Examples from the Web for lead screw
The carriage should not be disengaged from the lead-screw after starting the cut, the tool being returned by reversing the lathe.
As previously explained, lead-screw S is only used for feeding the carriage when cutting threads.
If, on the other hand, no movement takes place, it is an indication that the pitch of the lead-screw is correct.Measuring Tools|Unknown
This is not the case, however, when the number of threads being cut is not a multiple of the number on the lead-screw.
By referring to the last column, we find that the lead-screw gear should have 84 teeth.