Let's polish some rocks.
We had a brand new bathroom ventilation fan aching to get out of its box. With no bathroom in need of ventilation, we had to give it a new purpose. We decided to turn it into a rock tumbler. The results are marked V1 below. Unfortunately, this fan's motor had very little torque so it couldn't tumble more than one rock at a time. Also, I didn't have a good solution for attaching the tumbling jar; the motor axle was only attached to the extension by virtue of the friction of the rubber tire. This soon failed and the jar stopped turning. (By the way, gravity pulled the jar towards the motor and it would spin resting against the green stop you see in the photos.) V1 was a failure!
For V2, I got a 12V DC motor with more torque. We also got a power supply, a motor controller, a mount and an adapter for attaching things to the axle. Also, after watching this episode of How It's Made, I changed the design of the tumbler cradle. So far, V2 is much better than V1 and has a hope of polishing a rock!
Time passes...
Oh no, V2 has failed! I used nails to hold the rubber tire to the motor coupler. Well, one of those nails slowly worked itself out, caught a motor mount screw and jammed the motor. The motor burnt out. So on to V3! For V3, I also used nails to connect the coupler to the tire but I added a backing piece of plastic wood for the nails to securely embed in. This version of the tumbler was working very well until...
Catastrophe! With the other parts of the mechanism improving, the glass jar showed itself to be the weak link. Overnight, it cracked! Now we need to think about how to improve that part of the tumbler.
For V4, we have replaced the glass container with a plastic one. It's longer and tends to push itself towards the motor and so we added another restraint to keep it in place. How will this version fail?
24 hours in and no mechanical problems in sight, but looking at the stones, it's not clear if they are being ground or just packed with sand into smooth shapes? Perhaps the mix is too wet? Or perhaps it is too dry? Something to think about.