Drilling tool

A drilling tool comprises a drill tip, a chip removal part which axially adjoins the drill tip, and a drill shank which is positioned at the end of the chip removal part. Two chip flutes extend in a helical manner over the chip removal part starting at the main cutting edges of the drill tip. The drilling tool is separated into two parts in the region of the chip removal part and consists of a base body which is connected to the drill shank as one part and an exchangeable tip made of a harder material which is connected to the drill tip as another part. The exchangeable tip is connectable to the base body in a form-fitting and/or frictional-fitting manner at an axial separation point.

FIELD OF THE INVENTION 
The invention relates to a drilling tool, having a removable drill tip. 
Various connections between the drill shank and drill tip, as well as 
various drill tip features are discussed. 
BACKGROUND OF THE INVENTION 
The twist drill is the most widely used drilling tool for drilling into 
solid material and bores having a diameter up to approximately 18 mm. The 
material used in manufacturing twist drills is alloyed tool steel, 
high-speed steel, and hard metal. If needed, the drill can be coated with 
a wear-reducing layer of e.g. titanium nitride. Wear occurs mainly in the 
vicinity of the drill tip in the region of the main cutting edge and at 
the guide chamfer. In order to eliminate the wear, it has been hitherto 
known to re-grind the drill at the corresponding flanks. In this, it is 
found to be disadvantageous that the drill becomes shorter during the 
grinding operation. In order to avoid this disadvantage, it is further 
known (DE-C-37 09 878) that the cutting part consists of a base body 
connected to the drill shank as one part and an exchangeable tip connected 
to the drill tip as one part, which are connectable to each other in a 
form- and friction-fitting manner at an axial separation point. 
Based on this, it is the object of the invention to improve the known 
drilling tool of the type described above such that a reduction of wear 
and an improvement of the drilling quality can be obtained by simple 
productional measures. 
SUMMARY OF THE INVENTION 
The invention is based on the idea that with a separation into two parts of 
the drill in the region of the chip removal part the drill tip can be made 
of a different, harder material and easily exchanged when used to the 
limit of wear. In order to achieve this, it is proposed that the 
exchangeable tip as a whole consists of a material which is harder than 
the base body. The exchangeable tip advantageously consists of a cutting 
material from a group of hard metals or ceramics and may, in this case, be 
made as a sintered powder injection moulded part. The exchangeable tip can 
also be made of a wear-resistent coated tool steel. The base body on the 
other hand advantageously consists of tool steel or a high-speed steel. 
In a preferred or alternative embodiment of the invention, the main cutting 
edges each have two cutting edges which are formed into the exchangeable 
tip, sloped against each other in a roof shape, and aligned in a generally 
radial direction. The cutting edges of the two main cutting edges can be 
positioned in equal radial distances from the drill axis, forming a double 
cutter, so that they are engaged over their full length during the 
drilling operation. This is advantageous, though, only when one of the two 
inner cutting edges overlaps with the drill axis. The cutting edge corners 
which protrude radially over the outer circumference of the drill tip 
advantageously merge into a generally axially aligned guide edge. The 
guide edge is adjoined in the circumferential direction by a guide rib 
which radially protrudes over the outer circumference and extends over 
part of the circumference of the drill tip. The peak and the outwardly 
protruding cutting edge corners of the two main cutting edges are 
positioned in equal radial distances from the drill axis. Accordingly, the 
outer cutting edges of the two main cutting edges are of equal length, 
while the inner cutting edges are of different length. In a preferred 
embodiment of the invention, the cutting edges and a plane which is 
perpendicular to the drill axis include an angle of 2.degree. to 
30.degree., preferably of 8.degree. to 16.degree., so that the pairs of 
cutting edges of the main cutting edges include a roof angle of 
120.degree. to 176.degree., preferably of 148.degree. to 164.degree.. 
The cutting edges formed into the exchangeable tip may be at least 
partially bevelled and/or rounded-off and possibly wave-shaped. Further, 
indentations, raised portions, steps, or ribs, which preferably reach the 
cutting edges, can be formed into the cutting faces. It is especially 
advantageous for the formation of the chips when chip forming hollows are 
formed into the cutting faces, which are preferably positioned at an axial 
distance with respect to the cutting edges. The chip forming hollows may 
have sides corresponding in shape to the roof-shape of the cutting edges 
at least at their sides adjoining the cutting edges. The cutting faces 
which are generally axially parallel and radially aligned delimit a chip 
space which merges into the chip flutes in the direction of chip travel. 
In order to create the connection between the exchangeable tip and the base 
body, the exchangeable tip is advantageously connected in one piece to a 
coupling piece which protrudes over the side opposing the front rake. The 
coupling piece itself has a driver which meshes with a complementary 
driving part of the base body and may be fitted with an anchoring pin 
which protrudes centrally over the exchangeable tip and which can be 
inserted into the base body and anchored there in a form- and/or 
frictional-fitting manner. Advantageously, the coupling piece has two 
mutually opposed drivers which each mesh with a complementary driving part 
of the base body. 
In order to ensure an exact centering of the exchangeable tip on the base 
body, it is of advantage when the coupling piece has at least two 
partially cylindrical, convex centering sections which are positioned or 
distributed over or about the circumference and which fit exactly into a 
bushing at the base body, which has partially cylindrical, concave 
centering sections which are complementary to the convex centering 
sections. For the rotary drive there are provided at least two axially 
open radial recesses, which are positioned inbetween two adjacent convex 
centering sections of the coupling piece, for the engagement of a driver 
tooth which radially protrudes into the bushing between two adjacent 
concave centering sections of the base body. 
For the frictional connection between the exchangeable tip and the base 
body, it is of advantage when the exchangeable tip has a plane shoulder 
which protrudes generally radially over the coupling piece. The shoulder 
preferably is pressable against a plane face of the base body by means of 
a clamping mechanism. The shoulder and face are preferably each subdivided 
into two areas which are separated from each other in the circumferential 
direction by the flutes. A further improvement in this respect is attained 
in that the coupling piece has an end face which protrudes generally 
radially over the anchoring pin, the end face being pressable against a 
bottom face which delimits the bushing. 
For the creation of a form- and frictional-fitting connection between the 
exchangeable tip and the base body, at least one generally radially 
aligned conical countersink is advantageously positioned in the coupling 
piece, a threaded bolt having a conical tip, which is positioned in a 
generally radial threaded bore in the base body, being engaged in a form- 
and friction-fitting manner in the countersink. The conical countersink is 
advantageously formed into one of the drivers of the coupling piece, while 
the threaded bore penetrates one of the driving pieces of the base body. 
In a further possible connection between the exchangeable tip and the base 
body, the coupling piece of the exchangeable tip has a continuous cross 
bore, through which a clamping screw extends, which is led through a bore 
of the driving part of the base body and which is screwed into a threaded 
bore of the opposing driving part, by which a tensioning between the 
exchangeable tip and the base body without any free play in the axial and 
circumferential directions is created. 
Especially in drilling tools having a very small diameter, in which a 
mechanical connection between the exchangeable tip and the base body is 
difficult, the exchangeable tip and the base body are advantageously 
laser-welded or hard-soldered to each other at their joining locations 
which are positioned between the coupling piece and the bushing. 
In a further advantageous embodiment of the invention, the exchangeable tip 
has an axial snap-in reception which is delimited by a circumferential 
face and is located in the region of the separation point, into which a 
snap-in pin, which protrudes axially over a circumferential shoulder, can 
be inserted and locked by pressing the shoulder against the face. This 
ensures that the torque and retracting forces acting on the drill are 
taken up by the separating point. 
The snap-in pin may carry a catch spring which can be snapped into at least 
one radial undercut within the snap-in reception. Advantageously, the 
catch spring has two catch legs which radially protrude in opposite 
directions of the snap-in pin slanted in the direction of the shoulder, 
the catch legs being engageable into the undercuts of the snap-in 
reception while creating an axial pre-tension. The catch legs may be 
connected to each other in one piece by means of a spring pin which is 
connected to the face side of the snap-in pin. The catch spring can be 
screwed to the face side of the snap-in pin preferably by two screws which 
extend through two holes in the spring pin, or welded, soldered or glued 
to the snap-in pin in the region of the spring pin. It has proven to be 
especially advantageous when the catch spring is welded to the snap-in pin 
by means of laser welding. Additionally, the snap-in pin can have an 
undercut groove for the form-fitting acception of the complementary 
deformed spring pin. In principle it is also possible to directly form the 
catch legs, which are formed to be spring tongues, onto the snap-in pin. 
In a further preferred embodiment of the invention the radial undercut is 
formed by a transverse or slanted opening which penetrates the wall of the 
snap-in reception, wherein the transverse or slanted opening penetrates 
one of the side rakes of the exchangeable tip and the catch leg or the 
spring tongue can be unlocked from the outside through the corresponding 
transverse or slanted opening. 
For the transmission of force and torque in the region of the separation 
plane, it is of advantage when the snap-in reception and the snap-in pin 
have a generally rectangular cross section with two mutually opposing 
broad side faces and small side faces, wherein the broad side faces are 
located adjacent the outer chip surfaces within the chip removal part and 
the small side faces are located adjacent to the outer side rakes. 
In order to save space, when the catch legs overlap the snap-in pin in the 
region of its small side faces in the direction of the shoulder, pockets 
for the reception of the catch legs are left free between the small side 
faces of the snap-in pin and the snap-in reception. On the other hand, for 
the improvement of the rotary drive in the region of the separation plane, 
the snap-in pin is fit into the complementary snap-in reception with its 
opposing, parallel aligned broad side faces. With respect to the slanted 
outwardly directed snap-in legs of the catch spring, it is of advantage, 
when the broad side faces have a generally trapeze-shaped outline and that 
the small side faces are correspondingly positioned in a wedge shape with 
respect to each other. 
In order to achieve an axial friction-fit in the region of the separation 
plane, the catch legs of the catch spring have a snap-in base which can be 
snapped into the undercut under radial spreading and which are supported 
in the snapped-in state on a preferably slanted locking face of the 
undercut. The locking face advantageously includes an angle of 5.degree. 
to 15.degree. with a radial plane which is perpendicular with respect to 
the drill axis. 
In principle it is possible, to form the snap-in pin and the snap-in 
reception rotationally symmetrical with respect to the drill axis. In this 
case at least one fitting bolt is additionally provided radially outside 
the snap-in pin which engages in facing axially parallel fitting bores of 
the base body and the exchangeable tip for the transmission of torque. 
It is further possible to fix the snap-in pin in the snap-in reception by 
means of at least one locking screw which extends through a threaded bore 
in the wall of the snap-in reception. 
In an advantageous embodiment of the invention, the base body and the 
exchangeable tip are connectable to each other in a frictional-fitting 
manner by means of a fitting pin or bolt which is made of a shape memory 
alloy. The fitting pin or bolt can be screwed with its one end into an 
axial threaded bore of the base body and with its other end into a fitting 
bore of the exchangeable tip in a frictional-fitting manner. 
According to an advantageous embodiment of the invention, centering means 
are provided, which center the exchangeable tip on the base body in a 
non-rotatable manner. The centering means can have at least one centering 
cam which is positioned in the snap-in reception and which engages in an 
open-edged opening of the snap-in pin, or at least one fitting bolt which 
bridges the separating point and which engages in aligned fitting bores of 
the exchangeable tip and base body. 
In order to improve the drilling result, at least one coolant bore which 
penetrates the base body and the exchangeable tip axially or in a spiral 
manner and which bridges the separating point may be provided. 
In order to reduce the amount of stock, a base body having a given outer 
diameter may be fitted with exchangeable tips which have differing outer 
diameters.

DETAILED DESCRIPTION 
The drilling tools shown in the drawing are divided into two parts at a 
separation point 30 and consist of a base body 32 which carries a drill 
shank 14 and an exchangeable tip 34 which carries a drill tip 10, which 
are connectable to each other in a form- and frictional-fitting manner 
(FIGS. 1 to 9) or only in a frictional-fitting manner (FIG. 10) at the 
separation point 30. While the base body 32 is made of tool steel or a 
high-speed steel, the exchangeable tip in its entirety is a form part made 
of a cutting material selected from the group of hard metals or ceramics, 
which is manufactured as a sintered powder injection moulded part. In 
principle it is also possible to manufacture the exchangeable tip of a 
tool steel which is coated with a wear-resistant material. 
The drilling tool shown in FIGS. 1 to 4 has a drill tip 10, a chip removal 
part 12 which follows the drill tip and which is possibly formed to be a 
cutting part, and a drill shank 14 which is formed to the rear of the chip 
removal part. The drill tip 10 has two main cutting edges 16 and two front 
rakes 20 adjacent to the main cutting edges. Two helical flutes 22 extend 
from the main cutting edges 20 of the drill tip 10 over the chip removal 
part 12 up to the drill shank. The separation point 30 is located in the 
region of the chip removal part 12. 
The main cutting edges 16 each have two cutting edges 16', 16" which are 
formed into the exchangeable tip 34, pairs of which are positioned in a 
roof shape with respect to each other, and which are generally radially 
aligned. The cutting edges 16', 16" form double cutters and are positioned 
at the same radial distance with respect to the drill axis 78. As can be 
seen especially from FIG. 1b and d, only one of the inner cutting edges 
16' overlaps the drill axis 78, while the other inner cutting edge 16' is 
excentrically adjacent to a step 80. The peak 82 of the roof and the 
outwardly protruding cutting edge corners 84 of the two main cutting edges 
16 are positioned at the same radial distances from the drill axis. The 
outer cutting edges 16" are therefore of equal length and the inner 
cutting edges 16' of different length. The pairs of cutting edges 16', 16" 
include roof angles of 148.degree. to 164.degree. at the peak. They ensure 
that the drill tip is centered in the bore during drilling and does not 
run out. 
The cutting edge corners 84 which radially protrude over the circumference 
of the drill tip merge into a guide edge 86, adjoining which is a guide 
rib 88 which extends over part of the circumference of the drill tip 10 
and which protrudes radially over the circumference. 
The flutes 22 which begin immediately beyond the main cutting edges 16 in 
the direction of chip travel are delimited by the cutting face 90 at the 
side of the cutting edges 16', 16". In the case of FIG. 1a to FIG. 1e, 
chip forming hollows 92 are formed spaced with respect to the cutting 
edges 16', 16". The cutting edge sided borders have a shape that conforms 
with the roof shape of the cutting edges 16', 16" . In the embodiment 
shown in FIG. 2 a plurality of indentations 94 which penetrate the cutting 
edges 16', 16" are provided in place of the chip forming hollows. 
The exchangeable tip has a coupling piece 96 at its side opposing the front 
rakes 20, which has a rotary driver 100 which meshes with a complementary 
centering and driving part 98 of the base body 32. In the embodiments of 
FIGS. 1 and 2 there is additionally provided an axially protruding 
anchoring pin 104 which is insertable into a mounting bore 102 of the base 
body 32 and which can be anchored there in a form- and friction-fitting 
manner. 
The driver 100 has four partially cylindrical convex centering sections 106 
which are spaced from one another circumferentially, which fit exactly in 
a bushing 108 of the base body with partially cylindrical concave 
centering sections 110 which are complementary to the centering sections 
106. The rotary drive is effected by way of the flanks of the axially open 
radial recesses 112 each of which is positioned between two convex 
centering sections 106. A driver tooth 114 engages in each of the radial 
recesses 112, which protrudes radially into the bushing 108 between two 
adjacent concave centering sections 110 of the base body 32. 
The exchangeable tip has a plane shoulder 36 protruding radially over the 
coupling piece 96, which is pressable against a plane face 40 of the base 
body 32 by way of a clamping mechanism acting on the anchoring pin 104 
(FIGS. 1 and 2) or the coupling piece 96 (FIGS. 3 and 4). The shoulder 36 
and face 40 are each divided into two areas 36', 36" and 40', 40" which 
are separated from each other in the circumferential direction by the 
flutes 22. 
The clamping mechanism has a clamping screw 130 shown in examplary fashion 
in FIG. 4a. In the embodiments shown in FIGS. 1 to 3, the clamping screw 
is positioned in a threaded bore 120, 120' of the base body 32 and engages 
in an excentric conical countersink 122, 122' of the exchangeable tip with 
a conical tip. In the embodiment of FIGS. 1 and 2, the conical countersink 
122 is positioned in the anchoring pin 104 which is located in the 
mounting bore 102, while in the embodiment of FIGS. 3a to c the conical 
countersink 122' is positioned in the region of one of the radial recesses 
112 of the coupling piece, whereas the bore 122' penetrates the wall in 
the region of one of the driver teeth 114. 
In the embodiment of FIG. 4a and FIG. 4b the coupling piece 96 of the 
exchangeable tip 34 has a continuous cross bore 132, through which a 
sink-head screw 130, which is led through a continuous countersunk bore 
134 in a driver part 98, 114 of the base body and which is screwed into a 
threaded bore 136 of the diametrically opposed driver part 98, 114, 
penetrates between the exchangeable tip 34 and the base body 32 while 
creating play-free tension in the axial direction in the region of the 
faces 36, 40 and in the circumferential direction in the region of the 
mating flanks 138, 140 of the driver 100 and the driver parts 98. 
The coupling piece 96 further has an end face 116 which faces the bottom 
face 118 of the bushing 108. 
The twist drill shown in FIGS. 5a and b has a drill tip 10, a cutting 
portion 12 following the drill tip, and a drill shank 14 which is formed 
to the rear of the cutting portion. The drill tip has two main cutting 
edges 16, a chisel edge 18 which connects the main cutting edges at the 
tip, and two front rakes 20 adjacent to the main cutting edges and the 
chisel edge. Two helical flutes 22, which are delimited laterally by a 
minor cutting edge 24 with an adjoining guide chamfer 26 as well as by a 
side rake 28, extend from the main cutting edges 20 in the drill tip 10 
over the cutting portion 12. 
The twist drill is divided into two parts at a separation point 30 in the 
region of the cutting portion 12 and consists of a base body 32 carrying 
the drill shank 14 and an exchangeable tip 34 carrying the drill tip 10. 
A variety of coupling means in the region of the separation point 30 are 
shown in FIGS. 6 to 10. In the embodiments of FIGS. 6 to 9 the base body 
32 has a snap-in pin 38 which protrudes over a circumferential shoulder 
36. The snap-in pin 38 can be snapped into an axial snap-in reception 42 
of the exchangeable tip 34, which is delimited by a circumferential face 
40, while pressing the shoulder 36 against the face 40. The snap-in pin 38 
carries a catch spring 44, which can be snapped into two radial undercuts 
48 in the snap-in reception 42 with its catch legs 46 which radially 
extend slanted in the direction of the shoulder 36. The radial undercuts 
48 are formed to be slanted openings in the wall 50 of the snap-in 
reception 42, through which the catch legs 46 can be unlocked from the 
outside. The catch legs 46 have a snap-in base 53 which can be snapped 
into the undercuts 48 under radial spreading and which is axially 
supportable under tension in the snapped-in state on a slanted locking 
face 52. The locking face 52 and a radial plane 54 which is perpendicular 
to the drill axis include an angle .beta. of 5.degree. to 15.degree.. 
In the embodiments of FIGS. 6 to 8 the two catch legs 46 are connected to 
each other as one part by means of spring pin 56 which is connected to the 
front of the snap-in pin 38. In the embodiment of FIG. 6 the catch spring 
44 is welded, soldered or glued with its spring pin 56 to the front face 
of the snap-in pin 38, whereas in the embodiment of FIG. 7 it is screwed 
to the snap-in pin 38 by means of two screws 58. In FIG. 8 the snap-in pin 
has an undercut groove 60 for the form-fitting reception of the 
complementary deformed spring pin 56. 
In the embodiment of FIG. 9a the catch legs 46 are formed onto the snap-in 
pin 38 in the form of spring tongues. 
As can be seen from the section of FIG. 9b, the snap-in reception 42 and 
the snap-in pin 38 have a generally rectangular cross section or outline, 
wherein two mutually opposing broad side faces 62 and small side faces 64 
are formed. The broad side faces 62 each adjoin one of the outer flutes 22 
within the cutting portion and the small side faces 64 each adjoin one of 
the outer side rakes 28. The rotary drive between the base body 32 and the 
exchangeable tip 34 is effected mainly by way of the plane-parallel broad 
side faces 62, with which the snap-in pin 38 is fitted into the 
complementary snap-in recess. Suitable centering means are provided for 
centering the exchangeable tip 34 on the base body 32, which, in the 
embodiment of FIG. 9b, are formed by two centering cams 178 which have a 
semi-circular cross section. The centering cams 178 engage in 
corresponding open-edged openings 180 in the broad side faces 62 of the 
snap-in pin 38. In the embodiment of FIG. 9b two coolant bores 66 are 
additionally provided, which are aligned in pairs at the separation point 
30. 
In the embodiment shown in FIG. 10 the base body 32 has a threaded bore 68 
which is open toward the separation point 30, and the exchangeable tip 34 
has a fitting bore 70 which is aligned with the threaded bore 68. A bolt 
72 is screwed into the threaded bore 68 with its threaded part 74, which 
engages with its thread-free part 76 in the fitting bore 70 of the 
exchangeable tip 34 in a friction-fitting manner. The bolt 72 consists of 
a shape memory alloy. It is fitted into the fitting bore 70 at a low 
temperature and expands in the bore at room temperature to create a 
friction-fitting connection.