ROCK DRILL BIT AND A ROCK DRILLING TOOL

Abstract: A rock drill bit for percussive drilling comprising a head portion (1) with a front surface which has a number of inserts (7) provided in a peripheral ring, and a number of front inserts (9, 10) placed radially inside the peripheral inserts. At least a first surface portion (4) is provided on the front surface, which is recessed at a certain depth in relation to a surrounding second surface portion (5). One or more channels (12) for feed of flush medium to cool inserts and convey away drill cuttings between the drill bit and a bore wall terminates in the front surface. The second surface portion (5) is a part of an endless land portion which delimits and wholly surrounds the recessed space in connection to the first surface portion (4) in order to always retain a cushion of flush medium in this space for the purpose of flushing in a versatile manner and cooling said from inserts (9, 10). The present invention also relates to a rock-drilling tool for percussive drilling.

Description: ROCK DRILL BIT AND A ROCK DRILLING TOOL Background of the invention This invention relates to a rock drill bit for percussive drilling, including a head portion having a front surface, which comprises a number of inserts provided in a peripheral ring, and a number of front inserts placed radially inside the peripheral inserts, and which is formed with at least a first surface portion which is recessed in relation to a surrounding second surface portion, at least a channel for feed of flush medium in purpose to cool inserts and convey away drill cuttings between the drill bit and a bore wall, terminating in said recessed surface portion of the front surface, and said second surface portion constituting a part of an endless land portion which borders and wholly surrounds a recessed space in connection with the first surface portion in order to in retain a cushion of flush medium in this space with the object to flush and to cool said inserts. The present invention also relates to a rock drilling tool for percussive drilling. Prior art Through for example SE 359 350, US 3 388 756 and CA 1 300 124 are previously known drill bits which each has a recessed space associated with a flush medium channel which communicates with the peripheral part of head portion via a groove which is substantially just as deep as the recess itself. The object of this groove is primarily to guarantee an efficient feed of flush medium to the ring- shaped space between head portion and the surrounding bore wall for the major purpose of guaranteeing quick and efficient removal of drill cuttings. The circumstance however, that the evacuation grooves enables quick drainage of the flush medium in direction towards the cylindrical outer surface of the head portion, simultaneously leads to poor cooling of the front inserts, especially at the central front inserts. Research related to the present invention have shown that substandard cooling of the front inserts constitutes a decisive cause of development of so called reptile skin on inserts. Such a reptile skin consists of a pattern of micro cracks which successively develops in the cemented carbide containing inserts, due to great variations between the highest and lowest temperature in connection with the intermittent impact movements. In practice, rock drill bits of the discussed type is operated with a impact frequency of for example 50 Hz (3 000 im pacts/minute) and a speed of rotation of for example 80 revolutions/minute Even if the pressure of the flush medium is comparatively high (for example 8 bar for water) the cooling effect becomes deficient, specifically when the individual insert accomplishes an impact, something which shows in temperature rises by leaps in that instant The problems with high temperatures is particularly pronounced in connection with drilling in ore containing rock, magnetite for example. If the reptile skin formed by micro cracks are not removed through grinding within fairly short intervals (for example after 100 m of drilling), the micro cracks will develop to deep cracks which comparatively quick may result in individual insert breakage This can in its turn lead to breakage of the entire drill bit. With the purpose of improving cooling and cleaning of inserts on rotary drill bits US 4 776 41 1 has proposed to develop a number αf endless land portions on the front surface of the drill bit, which land portions frames and wholly surrounds a recessed space in which a channel for feed of flush medium terminates. When the flush medium is fed out through this orifice, a cushion is formed of flush medium in that recessed space However all front inserts on the drill bit are provided on the very land portions. This means that the cooling effect of the flush medium is improved only in limited dimension due to that all inserts, including the central inserts, are distant from the flush medium cushions, which are formed in a countersinking, each bordered by an endless land portion Similar drill bits for rotary drilling are described in US 3 322 218 and EP 0 325 271 Obiects and features of the invention The present invention aims to eliminate above mentioned drawbacks of earlier known rock drill bits and to create an improved drill bit for percussive drilling A primary object of the invention is thus to create a drill bit and a rock drilling tool the front inserts of which are cooled in a more efficient manner than corresponding inserts on earlier known drill bits in order to counteract development of reptile skin on the inserts, all in the primary purpose to increase life-span of the drill bit and the rock drilling tool. It is also an object to create a drill bit and a rock drilling tool, which in spite of improved cooling ability and increased tool life retains constructive simplicity and in such a manner that it does not result in substantial increases in manufacturing costs. According to the invention at least the basic object is reached by the features indicated in the characterizing part of the independent claims. Advantageous embodiments of the invention is furthermore defined in the dependent claims. Short description of enclosed drawings Fig. 1 shows a partial perspective view illustrating a head portion for a drill bit according to the present invention, Fig. 2 shows a plan view of a head portion according to Fig. 1 , Fig. 3 shows a section A-A in Fig. 2 and Fig. 4 shows a section B-B in Fig. 2. Detailed description of a preferred embodiment of the invention In the drawings a rock drill bit is shown, which in a usual manner comprises a substantially cylindrical head portion 1 and a more slender shaft 2. The head portion 1 comprises a front surface or a front side designated with 3, on which a number of inserts are mounted. More precisely the front surface includes a first surface portion 4 which is recessed to a certain depth in relation to a surrounding second surface portion 5. A third surface portion 6, between the surface portion 5 and the periphery of the head portion, is conically shaped. On this conical surface are provided a number of first inserts 7 in a peripheral ring on the head portion. As is distinctly evident from Fig. 2 for example, the inserts 7 in this case project somewhat outside the periphery of the head portion in order to machine a hole during drilling which has somewhat bigger diameter than the very head portion. Recesses 8 are provided in areas between adjacent peripheral inserts 7, through which flush medium can pass. A number of front inserts are provided also in the area inside the peripheral inserts 7, which depending of their position are designated 9 and 10, respectively. The rock drill bit should be coupled to a drill tube, not shown, in a drill string for example by means of a threaded connection. The tube include a channel for conveying fluid. The threaded connection comprises a male thread and a female thread, not shown When assembled, the rock drill bit and the drill tube constitute a rock drilling tool As evident from Figs 3 and 4, a main channel 1 1 for flush medium is provided inside the drill bit. This main channel surpasses at its forward end in a number of branch channels 12, which terminate in the first, recessed surface portion 4 In this connection it may be noted that one or more flush medium channels also can terminate in said recesses 8 The flush medium will in practice be constituted of water or air. The second surface portion 5 constitutes a part of an endless land portion or material portion 1 3 which in the example is substantially circularly ring-shaped By being endless and ring-shaped, this land portion 1 3 completely surrounds the recessed space 14 which connects to the first surface portion 4 In other words, the land portion 1 3 forms a circumferential edging which borders a in the example mainlv cylindrical, althougn shallow space 14, the bottom oi which consists of the first surrace portion 4 In the shown embodiment both surface portions 4, 5 are substantially planar and mutually parallel The surface portions extend essentially perpendicularly to the longitudinal central axis of the drill bit. By having the land portion 1 3 endless, i.e it lacks each disruption in the form of aπv evacuation groove, the flush medium that is fed out through channels 1 2 will form a cushion or pool, which always is retained bv the endless land portion The four front inserts which are designated by 9, are secured to the ring-shaped, planar surface portion 5 In the example these front inserts are spaced equi-distantlv The remaining four front inserts which are designated by 10, are secured to the recessed surrace portion or bottom 4, more exactly in an irregular pattern As evident from Figs 3 and 4 these front inserts 10 project more from the surface portion 4 than the depth with which this surface is recessed in relation to the surrounding, ring-shaped surface portion 5. In the shown example the front inserts 10 have a height which is substantially equal to the height of the front inserts 9. This means that the inserts 10 project to a common planar level which is separate from that the common planar level of the front inserts 9. In other words the central front inserts 10 are somewhat recessed in relation to the surrounding, more peripheral front inserts 9. In practice all inserts advantageously are made of cemented carbide. The shape of the inserts may vary considerably. They can thus be spherical, conical, ballistic or semi-ballistic. As evident from Fig. 2 a number of associated spaces 15 are recessed in the endless, ring-shaped land portion 13, which partly intersect said land portion. More exactly, four equi-distantly separated associated spaces 15 are provided in the land portion. Each individual associated space is substantially semicircular. An individual branch channel 12 for flush medium terminates at least partially in the planar bottom of each such associated space 15. It may also be noted that each individual associated space 1 5 is located about mid between two adjacent outer front inserts 9 The cushion of flush medium which is formed in the space 14 will thus continuously be filled with flush medium via four equi-distantly separate channels; something which guarantees an even discharge of flush medium to all parts of the space. In practice the area of the first, recessed surface portion 4 can amount to 30 - 65, suitably 40 - 55 % of the total area of the planar surface which exists inside the circular edge 16 which borders the surface portion 5 from the conical surface 6. In other words the space 14 occupies a relatively large part of the entire front surface. The depth of the space 14 may vary within fairly wide limits, for example in the area of 1 -7 mm for drill bits with ordinary diameters. In the shown example, where the drill bit has an operative outer diameter of about 100 mm, the depth of the pool-like space 14 amounts to about 2,5 mm. In connection therewith, the height of the inserts 9, 10 can lie in the area of 6-12, suitably 8-10 mm. Since the endless, ring-shaped land portion 13 guarantees the formation of a continuous, maintained cushion of flush medium, water for example, in the space 14, in particular the centrally positioned front inserts 10 will be flushed in a versatile manner and intensively with flush medium during efficient cooling of the inserts in all operational phases of the drill bit, i.e. during each part of an individual impact movement as well as during each part of a return movement. Each impact will further immerse the front inserts with flush medium. Since the endless land portion lacks deep evacuation grooves the flush medium will flow away from the cushion in a essentially evenly distributed radially directed flow from the recessed space 14 to the ring-shaped space between the bore wall and the surface of the head portion, more exactly via the planar surface portion 5 of the land portion. This means that also inserts 7 and 9 are subjected to an essentially evenly distributed cooling. Modifications of the invention The present invention is not limited to the described embodiment and the associated drawings. The front surface of the head portion can thus be recessed by two or more spaces. Furthermore the part surfaces which are separate in level and constitute bottom in the recessed space and constitute surface on the surrounding, endless the land portion need not necessarily be planar. For example the bottom can have a somewhat dome-shaped form in the recessed space. Furthermore the surrounding surface portion on the endless land portion can have a structure which differs from the absolutely planar shape. For example, the latter surface portion can be rough. An essential feature of the present invention is that the endless land portion which borders the pool-like, recessed space does not have any deep groove which allows concentrated evacuation of flush medium.

Claims: Claims 1. A rock drill bit for percussive drilling, including a head portion (1) having a front surface (3), which comprises a number of inserts (7) provided in a peripheral ring, and a number of front inserts (9, 10) placed radially inside the peripheral inserts (7), and which is formed with at least a first surface portion (4) which is recessed in relation to a surrounding second surface portion (5), at least a channel (12) for feed of flush medium in purpose to cool inserts and convey awav drill cuttings between the drill bit and a bore wall terminating in said recessed surface portion (4) of the front surface, and said second surface portion (5; constituting a part of an endless land portion (13) which borders and wholly surrounds a recessed space (14) in connection with the first surface portion (4) in order to retain a cushion of flush medium in this space with the object to flush and to cool said inserts, characterized i n that one or more front inserts (10) is provided in the first surface portion (4), which forms a bottom in the recessed space (14) and that at least one of said inserts (10) projects from said bottom a distance which is bigger than the depth of the recessed space (14) 2. Rock drill bit according to claim 1, characterized i n that front inserts (10) which are secured in the first surface portion (4) project to a level which is distant from corresponding level for the front inserts (9) which are secured in the second surface portion (5). 3. Rock drill bit according to claims 1 or 2, characterized i n that in the endless land portion (13) which surrounds the recessed space (14) is recessed at least at an associated space (15), which partly intersects said land portion and in which the flush medium channel (12) at least partially terminates. 4 Rock drill bit according to claim 3, characterized i n that a number of, for example four, equi-distantly separate associated spaces (15) are provided in the endless land portion (13) and in that a flush channel (12) terminates in each of said spaces. 5. Rock drill bit according to anyone of the preceding claims, characterized i n that the first and second surface portions (4,5) are substantially planar and mutually parallel and extend essentially perpendicularly to the longitudinal center axis of the drill bit. 6. Rock drill bit according to anyone of the preceding claims, characterized i n that the area of the first, recessed surface portion (4) amounts to 30-65, suitably 40-55% of the total area of the front surface (3). 7. A rock drilling tool for percussive drilling comprising a drill bit and a drill tube co-operating by means a threaded connection, said connection comprising a male thread and a female thread, said drill bit including a head portion (1) having a front surface (3) and comprising a number of inserts (7) provided in a peripheral ring, and a number of front inserts (9, 10) placed radially inside the peripheral inserts (7), and which is formed with at least a first surface portion (4) which is recessed in relation to a surrounding second surface portion (5), at least a channel (12) for feed of flush medium in purpose to cool inserts and convey away drill cuttings between the drill bit and a bore wall terminating in said recessed surface portion (4) of the front surface, and said second surface portion (5) constituting a part of an endless land portion (13) which borders and wholly surrounds a recessed space (14) in connection with the first surface portion (4) in order to retain a cushion of flush medium in this space with the object to flush and to cool said inserts, characterized in that one or more front inserts (10) are provided in the first surface portion (4), which forms a bottom in the recessed space (14) and that at least one of said inserts (10) projects from said bottom a distance which is bigger than the depth of the recessed space (14). 8. Rock drilling tool according to claim 7, characterized in that front inserts (10) which are secured in the first surface portion (4) project to a level which is distant from corresponding level for the front inserts (9) which are secured in the second surface portion (5). 9. Rock drilling tool according to claims 7 or 8, characterized in that in the endless land portion (13) which surrounds the recessed space (14) is recessed at least at an associated space (15), which partly intersects said land portion and in which the flush medium channel (12) at least partially terminates. 10. Rock drilling tool according to claim 9, characterized in that a number of, for example four, equi-distantly separate associated spaces (15) are provided in the endless land portion (13) and in that a flush channel (12) terminates in each of said spaces.

Inventors: LILJEBRAND PER-OLOF, OLSSON URBAN, LILJEBRAND, PER-OLOF, OLSSON, URBAN
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1997
Main classification: E21B10
IPC codes: E21B10