Minimalistic spool

Inspired by CCphoto's "Another spool" ( http://www.thingiverse.com/thing:8208 ), this spool is parametric, minimalistic, and designed to make changing filaments easy. To add a roll, hook the arm pieces around it, slide the rods into the slots in the arms, and tighten the nuts to clamp it. At the default size, it can hold a 5 lb roll as pictured, though it's probably better to use it with half the roll. On a Mendel, you could print one large enough for a 20 lb roll! It includes variables for the