Anyone ever fix the g0602 tailstock screw-out problem?

I can see that. That would be an excuse to start adding DRO.
Overextension in those cases has never caused harm to the lathe though, so I haven't given it any thought.

I was plunging a 37/64" drill bit into the stock to make a series of bushings that were on the order of 3/4" long. Drill out inside, turn outside, part off. I got greedy trying to go too deep in a single operation. I screwed the tailstock out too far, and that fat drill started spinning my entire quill. The threads on the lead screw and quill did not like that very much.
That would be a problem. Aside from that, it is not a good practice to run to the end of travel as the drilling torque could strip the threads of the lead screw or quill if only a single thread was engaged.

You can limit the travel quite easily by drilling a hole for a small dowel pin at the end of the keyway. Then end of the pin would be ground to slightly below the circumference of the quill. Remove the set screw "key" and insert the quill. Then install the key. This will of course reduce your total travel.
 
OK I can see how unscrewing it under load could cause damage, I am still surprised it is catastrophic damage but I can still see the senerio where it could be.
Without knowing exactly what is inside your tail stock I see several possible methods to prevent this in the future.
1. put a pin or screw in the end of the key way. This will loose travel in the fully extended position
2. Put a screw and a washer in the end of the quill to prevent the over extension, depending on internal layout this may reduce travel in the fully retracted position. unless you can add a clearance hole to the inside of the tail stock.
3. Add a dro or other external device to limit travel.
4, Find the max safe point of extension and mark it on the quill, remove the quill and cut a groove just a few thou deep and fill it with red paint. When the red appears you know to stop. On mine, even though there is about another inch of travel after the end of the scale on the quill I never go past the the engraved scale, That is the max extension that the quill was designed for.
5. After 2 times should be lesson learned, lazy is always more work in the end.
 
That would be a problem. Aside from that, it is not a good practice to run to the end of travel as the drilling torque could strip the threads of the lead screw or quill if only a single thread was engaged.
It may indeed be that the threads failed before I had it fully extended. I had never given that a thought, and it's a completely fair point.

OK I can see how unscrewing it under load could cause damage, I am still surprised it is catastrophic damage but I can still see the senerio where it could be.
I think it was a perfect storm. I've unscrewed it plenty of times, but never as violently as that time.
1. put a pin or screw in the end of the key way. This will loose travel in the fully extended position
After considering the danger of over-extension, even without fully unscrewing, which I had never really contemplated, it seems like what I want to do is position whatever stop so that it limits my travel to the end of the scale on the quill, period. I hadn't thought about calibrating it, and that would be a good idea. I have determined through experimentation that the last line on that scale is indeed the end of useful travel. I had everything back from their slathered in red Sharpie, for all the good that did me. It looked pretty while it was spinning. I might want to set it so I can't quite reach the end of the scale, for good measure.

4, Find the max safe point of extension and mark it on the quill, remove the quill and cut a groove just a few thou deep and fill it with red paint.
I tried a variant of that idea already, and found that I'm never looking at the quill itself when this happens. I'm always looking at whatever the tool in the tailstock is doing. I've tried improving my discipline on this, and I just find it difficult in practice to keep my eye on both things at once.
5. After 2 times should be lesson learned, lazy is always more work in the end.
Oh, I agree that my lesson should have been learned by now. I'm just trying to build a mechanical safety feature that wouldn't be necessary if my machine didn't have a stupid operator. But my machine has a stupid operator. It is what it is.

I never quite liked the "just put a set screw in there" idea, and this thread has given me food for thought. I think I will make a little steel or brass stop block, and secure that with a set screw, rather than brazing it in. If I need to adjust the length of travel, I can adjust the block. Start a little longer than what I think, and sneak up on the right dimension.

Also, I think the tailstock would perform better if I implemented a variation of the setup used on my dividing head tailstock. Make a key for that keyway, and machine a little diameter onto the end of a set screw or some custom made part to engage a hole in that key. A royal bear to assemble, but the key in there would create a lot more positive alignment action. With the bevel-end set screw in there, it's really quite sloppy now. I think this would firm up the action without making it too tight.
 
Here are some links dealing with the tailstock key issue. The last details my approach to the problem.
https://www.hobby-machinist.com/threads/tailstock-quill-distortion.46920/#post-398327 posts#2,7
https://www.hobby-machinist.com/threads/choosing-a-dro-for-a-lathe.62746/#post-517778 post#23
 
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