Drilling tool for producing an undercut bore

A drilling tool including a central, laterally flattened primary drill adapted to produce a cylindrical main drillhole and a secondary drill, pivotally displaceable relative to the primary drill, in a laterally outward direction to produce an undercut, a sleeve which concentrically surrounds the primary drill and is axially slidable on the shaft of the primary drill against the action of a spring and a slotted link system adapted to effect pivoting of the secondary drill relative to the primary drill. The sleeve is constructed as a cage having a rear portion adapted to guide it for axial movement along the shaft, webs extend from the rear portion in the general direction of the shaft axis and leave large passages in the axial and circumferential direction, and an end ring joins the webs at their outer ends.

This invention relates to a drilling tool for producing an undercut 
drillhole in soft masonary, especially gas concrete, comprising a central, 
laterally flattened primary drill for producing a cylindrical main 
drillhole and a secondary drill, pivotally displaceable relative to the 
primary drill, which secondary drill can be laterally pivoted outwards, to 
produce the undercut, from a sleeve which concentrically surrounds the 
primary drill and can slide axially against the force of a spring on the 
shaft of the primary drill, the pivoting being effected by means of a 
slotted link system. 
Such a drilling tool, albeit for drilling in stone and hard masonry, is 
known, for example, from German Offenlegungsschrift 2,331,467. The 
drillhole produced with a drilling tool of this type, the conical portion 
of which diverges in the drilling direction, serves to receive positively 
an anchor for a fastening element which is connectable thereto. Whilst for 
anchoring in hard stone and masonry the frictional locking of a radially 
expansible dowel in general suffices, a positively locking, conical anchor 
has proved to be superior in lightweight building materials. 
The drilling tool known from German Offenlegungsschrift 2,331,467 has, for 
a number of reasons, not been able to find practical acceptance. Its 
primary drill is unsuitable for working in lightweight building materials. 
Above all, the removal of the drillings is substantially impossible, even 
in the case of a hard material. 
It is therefore the object of the invention to provide a properly 
functional drilling tool of the type described in the preamble above, for 
working in lightweight building materials, that is to say, in particular, 
in soft masonry, such as gas concrete. 
This object is achieved, according to the invention, if the sleeve is 
constructed as a cage having a rear portion serving to guide it on the 
drill shaft, narrow webs extending from the rear portion in the general 
direction of the shaft axis and leaving large passages between the webs in 
the axial and circumferential directions, and an end ring which joins the 
webs at their other ends. 
In this way, the drillings which build up are provided with a path of 
sufficiently large dimensions to the exterior even if, when starting to 
drill the cone, the sleeve rests against the wall or the like which forms 
the workpiece, without interfering with the controlling function of the 
sleeve. 
The removal of the drillings, and the proper functioning of the sleeve (and 
hence of the entire drilling tool) are further improved if the front end 
ring (viewed in the drilling direction) of the sleeve, which serves as the 
engagement stop of the sleeve against the workpiece, is not constructed to 
be rotatable relative to the remainder of the sleeve, but instead, 
according to a further feature of the invention, an outer collar of the 
rear portion together with the webs and the end ring is mounted so as to 
be rotatable relative to an inner ring of the rear portion. In particular, 
a ball bearing race may be located between the collar and the inner ring. 
Preferably, the primary drill consists of a flat core drill with a 
centering tip and with radially extendible cutting edges on each side of 
the tip. The customary forward curvature of these cutting edges ensures 
that the drillings are accelerated, as if by an axial centrifugal pump, 
towards the cage which forms the sleeve. 
Further, it has proved advantageous if the secondary drill consists of two 
plates located one on each side of the primary drill and pivotable 
outwards in opposite directions. Since the primary drill is flat, the 
lateral offset of the two plates which form the secondary drill is 
immaterial in practice. On the other hand, contrary to the known drill 
mentioned above, with the provision of a secondary drill guided in a slot 
of the primary drill, there is no danger that drillings will jam the 
secondary drill relative to the primary drill. Furthermore, the 
diametrically opposed attack of the two secondary drill portions leads to 
a symmetrical load on the drilling tool and to a cleaner drillhole than 
when using the prior art secondary drill which is pivotable outwards on 
(only) one side.

The primary drill, in the form of a core drill 3, is fixed to the central 
shaft 1 of the drilling tool, which can be clamped by means of the shaft 
portion 2 in a drilling machine. The primary drill consists, as is made 
clear in particular by FIG. 2, of a relatively thin plate of substantially 
rectangular shape which, however, narrows slightly towards the rear end 
when viewed in the drilling direction. It carries at the front end, a 
centering tip 4 and, on each side thereof a radially extending cutting 
edge 5 which is slightly curved forwardly (FIG. 2). 
On each of the two large faces of the core drill 3, a cutting edge plate 6 
is mounted so as to be pivotable about the axis of a bolt 7 which passes 
through the core drill 3. These cutting edge plates can be pivoted 
outwardly in opposite directions as indicated by arrows 8, and together 
they form the secondary drill. Along their outwardly directed lateral 
edges, which in the rest position (FIG. 1) extend approximately parallel 
to the main axis 9 of the drilling tool, there are provided cutting edges 
10 which, in cross-section, are of similar shape to the cutting edges 5 of 
the primary drill, and are chamfered in the region of the front ends 11. 
A sleeve in the form of a cage 12 is guided on the shaft 1 so as to be 
slidable in the direction of the main axis 9. A compression spring 13 
urges the cage 12 axially into the rest position shown in FIG. 1. The cage 
12 consists of a rear portion 14, three webs 15, arranged symmetrically 
about the main axis 9, and an end ring 16 which joins the free ends of the 
webs and faces the working zone of the drilling tool (FIG. 3). This sleeve 
or cage 12 is preferably made of plastics material. The rear portion 14 
consists of an outer collar 17, which carries the webs 15 and is mounted 
by means of a ball bearing race 18 so as to be rotatable relative to an 
inner ring 19 of the rear portion 14. 
The inner ring 19 of the cage 12 locates several balls 20 which are 
distributed about its periphery and which co-operate with the shaft to 
ensure easy axial slidability of the cage 12 thereon. In the direction of 
the working zone of the drilling tool, the inner ring 19 is extended as a 
slotted sleeve, which defines, within the cage 12, two arms 21 which 
extend one on each side of the primary drill 3 and substantially parallel 
thereto. The free ends of these arms are linked by a pin 22, which passes 
through a longitudinal slot 23 in the primary drill 3, so that the 
longitudinal slidability of the cage 12 against the spring 13 on the shaft 
1 is not impaired. 
The pin 22 forms the control guide of the slotted link system, which 
controls the outward pivoting of the secondary cutting edge drill plates 
6. For co-operation with the pin 22 slots 24 are provided in the plates 6 
and, in the rest position of the secondary drill plates, extend at an 
incline to the main axis 9. Thus if the cage 12 is caused to slide on the 
shaft against the action of the spring 13, that is to say upwards in FIG. 
1, the slots 24 co-operate with the pin 22, to cause rotation of the 
slotted zone towards the main axis 9, as a result of which the other ends 
of the plates 6 are pivoted outwards in the direction indicated by the 
arrows 8. 
In the illustrative embodiment of FIGS. 4 and 5, it is only the 
construction and hinging of the secondary drill plates which is different. 
The pin 22 which links the arms 21 of the cage 12 in this case forms the 
pivot bearing for the secondary drill plates 6, which are suspended from 
the pin 22 by one of their ends. The pin 22 therefore passes through only 
one longitudinal slot 23 in the primary drill 3. The latter further 
carries a centrally located pin 25 which passes through the slots 24, 
extending at an incline to the main axis 9 in the drill plates 6 and, 
acting as the control guide, co-operates with these slots in the manner of 
a slotted link system. 
In contrast to the first illustrative embodiment, the secondary drill 
plates 6 are in this case L-shaped and have a cutting edge extending 
parallel to the slot 24 so that the undercut which results is not conical, 
as in the first case, but instead is cylindrical. This results from the 
fact that in the first described embodiment of the invention, the 
secondary drill plates move axially with the primary drill during the 
undercutting but in the case of the latter embodiment, the secondary drill 
plates are held axially fixed during the undercutting procedure. 
In this second embodiment of the invention, the secondary drill plates 
simply move arcuately outwardly about the pin 22 which is axially fixed 
relative to the cage 12, as a result of movement of the pin 25 along the 
slots 24 in the secondary drill plates. The latter movement results from 
the axial movement of the primary drill relative to the cage and thus 
relative to the secondary drill plates.