I have always dissagreed with "the nut is to tight' and you are bending the mandrel thread. You would realy have to crank on that nut, and something would have to be sloppy or out of round to begin with to apply any lateral pressure on the mandrel. I will use a pliers to tighten the brass nut if I am turning a very hard wood and it keeps slipping. I can't say I have never had a out of round piece, but I don't think it was from a to tight mandrel nut. If your ends of your blanks are round, and the centers are out of round, your mandrel is either bent, or you are apply to much pressure with tools that are not sharp enough as ED said. Here is a quick check you can do on your mandrel. Ingauge the tailstock. Need not be super tight. Move up your tool rest until it touches the mandrel. slowly turn your mandrel by hand. see if there are any gaps betreen the tool rest and the mandrel. If there is, that is the point where it is out of round. You can tap on this point with a brass hammer until it runs smooth. Another test you can do is run your tailstock up to, but not touching your mandrel. Turn the mandrel slowely by hand and see if the point stays lined up with the hole in the end of the mandrel. If not gently bend the mandrel until it lines up to the tailstock point as you turn it. After this adjustment go back and repeat the first test. You could also use a dial indecator to true your mandrel. I always keep spare mandrels handy if I don't want to spend the time truing the one I am using at the moment. I usualy turn both blanks at the same time, and need the full length of my mandrel on my mini lathe. I have an adjustable mandrel that I can turn shorter objects on if needed. One other test you can do is move your tailstock up to your headstock and lock it down. They should meet up center to center. Slowley turn your headstock by hand. If it wobbles out of center, you may have a bad bearing. Want to check your mandrel for straightness? Roll it on a flat surface. Jim S