Drill

The present invention relates to a drill comprising a shaft with chip-conveying flutes and a drill head with two or more cutting elements. Each of the cutting elements comprises a cutting edge which is delimited between a chip-breaking surface and a relief surface and which, at least in the proximity of the geometrical center or rotational axis of the drill, comprises a curved portion. The cutting edge of the individual cutting elements is located with its curved portion in such a way that a tangential point on a straight line that extends from the central axis and tangentially touches said curved edge portion, is provided distantly from the central axis of the drill. In the immediate proximity of this central axis, the cutting edge is terminated in a small material portion which is common for all cutting edges and which extends between the cutting elements in order to serve for centering the drill.

BACKGROUND AND SUMMARY OF THE PRESENT INVENTION 
The present invention relates to a drill comprising a shaft with 
chip-conveying flutes and a drill head at the front of the shaft, with two 
or more cutting elements. The cutting elements comprise at least one 
cutting edge which is delimited between a chip breaking surface and a 
relief surface and which, at least in the vicinity of the geometric center 
or rotational axis of the drill, comprises a curved section. 
A drill of this type is previously known from SE-B-440 324 (and 
SE-A-7812393-2). This known drill has good cutting ability and enables 
drilling with fast feed. However, in practice it is marred by the 
disadvantage of an inferior centering ability. In relation with a 
reference plane that cuts the central rotational axis of the drill at an 
acute angle of 60.degree. to 70.degree. to a main plane that is parallel 
with the straight main sections of the cutting edges (or about centrally 
in the sectors being defined by the ends of the chip-conveying flutes), 
the curved section of each cutting edge protrudes a bit past this 
reference plane, at the same time as the relief surface of each cutting 
element, which relief surface is situated behind the cutting edge in the 
direction of rotation, extends as an uninterrupted surface up to the 
curved section of the cutting edge. Moreover, it is preferred that the 
curved section starts to curve in the immediate proximity of the central 
axis of the drill. This geometry produces two cutting edges that form a 
chisel-like configuration with a pronounced longitudinal extension in the 
area where the edges converge. When this chisel configuration is put 
against a work piece for drilling and entering into the same, it tends to 
slide transversely upon the surface of the workpiece. 
In practice, the entering surface of the workpiece is rarely perfectly 
plane and rarely oriented absolutely perpendicularly to the rotational 
axis of the drill. On the contrary, the entering surface is often rough or 
uneven and can be inclined locally or entirely one or more degrees in 
relation to the drill axis. Also with a very forcible placing of the drill 
against the workpiece, the chisel configuration of the drill tends to move 
at least a little transversely, without any capacity of finding a distinct 
centration. This tendency is particularly pronounced just at the entering 
of the workpiece, but the generally bad centering ability is also present 
when the drill head has worked its way into the workpiece, causing 
vibrations in the workpiece and in the drill itself. These inconveniences 
also become noticeable when working with relatively long drills (drills 
whose length is equal to or exceeds 3.5 times the diameter of the drill), 
particularly in connection with crank fixations of the workpiece. When 
vibrations arise, these negatively influence the quality of the drilled 
hole and the durability of the drill. 
One object of the present invention is to eliminate the above mentioned 
inconveniences of previously known drills and create a drill with good 
centering ability. Thus, a primary object of the present invention is to 
confer to the head of the drill such a geometry that it becomes 
self-centering at the placing against and the initial penetration of the 
drill into a workpiece. The drill also maintains a good centering ability 
during the continuing penetration of the workpiece, whereby the good 
centering ability shall be possible to achieve without the necessity of 
large feeding or placing forces. Another object of the present invention 
is to attain a good centering ability without the necessity of 
substantially reducing the active cutting edge length. 
According to the present invention, at least the primary object is brought 
about by the drill as set forth in claim 1. Further preferred embodiments 
of the present invention are defined in the dependent claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The drill shown in FIGS. 1 and 2 comprises a shaft 1 and a drill head 
designated in its entirety by 2. Two helical or screw-formed flutes 3,3' 
are formed in the shaft 1, which flutes are delimited by analogous, 
helically formed protruding lands 4,4'. The drill head 2 comprises two, in 
the present case identical, but inverted cutting elements 5,5' which 
extend in each other's extension in a common main plane A--A that cuts the 
central or rotational axis of the drill (in FIG. 3 this axis is designated 
by a C). In FIG. 2, a reference plane B--B also extends through the 
central axis of the drill. However, the reference plane B--B extends at an 
acute angle in relation to the main plane A--A, more specifically at an 
angle of about 68.degree.. In this embodiment, the cutting elements 5,5' 
are made as parts of a common cutting body, e.g., of cemented carbide, 
which has been secured upon the shaft by a soldered or brazed joint 6 and 
thereafter ground to its final shape, as shown in the drawings. Although 
this embodiment is preferred in practice, it is within the scope of the 
present invention to form the two cutting elements 5,5' of the drill head 
as an integral part of the drill shaft 1 as such, i.e., as ground parts of 
the drill shaft. 
Each individual cutting element 5,5' comprises a cutting edge designated in 
its entirety by 7, which is generally delimited between, on the one hand, 
a chip-breaking surface 8 and on the other hand, a relief surface 9. In 
relation with the total width of the cutting element, the relief surface 9 
has a reduced width by the fact that the piece of which the cutting head 
has been produced, has a countersunk surface 10 in the area behind the 
relief surface, seen in the direction of rotation of the drill. By the 
reduction of the width of the surface 9 to about one-half of the total 
width of the cutting head, the necessary grinding of the relief surface is 
simplified and speeded up. 
Each individual cutting edge 7 comprises a main section 11 which in the 
present case is substantially straight and which extends from the 
periphery of the drill head and which continues into a curved section 12 
closer to the central axis of the drill. The chip surface 8 is also formed 
with a restricted width in order to simplify grinding of the same. For 
this reason, a recess is formed in the blank of the drill head, in the 
area below the peripheral part of the chip surface. This recess is 
delimited on the one hand by an inclined plane surface 13 and on the other 
hand by a vaulted surface 14. At its peripheral end, the cutting edge 7 
continues into an edge section 15 that is substantially axial, behind 
which section there is a flange surface 16 which has a restricted width by 
the fact that the end surface of the drill head has been formed with at 
least one countersunk surface 17 in the area behind a delimiting line 18. 
By the existence of this axial edge section, it is possible to grind the 
cutting elements without changing the effective diameter of the drill 
head. 
With reference to FIGS. 1 and 2, it should be noted that each individual 
chip-conveying flute 3 ends in a restricted space (19 in FIG. 1) in the 
area between the cutting elements 5,5'. More specifically, this restricted 
space 19 is situated between on the one hand the curved cutting edge 
section 12 of the one cutting element, and on the other hand the inner end 
of the countersunk relief surface 10 of the other cutting element. This is 
true for both sides of the central main plane A--A. 
FIGS. 1 and 2 provide a general overview of the configuration of the drill 
according to the present invention. Reference is now made to FIGS. 3 to 
10, which give a more detailed explanation of the geometric form of the 
drill. 
In FIG. 3, D--D is an imaginary plane extending through the tip of the 
drill, perpendicularly to the central axis C. The angle .alpha. between 
this transversal plane D--D and the straight cutting edge 11 according to 
the shown embodiment amounts to around 20.degree., which means that the 
tip main angle of the drill amounts to about 140.degree.. This is a 
conventional tip angle which can vary upwards and downwards. In this 
connection it should also be mentioned that the flange surface 16 is 
somewhat inclined in relation to the central axis C. More specifically, 
the central axis may be inclined in such a way that the rear section of 
the surface is situated somewhat nearer to the axis C than the fore 
section (not visible on the drawings), thus conferring a certain clearance 
to the surface. In practice, this clearance or relief angle (not shown) 
can be extremely small. 
With reference to FIG. 4, the primary relief surface 9 of each cutting 
element 5,5' comprises a secondary, ground relief surface 20 in the area 
of the curved cutting edge section 12, which surface is delimited by a 
break or delimiting line 21 which in the present case is substantially 
straight. 
With reference to FIG. 6, the general tip geometry of the present invention 
is illustrated. This figure shows the tip portion of the drill head 
amplified 8 times in comparison with FIG. 4 and illustrates how the curved 
section 12 of the cutting edge 7 extends along the curved line of a circle 
sector from the straight main section 11. The circle sector has the circle 
angle .beta. which in the shown embodiment is about 68.degree. (the 
complementary angle .psi. is thus about 22.degree.). Further according to 
the illustrated embodiment, the curved cutting edge section 12 continues 
into a second, straight cutting edge section 22 in the proximity of the 
drill central axis C. The reference plane B--B which was mentioned in 
connection with FIG. 2, extends according to the shown example obliquely 
towards the main plane A--A, more specifically at an angle that 
corresponds with the sector angle .beta., i.e., about 68.degree.. In a way 
that is characteristic for the present invention, the cutting edges 7,7' 
of the two cutting elements are so formed near to the center of the drill, 
that the inner, straight cutting edge sections 22,22' are distanced 
outwardly from the reference plane B (or backwardly from said plane when 
seen in the direction of rotation of the drill), whereby the cutting edges 
in the immediate proximity of the central axis of the drill are terminated 
in a common, diminutive material part 23 that extends between the two 
cutting elements 5,5' in order to serve as a drill-centering center punch. 
Hence, each curved cutting edge section 12 is so located that a tangential 
point 24 on an imaginary straight line E (shown as a dashed-dotted line in 
FIG. 6), which line E cuts the central axis C of the drill, is commonly 
touched by the curved cutting edge sections 22,22' of the two cutting 
elements. That tangential point is distanced from the central axis C of 
the drill. 
It can be seen in FIG. 6 that the straight cutting edge section 22 next to 
the drill's central axis extends substantially parallel to the reference 
plane B--B, whereby an imaginary extension line 25 to the cutting edge 
section 22 on the cutting element 5 extends parallel to the analogous 
straight cutting edge section 22' on the other cutting element 5' and is 
located at a certain distance a from the latter. In the illustrated 
embodiment, the distance a amounts to about 0.3 mm, although smaller and 
larger values are also feasible. However, in practice, the distance a 
should be between 0.05 and 0.50 min. 
The central punching section 23 comprises a break line 26 in the absolute 
center of the drill. The break line is formed at the grinding of the 
relief surfaces 9 of the respective cutting element. The measure b of the 
break or edge line 26 should be smaller than the gap a and amounts to 
about 0.1 min. Although this width measure of the center punch section may 
vary, it should be within the range of 0.05 to 0.25 mm. The measure c, 
which marks the distance between the points at which the straight cutting 
edge sections 22,22' transpose into the center punch section 23, can be 5 
to 10 times larger than the measure b. It should also be mentioned that 
the measure a preferably is 2 to 4 times larger than measure b. 
Although material section 23 thus has relatively small dimensions, it 
nevertheless forms a distinct center punch that hits the surface of the 
workpiece first and gets a hold in it. In practice, the relief angle 
.epsilon. of the surface 9 (see FIG. 8) amounts to about 8.degree.. 
Therefore, if the surface 9 were to be formed as an uninterrupted plane 
surface from its peripheral end to the curved cutting edge section 12 at 
the opposite, inner end, the curved cutting edge section could be located 
on the same level as the center punch 23, seen in the feeding direction of 
the drill. Particularly, the outer part of the curved cutting edge 
section, i.e., the section that is distanced from the main plane A--A 
could be arranged at the same level as the center punch. 
In order to avoid this, the previously mentioned, secondary relief surface 
20 is ground into the surface 9 adjacent to the curved cutting edge 
section 12. As can be further seen from the cross-section in FIG. 9, which 
is enlarged in comparison with FIG. 8, the secondary relief surface 20 
forms an angle .lambda. with the primary relief surface 9 that can amount 
to about 13.degree., although smaller and larger angles are also feasible. 
By grinding the surface 20, the drill head will also have an acute angle 
in a direction transverse to the main plane A--A. More specifically, each 
of the two straight, inner cutting edge sections 22,22' will form a 
certain angle .omega. with the transversal plane D--D, as can be seen in 
FIG. 10. In practice, the angle .omega. can be about 5.degree., which 
means that the secondary tip angle in the plane B--B will be about 
170.degree.. Of course, the angle .omega. can be smaller or larger than 
5.degree.. However, the thus formed secondary tip angle should be between 
160.degree. and 179.degree., suitable between 165.degree. and 
175.degree.. 
The cutting edge 7 on each cutting element does not have the form of a 
completely sharply ground edge line between the chip surface 8 and the 
relief surface 9. On the contrary, the cutting edge comprises a 
reinforcing face 27 (see FIGS. 6, 8 and 9) consisting of a special ground 
surface with an extremely small width in the area between the chip surface 
and the relief surface. In practice, the width d (see FIG. 8) of this 
reinforcing face can lie within the range of 0.01 to 0.30 mm, preferably 
around 0.10 mm. The width of the reinforcing face can vary along the 
cutting edge. Due to the existence of this reinforcing face, the cutting 
edge gets a considerably larger strength and longer durability than a 
sharp-ground cutting edge which is more apt to damages. 
In accordance with a preferred embodiment of the present invention, the 
cutting edge is formed with a varying chip angle along its extension from 
its peripheral section to its inner end adjacent to the center punch 
section 23. More specifically, the cutting edge has a negative chip angle 
in the area of its inner straight section 22 and along the innermost part 
of its curved section 12. As can be seen from the cross-section IX--IX in 
FIG. 9, which is cut around the area between the straight section 22 and 
the curved section 12, the cutting edge in this area has a negative chip 
angle .tau. of about 5.degree.. However, in the cross-section VIII--VIII 
(FIG. 8), which is taken at a larger distance from the center axis of the 
drill along the curved cutting edge section, the chip angle .tau.' is 
positive. However, this positive chip angle .tau.' should amount to 
maximally about 10.degree.. Such a chip angle of about 10.degree. can 
advantageously be formed along the whole extension of the straight main 
section 11 of each of the cutting edges. Hence, the chip angle is 
gradually changed from a negative angle in the area next to the center of 
the drill, to an increasingly more positive chip angle, from a turn point 
defined at a predetermined location along the curved cutting edge section 
12. Since the chip angle thus is negative adjacent to the center of the 
drill, the cutting edge is extraordinarily strong in the area where it is 
submitted to the largest strain. 
When the described drill is brought into contact with a workpiece, the 
center punch 23, which is formed as a distinct tip, will first enter into 
the workpiece, whereafter the two cutting edges, starting with the two 
inner cutting edge sections 22,22', which are straight and behind the 
reference plane B--B in the direction of rotation, gradually become active 
radially outward from the central axis C. This action continues until all 
the cutting edges are fully engaged with the workpiece. This gradual 
engagement is considerably simplified by the fact that the drill tip has a 
tip angle not only in the main plane A--A, but also a certain secondary 
tip angle in the reference plane B--B, as shown in FIG. 10. In this way, 
the central pressing zone of the drill is minimized when entering the 
workpiece, thus reducing the necessary feeding or pressing force. Tests 
have indicated that the feeding force can be considerably reduced in 
comparison with the force that has been previously considered necessary. 
Moreover, the center punch section 23, in combination with the geometry in 
general, guarantees that the drill is centered in a reliable way, not only 
at the entering of the workpiece but also during the continued drilling in 
the same. Also, at relatively difficult fixations of the workpiece, the 
drill becomes auto centering, hence minimizing any tendency of vibrations. 
This improves the quality of the drilled hole and increases the durability 
of the drill quite considerably. 
Obviously, the invention is not restricted to the embodiment described 
above. Thus, it is not absolutely necessary to make section 22 of the 
cutting edge, which section extends from the tangential point 24 to the 
central center punch section 23, as an absolutely straight cutting edge 
section. Within the scope of the invention, this section can also be more 
or less arch-formed. The only essential point is that the cutting edge 
section in question be located in the area behind the reference plane 
B--B. Further, although the described embodiment comprises a drill head in 
the form of a cemented carbide body that is fixedly joined with the shaft 
by a soldered or brazed joint, the invention also foresees detachable 
cutting bodies, for instance, by being provided with a central tap which 
can be introduced into a central seat at the end of the shaft and can be 
fixed thereto by one or several screws which can be tightened from the 
side of the shaft. Further, it should be noted that the cutting element 
can be formed with a broken or divided cutting edge, although the 
embodiment shown in the drawings comprises a continuous cutting edge on 
each cutting element 5,5'. Thus, the individual cutting element can have 
its main part 11 or parts of it countersunk in relation with the cutting 
edge in general, i.e., retracted a bit rearwardly in axial direction from 
a cutting edge section including the curved section 12. Further, it is 
understood that the invention is not restricted to drills comprising 
precisely two cutting elements. Thus, the invention can also be applied to 
drills with several cutting elements, i.e., with three or four cutting 
elements. 
The principles, preferred embodiment and mode of operation have been 
described in the foregoing specification. However, variations and changes 
may be made that fall within the scope of the appended claims.