EP1052367B1

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Info

Publication number: EP1052367B1
Application number: EP00303645A
Authority: EP
Inventors: Nigel Dennis Griffin; Dean Travers Watson
Original assignee: Camco International UK Ltd
Current assignee: ReedHycalog UK Ltd
Priority date: 1999-05-14
Filing date: 2000-04-28
Publication date: 2005-04-06
Anticipated expiration: 2020-04-28
Also published as: EP1052367A3; GB2350381A; DE60019189T2; GB0010226D0; DE60019189D1; GB2350381B; EP1052367A2; GB9911139D0

Description

The invention relates to preform cutting elements for rotary drag-type drill bits, and of thekind comprising a facing table of superhard material having a front face and a rear surface bondedto the front surface of a substrate which is less hard than the superhard material.
Such preform cutting elements usually have a facing table of polycrystalline diamond,although other superhard materials are available, such as cubic boron nitride. The substrate of lesshard material is often formed from cemented tungsten carbide, and the facing table and substrateare bonded together during formation of the element in a high pressure, high temperature formingpress. This forming process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generallycylindrical stud or post received in a pocket in the body of the drill bit. The carrier is often alsoformed from cemented tungsten carbide, the surface of the substrate being brazed to a surface onthe carrier. Alteratively, the substrate itself may be of sufficient thickness as to provide, in effect,a cylindrical stud which is sufficiently long to be directly received in a pocket in the bit body,without being first brazed to a carrier. As is well known, the body of the drill bit itself may bemachined from metal, usually steel, or may be moulded using a powder metallurgy process.
Cutting elements of the above-described kind are often in the form of circular or part-circulartablets. Each cutter is so mounted on the bit body that a portion of its periphery definesa cutting edge which acts on the surface of the formation being drilled. In the case of a circularcutter the cutting edge will be provided by a curved stretch of the circular periphery of the cutter.In some locations on the drill bit, such as in the gauge region of the bit, however, the cutting element will be formed with a straight cutting edge across part of its periphery to acton the formation.
When drilling some types of formation, however, for example chalk orlimestone, it may be more effective and efficient for the cutting elements to act onthe formation with more of a "scribing" action. For this purpose the cutting elementis shaped so that a part of its periphery is "pointed", i.e. is formed with a cuttingapex defined between two adjacent regions of the peripheral surface which aredisposed at an angle to one another. The cutting element may comprise a singlecutting apex, or a plurality of cutting apices disposed side-by-side on its periphery.
Although cutters of this configuration may cut some formations moreeffectively, they tend to suffer from accelerated wear and also from impact damageas a result of abrasive impact forces being concentrated on a comparatively smallarea of the periphery of the cutting element. The present invention sets out toprovide an improved preform cutting element of this type where this disadvantagemay be reduced or overcome.GB 2323110 describes a cutting element having a protrusion formed on therear of a table of a superhard material, the protrusion being located in a spacebetween two substrate components.
According to the invention there is provided a preform cutting element, for a rotary drag-type drill bit, including a facing table of superhard material having afront face and a rear surface bonded to the front surface of a substrate which is lesshard than the superhard material, the cutting element having a peripheral surface andcharacterised in that the peripheral surface includes at least one cutting apex defmedbetween two adjacent regions of the peripheral surface which are disposed at anangle to one another, the rear surface of the facing table being formed with at leastone protuberance which projects into a correspondingly shaped recess in thesubstrate, the protuberance forming part of the periphery of the cutting element atthe cutting apex and extending inwardly away from the cutting apex into the interiorof the substrate.
The protuberance of superhard material projecting from the facing table intothe substrate serves to strengthen the element and serves to reduce the rate of wearof the cutting apex and also to protect it against impact damage.
Preferably, the rear surface of the facing table and the front surface of thesubstrate of the cutting element are co-extensive, although this is not essential, andarrangements are possible where the facing table extends over only a part of thefront surface of the substrate.
The protuberance may extend across the full width of the cutting element soas also to form part of the periphery of the cutting element in a region thereof of opposite said cutting apex. The protuberance may have substantially parallel sidesurfaces bonded to the sides of a recess in the front surface of the substrate.
In any of the above arrangements, the cutting element may be formed witha plurality of cutting apices spaced apart around the periphery thereof. The part ofthe periphery between two adjacent cutting apices may be smoothly concave orangular. Preferably, where there are a plurality of cutting apices, imaginary linesbisecting the cutting apices are substantially parallel so that the cutting apices pointin generally the same direction.
In any of the above arrangements the periphery of the cutting element maybe part-circular. In this case one or both regions of the periphery on either side ofa cutting apex may extend generally tangentially to a part-circular portion of theperiphery of the cutting element.
The following is a more detailed description of embodiments of the invention,by way of example, reference being made to the accompanying drawings in which:
Figure 1 is a diagrammatic side elevation of one form of rotary drag-type drillbit of a kind with which the preform cutting elements of the present invention maybe used;
Figure 2 is an end view of the bit shown in Figure 1,
Figure 3 is a top view of one form of preform cutting element for a drag-type drill bit, in accordance with the present invention;
Figure 4 is a section on the Line 4-4 of Figure 1,
Figures 5-22 are similar views showing alternative forms of preform cuttingelement in accordance with the invention;
Figures 23 and 24 are diagrammatic perspective views of the substrates oftwo further forms of preform cutting element;
Figure 25 is a top view of a further form of preform cutting element,
Figure 26 is a side elevation of the cutting element shown in Figure 25,
Figure 27 is a top view of a further form of preform cutting element,
Figure 28 is a side elevation of the cutting element shown in Figure 26,
Figures 29-32 are diagrammatic perspective views of further forms of preform cuttingelement in accordance with the invention,
Figures 33-48 are top and sectional views of still further forms of preform cutting elementin accordance with the invention, and
Figures 49 and 50 are side elevation and top views of a cutting element in accordance withanother embodiment of the invention.
One typical and well known form of drag-type drill bit is shown in Figures 1 and 2 andcomprises abit body 1 on the leading surface of which are formed six circumferentially spacedupstanding blades 2 which extend outwardly away from the axis of rotation of the bit. A numberof preformcutting elements 3 are spaced apart side-by-side along eachblade 2. As previouslymentioned, each cutting element may be brazed to a carrier which is secured within a pocket intheblade 2, or the substrate of the cutting elements may be of sufficient length to be receiveddirectly in the pocket. The cutters are arranged in a generally spiral configuration over the leadingface of the drill bit so as to form a cutting profile which sweeps across the whole of the bottomof the borehole being drilled as the bit rotates.
In Figures 1 and 2 all of the cutting elements on the drill bit are shown as being "pointed"cutters, each being formed with a cutting apex defined between two adjacent regions of theperipheral surface which are disposed at an angle to one another. However, it will be appreciated that it is not essential, according to the present invention, for all of the cutting elements on the drillbit to be of this type and some of the cutting elements may be of other shapes, for example theymay be circular or part-circular cutting elements, particularly in the gauge region of the drill bit.
The bit body is formed with a central passage (not shown) which communicates throughsubsidiary passages with nozzles 4 mounted in the leading surface of the bit body. Drilling fluidunder pressure is delivered to the nozzles through the internal passages and flows outwardlythrough the spaces between adjacent blades for cooling and cleaning the cutters. The spacesbetween theblades 2 lead to peripheral junk slots 5, or to internal passages 6 in the bit body,through which the drilling fluid flows upwardly to the annulus between the drill string and thesurrounding formation, passing upwardly through the annulus to the surface. The junk slots 5 areseparated bygauge pads 7 which bear against the side wall of the borehole and are formed withbearing or abrasion inserts (not shown).
The bit body and blades may be machined from metal, usually steel, which may behardfaced. Alternatively the bit body, or a part thereof, may be moulded from matrix materialusing a powder metallurgy process. The general construction of such drill bits, and their methodsof manufacture, are well known in the art and will not be described in further detail.
Figures 3 and 4 show a preform cutting element according to the present invention whichmay be employed on a rotary drag-type drill bit of the above-described kind, or other form ofdrag-type drill bit.
The cutting element comprises a front facing table 10 of polycrystalline diamond bondedto asubstrate 12 of cemented tungsten carbide. The facing table 10 is co-extensive with thesubstrate 12 so that the whole of the peripheral edge of the facing table is exposed around the periphery of the cutting element.
As may be seen from Figure 3, oneportion 14 of the periphery of the cutting element ispart-circular and extends around approximately half of the periphery of the element. Theremainder of the periphery of the cutting element is defined by two substantiallystraight regions16 of the periphery which are arranged at an angle to one another so as to define between thema generallypointed cutting apex 18. In use, the cutting element is so mounted on the drill bit thatthecutting apex 18 engages the formation being drilled and, as the bit rotates, cuts the formationwith a "scribing" action.
In order to reduce the susceptibility of thecutting apex 18 to rapid wear and impactdamage, the facing table 10 is formed with a thickenedrim 20 which extends continuously aroundthe whole periphery of the facing table and thus forms part of the whole periphery of the cuttingelement. The thickenedperipheral rim 20 projects into a correspondingly shaped peripheral recessin thesubstrate 12 and is bonded to the substrate during the process of forming the cuttingelement.
The thickened rim may project from the rear surface of the facing table 10 by any desireddistance, but the distance is preferably at least equal to the thickness of the top part of the facingtable 10 and may be up to several times its thickness. As may be seen from Figure 4, therim 20extends more than halfway through the thickness of the cutting element as a whole.
Theportion 22 of the rear surface of the facing table 10 which lies within therim 20 iscircular, as best seen in Figure 3. However, this portion may be of any other shape. For example,theperipheral rim 20 may be of substantially constant thickness, in which case theregion 22 willbe of an overall shape corresponding to the overall outer shape of the facing table 10.
In the arrangement shown, theregion 22 of the rear surface of the facing table 10 is planar,but this surface might also be configured, i.e. formed with ribs or other protrusions on the rearsurface of the facing table which engage within correspondingly shaped recesses in the substrate.Similarly, the surface of the thickenedrim 20 remote from the facing table 10 may also beconfigured.
It will be seen that the provision of the thickenedrim 20 provides a more substantial bodyof polycrystalline diamond adjacent the cuttingapex 18 than would be the case if the facing table10 were of substantially uniform thickness. This extra body of polycrystalline diamond thusprotects the cuttingapex 18 against rapid wear and impact damage. At the same time therim 20serves to enhance the mechanical interlock between the diamond facing table and thesubstrate 12thus reducing the risk of separation of the facing table from the substrate under the hightemperatures and stresses to which such cutting elements are subjected in use downhole.
In the modified arrangement of Figures 5 and 6 theperipheral rim 24 is narrower in widththan in the arrangement of Figure 3 so that it does not extend continuously around the wholeperiphery of the cutting element but provides a thickenedportion 26 adjacent the cuttingapex 28of the cutting element, and a part-annular portion 30 extending around the part-circular portionof the cutting element.
In the arrangement of Figures 7 and 8 theprotuberance 32 projecting into thesubstrate34 from the rear surface of the polycrystalline diamond facing table 36 is generally triangular incross-section so as to provide additional strength to the periphery of the cutter only in the vicinityof the cuttingapex 38. In this case the flatinner surface 40 of theprotuberance 32 is inclined sothat the protuberance decreases in cross section as it extends away from the facing table 36.
Figures 9 and 10 show a modified arrangement where theinner surface 42 of the generallytriangular protuberance 44 is convexly curved as well as being inclined.
Figures 11 and 13 show a modification of the arrangement of Figures 9 and 10 where theprotuberance 46 has a convexly curved inner surface which is stepped, as indicated at 48, so thattheprotuberance 46 again reduces in cross section as it extends away from the diamond facingtable.
Figures 13 and 14 also show an arrangement where theprotuberance 50 has a steppedinner surface, but in this case theinner surface 52 of the protuberance is concave and is providedwith three steps rather than the two steps of Figures 9 and 10.
In the arrangement of Figures 15 and 16 theprotuberance 54 projecting from the rearsurface of the facing table 56 extends across the whole width of the cutting element from thecuttingapex 58 to aregion 60 of the part circular peripheral surface of the cutting elementopposite the cuttingapex 58.
Figures 17 and 18 show an arrangement in which the periphery of the cutting element isformed with two spaced cuttingapices 62, 64 separated by a concave portion 66 of the peripheryof the cutting element. In this case the protuberance is again in the form of a thickenedrim 68which projects into thesubstrate 70 from the rear surface of the diamond facing table 72. Thethickenedrim 68 extends around the whole periphery of the cutting element so as to providesupport for both cuttingapices 62 and 64.
As shown in Figure 17imaginary lines 74 bisecting the two cuttingapices 62, 64 aresubstantially parallel so that the apices point in the same direction and thus act on the formationsimultaneously.
Figures 19 and 20 show a cutting element similar to that of Figures 17 and 18 but formedwith six cuttingapices 76, 78. The two leadingapices 76 are connected by aconcave portion 80of the periphery of the cutting element whereas theother apices 78 are defined by generallystraight edges meeting at an angle in a saw-tooth configuration. In this arrangement also theprotuberance on the diamond facing table comprises a continuous thickened rim extending aroundthe whole periphery of the cutting element.
Figures 21 and 22 shows a cutting element formed with three cuttingapices 79. In thiscase there are provided three parallelelongate protuberances 80 projecting from the rear surfaceof the facing table 81. Eachprotuberance 80 extends across the whole width of the cuttingelement from an apex 79 to a region of the peripheral surface of the cutting element opposite thatapex.
Figures 49 and 50 show a cutting element similar to those shown in Figures 15 to 22. Inthis arrangement, however, theapices 200, 202 are spaced apart from one another, a relativelyflatsurface 204 being located therebetween. A pair ofprotuberances 206, 208 which extend awayfrom the table 210 correspond with theapices 200, 202. Eachprotuberance 206, 208 extends onlypart way across the cutting element. A cutting element of this type is particularly useful in drillingthe sidewall of a borehole, with the cutting element oriented such that the relativelyflat surface204 is arranged to contact the sidewall. Theprotuberances 206, 208 tend to help prevent spallingand chipping of the cutting element.
As previously mentioned in relation to Figures 3 and 4, the inner region of the rear surfaceof the facing table may be planar or configured, for example by being formed with ribs or otherprotrusions on the rear surface of the facing table which engage within correspondingly shaped recesses in the substrate. As is well known, preform cutting elements of the type to which thepresent invention relates are normally manufactured by forming the substrate in the requiredconfiguration from compacted powdered material, such as powdered tungsten carbide. A layerof diamond particles is then applied to the front surface of the substrate and the assembly issubjected to very high pressure and temperature in a high pressure, high temperature press so asto bond the particles together, and the layers to one another, to produce a unitary element. Anyrequired configuration of the interface between the substrate and the facing table is normallyeffected by pre-shaping the front face of the substrate to which the layer of diamond particles isapplied.
Figures 23 and 24 show the shapes of substrates designed to provide the requiredconfigured interface between the facing table and substrate in the finished cutting element.
In the arrangement of Figure 23, thesubstrate 82 is formed with aperipheral rebate 84which surrounds a circular central raisedportion 86. The raisedportion 86 is formed around itsperiphery with alternating long andshort grooves 88 which extend inwardly from the peripheryof the raisedportion 86 at an angle to the radial direction. Thegrooves 88 reduce in depth as theyextend inwardly, leaving a centralplanar region 90 at the centre of the raisedportion 86.
When the layer of diamond is applied to thesubstrate 82, the body of diamond filling therebate 84 forms a rearwardly projecting peripheral rim on the rear surface of the diamond facingtable, and the bodies of diamond filling thegrooves 88 provide protuberances in the form of ribson the circular region of the rear face of the facing table within the peripheral rim. These ribsincrease the mechanical interlock between the facing table and substrate and tend to increase theresistance of the cutting element to impact damage and to spalling or delamination of the facing table from the substrate.
In the arrangement of Figure 3, and indeed in any of the arrangements according to thepresent invention, the substrate may be initially in the form of a completely circular tablet to whichthe layer of diamond particles is applied so that the preform element, as it emerges from the highpressure high temperature press, is completely circular. The shaping of the periphery of theelement to form the cutting apex, or several cutting apices, may then be effected by a furtherforming operation in which parts of the periphery of the cutting element are removed and/orshaped by any appropriate mechanical shaping process, such as grinding or EDM. The shapeshown diagrammatically in Figure 23, therefore, will usually be the shape of the substrate portionof the finished cutting element, after machining, rather than the shape of the compacted powdersubstrate component to which the layer of diamond particles is applied prior to forming the basiccutting element in the press.
Figure 24 is a similar view to Figure 23 showing an alternative form ofsubstrate 92. Inthis case theinner wall 94 of theperipheral rebate 96 around the upper surface of the substrate isinclined so that the raisedportion 98 is generally fiusto-conical in shape, and the inner surface ofthe peripheral rim on the diamond facing table is inclined.
In this case,grooves 100 extending inwardly from the periphery of the raisedportion 98also extend across therebate 96 and intersect the outer surface of thesubstrate 92.
Figure 23 showed a comparativelythin substrate 82 suitable for use, for example, in thecase where the finished preform cutting element is to be brazed to a separate supporting post orstud. In the arrangement of Figure 24, thesubstrate 92 is of greater axial length so as to providea finished preform cutting element which may be directly mounted in a suitably shaped pocket in the bit body.
In all of the arrangements thus far described, the peripheral surface of each cutting elementis shown as being at right angles to the front exposed surface of the diamond facing table.However, additional strength and support may be given to the cutting apex by inclining theperipheral surface of the cutting element outwardly as it extends away from the front surface ofthe facing table on either side of the cutting apex. This increases the relative size of the body ofprotuberance material and substrate material which lies beneath the top of the cutting apex of theelement so as to support the apex against wear and impact damage. Figures 25-31 show preformcutting elements of this type.
In each case the cutting element is initially formed as a circular cylindrical element havinga circular front facing table bonded to a circular substrate. The required configuration of thefinished element is then subsequently achieved by grinding, EDM, or any other suitable form ofmachining.
In the arrangement of Figures 25 and 26 asingle cutting apex 102 is provided by formingtwoinclined flats 104 approximately at right angles to one another. As a result of the cylindricalshape of the basic element and the inclination of the flats, eachsurface 104 reduces in width as itextends away from the facing table 106 to therear surface 108 of thesubstrate 110. Thefrontface 106 of the diamond facing table is preferably chamfered around its whole periphery, asindicated at 112.
The included angle between theflats 104, defining the pointedness of the cutting apex,may be of any desired magnitude, for example it may be within the range of 60°-120°.
In the modified arrangement of Figures 27 and 28 the tip of the cutting apex is chamfered and radiussed as indicated at 114. In this case the periphery of the diamond facing table is notchamfered, although this may be effected if desired.
Figures 29-31 are diagrammatic perspective views showing further modifications of theshape of cutting elements shown in Figures 25 and 26.
In the cutting element of Figure 29, the machinedflanks 116 on either side of the cuttingapex 118 are cylindrically and convexly curved so that theperipheral stretches 120 of the facingtable 122 on either side of the cuttingapex 118 are also convexly curved.
In the cutting element of Figure 30 theflanks 124 on either side of the cuttingapex 126are cylindrically and concavely curved. Achamfer 128 is also formed on each side of the cuttingapex 126 andsteps 130 are formed between eachconcave flank 124 and the part-circularouterperiphery 132 of the cutting element. Figure 31 is a modification of the arrangement shown inFigure 30 where theflanks 124A are also concavely curved in the direction of the axis of thecutting element so as to provide a "hollow ground" effect on either side of the cuttingapex 126A.The flanks may also be "hollow ground" in the arrangement of Figures 25-28 where theflanks 104are straight in cross-section.
Figure 32 shows an arrangement where the machinedflanks 134 on each side of thecuttingapex 136 are more steeply inclined than in the previously described arrangements so as tointersect the bottom surface of thesubstrate 138 opposite the facing table 140. As a result thecutting apex 136 effectively extends for the whole axial length of the cutting element, but thepresence of a remainingportion 142 of the periphery of the cutting element below the cuttingapex136 has the effect that the cutting apex becomes increasingly broad as wear of the cutting elementprogresses in use.
In all of the arrangements of Figures 25-32 the rear surface of the facing table of thecutting element may be non-planar, i.e. provided with at least one protuberance which projects intoa correspondingly shaped recess in the front surface of the substrate, in accordance with thepresent invention. For example, the protuberance may be of any of the configurations previouslydescribed with reference to Figures 3-16. However, the configurations shown in Figures 25-32may also be employed in cutting elements where the interface between the facing table andsubstrate is not configured according to the present invention, for example where the interface issubstantially planar.
Figures 33-48 show further variations of preform cutting element in accordance with thepresent invention.
Figures 33-38 show variations of the arrangement shown in Figures 15 and 16 where theprotuberance on the rear surface of the diamond layer is thickest in the centre of the diamondlayer, extending diametrically across the cutting element from the cutting apex. In the arrangementof Figures 33 and 34 the underside of the protuberance ondiamond layer 144 has an inverted ridgeconfiguration formed by two flatinclined surfaces 146 on each side of thecentreline 148 of theprotuberance. The arrangement of Figures 35 and 36 is similar except that thesurfaces 150 oneach side of thecentreline 152 are cylindrically and concavely curved.
In the arrangement of Figures 37 and 38 the parallel-sided protuberance 154 extendingfrom the rear surface of thediamond layer 156 is generally triangular in cross-section. It extendsdiametrically away from the cutting apex of the cutting element and across only a part of the widthof thediamond layer 156.
Instead of the cutting apex of the cutting element being strengthened by a protuberance on the underside of the diamond facing table, the desired additional strength may be provided byforming the diamond table with a region of increased abrasion resistance which lies adjacent to thecutting apex and, preferably, forms part of the periphery of the cutting element at the cutting apex.Arrangements of this kind are shown by way of example in Figures 39-44.
In the arrangement of Figures 39 and 40 the diamond facing table 158 comprises acentralstrip 160 of greater abrasion resistance flanked by twoside portions 162 of lesser abrasionresistance. Thestrip 160 extends diametrically across the facing table 158 from the cuttingapex164 to a region opposite the cutting apex. The end of thestrip 160 is exposed at the peripheryof the cutting element and thus itself forms the cuttingapex 164.
Figures 41-44 show similar arrangements where the strip of greater abrasion resistance isof different cross-sectional shapes. In the arrangement of Figures 41 and 42 thestrip 166 ofgreater abrasion resistance is generally triangular in cross-section. In Figured 43 and 44 thestrip168 extends across substantially the whole width of the facing table 170 and comprises acentralridge 172 flanked by two cylindrical andconcave surfaces 174.
In any of the arrangements last described, the protuberance on the rear surface of thediamond facing table, and/or the more abrasion resistant strip included in the facing table, mayextend only partly across the diameter of the facing table. Alternatively or additionally the shapeof the strip may vary according to the diameter of the cutting element.
Thus, Figures 45 and 46 show an arrangement similar to Figures 33 and 34, but where theprotuberance on the underside of the diamond facing table 176 extends away from the cuttingapex178 across only a portion of the diameter of the cutting element, thepart 176A of the facing tablebeyond theprotuberance 176 being of substantially constant thickness.
In the arrangement of Figures 47 and 48 thestrip 180 of more abrasion resistant diamondmaterial incorporated in the diamond facing table 182 extends only across a part of the diameterof the cutting element as it extends away from the cuttingapex 184.
The feature of incorporating in the diamond facing table a portion of polycrystallinediamond which is of greater abrasion resistance may be provided in any of the cutting elementsaccording to the invention and as described above in relation to Figures 3-38.
The greater abrasion resistance of the portion of the diamond facing table may be achievedby any of the means of increasing abrasion resistance which are well known in the art. Forexample abrasion resistance may be varied by variation in the mean diameter of the particles ofdiamond, or other superhard material, from which the front facing table of the cutting element isformed and/or by variation in the packing density of the particles.
In all of the arrangements described above the protuberance on the diamond facing tableforms part of the peripheral surface of the cutting element adjacent the cutting apex of the element.However, the invention does not exclude arrangements where the protuberance is spaced inwardlyfrom the periphery of the cutting element but is still located in a position where it can providesupport and protection to the cutting apex. For example, the protuberance may be located in aposition where it is not exposed at the periphery of the cutting element and does not come intocontact with the formation until some wear of the cutting element has occurred.

Claims

A preform cutting element, for a rotary drag-type drill bit, including a facingtable (10, 36, 56, 72, 81, 144, 160, 180, 210) of superhard material having a frontface and a rear surface bonded to the front surface of a substrate (12, 34, 70, 204)which is less hard than the superhard material, the cutting element having aperipheral surface andcharacterised in that the peripheral surface includes at leastone cutting apex (18, 28, 38, 58, 62, 64, 76, 78, 79, 164, 178, 184, 200, 202) definedbetween two adjacent regions of the peripheral surface which are disposed at anangle to one another, the rear surface of the facing table being formed with at leastone protuberance (20, 26, 32, 44, 46, 50, 54, 68, 206, 208) which projects into acorrespondingly shaped recess in the substrate, the protuberance (20, 26, 32, 44, 46,50, 54, 68, 206, 208) forming part of the periphery of the cutting element at thecutting apex and extending inwardly away from the cutting apex into the interior ofthe substrate.
A preform cutting element according to Claim 1 wherein the rear surface ofthe facing table and the front surface of the substrate of the cutting element are co-extensive.
A preform cutting element according to Claim 1 or Claim 2, wherein theprotuberance (54, 80, 160, 166) extends across the full width of the cutting elementso as also to form part of the periphery of the cutting element in a region thereofopposite said cutting apex.
A preform cutting element according to any one of the preceding claims,wherein the protuberance has substantially parallel side surfaces bonded to the sidesof a recess in the front surface of the substrate.
A preform cutting element according to any of the preceding claims, whereinthe cutting element is formed with a plurality of cutting apices (62, 64, 76, 78, 79,200, 202) spaced apart around the periphery thereof.
A preform cutting element according to Claim 5, wherein the part of theperiphery between two adjacent cutting apices is smoothly concave.
A preform cutting element according to Claim 5 or Claim 6, whereinimaginary lines bisecting the cutting apices are substantially parallel so that thecutting apices point in generally the same direction.
A preform cutting element according to any of the preceding claims, whereinthe periphery of the cutting element is part-circular.
A preform cutting element according to Claim 8, wherein one or both regionsof the periphery on either side of a cutting apex extend generally tangentially to apart-circular portion of the periphery of the cutting element.