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.