16 tpi external, the dial doesn't land where I left it

Threading 1/2-20, which is 20 tpi, sorry, the title is from the last panic. The cross slide dial does not come back to the same number after I withdraw and return. I keep a note. The note and the dial disagree by a few thou. Thread gage is unhappy. Half nuts stayed closed.

If the half nuts stayed closed, the pitch is the leadscrew. The dial lying is backlash plus you reading it from the wrong side of the lash. Set the cut with an indicator on the cross slide, or always come in from the same direction and ignore the absolute number.

Stop trusting a dial you've already admitted doesn't return. The thread gage is unhappy because the depth is wandering, not because 20 tpi became a new religion overnight.

Indicator on the slide. Same direction every pass. Gage is less theatrical. The dial is now a decoration I still look at, like a clock in a room where I have a phone.