I am usually very thorough when I operate machinery and recheck my equipment. I make sure all is tight and guards are in place as it should be. I had turned a very thin finial   and took it out of the chuck, removed the chuck and put another project between centers. After I finished, I returned the chuck and finial to the lathe for polishing. I got a phone call and they returned to polish the piece. I turned the lathe on, the chuck unthreaded in a micro second, fell on the lathe bed smashing my finial. I gave myself a compliment by saying loudly

"You big dummy!"

I did something similar once. I partially threaded a chuck with bowl. The inertia caused the chuck to jam tight on the spindle. Ever since then I’ve always used a plastic spindle washer. 

Sorry about your finial but I would call this a very cheap lesson. Not too many happy endings with a chuck bouncing around the shop. For the record, have done it myself.

 

Steve

I have a big metal lathe. I have both a 3 jaw and a 3 jaw chuck.

My 4 jaw chuck weights a few hundred Lbs. So I don't like changing

chucks. So when I need a 4 jaw I sometimes put a smaller 4 jaw

chuck inside my larger 3 jaw chuck. Even the smaller 4 jaw chuck

is heavy. Once when I did this the weight of the smaller chuck sat

on only two of the three jaws of the 3 jaw chuck and tighten up.

The third jaw was not touching the smaller chuck. I turned on the

lathe and proceeded to do my work. The small 4 jaw chuck came

unglued and landed on the floor at a good RPM. Sparks flying off

the cement the chuck sat there spinning. It slowly started moving

across the floor until it came to my drill press. Then one of the jaws

caught on the base of the press and the chuck started bouncing off

the walls. Time to leave the room. Nothing got broken that I could find.

Someone was watching over me that day...