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When single-pointing metric threads  on a lathe with an Inferial leadscrew the inherited wisdom is to never disengage the half-nuts.  However, this makes threading to a shoulder or a runout groove difficult at best.  Thankfully, there is a technique that allows one to disengage the half nuts without losing registration on the thread.  It involves the use of the threading dial but not in conventional manner.

1. After setup (including checking the pitch) start the first pass by engaging the half-nuts on a selected number on the threading dial.  (Here we'll assume you engage on '1'.)

2. When you reach the runout groove/shoulder disengage the half-nuts and turn off the lathe motor.  Back the tool out but do not move the carriage.  You will note that the thread dial no longer points to '1' because the motor inertia continued to turn the lead screw after you turned the motor off.

3. Start the lathe motor in the reverse direction.  The '1' on the thread dial will begin to move back towards the index mark.  When it reaches the index reengage the half-nuts.  You're now back in sync with your thread.

4. Let the leadscrew drive the carriage to the right until the tool clears the workpiece.  Stop the lathe.  Put on the next cut increment and start the lathe in the forward direction to make the next cut on the thread.