Drilling tool

A drilling tool for drilling bores in solid metal, with a shank having at its front end several geometrically similar indexable inserts arranged at various radial distances and staggered with respect to each other by the same peripheral angle, whose working areas overlap, each of them having two equally long cutting edges, two of the respectively contiguous cutting edges inclined with respect to each other at an obtuse angle being simultaneously engaged, whereby the radial inner indexable insert extends slightly with one of its engaged cutting edges beyond the drill axis, at least one of the bisectors of the two pair set cutting edges of indexable inserts being inclined with respect to the longitudinal shank axis by an angle up to 25.degree. m and at least one of the bisectors running at an angle differing from zero with respect to another bisector, so that the sum total differing from zero of the radial force component (F.sub.R) resulting from all engaged effective cutting edges and the corresponding sum of the horizontal force components (F.sub.H) form a general resultant (F.sub.RES) acting upon the cutting edges and which is oriented in the direction of the radially utmost indexable insert at an angle (.beta.) differing therefrom by maximum 45.degree..

CROSS REFERENCES TO RELATED APPLICATIONS 
This application is a national phase of PCT/DE 91/00425 filed May 22, 1991 
and based upon a German application P 4018043.3 filed Jun. 6, 1990 under 
the International Convention. 
FIELD OF THE INVENTION 
The invention relates to a drilling tool for drilling bores in solid metal, 
with a shank having at its front end several indexable inserts arranged at 
various radial distances and staggered with respect to each other by the 
same peripheral angle, their working areas overlapping, whereby the 
radially inner indexable insert slightly extends beyond the drill axis 
with its engaged cutting edge. 
BACKGROUND OF THE INVENTION 
It is known to the state of the art that depending on the arrangement of 
the indexable inserts the asymmetrical cutting force conditions lead to an 
unfavorable radial action of the force on the drill shank, which can cause 
the drilling to deviate. For this reason, many times the operation is 
performed with one or more support rods fastened to the drill shank, which 
support the drilling tool against the bore walling. The drawback is that 
the support rods are highly subjected to wear and besides can not prevent 
the machine support of the drill from being exposed to a heavy load. Also 
surface quality of the bore is very defective. 
Therefore the DE 27 30 418 C2 proposes a drilling tool wherein the bisector 
of the engaged cutting edge of each indexable insert is arranged 
parallelly to the drill axis. As cutting inserts, preferably, indexable 
inserts, are used, wherein at each second cutting edge the neighboring 
cutting edges define an angle of 156.degree.. However, a force balancing 
of the drill shank in radial direction, which is supposed to be 
accomplished by this drilling tool, is only then possible when each time 
both engaged cutting edges are basically engaged over their full length. 
In other words, the working areas are not allowed to overlap. Such 
structure has the disadvantage that when the diameter of the bore is 
changed, a new set of cutting inserts has to be used. 
In order to eliminate this disadvantage, the EP 0 054 913 B1 proposes that 
the mounting for the individual indexable inserts be arranged so that the 
radial cutting force components of the engaged cutting edge are balanced 
for each individual indexable insert, in that the bisectors of these 
cutting edges are inclined with respect to the parallel to the drill axis 
by an angle which depends on the effective length of the cutting edge 
engaged along a partial segment. 
In order to balance the radial forces, in DE 27 51 255 C2 it has also been 
proposed to bend the radial plane of the inner cutting insert towards the 
rear, against the rotating direction of the drill, by a certain angle with 
respect to the peripherally arranged cutting insert. However, as a result 
the forces are merely parallelly oriented, but not balanced, since the 
angle of displacement can not have any influence on the size of the radial 
or cutting forces. This way the forces are maintained as a function of the 
cutting conditions and are independent from the geometry of the tool. 
Finally in the DE 38 02 290 C1 dealing particularly with drilling through 
stacks of plates but also through a single piece solid metal material, it 
has been proposed to arrange a first indexable insert radially outside on 
a diameter beyond the half defined by the drill axis and on the opposite 
side to arrange two smaller indexable inserts, whereby the effective outer 
cutting edge of the indexable insert arranged radially outside runs 
approximately on the same working cone and the same maximal radius as the 
radially outer cutting edge of the largest indexable insert. This 
arrangement is also supposed to achieve merely a balance of the radial 
cutting forces. 
OBJECT OF THE INVENTION 
It is the object of the present invention to further develope the 
aforementioned drilling tool, so that all influences beyond the radial 
force components be taken into account during drilling, with the purpose 
to cover the largest possible area of staggered bore diameters with a 
minimum of indexable insert sets, whereby scores are avoided in the bore 
hole. 
SUMMARY OF THE INVENTION 
This problem is solved by a drilling tool according to the present 
invention. Compared to the aforementioned drilling tools according to the 
state of the art, the difference resides in the conscious selection of an 
indexable insert arrangement wherein the sum of the radial force 
components resulting from the total of effective cutting edges in 
engagement does not equal zero. According to the invention, the remaining 
radial force components form together with the sum of all horizontal force 
components an overall resultant acting on the cutting edge, oriented in 
the direction of the radially utmost indexable insert or at an angle 
differing by 45.degree. from the latter, whereby bore holes can be cut 
whose diameter is only slightly larger than the drill diameter. This 
offers the advantage that the drilling tools can not jam in the bore hole 
and that no retraction scores occur when the drill shank is withdrawn. 
Thereby the inclination angle of at least the utmost indexable insert with 
respect to the longitudinal axis of the shank depends firstly on the apex 
angle, respectively apex angles formed by the effective cutting edge, 
respectively cutting edges of each indexable insert, further on the degree 
of overlapping of the working areas of the used indexable inserts as well 
as the drilling diameter, and finally also on the mutual relationship 
between the inclination angle of the bisectors of the used indexable 
inserts. Experience has proven that with the same apex angle of the used 
indexable inserts and with a fixed, preselected angle of inclination of 
the inner indexable insert, which can also be equal to zero, the 
inclination angle of the radially outer indexable insert has to be 
selected bigger when the drilling diameter decreases. The same applies 
also when indexable inserts with larger apex angles are selected. With the 
same apex angle and the same drilling diameter, the inclination angle of 
the inner indexable insert decreases when a larger inclination angle is 
selected for the radially outer indexable insert. 
While according to the aforementioned first solution each time two 
geometrically similar indexable inserts each with two cutting edges of 
equal length have been used, whereby the respective two contiguous cutting 
edges form together an obtuse apex angle, according to a second 
embodiment, the use of at least one indexable insert with a trochoidal 
cutting edge is provided. For the inserting position of this trochoidal 
indexable insert the same applies with respect to the median axis of 
symmetry of this indexable insert as for the aforementioned bisector of 
the apex angle of an indexable insert with two equally long cutting edges. 
The apex angles of the equally long cutting edges of the used indexable 
inserts should be selected between 115.degree. and 160.degree., preferably 
at 160.degree., 156.degree., 145.degree., 125.degree. or 115.degree.. 
Depending on the apex angle, the inclination angles of the bisector with 
respect to the longitudinal shank axis are selected so that with an apex 
angle of 160.degree. the inclination angle of the inner, respectively 
outer indexable insert will be of maximum 8.degree.. With smaller apex 
angles, e.g. 115.degree., the inclination angle, respectively inclination 
angles can be selected up to a maximum of 25.degree.. 
According to a further embodiment of the invention, the bisector or the 
axis of symmetry of the cutting edge(s) of the inner indexable insert is 
inclined by a smaller angle with respect to the longitudinal shank axis 
than the respective angle of the outer indexable insert. Hereby with 
clearly overlapping areas of the indexable inserts the particular case is 
addressed when the inclination angle of the inner indexable insert equals 
zero, and the inclination angle of the outer indexable insert lies between 
3.degree. and 5.degree.. When a total of two indexable inserts are 
arranged on the frontal end of the shank, it is further preferred that the 
inner indexable insert have two equally long cutting edges and the 
radially outer indexable insert use a trochoidal cutting edge. The 
inclination angle of the axis of symmetry of the trochoidal cutting edge 
can reach a maximum of 25.degree., and is preferably each time larger than 
the inclination angle of the bisector of the inner indexable insert. 
Finally the drilling tool can be designed so that the indexable inserts 
have different sizes, respectively cutting edges of different length, seen 
from one indexable insert to the next indexable insert. 
Preferably the length of the effective (engaged) cutting edge of the radial 
utmost indexable insert is by at a range of 8% to 15% longer than the 
length of the effective cutting edge of the radially innermost indexable 
insert. This is particularly the case where on one half of the drill more 
than one indexable insert is mounted and whereby the length of the 
effective cutting edge of the indexable inserts increases from innermost 
to the utmost indexable insert. 
According to a further embodiment of the invention, the overlapping of the 
working areas of the cutting edges can be established also due to the fact 
that the radially inner indexable insert protrudes by a maximum of 0.15 mm 
with respect to the other indexable inserts. Thereby during the start of 
the drilling operation a good centering of the tool results.

SPECIFIC DESCRIPTION 
In FIGS. 1a to b a drilling tool according to the invention is shown to 
whose frontal shank side two indexable insert staggered by 180.degree. are 
mounted. The hexagonal indexable inserts define together an apex angle 
.gamma. of 145.degree.. Each of these indexable inserts has respectively 
cutting edges 11, 12 and 21, 22, whereby however the cutting paths of the 
radially outer cutting edge 12 and the one of the radially inner cutting 
edge 21 overlap. The radially outer indexable insert 20 determines the 
diameter of the bore, which is clearer defined by the bore wall 30. The 
radially inner indexable insert is arranged so that its radially inner 
cutting edge 11 extends slightly beyond the drill axis, respectively the 
longitudinal shank axis 40. In the embodiments of FIGS. 1a and 1b both 
indexable inserts with a respective apex angle .gamma. of 145.degree. are 
inclined with respect to the longitudinal shank axis 40 by an angle 
.alpha., respectively .beta. of 20.degree. respectively at 
9.degree.-10.degree.. When the drilling tool is in use, at each cutting 
edge forces appear which can be taken up basically vertically to the 
cutting edges. In the example of the indexable insert 10 these cutting 
forces are taken up as F.sub.1 and F.sub.2 and can be decomposed into a 
vertical component parallel to the longitudinal shaft axis 40 and a radial 
component F.sub.r12 and F.sub.r11. A corresponding situation results from 
a consideration with reference thereto of the radially outer indexable 
insert 20. In addition to the forces acting vertically with respect to the 
cutting edges, also horizontal cutting forces occur, which can be seen in 
Fig. 1b marked with F.sub.12, F.sub.11, F.sub.21 and F.sub.22. While the 
horizontal cutting forces acting on the cutting edges 11 and 12 of the 
inner indexable insert are approximately equal (FIG. 1b) and correspond 
approximately to the horizontal cutting force F.sub.22 acting on the 
cutting edge 22, the horizontal cutting force F.sub.21 is slightly smaller 
because of the clearly shorter effective cutting edge 21. According to the 
concept of the invention, the inclinations of the bisectors W1 and W2 of 
the two indexable inserts 10 and 20 are selected so that the sum total of 
all radial forces F.sub.r11, F.sub.r12, F.sub.r21 and F.sub.r22 differs 
from zero. In this way, what remains with reference to all engaged cutting 
edges is a residual radial force component which together with all 
horizontal force components forms a resulting total residual force 
F.sub.RES, which differs from zero and which pushes the drill radially in 
the direction 50 of the indexable insert 20 towards the outside, whereby a 
slightly larger drilling diameter than the one corresponding to the radial 
distance of the indexable insert 20 results. This insures that the drill 
can not jam during its advance and also does not cause scoring when 
withdrawn. The angle .delta. formed by the total residual force F.sub.RES 
with the straight line 50 equals a maximum of .+-.45.degree.. 
Instead of the indexable inserts 10 and 20 with an apex angle .gamma. of 
145.degree., other indexable inserts can replace one or both indexable 
inserts according to FIG. 1. Such insertable indexable inserts are shown 
in FIGS. 2a to e with corresponding apex angles of 115.degree., 
125.degree., 145.degree., 156.degree. and 160.degree. or a trochoidal 
cutting edge (FIG. 2e). The above listed apex angles correspond to a 
maximum bisector angle respectively equal to 8.degree., 12.degree., 
15.degree., 20.degree. , or 25.degree.. 
An embodiment variant of the arrangement in FIG. 1 is illustrated in FIGS. 
3a and b. As opposed to FIG. 1, the radially inner indexable insert 10a is 
arranged so that their bisector W1 runs parallelly to the longitudinal 
shank axis 40. Besides, the cutting edge 11 extends beyond the drill 
middle by a far lager segment than the indexable insert 10 in FIG. 1a. The 
radially outer indexable insert, which has also an apex angle 
.gamma.=145.degree., is inclined by approximately 3.degree. with respect 
to the longitudinal shank axis. The two cutting edges 21 of the radially 
outer indexable insert 20a as well as the outer cutting edge 12 of the 
radially inner indexable insert 10a overlap also clearly, so that in 
neither of the indexable inserts a radial balancing of the cutting forces 
takes place. Besides it results here too that the sum total of the radial 
cutting forces differs from zero. The occurring horizontal forces F.sub.H1 
and F.sub.H2 are shown as hatched areas, whereby a resultant horizontal 
residual radial force component tending towards zero results, so that the 
residual radial force forms the total residual force FRES oriented in the 
direction of the indexable insert. 
A corresponding force relationship is also achieved with the arrangement 
according to FIG. 4, wherein the radially inner indexable insert 10b is 
arranged so that its bisector W1 runs parallelly to the longitudinal shank 
axis 40. In opposition thereto, the radially outer indexable insert 20b is 
inclined by an angle .beta. of 5.degree. with respect to the longitudinal 
shank axis. The cutting path of the radially outer cutting edge of the 
indexable insert 10b overlaps clearly with the cutting path followed 
radially by the inner cutting edge of the indexable insert 20b. Thereby 
the effective cutting edge 21b is almost reduced to half. Correspondingly 
lower is also the corresponding horizontal cutting force component 
F.sub.21b. Thereby the horizontal components compensate each other. 
Thereby from the radial force components of the indexable insert 20b a 
common component is formed. An embodiment variant corresponding to FIG. 4 
is shown in FIG. 5, whereby the apex angle of the used indexable inserts 
10c and 20c each have 160.degree.. The bisector W1 of the radially inner 
indexable insert 10c runs parallelly to the longitudinal shank axis 40, 
while the bisector W2 of the radially outer indexable insert is inclined 
by an angle of 9.degree. with respect to the longitudinal shank axis. 
Generally it becomes clear from FIGS. 3, 4 and 5 that with an increasing 
apex angle of the indexable inserts and a constant angle .alpha., the 
inclination angle .beta. of the radially outer indexable insert has to 
increase. 
In all aforementioned embodiment examples the same indexable inserts (with 
identical apex angles) have been used. According to the embodiment shown 
in FIG. 6a, a corresponding effect can be achieved when as a radially 
outer indexable insert a trochoidal indexable insert 23 is used. The 
trochoidal indexable insert 23 is inclined so that its axis of symmetry W3 
defines together with the longitudinal shank axis 40 an inclination angle 
.beta. of 5.degree.. Approximately the same inclination angle .alpha. 
forms the bisector W1 of the radially inner indexable insert 10d, whereby 
the cutting path of the trochoidal cutting edge 23 (see broken line 23') 
clearly overlaps with the cutting path of the radially outer cutting edge 
of the indexable insert 10d. Here too a residual radial force component 
remains, which together with the horizontal residual force component forms 
a general force acting on the cutting edge in the direction of the 
radially outer indexable insert. 
It is possible to use instead of indexable inserts of approximately the 
same size with approximately equally long cutting edges, also indexable 
inserts of different size, whereby the relative load on the cutting edges 
is lower, which has a wear-reducing effect on the cutting edges of the 
indexable inserts and also lowers the breaking risk of the indexable 
inserts. Here too the aim is to achieve an intended, strong overlapping of 
the cutting edge areas, which leads to the formation of residual radial 
force components, which according to the invention are equal but opposite 
to the horizontal residual radial cutting force components. As far as a 
remaining general resultant of radial forces and horizontal forces is 
accepted, this has in any case to be oriented in the direction of the 
radial line of an indexable insert. 
As indicated in FIG. 5a, the radially inner indexable insert protrudes by a 
magnitude s, which does not surpass 0.15 mm.