Machining Fundamentals 402 Copyright Goodheart-Willcox Co., Inc. Minimizing backlash is critical in CNC machines to ensure precise positioning as the axis changes direction. Lead screws and lead nuts are precision-ground to exacting tolerances to minimize backlash. Encoders help filter out minor backlash but, as these components wear, they often need to be replaced. Worn lead screws and lead nuts result in too much clearance for the encoder to compensate. The threaded rods and nuts do not fit tightly together. If they did, you would not be able to turn one while hold- ing the other. The clearance designed into the mating of the rods and nuts is called backlash. Backlash is present in all threaded applications. Goodheart-Willcox Publisher Figure 21-22. This servomotor controls the Y axis movement on a horizontal machining center (HMC). Servomotors use a closed-loop system, requiring encoders to provide feedback to the control. Haas Automation, Inc. Figure 21-23. A lead screw assembly consisting of a lead screw and a ball bearing nut. The lead screw translates the rotary motion of the motor into the linear motion that moves the nut.