Hole production apparatus for cutting a circular hole in a workpiece and removing the severed material from the formed hole, comprising:
- a rotatable, elongate, hollow drill shank having an annular cutting knife edge of a predetermined diameter at the axially outer end thereof for cutting a circular hole in said workpiece;
- drive means, detachably coupled to an axially inner end of said drill shank, for rotating said drill shank about its longitudinal axis and for relatively moving said drill shank and said object toward and away from each other;
- said drill shank including a hollow cylindrical tubular member having
- an inlet opening at the axially outer end thereof,
- an outlet opening at the axially inner end thereof, and
- an elongate, axially-extending passage for communicating material which has been severed by said drill shank from said inlet opening to said outlet opening;
- said tubular member including means for radially inwardly compressing and concurrently axially elongating the material severed from said object by said drill shank as said drill shank and said object are relatively moved toward each other, to form an elongate core of plastically strained removed material comprising
- an inner tapered cylindrical wall surface portion which radially inwardly converges in an axially inward direction from said annular cutting edge, and
- a right circular cylindrical wall surface portion extending axially inwardly from the axially inner end of said tapered wall surface to provide a reduced diameter throat for receiving and axially passing said core of removed material;
- said tubular member including means, axially inward of said throat, for receiving and axially freely passing said core of material, including an elongate tubular portion having an internal diameter which is greater than the diameter of said throat but less than the diameter of said annular cutting edge, to allow the said compressed core material to partially expand and freely pass axially inwardly of said throat;
- said internal diameter of said elongate tubular portion gradually increasing in a radially inward direction;
- said tubular member being of such construction that the included angle between said inner tapered wall surface and the outer surface of said tubular member lies within the range of 30.degree. to 45.degree.;
- the ratio of said predetermined diameter, less the diameter of said throat, relative to said predetermined diameter lies within a range of 0.25 to 0.36;
- the ratio of said internal diameter of said elongate tubular portion, less the diameter of said throat, relative to the diameter of said throat, lies within a range of 0.16 to 0.39;
- the ratio of said predetermined diameter, less the internal diameter of said elongate tubular portion, relative to said predetermined diameter lies within a range of 0.09 to 0.04;
- the ratio of said predetermined diameter, less the diameter of said throat, relative to said predetermined diameter less said internal diameter of said elongate tubular portion less within a range of 1.45 to 6.00;
- and the axial length of right circular cylindrical wall surface portion being substantially less than the diameter of said throat;
- said drive means including an axially extending internal passage in communication with the axially inner end of said passage in said tubular member for receiving and axially passing said core material, and a transversely extending discharge passage for receiving said core from the axial passage in said drive means and passing it radially outwardly.