Guitar Project

I like to play guitar, usually acoustic, and capoed at the 4th fret. However, acoustic guitars are big, mostly because the body is designed to resonate at the lowest note the guitar can play. I wanted something smaller, but with a neck and scale length equivalent to a full size guitar cut off at the fourth fret. Therefore I designed and made a short scale archtop, with a thin body, headless tuner, and fanned frets. The whole thing is printed in carbon fibre filled PLA, and backed with carbon fibre cloth. It turned out about like I expected for a first version. It's playable. I am using extra light strings, and the action is better than most guitars out of the box, but it could improve with a bit more reinforcement in the neck (maybe an extra layer of carbon fibre or three?). It's ergonomic, easy to hold, but I will be tweaking the body a bit in version 2. It sounds like a small body guitar (duh) not a full size guitar. However, it does sound much better than a cheap, similar sized, children's guitar. The intonation is on point, which means I got the scale lengths and fret spacing pretty close. Overall, not bad for a first attempt designing an acoustic guitar from scratch!

Details

After thinking about making a small guitar for a few months back in 2020, I had come up with a few requirements for the design:

• Headless, or placing the tuners at the bridge end or the strings. This greatly shortens the length of the guitar, and improves the balance (with most small body guitars, the heavy tuning machines on the headstock make the guitar unbalanced and uncomfortable to hold)
• Archtop, or placing the tuners below the plane of the fretboard so that the strings exert pressure against the bridge, and properly transmit the energy of the strings to the soundboard.
• Offset soundhole, to avoid any weakening of the soundboard (needed since I am barely using any bracing other than on the belly)
• Small body, while still comfortable to hold. Specifically a notch so that it rests on the leg easily, and a top corner that extends far enough to rest your arm on.
• Fanned frets, because why not!

Once I had a good idea of what I wanted, I made a rough sketch, and some cardboard cutouts to test for ergonomics:

Next, it was time to start modeling. The first thing I did was create the fretboard and bridge, as they were the only things that needed exact measurements. Fret placement in particular is important, and complicated. I did some research, and ended up using the formula from here: Liutaio Mottola

• distance_from_nut = s - (s / (2 ^ (n / 12)))
• s = scale length
• n = fret number

With fanned frets, you have two scale lengths (480mm and 510mm in my case) that determine the placement of the ends of each fret. There are 12 frets per octave, and I wanted at least an octave per string, so I went with a lucky 13 frets total. Not as many as you would find on a normal guitar (20+), but let's be honest, I almost never use frets higher than 12 and each fret only increases your range by a half step. The fretboard extends a bit beyond the last fret to add some strength to the soundboard. After the fretboard, I just did a freehand spline for the outline of the body, only making sure the arm rest and leg rest bits were in the right places. There are two braces running along the belly on either side of the neck to keep the body from folding under the string tension. In the side cutaway below, you can see I cut holes in the braces so that they did not section off the body into multiple resonating chambers (although that might be a fun idea to test later)

I am still working on this page, come back later.