Drill

A drilling head has a cylindrical body with a longitudinal axis, tapering towards a leading end thereof. The drilling head comprises at least two chip evacuation flutes and at least two lands therebetween. Each flute has a first side all facing in the direction of rotation of the drilling head and a second side wall diverging outwardly from the first side wall and merging therewith via a web-thinning recessed surface extending towards this leading end. Each land is bound by the first wall of one adjacent flute and the second wall of another adjacent flute and has a generally radially extending and axially inclined front flank face tapering towards the leading end. The front flank face of each land intersects the first wall of one flute at a primary cutting edge, the web-thinning recessed surface of this flute at a secondary cutting edge, and the front flank face of the other land at a chisel edge which is disposed at the leading end of the drilling head and which passes through the longitudinal axis thereof. The chisel edge is concave and merges with the secondary cutting edges of the two lands via axially outermost extremities thereof which are spaced from the longitudinal axis.

FIELD OF THE INVENTION 
This invention relates to a drill having a drilling head either mounted 
thereto or formed integrally therewith, and having a longitudinal axis of 
rotation. The invention particularly refers to the drilling head of such a 
drill, which is formed with chip evacuation flutes and lands therebetween 
and which tapers towards a substantially point-like leading end thereof. 
BACKGROUND OF THE INVENTION 
Different design modifications of the point-like leading end of a drilling 
head of the above specified kind are discussed in Metals Handbook (Ninth 
Edition), Vol. 16, pp. 226-227, 1989, USA. The particular type of the 
drill point design to which the present invention refers is shown in FIG. 
23 on page 227 of this Handbook. 
According to this particular design of the drilling head, each of its chip 
evacuation flutes has a first side wall facing in the direction of 
rotation of the drilling head, a second side wall diverging outwardly from 
the first side wall, and a web-thinning recessed surface formed 
therebetween adjacent the leading end of the drilling head and extending 
towards this leading end, so that the web thickness, defined at the 
leading end of the drilling head by the distance between the web-thinning 
recessed surfaces, is minimal. Each land of the drilling head is bound by 
the first wall of one adjacent flute and the second wall of another 
adjacent flute, and has a generally radially extending and axially 
inclined front flank face, which tapers towards the leading end of the 
drilling head. The front flank face of each land intersects the associated 
first wall of the flute at a primary cutting edge, intersects the 
associated web-thinning recessed surface of the same flute at a secondary 
cutting edge, and intersects the front flank surface of the other land at 
a chisel edge which is disposed at the leading end of the drilling head 
and passes through the axis of rotation of the twist drill. 
With the above geometry of the leading end of the drilling head, the width 
of the chisel edge is essentially reduced when compared with other drill 
point designs, whereby the thrust exerted on the leading end of the 
drilling head during drilling is reduced and the cutting efficiency at the 
center of the drilling head and the centering capability thereof are 
improved. 
Drilling heads similar to that described above are disclosed in further 
detail in EP 320 881 and KR 96-9591. 
However, since it is practically impossible to provide an ideal point 
chisel edge, i.e. having a zero width, the above drilling heads still 
encounter problems connected with their centering capability as well as 
with their cutting at the center of the drill. 
It is therefore the object of the present invention to provide a new 
drilling head having advantages in the above respect. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a drilling head 
having a cylindrical body with a longitudinal axis, tapering towards the 
leading end thereof, said drilling head comprising at least two chip 
evacuation flutes and at least two lands therebetween; 
each flute having a first side wall facing in the direction of rotation of 
the drilling head and a second side wall diverging outwardly from the 
first side wall and merging therewith, adjacent the leading end of the 
drilling head, via a web-thinning recessed surface extending towards said 
leading end; 
each land being bound by said first wall of one adjacent flute and said 
second wall of another adjacent flute and having a generally radially 
extending and axially inclined front flank face, tapering towards said 
leading end; 
said front flank face of each land intersecting the first wall of said one 
flute at a primary cutting edge, intersecting the web-thinning recessed 
surface of said one flute at a secondary cutting edge, and intersecting 
the front flank face of the other land at a chisel edge which is disposed 
at the leading end of the drilling head and which passes through said 
longitudinal axis; 
characterised in that 
said chisel edge is concave and merges with the secondary cutting edges of 
the two lands via axially outermost extremities thereof which are spaced 
from said axis. 
With the geometry of the chisel edge according to the present invention, 
the axially outermost extremities thereof function as `piloting` points 
and enhance the centering capability of the drilling head. Moreover, 
sections of the chisel edge extending between its innermost point and said 
outermost extremities effectively serve as chisel edge cutting sections. 
Preferably, an axial distance between the innermost point of the chisel 
edge and its outermost extremities is substantially less than an axial 
projection of the secondary cutting edges. 
Preferably, the chisel edge is provided with recesses formed in the front 
flank faces of the lands and extending away from the chisel edge, each 
recess providing one chisel edge cutting section with a chip rake surface 
and the other chisel edge cutting section with a relief surface. Still 
more preferably, the portion of each recess which constitutes the chip 
rake surface for its corresponding chisel edge cutting section, merges 
with an adjacent web-thinning recessed surface. Thereby, improved chip 
evacuation from the leading end of the drilling head and, particularly, 
from the chisel edge cutting sections, may be effectively obtained. 
Preferably, the recess merges with the adjacent web-thinning recessed 
surface at a ridge. Preferably, each recess terminates at a surface 
disposed remote from the chisel edge. 
Preferably, the front flank face of each land comprises a major flank 
surface disposed adjacent the first wall of said one adjacent flute and 
extending along the primary and secondary cutting edge and terminating at 
the chisel edge, and a minor flank surface disposed adjacent the second 
wall of said another adjacent flute, said recesses being formed in the 
major flank surface.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1 to 4 illustrate a drilling head according to the present invention. 
The drilling head has a longitudinal axis 0 and is designed to be 
coaxially coupled to a drill in any appropriate manner which does not 
constitute the subject matter of the present invention and, therefore, 
will not be described herein. When the drilling head is mounted on a 
drill, the longitudinal axis 0 constitutes the drill's axis of rotation. 
As seen in FIGS. 1 and 2, the drilling head has a cylindrical body 1 with a 
leading end 2, comprising a pair of identical chip evacuation flutes 4 and 
a pair of identical lands 6. 
Each flute 4 has a first side wall 10 facing in the direction of rotation 
of the drilling head and a second side wall 12 diverging outwardly from 
the first side wall 10 and merging therewith, adjacent the leading end 2 
of the drilling head, via a web-thinning recessed surface 14 extending 
towards the leading end 2 so that the web thickness defined by the 
distance between the web-thinning recessed surfaces 14 of the flutes 4 is 
minimal at the leading end 2. Thus, as seen in FIG. 2, the web thickness d 
at the leading end 2 of the drilling head is much smaller than the web 
thickness D remote therefrom. 
As seen in FIGS. 1 and 2, each land 6 is bound by the first side wall 10 of 
one adjacent flute 4 and the second side wall 12 of another adjacent flute 
4. The land 6 has a front flank face 15 which, as seen in FIG. 3, extends 
generally radially and is inclined to the axis 0, the front flank 15 
tapering towards the leading end 2 of the drilling head. With the geometry 
of the chip evacuation flutes 4 and lands 6 as described above, 
consequently, the drilling head has a shape generally tapering towards the 
leading end 2 thereof. 
As seen in FIGS. 1 and 2, each front flank face 15 comprises a major flank 
surface 16 disposed adjacent the associated first wall 10, a minor flank 
surface 18 disposed adjacent the associated second wall 12, and a bridging 
step 19 therebetween. 
The major flank surface 16 intersects the associated first wall 10 of one 
flute 4 at a substantially concave primary cutting edge 20, the associated 
web-thinning recessed surface 14 of the same flute at a substantially 
convex secondary cutting edge 22, and the major flank surface 16 of the 
other flute at a chisel edge 24 disposed at the leading end of the 
drilling head and constituting its axially outermost region. 
To provide the primary and secondary cutting edges with appropriate cutting 
conditions, the major flank surface 16 is inclined relative to a plane 
normal to the axis of rotation so as to provide these cutting edges with 
required clearance angles. Also, a portion 26 of the first side wall 10 
and a portion 28 of the web-thinning recessed surface 14 of each flute 4 
which are disposed adjacent respective primary cutting edge 20 and 
secondary cutting edge 22 and which constitute chip surfaces thereof, are 
inclined with respect to the axis of rotation 0 so as to provide these 
cutting edges with required rake angles. 
As seen in FIGS. 3 and 5, the chisel edge 24 is concave and has two axially 
outermost extremities A and B which are spaced from the axis 0 and an 
innermost point C which lies on the axis 0. As shown in FIG. 3, an axial 
distance h between the innermost point C of the chisel edge 24 and its 
outermost extremities A and B, is substantially less than the axial 
projection H of each secondary cutting edge 22. The width of the chisel 
edge 24 is defined by the distance between the extremities A and B. 
With the above geometry of the chisel edge 24, the axially outermost 
extremities A and B are capable of fulfilling a piloting function during 
drilling, and, thereby, to facilitate the centering capability of the 
drilling head, without unduly weakening its leading end. In addition, 
curved sections AC and BC of the chisel edge 24 are capable of cutting. 
As illustrated in FIGS. 1 and 2, the chisel edge 24 is provided with 
recesses 30 and 32 formed adjacent thereto in the major flank surfaces 16 
and extending radially away from the chisel edge 24. The recess 30 
provides the chisel edge cutting section AC with a chip rake surface and 
the chisel edge cutting section BC with a relief surface. Similarly, the 
recess 32 provides the chisel edge cutting section AC with a relief 
surface and the chisel edge BC with a chip rake surface. 
Each recess 30, 32 merges, with its portion constituting the chip rake 
surface of the respective chisel edge cutting section AC, BC, with the 
adjacent web-thinning recessed surface 14 at a ridge 34. In addition, with 
reference to FIG. 4, each recess 30, 32 terminates at a surface 36 which 
is disposed remote from the chisel edge 24. The recesses 30, 32 allow for 
chip evacuation from the chisel edge cutting sections AC and BC. 
The drilling head and, particularly, its leading end may have alternative 
designs not described above and not shown in the drawings. Thus, for 
example, the drilling head may be formed integrally with the drill rather 
than being adapted for mounting thereon. The drilling head may have more 
than two flutes and, consequently, more than two lands. The primary 
cutting edges do not have to be concave but rather may be straight or 
convex. The front flank faces of the lands may be continuous and do not 
necessarily have to be divided into minor and major flank surfaces. The 
recesses formed in the major flank surfaces adjacent the chisel edge may 
be of an asymmetric shape. The drilling head may have any other 
alternative features which are appropriate in view of the main concept of 
the present invention. 
LIST OF REFERENCE NUMERALS 
1 cylindrical body of the drilling head 
2 leading end of the drilling head 
4 chip evacuation flute 
6 lands 
10 first side wall of the flute 
12 second side wall of the flute 
14 web-thinning recessed surface 
15 front flank face of the land 
16 major flank surface 
18 minor flank surface 
19 step bridging the major and minor flank surfaces 
20 primary cutting edge 
22 secondary cutting edge 
24 chisel edge 
26 chip surface of the primary cutting edge 
28 chip surface of the secondary cutting edge 
30,32 recesses 
34 ridge 
36 surface of the termination of the recesses 30, 32 
A,B outermost extremities of the chisel edge 
C innermost point of the chisel edge 
O axis of rotation