So backlash is a pain. On a lead-screw driven cnc, there will be backlash. Any nut on a bolt will have space in the threads. When you reverse the direction on the screw, there's a moment where turning the screw just takes up the slack in the threads. This produces a no-movement moment, when the controller thinks that the machine is moving. This is called backlash. Anti-backlash nuts take two nuts and spread them with a spring. They are also bound together mechanically, so when one turns, the other nut also turns. I made these on our CNC laser at work using some delrin. Delrin is a self-lubricating very hard plastic. Perfect for this application.