(06-21-2016, 11:19 PM)Pete O Wrote: Sadly my lathe lacks the transposing gear to be able to cut metric threads so I don't think I can accurately fix the thread that way. One of the key missions for my new universal dividing head is to make the 127-tooth gear so I can do metric work on my lathe but that capability is a fair way off at present- especially given that I need to make several other change gears to make the 127 possible. I'm thinking I'll just acquire the appropriate non-standard tap and die for this job.
Do you have a feed rate of .0394" or very close to it on your lathe? If so set up like the usual for threading, set the feed rate at .0394 and use your longitudinal feed (not the half nuts) once you're aligned with the existing thread. Just remember to never disengage the feed lever until you're 100% complete, but stop the lathe, back the tool away from the work, and reverse the spindle to go back to your starting point.
BTW, go with very low RPM's and use layout dye on the existing threads so you can tell just how close to finishing you are.