A self-drilling screw including a drilling section (1; 4) adjoining a thread section (2; 5) of the screw and formed of first (11; 41) and second (12; 42) parts arranged one after another, within the drilling section (1; 4) having first and second cutting edges (15, 16; 45, 46) a radial extension of which is symmetrical with respect to the longitudinal axis of the screw, in the first, in a drilling direction, part of the drilling section (1; 4), with at least a portion of the second part (12, 42) of the drilling section (1, 4), which is located between the first part (11, 41) and the thread section (2; 5), having in a region of one of the first cutting edges (15; 45), a radial extension (R1; R3) corresponding, at most, to a half of a root diameter (D1; D3) of the thread section (2; 5), and with the second cutting edge (16, 46) having in the second part (12, 42) of the drilling section (1, 4), a radial extension (R2; R4) which is smaller than a half of a pitch diameter (D2; D4) of the thread section (2, 5) and corresponds at least to the half of the root diameter (D1; D3) of the thread section (2; 5).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a self-drilling screw including a head, a 
thread section adjoining the head, and a drilling section adjoining the 
thread section and formed of first and second parts arranged one after 
another, with the drilling section having two, diametrically opposite, 
discharge grooves extending parallel to a longitudinal axis of the screw 
at least along a portion of a longitudinal extent of the drilling section, 
and first and second cutting edges, and with a radial extension of the 
first and second cutting edges being symmetrical with respect to the 
longitudinal axis of the screw in the first, in a drilling direction, part 
of the drilling section, with at least a portion of the second part of the 
drilling section, which is located between the first part and the thread 
section, having in a region of the second discharge groove, a radial 
extension corresponding, at most, to a half of a root diameter of the 
thread section. 
2. Description of the Prior Art 
Attachment elements, which are used for securing, e.g., a thin-wall metal 
constructional part to a constructional component formed as a metallic 
support, can be formed, e.g., as a self-drilling screw of a type described 
in European Publication EP-0 535 193. This self-drilling screw has a 
drilling section which forms a hole in the constructional part and in the 
metallic support, into which a thread section of the self-drilling screw 
can be screwed in. 
The self-drilling screw has, beside the drilling and thread section, a head 
which adjoins the thread section at the side of the thread section 
opposite the drilling section, and which is provided, e.g., with drive 
surfaces engageable with a rotary tool. The drilling section has two, 
diametrically opposite, discharge grooves and two diametrically opposite, 
cutting edges, both extending substantially parallel to the longitudinal 
axis of the self-drilling screw. Both the discharge grooves and the 
cutting edges extend at least along a portion of the longitudinal extent 
of the drilling section. The discharge grooves and the cutting edges 
extend from the free end of the drilling section. The drilling section is 
formed of two, arranged one after another, parts, with the cutting edges 
being arranged, in the first, in the drilling direction, part 
symmetrically with respect to the longitudinal axis of the self-drilling 
screw, and with the second part of the drilling section having a diameter 
which is smaller than the root diameter of the thread section. 
Because of the smaller diameter of the second part of the drilling section, 
no adequate side guidance is provided when the second part of the drilling 
section of the self-drilling screw passes through the holes formed in the 
constructional part and its support. This results in an inadequate 
centering of the following thread section because of, e.g., an eccentrical 
alignment of the self-drilling screw with respect to the holes, or can 
result in an inclination of the self-drilling screw relative to the outer 
surface of the constructional part. This can cause a radial expansion of 
the holes or damage of thread flanks during driving-in of the thread 
section. This adversely affects the holding value of the self-drilling 
screw in the construction part and its support. 
Accordingly, an object of the present invention is to provide a 
self-drilling screw which would insure rapid setting of the self-drilling 
screw without a large expenditure of force, while providing for adequate 
guidance and centering of the drilling section. 
SUMMARY OF THE INVENTION 
This and other objects of the present invention, which will become apparent 
hereinafter, are achieved by providing the second cutting edge, in the 
second part of the drilling section, with a radial extension which is 
smaller than a half of the pitch diameter of the thread section and which 
corresponds at least to the half of the root diameter of the thread 
section. 
The first part of the drilling section primarily contributes to the 
formation of holes in the constructional part and its support. A portion 
of the second cutting edge, which is located in the second part of the 
drilling section, contributes to the hole formation and/or is used for 
side guidance of the self-drilling screw in the holes. 
In order to achieve a stable and uniform guidance of the self-drilling 
screw during the entire time the second part of the drilling section 
cooperates with the constructional part and its support, advantageously, 
the entire second part has, in the region of the first cutting edge, a 
radial extension which corresponds at most to the half of the root 
diameter of the thread section. 
With a self-drilling screw in which, e.g., the first part of the drilling 
section serves for forming holes in the constructional part and its 
support, and the second part provides for side guidance of the 
self-drilling screw in the holes, advantageously, the radial extension of 
the cutting edges in the first part of the drilling section is smaller 
than a half of the pitch diameter of the thread section and is larger than 
the half of the root diameter of the thread section. 
With a self-drilling screw in which, e.g., the first part of the drilling 
section serves for pilot-drilling of the holes in the constructional part 
and its support, and the second part serves for actually drilling the 
holes and sidewise guiding of the self-drilling screw in the holes, 
advantageously, the radial extension of the cutting edges in the first 
part corresponds at most to the half of the root diameter of the thread 
section. 
To insure a good removal of drillings from the drilled holes, 
advantageously, both the discharge grooves and the cutting edges extend 
over the entire drilling section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A self-drilling screw according to the present invention, which is shown in 
FIGS. 1-2, has a head 3, 6, thread section 2, 5, and a drilling section 1, 
4 which adjoins the thread section 2, 5, respectively. The drilling 
section 1, 4 of the self-drilling screw according to the present invention 
is formed of two parts 11, 12; 41, 42, respectively. The first part 11, 41 
extends from the free end of the drilling section 1, 4 up to the second 
part 12, 42 which adjoins the thread section 2, 5, respectively. 
The drilling section 1, 4 has two, diametrically opposite, discharge 
grooves 13, 14; 43, 44 and two, diametrically opposite cutting edges 15, 
16; 45, 46, respectively. The first cutting edge 15 of the drilling 
section 1 extends only along the first part 11 of the drilling section 1 
whereas the second edge 16 extends along the part 11, 12 of the drilling 
section 1. The edge 15' of the second part 12 of the drilling section 1 
,which is located inwardly of the first cutting edge 15 of the first part 
11, is not a cutting edge. In the drilling section 4, it is the first 
cutting edge 45 that extends along both parts 41, 42 of the drilling 
section 4. The edge 46' of the second part 42 of the drilling section 4 is 
offset outwardly with respect to the cutting edge 46 in the first part 41. 
The discharge grooves 13, 14; 43, 44 and the cutting extend over the 
entire drilling section 1, 4, respectively. The cutting edges 15, 16; 45, 
46 extend in the first part 11, 41 of the drilling section 1, 4 
symmetrically with respect to the longitudinal axis of the self-drilling 
screw. The second part 12 of the drilling section 1 has, in the region of 
the non-cutting edge 15, a radial extension R1 which, at most, corresponds 
to a half of the root diameter D1 of the second section 2 and extends over 
the entire second part 12 of the drilling section 1. The second cutting 
edge 16 has, in the second part 12 of the drilling section 1, a radial 
extension R2, which is smaller than a half of the pitch diameter D2 of the 
thread section 2, and is larger than the half of the root diameter D1 of 
the thread section 2. 
The second part 42 of the drilling section 4 has, in the region of the 
first cutting edge 45, a radial extension R3 which, at most, corresponds 
to a half of the root diameter D3of the thread section 5 and extends over 
the entire drilling section 4, and has, in the region of the second part 
46' of the second cutting edge 46, a radial extension R4 which is smaller 
than a half of the pitch diameter D4 of the thread section 5 and is larger 
than the half of the root diameter D3of the thread section 5. 
In the embodiment of a self-drilling screw shown in FIG. 1, the radial 
extension of the cutting edges 15, 16, in the first part 11 of the 
drilling section 1, is smaller than the half of the pitch diameter D2 and 
is larger than the half of the root diameter D1 of the thread section 2. 
In the embodiment of a self-drilling screw shown in FIG. 2, the radial 
extension of the cutting edges 45, 46, in the first part 41 of the 
drilling section 4, corresponds at most to the half of the root diameter 
D3of the thread section 5. 
Though the present invention was shown and described with references to the 
preferred embodiments, various modifications thereof will be apparent to 
those skilled in the art and, therefore, it is not intended that the 
invention be limited to the disclosed embodiments or details thereof, and 
departure can be made therefrom within the spirit and scope of the 
appended claims.