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Some people have the opinion that a 3 phase motor produces a smoother power delivery than a single phase one. It may be worth doing some research to see if this is perhaps a worthwhile upgrade whilst you are embarking on your quest for a better finish.Martin
Many years ago I had a Jet 12X36 lathe similar to yours. I chased “smoothness” like you’re about to do. At work we had Clausings, Monarchs & LeBlonds. The one thing that helped tremendously was to install a twist-lock adjustable drive belt.
What you need is a 50+ year old ex industrial lathe:http://www.youtube.com/watch?v=d9BX_XBiliYThis is what Jason terms a "Knackered old Lathe" JoP.S. I tried thumping the tailstock with my elbow while taking the video but that did not work either