Metric Threading on an Imperial Lathe

[Darren]

The dial stops turning when the half nuts are engaged because the carriage is travelling at the same rate as the leadscrew..

 

[DPittman]

The dial stops turning when the half nuts are engaged because the carriage is travelling at the same rate as the leadscrew..

OMG. That makes sense. I'm embarrassed I didn't know that. I'm going to go now to the corner and hang my head in shame.

 

[Susquatch]

One other comment/question on the reversing but did engaging the half nut method...
If you happen to blow it when re-engaging the half nut lever even once then you have to start all over? On my lathe and fine thread lead screw I often miss hitting the mark perfectly. It's not a problem when your threading the traditional way as you can usually disengage and try again before making contact.

No problem, just switch back to forward and catch it as it goes past again the other way. And if you happen to miss again, switch to reverse again and try again! You can switch back and forth as many times as you need to. The idea is to catch it so the exact same spot on the dial coordinates with the half nut engaging on the exact same spot on the lead screw.

Edit - basically, you don't want the half nut to even know it was ever disconnected.