I never thought to ask but are you just looking for the rotary to turn at a slow speed vs using a cordless drill? If so you could just use a signal generator ($15) going to a stepper driver connected to a stepper motor ($55 for both). From there you would need a bracket for the motor ($10) and a coupler ($10) to go from the stepper motor shaft to the 3/8" rod you were using in your first attempt. You would also need a power supply ($20) to drive the signal generator.