Dog Ball Launcher

Do you have a dog (or a ball-fetching human) but sometimes feel too tired to play with them?

Introducing the Fetch-O-Matic, an automatic ball launcher that takes the tedious task of picking up the ball and throwing it out of your playtime routine. Instead, you just need to drop the ball into the opening at the front of the mechanism and let the Fetch-O-Matic do the work for you!

Special thanks to Dean Segovis from Make Magazine for the inspiration behind this project, as well as Ms.Berbawy and Uncle Vijay for their assistance during the project.

This project was made by Praneel and Nakul as our SIDE project for Ms. Berbawy's PLTW Principles of Engineering Class.

Materials

• Plywood
• 2’x 4’ plywood (5 ply minimum, for box)
• Wiper Motor 
• Aluminum square tube
• ¾” diameter, 8 ¾” length
• Battery
• 12v, 7a
• Battery Charger
• 12v
• Toggle switch
• Limit action switch
• Hookup wire
• Blade connectors
• Conduit strap
• ¾”
• 2 hole
• Wood dowel
• ¾”
• Acrylic sheets 
• Flat Head Wood Screws
• M4 40 mm Length Screws
• M4 10 mm Length Screws
• M4 Nuts
• PLA Filament

Tools

• Circular saw
• Reciprocating saw
• Drill
• Wire Stripper
• Crimping Tool
• Screw Driver
• Soldering Iron Kit
• 3D Printer (PRUSA Mini)

Above is our entire project in Fusion 360 along with our two designs which needed to be 3D printed. (The tennis ball, imported for visualization purposes, was from GrabCAD.)

We decided to CAD the project first using Fusion 360 so that we would be able to confirm our dimensions and constraints.

First, we modeled the base of the Fetch-O-Matic by creating the four side boards. We aligned and grouped the four boards to the sides of the bottom board to form the open box. We then modeled the ball aligning wood pieces and aligned them to the opening at the front of the mechanism.

Then, we created the square aluminum tube that would act as the whacker on a separate document.

On another document, we created the motor attachment, which was later 3D-printed. We subtracted 2 mm from the hole for the motor shaft since we wanted a press-fit design. A press-fit design was necessary since there were no specifications detailing the exact type of threads on the motor shaft. Since the printed design was 2 mm less than the diameter of the motor shaft, with enough force, we were able to create our own threads in the print. Also, since we wanted a tighter fit, we added a clamp feature to the bottom of the design.

The final CAD object was the on/off switch holder. We had a 0.1 mm tolerance on both sides of the piece which allowed for a perfect fit for the switch so that it didn't move around.

Finally, we imported the on/off switch holder, the motor attachment, and the square tube into the document with the wood board. This way, each piece was a different component and we were able to alter the different components separately. Since they were all different components, we also added a rotational joint with the motor attachment+ square tube and the motor to visualize the rotating motion.

All CAD models are included below.

Once we had the CAD models done, we were ready to start building.

We cut the larger wood pieces to the dimensions specified in the drawing attached below. After we had 4 separate sideboards and the bottom board ready, we screwed countersunk wood screws through the edge of the bottom board into the side boards with a screwdriver to form the box seen above. We tried to avoid power tools when screwing in the countersunk screws since it could have resulted in the screw moving off center on accident.

Once the outer edges were ready, we cut out the aligning pieces for the ball to rest on. After cutting it out, we screwed the pieces into the board using the same model of countersunk screws used on the edges of the box.

Then, we moved on to the holes necessary for the motor and switches. At the center of the bottom board, we drilled three holes for the motor to be attached.

In front of the back aligning piece, we drilled a hole for the limit switch. Since the fit was already tight, there was no need for further support. If the switch was wobbling, we had considered adding a wood block underneath it to secure the limit switch. Before moving on to the next step, we made sure that the limit switch was activated when a tennis ball rolled onto it.

All dimensions for the wood boards are attached below.

We printed the motor attachment, made in Step 1, on the large circular face (depicted in the first picture).

Then, we cut the aluminum tube using a hacksaw and drilled in two holes that matched the holes on the motor attachment.

Before attaching the 3D print to the motor, we placed two M4 screws into the two holes of the aluminum tube and the motor attachment and tightened them with a nut on the other side. We made sure it was tight since the nut was hard to reach after we attached it to the motor.

Since the 3D-printed motor attachment was a press-fit design, we had to create our own threads on the print using the motor shaft. We placed the print right over the top of the hole, pressed it down with medium force, and twisted it. This created the threads in the print and kept the motor attachment in place for many runs. After twisting the attachment down to the bottom of the shaft, we used a 10 mm M4 screw and nut to tighten the clamp at the bottom of the 3D-printed piece for a tight fit.

To see if the spinning motion was in the correct direction, we hooked the motor to a variable power supply and tested it.

All dimensions for the aluminum square tube are attached below.

With the rotating piece working, we moved on to the wiring.

We needed two 10-inch wires with crimped-on female blade connectors on both sides and two 8-inch wires with crimped-on female blade connectors on one side.

To crimp the ends of the wires, we first stripped the end, ensuring that there was 6 mm of exposed strand. Then, we slid the blade connector onto the exposed wire, and with the crimping tool, we pressed down firmly using the "insulated" gap on the tool to secure the connector onto the wire. We repeated this process with both sides of the two longer wires and one side of the two shorter wires.

We then soldered the non-crimped sides of the shorter wires to the "Closed" and "Normally-Open" terminals on the limit switch.

Once the wire was soldered to the switch, we put the switch into the gap made for it in Step 2.

We printed the switch holder using a PRUSA Mini. We then found a suitable position in the middle of the right sideboard to attach the switch holder. We slid a M4 screw into the holder with the switch sitting on the print, placed it against the wood, and screwed it in. We made sure it was tight so that the switch wouldn't wobble when the mechanism was switched on.

After attaching both switches, we connected the different wires to the correct components according to the wiring diagram in Step 4.

Click on the video above to see our design in action. Thank you to Vaib Jay for being our "ball-fetching human" in our demo video!