Drilling tool having frictionally rotatable drilling head

A drilling tool is proposed which is preferably suitable for making apertures in concrete work or masonry is fissured rock. To avoid damaging an interchangeable conveying helix (4), the connection between the conveying helix (4) and drilling head (3) is made as a frictional-resistance connection (FIG. 2).

DESCRIPTION 
The invention relates to a drilling tool having a drill head axially 
supported by an interchangeable conveying helix. 
Drilling tools with interchangeable conveying helices are primarily used 
for making apertures while using electrically or pneumatically driven 
hammer drills. In these tools, the carbide-tipped drilling head is 
designed as a cross drilling head or solid drilling head. Tools of this 
type are shown, for example, in German Offenlegungsschrift 2,639,310, 
German Offenlegungsschrift 3,044,757 or German Offenlegungsschrift DE 
2,543,578A1. 
European patent 0,264,657A1, a counterpart to U.S. Pat. No. 4,852,670, and 
to Federal Republic of Germany Offenlegungsschrift 3,635,538 has disclosed 
a drilling tool having an interchangeable conveying helix in which a coil 
spring is provided between conveying helix and axial supporting ring, 
which coil spring permits certain axial play of the conveying helix 
relative to the drill shank. The conveying helix can move away from the 
drilling head against the force of the spring and can also fully rotate 
freely relative to the drilling head or the drill shank in particular in 
the event of jamming or tilting of the helix in the drilled hole. 
Destruction of the plastic helix can thereby be avoided. 
The contact force, produced by the supporting spring, between the conveying 
helix and the drilling head depends on the loading capacity of the plastic 
helix. If the contact force is set very high, the conveying helix is 
axially displaced and thus the positive-locking connection between 
conveying helix and drilling head is separated only during very high 
loading as a result of tilting or jamming. But this can lead to premature 
destruction of the conveying helix. Conversely, the contact force must be 
at least so large that there is always good positive locking between 
conveying helix and drilling head to bridge the play provided for the 
conveying helix. 
As a result of the drilling-dust grooves which are contained in the 
drilling head and in which the conveying helix is generally anchored in a 
positive-locking manner, the effective contact-pressure area or connecting 
area of the positive-locking connection between conveying helix and 
drilling head is kept relatively small. Here, therefore, high surface 
pressures and thus increased stress on the conveying helix occur. 
Furthermore, in the known arrangement, during sudden stressing of the 
conveying helix caused by jamming or tilting, the positive-locking 
connection is subjected to exceptionally high impact loading, since the 
axial displacement for bridging the play provided cannot make a sudden 
adjustment. This can also result in premature destruction or fracture of 
the conveying helix. 
The object of the invention is to improve a drilling tool of the type 
described above to the effect that the conveying helix is exposed to less 
forces in operation so that the wear on the conveying helix and thus the 
risk of fracture is reduced. 
Starting from a drilling tool of the type designated at the beginning this 
object is achieved by the provision of a spring-loaded conveying helix 
pressing against, supporting, and rotating a drilling head through 
frictional forces. Advantageous and convenient further development of the 
invention are as specified below. 
Compared with known devices, the drilling tool according to the invention 
has the advantage that the life in particular of a plastic conveying helix 
can be considerably increased in certain applications. According to the 
invention, no provision is made for an otherwise customary 
positive-locking connection between conveying helix and drilling head, but 
rather provision is made for a frictional-resistance connection. In this 
respect, the invention is based on the knowledge that it has proved to be 
safe in practice if the conveying helix, even during any slight jamming, 
rotates slightly relative to the drilling head or the mounting shank. This 
rotated position cannot generally affect unimpeded drilling-dust removal 
so that a positive-locking connection between conveying helix and drilling 
head is not imperative. On the contrary, when helices made in particular 
of plastic are used, the crucial factor is that sudden loading of the 
helix is to be avoided as far as possible during jamming or tilting, i.e. 
the helix should immediately disengage without a time lag during stressing 
of this type. However, according to the invention, this is only possible 
with a conveying helix which fully slips immediately and does not first 
have to be axially displaced. The connection between the conveying helix 
and the drilling head has therefore been made as a frictional-resistance 
connection, in which arrangement no significant axial displacement occurs 
between the conveying helix and the drilling head. On the contrary, the 
supporting spring now has the other task of achieving an adequate contact 
force of the conveying helix relative to the drilling head so that the 
conveying helix does not fully slip in normal operation. Tests have shown 
that this type of connection is adequate in practice, this advantage being 
thus associated with extremely careful treatment of the conveying helix. 
The bearing surface between conveying helix and drilling head can be 
designed so as to be flat, conical or even arched. The latter has the 
advantage that a larger bearing surface and thus better frictional 
resistance is ensured. 
To improve the frictional resistance, it can be convenient and advantageous 
for the contacting bearing surfaces between conveying helix and drilling 
head to be roughened. The roughening can, for example, be designed as a 
type of fluting. Furthermore, better frictional resistance can be achieved 
by the additional application of a friction lining or by an additional 
friction disk. 
Furthermore, it is advantageous that the spring force of the supporting 
spring and thus of the frictional resistance is made to be adjustable. 
This can be achieved, for example, by the supporting ring being adjustable 
in its axial position. Supporting springs of different hardness can also 
be used. 
Further features and advantages essential to the invention are obtained 
from the following exemplary embodiments described in greater detail with 
reference to the drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The drilling tool 1 shown in FIG. 1 consists of a drill shank 2 and a 
drilling head 3 which is designed, for example, as a cross drill bit as 
disclosed, for example, by German Offenlegungsschrift 3,426,977. The 
slip-on and interchangeable conveying helix 4 is designed as a plastic 
slip-on helix profiled from the solid, i.e. the conveying helix has its 
own drill helix profile. Solid plastic helices of this type are known, for 
example, from DE 3,614,010.4A1. 
Instead of the solid plastic helix 4, a wound conveying helix can also be 
used provided it addresses the same problem as is addressed by the 
invention. 
The conveying helix 4 is defined at the bottom in the axial direction by 
the supporting spring 5, the longitudinally supporting spring being 
supported on a supporting ring 6. The supporting ring 6 consists of a 
radially expandable, longitudinally slotted supporting sleeve 7 which can 
be snapped into a turn groove in the drill shank 2, the lower part of the 
supporting spring 5, to block the radial expansion of the supporting 
sleeve, surrounding an upper supporting sleeve area 7'. 
The supporting spring 5 has the same winding direction as the conveying 
helix 4 so that additional conveying action can be obtained for drilling 
dust. 
As apparent from FIG. 2a in sectional representation, the conveying helix 
4, in its upper area 8, is axially supported by means of a 
frictional-resistance connection at the contact surface 9. Here, in FIG. 
2a, as an alternative embodiment, the drilling tool is shown as having a 
conical or truncated-cone-shaped contact surface 9'. FIG. 2b shows the 
embodiment which has a flat contact surface 9". The conical design of the 
contact surface 9' has the advantage of a greater connecting area and thus 
an increased frictional-resistance action. Contact surfaces 9'" and 9"" 
surface can also be of arched design as shown in the third and fourth 
embodiments of FIGS. 3a and 3b. The mutual contact surfaces 9 between 
conveying helix 4 and drilling head 3 are conveniently roughened, as best 
seen in FIG. 2a. For this purpose, a type of fluting 10 is schematically 
indicated in the left hand half of FIG. 2a. Another type of roughening can 
also be used to increase the friction moment, e.g. in the form of an 
adhesive or a friction lining adhesively bonded or sprayed on. Also, as 
seen in FIG. 4, showing a still further embodiment, an additionally 
incorporated friction disk 12 can be used. 
The supporting spring 5, with a certain contact force or spring 11, presses 
the conveying helix 4 toward the drilling head 3. During normal loading of 
the drilling tool, this results in a frictional-resistance connection 
between conveying helix 4 and drilling head 3, thus ensuring that the 
conveying helix 4 is reliably driven along in a rotating manner on the 
drill shank 2. The contact spring force 11 can be changed by varying the 
spring hardness of the supporting spring 5. This would be ensured, for 
example, by axial displacability or adjustability of the supporting ring 6 
as indicated by the arrow in FIG. 2b or by springs 5 of different 
strength. 
The jamming action is described in European Patent 0,264,657 which is a 
counterpart to U.S. Pat. No. 4,852,670, and to Federal Republic of Germany 
Offenlegungsschrift 3,635,538. This occurs in particular when making 
apertures in fissured rock. 
As soon as the conveying helix 4 in the drilling tool according to the 
invention jams, the frictional-resistance connection between conveying 
helix 4 and drilling head 3 can be released by slight axial displacement 
so that damage to the conveying helix is impossible. As soon as the 
jamming between conveying helix and drilled hole has been released, the 
supporting spring 5, via the contact spring force 11, again pushes the 
conveying helix 4 to a sufficient extent against the drilling head 3 so 
that a frictional-resistance connection is created at the various contact 
surfaces 9', 9", 9'", and 9"".