And he should no better lol

[Dr Stan]

Think about the direction of rotation in relationship with the threads.  I did something similar when cutting a hex while holding the part with its internal threads on a stud in an index head.

I'd forgotten an incident while I was working in a tool & die shop and the foreman was cutting the OD of a ring using a 18" RT.  He was climb milling to make it easier to turn the RT the EM grabbed and wiped out the worm and worm wheel.