In article , Harold & Susan Vordos
says...

... When you plunge cut with an end mill
of the needed size, especially when it's held in a drill chuck, which is NOT
intended to hold heat treated items like end mill shanks, the chances of the
hole coming out even remotely the right size is almost non-existent.

Plus there's a good chance that the end mill will come uncorked
out of the chuck - as you say, drill chucks are not designed for
this use!

Even running a horizontal milling machine with an end mill
in the spindle, where the setups are pretty rigid, one can
still not depend on the hole being any particular size when
plunging. I don't do this unless I'm just spot facing
something.

Jim

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