need help with lathe chuck

[rogm1017]

Hi, i am fairly new to turning and need a little help with lathe chuck. Purchased chuck and placed into head stock. Used 1/4 threaded rod to thread into mandrel to keep mandrel from coming out which works, but the chuck keeps wanting to run off the mandrel. If anyone has any ideas, i would be grateful. Not that it matters but i am trying to turn bottle stoppers, and i can't use the tailstock when i need to turn the top of the stopper, and i can't turn the top of bottle stopper because of the above problem. thanks