DIY String Shooter - Make String Move

A while ago I decided to try making a string shooter because I wanted to see if I could make one optimized for 3D printing and cheaper than the 15-30 dollar versions on amazon. But little did I know that this would become one of my most difficult projects to get working. Along the way I learned new ways to incorporate electronics in my projects and how to optimize my designs for a more professional look.

Follow along to see how you can make your very own String Shooter to make string move in ways you thought impossible.

You can download all the files for printing here : https://www.printables.com/model/1147007-string-sling-the-3d-printed-string-shooter

I am a 8th grade student at the Maria von Linden Gymnasium.

Supplies

1. For V2 (AA battery powered)
2. PLA/PETG filament
3. 2 regular hobby motors
4. electrical cables
5. springsteel wire
6. 2 batteries
7. aluminum/copper foil
8. switch
9. string
10. hot glue

1. For V3 (Supercapacitor powered
2. PLA/PETG filament
3. 2 regular hobby motors
4. electrical cables
5. springsteel wire
6. 2.7V 500F Supercapacitor
7. spring steel sheet (I got mine from and old phone)
8. 2 terminal jst/dupont connector (jst is best)
9. 3V Solar panel
10. diode with a voltage drop of around 0.5V (you may connect two 0.25V voltage drop diodes in series as well)
11. switch
12. string
13. hot glue

Tools

1. 3D Printer
2. Soldering Iron
3. Hot glue gun

For this project I have several goals in mind:

1. Get the price under 10 dollars (most commercial ones were around that price range)
2. Being able to easily remove and insert new string
3. Look somewhat stylish

Next I drew a rough sketch of the design. In the beginning I wanted to use only a single motor and have two gears linked both pushing the (similar to the Bondtech dual gear extruder found on many 3D printers). However, considering the enormous friction that usually comes with 3D printed gears, I decided to go for a two motor design. In addition to contemplating whether to use one or two motors, I also planned out the general location of the batteries and the on and off switch

1. To create the battery holder, I created a box by create a rectangle and extruding it.
2. To get the right dimensions of this box, I took measurements off of my injection moulded battery holders that I had lying around
3. Next I created a sketch on the side plane and drew two congruent circles as shown in the picture. I used lines to dimension these circles to be in the middle
4. Finally I extruded both circles and the upper small triangle shaped portion between them to add space for the batteries

1. To add a lid, I edited the previous sketch and drew small dovetail shaped lines, which I then extruded as a surface and used as a cutting tool to split the battery holder into two parts
2. Then I created components out of both bodies
3. To add the terminals to connect both batteries with each other using copper foil, I extrude cut a small rectangle on both sides of the battery holder, to allow space to slide in small copper strips
4. In addition I also used several small circles to cut out small holes to pour hot glue into and secure the strip in place
5. Finally I created both batteries using circles which I extruded out 50mm

Next I created the compartment for the switch

1. To do this I elongated one of the ends of the battery case using the extrude function
2. Then I drew a smaller rectangle on the front face and dimensioned it to be slightly smaller that the face
3. I used this rectangle then to create a pocket for the electronics
4. Next I created a sketch on the side and drew a rectangle matching the dimensions of my switch (plus a little bit of clearance)
5. Finally I used this sketch to create the pocket for the switch

Next I created the motor mount

1. To do this I started by sketching both motors using a circle and a rectangle for each motor, making sure to fully constrain them
2. Next I created the outer wall, by using two circles, connecting them with tangent lines and trimming off the rest
3. After that I drew some smaller circles for the motor shaft to poke through
4. Then I used several extrude features to first create the lower bed for the motor to rest on, then the walls and then a cap on top
5. After completing that I added a few fillets to smooth out the design
6. Then I drew a line, extruded it, and used it as a cutting too to cut the main body into two parts
7. Finally, I hollowed out some space to allow for the fitment of the motors using various extrudes and fillets

1. To create the rollers, I started by creating an axis through the motor shaft hole and then created a plane on this axis
2. Next I drew a line with a V shaped cutout at the center (to hopefully guide the string to the center) and then connected this line with the motor shaft axis to create a closed profile
3. After that I filleted the edges on the v shaped cutout
4. Next I created a circle on the front face of the main body
5. After finishing that sketch I swept the profile along the center motor shaft and used the first sketch as a guide rail
6. To finish the roller, I cut a hole into the center of the roller for the shaft to fit through
7. I duplicated this roller and placed it on the other motor shaft and used both wheels as a template for my roller cover.
8. The resulting sketch was then extruded and a cap was added using another extrude function

My friend came up with the logo name of String Sling for the string shooter and I proceeded with designing the logo next.

1. To design the logo I used Powerpoint and started brainstorming different design ideas. I wanted to emphasize the alliteration in String Sling, so I decided to write both words underneath each other and use only a single S which I extended on the bottom to give the idea of a string. I also chose a cursive font and exported the design as a svg.
2. Next, I imported the svg into Fusing 360 and scaled it to the right size on the front face
3. This was then used to cut a 0.4mm tall cavity into the face and then it was filled with a new component : the logo. The plan is to use the same method I used in my Compass Case Repair Instructable to print the logo.

Finally, I created a parameter called clearance and used this variable to press pull every face that in some way interfaces with another 3D printed part or electrical component. I used a parameter instead of just a number, so that I can easily update this in case I realize the parts don't fit. In addition, I used the fillet tool to round off most of the sharp edges to give a more streamlined look and feel.

I printed the main part first and used minimal setting (2 walls, 5% lightning infill) to save on material. I knew that my first prototype probably wouldn't be perfect, but I was pleasantly surprised to see the batteries snap into place. The switch opening was also the right size, but the motor housing was a bit too tight. After adjusting all these things and printing it again, I placed the motors in the corresponding housings and connected everything up. I also printed the rollers which were a bit tricky to get the tolerances right so that they were a press fit.

I made a makeshift loop of string by gluing the two ends together using Uhu glue and placed the string between the rollers. I hooked up 2 AA batteries from a regular case using alligator clips and started the motor only to be disappointed by the fact that the string instantly tangled up in the motors. I repeated this test several times, but the string only either got tangled up in the motors or got ejected from the rollers. I tried adding the housing that I designed, but that didn't help either. Next, I tried to thicken the rollers to allow for a greater tolerance of string slippage. I scaled the rollers up 200% in Z in the slicer and replace the new smaller rollers with the updated ones. This also didn't help, but once I tried guiding the string with a loop of wire to the center of the roller, I had string that was looping around the rollers.

Although the string wasn't being thrown off the rollers anymore, the string still wasn't being thrown forwards like it was supposed to. I thought that this could be because the rollers weren't gripping the string, so I started searching for easy ways to add grip to 3D prints. I could use a special grip spray, but that would drastically increase the cost of this build, if you would need to buy an entire spray can. In the end I decided to use hot glue. However, I couldn't just apply hot glue onto my existing rollers, otherwise they would be too thick and so I had to design a special roller. For the roller, I took the existing design and enlarged the V shaped cutout all the way to both sides and printed them. Applying the grip wasn't easy, but it was doable. I slowly piped the hot glue around the roller and let it cool. This was my rough pass. Then I added extra glue and slowly evened out the layer using a flat object. Finally, I removed excess material by quickly scraping the roller on some paper. As soon as I installed these and turned on the motor, the string readily shot out and formed an elegant loop.

After doing some smaller tests on the functioning string shooter, I noticed the motors slowing down and the string getting less and less agile. I checked the batteries with a multimeter and they both still had around 1.35 V. I reasoned, that the 3V provided by my two AA Batteries was at the bare minimum for the string shooter to function. This meant redesigning the entire main part to accommodate a third cell. I wanted the handle of the string shooter to not be too long and brainstormed ideas. The simplest possible arrangement would be three batteries side by side, but that would make the handle too wide. I also thought of using one battery next to two batteries end to end. However, that would elongate the handle too much. I also considered a triangle arrangement, but the wiring would require me to make a separate battery barrel which, in total, would require 8 terminals = 2 (terminals for a single battery) x 3 (amount of batteries) + 2 (connection needed to join the separate battery barrel). In the end I settled for the arrangement with on battery on one side and two end to end on the other side. To reduce the size however, I would have to place the third battery's compartment into the compartment for the switch.

The V2 design was similar to the previous version and only have a few minor tweaks and improvements:

1. The compartment for the switch was elongated and an extra battery holder was put in place there too
2. The roller cover was removed
3. Shadow lines for a more premium look were added on the electronics cover

You can download all the files for printing here : https://www.printables.com/model/1147007-string-sling-the-3d-printed-string-shooter

I printed everything and put it all together an discovered that it worked quite nicely. However assembling the battery holder so that the batteries were securely held in place was very difficult. Originally I planned to use the spring of an empty pen, but soon discovered that this spring was too weak and wouldn't properly grip the batteries. In addition, the spring, made of spring steel, was impossible to solder to and the only method that partially worked, was to drop a blob of molten solder down through the center of the spring. This would clamp the spring in place. I also had some steel wire lying around and decided to try that as well. I bent the wire in a U shape and bent the ends of the U back down. The wire was covered in a layer of paint, and soldering directly to it was impossible. So I therefore took a sheet of copper, which I could solder to, and glued it onto the wireframe structure. Once I had properly installed everything however, I realized that the spring of the new version was not adequate either. The little flex this makeshift spring enabled was too little and so the design, once tuned properly, could only accept one AA battery type. (Yes different manufactures have different AA battery sizes). Next I tried creating a compliant mechanism, but this one suffered similar issues as the previous design. I could have tuned the spring to be more springy, but as I currently do not have PETG filament, I decided to find a different solution as my PLA spring would degrade quickly. After a few days I decided to try a sheet of thin spring steel that I had harvested out of an old phone. This provided the perfect amount of flex, but was basically impossible to solder to. So I created a makeshift crimp to the wires, by bending a flange with the spring steel sheet and sliding the wire under it.

1. Attach a loop of wire on the 3D print using hot glue
2. Create two spring terminals by bending a spring steel sheet in a 'N' shape
3. Lay out all your components around the 3D print and cut wires to the appropriate lengths
4. Solder everything together and connect the spring terminals by clamping them in between a fold of springsteel and adding solder to fill in the gaps
5. Glue everything in place

In all the commercial version of a string shooter, the supplied loop of sling looks seamless. However creating one at home is basically impossible. My previous loops were simply glued together, but that clearly left a visible seam. A knot would either be too weak, or too visible and would hinder the strings movement. In the end I settled on a tedious, but best looking alternative. To do this, I had to unravel the individual fibers of the string on both sides and weave them each tediously back and forth through the other side of the string.

The current design worked flawlessly, but I still wanted to make a few minor tweaks for an alternate design. The main one being the removal of regular non-rechargeable batteries for rechargeable supercapacitors. I wanted to try supercapacitors instead of regular lithium based rechargeable batteries because they have a few advantages :

1. The can provide heaps of current
2. They can't be over discharged
3. They don't require complex charging circuits that precisely regulate current flow and voltage

However they do have a few disadvantages over regular lithium ion batteries

1. They lose their charge rather quickly
2. Their capacity (measured in Farads) is much lower than lithium ion batteries
3. They usually only have a voltage of 2.7V and can be charged to around 2.5V

For these reasons I bought a 500F 2.7V capacitor

Firstly I wanted to get a feel for supercapacitors and decided to run some tests. I noticed that their change storage is way less permanent than with lithium batteries and decided to graph this loss of electric charge first. To do this I charged the capacitor to 2.5V and let is slowly discharge to the environment. I regularly checked the voltage and was pleasantly surprised to find a nice graph showing the exponential decay of the voltage in the supercapacitor. Next I tested the amount of amps drawn by the supercapacitor when charging from two AA batteries. This also showed an exponential decay graph, but this time it was not as pronounced. And so with a rough feeling of supercapacitors, I started designing.

I kept the front part of the design from V1 mostly intact and basically just changed the size of the switch opening to fit a different switch I had laying around. The back however required a complete redesign to accommodate the large 2.7V 500 Farad capacitor. Nevertheless, the design process was very similar.

1. I edited the first sketch so that instead of being two circles enclosed by a box, it would be just one large circle enclosed by a box.
2. Then I extruded this sketch the length of the supercapacitor and added a small hole for a JST connector for charging
3. After that I cut out a section to allow the wires to go from the JST/DuPont connector all the way to the anode and cathode of the supercapacitor
4. Once that was done, I lofted the resulting body with the front part of the V1 to get a smooth transition between both
5. Finally, I added a finger grip pattern by unstitching the body in the surface workspace, deleting the face where the grip was going to be applied, creating a wavy spline and patching the hole again, but this time by following the wavy spline.

You can download all the files for printing here : https://www.printables.com/model/1147007-string-sling-the-3d-printed-string-shooter

Next I soldered everything together according to the following procedure

1. First I placed both motors into the housing and soldered the leads together and a cable for the positive and one for the negative
2. Next I soldered the positive wire to the normally open part of the switch
3. After that I soldered the negative wire from the motor to the negative terminal of the supercapacitor and soldered another connection from the switch to the positive terminal of the supercapacitor
4. Next I soldered some wires from the 2 terminal JST/DuPont connector to the terminals of the supercapacitor making sure that the anode (-) was connected to the right side of the 2 terminal JST/DuPont and the cathode (+) to the left side.

1. To solder the solar panel I first placed it in the dedicated stand and fed the cables through the dedicated holes on the back
2. Next I soldered a diode with a forward voltage drop of 0.5V to positive terminal of the battery. This will prevent current from flowing back through the solar panel producing light and will regulate the voltage from 3V to a more manageable 2.5V for the supercapacitor
3. Next I soldered the 2 terminal JST/DuPont connector to these cables, making sure that the positive terminal will connect to the positive terminal of the supercapacitor and vice versa

Diodes are little electronic components that allow the flow of current in one direction, but not the other. Kind of like a check valve in water systems. These diodes always have a forward voltage drop, so a constant drop in voltage at the output. For example if a 9V battery is connected to a diode with a forward voltage drop of 0.3V then the voltage at the output of the diode will be around 8.7V. (In reality this voltage drop will vary a bit and get slightly larger with more current). The voltage drop can be measured using a multimeter in the diode mode or by doing a similar test as in the example above. This voltage drop happens as a consequence of how diodes work. They are composed of two doped semiconductors (a positive and negative one) joined together. These doped semiconductors are still neutral, but at the interface where they are joined together a few electrons from the N type (negative) semiconductor fuse into the 'holes' in the P type (positive) semiconductor and create a charge imbalance called the depletion region. When the diode is powered the wrong way the depletion region just grows and no charge can flow, but when powered in the correct way the depletion region shrinks and current can flow. This shrinking of the depletion region requires voltage and the voltage required is called the forward voltage drop. (If you want to learn more, I recommend watching the Veritasium Video about LEDs ,which are a type of diode)

1. To prepare everything for charging I glued the top cover onto the base of the string shooter and made the cables neater and less likely to short using electrical tape
2. Then made sure to plug the cable in correctly by connecting + to + and - to -.
3. After that, I waited

Once it was fully charged, I tested the string shooter and was filled with joy once the string got shot out of the device purely from the sun's energy. It worked great until eventually the supercapacitor dropped to around 1.8 V. Even though the capacitor was able to provide more current than regular AA batteries, the motor still slowed down and had to be recharged. Even though I could only use a fraction of the store energy, I still considered this as a win, because I accomplished my goal of powering it with the sun. Next time, I will use a boost converter or a joule thief to be able to use those last few farads and get a longer runtime.

I really enjoyed this project and learned a lot including how to design shadow lines for a more professional look, how to use diodes and their forward voltage drop, and what material is best for creating springs. In addition I learned about different connector types like JST and DuPont and the advantages and disadvantages of supercapacitors.