What this whole thread is overlooking is the fact that the OEM bearings are deep groove ball bearings; the cheapest option! An upgrade to angular contact bearings gives more thrust resistance to the spindle, resulting in the lathe becoming more rigid. Tapered roller bearings would be the most rigid, but, sealing them is much more complex.
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I upgraded mine to angular contact bearings years ago & have been happy with the results! My preload setting method is to simply adjust preload for slight heating at full speed.
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The only benefit to higher precision bearings in the spindle is reducing wallet weight!?
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