PERCUSSION DRILL BIT, AN INSERT, A USE AND A METHOD OF MAINTAINING THE DRILL BIT DIAMETER

Abstract: The present invention relates to a percussion drill bit (10) for drilling bores, comprising a steel body (12), means at one end of the steel body for connecting the bit to a percussive unit such as a down-the-hole hammer or a drill string for a jack hammer, a plurality of inserts embedded in the other end of the steel body. At least the peripheral inserts or gauge row inserts comprise a cemented carbide body having a rear mounting portion embedded in the steel body and a front end protruding from the steel body. A polycristalline diamond layer is provided on the front end of the carbide body. The layer has an edge along a border in the vicinity of the maximum diameter of the insert. The edge (22) is kept away a first distance (L1) from the wall of the bore during drilling by tilting the gauge row insert (16) an acute angle (a) of 41 DEG to 51 DEG relative to the rotational axis (11) of the drill bit (10). The invention further relates to a gauge insert, use of a diamond-coated insert as a gauge insert in a drill bit and a method of maintaining the diameter of a drill bit during percussive drilling of a bore in a rock material.

Description: PERCUSSION DRILL BIT, AN INSERT, A USE AND A METHOD OF MAINTAINING THE DRILL BIT DIAMETER Technical background The present invention relates to a percussion drill bit for drilling bores in rock. A typical percussion drill bit comprises a steel drill body, means at one end of the drill body for connecting the bit to a percussive unit such as a down-the-hole hammer or a drill string for a jack hammer, and a plurality of inserts embedded in the other end of the drill body. At least the peripherally located inserts, or gauge row inserts, comprise a cemented carbide body having a rear mounting portion embedded in the drill body and a converging front end protruding from the drill body. A polycristalline diamond layer is provided on the converging end of the carbide body, the layer having an end edge disposed in the vicinity of the maximum diameter of the insert. The invention further relates to a gauge insert, use of a diamond-coated insert as a gauge insert in a drill bit and a method of maintaining the diameter of a drill bit during percussive drilling of a bore in a rock material. Known percussive drill bits, e.g. in applicants US-A-5,335,738, that are provided with diamond-coated gauge inserts often have a disadvantage in that premature failures occur due to fatigue of the diamond-coating. A reason for the failure is that the point of contact between the wall of the bore and the gauge insert is close, or on, the rear edge area of the diamond-coating. The coating is very thin at that area and thus has a comparatively poor resistance to wear. The known drill bits more or less drill the bore diameter with the thinnest part of the gauge insert since their gauge inserts are inclined by about 35° relative to the center axis of the drill bit. Objects of the invention An object of the present invention is to provide a drill bit for percussive drilling, wherein the life of the bit is extended. Another object of the present invention is to provide a drill bit for percussive drilling, wherein the gauge inserts are angled relative to the rotational axis of the bit to keep the edge of the diamond-coating away from the wall of the bore during drilling. Still another object of the present invention is to provide a drill bit for percussive drilling, wherein each gauge insert has a diamond-coating, the thickness of which varies and wherein a portion of the diamond-coating thicker than the edge determines the diameter of the bore. Still another object of the present invention is to provide an effective gauge insert for a drill bit for percussive drilling. Still another object of the present invention is to disclose an effective use of a gauge insert for a drill bit for percussive drilling. Still another object of the present invention is to disclose an effective method of maintaining the diameter of a drill bit for percussive drilling. These and other objects of the present invention have been attained by a drill bit, a gauge insert, a use and a method of maintaining the diameter of a drill bit as defined in the appended claims. 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, and in which: Fig. 1 shows one half of a drill bit according to the present invention in longitudinal section; Fig. 2 shows a sectional view of a gauge insert according to the present invention, in relation to a bore wall; and Fig. 3 shows a gauge insert according to the present invention in longitudinal section. Detailed description of the invention In Fig. 1 there is shown one longitudinal half of a drill bit 10 according to the present invention, the bit being generally symmetrical about its rotational axis 1 1. The bit comprises a steel drill body 12 which has external splines 13 on the shank 14 thereof. The splines shall be connected to corresponding splines in a driver sub of a down-the-hole hammer, not shown, so as to transfer rotational movement in the usual manner. Alternatively the spline can be replaced by other means, such as threads, for connecting the bit to a drill string, not shown. The drill bit 10 further includes a central passage 30 for a pressurized medium such as air, connected to an angled fluid passage 40 which is further connected to a chipway or return groove 50. A conventional plastic valve 60 is disposed in the passage 30. Provided in the front face 15 of the drill body is a number of button inserts, not shown. The button inserts are made of cemented carbide and are secured in borings preferably by press fit. A number of gauge row inserts 16 are positioned at the periphery of the bit 10, the radially outer portions of which define the diameter of the bit and thus the diameter of the bore being drilled. The gauge row inserts 16 are tilted in such a way, that a longitudinal centre line (center axis) 17 of each insert 16 diverges by an angle α from the longitudinal centre axis 1 1 of the drill bit in a forward direction of the bit. The tilting of the gauge row insert can also be expressed in such a way that the point where the longitudinal centre line 17 of the insert 16 intersects the converging end 18 is located farther from the longitudinal centre axis 1 1 of the drill bit than the point where the longitudinal centre line 17 intersects the rear end 19 of the insert. Several gauge row inserts 1 6 may be embedded in the front end of the drill body 12. Each insert 16 comprises a cemented carbide body having a grip portion or mounting portion 20 embedded in the drill body and a front converging end 18 protruding from the drill body. A polycristalline diamond layer 21 is provided on the converging end of the carbide body to define the cutting end 18 of the insert. The layer 21 has a rear edge 22 substantially lying in a plane P, see Figs. 2 and 3, the plane P preferably forming an acute angle with the center axis 1 1 , that angle being equal to the above-mentioned angle α. The edge 22 is disposed on the cylindrical jacket surface 25 of the insert at or in the vicinity of the maximum diameter of the insert, preferably at a junction between a curved front surface of the carbide body, and the cylindrical jacket surface 25. The longitudinal centre axis 1 1 of the drill bit and the longitudinal centre line 17 of the inserts 16 include the angle α therebetween. The angle α is in the range of about 41 ° - 55°, in order to space the edge 22 by a distance L1 from the wall of the bore 24 during drilling. More preferably, the angle α is about 41 ° - 51 °, and most preferably is about 45°. Such inclination of about 41 ° - 55° results in that the radially outermost part of the rear edge 22 relative to the axis 1 1, is supposed to be passive, i.e. spaced from the wall, during drilling. The distance L1 is at least 1.5 mm. For example the distance L1 is preferably about 1.7 mm for an insert diameter D of 18 mm. The cemented carbide portion of the converging end 18 is semi-spherical ly curved and defined by a radius R lying on the center axis 17, and the origin C of the radius R is positioned axially rearwardly of the plane P. The plane P intersects the centre axis 17 of the insert at a point A. The distance L3 between the point A and the radius origin C is 5 to 30 %, preferably about 20 %, of the radius R. The radius R is about 50 % to 52 % of the diameter D of the rear mounting portion 20, preferably 50 % to 51 %. During drilling the point of contact B, see Fig. 2, between the wall of the bore 24 and the insert 16 is axially displaced by a distance L2 from the edge 22. The distance L2 is 50 % to 70 % of the radius R of the insert, preferably about 60 % for an insert diameter D of about 18 mm. The diamond layer 21 has an outer surface defined by a radius R' of the same magnitude as the radius R of the cemented carbide converging end 18. The origin C of the radius R' lies on the center axis 17 and is displaced axially forwardly a distance L4 relative to the origin C of the radius R. The size of the distance L4 is less than 1 mm but larger than 0.1 mm, preferably about 0.6 mm. Thus the thickness of the layer varies from 0.1 mm at the edge to 1 mm at its intersection with the center line 17 (most preferably from 0.2 at the edge to 0.8 mm at the center line). The diamond layer 21 comprises polycristalline diamond (PCD), preferably with a binder content between 1 % and 15 %. Since the new inventive drill bit will drill longer than hitherto known drill bits it is favorable if the drill body, at least in the vicinity of and around the gauge row inserts, is provided with a wear resistant coating, such as a hard metal coating. The thickness of such a coating is preferably about 0.1 mm. That avoids steel wash-out and thereby prevents the gauge row inserts from losing steel support and from falling off the drill body. Drill bits according to the present invention, with a 45° inclination of the gauge inserts have experienced 50-70% increased life of the bits compared to prior art drill bits, with gauge inserts inclining 35° relative to the rotational axis of the drill bit. The present invention further relates to a method of maintaining the diameter of a drill bit during percussive drilling of a bore in a rock material. The method comprises the following steps: providing a drill bit having the geometry described above, connecting the bit to a percussive unit and drilling a bore, while spacing the rear edge of the diamond layers of the gauge row inserts from the wall of the bore during drilling so as to maximize the duration of the life of the bit. The present invention further relates to the use of a diamond-coated cemented carbide insert as a gauge row insert in a drill bit for percussive drilling. The invention can be varied freely within the scope of the appended claims. For example the shape of the cutting end of the insert can be semi-spherical or bullet-shaped. Although the present invention has been described in connection with a preferred embodiment 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.

Claims: Claims 1. A percussion drill bit for a drilling bore, comprising a drill body having a connection section at a rear end for connection to a percussive unit such as a down-the-hole hammer or a drill string for a jack hammer, and defining a rotational axis (1 1) of the drill bit (10) and a plurality of gauge row inserts (16) embedded in a front end of the drill body, the front end being rigid with respect to the connecting section, each gauge row insert (16) comprising a cemented carbide body having a rear mounting portion (20) embedded in the drill body, and a front end (18) protruding from the drill body, there being a diamond layer (21) disposed on the front end of the carbide body, the layer having a rear edge (22) disposed adjacent a maximum cross section (D) of the insert, wherein the gauge row inserts (16) are oriented at an acute angle (α) relative to a rotational axis (1 1 ) of the drill bit (10) and the angle (α) is from about 41 ° to about 51 ° for spacing the rear edge (22) of the diamond layer a first distance (L1) from the wall of the bore (24) during drilling. 2. A drill bit according to claim 1 , wherein the front end (18) is curved and defined by a radius (R), an origin (C) of the radius being disposed axially rearwardly of a plane (P) containing the rear edge (22) of the diamond layer (21). 3. A drill bit according to claim 2, wherein the radius (R) of the front end (18) is 50 % to 52 % of a diameter (D) of the rear mounting portion (20), preferably 50 % to 51 % thereof, and wherein the distance (L1 ) is at least 1.5 mm, preferably about 1.7 mm for a diameter (D) of 18 mm. 4. A drill bit according to claim 1 , 2 or 3, wherein the point of contact between the wall of the bore (24) and the insert (16) is displaced a second distance (L2) axially forwardly from the rear edge (22) of the diamond layer (21). 5. A drill bit according to claim 4, wherein the second distance (L2) is 50 % to 70 % of the radius (R) of the insert (16), preferably about 60 %, with a maximum diameter (D) of the insert being of about 18 mm. 6. A gauge row insert for percussive drilling, comprising a cemented carbide body having a rear mounting portion (20) and a front end (18), a diamond layer (21) disposed on the front end (18) of the carbide body, said layer (21 ) having a rear edge (22) situated in the vicinity of a maximum diameter (D) of the insert (16), wherein the front end is curved and defined by a first radius (R), and an origin (C) of that radius being disposed axially rearwardly of a plane (P) containing said rear edge, the length of the first radius (R) being from about 50 % to about 52 % of the diameter (D) of the rear mounting portion, a part of said rear edge (22) being supposed to be passive during drilling. 7. An insert according to claim 6, wherein the diamond layer (21) is curved and defined by a second radius (R * ) which is in the same magnitude as the first radius (R) of the cemented carbide front end (18) and wherein an origin (C) of the second radius (R') is spaced axially forwardly by a fourth distance (L4) from the origin (C) of the first radius (R). 8. An insert according to claim 7, wherein the size of the fourth distance (L4) is less than 1 mm but larger than 0.1 mm, preferably about 0.8 mm and wherein the diamond layer (21 ) comprises polycristalline diamond (PCD) preferably with a binder content between 1 % and 15 %. 9. Use of diamond-coated cemented carbide insert as a gauge row insert in a drill bit for percussive drilling, said drill bit (10) having a rotational axis (1 1 ) and a plurality of gauge row inserts (16) embedded in a front end of the drill body, the front end being rigid with respect to the connecting section, wherein the insert comprises a cemented carbide body having a rear mounting portion and a front end protruding from the rear mounting portion, a polycristalline diamond layer on the front end of the carbide body, said layer having a rear edge (22) disposed adjacent a maximum cross section (D) of the insert, wherein the front end is curved and defined by a radius (R) and an origin (C) of the radius is axially rearwardly of a plane (P) containing said rear edge (22), the gauge row inserts (16) are oriented at an acute angle (α) relative to a rotational axis (1 1) of the drill bit (10) and the angle (α) is from about 41 ° to about 51 ° for spacing the rear edge (22) of the diamond layer a first distance (L1 ) from the wall of the bore (24) during drilling. 10. Use of an insert according to claim 9, wherein the radius (R) is 50 % to 52 % of the diameter (D) of the rear mounting portion. 1 1. Method of maintaining the diameter of a drill bit during percussive drilling of a bore in a rock material, the method comprising the steps of: - providing a percussive drill body (12) having means (13) at a rear end for connecting the bit (10) to a percussive unit, and defining a rotational axis (1 1 ) of the drill bit (10), the front end being rigid with respect to the connecting section, a plurality of gauge row inserts (16) embedded in the front end of the drill body, each gauge row insert (16) comprising a cemented carbide body having a rear mounting portion (20) embedded in the drill body and a front end (18) protruding from the body, a diamond layer (21) on the front end of the carbide body, said layer having a rear edge (22) disposed in the vicinity of a maximum diameter (D) of the insert; -rotating the drill body about a rotational axis (1 1 ) such that the gauge row inserts (16) define a diameter of a bore (24) being drilled, and - spacing the rear edge (22) of the layer (21 ) a first distance (L1) from the wall of the bore during drilling by tilting said gauge row inserts (16) at an acute angle (α) of about 41 ° to about 51 ° relative to the rotational axis (1 1 ) of the drill bit (10).

Inventors: AASBERG BENGT, HEDLUND JAN-GUNNAR, AASBERG, BENGT, HEDLUND, JAN-GUNNAR
Applicants: AASBERG BENGT, HEDLUND JAN GUNNAR, SANDVIK AB, AASBERG, BENGT, HEDLUND, JAN-GUNNAR
Application year: 1996
Main classification: E21B10
IPC codes: E21B10