EP0032428B1

EP0032428B1 - Rotary drill bits

Info

Publication number: EP0032428B1
Application number: EP81300062A
Authority: EP
Inventors: John Denzil Barr
Original assignee: Stonehouse Uk Ltd
Current assignee: Stonehouse Uk Ltd
Priority date: 1980-01-10
Filing date: 1981-01-08
Publication date: 1986-08-27
Also published as: EP0032428A1; AU6618681A; AU547017B2; DE3175194D1

Description

The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations comprising a bit body with a shank and an inner channel for supplying drilling fluid to the face of the bit. The bit body carries a plurality of so-called "preform" cutting elements. The preforms are shaped items of hard material and may be moulded of hard abrasive particles or of a hard homogeneous material. For example, they can be moulded of particles of natural of synthetic diamond, secondary abrasive particles and metal bonding agents. The preforms often comprise a thin diamond facing layer and a thicker cemented tungsten carbide backing layer. This construction provides a degree of self-sharpening in that, as the preform wears away in use, the tungsten carbide layer wears away more easily than the diamond layer. The preforms are usually mounted on or in the rotary drill bit by being bonded, e.g. brazed, to a support member which may be of steel or a matrix of tunsten carbide particles infilled with a metal alloy or of cemented tungsten carbide. The use of such preforms, their manufacture and mounting on rotary drill bits are disclosed in the following U.S. patent Specifications Nos. 3 743 489, 3 745 623, 3 767 371, 4 098 362, 4109737, 4 156 329. Preforms may also be made of boron carbide, boron nitride, titanium diboride, silicon nitride or mixtures thereof or of "Sialon" or of other extremely hard material. Typically the particles of abrasive material in the preform are large so that the preform cutting element can act along one edge, i.e. a cutting edge.
In use the cutting forces cause bending stresses in preforms mounted on deep hole rock drilling bits. Where the drilling is being carried out in nonhomogeneous formations, inclusions of hard material in the formation can increase the cutting forces and hence the stresses. The value of the bending stress in the preform depends partly on the deflection of the material on which it is mounted. The bending stress is sometimes sufficient to fracture the preform, particularly if the formation contains inclusions of hard material. This problem may be reduced by mounting the preforms on more rigid supports of material having a high modulus of elasticity such as cemented tungsten carbide. For example, each preform may be brazed to an underlying support stud of cemented tungsten carbide mounted on a . tool body of steel or matrix. Such an arrangement reduces bending deflection and can prevent fracture of the preform but gives rise to other disadvantages which arise because materials of high modulus of elasticity are normally hard and relatively wear resistant. As the preform wears down, the hard material of the underlying support begins to rub on the formation behind the cutting edge. This increase the normal force required to achieve a given depth of cut and the resulting friction force is added to the tangential cutting force, increasing the specific energy, i.e. the energy required to drill a unit volume of formation. Heat is also generated near the cutter and this heat may weaken the brazed joints securing the preforms to the supports and damage the diamond layer. One result of this is that the rate of penetration of a drill bit is reduced as the cutters become worn, and the specific energy increases.
It is an object of this invention to provide a rotary drill bit for deep hole drilling or coring including preform cutter elements which are mounted on the bit in such a manner as to reduce the bending stresses in the preforms generated by deflection of the supports, thereby permitting the use of relatively soft and/orthin supports. This tends to avoid the above-mentioned disadvantages of using supports of high rigidity and, therefore, high wear resistance; and also reduces the cost of manufacture of the bit.
U.S. Patent Specification No. 3,311,181 discloses a rotary drill bit in which each drill tooth comprises a leading holding section and a following working section, the two sections being mounted within a socket in the body of the drill bit and projecting partly from the socket. The two sections are cemented together before being pressed into the socket. In view of this, and the fact that the portion of the working section projecting from the socket is less well supported than the portion within the socket, it has the effect that the working section cannot accommodate high bending stresses which may lead to fracture of the working section as described above.
U.S. Patent Specification No. 3,717,209 discloses a drilling bit in which generally cylindrical cutting elements are supported in sockets by split sleeves. The cutting elements, being cylindrical, operate on the subsurface formation in a different manner from the preform cutting elements of the drill bit of the kind to which the invention relates and are not subjected to the bending stresses, with the consequent risk of fracture, to which the preform cutting elements are subjected in a bit according to the present invention.
According to the invention there is provided a rotary drill bit for deep hole drilling or coring in subsurface formations of the type disclosed in U.S. patent Specification No. 3 362 489 comprise ing a body with a shank having a fluid bore, said body carrying a plurality of preform cuttingi elements, each having an associated support member carried by the body and disposed on the rearward side of the cutting element with respect to the normal direction of rotation of the bit, characterised in that each cutting element includes, in known manner, a thin hard facing layer and a thicker, less hard, backing layer so that the cutting element is self-sharpening, and in that there is provided between each cutting element and its associated support member a structural link which has high compressive strength but low tensile strength, whereby to reduce bending strain in the cutting element caused by deflection of the support member in use of the bit.
In the known bits, the preform cutting elements fastened or bonded to solid support elements will be constrained by the fastening element or the bond to suffer deformation along the element or the bond surface nearly equal to that occurring near the element or the bond surface in the support member due to strain in the support material caused by the cutting forces. The lack of tensile strength in the link between the cutting element and the support member reduces this constraint and/or its effect.
In use, the preform cutting element has a cutting edge. The low tensile strength in the structural link between the preform cutting element and the support member has the effect that the remainder of the preform can more easily move away from the support member. In this way the geometry of the preform cutting element is isolated from deformations of the support member in use of the bit. The remaining linkage between the preform cutting element and the underlying structure must be sufficient to hold the element to the structure while the tensile strength must be sufficiently low to allow the structure to deform without causing damaging deflection in the preform cutting element. In practice, there may be a gap between the facing surfaces of the preform cutting element and the support member, which gap member be either closed or open depending on whether the surfaces are in contact or not. Such arrangement provides high compressive strength but low tensile and shear strengths. (A temporary bond between the two surfaces which can be allowed to fracture in use is included within the scope of the invention if other means are provided to retain the preform.)
In one preferred aspect, the preform cutting element is separate from the support member, and locating means are present to hold the preform to the support member. In another preferred aspect, the preform is secured, e.g. by brazing, to an intermediate backing member.
Where the cutting element is separate from the support member, the locating means is arranged so that loads imparted to the preform during use of the tool are transmitted to the support member, the locating means being arranged to permit limited movement of the preform as a whole relative to the support member. The cutting forces which may be temporarily increased by impact loads or hard inclusions in the formation cause deformation of the support member but the preform will move relative to the support as it deflects and there will be a reduction in the bending stresses which would otherwise be imposed on the preform. As a result - and this is a particular advantage of the invention - use may be made of a support structured of a material which has a lower wear resistance, e.g. of steel and/or a support which is thinner than usual. In both cases, frictional forces are reduced.
In one embodiment, one end of an elongate locating means is fixed to the bit body at a position spaced from the support and the other end abuts the cutting face of the preform cutting element. Preferably the locating means comprises an elongate resilient metal strip and preferably two such strips are used. The locating means may take the form of a leaf spring. The support assembly preferably further includes additional means to locate the preform on the support to prevent translatory movement of the preform along the fixed surface of the support. The additional means may include a pocket formed in the support member, the sides of which partially enclose the preform. The additional locating means may also include a projection extending from one surface into the other while permitting relative movement between the surfaces in a direction normal thereto. For example, the projection may comprise a peg formed separately from both the preform and the support and, optionally, secured to one of them.
In any of the above arrangements, the support and the retaining element may be provided as a sub-assembly for attachment to the bit body. For example, the sub-assembly may be in the form of a stud to be received in a socket formed in the bit body. The stud may be formed in at least two separately formed abutting parts, at least one of which parts may be wedge-shaped whereby the stud may, in use, be wedged within the socket in the bit body.
In another preferred feature of the invention, the preform is mounted on a backing element and locating means are provided comprising a resilient member connecting the backing element to the bit body. In such an arrangement the movement of the preform as a whole is permitted by deflection of the connecting member, and since the preform is not directly connected to the support member, there will be a reduction in the bending stresses which would otherwise be imposed on the preform by deflections of the support. Preferably the resilient connecting member is stiffer in directions parallel to the front surface of the preform than it is in a direction normal to the front surface of the preform.
The backing element and resilient connecting member may be integrally formed with one another, and they may also be integrally formed with the support.
The backing element, resilient connecting member and support may be provided as a sub-assembly for attachment to the bit body. For example, the sub-assembly may be in the form of a stud to be received in a socket formed in the bit body. The stud may be formed in at least two separately formed abutting parts, at least one of which parts may be wedge-shaped whereby the stud may, in use, be wedged in the socket in the bit body.
According to another preferred feature of the invention, the support member includes one or more slot(s) and/or one or more aperture(s) so shaped and positioned in relation. to the preform as to reduce the tensile strength of the structural link between the cutting element and the support member and thereby modify the deformation of the portion adjacent the preform under cutting loads, so as to reduce the bending stresses which would otherwise be induced in the preform by said deformation.
In order that the invention may be well understood, it will now be described by way of example with reference to the accompanying diagrammatic drawings, in which:

Figures 1,4 and 9 are each a sectional view and front elevation of preform cutting elements arranged according to the invention, and
Figures 2, 3, 5, 6, 7 and 8 are each sectional views of different preform cutting elements arranged according to the invention.

In the following description, the same references used to describe the different embodiments indicate the same parts.
In the embodiment of Figure 1, a disc-shaped preform cutting element C is located in a semi-circular pocket P on a support surface 1 of the body of a drilling bit B. Two holes are drilled at an angle into the bit B a short distance from the support surface 1. Within these holes are located elongated metal rods 2 such that the free ends thereof abut the exposed face of the preform C. A short hole is present in the rear face of the preform C to receive a peg 3 extending from a hole in the support surface 1. There is a small annular gap between the peg 3 and the hole in the preform C. In use, a loose preform C is fitted into the pocket P by reception of the peg 3 in the hole of the support 1 and then the rods 2 are fitted in place. In this way the preform C is held to the support 1. When the drill bit is rotated for example to drill a hole in soft formation and a harder piece of formation is struck, the cutting force or impact load causes elastic deformation of the support 1 and this deformation tends to lift the upper part of the preform C from the support 1 without however damaging either of these parts. If the preform C were simply brazed to the support surface 1 there is a risk that the deformation would fracture the preform or the braze.
In the embodiment of Figure 2, a socket 4 in the drill bit B receives a support stud assembly 5. The stud 5 comprises a support 1 and a complementary wedge part 8 and they mate in wedging manner to receive between them the end of a leaf spring 6 a portion of which extends to abut the preform C. This preform C is held to the support 1 by a peg 3 but is able to move away from the support when the latter deflects under stresses generated in use of the bit.
In the embodiment of Figure 3, the preform C is brazed in a recess in the backing element 7 of a backing member assembly 7, 8, 9, 10. The element 10 of the backing member abuts the support 1 and these are wedged by a wedge member 8 in the socket 4 of a bit B. The backing member includes a neck 9 shaped to resist lateral movement of the preform C relative to the bit B. The discontinuity between the backing element 7 and the support 1 allows relative movement and reduces the effect on the preform C of deformation of the support 1 thus permitting the support 1 to be made of a softer material and/or smaller section. This wears away more easily and reduces friction, saving energy and reducing the heat generated. This is true even when the support 1 and the element 10 are integrally formed.
In the embodiment of Figure 4, the preform C is brazed to a backing element 7 which is abutted against a support surface 1. Two rods 2 extend from aligned holes in the bit body B and the backing element 7 to hold the preform C in position and isolate the preform C from deformations imposed on the support 1 in use of the drilling bit.
In the embodiment of Figure 5 a semi-circular or part-circular preform cutting element C is bonded to a backing element 7 which is preferably integrally formed with the flexible neck 9 and the support member 1. A slot 11 is formed between the element 7 and the support member 1 and an intermediate spacer 12 is provided to transmit the cutting forces from the backing element 7 to the support member 1. (Although a gap G is shown for clarity, the spacer 12 is preferably normally in contact with the support 1.) The discontinuity between the spacer 12 and the support 1 reduces the effect on the preform C of deformation of the support 1 and permits the support 1 to be made of a material softer than cemented tungsten carbide (e.g. steel) giving the same benefits as the previous embodiments.
In the embodiment of Figure 6, the circular preform cutting element C is bonded into a pocket P in the support element 1 which incorporates a partial structural discontinuity in the form of a slot 11. A spacer 12 is preferably but not necessarily present in the slot 11 to reduce vibration and deflection from near vertical forces, particularly when the cutter is partly worn. Cutting forces and horizontal impact forces cause deformation of the support member 1 which, without the slot 11, would cause curvature of the bond surface 13 between the preform C and the facing wall of the pocket P and bending and tension in the preform C. The slot permits the gap G to open, and a reduction in the bending and tensile strains in the preform C.
The embodiment of Figure 7 incorporates the feature of the embodiments of Figures 5 and 6 and is designed to ensure that the advantages of the invention are obtained as the cutting edge of the preform cutting element C is worn away. When the drilling bit is new, it behaves like the embodiment of Figure 5. As the preform C and the support 1 wear away, the behaviour approaches that of the embodiment of Figure 6.
In the embodiment of Figure 8 the lower slot is replaced by a hole or aperture 17 which provides a structural discontinuity to reduce the rigidity of that part of the support 1 behind the central region of the preform C, thus reducing the bending forces transmitted to the preform when the support 1 is deformed by the cutting forces.
In the embodiment of Figure 9 the preform C is brazed to one end of a support peg 5, the other end of which is received in a socket 4 in a bit body B. A hole 14 extends through the thickness of the peg 5 substantially parallel to the front surface of the preform C. The hole 14 provides a structural discontinuity to reduce the rigidity of that part of the support 1 behind the central region of the preform C, thus reducing the bending forces transmitted to the preform C when the support 1 is deformed by the cutting forces. More than one hole 14 may be present and the hole may be of any suitable cross-sectional shape. The hole may be disposed adjacent to the braze bond line. The percentage reduction of stress in the preform cutter provided by the embodiment of Figure 9 will depend on the particular shape and location of the hole but it is found in practice that a reduction of stress in the cutter of as little as 20% is worthwhile if it prevents fracture. Similar reductions in stress have been achieved, with steel pegs, by increasing the size of the pegs in relation to the size of the cutters, but such arrangements increase resistance to the operation of a worn bit, the cost, the distances between cutters and interfere with the efficiency of the hydraulics associated with the cutters.

Claims

1. A rotary drilling bit for deep hole drilling or coring in subsurface formations comprising a body (B) with a shank having a fluid bore, said body carrying a plurality of preform cutting elements (C), each having an associated support member (1, 5) carried by the body and disposed on the rearward side of the cutting element with respect to the normal direction of rotation of the bit, characterised in that each cutting element (C) includes, in known manner, a thin hard facing layer and a thicker, less hard, backing layer so that the cutting element is self-sharpening, and in that there is provided between each cutting element (C) and its associated support member (1, 5) a structural link which has high compressive strength but low tensile strength, whereby to reduce bending strain in the cutting element caused by deflection of the support member in use of the bit.

2. A bit according to Claim 1, characterised in that the cutting element (C) is separate from the support member (1) and locating means (2, Fig. 1; 6, Fig. 2) are provided to hold the cutting element in contact with a surface on the support member.

3. A bit according to Claim 2, characterised in that said locating means comprise an elongate resiliently flexible element (6, Fig. 2) one end of which is fixed to the bit body at a position spaced from said surface of the support member and the other end of which urges the cutting element towards the surface of the support member.

4. A bit according to Claim 3, characterised in that the resiliently flexible element (6) comprises a leaf spring.

5. A bit according to Claim 3 or Claim 4, characterised in that the fixed end of the resiliently flexible element (6, Fig. 2) is wedged between two separable parts (1, 8), one (1) of which defines the support member for the cutting element.

6. A bit according to any of Claims 2 to 5, characterised in that there are provided additional means (3) to locate the cutting element on the support member to prevent translatory movement of the cutting element along the fixed surface of the support.

7. A bit according to any of Claims 2 to 6, characterised in that there are provided additional means (3) to locate the cutting element on the support member to prevent rotation of the cutting element in the plane of its cutting surface.

8. A bit according to Claim 7 or Claim 8, characterised in that said additional means comprises a peg (3) formed separately from both the element and the support and, optionally, secured to one of them.

9. A bit according to Claim 1, characterised in that the cutting element (C) is bonded to a backing element (7) which is separate from the support member, and locating means (9, Fig. 3; 2, Fig. 4; 9, Fig. 5) are provided to hold the backing element in contact with a surface on the support member.

10. A bit according to Claim 9, characterised in that said locating means are resiliently flexible.

11. A bit according to Claim 9 or Claim 10, characterised in that said locating means comprise a neck (9, Fig. 3) integral with the backing element.

12. A bit according to any of Claims 9 to 11, characterised in that said locating means comprise elongate rods (2) extending between the bit body and the backing element.

13. A bit according to any of Claims 9 to 12, characterised in that the locating means (9, Fig. 5) are integral both with the backing element (7, Fig. 5) and with the support member (1, Fig. 5).

14. A bit according to Claim 1, characterised in that said low tensile strength in the structural link between each cutting element and its associated support member is provided by including at least one aperture (11, Fig. 6; 11, Fig. 7; 14, Fig. 8; 14, Fig. 9) in the structural link.

15. A bit according to Claim 14, characterised in that the aperture comprises a slot (11, Fig. 6; 11, Fig. 7; 14, Fig. 8) disposed parallel to and behind the major face of the preform.

16. A bit according to any preceding Claim, characterised in that the support member (1, Fig. 2; 1, Fig. 3; 1, Fig. 5; 1, Fig. 6; 1, Fig. 7; 1, Fig. 8; 5, Fig. 9) is separately formed from the bit body and is attached thereto.

17. A bit according to Claim 16, characterised in that the bit body includes a socket (4) adapted to receive a stud (5), constituting the support member to which the preform cutting element is attached.