Robotic Thrower Documentation

This project was a minimal cost attempt at making an electric, repeating thrower. The project is designed to run from a DC motor controlled by a standalone motor controller that pulls back a rubber throwing arm and releases. The motor mechanisms were based off that of an Airsoft gearbox seen below, although, instead of compressing air to fire a plastic bb, the piston would pull back a rubber throwing arm to release an arbitrary light object.

The annotated result I constructed can be seen here.


Materials used:

  1. Motor controller (controller)
  2. 6V DC motor (motor)
  3. Additional copper wire (wire)
  4. I used a plastic gear set to pull the rubber arm down (gears)
  5. Power for the motor controller (Power adapter)
  6. Adapter for barrel jack from power source to pos/neg wires for motor contgroller (adapter)
  7. Ecoflex 30
  8. Duct tape
  9. Hot glue
  10. Zip ties
  11. Cardboard box
  12. Tacks
  13. Presta bike tire valves
  14. Fishing line


I began constructing the thrower from the motor to the gear system due to the precise nature of gears and the imprecise nature of hot glue and cardboard, which I combined for the gear system. I had hoped the motor would have the torque to pull the slide and arm back directly, however, this was not the case and the motor is designed for high speed instead of torque. I overcame this by using an additional gear that helped convert this speed into torque, that is called a worm.

Image result for worm gear images

I attached this worm to the motor and then made a hybrid gear between a spur gear and a sector gear as seen here.

Image result for names of gears images

The worm attached to the inner ring of the spur gear, and I filed away teeth from the outer ring of the spur gear so that it had the appearance and functionality of the sector gear, which can be seen in action in the first image pulling and releasing the piston. The piston is then attached to fishing line which is then angled to pull the rubber arm down and I hoped the stretching the arm back would provide enough return force on the piston to slide to its original position, as the spring does in the Airsoft gearbox. The rubber arm was created with Ecoflex and a cardboard mold to get a rectangular shape. I initially created a prototype to see how thick the arm should be, which led me to the realization that Ecoflex carries very little tension and would have no chance of throwing anything anywhere, unless it was very thick. My solution was to modify my mold by adding two air channels that I could inflate that would increase the stiffness and create a more firm shape out of the Ecoflex. My first prototype with this design was successful in regards to stiffness by pumping air through Presta bike valves, however, the mold I created left a weak point in the Ecoflex, so with air pressure the Ecoflex popped. I attempted this process again but with my imprecise mold one of the channels leaked. The arm held long enough to test the whole project together, but this led to the realization that the combination of high speed motor and insufficient removal of gear teeth, the piston could not release. The motor controller does support bi-directional rotation so I could see that the mechanisms worked, but the motor is not fast enough to push the arm forward with any reasonable force.

Although I did not have time to fix these issues some solutions would be to eliminate the sector gear and simply use the motor to pull the piston back, then use a mechanism to manually detach the slide from the mounted gear to release the arm. Another option would be to use a higher torque and lower speed motor. This would allow the slide to be pulled back without obfuscation gears and the slide would have time to return to its original position before the sector gear rotates.


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