Patent ID: 7195428

Claim:
A gundrill for forming deep holes in a body of material as the gundrill is relatively rotated, axially advanced and supplied with drilling fluid, the gundrill comprising: an elongate tubular shank having a driven end, a distal end and a central region extending therebetween along a central axis, the tubular shank having a cross-section defining a shank flute extending from the distal end for at least a substantial portion of the length of the central region, providing a portion of an elongate fluid return path between the hole being drilled and the shank flute, allowing drilling fluid, which is pumped into an internal fluid passage formed through the tubular shank to exit the hole being drilled removing chips as they are formed; and a cutting tip affixed to the distal end of the tubular shank, the cutting tip having an internal fluid passageway which is coupled to the tubular shank internal passageway and terminates in an orifice formed in a tip end surface, a tip flute extending axially from the tip end surface toward and generally aligned with the shank flute providing a portion of the elongate fluid return path, the tip flute defined by a secondary flank surface and a generally radially extending primary rake surface having a peripheral rake edge lying on a cylindrical surface coaxial with the central axis, and a generally radially extending cutting edge at the tip end which defines a radially offset point; wherein the surface of the cutting tip circumferentially behind the peripheral rake edge as the gundrill rotates deviates inwardly sufficiently from the cylindrical hole to form an enlarged localized relief passage which provides an alternative exit path for drilling fluid, the relief passage extending from the tip end toward the tubular shank member; wherein the tip end surface of the cutting tip cooperates with the hole being drilled to define a toroidal bottom space area having a portion thereof which lies between the end of the tip and hole bottom forming a pressurized end clearance volume which receives drilling fluid through the tip orifice and discharges drilling fluid through the relief passage and an outlet passage, which is generally bounded by the distal edge of the secondary flank surface and the bottom of the hole being drilled; wherein at least one of the tubular shank and the cutting tip is provided with a crossover port connecting the relief passage to the elongate fluid return path; and wherein the crossover port is primarily formed in the tubular shank.