This is an upgrade for my Printrbot Metal Plus spool holder (http://www.thingiverse.com/thing:675439) The narrow roller bearings tend to lean over, and occasionally the spools become dislodged. These rollers overcome those shortcomings and are much easier to load.First to print my original spool holder: http://www.thingiverse.com/thing:675439 Purchase 80 (for a single spool) or 160 (for dual spools) 3/16-inch bearing balls. Amazon has packs of 144 for $5.45: http://www.amazon.com/gp/product/B000AO5KOW/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1 For one roller pair (front and back) print: 2 x 'spool_holder_roller_bar' (I used 0.3mm layers, 25% fill, 3 perimeter shells) - green in the exploded view 4 x 'spool_holder_roller_bearing_p1' (I used 0.2mm layers, 25% fill, 3 perimeter shells) - magenta in the exploded view 4 x 'spool_holder_roller_bearing_p2' (I used 0.2mm layers, 25% fill, 3 perimeter shells) - yellow in the exploded view 4 x 'spool_holder_ring' (whatever layer height you want) - cyan in the exploded view Insert your balls (note: I am not responsible for any injuries caused by misinterpretations of this step) through the small hole in 'spool_holder_roller_bearing_p2', and place 'spool_holder_roller_bearing_p1' over the top. In my print, the fit was tight and no glue was needed. Place a 'spool_holder_ring' over this sub-assembly with the tapered edge towards the conic part (so they match). Then insert this in to one end of the 'spool_holder_roller_bar'. Repeat for the other end of the roller. If you have a dual extruder set up, or just want to have two spool holders, then double everything above to make 4 rollers, and print 6 x 'spool_holder_roller_spacer'. One spacer goes between each side-by-side roller pair to prevent the rollers from rubbing against each other, and then one on each outside end to prevent the rollers from rubbing against the main spool holder pieces.