Classical travel guitar - version 2

This is an updated 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.

Modifications from the first design

While the overall shape of the guitar is the same as the earlier design, the changes affect almost every printed part. Therefore, I have opted for a complete new version, rather than updating the design.

For better sound, print in PETG (and the consequences for the design)

The description of the earlier design already contains the recommendation to use PETG or ABS instead of PLA, but that was because of PLA is more prone to creep. In addition, I perceived the sound of the PLA-printed guitar to be dry and lacking clarity. A dry sound was expected: all solid-body guitars sound dry; but the dull sound was a surprise. I suspect that the particular plastic (PLA) was the cause. A few experiments (that I did afterwards), suggested that PETG might have better acoustic properties than either PLA or ABS. This was echoed in the research article:

Zvoníček T, Vašina M, Pata V, Smolka P. Three-Dimensional Printing Process for Musical Instruments: Sound Reflection Properties of Polymeric Materials for Enhanced Acoustical Performance. Polymers (Basel). 2023 Apr 24;15(9):2025. doi: 10.3390/polym15092025. PMID: 37177173; PMCID: PMC10181013.

A minor drawback of PETG is that it is not as stiff as PLA (and stiffness is essential for the neck). This was countered in the design by relying more on the aluminium bars running through the neck and body. With the new support bar structure, a single bar set runs through both the neck and body. The neck is not detachable from the body; the two are glued together.

A more significant issue, however, is that PETG is a difficult material to glue. Normal epoxy does not adhere well to PETG. Cyanoacrylate glue (superglue) has low "gap filling" capabilities, and it is difficult to work with on large surfaces (apart from that, for strong joints with cyanoacrylate on PETG, you should use a primer). For this design, I have opted for Bison Kombi Power, a two-component polyurethane glue for "difficult joints", but moreover, the design was tweaked to be less dependent on strong glue joints.

Other changes

• The thickness of the body is now 40 mm (changed from 45 mm). The main motivation is to reduce weight and filament requirements a little.
• This design is now pre-configured for the ARTEC PG-333 dual piezo pickup (the OpenSCAD design files allow you to adjust the bridge and saddle for other piezo pickups).
• Option for a simple (detachable) stand. The stand snaps onto the guitar body with magnets.

Printing and Post-processing

All parts are printed in PETG. I used AzureFilm "Skin Latte" and "Skin Cappuccino". The main parts are printed with a 0.6 mm nozzle and a 0.25 mm layer height (my printer is a Prusa XL). Furthermore:

• For the neck and body: 4 perimeters (2.6 mm vertical walls), 8 top and bottom layers, and a 25% infill ratio of type "cubic" (as a compromise between print speed and strength).
• For the head: 6 perimeters (3.8 mm walls), 8 top and bottom layers, and a 40% infill ratio (of type "cubic").
• For the wings (sides): 4 perimeters, 8 top and bottom layers, and a 15% infill ratio (of type "cubic").
• For the tie block: 100% infill.
• For the bridge, I used 4 top layers & 8 bottom layers. 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.
• The guitar was printed over the course of months (when the printer was not busy doing "real" work), and somewhere along the line I switched to a 0.4 mm nozzle. The nut and saddle are printed with this nozzle, a 0.1 mm layer height (for a smooth surface), and solid infill. Update 24 July 2024: see the note in the section on the nut and saddle for why I now recommend to use a non-printed saddle.

The gluing mould is an exception; print this with the default settings and in your cheapest filament. The mould holds the parts of the wings in place while these are glued. It is a "throw away" part, only used during construction (if you plan series production of this guitar, then, of course, don't throw away the mould).

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. I did this for the entire height, but 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.

You need a glue suitable for PETG; Bison Kombi Power (a two-component polyurethane glue) appears to be adequate.

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) are an exception: I drew them 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 making thin layers at the top and bottom sides that simulate the round shape. It is coarse, but I always planned to smoothen the edges and round surfaces anyway.

Not everything is 3D printed. The fingerboard is wood; ebony in my case. A set of aluminium bars runs through the neck and body, for the required mechanical stiffness. This set consists of two L-shaped bars and one solid flat bar.

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 customisation, the first stop is the file settings.scad that has most of the shared dimensions. However, when changing one value, others may need to be tweaked as well.

The aluminium bar support

The support structure consists of two parallel L-bar profiles, plus a flat bar wedged in the middle at one end. The flat bar also extends from the L-bars. The L-bars run through the neck and body. The flat bar runs through the head. There is overlap of a 100 mm between the flat bar and the L-bars, and along this overlap, the bars should be glued. The mechanical strength of the guitar depends on the strength of this joint, so a good quality glue should be used. I used Bison Kombi Power for this as well. It is also recommended to roughen up the surfaces that will be glued a bit (with coarse sandpaper), and to make sure the surfaces are free from dust and grease.

At the end of the neck, the L-bars need to have a chamfer at the bottom, over a length of 10 mm (or a bit more). The neck ends in an asymmetric taper, so the support bar structure should at that point be tapered too.

Neck

I printed the neck with the fingerboard surface up. Printing therefore requires supports. It therefore also requires post-processing, but that will not come as a surprise: you will want the neck smooth, so you'd post-process anyway. See the section "Printing and Post-processing" (above) for some tips.

The neck is glued onto the body, and the aluminium bar structure is inserted (and glued) into the combined neck/body. Obviously, this must be done before the fingerboard is glued onto the neck.

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.

The holes in the tie block (for the strings) extend into the neck. After glueing the tie block on, these holes likely contain some glue too. After the glue has hardened, you can open up the holes with a 2 mm drill.

Body

The body is printed with the top side up (bottom side on the build plate). It requires no supports. The aluminium support structure is glued into slots in the top surface of the body, and then a cover is glued on top, to hide the support structure. This cover is a separate object (in other words, the body consists of two parts).

The part that is called the "case" is a teardrop-shaped hollow box that contains the 6.35 mm TRS socket and volume knob, plus electronics. 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 (if the guitar is standing upright) instead of at the 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. Two alignment pins help glueing the two parts. The head then slides onto the the aluminium bar that extends from the body, and it is glued onto the body.

Since the head is tilted, and the aluminium support structure is straight, the aluminium bar "surfaces". There is a separate print for a cover to put over this part of the bar.

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.

Bridge

In the bridge is a slot for the saddle. The piezo pickup also sits in this slot. This design is for a dual pickup (separate pickups for the 3 bass strings and the 3 treble strings), so the slot is split in two parts. At each end of the slot, there is a hole for the cables of the pickup.
The holes align to cable gutters that run through a section of the head, the body and then end into the case (below the pickguard). It is called the "pipe" internally, as it is constructed as a pipe assembly in OpenSCAD. The cables of the piezo pickup run through this pipe.

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. You won't be able to hide the "cut" lines, and the ancient trick is: emphasise 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 I wanted the cuts to be aligned on both wings, doing the cutting in OpenSCAD is easier than in the slicer.

To glue the three parts of each wing, while keeping the correct shape, also print the gluing mould. The mould has holes for the alignment pins at the correct positions. So, first glue the alignment pins in the sections 1 and 3 of each wing. When that has dried, you can glue section 2 to the other two, and fit the entire wing on the mould.

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 figured that PETG is "good enough", and so a 3D printed nut and saddle are in the design. As the bridge has a split slot, the saddle consists of two parts. Update 24 July 2024: after various experiments, I am having to backpedal on this: there were persistent problems with the balance between strings, and I have now settled for a aluminium saddle.

The 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.

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". 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 choice is for the bridge pickup 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.

My preamplifier is a dual-channel design. It has two separate (nearly identical) channels for the two pickups, and a third opamp stage to mix the two. A trimmer potentiometer allows you to adjust the balance between the treble and the bass strings. Update 24 July 2024: the most recent update of the design has a bass-boost of frequencies below 300 Hz of about 5 dB. This mimics the resonance of the acoustic chamber (of a hollow-body guitar) when that is modelled as a Helmholtz resonator.

The voltage range of this preamplifier is 4.5 V to 36 V. 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

See the schematics for the preamplifiers for how to wire the jack (TRS), potentiometer and preamplifier. The potentiometer is specified as 20 kΩ in the schematic, but 50 kΩ is fine too. The TRS socket and the battery are wired 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

The list below is for the guitar. The list excludes all the printed parts.

The stand

The (optional) stand consists of a block with two magnets, that lines up with two (recessed) magnets in the back of the body. Two legs pivot on this block (the legs can be folded opened or closed). The legs are attached to the block with an M4 screw (and a plastic washer). There are two enclosed M4 nuts in the block.

In the design, there is a 0.15 mm think layer on top of the nuts. This is to avoid floating bridges. I have read that this technique is called "sacrificial bridging".

Additional items for the stamp:

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, and distributed under the GPL 3 license.

And to address the elephant in the room, a direct inspiration for this design is the Yamaha Silent Guitar.
 

Many tweaks, to make the design suitable to be printed in PETG.