As I previously mentioned, I finally got the round column mill from my brother, after months of other projects taking priority, then building the bench for it and arranging the garage to accommodate it. I doubt it had been turned on in a decade, thus he talked like I was doing him a favor for taking it. Ha! I'm happy to help!
Because he was trying to use it, after getting it from our father, in the days before YouTube, this forum, and most other accessible resources we now enjoy, he was flying completely blind. So it's understandable that he tried to use the drill chuck to hold an end mill, as there was no other tool holding options included when he got it. (And I recently learned that my father never even used this mill, as my parents moved to a smaller home before he had a chance. He got it, the surface grinder, and the lathe, all of which now live in my garage, as payment from a client who was unable to pay their original terms. He's excited to now come and use it in my garage.)
Anyway, this is what happens when you try to mill with an end mill in a drill chuck (not to mention the chuck coming off of its arbor, which REALLY confused my brother):
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The chuck is broken (anyone think of a use for it now??), and the end mill is thoroughly beat up. Sides still look good so I'll keep it around for side milling something someday.
Thanks to a tip from John HMan, I got the practically glued-in old drill arbor removed, and a new Shars keyless drill chuck installed. I went with the smaller size because I don't have any bits with shanks over 1/2", and I have plenty of smaller ones. JT33 arbor, just like the lathe chuck, so I can use this one (and the keyed one I've been using on the lathe, with its 5/8" capacity) on either machine.
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A few more things I have to prioritize before I can allow myself to turn it on and make some chips, but it's good to finally have it here so I can as soon as I'm ready.