Abstract:
An insert ( 10 ) for a cutting tool ( 100 ) for use down a well bore comprises a body of a hard material (tungsten carbide) suitable for cutting steel. The body is shaped for formation in a mould that comprises a die ( 70 ) and first ( 74 ) and second ( 78 ) punches and arranged so that the first punch can eject the body after formation from an opening of the die closed during formation by said second punch. The insert has first ( 12 ) and second ( 14 ) ends whose faces are defined, at least in part, by corresponding faces of the first and second punch. Between them is a longitudinal axis ( 24 ) of the insert. The area of the first end is less than the area of the second end. The body has flanks ( 16 ) that form ridges ( 18 ) extending between the first and second ends. The ridges form cutting edges of the insert. They are separated by V-shaped troughs ( 20 ) of said flanks. The ridges taper and spiral about the axis.

Description:
[0001]    This invention relates to cutting tool inserts, in particular to random distribution inserts for use on cutting tools adapted to remove casings from well bores. 
       BACKGROUND 
       [0002]    Down hole mills are known for removing casing, packers and other debris down hole for the purpose of renovating the hole. Such tools are also referred to as fishing tools and may comprise a cylindrical body adapted to be rotated about their longitudinal axis having cutting faces either arranged on the face of peripheral blades, in which event, the face is perpendicular to the cutting action (that is, parallel to, or possibly helically inclined to, the axis of rotation of the tool) or on an end face of the tool, in which event, the face is substantially parallel to the cutting action (for example, lying on a radial plane of the axis of rotation of the tool). 
         [0003]    In either case, the body of the tool is protected by cutting elements fixed thereto that are made of a material (usually tungsten carbide composite material) harder than the metal (usually steel) forming the casing or other component to be cut. The cutting elements not only protect the tool, but also of course machine the body being cut. Consequently it is desirable that the cutting elements present an effective cutting profile to the workpiece. On the other hand, there are two considerations. Firstly, the milling of casing and packers by machining them is abrasive to the tool. The cutting inserts and blades etc on which they are mounted wear away rapidly in the aggressive environment. Secondly, there is no requirement for great precision—the application is generally just the removal of a well casing, and not precision machining. 
         [0004]    There are three currently known options. In a first arrangement, an array of cutting inserts are carefully oriented and disposed on the tool in a pattern that results in a most efficient cutting tool. Each element is aligned so that a sharp edge of the element, with a desired rake angle, is presented to the workpiece. The face of the element above a cutting edge may be provided with surface features that results in shavings from the workpiece breaking, so that long spirals of shavings are avoided. Such long shavings run the risk that they bundle together in a “bird&#39;s nest”, making removal of the cuttings potentially problematic. If short chips a few millimeters long break off quickly, bird-nesting can be eliminated or at least reduced. Furthermore, with regular patterns it is easy to ensure that, when one element wears away and breaks off, a following element is equally presented in the most effective cutting position. However, while the tool is ideal, it is time-consuming, and thus expensive, to construct. 
         [0005]    A second arrangement is at the other extreme. Old tungsten carbide inserts recovered from a large number of tools can be crushed and broken into irregularly shaped fragments. These can be sorted for size and then randomly distributed over a surface of the tool that will form the working bit of the tool. The crushed tungsten carbide is generally attached to a cutting tool by a brazing process. In this embodiment, the random inserts may be supplied in a rod of brazing material, and applied to the tool simply by progressively melting the brazing rod and bonding the crushed tungsten carbide inserts to the tool. Alternatively, the inserts may be embedded in a bar, a number of which may themselves be fixed (eg by brazing) to the tool body. In either case, the disposition of the crushed tungsten carbide inserts on the tool is entirely random, as is the presentation of each insert to the workpiece when cutting. Consequently, despite the obvious cost savings to be had by this arrangement, the cutting and wear resistant performance is certainly compromised. Any given insert is unlikely to present a clean cutting edge, and if it does, long shavings of the work piece may result. Of course, it has to be said that clean cutting, as mentioned above, is not really the issue. With sufficient pressures and torques applied, it is not essential to have clean cutting edges. It is not necessarily the case that the grade of material of the all of inserts will be specified for cutting—some may come from wear applications which use different grades of tungsten carbide (tungsten carbide is generally the material of choice at present times). Consequently, while inexpensive, this arrangement does not perform as well as the first arrangement described above. 
         [0006]    The third arrangement, however, is a compromise, in which geometrically shaped inserts are used that have a plurality of cutting edges and faces so that, even with a random distribution of them on a tool, they are likely to present an effective cutting face to the workpiece. Of course, merely square inserts are likely to simply “tile” onto the tool so that no cutting faces are presented. Instead the inserts provide a wear resistant face that does little cutting. The availability of shapes is of course limited, to some extent, by the process by which inserts are routinely made. In the case of tungsten carbide inserts, these are conveniently made by mixing tungsten carbide and other metallic carbide powders with a binder substance such as cobalt. The mixture is then filled in to a die and, under high pressure, the tungsten carbide is pressed to a solid block having the shape of the die and end punches. Generally, the die is tubular and each end is closed by a punch that provides the pressure. In order to extract the formed insert from the mould, the bottom punch is used to push the insert out of the mould through the mouth of the die. Consequently, inserts generally have a constant cross-section to enable ejection from the die. The inserts are then sintered at high temperature 1300-1500 Deg C. to give the final physical properties. 
         [0007]    GB-A-2378670 discloses an insert that is a constant cross section prism whose ends are rendered concave and, optionally, provided with dimples or other surface irregularities. The cross section is a complex polygon having both internal and external angles of less than 180 degrees that provide corrugations of ridges separated by grooves in the sides of the insert. 
         [0008]    It is an object of the present invention to provide an insert that can be made in a conventional manner but which provides at least one of the following advantages: 
         [0009]    a tendency to generate a more aggressive cutting action, which may be beneficial for cutting not just metal but other non-metal components frequently encountered downhole when milling; 
         [0010]    a tendency to reduce vibration during milling; and 
         [0011]    promotion of tighter coils of cuttings that encourage break-off and discourage bird-nesting or production of a multiplicity of fine cuttings which are easily broken up and circulated out of the hole with the drilling mud. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0012]    In accordance with the present invention there is provided an insert for a cutting tool for use down a well bore, comprising a body of a hard material suitable for cutting steel, the body being shaped for formation in a mould that comprises a die and first and second punches and arranged so that the first punch can eject the body after formation from an opening of the die closed during formation by said second punch, said body having: 
         [0013]    first and second ends whose faces are defined, at least in part, by corresponding faces of the first and second punch and which between them define a longitudinal axis of the insert, the area of the first end being less than the area of the second end, and 
         [0014]    flanks of the insert extending between the first and second ends comprising ridges, said ridges forming cutting edges of the insert and being separated by troughs. 
         [0015]    The insert may have a plurality of tiers that are of different cross sectional area with steps between them, each tier having said ridges and troughs that extend between first and second faces of the tier. Thus the insert tapers from the second end to the first end in a stepwise fashion. 
         [0016]    Additionally, or instead of having tiers, said ridges may taper from said second end to said first end. When tiers are present, the tapering of the ridges is of each tier. 
         [0017]    Optionally, whether there are tiers or not, said ridges may spiral about said axis. 
         [0018]    Thus, in another aspect, the present invention may provide an insert for a cutting tool for use down a well bore, comprising a body of a hard material suitable for cutting steel, the body being shaped in a die and first and second punches and arranged so that the first punch can eject the body after pressing from an opening of the die closed during formation by said second punch, said body having: 
         [0019]    first and second ends whose faces are defined, at least in part, by corresponding faces of the first and second punch and which between them define a longitudinal axis of the insert, the area of the first end being less than the area of the second end, and 
         [0020]    flanks that form ridges extending between the first and second ends, said ridges forming cutting edges of the insert and being separated by troughs of said flanks, wherein 
         [0021]    said ridges taper from said second end to said first end, and spiral about said axis. 
         [0022]    The degree of spiral may be sufficiently limited, and the degree of taper sufficiently large, and the angular separation of a ridge and trough in the radial plane of the axis is, such that the insert is not required to rotate on ejection from the die. In this event, ejection is, of course, in the axial direction. Other directions are feasible, for example in the direction of taper of the ridges. 
         [0023]    Preferably, the tapering is not linear. Preferably, it is convexly curved with respect to the axis. 
         [0024]    In accordance with a third aspect of the present invention there is provided an insert for a cutting tool for use down a well bore, comprising a body of a hard material suitable for cutting steel, the body being shaped in a die and first and second punches and arranged so that the first punch can eject the body after pressing from an opening of the die closed during formation by said second punch, said body having: 
         [0025]    first and second ends whose faces are defined, at least in part, by corresponding faces of the first and second punch and which between them define a longitudinal axis of the insert, the area of the first end being less than the area of the second end, and 
         [0026]    a plurality of tiers of the insert that are of different cross sectional area with steps between them, each tier having ridges that extend between first and second faces of the tier, said ridges forming cutting edges of the insert and being separated by troughs that also extend between said first and second faces of the tier. 
         [0027]    In any case, preferably, said troughs are V-sections. The troughs may diminish in depth, and said ridges correspondingly diminish in height, from said second end to said first end. Of course, the depth of a trough is the same as the height of a ridge, both being the difference in radial distance from said axis of the base of the trough and the peak of the ridge. Said ridges may terminate at or before said first end. 
         [0028]    Preferably, the ridges on each tier spiral between the first and second faces. Preferably there are three tiers. Preferably, the steps between them form additional cutting faces of the insert. Preferably the profiles of the tiers are the same. Preferably, they are rotationally offset about said axis with respect to one another. Preferably, the ridges of each tier taper between said first and second faces. The first face of the smallest tier of the insert forms said first end of the insert and the second face of the largest tier of the insert forms said second end. Where a first and second face of a tier intersect, said second face is internal of the insert and only exists in a geometric sense because it is entirely within the confines of the first face of the adjacent tier. The step is formed where a first face overlaps an adjacent second face. 
         [0029]    When scattered randomly on a tool, each insert will generally come to a position in which its flanks rest on the tool surface, assuming the tool surface is flat and horizontal when the inserts are applied. The orientation, however, of an insert about an axis perpendicular to the tool surface is random. Where the direction of approach of the tool surface with respect to a workpiece during cutting is perpendicular to the tool surface, this randomness of orientation is largely irrelevant. Indeed, it is largely irrelevant, in any event. However, when the direction of approach of the tool surface to a workpiece is parallel to the tool surface, that randomness of orientation results in the end faces of inserts frequently providing the cutting function. Therefore, the end faces also provide cutting faces, as well as the ridges of the flanks. The points of the ridges of second end present an aggressive cutting face. The reduced size of the first end also presents an aggressive cutting face. Also by curving the flanks, a shorter cutting edge is presented, which is also more aggressive. Consequently, inserts shaped as defined are effectively more pointed, whatever their orientation, leading to a more aggressive cutting environment that is especially useful where not only the metal of casing is encountered, but also potentially surrounding well bore material or backfilled cement. Also, the more pointed cutting face results in less vibration that occurs when a face attempts to cut too much material in one go. That leads to snatching of the tool and, frequently, a consequent premature break-off of inserts from the tool surface. Finally, by spiraling and tapering the flank ridges, generally the cuttings that do not break off are wound into tight curves that more readily break off and, when they do, are less likely to entangle with other cuttings forming bird nesting. 
         [0030]    The invention also provides a tool comprising a substantially cylindrical body having a working face, wherein the working face has applied thereto within a fixing matrix a plurality of cutting inserts as defined above disposed on the face in a random distribution. Preferably, the working face is arranged in a radial plane of the tool. Alternatively, the working face is arranged substantially perpendicularly to a radial plane of the tool. Said working surface may comprise blades attached to the tool. Said working surface may comprise flutes formed in the side of the tool. Said matrix may be braze material. 
         [0031]    Said hard material is preferably tungsten carbide optionally including other metallic carbides. Preferably, a binder for the tungsten carbide comprises cobalt. Preferably, said tungsten carbide insert is formed by pressing and sintering. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0033]      FIGS. 1(   a ) to ( d ) are respectively a perspective view, a section on the line A-A in  FIG. 1(   d ), an end view in the direction of arrow C in  FIG. 1(   d ), and a side view, of an insert according to a first embodiment of the present invention; 
           [0034]      FIGS. 2(   a ) to ( c ) are respectively an end view in the direction of the arrow A in  FIG. 2(   b ), a side view and a perspective view, of a second embodiment of an insert according to the present invention; 
           [0035]      FIGS. 3(   a ) to ( d ) are respectively a perspective view, an end view in the direction of the arrow B in  FIG. 3(   c ), a side view, and an end view in the direction of the arrow D in  FIG. 3(   c ) of a third embodiment of an insert according to the present invention; 
           [0036]      FIGS. 4(   a ) and ( b ) are respectively side views of a tool in which a) inserts are disposed on a perpendicular face with respect to a work piece surface and b) where inserts are on a parallel face of a tool with respect to a work surface; 
           [0037]      FIGS. 5(   a ) and ( b ) are schematic representations of typical tools according to the embodiments shown in  FIGS. 4(   b ) and ( a ) respectively; 
           [0038]      FIG. 6  is a schematic representation of a sintering die for producing inserts according to the present invention; 
           [0039]      FIGS. 7(   a ), ( b ) and ( c ) are respectively a perspective view, a side view and an end view in the direction of the arrow E in  FIG. 7(   b ) of a fourth embodiment of an insert according to the present invention; and 
           [0040]      FIGS. 8(   a ), ( b ) and ( c ) are respectively a perspective view, a side view and an end view in the direction of the arrow F in  FIG. 8(   b ) of a fifth embodiment of an insert according to the present invention. 
           [0041]      FIGS. 9(   a ), ( b ) and ( c ) are respectively a perspective view, a side view and an end view in the direction of the arrow G in  FIG. 9(   b ) of a sixth embodiment of an insert according to the present invention. 
           [0042]      FIGS. 10(   a ), ( b ) and ( c ) are respectively a perspective view, a side view and an end view in the direction of the arrow H in  FIG. 10(   b ) of a seventh embodiment of an insert according to the present invention. 
           [0043]      FIGS. 11(   a ), ( b ) and ( c ) are respectively a perspective view, a side view and an end view in the direction of the arrow J in  FIG. 11(   b ) of a eighth embodiment of an insert according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Referring now to  FIG. 1 , an insert  10  is illustrated comprising end faces  12  and  14 , and flanks  16  consisting of ridges  18  separated by troughs  20 , forming faces  19  between them. The insert  10  has an axis  24  extending between the end faces  12 , 14 , which are shown as being flat faces perpendicular to the axis  24 , although either face may be concaved, convex or faceted, internally or externally, or inclined with respect to the axis  24 . A section taken perpendicular to the axis  24 , such as the section A-A, has a regular six pointed star shape profile  30  that is the same from the small end  12  to the large end  14 . The angle between faces  19  is α, which typically is 60°. However, the ridges  18 , and corresponding troughs  20 , spiral about the axis  24  between the ends  14  and  12 . Thus, the ridges both taper and spiral between the ends  12 , 14 . 
         [0045]    The degree of taper is, in the case of  FIG. 1 , non-linear. That is to say, the radial distance R of the ridge  18  from the axis  24  decreases at an increasing rate from the large end  14  towards the small end  12 . Indeed extrapolating the flange  18  until the extrapolation coincides with the axis  24  defines a start point or origin Ø. The taper provides a curved profile to the insert  10 , so that it is not stable in any position when resting on its flanks and consequently adopts easily a number of dispositions when lying on its flanks  16 . 
         [0046]    The degree of spiral is such that the ridge  18  (and trough  20 ) turns through an angle θ between the ends  12 , 14 . This angle is about 20 degrees, but may be between a greater or less angle, depending on the requirements of manufacture and the ease with which it is desired that the insert can be ejected from the die which forms it. This is discussed further below. 
         [0047]      FIG. 2  shows a variation being a second embodiment of the present invention where insert  10 ′ has tapering ridges  18 ′, as in the first embodiment, but less tapering troughs  20 ′ so that, at small end  12 ′ of the insert the section through the insert is circular. The distance x from the face  12 ′ to the origin Ø,Ø′ will be different for each of the trough  20 ′ and ridge  18 ′. 
         [0048]    With reference to  FIG. 3  an insert  10 ″ is shown being a further embodiment of the present invention, which differs from the above embodiments in that the ridges  18 ″ and troughs  20 ″ have a radial distance R with respect to the distance x from its origin Ø that is linear. 
         [0049]      FIG. 6  illustrates one possible manufacturing arrangement for the inserts of  FIGS. 1 to 3 , or, indeed, those of  FIGS. 7 and 8  described further below. The manufacture of inserts according to the present invention employs conventional techniques, but with special considerations. A die  70  is formed having the desired profile  72  of the flanks  16  of the insert to be formed. An top punch or ejector pin  74  closes a bottom end of the die and has an end form  76  to form end face  12  of the insert. A bottom punch (plate)  78  closes a bottom end of the die  70  and has an end-form  80  to form end face  14  of an insert. The joint  82  between die and punch  78  is shown extending perpendicularly to the axis  24  of the insert. However, it could be parallel, as is the joint between the pin  74  and die  70 . 
         [0050]    Tungsten carbide inserts are generally made in three steps. Firstly, the tungsten along with other metallic carbides are milled along with the metallic binder (usually cobalt) and a wax. This is then granulated (to give good flow characteristics). The second step is to press it in a die with a top and bottom punch at room temperature. The last step is to sinter, in a first stage to drive off the wax binder, and in a second stage to fully sinter the carbide and which results in a 20% reduction in volume. 
         [0051]    The die  70  is filled with tungsten carbide and other metallic carbide powders of appropriate grade already mixed with binder composition (eg cobalt). The composition of such material is known per se in the art and needs no further description herein. The bottom punch  78  closes the die. Punch  74  is pressed against the material in the die to compress it. After pressing, the die is opened by withdrawing the bottom punch  78  and ejecting the insert with the top punch  74 . To the extent that the geometry of the formed insert is such that some rotation of the insert is necessary during ejection along the axis  24  (in order for the formed ridges  18  to clear the flutes of the die  70 ) such rotation may be self-effecting by sliding interaction between the flutes of the die and the ridges  18  of the insert. Also, however, such rotation can assisted by corresponding rotation of the pin  74 . Some formations on the end face  12  that assist frictional grip between it and the pin  74  may in this instance be advantageous. 
         [0052]    Turning to  FIGS. 5   a,b  two alternative arrangements of a tool incorporating the inserts of the invention are illustrated. 
         [0053]    In  FIG. 5   a , a tool  100   a  has a substantially cylindrical body  102  which is fluted at  104  adjacent an end face  106  of the tool. The end face is coated with multiple inserts  10  (or  10 ′ or  10 ″, future references to the insert being only made to the insert  10 , but it is to be understood that the inserts  10 ′ and  10 ″ are equally applicable, unless the context discusses otherwise). The inserts  10  are held in a matrix  108  of braze material, also known in the art and not described further herein. One method of application is to supply inserts  10  to the tool manufacturer in rods of braze material which then need only melting against the face  106  for inserts to be delivered in approximately the right volume ratio of braze to insert so that the face  106  can be covered with at least one layer  110  of randomly arranged insert  10 . (In fact, and insert of the form  10 ′, as shown in  FIG. 2 , is illustrated in  FIG. 4   b , although here they are labeled  10 ). More than one layer may be provided. In applying the inserts to the face  106 , this is done with the face  106  uppermost and substantially horizontal. Most of the inserts end up on their sides, as shown at  10   a,c  in  FIG. 4   b , but occasionally some will end up standing on either end  12 ,  14  (as at  10   b ) or at some other angle. The randomness of the arrangement ensures that plenty of sharp edges and points of the inserts  10  are presented to a work piece  200   a.  In the case of the tool  100   a,  the workpiece  200   a  is parallel the tool face  106  carrying the cutting elements  10 . In this case, the tool  100   a  is rotated about its longitudinal axis so that the working face  106  moves in the direction of the Arrow X in  FIG. 4   b , that is, parallel the workpiece surface  200   a    
         [0054]    In  FIG. 5   b , tool  100   b  also comprises a substantially cylindrical body  122 , but is here provided with a pilot nose  120  designed to fit in and slide inside a casing sleeve (not shown) to be milled away. The body  122  is provided with blades  124  that are here shown spiraling around the longitudinal axis of the tool  100   b,  but equally they can be parallel that axis, and indeed, this is simpler to construct. One face  126  of the blades  124  is coated with randomly distributed inserts  10 , distributed in similar manner to the application to face  106  of tool  100   a.  Here, however, the coated face  126  moves with respect to a work piece surface  200   b,  also in the direction X parallel with the workpiece  200   b,  but in this case perpendicularly to the face  126 . Thus fewer inserts perform the cutting function of the workpiece and accordingly wear away more rapidly, but they are constantly replenished by new inserts as the blades  124  are eroded. 
         [0055]    By virtue of the tapered form of the inserts, especially with exponentially tapering ridges, numerous different orientations of the insert is possible that present a sharp cutting edge to the workpiece. Long edges, that might increase the risk of tool vibration, are substantially eliminated. Nevertheless, a more aggressive cutting profile is achieved that may be more effective in respect of mixed workpiece cutting, especially those involving mineral formations/debris/concrete etc. Moreover, given that a sloping edge is almost inevitably presented to the workpiece, in the case of metal cutting, tighter coils of the cut material are likely to result, whereby the problem of nesting of cuttings can be reduced. 
         [0056]    Turning to  FIGS. 7 and 8 , two further alternative arrangements of inserts  110   a,b  are illustrated and which differ from one another only in degree. Here, instead of straight ridges  18 , stepped ridges  118  are presented, along with stepped troughs  120 . Thus the inserts  110   a,b  as a whole are stepped having a plurality of tiers  140   a,b,c.  Each tier tapers and spirals in a similar manner to the entire insert  10 ″ described above with reference to  FIG. 3 . However, there is no necessity for the tapering to be linear and it could be exponentially convex, as per the  FIGS. 1 and 2  arrangements. Indeed, the tapering could be different for each tier. The degree of spiral is shown being the same for each tier. Indeed, the start points  142   144  of each ridge  118  and trough  120 , respectively, of each tier is shown at the same angular position as the end points  146 , 148  of the corresponding ridge and trough respectively of the adjoining tier. This is shown by the lines  150  in  FIGS. 7 and 8  ( c ) (the start points  144  of the troughs  120  are not visible in the end views, although the end points  148  are). 
         [0057]    This arrangement enjoys the benefits described above but also has two the further advantages compared with the earlier embodiments described above. The first advantage is realized on those occasions when the end face  112  of the insert faces the workpiece during cutting operations. While the end faces  12  in the embodiments described above work perfectly effectively, the ridges  18  behind tend to plough into the workpiece without effecting a cutting action. This is not especially problematic, since the ridge in that event quite soon wears down and an insert behind will pick up the cutting action. However, with the inserts  110   a,b,  there is not a long ridge behind but a relatively short one followed by a further effective cutting face  112   a,  and behind that another face  112   b.  Accordingly, the problem, such as it is, of the ridge  18 , (or  118  in the case of these embodiments), ploughing into the workpiece and failing to effect a cutting action is avoided. 
         [0058]    This leads also to the second advantage, which is also experienced when the insert presents other faces to the workpiece, is that cross-sectionally smaller chips are cut from the workpiece. That is, instead of one chip being cut by the end face  112 , three chips are cut with the same depth of penetration into the workpiece. Likewise, three ribbons are cut by the tiers  140   a,b  when the side of the insert faces the workpiece. Consequently, even if long turns are cut, these are thinner and break up more easily in the melee around the tool during its operation. 
         [0059]    As mentioned above, the only difference between the embodiments of  FIGS. 7 and 8  is in the degree of taper (that is to say, the size of the steps ( 112   a,b ) between tiers). Otherwise they are the same. It should of course be appreciated that more than three tiers may be provided, although the more that are provided, the more the insert approximates the earlier embodiments of the present invention. It should also be made clear that the step in the ridges need not be the same as the step in the troughs. Indeed, there could be no step in the troughs and only steps in the ridges, and vice versa. Finally, although the start  142 , 144  and finish  146 , 148  of ridges and troughs of adjacent tiers are shown in the same angular position, this is not essential. Any angular displacement between is permissible provided that the entire rear face ( 112   x ) of a smaller tier is within the profile ( 112   a ) of the adjacent tier (see rear face  112   x  partially shown in dotted lines in  FIG. 7(   c ), which face is “virtual” being internal of the insert  110   a ). 
         [0060]      FIGS. 9 to 10  show further embodiments of the present invention. In  FIG. 9 , insert  910  has two tiers  940   a,b,  each of whose cross-sections are the same (that is, there is no taper) and their ridges do not spiral from a first face  912   a,b  to a second face  914   a,b  of the respective tiers. The cross section is, in each case, a six-pointed star, but rotationally offset with respect to one another. Of course, a regular six-pointed star having 60° angle ridges and 120° angle troughs is not essential, any more than it is for the embodiments described above. 
         [0061]    In  FIG. 10 , the only difference of the insert  1010  shown with respect to that of  FIG. 9  is a third tier  1040   c.  In  FIG. 11 , the only difference of the insert  1110  shown with respect to that of  FIG. 10  is a middle tier  1140   b  that is a five pointed star compared with the tiers  1140   a  and c that are six pointed stars. Any combination of cross-sections is possible and, indeed, a variation in the shape changes the cutting profile of the insert with respect to the workpiece. The variations shown in  FIGS. 10 to 11  in respect of tier numbers and profiles are, of course, equally applicable to the embodiments described above with reference to  FIGS. 7 and 8 . 
         [0062]    While the end faces  12 , 14  in all the embodiments described above are shown flat, as well as the intermediate faces  112   a,b  of the embodiments described above with reference to  FIGS. 7 to 11 , it is feasible to include features in those faces. For example, they may have dimples or bumps. Likewise, they may be dished, as indicated at  1175  in  FIG. 11(   b ) in order to improve cutting performance when the end faces  1112 , 1114  face the workpiece. A more aggressive, pointed, cutting profile is achieved. 
         [0063]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0064]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0065]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.