My Stuff alright (nobody else to blame: spindle)

I was actually talking to my spouse about the absolute importance of tightening a collet, especially when a 1/2" endmill is the occupant.I don't remember much after that--I must have fallen asleep mid-tighten.

When I hit the Run button and the machine started moving toward the start of the roughing pass, I realized I had forgotten ear protection, and was in the midst of putting them on when the steppers just started making all kinds of blood-curdling wailings.  It wasn't five seconds into the cut when I hit the E-stop--but it was enough.

The endmill was buried in the oak plank, well below 1/2" even though the pass-depth was set at 0.1", and immovable.  I was able to loosen the collet nut just enough to jog the spindle up off the cutter, but only just.Next came 2 hours of pounding on wrenches to actually free the collet nut from the spindle.  In the end I used a chain wrench between the box ends of two wrenches to "influence" them towards each other, one silly micro-degree at a time.  The final 20 minutes were spent hammering them towards each other, ripping of some threads and folding others flat.

I post a picture of "My Stuff". This was after I got the Bright Idea that--perhaps--I could grind off the ruined threads enough to get a nut on; it shows evidence of at least one of the flattened threads and of the grinding. (I ran the spindle at its lowest end, M3 S1200, and just set the grinding wheel of an unpowered grinder in contact with it.)

Before I completely followed through on that idea, it dawned on me that a) the spindle shaft had taken a pretty good lick in order for the nut to weld itself to the threads, and as a result might not be quite plumb, and b) both the length of the shaft and the number of threads might be the result of actual computations, and could only be changed at one's peril.

After a brief search showed near-exact replacements[1] could be found for a few hundreds, I re-assessed the worthiness of my task--and dropped it.  Surprisingly, a spindle ordered from ebay on Sunday evening arrived Tuesday afternoon.  What should have taken 15 minutes took a couple of hours, but the machine is back up and cutting. (And I am pondering the value of keeping a hot-spare lying around.)

[1] Although spindles look identical, manufacturers maintain different favorites for electrical connectors which, really, ought to have been standardized by now...

I read elsewhere that I might have been overtightening the collet nut for years, based on the informal advice I received from an amateur machinist to "tighten the nut with wrenches with my bare hands, and try to bend the wrenches."

Perhaps, over time, I weakened or deformed the threads so that, at one point, they "felt tight" but then failed to hold the tool.

Wow, that sucks. The F=MA thing can get pretty cruel. I don't know too many machinists, professional or amateur, that don't have at least one story of a catastrophic machine crash. I've had some Lu-Lu's.
Trying to "bend the wrench" on any fastener is always a big no-no but the odds of your deforming a 25mm+ hardened thread diameter with a couple of box end wrenches are low. I think that you'd be more likely to break your knuckles than the threads. A decent rule of thumb is "snug plus a quarter turn". If it feels like it's scraping or hanging or just looks wrong, take it back apart to clean and inspect it. Usually, there's some swarf that got up in the threads or the collet's not quite seated on its retainer. If the collet's deformed, throw it away.

And the last thing we all should do before hitting the "go" button is check our tool holding (the collet) and our work holding (clamps and fixtures).
Always.

Its much more preferable to bury a milling cutter into a piece of oak then a finger like I did!
Required a specialist hand surgeon and general anesthetic to put me together!
But the spindle collet was fine!

@Gorts Thanks for the sympathy and the empathy.

AND thanks for the suggestion: "CTFC: Clean the frippin collet." IT NEVER EVER OCCURRED TO ME. Definitely on my punchlist.

Re: F=MA It's almost too easy to settle into a mindset where "no accidents implies everything perfect." I wish everyone a couple-hundred dollar crash like this that reinforces the FACT that "that hunk of iron over there, no matter how long your association, is NEVER going to overlook your mistake." My father tried to drum the idea into my head about power-take-off/PTO when he said, repeatedly, "Torque is merciless."

@Rod
After reading your post and creating an image, I had to go sit down from loss-of-blood--it drained to my ankles. I am sorry for having been the cause of reliving it[1], but I hope all future readers will "check their assumptions."

I'm going to paint an Angry Face on my machine.

[1] Yogi Berra is supposed to have said[2] once, at an awards ceremony where his parents were in attendance: "I'd like to thank my parents for making this evening necessary."
[2] He also is reported to have said, "Half the things I said, I never did."

Yes, you are right that collet nuts have to be tightened more than most people think is enough.
And yes again, cleaning collet, nuts and seat is VERY important. There's even some specialized 'cleaning oil' just for that purpose.
Here is a chart in lb/feet but you can easily find the same in Nm over the internet.