I like the fix, but just want to warn that while you don’t see or feel the vibration, it may still be there.Lathe vibrations are fixed. Sometimes the answer is so simple. Not the first time I've over complicated or over diagnosed something. Added a brace to the corner and BOOM, no more shake rattle and roll. Smooth all the way up to 1265 rpm. Best part, I used an existing piece of flat metal I had and used pre-existing holes.
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You may want to go ahead and change the belt. Or not.
Just coming from a guy that has torn a lot of stuff up.