The Fetch-o-Matic

The Fetch-o-Matic is an automatic tennis ball launcher. I used two separate designs for the Fetch-o-Matic. There was one design which used the motor to wind up the spring and then uses the combined forces of the spring and motor to move the whacker at high speeds toward the ball. The other design, which has much less potential to hit the ball with force but is more consistent is the power doorstop design. This design uses the motor to pull back a spring that flicks the ball out of the Fetch-o-Matic.


Jack M.

Area of interest

Mechanical engineering


Fremont Highschool


Rising Sophomore


The Mechanical Drawings

Bill of Materials


Overall, my time at BlueStamp opened my eyes to multiple aspects of engineering I wasn’t privy to beforehand. I realized that engineering is not building things but rather solving the nearly infinite amount of problems that get thrown at you as you’re building it. I also realized that project-wise, mine could have been extremely better if I had realized that I had springs of doom and destruction instead of the ones I intended to order. If I had realized this beforehand, I might have had the correct spring to use and then I may have gotten my project to be more powerful. However, I plan to continue working on this until it works how I want it to.

Final milestone

For my final milestone, I got the ball to fire out of the Fetch-o-Matic at the minimal distance of 3 inches. I did this by completely changing my design from my previous slingshot style firing mechanism to making it more of an extremely powerful flicking mechanism. I decided this after creating the motor mount for the slingshot style build and then realizing that the motor’s torque was going against the energy coming from the spring. Because of this change, I decided to switch the firing mechanism to using the motor to just pull back the spring and have the ball fly solely from the energy in the spring. Unfortunately, this occurred to me 3 hours before demo night would begin which meant that I had to push through the problems that inevitably got in my way  at an exorbitant rate. Thankfully, I got it done in time and also got to practice my speech until it almost made up for the anticlimactic results of my project. I used a door hinge to hold the spring above the firing area. I also got rid of the micro-switch since instead of helping the circuit become more automated, it just became an unnecessary constraint in the other design. It took me quite a while and lots of hard work to even come up with a simple thing such as a motor mount, which I created using the pipe routers I used to hold the dowel, and some balsa wood to hold up the end of the motor that wasn’t flush with the bottom of the assembly I had made. I definitely intend to make my project better by switching back into slingshot mode but using a better spring that would compliment the motor rather than go against it. Another, much more complicated modification would be to make a hopper that would drop balls in rhythm to the whacker’s movement. Overall, even though I managed to get the ball to go out of the assembly, I am still going to continue working on it with out documentation.

Third Milestone

For my third milestone, I combined my circuit and my casing. I glued the switches into their respective positions on the casing but the main problem was making the motor assembly. I spent approximately a day and a half trying to figure out a way to complete my pulley and motor assembly. One of my designs was to buy a threaded spacer and then just use a really large screw to bind everything together. I also had a crude idea of just using superglue to solve all my problems. In the end, I made a thin wooden dowel into a makeshift spacer that went through the pulley and connected to the motor shaft. I added some spacers before I put the whacker on and then screwed the whacker onto the wooden dowel. Finally, I plan to completely bind the motor to the dowel via wood glue. While going through this process, I split multiple dowel pieces while trying to drill the area where the shaft of the motor would get inserted. After multiple dowel pieces, I realized that the dowels wouldn’t split as long as I was drilling the part of the dowel that was inside the pulley.

Second Milestone

For my second milestone, I completed building the outer casing of my project. One of the challenges I faced while building was the low-quality wood I was using, which led to multiple splints in the wood and having to drill my wood multiple times. Another thing I had to deal with was the fact that we had no Philips tip for the drill, which made it extremely aggravating to screw parts together. This means I had to make a pilot hole for every screw before I manually put it in with a screwdriver. Since some sides of the wood weren’t straight, I had a hard time clamping them to the workstation.

However, I ended up completing the box and will combine my circuit with the casing next milestone. I started this case by first finding the base schematics and then adding my modifications so that it better fit my situation and my motors. After planning out the cut-out, I had an instructor use a jigsaw to cut out the pieces. I then marked all the places I would drill and eventually inserted screws as my connectors. Finally, I had completed my outer casing.

First Milestone

For my first milestone, I have the completed circuit of my Fetch-o-Matic that makes the motor trigger at the correct moments. In this circuit, I have a toggle switch and a micro-switch. A micro-switch is a switch that automatically is in the off position if no force is being applied onto the button. A toggle switch requires force to turn on and off. When I flip the toggle switch I prime the Fetch-o-Matic. Then a tennis ball falls into place and triggers the microswitch and sends power to the motor. The motor then pushes the whacker against 2 springs where potential energy is stored. Then, once the whacker passes the halfway point, the springs transform their potential energy into kinetic energy which makes the whacker speed up greatly when it hits the ball. While building the circuit, I used a multimeter and trial and error to figure out how to attach the wires to the leads correctly.

Useless machine

My starter project was the Useless Machine which is a box with a single switch. When one flips the switch, half the box opens up and a plastic arm comes out and turns off the switch before retreating back into the box. In order to make this, I had to learn about multiple components such as a PCB, two separate types of switches, and resistors. Everything gets soldered onto the PCB because the power gets distributed to all the pieces on the board. I attached two switches to the PCB, one which required force to turn on and off and another that automatically goes to its off position if no force is being applied. I also learned about resistors which reduce the current passing through the circuit.

After familiarizing myself with the layout of the PCB, I began soldering the aforementioned pieces onto it. After soldering the pieces on, I started attaching the plastic pieces to the motor casing and putting the PCB onto the motor. The next day, I quickly completed the battery pack related instructions and attached everything to the bottom of the box. My first challenge was attaching posts to the base. Because I was using self-tapping screws, I had to find a way to steadily insert the screws into the posts. I solved this problem by using the pliers to get a steady grip on the posts while my other hand applied force into the screwdriver. When I completed attaching the posts, I moved on to putting all the walls, the top, and the lid onto the box. This presented a challenge since there were so many pieces to manage and keep track of. I solved this by placing the pieces in groups. This allowed me to deal with the group connections while they are braced by screws. Finally, I had built the machine.

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