Spin the thing, cut away little bits at a time, the result is sure to be perfectly round.
See also:
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https://www.youtube.com/watch?v=vmIkQV--rh4
Just a fantastic video about making a super low cost minimal lathe out of a motor, section of a large metal tube, some misc hardware, and a pair of really beefy bearings, using only an arc welder and hand tools. Wait 'till you see how he uses it to make a threaded rod. Genius!