FIELD OF THE INVENTION The invention relates to the field of single crystal synthetic diamond, in particular single crystal synthetic diamond for use as part of a tool.

BACKGROUND Cutting tools are used to form, bore or degrade workpieces or bodies by removing material from them. Examples of cutting operations that use cutting tools are turning, milling or drilling tools. Such operations typically comprise one or more cutting tools or inserts affixed in some way to a tool holder. The inserts typically each comprise at least one cutting edge at the periphery of a surface of the tool.

For a cutting tool, the workpiece material removed by the cutting action is typically in the form of pieces called "chips". A rake surface is understood to mean the surface of a cutting insert over which the chips flow. When the rake surface is composed of a number of surfaces inclined to one another, these are designated first face, second face, and so forth, starting from the cutting edge. A clearance surface is sometimes referred to in the art as a flank surface, and may also be composed of a first face, second face and so forth, starting from the cutting edge. The cutting edge is the point at which the rake face and the flank face meet. A rake angle is the inclination of a rake face relative to the workpiece surface. Recommended rake angles can vary depending on the material being cut, tool material, depth of cut, cutting speed, machine, and setup. A positive tool is understood to mean that the tool is capable of being positioned in use such that the angle between a rake face and the workpiece is greater than 90 degrees. A negative tool is, on the other hand, is positioned in use such that the angle between a rake face and the workpiece is less than 90 degrees. For machining applications where high precision and tolerance is required, a cutting element may be formed from a single crystal diamond, as illustrated in Figure 1. A single crystal diamond may be formed from synthetic diamond. The main processes for forming synthetic single crystal diamonds are high pressure high temperature (HPHT) and chemical vapour deposition (CVD).

A single crystal diamond is usually oriented along crystallographic planes. These planes are usually determined by the orientation of a seed diamond from which the single crystal diamond is formed, and/or the conditions of synthesis.

Figure 1 shows a typical single crystal diamond tool element 101 . The single crystal diamond tool element 101 has a first major surface 102. This is typically oriented along a crystallographic plane, such as {100} or {1 10}. A toolmaker is usually provided with a blank with no chamfers cut into it (as shown by the dotted line in Figure 1 ).

A toolmaker forms one or more chamfers into the blank to form the single crystal diamond tool element 101 . In the example of Figure 1 , a rake face 103 and a flank face 104 are formed by cutting the blank. The single crystal diamond tool element 101 can then be affixed to a tool and used in a cutting operation.

Referring now to Figure 2, the single crystal tool element 101 of Figure 1 is shown in a turning operation. The rake face 103 is brought into contact with a workpiece 201 . In this example, the workpiece 201 is cylindrical and rotates in direction R about an axis. The workpiece 201 and the cutting face are moved in a feed direction relative to one another.

SUMMARY It is an object to provide an improved single crystal diamond cutting tool element. It is known that the diamond crystal structure is anisotropic, and particular crystallographic orientations have different properties. For example, the {1 1 1 }, {100} and {1 10} planes, in order, demonstrate decreasing abrasion resistance in their respective "easy" material removal directions, as described in Yuan et. al., "Lapping of single crystal diamond tools", CIRP Annals, Manufacturing Technology 52, pp 285- 288, 2003. Furthermore, tool elements with one long edge length may be required in order to facilitate attachment to a tool.

It has been realised that when a toolmaker forms one or more chamfers into a blank (or single crystal diamond precursor structure), the chamfer typically does not lie on a tool-working crystallographic plane, because the blank has been provided with a flat surface already on (or close to) a major crystallographic plane. The rake face or the flank face (or chamfered face for another purpose) may therefore not lie on the best crystallographic plane for the purpose of the tool. US2010/0003091 and JPH02145201 describe tools that have a tool-working surface on a tool-working crystallographic plane. However, the surface of the single crystal that is in contact with the tool holder is also on a major crystallographic plane, and so the single crystal must be held using a non-standard tool holder that holds the diamond off-axis (see, for example, US2010/0003091 Figure 3). Systems such as these therefore cannot be used with standard tool holders, as they require a misaligned holder to allow the chamfer to lie on a tool-working crystallographic plane. According to a first aspect, there is provided a single crystal synthetic diamond tool precursor structure. The single crystal synthetic diamond has a first surface, wherein the first surface is arranged to align with a holder and is oriented more than 5°away from a major crystallographic plane such that when a chamfer is formed in the single crystal synthetic diamond tool precursor structure at a pre-determined angle relative to the first surface to form a tool-working portion, the tool-working portion is oriented within 5° of a tool-working crystallographic geometry. An advantage of this is that when a chamfer is cut into the single crystal synthetic diamond, the chamfer lies on or close to a tool-working crystallographic plane, which can be optimized for a particular tool operation. For example, a selected crystallographic plane may be chosen for its hardness or wear resistance.

As an option the single crystal diamond tool precursor structure is made using either a high-pressure, high-temperature process or a chemical vapour deposition process. Optional examples of the tool-working crystallographic geometry include a {100} plane, a {1 10} plane, a {1 1 1 } plane, a {1 13} plane, a <100> direction, a <1 10> direction, a <1 1 1 > direction, a < 1 13> direction and an intersection of any two of a {1 1 1 }, a {1 10}, a {1 1 1} and a {1 13} plane.

As an option, the major crystallographic plane is selected from any one of a {100} plane, a {1 10} plane, a {1 1 1 } plane, and a {1 13} plane. As an option, the predetermined angle is between 6° and 35°.

The tool-working portion is optionally arranged to be oriented to within 2° of the tool- working crystallographic geometry. The tool-working portion is optionally selected from any of a cutting edge and a working surface.

As an option, the chamfer is arranged to provide either a rake face on a cutting tool or a flank face on a cutting tool.

The single crystal diamond tool precursor structure is optionally shaped from a precursor single crystal diamond to provide the crystallographic orientation of the first surface. As an alternative option, the single crystal diamond tool precursor structure is grown from a seed diamond to provide the crystallographic orientation of the first surface. This may be subsequently finished by shaping to further refine the orientation of the single crystal diamond tool precursor structure.

The single crystal diamond tool precursor structure is optionally configured to be used in any of a cutting tool, an engraving tool, a wire drawing die, a dresser, a wear part, and a fluid jet nozzle.

According to a second aspect, there is provided a single crystal synthetic diamond tool element. The single crystal synthetic diamond comprises a first surface, wherein the first surface is arranged to align with a holder, and wherein the first surface is crystallographically oriented more than 5° away from a major crystallographic plane. The single crystal diamond is also provided with a chamfer formed at a predetermined angle relative to the first surface to form a tool-working portion, wherein the chamfer is oriented within 5° of a tool-working crystallographic geometry.

As an option, the single crystal diamond is any of a high-pressure, high-temperature single crystal diamond and a chemical vapour deposition single crystal diamond. The tool-working crystallographic geometry is optionally selected from any of a {100} plane, a {1 10} plane, a {1 1 1 } plane, a {31 1 } plane, a <100> direction, a <1 10> direction, a <1 1 1 > direction, a < 1 13> direction and an intersection of any two of a {1 1 1 }, a {1 10}, a {1 1 1} and a {1 13} plane. As an option, the major crystallographic plane is selected from any one of a {100} plane, a {1 10} plane, a {1 1 1 } plane, and a {1 13} plane.

As an option, the predetermined angle is between 6° and 35°. The tool-working portion is optionally arranged to be oriented to within 2° of the tool- working crystallographic geometry.

As an option, the tool working portion is selected from any of a cutting edge and a working surface.

The chamfer surface is optionally arranged to provide either a rake face or a flank face on a cutting tool.

As an option, the single crystal diamond tool element is shaped from a precursor single crystal diamond to provide the crystallographic orientation of the first surface. As an alternative option, the single crystal diamond is grown from a seed diamond to provide the crystallographic orientation of the first surface. This may be subsequently finished by shaping to further refine the orientation of the single crystal diamond tool precursor structure.

The single crystal diamond tool element is optionally configured to be used in any of a cutting tool, an engraving tool, a wire drawing die, a dresser, a wear part, and a fluid jet nozzle.

According to a third aspect, there is provided a tool comprising the single crystal synthetic diamond tool described above in the second aspect.

The tool is optionally selected from any one of a cutting tool, an engraving tool, a wire drawing die, a dresser, a wear part, and a fluid jet nozzle.

According to a fourth aspect, there is provided a method of forming a single crystal diamond tool element. A single crystal synthetic diamond tool precursor structure is provided, which has a first surface, the first surface being oriented more than 5° away from a major crystallographic plane. A chamfer is formed in the single crystal synthetic diamond tool precursor structure at a predetermined angle relative to the first surface to form a tool-working portion such that the tool-working potion is oriented within 5° of a tool-working crystallographic geometry to form a tool element. The chamfer may be formed before or after the diamond is fixed to a tool.

As an option, the method further comprises providing a single diamond using any of a high-pressure, high-temperature technique and a chemical vapour deposition technique.

The chamfer is optionally formed such that the tool-working crystallographic geometry is selected from any of a {100} plane, a {1 10} plane, a {1 1 1 } plane, a {31 1 } plane, a <100> direction, a <1 10> direction, a <1 1 1 > direction, a < 1 13> direction and an intersection of any two of a {1 1 1 }, a {1 10}, a {1 1 1 } and a {1 13} plane.

As an option, the major crystallographic plane is selected from any one of a {100} plane, a {1 10} plane, a {1 1 1 } plane, and a {1 13} plane. The predetermined angle is optionally between 6 and 35°.

The chamfer is optionally formed such that the tool-working portion is oriented within 2° of the tool-working crystallographic geometry.

As an option, the chamfer is arranged to provide either a rake face or a flank face on a cutting tool. The single crystal diamond tool precursor structure is optionally shaped from a precursor single crystal diamond to provide the crystallographic orientation of the first surface. As an alternative, the single crystal diamond tool precursor structure is optionally grown from a seed diamond to provide the crystallographic orientation of the first surface.

As an option, the method comprises attaching the single crystal diamond tool element to any of a cutting tool, an engraving tool, a wire drawing die, a dresser, a wear part, and a fluid jet nozzle. BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting example arrangements to illustrate the present disclosure are described hereafter with reference to the accompanying drawings, of which: Figure 1 shows a side elevation view of a known single crystal diamond for use in a machining tool;

Figure 2 shows a perspective view of a known single crystal diamond machining a workpiece;

Figure 3 shows a side elevation view of an exemplary single crystal synthetic diamond tool precursor structure; Figure 4 shows a side elevation view of an exemplary single crystal synthetic diamond tool element for use in a tool;

Figure 5 shows a side elevation view of a further exemplary single crystal synthetic diamond tool element for use in a tool;

Figure 6 shows a side elevation view of a further exemplary single crystal synthetic diamond tool element for use in a tool; Figure 7 is a flow diagram showing exemplary steps;

Figure 8 is a perspective view showing one example of how a synthetic diamond tool precursor structure may be obtained from a precursor single crystal diamond; and Figure 9 is a perspective view of an edge of an exemplary cutting tool.

DETAILED DESCRIPTION

It is known that the mechanical properties of diamond are anisotropic, as discussed in Uddin et. al., "Effect of crystallographic orientation on wear of diamond tools for nano-scale ductile cutting of silicon", Wear 257 (2004) 751-759, Elsevier. For example, it has been reported that with respect to cleavage fracture along the {1 1 1 } plane, the {100} set of planes have a higher strength than the {1 10} or the {1 1 1 } planes. Similarly, different planes have different abrasion resistance. It is therefore appreciated that a cutting tool manufactured using a single crystal synthetic diamond will have different wear characteristics depending on the crystallographic planes exposed during a cutting operation. It is therefore advantageous to align a chamfer with a particular crystallographic geometry, such as a crystallographic plane, a crystallographic direction or an intersection of two crystallographic planes.

As described above, where a single crystal synthetic diamond is used as a cutting (or other form of shaping) element, a toolmaker is typically provided with a single crystal synthetic diamond tool precursor structure. This is in the form of a block, with the major surfaces corresponding to major crystallographic planes. This is because the single crystal synthetic diamond tool precursor structure is obtained from a seed crystal in a high pressure high temperature (HPHT) process or a chemical vapour deposition (CVD) process, and adopts the crystallographic orientation of the seed crystal. A consequence of the crystallographic orientation of the synthetic diamond tool precursor structure is that when a chamfer is cut into it to form a rake face or a flank face, the chamfer is not aligned with a crystallographic plane. The resultant single crystal synthetic diamond tool element may therefore not have optimised properties.

Turning to Figure 3, a single crystal synthetic diamond tool precursor structure 301 is shown wherein a first surface 302 or second surface 303 is oriented such that it does not coincide with a major crystallographic plane. However, the orientation of the first surface 302 relative to the crystallographic orientation is selected such that when a chamfer is applied (for example by cutting, grinding an polishing, laser cutting, chemical etching, scaife polishing and so on) at a predetermined angle a relative to the first surface 302, a working portion formed by the chamfer is oriented to substantially coincide with a tool-working crystallographic plane. A tool-working crystallographic plane is defined herein as a crystallographic plane that has beneficial properties for the particular tooling operation that the synthetic diamond tool element is used for. A working portion may be a cutting edge, a chamfer surface, a die surface and so on, again depending on the tooling operation.

Figure 4 shows a single crystal synthetic diamond tool element 401 formed by cutting a chamfer 402 into the single crystal synthetic diamond tool precursor structure 301 of Figure 3. In this example, the tool-working portion aligned with a tool-working crystallographic plane coincides with the chamfer surface 402. The chamfer 402 is therefore cut at a predetermined angle a relative to the first surface 302 such that the chamfer 402 is oriented to substantially coincide with a major crystallographic plane. In the example of Figure 4, the chamfer 402 is oriented to coincide with a major crystallographic plane which is provided for a different purpose or cutting application. The term "substantially" oriented on a major crystallographic plane in this context means that the chamfer 402 is oriented to within 5° of the major crystallographic plane, although the closer the chamfer 402 is to the required crystallographic plane the better. A tolerance of plus or minus 2° may be achievable within normal manufacturing constraints.

Examples of tool-working crystallographic planes for diamond are {100}, {1 10} and {1 1 1}. The example of Figure 4 shows the single crystal synthetic diamond tool element 401 for use as cutting element in a tool, with the chamfer 402 providing a rake face, and a second chamfer 403 also provided. It will be appreciated that the first surface 302 of the single crystal synthetic diamond tool element 401 may be oriented relative to the crystallographic orientation such that more than one chamfer, when cut into the single crystal synthetic diamond tool element 401 , is oriented to substantially coincide with a major crystallographic plane. Chamfers for cutting operations are typically cut at an angle up to 35°.

Note that a plan view of the tool element 401 of Figure 4 (not shown) typically has a radius at the chamfer edge. However, a single crystal synthetic diamond tool element 401 need not be used as a cutting element, but could be any type of shaping element. For example, the single crystal synthetic diamond tool element 401 could be used in an engraving tool, a wire drawing die, a grinding wheel dresser, a wear part (including water nozzles) and so on, and is shaped accordingly.

Figure 5 shows a further example where the chamfer does not intersect the first surface 302. In this example a single crystal synthetic diamond tool element 501 has a chamfer 501 provides a flank surface but does not intersect the first surface 302. Nevertheless, the angle of the chamfer is relative to the first surface 302 as the first surface 302 is used to align the single crystal synthetic diamond tool element 501 relative to the tool. Similarly, a single crystal synthetic diamond tool element may be provided with multiple chamfers, but the chamfer providing the tool-working portion oriented substantially on a crystallographic plane is formed at an angle relative to the first surface 302 regardless of whether that chamfer intersects the first surface 302.

Figure 6 shows a further example in which chamfer does not intersect the first surface 302. In this example a single crystal synthetic diamond tool element 601 has multiple chamfers. In this instance, a chamfer 601 provides a flank surface but does not intersect the first surface 302. Nevertheless, the angle of the chamfer is relative to the first surface 302 as the first surface 302 is used to align the single crystal synthetic diamond tool element 501 relative to the tool.

Similar principles apply where the tool is used for a different purpose. The tools of Figure 4, 5 and 6 are used as a cutting tool and have a cutting edge that requires mechanical strength, toughness and chip resistance. However, where a tool is used as a die (for example in a wire drawing operation), the crystallographic orientation of a surface may require different properties.

Figure 7 is a flow diagram showing exemplary steps, with the following numbering corresponding to that of Figure 7: S1 . A single crystal synthetic diamond tool precursor structure 301 is provided. The single crystal synthetic diamond has a first surface 302 that is oriented away from a major crystallographic plane.

52. The single crystal synthetic diamond tool element 401 is affixed to a tool either directly or indirectly using the first face 302 to align the single crystal synthetic diamond tool element 401 relative to the tool.

53. A chamfer 402 is cut into the first surface 302 at a predetermined angle a relative to the first surface 302, although it need not necessarily intersect the first surface 302. This ensures that the chamfer surface 402 is oriented within 5° of a major crystallographic plane. This forms a single crystal synthetic diamond tool element 401 with a tool-working portion that is more efficiently crystallographically oriented with respect to a workpiece. Note that steps S2 can happen before step S3, or step S3 can happen before step S2. As shown in Figure 8, the correct orientation of the first surface 302 relative to a major crystallographic plane may be achieved by providing a precursor single crystal diamond grown 801 in a conventional manner. The surfaces of the precursor single crystal diamond 801 correspond with major crystallographic planes (e.g. {100}, {1 10} and so on). The precursor single crystal diamond is then shaped to provide the single crystal diamond tool precursor structure 301 with the first surface 302 at the required orientation. Shaping may be achieved by any conventional means, such as mechanically cutting, grinding, polishing, laser cutting, chemical cutting and any combination thereof. As an alternative, the single crystal diamond tool precursor structure 301 may be grown directly in an HPHT or a CVD process using careful orientation of seed diamonds and careful control of growth conditions.

However the single crystal diamond tool precursor structure 301 is obtained, it is typically derived from a synthetic diamond process such as HPHT synthesis or CVD deposition. HPHT synthesis of single crystal diamond material is well known in the art. Standard processes for manufacturing small crystals of diamond involve mixing a graphite powder with a powdered metal catalyst comprising, for example, cobalt and iron (advantageously in a ratio at, or close to, the eutectic composition - 65% Co : 35% Fe). Other catalyst compositions are also known comprising, for example, Co, Fe, Ni, and/or Mn. A micron scale diamond powder may also be included in the reaction mixture to form seeds for diamond growth although spontaneous nucleation is possible. The reaction mixture is transferred into a capsule and loaded into a press where it is subjected to a pressures and temperature in the region of the carbon phase diagram where diamond is the thermodynamically stable form of carbon and diamond growth occurs. With careful control of the synthesis conditions, large single crystal diamond can be produced. The application of the tool has an effect on the angle used and which crystallographic geometry the chamfer is aligned with. The geometry of the chamfer may be selected to align with a plane, a crystallographic direction (for example where the properties of an edge is important) or the intersection of two planes.

By way of example, Figure 9 shows a perspective view of the cutting edge of a tool. In this example, the tool has a rake face 901 , a cutting edge 902 and a flank face 903. As mentioned above and shown in Figure 9, the cutting edge 902 is typically curved. As the cutting edge 902 is curved, the selected crystallographic geometries may lie on a tangent of the curve.

In tool applications where it is important for the cutting edge to be chip resistant, the chamfer may be selected so that the crystallographic direction at the edge optimises the toughness of the diamond at the edge. In applications where it is important for the flank face to have improved wear resistance, the chamfer can be cut such that the flank face is on a crystallographic plane (e.g. {1 13}) that gives particularly wear resistant properties. In some instance, optimized properties such as wear resistance may be achieved if a face lies perpendicular to a particular crystallographic plane. Similar consideration can be given to other types of machining operations.

The crystallographic notation described above is based on Miller indices. Indices in parentheses such as (100) denote a crystallographic plane. Indices in curly brackets such as {100} denote a family of planes related by symmetry. Indices in square brackets such as [100] denote a direction vector (and may be used to describe an edge where two planes meet). Indices in chevrons such as <100> denote a family of directions that are related by symmetry.

The invention as set out in the appended claims has been shown and described with reference to embodiments, in particular where the single crystal synthetic diamond tool element is used in a cutting tool. However, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims. For example, other types of shaping tool that use single crystal diamond may be used, and the crystallographic plane on which a chamfer is formed may be selected depending on the operation to be performed. Furthermore, the single crystal diamond may have more than one chamfer.