The holder is designed to hold common spools (makerbot/esun).

Print instructions

I printed the 'Parametric universal spool holder' (http://www.thingiverse.com/thing:767317) and I noticed how cumbersome was to replace the spool because you had to untight the nuts, then align the parts, etc... I just wanted to slide the spool on the holder and get back to print.

Optionally there are 2 nut holders for the specific metallic beams included in my hictop prusa i3