Actually; the Chinese made machines with single phase motors often show a very fine sine wave pattern on finish cuts and a slightly "frosty" appearance on threads. This seems to come from the vibration pattern of the motor which is rigidly mounted to the headstock. The 3 phase motors don't seem to cause the same issue. Isolating the motor from the headstock with rubber washers and bushings can make the problem almost disappear. I have heard that changing to a higher quality single phase motor (Baldor for example) also improves the smoothness a lot.
