A typical blade grinder produces a mind-shattering crackle when it first turns on and can burn the beans. You also have to scrape out the oily grounds that get stuck in the chamber. A burr grinder is better, but good electric ones are expensive and a pain to clean. Luckily, Julie had a grinder made for camping, and it heroically donated its ceramic burrs to the cause.
To power it, I used a Pittman 12v gearmotor I'd been tripping over for years. At 218.4:1, the speed was far too slow but since they come in several speeds I figured the gearboxes were modular. Sure enough, a bit of rearranging of the gears yielded a usable ratio.
The grinding chamber is three pieces of machined acrylic bonded together. The top of the chamber locates the motor and has a complex "funnel" opening to accept the beans. The bottom locates the grinder and also tapers to guide the beans into the burr. Coupling and shaft are stainless steel.