The half-nut lever is a control on a lathe used to engage or disengage the carriage from the leadscrew during threading operations. Inside the apron, it operates a pair of split nuts—called half-nuts—that clamp around the rotating leadscrew when the lever is engaged. This mechanical connection allows the carriage to move precisely in sync with the spindle, which is essential for cutting accurate threads.
Unlike the feedrod, which drives the carriage through gears for general turning, the leadscrew and half-nut system provides a direct and exact relationship between tool movement and spindle rotation. The lever must be engaged at the correct moment to align with the threading pitch, and disengaged carefully to avoid disrupting the thread or damaging the machine. Its proper use is critical for successful manual thread cutting on the lathe.
