*** Updated version with additional bracing for stability ***

I had good success with my steel string guitar, but it required a huge printer. For folks wanting a more manageably sized project, I decided to try a mandolin instead. The main issue for any plastic instrument with steel strings is the tremendous string tension involved - about 150lb total (!) for this mandolin. Not only does it have to not implode when the strings are at full tension, but it also can't creep over time. In this updated version, I have added aluminum ribs to the bracing of the soundboard, and have also added aluminum stiffening ribs in the neck (essentially a fixed truss rod), all formed from 1/16" thick sheet aluminum. I've also added some internal diagonal struts inside the body to prevent body flex.

The other big challenge was to create a soundboard that would fit on a a Prusa Mini+ bed (180x180mm) and still resonate well. I still wanted a full-scale mandolin (13.5"), so this meant that the bridge would need to be mounted near the end of the body. Rather than use a traditional floating bridge and tailpiece, I opted for a guitar-style bridge that excites the soundboard in a bending mode which can still provide higher amplitude vibrations when the bridge is not centered on the soundboard.

The last challenge was creating a soundboard light and stiff enough to project well. I kept the top very thin (0.6mm/3 layers). But then I added triangular lattice bracing across the top to distribute the energy. The lattice bracing (shown in the next to last photo) is actually an exposed triangular infill pattern that prints more efficiently and with greater integrity than if I had modeled the ribs discretely.

Additional Parts Needed
6-32 x 3/8" screws (3) for securing the bridge
8-32 - ½" screws (2) for the strap pins
¼-20 x 1.375" socket cap bolt (1) for the bolt-on neck
¼" x 0.55"OD washers (2)
¼-20 nut (1)
1/16" sheet aluminum (approx. 12" x 6")
Straight fret wire, like this from Amazon
Tuners, 25mm post spacing like these
Strings: 10-34 light 80/20 strings, loop end (D'Addario, Martin or Ernie Ball)
2-part epoxy, 30 in. or longer set time (I used Devcon 2-ton epoxy)

Filament Choice
I first printed this using Sunlu Carbon Fiber PLA because when I printed my guitar many years ago, the CF PLA filament I used added significant stiffness. After printing several versions of this mandolin, though, I determined that the Sunlu CF PLA was adding no stiffness, strength, or creep stability. I ended up reprinting it using standard PLA which worked maybe even better. Still, there are some CF reinforced filaments (PLA, PETG or PC) that might provide better stiffness and creep resistance. Unfortunately, hardly any manufacturers (except for makers of hideously expensive engineering grade materials) post data on parameters such as the Young's Modulus or yield strength. (Hint, hint, Prusa.)

Plain PLA is one of the stiffest non-reinforced filaments, but unfortunately, it has low thermal resistance. Therefore, you do not want to leave your mandolin in a hot car, or even sitting out in the sun on a warm day. I also recommend de-tensioning the strings if you aren't going to be playing for a while to reduce the chance of the plastic parts creeping over time. (Same as they recommend for wooden instruments.)

Printing
All of the parts, due to the loads on them, have pretty specific print settings (number of walls, in-fill density and pattern, etc.) so I've provided .3mf for all the parts.

The back, top and headstock plate all need some special handling:

Headstock Plate: This consists of the plate itself and the color-contrast inlay that is printed separately using a print-on-print technique. You first print the 2-layer thick inlay in one color, then change the filament and then start a second print job of the plate itself that prints on top of the inlay. But between the print jobs, you have to keep the motors and heaters turned on after the inlay print and also prevent homing and bed levelling before the start of the headstock print. The .3mf files I've provided have modifications to the starting and ending g-codes. Just start the inlay print and just before it ends, you'll be prompted to change filament. Then just start the plate print job. You'll notice that there is a rectangular fiducual boarder around both the inlay and the plate. This border insures that when the two parts are centered on the build plate, they will be correctly aligned with each other. For various reasons, you can't really do this any other way (that I've found) using PrusaSlicer.

Back: The back is actually 2 separate parts contained in a single object the back itself, and a separate object that becomes lattice bracing. The solid part of the back has one set of printing parameters, but the bracing part gets printed with zero perimeters, zero top layers and zero bottom layers – nothing but infill (10% triangular). This infill forms a lattice bracing, but also supports the curved arch of the back.

Top: The top has both the print-on-print inlay technique used with the headstock plate, and the multi-part lattice bracing used with the back. Just print the top-inlay first, change filament as prompted, and then print the multi-part top object.

Aluminum Rib Fabrication
The layout for the aluminum ribs that for the soundboard bracing and truss rod is in the included BRACING.PDF file. Sheet aluminum can be cut with wood working tools (be very careful!) or can be cut out with a hacksaw and filed to shape. It helps to have your printed parts ready so that you can test-fit the aluminum parts as you fabricate them.

Assembly

1. Start by epoxying the aluminum bracing ribs to the underside of the soundboard. It's best to apply the glue to the rib and then press it in place. If you fill the slot with glue, you will have trouble pressing out any excess when you insert the rib. Excess epoxy can be wiped away with isopropyl alcohol before it has set.

2. Next epoxy the struts in place as shown. Make sure to dry-fit the struts and back piece first to make sure it's all fitting together properly.

3. Next screw the bridge to the soundboard with the three 6-32 screws but without any glue. With the bridge in place, use masking tape to mask of the area around the bridge for gluing. Remove the bridge, apply a thin layer of epoxy and then screw back in place. Wipe off any squeeze-out epoxy and then remove the masking tape before the epoxy sets.

4. Mask off the bottom edge of the top and around the edge of the back. Apply epoxy to the inner lip of the back and to the flat section where the meets the body. Wipe off any excess glue and then remove the masking tape. Use several (many) more strips of masking tape to secure the back to the top while the epoxy sets.

5. Use epoxy to glue the fretboard dots into the fretboard. Cut the fret wire to length (see fretlengths.pdf) using a Dremel cutoff wheel, dress the ends of the frets to round the ends, and then tap into the slots on the fretboard. This is tedious.

6. Remove the supports from the neck head and sand the top surface smooth. (You don't need to be too fussy as the headstock plate will get glued on top.) Epoxy the ends of the neck (heel and head) together. Place the two parts on a flat surface before the glue sets to ensure that the fretboard surface is dead-flat. Make sure to wipe out any glue that gets into the truss rod slot.

7. Epoxy the two truss rod ribs side-by-side into the slot in the neck. Make sure the top edges of the ribs do not rise above the flat fretboard mounting surface or above the headstock surface.

8. Epoxy the headstock plate onto the headstock, making sure the tuning post holes line up exactly. You can press in the ferrules provided with the tuners to make sure the holes line up.

9. Epoxy the fretboard to the neck, masking off adjacent areas as described earlier. The fretboard should butt up against the headstock plate.

10. Assemble the tuners. First press the ferrules into the holes in the top of the headstock (if you haven't pressed them in already) and then screw the right and left tuner sets into the back of the headstock.

11. Bolt the neck onto the body using the ¼-20 bolt and nut and a washer on each side. It's a little tricky getting nut and washer on in through the sound hole, but it helps if you have the neck mating surface pointing downward. You'll need a ¼-20 open-end wrench to hole onto the nut through the sound hole as you tighten the bolt with an Allen wrench.

12. Strings. The bridge is designed for loop-end strings, but it turns out the loops in most strings are too long and interfere with the saddle. What is did was form my own, shorter loops in the other ends of the strings and then cut off the factory loops. To help form the loops, I used a 1/16" wide by 0.25" deep mandrel to wrap the wire around before twisting it together. For good measure, I also soldered the twisted section.

At this point you can insert the saddle and then string up the mandolin. If the action is too high or low (it should be about 0.060" at the 12th fret), re-print the saddle with the Z height scaled up or down as needed.

Happy picking!

Now if I only knew how to play the mandolin…

And finally, the bone yard: