After you turned the commutator, did you undercut the mica between the
commutator segments? It is critical that the insulation be below the
surface of the copper to ensure good contact between the brushes and
the copper. If there is still sufficient undercutting [you only
skimmed the surface of the comm] then be sure that there is no copper
shorting between commutator bars. The copper often 'smears' a bit when
being turned and will bridge the gaps here and there. A "pole growler"
will show up any such shorts.

Neil S.

Thanks for that info.

I did not undercut mica. I presumed that since, before turning on the lathe,
the motor was functioning well enough (sans bearing noise) with not-undercut
mica that its purpose was not important.

What's the best way to undercut? Using an exacto or utility knife in my hands
gets quickly messy...

I looked closely and eliminated any segment shorts on the commutator.

Thanks,
Dave