Trepanning is the process whereby a hole is made by cutting out a ring of material from a sheet. It can also be used to produce a round shape out of a sheet. This can be seen as being a process whereby a piece of metal is separated into two parts.
This process can be performed on the lathe by using a suitable cutter to cut a ring. It can also be done on a milling machine by using just an endmill to cut out a ring of material. It can also be done on either a lathe a milling machine or a drilling machine by using a hollow round cutter.
trepanning using a round hollow cutter