NERF Gun Upgrade: Semi-Automatic to Automatic Modification
By Hunter Black – Mechanical Engineer
A few months ago, a co-worker shot me with a dart from across the room to get my attention. Jumping up out of my seat and removing my headphones, I realized that I could get a NERF® gun and I could improve it should I ever be on the receiving end of a dart in the future. I would have the last laugh. Such a project would appease my inner child and stimulate my intellectual curiosity.
All at once I had a rush of ideas flood into my head…
“Make the gun shoot around corners!”
“What if you turned a semi-auto NERF® gun into an automatic?”
“What if you completely redesigned the wiring to upgrade the motors and get more range from each dart?”
For anyone who has ever experienced scope creep, this was an optimal setting for me to over-commit and ultimately fail. I had no idea how a semi-automatic NERF® gun worked, nor did I have any idea what the specifications for such a project would be. I was also unsure if it would be possible to use our MakerBot 3D printer to print the gears within a necessary tolerance to function properly. However, at the same time, I was convinced that it would be easy. I had built a few gearboxes and designed a basic transmission in college. How hard could this be?
Long story short, if I did not have nearly unlimited access to the MakerBot, helpful co-workers, multiple design iterations, and the internet, this post would be about learning from your failures. Fortunately, at the eleventh hour, I was successful.
In that which follows, I will do my best to take you through the process and its limitations. Last but not least, I will include the CAD for anyone who would like to give this a try. At minimum, you will need access to a 3D printer, a range of drill bits, a soldering iron and a set of hex keys. You can either laser cut your side panels out of acrylic (like I did) or you can print them just the same. The total cost of materials, besides MakerBot ABS, is under $20 (this does not include the purchase of the NERF gun), and it is all you need to upgrade any semi-auto NERF gun to an automatic version.
Warning: To perform this modification requires working with power tools and electronics. Modify NERF® products at your own risk and always utilize appropriate safety equipment when operating these tools (e.g., respirator, eye protection, ear plugs, parental supervision, the buddy system, etc.).
MODIFYING THE NERF® DESOLATOR
Step 1: Research and Conceptualize:
On a Saturday afternoon, I went to Target and picked up the Desolator – a recently released product from NERF®. I chose the Desolator because I could not find any available modification kits for this gun online, so it seemed like a good niche to fill.
Figure 1. Inside the Desolator
Figure 1 (above) shows my first look inside the gun. I would like to comment that I was very impressed with how compact the mechanisms of this blaster are. I was expecting a much emptier, or at least less used, space to work with and did not realize how compact the actual firing mechanisms would be. I had expected the design to be more similar to airsoft guns, which compress a spring when they fire each round. However, semi-auto NERF® blasters function using “flywheels” and not springs (this concept will be explained further as we progress).
There are three features to this blaster that perform the semi-automatic function:
The flywheel cage that accelerates the darts similar to how a baseball pitching machine works (Figure 1, left of image).
The pusher that moves the darts to the flywheels from the clip to launch darts in rapid succession with minimal effort – no preloading a spring, just a smooth easy trigger pull (Figure 1, middle of image).
A few safety mechanisms that ensure that the gun only operates under certain conditions: if the clip is inserted, if the magazine cover is closed and if the flywheels are spinning. Below is a detailed explanation of these features.
Characterizing the Desolator’s Features:
First off, the flywheel motors spin in opposite directions to pull, accelerate, and then push darts through the barrel. A common first NERF® modification is to upgrade these motors and flywheels to get a faster output from the gun. These upgrades increase the initial speed of the dart and therefore improve maximum distance a dart can be shot. However, there are a few existing modifications that can be found on reddit for this, and 3D-printed as well as machined flywheel cages are already available online, so I wanted to push for something new.
Next, looking at the dart pushing mechanism, there is a lever on the trigger that translates the roughly 0.75 inch trigger pull into roughly 1.25 inches of linear mechanical motion to push each dart from the clip in contact with the flywheels that will later be launched from the blaster. After searching the internet for a non-invasive design, I found a gearbox for a NERF® gun that would require cutting out the sides of the blasters to fit the gearbox (https://www.thingiverse.com/thing:2095251). After printing the gearbox on our MakerBot, I quickly approached upon the printing limitations of the machine and became unhappy with the design. As a result, I figured this would be the first place I would make my modifications. Later, if I would have the time, my next step would be to upgrade the flywheel system.
Finally, the safety mechanisms are built into the wiring to ensure proper usage of the blaster right out of the box. I came across a great explanation of how this wiring works on an automatic NERF® gun, so I have included the link: http://torukmakto4.blogspot.com/2015/06/tech-rapidstrike-control-circuits-part-1.html. I used this information to help me get an understanding of how NERF® gun wiring works so that I could design the wiring modification. In my final design, these safety mechanisms are removed, minus the mechanical latch that requires the flywheel accelerator microswitch to be on before the trigger can be pulled. I did this to reuse parts, decrease modification cost and maximize my building envelope for the gearbox.
Step 2: Design Development
At this point, I knew that the gearbox was the best place to focus my efforts. As mentioned in the link relating to the existing gearbox, the gearbox would require a more powerful circuit and motors to work. The motor recommended for the existing gearbox has a 20A stall current draw, meaning that it would require the use for LiPo batteries. LiPo batteries can be finicky and can explode for any range of reasons from being under charged to over charged, to being left fully charged for too long. (If you want to see LiPo batteries exploding just look up “exploding hoverboard” videos on YouTube).
Instead of upgrading the motors and batteries, I decided to upgrade the gearing so as to achieve a higher torque efficiency by using worm gears to create my gearbox. By using a worm gear, I could get a much higher reduction than your typical spur gear in a single stage. (If you want to know more about some different types of gearing look here: http://www.explainthatstuff.com/gears.html.) After finding a source for worm gear CAD online, I printed out an initial concept to be turned using a drill at a 50:1 gear reduction with 3D-printed gears.
Figure 2. Initial Proof-of-Concept:
Worm gear driven pushing mechanism
Step 3: Design Refinement
With this successful design as an initial concept, I began to focus on my motor and power requirements. I decided on a 3V motor that at optimal efficiency runs at 12000 RPM. Using this 12000 RPM as my baseline, I designed a gearbox with a 30:1 reduction to result in firing 6.7 darts per second (12000 [Rev/min]/30[unitless reduction]/60[seconds/min]=6.667 shots/second). My final design would have 1.1 inches of total travel of the pusher.
Figure 3. Second design iteration where my gearbox is significantly smaller and powered by a 3V motor.
After printing out these gears, the gearbox performed mechanically as expected and held the motor exactly how I wanted it to, but it failed to achieve the necessary force to push a dart out of the clip. I had been so confident in my high reduction gears ratio that I expected to have more than enough torque to fire a dart. However, I was wrong. My design was small and clean, but I needed to generate more torque without generating any more friction.
Figure 4. Test fit of the 2.0 gearbox.
My next step was to increase the gear reduction and therefore increase the torque output. I used a force gauge to check how much force it would take to push a dart from the clip and I needed to generate 1 pound of force. Running the math, I was generating roughly 0.85 pounds of force at stall based on my motor’s spec sheet and the 30:1 reduction; therefore, I needed to generate more torque (neglecting gear friction). Out of fear of friction, which would require an additional iteration of design, I increased my gear reduction to 50:1. With this design, I could have a 30 percent loss of torque due to friction and still have more than 1 pound of force from my motor. As a trade-off, my rate of fire would be 4 darts per second. The largest drawback was that this design would increase the pitch diameter of my worm gear, increasing the size of the gearbox and requiring a modification to the outside shell of the blaster – something I was attempting to avoid. However, a shell modification is common in NERF® modifications, so I figured this was the ideal method to finalize my design.
After printing out this new gearbox, I was initially unsuccessful in generating the torque requirements. As a last ditch effort, I upped the voltage to 6V. I had initial success (see my YouTube video). However, I was unable to get dependable operation of the gearbox. The gears kept jamming and the pusher did not always reliably fire a dart with each cycle of the pusher. In short, nothing seemed to work properly.
That night, after going home defeated, I woke up in the middle of my sleep realizing that I had forgotten to lubricate my gears as I had in previous trials. After returning early the next morning, I soaked a fresh gear and worm in spray-on silicone grease and my gearbox operated perfectly. Lubrication made all the difference. From there it was all about polishing up the final details.
Step 5: Final Steps and Modifications
My next steps were to hollow out the NERF® gun to make room for my gearbox. I cut out and hollowed out the dart pushing portion connected to the trigger, and hollowed out the shell around the pusher to give my gearbox a home. Then, I glued a micro switch to a place where the trigger would contact it when pulled, and used a spring from one of the removed micro switch assemblies to make the trigger return properly. Lastly, to illustrate how the gearbox works, I cut a hole in the side of the shell as a viewing window for the gearbox. When you build your gun you can forgo this option.
Figure 5. Empty shell prior to installing gearbox.
From there I cleaned up my wiring to be similar to the diagram in Figure 6. The green circuit connected one of the removed micro switches from the safety circuit behind the trigger to power up the gearbox when the trigger was pulled. This recycling decreased my added cost and eliminated complexity in the circuit. The blue circuit connected to the existing micro switch at the rev up trigger to the motors, which is very similar to the initial wiring.
Figure 6. Wiring diagram of the NERF® gun automatic modification.
After all of the shell modifications and re-wiring, I glued the gearbox in place with hot glue and closed up the NERF® gun for testing. The final result is a gun that fires roughly 6 darts per second (thanks to the use of 6V vs 3V). With the included CAD, Bill of Materials, wiring diagram, and modification instructions, you should have all you’ll need to perform the same modification. Enjoy!
Figure 7. Final design layout: NERF® shell modified with gearbox installed.