QUBD OneUp Gantry Belt Tensioner

While helping out a friend with his QUBD OneUp printer, I found the need to put a bit more tension in the gantry belt after upgrading to a remix of this [gantry assembly](http://www.thingiverse.com/thing:952796).

This mounts the bearing for the belt offset from a smaller bolt holding the bearing and washer assembly in place. Varying belt tension can be achieved by rotating the assembly which will move the bearing further away. This worked pretty well, but I still wasn't able to get the belt a