I Heart Door Harps

I have always loved door harps. 

They make a delightfully slightly-in-tune chord any time you close your door. Nothing says home like the 'ting-ting-a-ting-ting' of a D# augmented seventh diminished flat chord. (I made that up.)

I designed my first 3D printed door harp in January 2023, and designed this heart-shaped harp variation for the Printables 2023 Valentine's Day contest. Shoot, we should make a door harp for every holiday or season, then you can just swap them out every few months! Someone help me out with this! I call dibs on Halloween. ;-) You are welcome to print these and sell them, just make them well. Attribute whenever possible, please. 

The purchased parts, weights, and string mounts are the same between all of my door harp designs. Only the base and soundboard are different.

There are two base files – print one! The HeartBase-R2 has 0.25mm clearance between soundboard and base, and similar clearance on the string mounts. The HeartBase-tighter-R2 has no clearance on the string mounts and 0.1mm clearance on the soundboard. Print the one that you think will work best with your printer/settings/materials. 

Purchased parts 

• Zither pins (quantity six) -- any zither pin that is about 5mm in diameter should work great. I first used some from Amazon and then I ordered these in bulk. 
• Tuning tool -- A printed design is included and it sort of works, but to fine-tune the harp once it is complete you will want a metal tuning wrench. 
• Thread or monofilament
• Music wire (guitar, piano, harp, whatever) about 0.014" thickness.
• Toothpicks 

Tools and Other Supplies

• Drill with 3/16" bit, and something ~1-1.5mm in diameter (1/16" is good)
• Wire cutters, needle-nose pliers, maybe an exacto for whatever you might need an exacto for
• Your favorite glue for wood/PLA (I use CA but you might consider something slower curing for your first build) 
• Files and or sandpaper or sanding blocks
• A wood screw for your door that fits the harp (head diameter of 6-9mm, screw diameter (<5mm)

Here's a photo of most of what you need:

Assembly is easy. It goes like this:

1. Print one base, one soundboard, three weights, and three string mounts. 

The base I print with three perimeters and 30% infill using a 0.5mm nozzle. 

The weights and string mounts should be printed solid or very close to solid. 

The soundboard is also printed close to solid, with just a few layers of 50% infill in the middle of the part. 

Check that the soundboard inserts into the base. It should fit with just a bit of friction. If it doesn't, you might need to use a bit of non-permanent glue to hold it in place. 

2. First, install the zither pins to the base. Drill out the holes as needed (3/16" bit) so that you don't break the base while screwing in the zither pins. That said, they should be quite tight and might even heat up during installation such that they melt the plastic a bit (but not visibly). That's OK. 

On the left hand side of the harp, thread the pins in until no thread is showing.

On the right hand side, thread the pins in until there are 3-4 threads showing.

If you drill it out too much and the pins thread in loosely you have a few options. (I did this actually for the pink frame shown here so I know from experience.)

• Print the part again and go easy with that drill! Or adjust your settings if needed.
• Use ‘threadlocker.’ Real thread locker is CA based, so I figured I could just put some CA glue in the hole and use it to take up space and add some friction. It worked great! I applied CA, tightened down the pin, let it set, and broke it free by threading it back out. Then I repeated this two or three times until I felt like I had a good fit all around the pin (and with some depth) and not just in one section. 

3. Let's go ahead and fit the string mounts. I also drill out these holes in the base with a 3/16" bit. The printed string mounts I actually chuck up in my drill and use it like a lathe to smooth the shafts. File or sand the shaft until it inserts with some resistance to the base. Remove the string mounts. If needed, drill the small thread hole out with a 1-1.5mm drill bit. You'll need to pass thread through this hole. Now set the three string mounts aside. 

4. String the harp. If you haven't yet, go ahead and put the soundboard in place. Once the strings are tight, it should be squeezed a bit and held in place firmly. 

Cut a section of music wire that is about 2" longer than the distance between the zither pins. First, insert into the hole on the left pin, wrap the wire once above where it is inserted, and then wrap it twice more but under where it was inserted into the pin. Now pass it through the right pin and do the same, but do it by turning the pin instead of moving the wire around the pin. 

Here's a closeup of the pins once fully set up:

5. With the harp wired, let's prepare the weights. Drill out the string hole as needed with a 1-1.5mm bit. To fix the thread to the weight, insert the thread. Then get a dab of glue on a

 toothpick, and jam the toothpick into the hole, sandwiching the thread in place. Try to position the thread in the center of the weight (the hole is offset). Cut the thread to leave about 6" length. You'll trim it more later.  

Repeat this for the three weights. 

6. Final assembly! 

Install the weights one-at-a-time. Insert the string mount into one of the upper holes. Then pass the thread from one of the weights through the string mount. The height of the weight relative to the string is CRITICAL for good performance. It will work best if it bounces right on the center of gravity of the weight. Test it -- pull it a little high, see how it bounces. Go a little low, same thing. Find the sweet spot. Then get a dab of glue on a toothpick and jam it into the string mount hole from the top to lock in the thread. Repeat for the other weights.

Which weight strikes which string is up to you, no wrong answer there. 

If your string mounts are a bit loose in their holes, you can use an exacto to raise a bit of material on the base or on the mount. You can also use a non-permanent adhesive or RTV (I used E6000 on this one) to tighten things up. 

7. Mount on a door. Using a wood screw that fits the mounting hole, mount the harp on a door. It is best if it mounts snugly, with no looseness in its connection. So adjust the depth of the screw into the door until you have a nice fit. Note that you probably need to pre-drill the door so you don't crack anything! I don't know much about wood, but I know this.