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.
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.
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:
- 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).
- 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.
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.
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.
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?
See the description above for the list of hardware needed to build each instrument. The total cost comes out to just under $7/harp.
- Music Theory
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:
- the string vibration frequency equation explains the relationship between string length, tension, and string mass per unit length.
- 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.