Rock Tumbler

http://www.youtube.com/watch?v=Vzuz8jFhXm8&feature=youtu.be
This is a rock tumbler that I designed as a birthday present for my daughter. It uses a 3000 RPM AC vent fan motor to drive it and the container is a mayonnaise jar. It's very powerful and spins easily with a load of rocks at 60 RPM. In my initial try with only rocks and water (no compound) the rocks were noticeably smoother in only 2 hours.
The motor gets warm so I put a fan on the motor pulley to blow air on it. The fan is thin enough that if you stick your finger in it, it will bend or break and hopefully not slice off your finger. (I did it once and I can still count to 10 without using any toes!) Someday I may add a guard.
The belt between the motor and gear pulley is a standard 3.5" rubber band.
I use thick rubber bands on the jar for traction. The rollers are also designed so a standard size wide rubber band can go on them.
The gears are true involute gears so they should not wear quickly. Except for