RichardS wrote:
As others have said, it is screwed on with a reverse thread screw.


Huh? A drill normally rotates clockwise (viewed from the rear) so if the
bit snatches, such as when breaking through when drilling metal, it
would tend to undo the chuck if it were a LH thread. I bet it's the
locking screw that's a southpaw...hang on....just been out to the garage
(any excuse to get out of the house with this heat :-) )...yep, the
locking screw is LH thread on my old B&D Quattro.