Legs for the printrbot in order to stabilize it when using a larger print bed.
Just print two and put the printrbot on them. Optionally you can fasten them with extra M8 nuts as well.
Please note that this specific version is generated for the pb-Base-v2 from http://www.thingiverse.com/thing:16990, which has a rod distance of 60 mm center to center. If your printrbot differs you can change the distance in the scad file.