|Both man and machine are exceptionally photogenic|
|With this, even $13 ebay leadscrews can become precision items!|
|As the screw (grey) rotates, a nut (blue) moves linearly along the shaft.|
Usually this nut is constrained to prevent it from spinning in addition to axial motion
gif from W.Rebel, Wikipedia
Since nothing is perfect, there's some clearance between the threads when the nut is threaded onto the shaft. This is fine when you only move in a single direction - the nut will contact on one side and the clearance does nothing.
However, when you want to switch directions, there will be some dead space where the shaft turns and the nut doesn't move until the other side of the thread contacts the shaft. This backlash positioning error induces sadness.
Austin's preload mechanism allows him to tune the position of Brass Nut 2 such that backlash in the carriage is eliminated in either direction, without causing binding.