of the drill bit and adjustment of the dwell time to consistently produce identical countersunk holes in a variety of workpiece materials application_number US 39210006 A description referring first to fig1 7 a pneumatically driven countersink drill 10 has a handle 12 connected with a pneumatic drill motor 14 and a pneumatic coupling 16 adapted to be coupled with a source of pressurized air the drill motor 14 includes a cylindrical body which passes through and is secured to a generally rectangular bulkhead 18 an outer cover 20 protectively encloses internal parts of the drill 10 and is secured to the bulkhead 18 as with screws not shown a pair of pneumatic cylinders 22 are secured to opposite sides of the bulkhead 18 and have corresponding linear drive shafts 24 secured to a nosepiece 26 a pair of hydraulic check cylinders 28 are also secured to the bulkhead and have output shafts 30 secured to the nosepiece 26 the bulkhead 18 is slideably mounted on the nosepiece 26 by means of shafts 24 30 the outer end of motor 14 is provided with a drill motor gearbox 32 which gears down an output shaft 34 to a desired speed the output shaft 34 is coupled to an arbor 36 by means of a threaded sleeve 38 a generally cylindrical aft stop 40 is.