Hollow cutters are intended for drilling incandescent heaters in case a drill bit remains in them or tap. The cutter drills the outer casing of the glow plug together with the thread while the interior remains undamaged even with the tool bent. At this operation will damage the thread in the motor head and it is necessary use threaded inserts. The advantage is a big time saving as such a seized glow plug is very difficult demountable.
Milling machines can also be used for drilling bent screws in aluminum (eg engine block), if it is original thread insert.
Total length of cutters 100 mm, number of teeth 5.