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For many years ball bearings could not equal the precision of cone bearings. This is why all of the old watchmaker lathes used cone bearings. The advantage of a ball bearing spindle was more RPMs, but sacraficing precision for more speed was unacceptable. This was a particular problem in the drilling of extremely small holes where more speed was needed to get the proper SFM and prevent drill breakage. This situation changed around WW-2 when manufacturing methods allowed ball bearing precision to surpass that of the cone bearings. As far as I know the first watch lathe company to pick up on this was Derbyshire. Now it is ball bearings that have reached their upper end of precision and have been surpassed by air bearings. The machines that use these spindles are so accurate they can produce a black polish finish with a turning tool.