Re: chucking a rectangular bar
Oh, I might be wrong about this, but I've found that most stock is round, but not straight. That is, unless you get the precision stuff. What I do is plan my work around a piece.
Once you have the piece chucked, the part of it that you turn will be as accurately round as your machine is capable of making it. I do all work possible before I part off the piece that I was working on. If I have to re-chuck after parting, I just do what cy said. I tighten and check. If it's a bit off, I loosen and turn the piece a bit.
