Re: Lee collet die problem
"The mandrel has pushed the aluminium top up and stripped the threads that hold it in the die body... I was applying some force as the instructions say at least 25lb but I am not a gorrilla and was really surprised. "
It was too much lever pressure. And it was too much on the collet, not the mandrel. Lee's instructions are for Lee's presses, they can't tell each of us how much pressure to apply with other pressses. Case length won't have anything to do with blowing the top off. The leverage issue is how much upward force we put on the base of the collet anyway, not how much we push the lever. Lee's instructions say to adjust the die down until the lever is at right angle to the ram and then use about 20# of pressure but that obviously changes from one press to another. Once we form the necks to the mandrel there is no point to using more force, that can only push the cap out as your's did - it's designed to do that instead of busting the press or die body.
Starting by having the die UP until the lever is nearly full down before applying 20# of pressure increases the ram force tremendously; that's the point of a toggle lever system on a press.