This little marble machine uses 6mm Airsoft BB's for the balls. When they get to the bottom, just tap the little thwacker and send them back over the top to the start. It's actually a little more challenging than it looks! See how many balls you can keep going at the same time.
Plus, it prints in one piece (except for the feet) and with no supports!
Watch it go: https://youtu.be/Q6aKoDP1uTw
Print OTTMM.STL plus four of the OTMM_foot.STL. The feet should press into the divots in the bottom of the marble machine, but if they are loose, secure them with a drop of superglue.
After printing, you need to run some fine sandpaper or steel wool through the track to make it very smooth - the slighted bump will cause the balls to hang up. Keep at it until the balls run the entire length of the track without hesitation.
Playing with this is a little trickier than is seems, due to the lively action of the thwacker.
Operating the thwacker takes a little technique. For reasons I don't quite understand, it is much easier to press and release the thwacker with a hard object (like a stick or pen tip) than it is with your finger. I'm using a tongue depressor in the video.
If you thwack the ball too hard, it will ricochet out of the track at the far end. There is a sweet spot in the press and release that you'll have to get a feel for.
Lastly, if more than one ball is stacked up at the bottom, the thwacker will launch the first ball upwards, but also send the other balls behind it flying the other way. So as soon as a ball reaches the bottom, send it on its way. The challenge is to see how many balls you can keep moving at the same time.