Neil Mendoza created this fantastic electromechanical band as part of the artist-in-residence program at Autodesk:
The rock band is composed of electromechanical instruments that make music with rocks by throwing them through the air, slapping them and making them vibrate. The song that they're playing, Here Comes the Sun, is biographical, describing the daily experience of a rock sitting on the ground. The rock band is made up of the following members...
Pinger - fires small rocks at aluminium keys using solenoids. Spinner - launches magnetic rocks, Hematite, at pieces of marble. Rocks are launched by spinning magnets using Applied Motion stepper motors. Slapper - slaps rocks with fake leather. Buzzer - vibrates the plunger of a solenoid against a piece of marble. The whole project is controlled by a computer running a MIDI player written in openFrameworks talking to a Teensy. The machines were designed using Autodesk Fusion 360 and Autodesk Inventor.
He posted plans to make your own mechanical xylophone at Instructables.