A spool spacer to be used with any existing filament holder (I use this one for my printrbot simple metal: http://www.thingiverse.com/thing:355889/#files ) Allows better stability and control during unspooling, drops the running diameter from 52.75 down to 30mm. Good 'n chunky, it drops and locks onto the spools inner ribs, spins up a treat! Enjoy! For best results, print with the largest diameter on the print bed. Infill around 25% is sufficient to ensure correct dimensions and withstand the weight of your biggest reels!