DIY Grinder Disk
Nice work - *Ingenious mechanism for tensioning sand paper around a drum. Minute 2:52 shows how to use a slit screw to tension the belt. The belt goes in the slit, and the screw is rotated.
This can be redone in STEAM Camp 2020 as a simple prototype with 555 motor, 3D printed geardown belt (20x) and pulley, and a strip of sand paper. The tensioning bolt can be 3D printed, along with a counterbalancing bolt.