Solid body classical guitar

This is a design for a nylon-stringed electric "travel" guitar. It follows the traditional shape of a concert guitar, with the neck joining the body at the 12th fret (though the cutaway is a deviation of the traditional design). The guitar has a scale length of 650 mm. An additional design goal is to make it easy to transport, so I left the sides detachable.

This is an experimental design. I will likely revisit it with modifications and adjustments.

Nota Bene: I finished this guitar in the last week of December 2023. At the end of January 2024, I observed that the neck and body had slightly bent under the string tension. PLA is prone to creep under continuous stress. The effect was minor, and I could fix it by lowering the bridge (or more accurately: reducing the saddle height). Since then, there has been no further bending (I am writing this end of April 2024). If you set out to build this instrument, keep the effect of creep in mind.

Updates

• 28 December 2023:
  First upload.
• 6 January 2024:
  Made the saddle 4 mm wide, and adjusted the bridge accordingly. Since the tuners (machine heads) are now behind the saddle (instead of behind the nut), it is beneficial to have a wider saddle. The drawback is that this is an uncommon saddle size, so if you set out to create a saddle from bone or TUSQ, it will involve a bit more work to file or mill the material to the correct thickness.
• 29 January 2024:
  Added a design for a bridge that is custom-fit for the ARTEC PG-333 piezo pickup. The STL for this bridge is in a separate folder, though the ARTEC PG-333 is now the preferred pickup for this guitar.
• 4 July 2024:
  A full update, as a new version “Classical travel guitar - version 2”.
   

Printing and Post-processing

I printed the guitar in the pictures in PLA (Prusament Vanilla White). In hindsight, this was a mistake (see the “caveat” near the top of this decription). I chose PLA because of its stiffness. However, a drawback of PLA is that it is prone to creep (especially at higher temperatures). All polymers creep to some degree, but PETG, ASA and PC less so than PLA. Therefore, I recommend to use PETG.

My printer is a Prusa XL. I used a 0.6 mm nozzle, and overall a 0.25 mm layer height (and everything was printed on the “satin” sheet). Furthermore:

• For the neck, body and head: 6 perimeters (3.6 mm vertical walls), 8 top and bottom layers, and a 40% infill ratio of type "cubic infill" (as a compromise between print speed and strength).
• For the wings (sides): 5 perimeters, 8 top and bottom layers, and a 20% infill ratio (of type "cubic infill").
• For the bridge: 4 perimeters, 4 top layers & 8 bottom layers, 40% infill ("cubic"). The reason for the large number of bottom layers, is to fill up the area below the slot for the saddle, so that the saddle (and the piezo pickup) rests on solid infill.
• For the tie block (tail piece): 4 perimeters, 100% infill (rectilinear).
• For the nut and the saddle: 2 perimeters, 5 top layers & 4 bottom layers, 100% infill (rectilinear). For these two models, I used a 0.15 mm layer height, to get smoother surfaces.
• For the pickguard (cover): 2 perimeters, 5 top layers & 4 bottom layers, 20% infill.

Especially on the wings, I have noticed that PrusaSlicer places the seam at the corner with the most acute angle. But corners with acute angles also have the biggest risk of coming loose from the build plate (first layer), and a seam point increases that risk. So, for the wings, I have routinely set the seam by hand (seam painting tool), to a corner with an obtuse angle, or in the middle of a flat surface that will be glued. I did this for the entire height, though it is only critical for the first layer.

After printing, you will want to smoothen the surface (especially on the neck). For this purpose, I recommend a card scraper; it gives quicker (and better) results than sanding. You still need to do some sanding after scraping, and for that, use "wet sanding" in preference to dry sanding.

I recommend epoxy for all glued parts, but choose an epoxy glue that is suitable for plastics (I used Bison Kombi Power).
 

Design & Construction

The design is in OpenSCAD, and most parts have a design file of their own. The general file "guitar.scad" includes the others and lets you select the part to print; it can also show you the assembly. The body wings (sides) and the pickguard are an exception: I drew their basic shapes in Inkscape, and then imported the SVG files in the OpenSCAD script. A problem was that the Minkowski operator does not work on the imported SVG (I intended to use the Minkowski operator for the edge rounding). I therefore simulated the rounding by stacking slices of the (extruded) SVG with the same height as the layer height. It came out fairly well (needing a little postprocessing witht the card scraper).

Not everything is 3D printed. The fingerboard is wood; ebony in my case. An aluminium bar runs through the neck, for extra stiffness and to (hopefully) compensate for material creep in the plastic. Another bar runs through the body; this might not be needed, as the body is more solid, but the bar also runs through the head which may need the extra strength.

The glued parts often have holes for pins. These pins serve for both for alignment and for extra strength. The idea is that the pins are cut from a metal rod (4 mm diameter). All pins in this design are 10 mm long.

When it comes to customization, the first stop is the file "settings.scad" that has most of the dimensions and options. However, when changing one value, others may need to be tweaked as well. Some dimensions or shapes are also hard-coded in the various other scripts.

Neck

The neck is printed with the "fingerboard surface" on the build plate.

The neck is screwed onto the body. The neck holds four M6 nuts. There are also washers below the nuts, and these are enclosed. The printer must therefore be paused at the right height to insert the washers. When using 0.25 mm layer height, the pause point is at 6.95 mm. (The nuts are also enclosed after the fingerboard is glued on.) Also, there is a single bridging layer above the washers (so the hole is not entirely through); this is to avoid "floating bridges" in the sliced print. (I have heard this trick is called sacrificial bridging.) You have to use a 6 mm drill to finish the holes.

The aluminium bar (4 x 10 x 390 mm) must be glued into the neck before the fingerboard is glued on. And before glueing the fingerboard, insert the M6 nuts. Tip: put small round stickers on top of each nut, to avoid any glue drooping into the nuts.

I usually don't care much about visible layer lines, but you will want the neck smooth. So post-processing is required. See the section "Printing and Post-processing" for some tips.

Tie block

The tie block is the section at the end of the neck, where the strings are tied off. This part is glued onto the neck. There are two alignment pins for keeping proper position and to make the construction stronger.

Body

The body is printed with the top side up (bottom side on the build plate). It requires no supports, but the four 6 mm holes (for screwing the neck onto the body) need to be drilled-through; to avoid floating bridges, I have used the same trick as with the neck.

An aluminium bar (4 x 10 x 480 mm) runs though the entire body. In fact, it sticks out of the body, because the head fits on this bar too.

The part that is called "case" is a teardrop-shaped hollow box that contains the 6.35 mm TRS socket, volume knob and battery (plus possibly a preamplifier). It is printed as an integrated part of the body. I printed the narrow part of the case with supports (a "support enforcer" in PrusaSlicer), to support the top surface. That said, I have not tested whether supports are really needed.

The pickguard serves as the cover for the case. The pickguard snaps onto the case with magnets; there are three magnets in the top ridge of the case, and three corresponding magnets in the pickguard.

Head

The "head" is at the bottom of the body, instead of at the end of the neck; the "tail" might be a better term for it, but I have kept calling it the "head".

The head is printed in two parts, so that no supports are needed, and so the top surface quality is best. Two alignment pins help glueing the two parts. The head then slides onto the the aluminium bar that runs through the body, and is glued onto the body.

At the back of the head is a cover to put over the aluminium bar. I printed it with supports and with a 0.15 mm layer height, to get a fairly smooth surface. This reduces the amount of post-processing (to get a truly smooth surface).

Machine heads (tuner mechanisms) for classical guitars often have an ornament at to top end. On a standard guitar head, there is space for that. For our inverted head, we are cramped for space, though, and you'll have to find more compact (in length) machine heads. Two suitable options are in the bill of materials below. The individual machine heads of the "Gebr. Van Gent" model "56.01.G" are interesting in that they allow you to put the tuners closer together than the standard 35 mm pitch. I happened to get Rubner "straight style" tuners for a bargain price, so I decided to use those and stick with the standard pitch.

There is a slot in the head that will be covered by the bridge. This is called the "pocket" in the source code. The preamplifier for the custom piezo pickup sits in this pocket (so that the high-impedance cable is as short as possible). If you use an off-the-shelf preamplifier (or omit the preamplifier altogether), you do not need a pocket either. You can set the pocket "depth" setting to zero to remove the pocket (see also the notes on the "Bridge" section, below).

Bridge

In the bridge is a slot for the saddle. The piezo pickup also sits in this slot. At one end of the slot, there is a hole for the cable of the pickup. Depending on whether there is a "pocket" below the bridge (see the notes for the "Head", above), this hole is at a different end of the slot. When there is a pocket, the hole is near the bass strings; when there is no pocket, it is near the treble strings. As a side note, there is a kind of cable gutter that runs through a section of the head, the body and then ends into the case (below the pickguard). It is called the "pipe" internally, as it is constructed as a pipe assembly in OpenSCAD. The cable that runs from the piezo pickup and/or preamplifier runs through this pipe.

Update 29 January 2024: There is also an option for a "3+3 split pickup", such as the ARTEC PG-333. This is a pair of piezo pickups, each of which span three strings. The split-pickup option is enabled by setting the Saddle_Split dimension to a non-zero value (in settings.scad). There will then be two holes in the bridge for the pickup cables, one at either end. You need to also have a pocket in the head, to route one of the cables through.

Note: the STL file for this bridge is in a separate folder: “STL files - option: dual bridge & saddle for ARTEC PG-333”.

At the back of the bridge are a series of merlon-like shapes, which serve to keep the strings spaced correctly. In the source code, this part is called the "comb".

I glue the alignment pins for the bridge into the head, but I do not glue the bridge to the body/head. Instead, I leave the bridge detachable, so that I can still access the wiring to the piezo pickup.

Wings

The detachable sides of the body are called "wings" in the OpenSCAD source code. The "left" and "right" qualifications are relative to looking at the guitar when it is standing vertically (and from the frontal direction). In other words, the "right" wing is near the treble strings, the "left" wing is near the bass strings.

The wings use the pins to align to the body. The pins are supposed to have a tight fit on the wings, but to slide in easily into the corresponding holes in the body (the body therefore has more tolerance on these pins). The wings then also hold onto the body with neodymium magnets.

The wings are too large to print as a single unit, even on a Prusa XL. Therefore, I cut them in parts. You can opt to use the cutting tool in the slicer, but I used OpenSCAD. The rationale is that you won't be able to hide the "cut", and the ancient trick is: emphasize what you cannot hide. So the idea is to make the cut part of the design; in this case, with a colour change. And as a result, I want the cuts to be aligned and parallel on both wings. And this was easier to do in OpenSCAD than in the slicer.

Nut and saddle

In general, luthiers will recommend against a nut or saddle made from plastic; the favoured materials are bone and TUSQ (a synthetic material that supposedly mimics the tonal qualities of bone and ivory). You can, of course, follow that recommendation, and get a set of semi-finished nut and saddle from bone or TUSQ, and shape these to the final form.

However, I have still included a nut and a saddle in the design. The motivation is that "plastic" is a very generic term. There are many kinds of synthetic polymers, with wildly different properties, all called "plastic" (indeed, TUSQ is a synthetic polymer too). When you choose to print the nut and saddle, I recommend PLA over PETG, considering the hardness of the material; and obviously, I recommend 100% infill.

Another side note: according to the information that I found, PETG scores higher on "Rockwell" hardness than PLA, but a simple scratch test indicates the inverse. More concretely, I printed a nut in both Prusament PLA and Prusament PETG. The PLA nut scratches the PETG nut, but the PETG nut does not scratch the PLA nut.

The printed saddle is dimensioned to give an action of 3 mm at treble E string and 4 mm at bass E string. These are standard values for a classical guitar (the "action" is the clearance between the string and the fret, measured at the 12th fret). However, the final height of the saddle also depends on the thickness of the bridge pickup. I used a Shadow SH097 bridge pickup, which is what the saddle is based on. When using a different pickup, you may need to adjust the saddle dimensions (you may also need to adjust the bridge dimensions in that case).

Update 6 January 2024: The original design is for a saddle that is 2.5 mm thick. This is a quite common value for classical guitars. However, with the machine heads behind the bridge, there is more friction from the strings on the saddle. Therefore, I changed the design to use a saddle that is 4 mm thick. The STL files have been updated. The original designs for the bridge and the saddle are in a separate folder. (Of course, you can also customize the width of the saddle in the OpenSCAD script.)

Update 29 January 2024: See the folder “STL files - option: dual bridge & saddle for ARTEC PG-333” for the bridge & saddle for the preferred pickup for this guitar.

Knob for volume control

I did not design a knob for potentiometer, because there exists a suitable design already. Actually, there's more than one: search for "EC11 knob" on Printables.com or elsewhere. The design I ended up with is generated with the "Knurled knobs generator" script by Francois Polito, available on Printables.com. That script is actually for machine screws with a hexagonal head, but you can tweak the settings to create one that fits the knurled EC11-style axis of the potentiometer.

Electronics

My initial choice for the bridge pickup was the Shadow SH097. The Fishman AGX094 is an alternative with almost the same dimensions. You may use a different pickup, but the length of the slot (for the saddle) and the position of the hole in the bridge for the cable may need to be adjusted.

My current choice is an ARTEC PG-333. This is actually a pair of pickups that each cover three strings. Thus, you have a separate pickup for the bass and the treble strings.

These bridge pickups are piezo elements. When a piezo pickup is connected to a (pre-)amplifier with a standard input impedance of 20 kΩ to 50 kΩ, it tends to sound rather thin. This is because the pickup has an implicit series capacitor of roughly 1 nF, so it forms a high-pass filter in combination with the relatively low amplifier impedance. This eliminates the bass. What is needed, therefore, is a preamplifier with high input impedance. I provide the schematics and PCB layout for my preamplifier designs on GitHub. The designs are in KiCAD, but Gerber files for the PCB, and a PDF for the schematic, are also included. You can choose an off-the-shelf preamplifier too, such as the MicroPre from Schatten-Pickups.

Single-channel

This preamplifier is suitable for the Shadow SH097 or Fishman AGX094 (or similar). It is a straightforward circuit based on an opamp. The circuit is designed to be as compact as possible, in order to fit in the pocket below the bridge. The rationale is that high-impedance inputs are prone to pick up radio wave interference, and a long wire is great as an antenna. This is why I wanted the preamplifier close to the pickup, directly below the bridge.

The voltage range of the opamp is 4.5 V to 36 V, so you can use either a 9 V battery block, or four 1.2 V NiMH (rechargeable) penlights. Power consumption is low at roughly 2.5 mA, so the battery should last long.

Dual-channel

The dual-channel preamplifier is for the ARTEC PG-333. It is a lot bigger, as it uses three opamps. Hence, it will not fit below the bridge. The preamplifier has two separate (nearly identical) channels for the two pickups, and a third opamp stage to mix the two. The "treble" channel has a trimmer potentiometer, so that you can adjust the gain of the treble strings can be adjusted (within limits) relative to the bass strings.

The voltage range of this preamplifier is the same as that of the single-channel one: 4.5 V to 36 V. Due to there being three opamps, the power consumption is roughly 6 mA. With a 9 V alkaline battery, one may expect 80 hours of play time.

Volume control and TRS socket

When using the single-channel preamplifier that is mounted below the bridge, the cable that runs from the output of the preamplifier to the potentiometer (volume control), output jack and battery, needs to have two conductors and shielding. One conductor carries the audio signal, the other the power for the preamplifier.

See the schematics for the preamplifiers for how to wire the jack, potentiometer and preamplifier. The potentiometer is specified as 20 kΩ in the schematic, but 50 kΩ is fine too. Note that I have used a stereo TRS socket, and wired the battery such that the preamplifier is only powered when a mono 6.35 jack is inserted (i.e. if you pull out the cable, the preamplifier is disconnected from the battery).
 

Bill of Materials

This list excludes all the printed parts.

Acknowledgements

The Bézier drawing code is written by William A. Adams (https://www.thingiverse.com/thing:8443).

The knob for the potentiometer is generated with the "Knurled knobs generator" script by Francois Polito (https://www.printables.com/model/167675-knurled-knobs-generator), and distributed under the GPL 3 license.

The Yamaha Silent Guitars series inspired this design.