Project Mike: Marble Launcher

Project Date: Jan 2012

This is my all-new Lego gun that shoots marbles. It is built machine-gun style, with a quad-pod and vertical handles.

It accelerates the marbles using two wheels that are spun by 6 PF M-motors. A geartrain increases the rotation speed of the wheels to around 3000 rpm. Two battery packs mounted at the back to counterbalance the weight of the motors powers them. Because of the increase in rotation speed, torque is severely reduced. This means that the wheels need some time to recover and speed up after every shot. The original concept of "feed in the marbles and let the gun do the work" is made complicated by this. I devised a mechanism to feed in the marbles one-by-one at a controlled rate. This way, the marbles do not go in one immediately after another and potentially jam the gun. It's possible to solve this problem by either using more powerful motors (which I don't have) or by sacrificing projectile power by decreasing the rotation speed (which I don't want to).



The mechanism I used consists of three simple parts: a simple, tilted, funnel-like magazine, a vertical S-bend and a flipping switch to control marble feed rate. You can see the large magazine in the pictures at the top. Marbles are poured into this magazine and are funneled into a single row by the two angles beams. They roll down  the track in the picture at the left, drop down into a hole, and continue rolling. The purpose of this vertical S-bend will be explained later. The first marble will roll into a switch, made up of two peg connectors, which can be seen in the top left picture, and get stuck. This stops the marbles from rolling further. Once the trigger is pressed, a motor periodically flips this switch (mechanism explained later) and allows one marble to go through at a time. This marble then rolls down a ramp and into the wheels.

The flipping action of the switch is violent and will force the marbles behind to roll backwards, up the ramp. The purpose of the vertical S-bend is to limit the distance the marbles can roll backwards and to reflect them back towards the switch. If there were no S-bend, the fire rate of this gun would be severely reduced since the marbles would roll back, stop, and return to the switch every time the switch is flipped. The switch can be more clearly seen in the left picture below.



The mechanism I used to flip the switch can be seen in the picture at the right. It requires a seventh motor. I used a PF XL-Motor for this purpose. It spins an axle connected perpendicular to the rotation axis, which at the 10 o'clock position, will strike a Lego piece, tilting it anticlockwise. This Lego piece is connected to the switch. Since the motor spins at about 200 rpm, the switch flips about 200 times a minute, which translates to a firing speed of around 200 rpm.

The trigger is extremely primitive in this gun. To fire, the handle at the right is tilted forwards, triggering an electronic switch and activating the motor that flips the switch, enabling marbles to proceed to the launching wheels.

Having said so much about the gun, it's time for some description for the quadpod.


The front legs are tilted sideways, but to save time, I constructed the back legs straight. To increase the strength, I used dental floss to chain the legs together (visible in the left picture). Otherwise the legs would spread out under stress and break.

This gun is supposed to fire at 200rpm, but the actual value is at 250 to 300 rpm. Likely reason for this is that some marbles managed to escape from my switch.

And lastly a picture of me and my gun:

And the video from my YouTube channel (didn't want to upload on blogger since the quality is so bad here):