove disastrous, but what the heck, right? Target thickness was around 100 thou inches, which might be a little thin. I’ll address that when I brace the soundboard.

Here’s a video of the machine running, working the soundboard down to size…

I’ll definitely reset the drum when I do the back and sides and the materials for inlays, headstock veneers and binding/purflings; but its something that will be a quick fix and doesn’t really add up unless sanding boards the entire width of the drum.

Closing Thoughts

Ambitious, huh? We’ll see if I can pull off that rosette design and truly make it as elegant and intricate as I hope. I’ll be taking my time with that and I’ll certainly be practicing a lot. But that’s kinda the point, right? I mean, I know I can build a guitar. I just don’t know if I can build a really good one, so I want to get a good grasp on the methods and tools required to make that possible. The result should be a decent looking and sounding guitar, with enough of a satisfying and educational experience to make it possible to produce more (and better) guitars in the future.

NEXT BLOG: Inlaying the Rosette, part 1

-- jay,

9 comments so far

View Cosmicsniper's profile


2202 posts in 3358 days

#1 posted 10-16-2012 05:25 PM

Sorry about the pictures. I did this entry with the iPad and it looks like landscape mode wins out. I can fix that later on the PC.

-- jay,

View Cosmicsniper's profile


2202 posts in 3358 days

#2 posted 10-16-2012 07:39 PM

Pictures fixed!

-- jay,

View Swyftfeet's profile


170 posts in 2371 days

#3 posted 10-19-2012 07:21 PM

I’ll be following this. I might try to make a guitar someday. I like how you’re incorporating math into the instrument, human understanding of sound depend so much in the region of 0 to 2Pi ;)

I like Tau better but hey…

-- Brian

View Cosmicsniper's profile


2202 posts in 3358 days

#4 posted 10-19-2012 10:54 PM

OMG, Brian, tell me you’re not one of those radius-loving Pi bashers! Do you have nightmares of unit circles? LOL!

Glad to have you on board!

-- jay,

View Swyftfeet's profile


170 posts in 2371 days

#5 posted 10-22-2012 01:54 PM

You have no idea!, (exclamation comma is a real punctuation mark, but cant find it on my keyboard…)well actually I am sure that you do understand. There are certain formulas(given constraints) in EE that would have been significantly easier to use Tau rather than Pi. Unfortunately, I don’t use my math skills much any more its more like a swiss army knife in a fully equipped tool shop for when nothing else fits, I am in applications and pre-sales work now.

3rd year was something like this for me:
A few simple series questions (mmmhmmm)
A few integrations (la de da)
Laplace transforms (still getting it)
OMG-WTF-BBQ-BRAIN-EXPLODES. (3 hour tests based on 3 out of 5 questions, your pick! Ugh I hated those)

I don’t know who to blame Pi or my self-diagnosed medication routine of maintaining a 0.10 BAC… I choose to blame Pi.

-- Brian

View Loren's profile


10477 posts in 3848 days

#6 posted 04-01-2013 06:16 AM


That’s the problem with building your first few
acoustic guitars. Most students stall and never

View Cosmicsniper's profile


2202 posts in 3358 days

#7 posted 04-01-2013 12:57 PM

Hey, Loren! Thanks for popping in!

It’s not stalled because of lack of interest. Much to the contrary. It’s just hard to find times when I can do the selfish, all about me types of projects. They are viewed as unproductive by my SWMBO. So currently, this is the “productive” project…

I’ll definitely get back to the guitar soon, for sure over the summer months when I have time off (being a teacher is great). I would like to get the sides bent and make some rosette progress before then, so hopefully I can post a new blog soon.

-- jay,

View rhybeka's profile


4387 posts in 3321 days

#8 posted 04-27-2014 01:55 AM

awesome work so far, Jay! give me hope that a newbie like me might be able to take on a similar project in a year or two :) can’t wait for the next installment, but in the meantime, good luck on your house renos!

-- Beka/Becky - aspiring jill of all trades, still learning to not read the directions.

View Cosmicsniper's profile


2202 posts in 3358 days

#9 posted 05-18-2015 02:36 PM

Just an update in case people are wondering about my progress…

There is no progress.

I still find myself doing other major projects that prevent me from spending time on the smaller, more fun projects like this guitar. I am hoping that I can get a few of them finished this year. I will definitely post to this blog when I do take the next step.

In the meantime, FYI, I made a rookie mistake when I thicknessed my soundboard. I should have done the rosette first. As it is, I made this soundboard too thin…and now there isn’t enough left to run the soundboard through the drum sander to flatten the rosette. Also, inlaying into a thicker board would have made the process much easier.

Funny, but I look back on this today and I am disappointed in myself. I should have known that. Sometimes, despite my planning, I just don’t make smart decisions. Of course, welcome to woodworking, right?

-- jay,

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