Fits atop the smooth rods to be joined by a threaded rod for hopefully increased stability.
Do NOT use as a handle - it fits onto the smooth rods, it does not in any form or fashion clamp to them, and should not be used to lift the bot.
Print two, screw onto threaded rod, fit onto smooth rods. This design choice is because my kit came with an extra threaded rod, but the kit is parametric, and you can swap the 5/16 rod size for a millimetre size, and comment out the nut if you don't want it.
It includes an embossed Printrbot logo on both sides, which again can be commented out if you prefer.
If you prefer the nut be printed flat on top and bottom, you can add a 30 degree rotation on the Y axis just before nut() is called - however, be aware that this requires bridging a gap. Point up and down, as in the current file, only needs to bridge the center hole, and there is plenty of depth involved to manage it even on an ill-tuned printer.
There is a tolerance parameter that adds a settable amount of additional space around the various openings.
I started from Siorinex's Printrbot rods end hat file, so thanks to Siorinex for that.