You can buy a rock polisher on Amazon but they run 40-50 quid and have a pretty small drum for parts. I have all kinds of motors and electronics, and a 3D Printer, so I decided to design and build one.
Above is the assembled and operating tumbler polisher.
I printed the files you see with an 800 micron nozzle and a layer height of 400 microns. Any resolution will likely work. Note that inside the drum there are five printed pieces that assemble to form a mixer. It should be inserted into the drum such that it scoops up the little brass bits as the drum rotates. I glued mine into the drum.
The above image shows a close up of the base with the electronics and stepper motor.
The above video from adafruit walks you through the polishing process.
Above are the before and after pictures of some parts done in Colorfabb Copperfill and then polished for seven or so hours.