TWIST DRILL GEOMETRY.

Twist Drill Geometry
Twist drill geometry and its nomenclature are shown in Fig. 22.5. A twist drill has three
principal parts:
(i) Drill point or dead center
(ii) Body
(iii) Shank.

Drill axis is the longitudinal centre line.
Drill point is the sharpened end of the drill body consisting of all that part which is
shaped to produce lips, faces and chisel edge.
Lip or cutting edge is the edge formed by the intersection of the flank and face
Lip length is the minimum distance between the outer corner and the chisel-edge
corner of the lip.
Face is that portion of the flute surface adjacent to the lip on which the chip impinges
as it is cut from the work.
Chisel edge is the edge formed by the intersection of the flanks.
Flank is that surface on a drill point which extends behind the lip to the following flute.
Flutes are the grooves in the body of the drill, which provide lips, allow the removal of
chips, and permit cutting fluid to reach the lips.
Flute length is the axial length from the extreme end of the point to the termination
of the flutes at the shank end of the body.
Body is that portion of the drill nomenclature, which extends from the extreme cutting
end to the beginning of the shank.
Shank is that portion of the drill by which it is held and driven,
Heel is the edge formed by the intersection of the flute surface and the body clearance.
Body clearance is that portion of the body surface reduced in diameter to provide
diametric clearance.
Core or web is the central portion of the drill situated between the roots of the flutes
and extending from the point end towards the shank; the point end of the core forms the
chisel edge.
Lands are the cylindrically ground surfaces on the leading edges of the drill flutes. The
width of the land is measured at right angles to the flute.
Recess is the portion of the drill body between the flutes and the shank provided so as
to facilitate the grinding of the body. Parallel shank drills of small diameter are not usually
provided with a recess.
Outer corner is the corner formed by the intersection of the lip and the leading edge of
the land.
Chisel edge comer is the corner formed by the intersection of a lip and the chisel edge.
Drill diameter is the measurement across the cylindrical lands at the outer corners of
the drill. .
Lead of helix is the distance measured parallel to the drill axis between corresponding
points on the leading edge of a flute in one complete turn of the flute.
Helix angle is the angle between the leading edge of the land and the drill axis.
Rake angle is the angle between the face and a line parallel to the drill axis. It is bigger
at the face edges and decreases towards the center of the drill to nearly 0°. The result is that
the formation of chips grows more un-favorable towards the centre.
Lip clearance angle is the angle formed by the flank and a plane at right angles to the
drill axis; the angle is normally measured at the periphery of the drill. To make sure that
the main cutting edges can enter into the material, the clearance faces slope backwards in
a curve. The clearance angle is measured at the face edge, must amount to 5° up to 8°.
Point angle is the included angle of the cone formed by the lips.