Rock drill bit having retrac teeth and method for its manufacturing

RELATED INVENTION

This application claims priority under 35 U.S.C. §§ 119 and/or 365 to Patent Serial No. 0000688-2 filed in Sweden on Mar. 2, 2000, the entire content of which is hereby incorporated by reference.

1. Technical Area of the Invention

The present invention relates to a rock drill bit for percussive drilling, especially for top hammer drilling, and a method for the manufacturing of a rock drill bit.

2. Prior Art

A percussive drill bit and its associated drill string comprising drill rods shall be removed from a drilled hole after the hole is completed. During removal, the drill bit and the drill string are usually rotated in an opposite direction compared to the direction employed for drilling of the drilled hole. However, the removal of the drill bit and drill string from the drilled hole is impaired by loose rock material from the bore wall. To correct this problem it is customary that the drill bit has at its rear end, i.e., at the end surface facing away from a drill bit front surface, inserts or chisels, hereinafter called retrac teeth, which during rotation of the drill string and the drill bit, in connection with retraction of the drill string and the drill bit from the drilled hole, crush the rock material which has become loosened from the bore wall. The retrac teeth are made by means of special milling operations, which consequently constitutes additional manufacturing operations besides the usual operations for the manufacturing of a rock drill bit of the current type. However, the shaping of these known teeth at the rear end of the drill bit results in the formation of teeth having sharp corners. This promotes the generation of cracks in the teeth during operation of the drill bit.

U.S. Pat. No. 5,743,345 shows in

FIG. 3

thereof a rock drill bit with an associated drill rod, where the drill bit is provided with special cemented carbide buttons at its rear end, which function in a corresponding manner to the above-described retrac teeth. The provision of special cemented carbide buttons at the rear end of the drill bit makes the manufacturing of a rock drill bit of the current type more expensive.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to provide a rock drill bit of the above-captioned type, wherein the retrac teeth of said rock drill bit are formed such that crack formation is avoided to a high degree in connection with said retrac teeth.

Another object of the present invention is to provide retrac teeth which permit the transfer of impact energy between the drill bit and the drill rod situated closest to the drill bit by means of so-called shoulder abutment.

The objects of the present invention are realized by a rock drill bit for percussive drilling comprising a head which defines a longitudinal center axis. The head includes an axially forward rock-crushing surface and a skirt extending axially rearwardly from the rock-crushing surface. An outer circumferential surface of the skirt is formed with circumferentially spaced apart grooves which form lands therebetween. The grooves and the lands extend at least generally in an axial direction. An axial rear portion of at least some of the lands form retrac teeth for crushing rock during withdrawal of the bit from a hole. Each retrac tooth includes a cutting edge extending along an outer circumference of the skirt.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawings, in which like numerals designate like elements, and in which:

FIG. 1

shows a perspective view of a rock drill bit according to the present invention, wherein a drill rod is connected with the drill bit;

FIG. 2

shows a longitudinal cross-section through the drill bit according to

FIG. 1

, wherein the cross-section runs through a retrac tooth, and an associated drill rod is shown schematically;

FIG. 3

shows a perspective view of a rear part of the drill bit according to

FIG. 1

; and,

FIG. 4

shows a longitudinal cross-section through the drill bit according to

FIGS. 1-3

according to the present invention, wherein the cross-section runs through a retrac tooth, and an alternative associated drill rod is shown schematically.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The drill bit

1

shown in

FIGS. 1-3

comprises a bit head

3

and a shank or a skirt

5

, said bit head

3

and the skirt

5

being integral with each other. A schematically shown drill rod

7

is connected with the drill bit

1

via a thread joint. A through-going flush channel

8

is in a usual manner provided in the drill rod

7

along a longitudinal center axis

2

common for the drill bit

1

and the drill rod

7

.

As is most visible in

FIG. 2

, the drill bit

1

is provided with an internal recess having a female thread

9

, which receives an external male thread

10

disposed at one end surface of the drill rod

7

.

The bit head

3

of the drill bit

1

according to the present invention is in a usual manner provided with rock crushing means, in the shape of cemented carbide buttons in the shown embodiment, of which several annularly positioned peripheral buttons

11

are shown. Several flushing channels

12

extend between the internal space of the drill bit

1

defined by the female thread

9

, and the front of the bit head

3

. There is also provided an axially rearwardly facing first abutment surface

13

in said internal space, a so-called bottom abutment, for abutting the free end of the drill rod

7

.

There is provided an axially rearwardly facing second abutment surface

14

, a so-called shoulder abutment, at the rear end of the drill bit

1

, which is best visible in

FIG. 3

, which is intended to cooperate with a shoulder

15

on the drill rod

7

. In the embodiment shown in

FIGS. 1-3

there is normally used a so-called shoulder-bottom abutment, which means that the drill rod

7

is made with such tolerances during forming of the threaded connection between the drill bit

1

and the drill rod

7

that the free end of the drill rod

7

initially engages the bottom abutment before engaging the shoulder abutment

14

. After a relatively short time, the wearing of the threaded connection causes the shoulder

15

of the drill rod

7

to engage the shoulder abutment

14

, whereby a so-called shoulder-bottom abutment has been formed, i.e., abutments between the drill bit

1

and the drill rod

7

occur at both the bottom abutment surface

13

and the shoulder abutment surface

14

. This means that transfer of the impact energy from the drill rod

7

to the drill bit

1

will occur at both the bottom abutment

13

and the shoulder abutment

14

.

As is best seen in

FIGS. 1 and 3

, the drill bit

1

is provided with a number of straight grooves

16

for conducting drill cuttings on its external surface, which grooves extend in the axial direction of the drill bit

1

and therebetween define a number of straight protruding lands

17

, which likewise extend in the axial direction of the drill bit

1

, i.e., parallel to the center axis

2

. As is seen in

FIGS. 1-3

, each protruding land

17

has a reduced height (radial dimension) along an intermediate part of its length. This reduction of material at intermediate parts of the protruding lands

17

ensures that guiding of the drill bit

1

in the drilled hole occurs by means of the full-height parts of the protruding lands

17

situated at respective ends of each land. The grooves

16

are in the usual manner intended to transport away the drill dust generated at the front of the drill bit

1

.

As can be seen in

FIGS. 1-3

, the protruding lands

17

are formed with retrac teeth

18

at their rear ends, i.e., the ends facing away from the rock crushing end of the drill bit

1

, which teeth are intended, during retraction of the drill bit

1

from a drilled hole, to crush the material which has loosened from the bore wall. Each retrac tooth

18

is provided with a cutting edge

19

, which is situated on the respective protruding land

17

at a location situated farthest from the central axis

2

. The cutting edges

19

extend along, i.e., lie on, an outer circumference of the skirt and coincide with an imaginary cylinder that intersects the peripheral buttons

11

of the drill bit

1

.

The retrac teeth

18

according to the present invention can be formed in a simple and rational manner in connection with the manufacture of the drill bit

1

according to the present invention. The end where the retrac teeth shall be formed is turned in a lathe during said manufacturing, whereby to form the second abutment surface

14

and a concentrical outer rear surface

20

, which forms an angle &agr; of less than 180 degrees with the inner second abutment surface

14

. In a subsequent step during manufacturing of the drill bit

1

according to the present invention the grooves

16

are formed on the outside of the drill bit

1

, preferably through milling. Thereby the protruding lands

17

situated between the grooves

16

for drill cuttings are formed automatically. Reduction of the height of the intermediate portions of the lands

17

is suitably made by means of milling.

During the formation of the protruding lands

17

, the retrac teeth

18

are inherently formed at the ends of the protruding lands

17

which face away from the rock crushing end of the drill bit

1

. The retrac teeth

18

will be formed with the edges

19

that are situated on the outer diameter of the skirt

5

since the concentric, rear surface

20

forms the angle &agr; that is less than 180 degrees, with the inner second abutment surface

14

. Thus also each retrac tooth

18

will include a portion of the surface

20

facing generally away from the rock crushing end of the drill bit

1

. The rear surfaces

20

coincide with a cone having an imaginary cone apex directed towards the rock-crushing end of the drill bit

1

. That is, each surface is inclined in an axially forward and radially inward direction. A substantial advantage of the retrac teeth

18

formed in that manner is that there are no sharp corners associated with said teeth

18

. Thereby the danger for crack formation in connection with said retrac teeth

18

is reduced to a high degree.

In other words, the method for manufacturing of the rock drill bit from a cast blank, not shown, comprises the following steps:

A) turning the rock crushing end surface and the rear surface

20

, whereby the end surface forms the angle &agr; of less than 180 degrees with the plane

21

, and

B) forming the grooves

16

for drill cuttings on the outside of the drill bit

1

, preferably by milling, which grooves are made to extend between the rock crushing end surface and the surface

20

of the drill bit. The intermediate protruding lands

17

are formed due to the development of the grooves, wherein the rear end portions of the lands define the retrac teeth

18

, which have edges

19

extending along the outer circumference of the skirt

5

. In addition, portions of the surface

20

are formed on the respective retrac teeth. Furthermore, when the surface

20

is turned, a second abutment surface

14

is achieved, which is concentric with and situated inside the surface

20

. Also, some of the retrac teeth may be removed by means of a subsequent machining, preferably by milling.

In

FIG. 4

is shown a rock drill bit

1

according to

FIGS. 1-3

connected to a drill rod

7

′, which has an alternative design when compared to the drill rod

7

according to

FIGS. 1-2

. The principal difference between the drill rod

7

according to

FIGS. 1-3

and the drill rod

7

′ according to

FIG. 4

is that the latter is designed without a shoulder

15

. In a joint of the drill bit

1

and the drill rod

7

′ according to

FIG. 4

, the transfer of shock wave energy exclusively occurs through a so-called bottom abutment, i.e., where the free end surface of the drill rod

7

′ abuts against the first abutment surface

13

of the drill bit

1

. Even if the drill bit

1

according to

FIG. 1

is basically the same as the drill bit

1

according to

FIG. 4

, the axial distance between the first abutment surface

13

and the second abutment surface

14

can be larger if a drill rod

7

′ according to

FIG. 4

is used. The reason thereto is of course that the drill rod

7

′ does not have any shoulder, i.e., the second abutment surface

14

is in principle not used.

CONCEIVABLE MODIFICATIONS OF THE INVENTION

In the above-described embodiment of the drill bit

1

, the grooves

16

and the protruding lands

17

are straight and extend exactly in the axial direction of the drill bit

1

. However, within the scope of the present invention the grooves and the associated protruding lands may, for example, extend helically on the surface of the drill bit and thus would be considered as extending generally in the axial direction. Alternatively, the grooves and lands could extend straight, but at an inclination relative to the axis and thus would also be considered as extending generally in the axial direction

According to the above-described embodiment of the drill bit

1

, retrac teeth

18

are formed between each adjacent pair of grooves

16

. Within the scope of the present invention for example every second retrac tooth

18

may be removed, e.g., by milling-away the rear portions of respective lands. The number of retrac teeth

18

that a rock drill bit

1

according to the present invention could exhibit is based upon many different parameters, wherein for exemplifying and not limiting purpose can be mentioned: (i) the diameter of the drill bit, (ii) the type of rock being drilled, and (iii) the type of drilling-rig being used.

In the embodiment according to

FIG. 4

, the abutment surface

14

is not needed, as noted earlier, so the surface

20

could extend all the way to the inner diameter of the drill bit. Thus, the angle a would be formed by the surface

20

and the imaginary plane

21

which extends perpendicularly to the center axis

2

of the drill bit (i.e., the plane

21

is parallel to the surface

14

which also extends perpendicular to the axis

2

).

Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.