Backlash is the slop you see when changing direction on a leadscrew. It is usually caused by the gap between the threads on a threaded rod and the drive nut (can also be in the bearings for the rod). Even expensive ballscrews can have backlash. CNC software packages generally have "leadscrew compensation" or "backlash compensation" (the user saves the value of the amount of slop which can be easily determined) so that when changing directions, the slop is taken into account and the table ends up where you want it to be.
Backlash prevention methods:
Half-nut: Nut with a slot cut in it almost cutting it into two nuts with a set-screw forcing the cut open thus reducing backlash.
Coupling nut: This looks the same as 3 or so standard nuts all stacked end for end. The intended use is for joining ready rod (otherwise known as "allthread") to a protruding stud with the same thread. I think that is one of the tricks that John Kleinbaurer uses but I saw mention of it long before I ran into his website.
Custom nut: Making a nut that exactly matches the threads of the rod would seem an ideal way to reduce backlash. The trick is that threaded rod is never absolutly perfect and so a "perfect" nut will alternatly bind and loosen as it travels. A longer nut that covers more threads helps to "average" the imperfections and reduce this variablility, but that will also increase friction. To reduce friction while still allowing for a very long nut, it should be made from a material with a low coefficent of friction. Examples inlcude Delron or UHMW (Ultra-High Molecular Weight) Polyethylene.
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