TuneFast Harp

by makefast, published

TuneFast Harp by makefast Dec 12, 2017

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The TuneFast Harp is a 3D printed musical instrument that uses a single-string to create an 8-note diatonic harp.

The total cost with all hardware is under $7 per harp and only takes around 3 hours to print.

Hear what it sounds like with a metal string https://youtu.be/fx9zlmGFXBQ and with a nylon string https://youtu.be/yGjTQOmBWsM.

UPDATE: A new version of the harp (AHarp22_v04.stl, shown below with blue PLA) has been modified from the original to include a parabolic sound reflector (focusing the sound 8-10cm away from the string) and a clipped corner (to reduce total material used). Files are also included to print your own M3 washers and the final M3 spacer to reduce the parts list a bit.

updated TuneFast Harp with sound reflector and clipped corner


We designed the harp to be easy to assemble and fairly quick to print so that even young makers with shared resources could try it out as makerspace/classroom projects.

Parts List

You’ll need a handful of parts to complete this project. Everything is reasonably inexpensive, but many of the parts come in larger packs so you may want to plan to make a few and scale your quantities accordingly.

Here’s what you’ll need for each harp:

  • 1 x Right-handed Guitar Tuner ($1.25/each, typically comes in a pack of 6)
  • 9 x M3x20mm bolts/nuts (~$0.10/each for large quantities; or consider a M3 bolts/nuts multipack)
  • 8 x 3124mm V-Groove Bearings ($0.33/each, comes in pack of 20 but account for the possibility of a couple bad bearings per pack at these prices)
  • 1 x Nylon guitar string ($0.50/each, or use 25-50 lb test fishing line). You can also use a thin metal guitar string (9-17 gauge) but be careful not to over tighten.
  • ~100g of PLA (or similarly rigid filament) to print the parts:
    • 1 x harp base (file: AHarp20_v07.stl)
    • 1 x final spacer (file: AHarp21_v03_M3Bushing_6x4.stl)
    • 9 x washer (file: AHarp21_v03_M3Washer.stl)

Optional parts (for those with washers/spacers around):

Print It Out

Our print time came out to around 3 hours total on a TAZ6 printing PLA with high speed settings and 30% infill.

Generally, the harp works well printed with fairly large layer heights (i.e. coarse/fast settings), but make sure you use at least 30% infill (to increase strength) and print with stiff plastic (PLA works well; ABS is strong, but it’s significantly less stiff than PLA and that’s what we’re looking for here).

Assemble the Harp

After the print is complete, remove it from the print bed and clean up any defects from the print process (we didn’t really need to do anything, but your mileage may vary depending on which printer/filament you use). In particular, make sure there aren’t any extra strands or printer blobs that would interfere with the bearings.

Add the 8 bearings using an M3x20mm bolt, nut, and washer (we prefer metal belleville washers, but you can even print your own M3 washers). The washer should go between the bearing and the printed part to keep the bearing from touching and interfering with its ability to smoothly rotate. Tighten the bolt firmly, but not so tight that it causes the bearing to rub.

Next, add the spacer for the shortest/highest note (we don't use a bearing here because of space constraints and the minimal displacement under tension so close to the end of the string).

Now, double check that each of the bearings can rotate smoothly. If there’s additional friction in any of the bearings, the string won’t be under uniform tension and thus alter the relative tuning of some of the notes. If any bearing is rubbing, just loosen it, inspect/clean any surfaces, and slowly re-tighten.

Then install the guitar tuner in the remaining hole. Hand tighten in place with the knob facing outward. Screw it in place from the bottom and fully tighten the nut.

Finally, string it up. If using a ball end string (common for electric guitar strings), just feed it through the small hole by the spacer. If using a nylon string, tie a knot in it first near the end then feed it through.

Now weave the string back and forth, holding it firmly enough that it doesn’t slide off of the bearings as you go. After the last bearing, thread the string through the hole in the metal shaft of the tuner and turn the knob to tighten it in place. Bring the string just upto tension so it holds in place for tuning. Trim off any excess string that sticks out past the tuning peg so it doesn’t get in the way or poke you later.

Tune It Up

Since all of the notes are produced from a single string under constant tension, all of the notes should be relatively in tune as soon as the string is somewhat tight. By further tightening the string, all of the notes will increase in frequency, allowing it to be tuned to any desired key.

Two things to keep in mind as you’re tuning:

  1. You may need to sequentially push on each of the strings after adjusting the tuning to make sure the tension is even across all the notes. This is particularly important if any of the bearing are rubbing (i.e. have significantly higher friction).
  2. You can only tighten the string so much before something bends/breaks, so adjust it slowly at first to understand its range for a given string size.

Note that the harp also works well with steel strings, but they require about twice as much tension which can deform the printed part a bit more and make it trickier to play in-tune (especially when using thicker low strings). The other downside of using a metal string is that it can be pokey/dangerous when things break or things are flopping around as you string the instrument. Just work carefully and be mindful of others around you.

Play It!

We hope you enjoy playing this experimental single-string diatonic harp. When properly built, it produces a pleasant sound and holds its tuning quite well.

The TuneFast Harp is intended to be a hands-on tool for learning about how stringed instruments work as well as a springboard for designing and printing your own custom stringed objects.

A final note for the musically inclined that feel limited by only having 8 notes to choose from: keep in mind that you can use chord inversions to play chords that go beyond the harp's limited range. For example a G Major chord (G-B-D) can be played in the 1st inversion (B-D-G) or 2nd inversion (D-G-B) to fit within the harp.

Print Settings

Printer Brand:








0.38mm layer height


30% minimum


This is designed to print quickly, so feel free to use coarse/fast settings. Just make sure to set the infill to at least 30% to improve the rigidity of the instrument (and use stiff plastics like PLA).


Clean up any rough edges and make sure all holes are relatively clean. You shouldn't have to do much here, but double check before assembling.

How I Designed This

This harp was designed in Fusion 360.


Overview and Background

Tuning musical instruments can be a pain. This is especially true with instruments like pianos or harps, which can have dozens of strings (all of which slightly deform the instrument as they are tightened, affecting the other strings around them making tuning a long and tedious process).

Which raises the question: can you use a single string that zig-zags back and forth to drastically reduce the number of tuners necessary in an instrument? This harp is one example of such an instrument.

Lesson Plan and Activity

For younger students

Learn hands-on how stringed instruments are made by printing and assembling this harp. Intuitively explore the relationships between the sounds produced and changes to string tension, string length, and string thickness.

For older students

Print and assemble this harp, discussing the mathematical relationships between the various design elements (placement of bearings, string tension, string material, etc.).

Then look at edge cases and make hypotheses for questions such as:

  • What will happen to the sound if one of the bearing is rubbing (has increased friction) and the tuning is changed?
  • What sound will result if multiple notes are played and the string is stretched simultaneously?
  • What specifically will happen to the instrument if the string is over/under tightened?

Next, look at failure modes. Wearing safety glasses and gloves, tighten the string and watch what happens to the instrument as it surpasses its designed limits. Does the instrument start to bow? If so, where? Does the string break first or the instrument?

Finally, sketch (or fully design and print if you have the resources/knowledge) new simple stringed instruments that build on the underlying concepts of the harp but look/function differently.

Some ideas for new designs might include:

  • Alternative string materials? Ribbons? Rubber bands?
  • Incorporating more than 1 string?
  • Non-parallel string arrangements?
  • Allowing the string to pull on both the front and back sides of the instrument to distribute stresses?

Materials Needed

See the description above for the list of hardware needed to build each instrument. The total cost comes out to just under $7/harp.

Skills Learned

  • Math
  • Engineering
  • Music Theory

Duration of Lesson

3 hours to print, 20 minutes to assemble, 1 hour+ to play/experiment


While anyone can build this instrument by printing and screwing the parts together, to understand how it was designed you'll want to consider two equations:

  1. the string vibration frequency equation explains the relationship between string length, tension, and string mass per unit length.
  2. the equal tempered scale explains the relative change in frequency between notes in the scale.

Also consider reading more details about the math involved in making the harp.

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I'm trying to source alternative bearings (Amazon does not ship this item globally).

The 3124 pulleys I'm finding have a notch that is 2.8mm wide and 1.4mm deep. Will this work or mess with the tuning?

The diameter of the bearing where the string touches is the only critical element to maintain tuning. The 3124 bearings have a v-groove diameter of 9.7mm, so getting as close to that as possible will help (for example, 10mm diameter should still sound okay albeit slightly trickier to tune).

You can also use flat sided bearings, but then you may accidentally push the string off the side without the groove and need to completely restring/tune it. Once the string is under tension, it stays put pretty well, but as you're stringing it or playing it for longer amounts of time, the V groove really helps.

After a ton of searching I finally found something I think will fit.
This: https://www.aliexpress.com/item/10pcs-V623ZZ-V-groove-roller-wheel-ball-bearings-3-12-4-mm-embroidery-machine-pulley-bearing/32769540034.html

It seems to have an inner diameter of 9.8mm. Weridly, other v623zz berings have an inner diameter of 9.2. But this one shows a different diagram.

Do you have any recommendations on substitutions for the 3124 mm v groove bearings? I am going out of town this weekend and was hoping to make this as a gift for someone to take with me. I ordered the other materials on amazon but the bearings were out of stock until yesterday so they won't be arriving in time. The hardware stores around me don't seem to carry those specific bearings.

Woo Hoo! I was your 1,000 like! I'm printing this right now but I have to wait till the bearings come are delivered on Tuesday to play it. I can't wait to try it! Looks like an awesome design!

Comments deleted.

GREAT project and easy to assemble! If the holes are reamed after printing, older kids can put together with little supervision. Nylon strings seem to be stocked only in sets, but select "low tension for acoustic guitars" label on the package. The 4 thinnest ones work well. Appreciate that the base print is barely thick enough for the washer and nut on the tuner piece. If that part of the print lifts, you will have assembly issues. I get much better print on rafts, plus the back of the base is no longer slick from printing on your plate. Cut rounded triangles from old credit card plastic for picking the strings and loudest sound, if you have problems printing one that works.

Editing to add, great design! Ordering up some hardware now. Quick thought on reducing the parts list... did you try a version where the spacer/standoff is part of the printed part itself? I know the small diameter standoff would be tricky to print, but since there are 8 of them the layers should have time to cool as it prints. If you made them conical it would help with the flex under tension too.

DUDE! Way cool! When I was a kid we had a harp just like this but it was made out of wood, had a resonance chamber and about 15 strings or more. What was cool about it was that you had paper "music sheets" that slid in under the strings that were kind of a "connect the dots" song. So all you had to do was start on one side, pluck the first note and follow the lines/dots to play a song. All you need for this is a basic template and you could start printing on paper all sorts of little short songs. Might be cool start a google docks type thing that people could start making/sharing songs to print/cutout on paper. Definitely going to be printing/building some of these for my kids!

That's super cool, and you can even still buy those little multi-string harps with music that slide underneath if you're feeling nostalgic. But making your own (especially with the this harp's bearing configuration) would be even cooler!

One thing to consider is that the friction from the bearings (especially lower-cost version) will accumulate as you pass the string over more zig-zags back and forth. We found a practical limit of 8-10 notes per string before things get difficult to tune, so if you do want to make a 15-note version, consider using two strings/tuners.

if one were to CNC this out of wood, do you think it would still be effective? or does the infill or PLA material lend itself to acoustics?

See the other comment below about CNCing the harp out of wood (or manually just cutting/drilling it); in short, go for it!

A PDF for a paper template to trace is included in the files for download.

Wouldn't you want to make a cavity under the strings for the sound to be amplified ?

Ok, I'm pretty inspired. Really thinking about making one, then winding a thin pickup to slip underneath it, run it into my DAW and turn it into some kinda drone/ambient thing.

That would be awesome. You could probably even use an off-the-shelf pickup (perhaps a "rail" style so that the strings don't need to be placed exactly over particular points), just make sure it's wide enough to sense all of the notes. Or if it's primarily for drone sounds, just place it under the lowest 4 or 5 notes and don't worry about the higher ones.

Just ordered a bunch of hardware on Amazon. This is great. I am making a bunch of these for my wife and her new music class. This is absolutely perfect. I also have some color changing PLA, that will be fun for a handheld instrument...

Thanks for your work on this. I will post the pics when Amazon sends my stuff...

Happy to hear it! Let us know if you have any questions once you get the parts and are making/assembly it. It should (hopefully) be a pretty quick print-and-play instrument.

Quick design question. When using the formula to calculate proper string lengths. Do you use these exact values to space the centers of the holes for the screws/Vbearings OR do you adjust the distance between the holes based on the Vbearings radius?
I am guessing that the string stops touching the vBearing basically at the center, so you would used the exact length calculated for the center of the holes, but I could be wrong and you've done all the prototyping.

You're on the right track. In the end, all of the string segments can be placed such that the end of each is measured from the center of the bearing (off to the side by the radius of the bearing at the center of the v-groove) as you mentioned... plus a minor adjustment to makeup for two non-ideal factors:

  1. The slight tilt of each bearing when the string is under tension (i.e. the bearing/bolt each tip in a bit toward the center of the harp as the string is tightened).
  2. The overall bow of the instrument as the plastic bends depending on how tight you make the string.

For the bearing/bolt tilt, all of the bearings are the same except for the one closest to the tuner (since the tuner pulls the string to the side) and closest to the top of the high note (the final bushing has almost no tilt since the string is just guided along at a low angle). The overall bow is fairly uniform, but you may need to make some minor tweaks depending on how tightly you intend to string the harp.

Hope that helps!

thanks for the details. If I understand, both these factors shorten the actual length so you add a constant to make the holes farther part.
And for strings numbered 1-8.
B=bow factor
T=tilt factor
string1 and string8=lenght+B+T/2
string2 thought string7=lenght+B+T

Yes, that's correct. We ended up applying a constant to the middle string segments (on the order of 0.5-1.0mm was enough) to account for bow/tilt, and treated string segments 1 and 8 with slightly less (as you point out, half of the tilt is a good ballpark). In practice the first/longest segment is pretty comparable to the others and can probably use the same factor as the middle strings.

However, the shortest string segment (#8) was noticeably out of tune without compensating for the bushing (which doesn't tilt much at all). In the end we settled on moving it an additional 0.5mm closer than the theoretical placement (and could probably have gone even a bit more, maybe 1.0mm, since the final note only ever feels slightly flat depending on string type/tension).

3 hours to print? That seems a bit much. What layer heights did you use?

We wanted to give a realistic/conservative estimate on the print time since there's so much variation between printers and settings.

For our prints, we used 0.38mm layer height (0.5mm nozzle) at medium/high speed (but probably could have pushed it harder if pressed for time). If you do print one out, let us know how fast you're able to do it (and what settings you used)!

While they're super cheap I decided to just print some groove wheels for mine to just get started. Here's the thing model in case anyone else wants some or has a hard time getting them for whatever reason:

VGroove Pulley

Do you think there would be any way to make semitones? Im trying to make a 3 octave dovetailed set and I think that would be amazing. The math is already beyond me, or I'd work on experimenting with it.

A dovetailed set would be super cool. Go for it! Just keep in mind that you'll likely only want 7 notes per string (instead of 8) or you'll have a repeated octave note (which could actually sound nice, so maybe that's not an issue at all).

As for using semitones, we tried a version (see the really wide harp in the upper right of the picture under the "Experiments Along the Way" section in the original write-up) and the complexity grew pretty quickly as the tension from tuning each string had a slight effect on the neighboring strings.

Also note that since a guitar string is a fixed length, you're limited by the number of zig-zags back and forth before you run out of string. And even if you do have really long string, the little bits of friction in each bearing start to become hard to manage (i.e. it's difficult to tune) when you get over 8 notes. So for our semitone experiment we had two strings per octave (6 semitones) and 3 string total (for 18 semitones total).

Probably if you rise string a bit it would be more comfortable to play. If you make it holow (acoustic guitar like) it will sound better and louder.

Just added a new version (AHarp22_v04.stl) that includes a parabolic sound reflector under the strings. This more than doubles the gap below the string and also projects the sound more effectively. You can see what it looks like in the picture of the harp printed in blue PLA above.

Thanks for the suggestions!

Making the strings higher does make it easier to play, but it also causes the instrument to bow/bend significantly more. That could be offset with an even thicker base, but we settled on a middle ground for the final design. Alternatively, there could be some structural reinforcements (like a truss rod or bolt the goes across the back side) to allow for the increased strain on the printed part from raised strings. If you want to try it yourself, you can easily raise the strings by just using a longer bolt and adding a spacer between each bearing and the base.

Increasing the volume would be a great next step, too. We considered making it hollow and adding a sound hole early on but settled on prioritizing the structural rigidity for the instrument, especially since it's designed with novices in mind with a lot of wiggle room for different printer capabilities.

edit: We're putting together a version now that includes a parabolic reflector to project the sound a bit. It adds a shallow dip under the center of the strings which hopefully addresses both of your suggestions (more comfortable to play and louder).

Hi there, do you think this would work in wood or MDF? I was thinking of making a paper template printed at scale to help drill holes in the right position.

Since the final design is rather flat, you could definitely make a version out of wood! The hole placement is critical, and the 3D printed part has a couple small details (like a raised guide to support the shorted string segment and recessed pockets to capture the nuts on the back side) that may be somewhat tedious to do by hand.

Printing a paper template sounds like a fantastic idea to line up all the holes. We'll take a look at adding a template file so that others can do the same. Note that you could even scale it up or down and the ratio of distances for each string should remain the same. However, the current design is about a large as you can make it where a standard guitar string is long enough to work.

In fact, for education/learning it'd be cool to make both 3D printed and wood versions to compare the building techniques, difficulty, strength, overall sound, etc..

edit: A PDF for a paper template is now included in the files for download. Let us know how it works out!

Good to know it could be scaled up/down, that hadn't occurred to me.
I had some other ideas for a wooden version:

  1. To avoid having the nuts standing proud on the back surface, you could perhaps have countersunk holes on the rear, and have the nuts on the front.
  2. For a raised guide to support the short string segment, you could perhaps have the string sit in a slotted screw, or the head of a hex bolt in that position.

1st to comment, this is actually really lit tho

congrats on the feature bro, you deserve it

I would make this if I had the supplies for it.

We've been thinking through what it would take to reduce the parts list. The washers can definitely be printed, and probably the smaller bushing that supports the final note (shortest string segment) could be printed too. If plastic bearings had low enough friction, that'd be super cool, but too much friction means that all the string segments aren't necessarily under uniform tension which is important for it to stay in tune.

IS that what you are going to try and tackle next?

Just included the files for printing out the M3 washers and M3 6x4 spacer to reduce the parts list.

Also tried printing out a version of the V-groove bearing but the significantly higher friction negates all of the tuning advantages (i.e. it takes a long time to tune, and if you pull on any note it shifts the tuning of neighboring notes until you carefully go back and re-tune it all). So at least for now, you'll probably need the metal bearings.

That said, if you want to give printed bearings a try (perhaps with a little oil/grease to let things slide under load), there's a nice version by geartechbrandon over here: https://www.thingiverse.com/thing:2750787

VGroove Pulley

Aw, you did that just for me?