When threading on a manual lathe, you can EITHER always engage the half-nut on the lead screw and return the carriage by reversing the spindle rotation through the motor drive …
… OR, you can use a thread dial or indicator which meshes with the lead screw and shows when the half-nut can be engaged, so that the tool will cut along the same thread.
The thread dial consists of a worm wheel meshing with the lead screw. It is connected by a short shaft to the indicating dial. The dial is calibrated with four numbered lines and four others midway between them.
These lines are used when cutting threads on a manual lathe. When set to a position, the line becomes your starting point to pick up the lead of the thread and, during additional passes, to achieve the proper depth of thread.
