New versions of the Printrbot Simple include aluminum handles that mount at the top of the z-axis rods. These handles support the weight of the machine, and they also hold spool racks so that filament does not have to be mounted separately. Older versions do not include these racks, but the upgrades are sold on the Printrbot site. The rack works for many common filament spools, but the end is too large for spools from producers like Hatchbox and Reprapper. This model is for 3D printing enthusiasts with older models of the Printrbot Simple, or those who want to hold filament spools with smaller central diameters!