GB2512978A

GB2512978A - Rotary tool

Info

Publication number: GB2512978A
Application number: GB201401798A
Authority: GB
Inventors: Mark Francis; Graham Richard Watson; Haydn G Smith
Original assignee: NOV Downhole Eurasia Ltd
Current assignee: NOV Downhole Eurasia Ltd
Priority date: 2013-02-05
Filing date: 2014-02-03
Publication date: 2014-10-15
Also published as: WO2014122440A2; GB201401798D0; WO2014122440A4; GB201306496D0; WO2014122440A3; GB201302028D0

Abstract

A drill bit includes a body which is rotatable, in use, about a body axis for drilling, and a cutting element 10 comprising a superhard portion 4 with a cutting face and fixed to an underlying substrate 5. The cutting element 10 is secured to the drill body so as to be moveable in a direction of movement when the drill bit is rotates, the cutting element 10 further being rotatable about a cutting element axis 7 thereof. The cutting element axis 7 extends away from the superhard material 4 through the substrate 5, and the cutting element axis 7 is tilted towards the direction of movement. The arrangement is such that the cutting element 10 has a leading surface (2a, fig 8) which bears against the formation and a trailing surface (2b, fig 8) which is spaced from the formation.

Description

Rotary Tool This invention relates to a rotary tool, and in particular to a rotary tool suitable for use in the formation or enlargement of boreholes in earthen formations.

A typical rotary drill bit comprises a bit body upon which are carried a series of cutting elements. The cutting elements may take a range of forms, but one form in common use comprises a substrate of, for example, tungsten carbide to which is bonded a layer of a superhard material such as polycrystalline diamond. Each cutting element is typically of generally cylindrical form, one generally circular end face and the adjacent part of the circumferential wall of the cylindrical element being of the superhard material. The cutting elements are typically fabricated by sintering under high pressure, high temperature conditions. This type of cutting element and the method of manufacture thereof is well known and so will not be described in further detail herein.

In a known arrangement, the bit body is shaped to define a plurality of generally radially extending blades, each of which carries a plurality of cutting elements. The cutting elements are typically brazed or otherwise secured in position in pockets formed on the blades, and the orientation of each cutting element is such that the superhard generally circular end face of each cutting element faces forwards in the drilling direction of rotation of the drill bit.

Figure 1 illustrates a drill bit of this general form, and Figures 2a and 2b illustrate, diagrammatically, one of the cutting elements of the drill bit of Figure 1 in use. The drill bit illustrated in Figure 1 comprises a bit body rotatable, in use, about its axis 18. The bit body includes a plurality of integrally formed, generally radially extending blades 16 spaced apart by channels 20. Each blade 16 has mounted thereon a series of cutting elements 10. As illustrated in Figures 2a and 2b, each cutting element 10 comprises a substrate 5, for example of tungsten carbide form, to which a table 4 of a superhard material such as polycrystalline diamond is integrally bonded. The cutting element 10 is of generally cylindrical form and is arranged such that table 4 defines an end face 2 of generally circular shape. The manner in which each cutting element 10 is mounted upon the bit body is such that the generally circular end face 2 of each cutting element is angled slightly relative to the direction A in which the cutting element 10 moves, in use. This angle is denoted by angle B in Figure 2. Commonly, the angle B is referred to as the negative or back rake angle.

In general, low back rake angles, such as shown in Figure 2a, are associated with aggressive drilling, and are more suitable for softer earth formations. Increasing the back rake angle, for example as shown in Figure 2b, tends to increase the volume of superhard material available for wear, increasing the durability of the cutting elements.

The back rake angles of this type of cutting element typically fall within the range of 100 to 30. Hereinafter, this type of cutting element is described as a conventional, forward facing culling element.

In use, the drill bit is rotated about its axis whilst a weight on bit loading is applied to the drill bit. As the drill bit rotates, the cutting elements thereof bear against and dig into the adjacent formation, gouging, scraping, abrading or otherwise removing formation material, which is subsequently carried to a remote location by a flow of drilling fluid, to form or extend the borehole. The part of the cutting element in contact with the formation, referred to herein as the shear area, will typically wear over time. The rate of wear will depend upon a number of criteria. For example, where used in drilling or culling highly abrasive materials or strong rocks, or when used in applications in which cooling of the drill bit is restricted, relatively high wear rates may be experienced. The wear will typically result in the formation of flats as the superhard material wears away.

As the wear continues, a point may be reached at which pad of the substrate upon which the superhard material is carried is exposed in the drilling direction. It is an object of the invention to provide a cutting element in which the volume of superhard material available for wear is increased.

The wear can also result in increased heat buildup and accelerated thermal failure.

The use of cutting elements of increased thermal stability, for example those where the, or some of the, catalyst material is removed or leached from the superhard material, can reduce this problem. It is another object of the invention to provide a cutting element in which cooling is enhanced.

U37726419 and U37455126 both describe drill bits in which part of the culling action involves a percussive action with the drill bit repeatedly engaging the bottom of the borehole. In these arrangements, a number of cutting elements are provided with their axes parallel to the axis of the drill bit to bear the cutting loads during such percussive cutting.

U32008/01 90670 describes a drill bit in which the back rake angle of the cutters is increased. For example, the gauge cutting elements thereof may be arranged with a back rake angle in the region of 40° to 70°. As mentioned hereinbefore, typically the back rake angle for a forward facing cutting element is in the regions of 10° to 30°, and so the back rake angle of the arrangement described in US2008/0190670 is unusually high. US6050354, US5467836 and US5655612 describe a drill bit in which gauge cutters are provided, the axes of the gauge cutters extending substantially radially of the drill bit. The back rake angle of such an arrangement is 90°.

US6332503 describes drill bits with cutting elements which have a non-circular cutting face. In one arrangement, the cutting element is essentially a cylinder with an extra thick diamond table which is mounted substantially vertically. The cylinder axis is at an acute angle to the direction of rotation of the drill bit. This arrangement can therefore be considered to have a back rake angle of at least 90°. The diamond table has a bevelled edge, and the cutting face is therefore frustoconical, and the clearance face is substantially flat and circular. US6003623 discloses similar arrangements with cufting elements described as having negative back rake angles, in which substantially cylindrical elements are mounted with their axes at an acute angle to the direction of rotation of the drill bit. These vertically mounted cufting elements increase the resistance of the drill to wear by presenting a larger volume of diamond behind the cutting surface that is available for wear. The underlying carbide substrate is thereby rendered remote from the wear at the cutting surface.

US4553615 describes drill bits with cylindrical cutting elements with thin flat circular cutting faces that are free to rotate. The cutting elements are mounted conventionally, with a shallow back rake angle, and it is further disclosed that the elements may be mounted at a side rake, so that the flat cutting face is not tilted perpendicular to the direction of rotation. This side rake helps to cause the cutting elements to rotate during use of the bit. The rotation of such cutting elements may be inconsistent, and the drilling environment may prevent or inhibit the rotation of such drilling elements.

US7762359 proposes that the cutting element may include surface features that promote rotation of the cutting element during use, for example helical features machined into a diamond table of the cutting element and/or substrate on which the diamond table is mounted.

US200B/0017419 discloses a drill bit in which cutting elements with flat circular cutting faces are actively rotated by a mechanism within the drill bit. It is shown that actively rotating the cutting elements results in improved wear resistance over cutting elements that are merely free to rotate (as described in US4553615). The mechanism proposed for rotating the cutting elements considerably increases the complexity and cost of the drill bit.

Other forms of rotary tool include, for example, reamers and under reamers, concentric hole openers and eccentric hole openers for use in the enlargement of bores.

There is a need for rotary tools with improved performance, particularly with improved resistance to wear. It is preferable that such improved tools are economic and of simple construction.

According to one aspect of the invention, there is provided a rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element having a cutting element axis and comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting element axis extending from the end face through the superhard material portion, and wherein the cutting element axis is tilted towards the direction of movement such that a part of the cutting element axis remote from the end face lies ahead of a part of the cutting element axis on the end face relative to the direction of movement.

The cutting element may be rotatable relative to the body about the cutting element axis thereof or may be fixed relative thereto.

According to another aspect of the invention there is provided a rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element having a cutting element axis and comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting element being rotatable relative to the body about the cutting element axis thereof, the cutting element axis extending from the end face through the superhard material.

According to a further aspect of the invention there is provided a rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting element being orientated such that a leading part of the end face of the superhard portion thereof bears against formation material, in use, and a trailing part thereof is spaced from the formation material.

According to another aspect of the invention there is provided a rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cufting face being tilted towards the direction of movement.

The cutting element preferably includes a part having a circular cross section perpendicular to the cutting element axis, and the said part is received by a corresponding recess in the body. The surface of the said part and/or the recess is preferably coated with a low friction material such as molybdenum disulphide, tungsten disulfide, graphite and/or diamond like carbon.

The cutting element may further be inclined to the side by a side rake angle.

The superhard portion may have a circular cross section, with the cutting face on the circumferential periphery thereof. The superhard portion conveniently has a chamfered edge, thereby modifying the shape of the cutting face. The chamfered edge may be angled to match the helix angle of the path followed by the cutting element, in use.

The cutting element is conveniently arranged so that, in use, cutting forces tend to rotate the cutting element about its axis.

A plurality of cutting elements may be provided, and the plurality of cutting elements may be arranged so that at least one cutting element is an offset cutting element, wherein the cutting face of the offset cutting element is offset from the offset cutting element axis to promote rotation thereof, the cutting face being asymmetric with respect to the cutter axis. For example, the cutting elements may be arranged with cutting paths that overlap so as to effectively offset the cutting face of at least one element.

Preferably, the superhard portion comprises polycrystalline diamond. It may be substantially free of a catalyst material in a region adjacent to the cutting face.

The invention further relates to a cutting element comprising a substantially disc shaped polycrystalline diamond table bonded to a substrate, wherein the polycrystalline diamond table is substantially free of a catalyst material in a region with a predetermined depth adjacent to at least a part of the circumferential side wall of the polycrystalline diamond compact.

The predetermined depth is preferably at least 100pm. By way of example, it may be in the range of 300-400pm.

The cutting element may include a substantially circular end face, and the pad of the circumferential side wall that is substantially free of catalyst material may extend from the end face to a position at least 50% of the thickness of the table. The portion of the side wall that is substantially free of catalyst material is preferably greater than this.

For example, it preferably makes up at least 75% of the thickness of the table, more preferably makes up at least 95% thereof, and ideally is 100% or thereabouts.

The diamond table may have a chamfered edge, which may be angled to match the helix angle of the path followed by the cutting element, in use.

The rotary tool may comprise a drill bit. Alternatively, it may be, for example a reaming tool or other bore enlargement tool. The cutting element may be a gauge cutting element. The bit body may comprise diamond impregnated portions.

One or more conventional cutting elements that do not rotate, and/or that have their cutting element axis tilted away from the direction of movement due to rotation of the tool about its axis while drilling may further be provided.

The invention will further be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic perspective view illustrating a typical rotary drill bit; Figure la is a view similar to Figure 1 illustrating a rotary drill bit in accordance with an embodiment of the invention; Figures 2a and 2b are schematic views illustrating the back rake angles of known cutting elements; Figure 3 is a schematic sectional view of cutting element according to an embodiment of the present invention; Figure 3a illustrates a modification; Figure 4 illustrates the difference between the available superhard material for wear according to prior art cutters and cutters according to an embodiment of the present invention; Figure 5 is a schematic view of an embodiment, with a cutting element in a recess of the drill bit body; Figures 6 to 9 are schematic views of cutting elements according to other embodiments; Figures 10 and 11 are views illustrating other forms of rotary tool to which the invention may be applied; Figures 12a and 12b illustrate techniques for securing cutting elements in position; Figure 13 illustrates an effect of the use of the invention; Figures 14 illustrates some alternative cutting element designs; and Figures 15 and 16 illustrate alternative configurations of cutting elements on blades of a tool.

Referring to Figure 1, as outlined hereinbefore, a prior art fixed cutter type rotary drill bit is shown, comprising a body 1 having an axis 18. The body 1 comprises a plurality of substantially radial blades 16 arranged around the distal end of the drill bit body 1, with channels 20 therebetween. Cutting elements 10 are fixed to each blade 16. A threaded portion is provided at the proximal end of the drill body for attachment to a drill string (not shown).

A typical cutting element 10 may have a body in the form of a circular disc having a thin front facing table of a superhard material such as polycrystalline diamond bonded to a substrate of less hard material such as cemented tungsten carbide or another metallic material. The cutting element 10 may further comprise a generally cylindrical carrier which may also be formed from cemented tungsten carbide.

In use, the drill bit body 1 is rotated about its axis 18 in the direction indicated by A (see Figure 2), so that the cutting elements 10 cut through the earth formation. The cutting face 3 of each cutting element 10 is a lower portion of the flat circular end face 2 of the diamond table 4 of each cutting element.

Referring to Figures 2a and 2b, individual cutting elements 10 are illustrated, comprising a diamond table 4 and substrate 5, cutting an earth formation 6. Figure 2a shows a cutting element 10 with a relatively low back rake angle e, and Figure 2b shows a cutting element 10 with a higher back rake angle e. The volume of the diamond table 4 that is available for wear in cutting the earth formation is increased with increasing back rake angle 9.

Referring next to Figures la and 3, a drill bit according to an embodiment of the invention is shown. The drill bit is, in many ways, similar to that shown in Figure 1 and described hereinbefore and only the significant differences therebetween are described herein. Like reference numerals are used to denote like parts where appropriate. The drill bit includes a bit body 1 upon which is mounted a number of cutting elements 10.

As shown in Figure 3, each cutting element 10 comprises a diamond table 4 and substrate 5, cutting an earth formation 6. The cutting element 10 is substantially cylindrical, and the diamond table 4 is substantially disc shaped. The cutting element is mounted upon a bit body, for example of a form similar to that shown in Figure 1, such that a cutting element axis 7 thereof, which extends rearwardly from the diamond table 4 through the centre of the cylindrical cutting element 10, is arranged to be inclined by an angle 13 towards the direction of movement A of the culling element 10 in use. In Figure 3, the rake angle e (which is analogous to the rake angle B for a conventional, forward facing cutting element such as shown in Figures 2a and 2b) is at least, and preferably greater than 90. By way of example, the rake angle B is preferably greater than 130. Accordingly, a part of the culling element axis 7 lying within the substrate 5 lies ahead of a pad lying within the diamond table 4 in the direction of movement of the drill bit and culling element, in use. However, it will be appreciated that this is merely one example, and that other arrangements are possible without departing from the scope of the invention.

Figure 4 compares the available volume of the diamond table 4 for wear of a culling element 10 according to an embodiment of the invention, and a conventional, forward facing cutter of the prior art. Orienting the cutting element 10 in this way significantly increases the volume of the diamond table 4 that is available for wear. Furthermore, the culling face 3 in this embodiment comprises the curved perimeter wall of the diamond table 4, rather than the flat circular face 2, which in this embodiment forms the clearance face. The curved cutting face 3 may provide a number of advantages including assisting in channelling chips from the earth formation away from the drill bit body 1.

The cutting elements may be secured in position using any suitable technique. For example, they may be brazed in position in the traditional manner. Alternatively, as the substrate 5 is supported by the bit body about the full circumference of the substrate, a number of other fixing techniques could be used. For example, as shown in Figure 12a, the substrate 5 may be provided or formed with a screw thread 5a arranged to mate with a corresponding thread provided in a socket formed in the bit body 1.

Alternatively, the culling element 10 and socket may be formed with recesses 5b arranged to accommodate a circlip 5c or the like to secure the culling element 10 in position. Further alternatives include the use of a grub screw or the like. Avoiding the use of brazing is thought to be advantageous in that it avoids applying heat cycles to the bit body 1 and cutting elements 10 during assembly.

As shown in Figure 3a, it may be desirable to provide a support region 16a of the blade 16 behind at least some of the cutting elements 10, in the cutting direction, in use, the support regions 16a providing additional support to the cutting elements 10. The support region 16a conveniently extends to a depth adjacent the table 4 so that it can assist in bearing loads tending to shear the table 4 from the substrate 5. The support regions 16a conveniently extend to a depth substantially coplanar with the end face 2 of each cutting element 10. A hard facing material coating 16b may be applied to the support regions 16a to enhance the weal resistance thereof Similar support regions (not shown) may be provided adjacent the sides of the cutting elements 10 relative to the cutting direction, these support regions serving to enhance the ability of the cutting elements 10 to withstand loadings substantially parallel to the axis of the bit 1 which may be experienced during tripping of the bit. As a result, the likelihood of the cutting elements 10 being damaged during tripping is reduced, and so the working life of the drill bit 1 may be enhanced.

This orientation of the cutting elements 10 further has the advantage that loads in the direction of the drill bit body axis 18 tend to compress the diamond table 4 onto the substrate 5. The cutting element 10 is thereby relatively resistant to axial shocks that may arise during drilling. The cutting element 10 is also less susceptible to spalling resulting from excessive point loadings than is the case with traditional forward facing cutting elements.

The applicant has tested this orientation of cutting element, and found that similar forces are generated at similar cutting conditions to a conventional orientation of cutter (as shown in Figures 1 and 2). Shear length is slightly increased for a given shear area so efficiency is reduced, but this is offset by the increase in life to be gained.

Furthermore, aggressive cutting orientations, in which the cutting face is substantially perpendicular to the direction of movement, tend to increase the volume of diamond table available for wear.

As mentioned above, the cutting elements 10 include a cutting face 3 of curved rather than planar form. As a consequence, compared to an arrangement in which the cutting faces are generally planar and face the direction of rotational movement of the drill bit, and so the cutting forces act approximately in the direction of movement, the cutting elements 10 orientated in accordance with the invention apply cutting forces in a range of directions.

Furthermore, substantially the full cross-sectional area of the cutting element 10 or the full area of the end face 2 is capable of bearing against the formation, serving to control depth of cut and to bear the applied weight on bit loads, making the bit less susceptible to issues associated with fluctuations in the applied weight on bit loads.

Where the depth of cut does change, the profile cut by each cutting element does not change considerably and so the shear length associated with each cutting element remains substantially fixed compared to an arrangement using traditional forward facing cutting elements. This effect is illustrated diagrammatically in Figure 13, the left hand side of which shows that a change d in the depth of cut does not significantly impact upon the shear length s of a cutting element 10 orientated in accordance with the invention whereas for a traditionally orientated forward facing cutting element the shear length is increased from s' to s'.

The cutting elements 10 may be arranged so that parts of at least some of the cutting elements 10 are shielded behind others of the cutting elements 10. As a consequence, the interaction between each partially shielded cutting element 10 and the formation may result in the application of a side load to the drill bit 1. It will be appreciated that the positioning of the cutting elements 10 may thus be used to achieve management or control over, or balancing of, other side loads that will be experienced by the drill bit 1 in use. By way of example, the layout of the cutters in a bicentre bit could be used to correct or partially correct the out of balance forces experienced in certain drilling modes thereof.

Compared to a traditionally orientated foiward facing cutting element, the tip profile of a cutting element positioned as described herein will have a relatively long shear length s. Where this is not desired, tilting of the cutting elements 10, or some of the cutting elements 10 may be used to reduce the effective shear length.

In accordance with one aspect of the invention, the cutting element 10 is arranged to be rotatable about its axis 7, so that different portions of the diamond table 4 form the cutting face 3 at different times. The rotation of the cutting element 10 makes a greater volume of the diamond table 4 available for wear during cutting, as the full periphery of the cutting element 10 is available for use. Furthermore, the rotation of the culling element 10 reduces temperature buildup at the culling face, reducing the wear rate of the diamond table 4, as the part of the cutting element 10 forming the cutting face is continually changing.

A number of arrangements are possible for securing each or at least some of the cutting elements 10 to the bit body to allow the cutting elements 10 to rotate about their respective axes 7. For example, the techniques outlined in some of the prior art documents mentioned hereinbefore in which substantially horizontally oriented culling elements are able to rotate about their axes may be modified for use in the invention.

One possibility is shown in Figure 5 in which the cutting element 10 is disposed within a corresponding cylindrical recess 11 of the drill bit body 1. The cutting element 10 will thereby be supported against lateral loads arising from drilling, and from axial loads resulting from drilling. Encapsulating the cutting element 10 within the recess 11 means that retention is only needed to prevent the culling element 10 from falling out of the recess 11, and any suitable retention means may be used for this purpose, for example a spring clip, grub screw or other retainer may be used. Alternative arrangements are also possible. For example, the substrate could be formed with an axially extending recess arranged to receive a generally cylindrical projection formed on the bit body to support the culling element on the bit body for rotation about the axis of the cutting element. In the case shown in Figure 5, it will be appreciated that it is the orientation of the recess 11 formed in the bit body 1 that governs the angle j3 of the associated culling element 10. The nature of the retention means is such that, whilst retaining the cutting element 10 within the recess 11, the cutting element 10 is free to rotate about its axis 7.

In some arrangements, it may be desirable to reduce wear between the culling element and the bit body 1. One way in which that may be achieved is to apply a low friction material to the journal surface of the culling element 10, and/or on the adjacent bearing surface of the drill bit body recess 11. The low friction material may comprise a solid lubricant, and may comprise molybdenum disulphide, tungsten disulfide, graphite and/or diamond like carbon. However, other bearing arrangements may be used without departing from the scope of the invention.

The cutting elements 10 may be placed or positioned upon the bit body such that, in use, a load is generated on at least one of the cutting elements 10 that urges that cutting element 10 to rotate. This may be achieved by arranging for the loads arising from cutting to be asymmetric relative to the cutting element's axis 7. The substantially vertical orientation of the cutting element 10 makes this much easier to achieve than in prior art rotating cutting elements that are forward facing, substantially horizontally arranged. By way of example, the cutting paths of different cutting elements 10 may be overlapped so that the cutting face 3 of each overlapping cutting element 10 comprises a portion of the front facing curved perimeter wall of the diamond table 4 which is offset from the cutting element axis 7. This will result in the application of a load to the cutting element 10 urging rotation thereof, in use.

Whilst this arrangement results, automatically, in rotation of the cutting elements, it will be appreciated that other techniques, including those in which the cutting elements 10 are positively driven for rotation by appropriate drive means fall within the scope of the invention.

The applicant has found in experiments to test the concept that off centre forces generated according to an embodiment of the invention were sufficient to overcome efforts made to secure the cutting element 10 from rotating, and so are sufficient to drive the cutting elements 10 for rotation about their respective axes, in use. In a further experiment a cutting element 10 was used to cut a granite cylinder under conditions used for standard cutter evaluation. Three times the normal amount of rock was removed when the test was stopped due to lack of wear on the cutting element. It is thereby clear that increases in wear resistance considerably in excess of 300% arise according to embodiments of the invention.

The arrangement outlined hereinbefore is advantageous in that a large proportion of the diamond table 4 is available for use before the cutting element and/or drill bit require replacement. The rotation of the cutting element 10 further serves to allow or enhance cooling of the cutting element 10 which may also have reduced wear and enhanced service life benefits.

In the context of cutting elements, it is known to leach catalyst material from a region of the diamond table adjacent to the cutting face to enhance thermal stability without compromising toughness. In known cutting elements the cutting face is a portion of the flat circular front face of the cutting element. Prior art cutting element leaching processes therefore leach the front face thereof to remove a catalyst material to a predetermined depth therefrom. Limited leaching of catalyst material from the peripheral surface may also take place.

In accordance with another aspect of the invention, as the cutting elements 10 are orientated generally vertically with the peripheral surface of the table 4 serving as the cutting face 3, the catalyst material is preferably leached or otherwise removed from the parts of the table 4 adjacent at least pad of the peripheral wall, either in addition to the removal of catalyst material from adjacent the end face 2, or as an alternative to removal of the catalyst material from adjacent the end face 2. By way of example, as shown diagrammatically in Figure 6, the table 4 may include a portion 4a adjacent the end face 2 from which catalyst material is removed, and is may include a peripheral portion 4b from which catalyst material is removed. The peripheral portion 4b may extend inwardly from the peripheral wall to a depth of at least 200pm. The portion 4b serves, in use, as the cutting face 3. The table 4 conveniently includes a further peripheral portion 4c adjacent the substrate 5 and from which the catalyst material is not removed.

The portion 4b conveniently extends over at least 50% of the thickness of the table.

However, the portion 4b of the side wall that is substantially free of catalyst material is preferably greater than this. For example, it preferably makes up at least 75% of the thickness of the table, more preferably makes up at least 95% thereof, and ideally is 100% or thereabouts.

The cutting element 10 of the type shown in Figure 6 may be mounted so as to be rotatable about its axis 7 as described hereinbefore. However, the invention is not restricted in this regard, and also relates to the use of such a cutting element when rigidly mounted upon the bit body against rotation about the axis of the cutting element.

As shown in Figure 7, the cutting element 10 is conveniently shaped to be of bevelled form, having a bevel or chamfered poition 20 formed at the intersection between the end face 2 and the peripheral wall of the superhard material table 4. The chamfered portion 20 is conveniently angled such that the part thereof adjacent the cutting face 3 at any given time lies flat against the bottom of the borehole being formed. The engagement between the chamfer 20 and the bottom of the borehole serves to limit the distance by which the cutting element 10 can dig into the formation, and so serves to control or limit the depth of cut. In order to operate in this manner, the chamfer 20 is preferably angled so as to match the helix described by the cutting element 10, in use, as the drill bit rotates and progresses forwards. In embodiments with substantially cylindrical cutting elements, this may be achieved by selecting an appropriate angle for the cutting element.

It will be appreciated that the depth of cut control technique achieved by the provision of the chamfer 20 may be used both in conjunction with rotating cutting elements, and with cutting elements which do not rotate, in use. Furthermore, it can be used with cutting elements in which the catalyst has been leached or otherwise removed from the peripheral surface, and with ones in which such removal has not taken place.

The present invention provides a rotating cutting element without the need for complex engineering. Orienting the cutting element so that the sidewall of the superhard element forms the cutting face, with the substrate leading the superhard element allows the cutting elements to be arranged so as to offset the shear area from the cutting element axis, thereby resulting in consistent and reliable rotation. Orienting the cutting element axis at angles of greater than 45° to the direction of movement of the cutting element in use tends to increase the tendency of any off-axis forces arising from cutting to rotate the cutter. The invention allows the cutting of earth formations currently at the limit of polycrystalline diamond compact fixed cutter drill bits by extending the cutter life, and further in applications where a low depth of cut is currently required in order to maximise the lifetime of the prior art cutting elements by ensuring that substrate is not exposed to rock by wearing of the diamond table. The rotating cutting element will allow higher rotation speeds of the drill bit without causing premature failure of the diamond table, thus allowing high rotation speed, low depth of cut (and hence low force) operation to achieve improved performance in hard rocks. Furthermore, reduced cutter wear allows fewer cutting elements to be used, minimising the weight on bit required to drill by spreading the load over fewer cutting elements. Embodiments of the invention extend the limits of existing PDC cutter operation into applications where diamond impregnated bits or insert bits have previously been favoured.

In the arrangements described hereinbefore, the cutting elements are of cylindrical or substantially cylindrical form, having an end face and a peripheral cutting face, the end face being normal to an axis of the cutting element. This need not always be the case and Figures 8 and 9 illustrate two alternatives. In Figure 8, the cutting element 10 is of substantially cylindrical form, but the end face 2 thereof is angled or relieved relative to the axis of the cutting element. Such a cutting element may be mounted upon the bit body in such a manner that the axis 7 thereof is not tilted towards the direction of movement, whilst still resulting in the provision of a back rake angle of greater than 9O.

Clearly, such an arrangement has a leading pad 2a of the end face 2 bearing against the formation material whilst a trailing part 2b thereof is spaced from the formation material.

Figure 9 illustrates an arrangement which, whilst of generally cylindrical form, is machined or otherwise treated to as to result in the formation of a flat 3 which serves, in use, as the cutting face. The flat 2 forming the cutting face is conveniently orientated so as to be perpendicular to or tilted forwardly relative to the direction of movement of the cutting element, in use.

Another alternative is for the cutting elements to be of polygonal cross sectional shape.

For example, they could be of substantially hexagonal cross-sectional shape.

Furthermore, the end face, and/or the peripheral cutting face, may be of concave form.

In many of the arrangements described hereinbefore the cutting element 10 is of generally cylindrical form. Where the cutting elements 10 are mounted upon a blade 16 of curved form it will be appreciated that the axes of adjacent ones of the cutting elements 10 will not be parallel, and in order to accommodate the substrates thereof, the exposed tables 4 thereof will be spaced apart by a significant distance. In order to allow the cutting elements 10 to be packed relatively closely to one another, each cutting element 10, or at least some of the cutting elements 10, are conveniently of tapering or stepped form so as to be of reduced diameter at the end thereof remote from the table 4. Figure 14 illustrates this, the left hand part of Figure 14 illustrating the spacing of cylindrical cutting elements 10, the centre part illustrating the use of tapered cutting elements 10 and the right hand part illustrating the use of stepped diameter cutting elements 10. Whilst in Figure 14 just the substrate is of stepped or tapering form, the shape of the table may also be modified if desired.

Whilst in some arrangements close packing of the cutting elements 10 is preferred, for some formation types there may be a need to provide grooves 50 (see Figure 15) or the like between adjacent ones of the cutting elements lOso as to increase the flow of drilling fluid between the cutting elements 10 and enhance the removal of cuttings therefrom, especially cuttings formed away from the front of the cutting elements 10.

Depending upon the arrangement of the cutting elements 10, the formation of cuttings may not occur symmetrically around an individual cutting element 10. In such an arrangement, the grooves 50 may be arranged asymmetrically, positioned to correspond with the locations in which the cuttings are formed, in use. By way of example, the grooves 50 may be provided adjacent just the radially outer sides of the cutting elements 10, or may be deeper there than at the radially inner sides.

Where close packing is desired, this may be achieved by placing cutting elements 10 in a staggered pattern or the like (see Figure 16) on each blade 16, rather than simply placing the cutting elements in rows as is traditional. Such close packing allows part of each cutting element 10 to be shielded by part of another cutting element 10. Such an arrangement achieves a degree of redundancy and so enhances the working life of the bit as if one of the cutting elements fails, at least part of the function thereof can be adopted by one or more of the other cutting elements 10.

In an arrangement of the type in which each blade is provided with a series of traditional forward facing cutting elements, the superhard material tables of the cutting elements form a highly wear resistant leading edge to the blade. Where the cutting elements are arranged as described hereinbefore the wear resistance of the leading edge of the blade may be reduced, and so the blade may be susceptible to increased erosion due to the interaction of the drilling fluid therewith. In order to resist or reduce such erosion in the vicinity of the cutting elements 10, the drilling fluid is conveniently directed, at least in part, away from the cutting elements 10, for example toward spaces or grooves 50 therebetween. To achieve this, the front faces of the blades 16 may be provided with ridges 52 (see Figure 15) or other formations serving to direct a proportion of the drilling fluid towards a desired location. Alternatively, the nozzles through which the drilling fluid is delivered may be shaped to direct the drilling fluid in a desired direction to reduce such erosion. Other options include applying a wear resistant material to the susceptible pads of the blades, or to shape the blades in such a fashion that the fluid velocity in the regions susceptible to wear is reduced, thereby reducing the likelihood of erosion thereof.

Whilst the cutting elements 10 described hereinbefore include a substrate 5, as the invention permits the use of securing techniques other than brazing there may be arrangements in which there is no need for the cutting elements 10 to include a substrate 5. For example, the cutting elements 10 could be of a binder free Nano polycrystalline diamond form with no substrate 5.

Where a substrate is present, there may be arrangements in which it is desired for the thickness of the table to be considerably greater than is the case with traditional forward facing cutting elements. For example, the table thickness could be in the region of 4mm.

Various aspects of the cutter element need to be optimised for best performance of the cutter including, for example: back rake angle, side rake angle, chamfer, cutter placement, and force balancing. Some of these aspects are intrinsic to the cutter, for example the chamfer, and others are dependant on the arrangement of the cutting element(s) on the drill bit. The appropriate parameters for a rotatable cutting element according to an embodiment of the present invention may differ from those suitable for

prior art cutting elements.

Although embodiments have been described in which the superhard element comprises diamond, it will be appreciated that other suitable materials may used, such as cubic boron nitride.

Whilst the description hereinbefore relates to a bit in which all of the cutting elements are orientated in the manner described, the invention also applies to drill bits 1 including one or more forward facing cutting elements. For example, this may be advantageous towards the centre of the bit where there is only space to provide a few cutters and so the depth of cut of each cutting element may be such that it is greater than the thickness of the table 4 and so would be difficult to achieve with a cutting element orientated as described hereinbefore.

For convenience, the description hereinbefore relates primarily to cutting elements 10 mounted at the nose part of the drill bit It should be appreciated, however, that the invention is not restricted in this regard and is also applicable to arrangements in which the cutting elements are located at or close to the bit gauge, or elsewhere on the bit.

Whilst these cutters are not orientated generally vertically, ie parallel to the axis of the bit body when the bit body is orientated vertically, they are still orientated such that the axis of the cutting element is tilted towards the drilling direction as described hereinbefore.

Furthermore, whilst the description hereinbefore relates primarily to the application of the invention to a rotary tool in the form of a rotary drag type, fixed cutter drill bit, the invention is also applicable to other forms of rotary tool. By way of example, it may also be applied to tools employed to enlarge an existing bore, such as reaming tools, under reamers, and eccentric and concentric hole openers, bicentre bits and bits with concentric pilot and reamer sections. It may be applied to bits with diamond impregnated material bodies. Figure 10 illustrates the invention employed in an adjustable reaming tool of the type including a series of radially movable blades 30, each of which carries a series of cutting elements 32. The blades 30 are radially movable between a retracted position in which they are housed largely within an associated tool housing 34, and an extended position in which they project from the housing 34. In accordance with the invention, the cutting elements 32 are arranged substantially as hereinbefore described such that a cutting element axis thereof is tilted towards a direction of movement of the tool, in use with the result that a leading part of an end face superhard portion thereof bears against the formation material whilst a trailing part thereof is spaced from the formation material, and a cutting face of the cutting element is tilted towards the direction of movement. Alternatively, rather than orientating the cutting elements 32 to achieve this effect, they may be shaped to achieve this effect. If desired, the cutting elements 32 may be mounted in such a manner as to be rotatable relative to the blades 30.

Whilst Figure 10 illustrates one form of reamer, it will be appreciated that the invention may be applied to a range of other designs of reaming or under reaming tool.

Furthermore, as shown in Figure 11 the invention may be applied to a hole opening tool. In the tool of Figure 11, a pilot drill bit section 40 is connected to a main drill bit section 42. The pilot and main bit sections 40, 42 are arranged concentrically. Each bit section 40, 42 is provided with a series of cutting elements 44 orientated or shaped as outlined hereinbefore. Either one or the other, or both, of the pilot and main sections 40, 42 may be provided with cutting elements 44 orientated or shaped in accordance with the invention, if desired. Whilst Figure 11 illustrates a concentric hole opener, it will be appreciated that the invention may also be applied to eccentric, or bi centre drill bits, if desired.

With a drill bit of the type described hereinbefore, it is thought that the cutting elements need to be positioned with a relatively high degree of accuracy. Accordingly, it may be the case that traditional manufacturing techniques such as mud moulding may not be suitable as the sockets in which the culling elements are fitted may not be sufficiently accurately formed. Accordingly, it may be preferable to manufacture the bit body of the tool by, for example, machining the body from a steel billet or by the use of a hard carbon moulding technique to form a matrix material body.

Where the tool body is provided with a wear resistant coating, for example using a flame spraying, welding or other thermal based process, a graphite plug is preferably inserted into each socket prior to the application of the wear resistant material. The plugs are conveniently dimensioned to accurately reflect the dimensions of the cutting elements to be used. Accordingly, it can be ensured that the coating thickness is such that the surface of the coating will be substantially coplanar with the end face of each cutting element 10 in the finished tool.

Whilst specific embodiments of the invention are described hereinbefore, it will be appreciated that a number of modifications and alterations may be made thereto without departing from the scope of the invention.

Claims

CLAIMS: 1. A rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element having a cutting element axis and comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting element axis extending from the end face through the superhard material portion, and wherein the cutting element axis is tilted towards the direction of movement such that a pad of the cutting element axis remote from the end face lies ahead of a part of the cutting element axis on the end face relative to the direction of movement.
2. The tool of claim 1, wherein the superhard portion of the cutting element is fixed to an underlying substrate.
3. The tool of claim 1 or claim 2, wherein the cutting element is rotatable relative to the body about the cutting element axis thereof
4. The tool of claim 3, wherein the cutting element is arranged so that, in use, cutting forces tend to rotate the cutting element about its axis.
5. The tool of claim 4, comprising a plurality of cutting elements.
6. The tool of claim 5, wherein the plurality of cutting elements are arranged so that at least one cutting element is an offset cutting element, wherein the cutting face of the offset cutting element is asymmetric relative to the cutting element axis thereof to promote rotation thereof.
7. The tool of claim 6, wherein cutting elements are arranged with cutting paths that overlap so as to effectively offset the cutting face of at least one element.
8. The tool of any of the preceding claims, wherein the peripheral cutting face is of substantially cylindrical or pad cylindrical shape, or is of polygonal shape.
9. The tool of claim 8, further comprising channels to direct drilling fluid adjacent at least a side of the cutting element.
10. The tool of claim 8, further comprising flow directing means for directing drilling fluid.
11. The tool of claim 10, wherein the flow directing means comprise ridges provided upon the bit body.
12. The tool of any of the preceding claims, wherein the cutting element is of substantially tapering diameter.
13. The tool of any of the preceding claims, wherein a part of the cutting element is of substantially cylindrical form and another part thereof is of substantially tapering diameter.
14. The tool of any of claims 1 to 11, wherein the cutting element is of stepped diameter.
15. The tool of any of the preceding claims, wherein the cutting element is provided with a screw thread formation for cooperation with a corresponding formation provided in a socket of the bit body.
16. The tool of any of the preceding claims, wherein the cutting element is provided with a groove adapted to accommodate a circlip to secure the cutting element to the body.
17. The tool of any of the preceding claims, wherein a support region is provided behind the cutting element in the direction of movement, the support region lying adjacent at least a part of the superhard portion.
18. A rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element having a cutting element axis and comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting element being rotatable relative to the body about the cutting element axis thereof, the cutting element axis extending from the end face through the superhard material.
19. The tool of claim 18, wherein the cutting element axis is perpendicular to the direction of movement.
20. The tool of claim 18, wherein the cutting element axis is tilted towards the direction of movement.
21. The tool of claim 18, wherein the cutting element is arranged so that, in use, cutting forces tend to rotate the cutting element about its axis.
22. The tool of claim 21, comprising a plurality of cutting elements.
23. The tool of claim 22, wherein the plurality of cutting elements are arranged so that at least one cutting element is an offset cutting element, wherein the cutting face of the offset cutting element is asymmetric relative to the offset cutting element axis to promote rotation thereof.
24. The tool of claim 23, wherein cutting elements are arranged with cutting paths that overlap so as to effectively offset the cutting face of at least one element.
25. A rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element comprising a superhard portion with an end face and a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting element being orientated such that a leading part of the end face superhard portion thereof bears against formation material, in use, and a trailing pad thereof is spaced from the formation material.
26. The tool of claim 25, wherein the cutting element has a cutting element axis orientated such that the leading part of the end face bears against the formation material and the trailing part thereof is spaced from the formation material.
27. The tool of claim 26, wherein the cutting element has a cutting element axis, and at least part of the end face is tilted relative to the cutting element axis.
28. A rotary tool comprising a body which is rotatable, in use, about a body axis for drilling, and a cutting element comprising a superhard portion with an end face of a peripheral cutting face, the cutting element being secured to the body so as to be moveable with the body in a direction of movement when the tool is rotated, the cutting face being tilted towards the direction of movement.
29. The tool of claim 28, wherein the cutting face is of cylindrical or part cylindrical form.
30. The tool of claim 28, wherein the cutting face comprises a flat.
31. The tool of any of the preceding claims, wherein the cutting element includes a part having a circular cross section perpendicular to the cutting element axis, and the said part is received by a corresponding recess in the body.
32. The tool of claim 31, wherein a surface of the said part and/or recess is coated with a low friction material.
33. The tool of claim 32, wherein the low friction material comprises molybdenum disulphide, tungsten disulfide, graphite and/or diamond like carbon.
34. The tool of any preceding claim, wherein the cutting element is further inclined to the side by a side rake angle.
35. The tool of any preceding claim, wherein the superhard portion has a circular cross section, with the cutting face on the circumferential periphery thereof.
36. The tool of any preceding claim, wherein the superhard portion has a chamfered edge, thereby modifying the shape of the cutting face.
37. The tool of claim 36, wherein the chamfered edge is angled to match the helix angle of the path followed by the cutting element, in use.
38. The tool of any preceding claim, wherein the superhard portion comprises polycrystalline diamond.
39. The tool of claim 38, wherein the polycrystalline diamond is leached to be substantially free of a catalyst material in a region adjacent to the cutting face.
40. The tool of any preceding claim and comprising a rotary drill bit.
41. The tool of any of Claims ito 39 and comprising a bore enlargement tool.
42. The tool of any of the preceding claims, wherein the end face is of concave form.