P,


  You may want to check out "trepanning".
  Trepanning is a machining method of producing larger diameter, deep
drilled holes which can be more cost effective than typical gundrilling
techniques.  It is best described as core drilling.  Unlike gundrilling,
Trepanning leaves a core from the center of the bar instead of turning it
all into chips.


    One vendor, easily found with a Google search is:

Trepanning Specialties, Inc.
16201 Illinois Ave., Paramount, CA 90723
Voice (562) 633-8110 . FAX (562) 633-0141
     More than likely... many others.

Jack Beale

According to Google this is a methods of drilling holes
in skulls.

Paul Lamar

P,

   Look it up again in the "machining" context.  Been done for decades in machine shops...

Best...

Jack Beale