We had the spindle rebuilt a few thousand hours ago and they put in a pretty stout Belleville washer spring stack for the draw bar pull studs. The retention is very good, but it's all the system can do to compress that new pull stud retainer and this coupler sees all that force as tension for each tool change. It's an old mill and they weren't built for the kinds of pull stud tension used today. I guess this was a weak link. It's a nice piece, the bit I mill as a replacement probably won't be quite as "nice" but it will be a fat piece of S7 with fillets in stressed areas and a very good heat treat so I don't expect it will fail again.
OMG what a pain in the ass though
This is the pig. My 4th CNC mill. I have a love-hate relationship with this machine. It was an old low hour mill when I bought it. Very old and very low hour. Early 90's. Less than a year of production on it (when I got it, lol). Almost 9,000 pounds of iron and well made. Slow and pokey as hell. I love this machine, it's almost like an old friend at this point. It's slow as hell and can't get out of it's own way, but the screws and spindle are silent and smooth and you can sweep the entire table and not see more than .0001" deviation. <--- that's really uncommon. The lubrication in the spindle was old when I got it and it didn't last long so we had the spindle rebuilt about 8 years ago, but other than that it has been steadfast reliable. It took a lightening strike about that same time frame and suffered some real damage to an input-output board with the control parameters on it that bricked it and I had to "nurse it back to health" (involved soldering in some new microchips on a board and reloading all of the system and diagnostic parameters from an old yellowed paper copy). I've made so much stuff with this old mill and it has earned it's keep and paid for itself so many times. I've run new Haas, Onsrud and Romi machines and they are linear way, noisy and rough, and lacking in the smooth precision of the Pig. Even the Mori, as badass as it is, it has 9,000 hours on it and can't hold a candle to the quiet, tight precision of this old pig. It has had a few hiccups here and there, and it can't get out of it's own way sometimes requiring slow feedrates to accommodate its prehistoric electronics (which is frustrating when you have a schedule to keep), but it decks difficult alloy steel to a dead flat mirror finish, can plow a straight even fuller across anything without any wobble anywhere, and holds tenths when I need it too. I really ought to replace it with something more modern but it's almost like family. Jo addresses it in the morning "hello Bridgy!" When my kids grow up, they might not remember much about their dad's shop, but they'll remember this old mill.
But goddamn it's a pain in my ass...