Metric threads on an Imperial lathe

I believe my Grizzly 12 x 36 uses a 91/86 transposing gear. I've used it a half dozen times without a problem. Of course I never release the half nuts.
 
I am not good with the threading gage, regardless of the politics of the thread I leave the half nut engaged and reverse in, finish out and everything lines up.
 
I am not good with the threading gage, regardless of the politics of the thread I leave the half nut engaged and reverse in, finish out and everything lines up.
This is also a valid method of threading. You are 100% always able to pick up the thread.
 
One thing I didn't see mentioned (maybe I missed it) was that your threading dial won't work since it's designed for cutting imperial threads. You will either need to never disengage the half nuts or use one of the other workarounds to be found on the internet. Joe Pi has a video on the subject.

Dis-engaging the half nuts, running the carriage back and re-engaging the half nuts will result in cuts that are not synchronized, screwing up your threads.

You probably know that but it's worth mentioning. It's what screwed up my first attempt(s) cut cut metric threads on my imperial machine.
Tom Lipton, OxTools, did a video on cutting metric threads on an imperial lathe. The problem with not being able disengage the half nuts due to losing synchronization is that you have to be able to stop the lathe quickly enough to avoid running past the gutter. For lathes with a VFD and dynamic braking,it isn't that big an issue but for a lathe with an induction motor, stopping when the you hit the gutter but avoiding running into a shoulder can be tense.

However, if you disengage the half nuts and shut the lathe down, then reverse the lathe and reengage the half nuts when the same number on the thread dial hits the index mark, you will not lose synchronization. You still have to run the lathe in reverse to the starting point but it eliminates the butt clenching for metric threading.
 
This is important to know. I leave my half nut engaged and run the carriage back and forth, after disengaging the tool. I turn the power off when hitting the thread gutter. Then extract the tool, reverse the lathe to go back to the start and stop the spindle. Then set the tool depth and turn on the spindle forward.

There should be a way to resynchronize the spindle and carriage thread dial. It's going to be some modulo 127 or 100 ratio, but I haven't figured it out yet. I have an ELS and I'd like to do this. Have an encoder on the spindle and I can count steps for the lead screw. One of my round tuits that I'd like to figure out this year. I need to just work out the math - hopefully it will become obvious once I plot some stuff.
IIRC, your ELS utilizes the z axis position. In order to be able to disengage the half nuts without losing synchronization, you simply have to be able to compare the spindle rotation angle to the z position. While there are a number of z positions tha correspond to a given rotation angle for any given thread pitch, there is a unique rotation angle for a given z position. You could either advance the carriage to a given start point and hold there until the spindle rotation angle was correct or you could track the spindle angle and increase or reduce the lead screw speed to reach a synchronization point. In either method, the thread dial is no longer relevant and the half nuts can be engaged at any convenient point.
 
It looks like you can buy a 80/63 transposing gear on ebay for about $5. Dave
 
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