Milling cutter

Abstract

A milling cutter comprising: a tool shank (12) having a longitudinal axis of rotation; and a tool head (16) attached to one end of the tool shank, the tool head comprising first, second and third tiers (18a, 18b, 18c) extending around an outer surface of the tool head; wherein the tiers are axially displaced from each other along the longitudinal axis; wherein the first tier of the tool head comprises a plurality of axially extending first cutting surfaces, each first cutting surface having a first profile; the second tier of the tool head comprises a plurality of axially extending second cutting surfaces, each second cutting surface having a second profile; and the third tier of the tool head comprises a plurality of axially extending third cutting surfaces, each third cutting surface having a third profile; wherein the first tier is closer to the tool shank than the second tier; and wherein the third tier is further from the tool shank than the first tier and the second tier; wherein, when the first, second and third profiles are superimposed, the first profile nests within the second profile, and the second profile nests within the third profile; wherein the first profile differs from the second profile, and wherein the third profile differs from the first profile and the second profile; and wherein the first, second and third cutting surfaces comprise superhard material.

Description

[0001] MILLING CUTTER

[0002] FIELD OF THE INVENTION

[0003] This disclosure relates to a cutting tool which is particularly suited for milling brittle materials, and a method of use thereof.

[0004] BACKGROUND

[0005] Brittle materials, such as glass, are used in many consumer electronics applications. For example, glass is used for smartphone and tablet screens. To enhance their aesthetic qualities, improve part surface integrity, and to promote brand recognition, manufacturers of smartphones and tablets often machine particular shapes around the edges of their products. This machining can be performed using cutting tools such as end mill cutting tools (also known as end mill cutters).

[0006] End mill cutting tools are the most common form of milling cutting tools and they are available in a wide variety of heights, diameters, features and types. End mill cutters are used for machining the faces and sides of a workpiece, like a smartphone or tablet screen. During a typical milling operation, the cutter moves perpendicularly to its axis of rotation, allowing it to remove material form the workpiece at the perimeter of the cutter.

[0007] A problem which arises when machining brittle materials, like glass, is that there is a tendency for the workpiece, in this case the smartphone or tablet screen, to fracture. There is therefore a need for improved cutting tools and methods to enable complex shapes to be formed on glass (and other brittle materials) while mitigating against the tendency for the brittle glass to fracture during machining.

[0008] The present invention provides a solution to the above-mentioned problem.

[0009] SUMMARY OF THE INVENTION

[0010] According to a first aspect, there is provided a cutting tool comprising: a tool shank having a longitudinal axis of rotation; and a tool head attached to one end of the tool shank, the tool head comprising first and second tiers extending around an outer surface of the tool head; wherein the tiers are axially displaced from each other along the longitudinal axis; wherein the first tier of the tool head comprises a plurality of axially extending first cutting surfaces, each first cutting surface having a first profile; and the second tier of the tool head comprises a plurality of axially extending second cutting surfaces, each second cutting surface having a second profile; wherein the first profile differs from the second profile.

[0011] As an option, the sizes of the first and second profiles are different to each other.

[0012] As an option, when the first profile and the second profile are superimposed, the first profile nests within the second profile.

[0013] As an option, the first profile and the second profile comprise cut-out portions, and, when the first and second cutting surfaces are viewed in longitudinal cross-section, the cut-out portion of the first profile has a smaller area than the cut-out portion of the second profile.

[0014] As an option, the first tier is closer to the tool shank than the second tier.

[0015] As an option, the axial extension of the first cutting surface is inclined from the longitudinal axis by up to 60 degrees and / or wherein the axial extension of the second cutting surface is inclined from the longitudinal axis by up to 60 degrees.

[0016] As an option, the tiers are separated by a non-cutting portion of the tool head.

[0017] As an option, when the first and second cutting surfaces are viewed in longitudinal crosssection, the first profile comprises a cut-out portion with a substantially planar base, and the second profile comprises a cut-out portion with a substantially planar base, wherein the depth of the cut-out portion of the first profile is less than that of the cut-out portion of the second profile.

[0018] As an option, when viewed in longitudinal cross-section, the cut-out portion of the first and / or second profile further comprises angled sides inclined from the outer periphery of the first and / or second cutting surfaces and towards the substantially planar base.

[0019] As an option, the angle of inclination of the first profile is less than that of the second profile.

[0020] As an option, the approximate increment of the angle of inclination between the profiles of adjacent tiers of the tool is calculated by dividing the chamfer angle to be formed on the workpiece by the number of tiers on the tool head. As an option, the percentage difference between the angle of inclination of the profiles of adjacent tiers is in the range of 20-60%, for example 20-50%, for example 20-40%.

[0021] As an option, the angled sides are symmetrical when viewed in longitudinal cross-section.

[0022] As an option, the angled sides are asymmetrical when viewed in longitudinal cross-section.

[0023] As an option, when the first and second cutting surfaces are viewed in longitudinal crosssection, the first profile comprises a first curved portion with a radius of curvature, and the second profile comprises a second curved portion with a larger radius of curvature than that of the first curved portion.

[0024] As an option, when the first and second cutting surfaces are viewed in longitudinal crosssection, the first profile and / or the second cut-out portions comprise cut-out portions, wherein the cut-out portion has a triangular, rectangular, square, semi-circular, pentagonal, hexagonal, heptagonal or octagonal profile.

[0025] As an option, the cut-out portion has a triangular profile selected from an equilateral triangle, an isosceles triangle, and a scalene triangle.

[0026] As an option, the tool head further comprises a third tier, wherein the third tier of the tool head comprises a plurality of axially extending third cutting surfaces having a third profile which differs from the first profile and the second profile.

[0027] As an option, the tool head further comprises a third tier, wherein the third tier of the tool head comprises a plurality of axially extending third cutting surfaces, each third cutting surface having a third profile.

[0028] As an option, the sizes of the first, second and third profiles are different to each other.

[0029] As an option, the first, second and third profiles are superimposed, the first profile nests within the second profile, and the second profile nests within the third profile.

[0030] As an option, the third tier is further from the tool shank than the first tier and the second tier. As an option, when the third cutting surface is viewed in longitudinal cross-section, the third profile comprises a cut-out portion with a substantially planar base and angled sides inclined from the outer periphery of the third cutting surface and towards the substantially planar base.

[0031] As an option, the cutting tool comprises at least three tiers.

[0032] As an option, the tool head is cylindrical and non-tubular.

[0033] As an option, the first and / or second cutting surfaces comprise superhard material.

[0034] As an option, the superhard material is monolithic polycrystalline diamond. As an alternative option, the superhard material is polycrystalline diamond adjoining a carbide backing portion.

[0035] As an option, the superhard material comprises any of high-pressure high-temperature polycrystalline diamond, chemical vapour deposition diamond, and polycrystalline cubic boron nitride.

[0036] As an option, the cutting tool is a milling cutter.

[0037] As an option, the milling cutter is a profile milling cutter.

[0038] As an option, the cutting tool is an end mill cutter.

[0039] As an option, the cutting tool is a micro end mill cutter.

[0040] As an option, the cutting tool is a micro end mill cutter having an outer diameter selected from any of no more than 15 mm, no more than 10 mm, no less than 6 mm and no less than 2 mm.

[0041] As an option, the cutting tool is a micro end mill cutter wherein the tool head has an outer diameter selected from any of no more than 15 mm, no more than 10 mm, no less than 6 mm and no less than 2 mm.

[0042] As an option, the tool shank comprises cemented carbide.

[0043] According to a second aspect, there is provided a method of cutting a workpiece comprising: providing a cutting tool as described herein; contacting the workpiece with the plurality of first cutting surfaces to form a first cut workpiece surface; and contacting the first cut workpiece surface with the plurality of second cutting surfaces to form a second cut workpiece surface.

[0044] As an option, the tool head further comprises a third tier, wherein the third tier of the tool head comprises a plurality of axially extending third cutting surfaces having a third profile which differs from the first profile and the second profile; and wherein the method further comprises contacting the second cut workpiece surface with the plurality of third cutting surfaces to form a third cut workpiece surface.

[0045] As an option, the workpiece is contacted by rotating the cutting tool about its longitudinal axis of rotation and bringing the workpiece into contact with, sequentially, the plurality of first cutting surfaces, followed by the plurality of second cutting surfaces, and, when present, followed by the plurality of third cutting surfaces by axial displacement of the cutting tool and / or the workpiece.

[0046] According to a third aspect, there is provided a use of the cutting tool as described herein in a method of cutting a brittle material.

[0047] As an option, the brittle material is any of glass, ceramic, polymer, composites, metallic materials and metal-ceramic composites.

[0048] BRIEF DESCIPTION OF THE DRAWINGS

[0049] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0050] Figure 1 is a perspective view of a tool in accordance with the invention, with a first embodiment of a tool head;

[0051] Figure 2 is an enlarged view of an edge part of the first embodiment of the tool head of the tool depicted in Figure 1 ;

[0052] Figure 3 is a schematic cross-sectional view of the cutting profile of the tool head depicted in Figures 1 and 2; Figure 4 is an enlarged view of an edge part of a second embodiment of the tool head;

[0053] Figure 5 is a schematic cross-sectional view of the cutting profile of the tool head depicted in Figure 4;

[0054] Figure 6 includes three drawings which highlight the first, second and third cutting surfaces, respectively, of an example three-tier tool;

[0055] Figure 7 is an example workpiece; and

[0056] Figure 8 is a series of drawings depicting the evolution of a pair of chamfers on the workpiece as depicted in Figure 7 using the three-tier tool as depicted in Figure 6.

[0057] Throughout the embodiments, similar parts are denoted by the same reference numeral and a further description is omitted for brevity.

[0058] DETAILED DESCRIPTION

[0059] Referring firstly to Figure 1 , a milling tool is indicated generally at 10. The tool comprises a tool shank 12 having a longitudinal axis of rotation 14, and further comprises a tool head 16 at one end of the shank 12. In this embodiment, the tool shank 12 comprises a cemented metal carbide, for example tungsten carbide, although other suitable materials are envisaged. In this embodiment, the tool head 16 is cylindrical and non-tubular. The tool head comprises a superhard material. The tool head 16 in this example comprises polycrystalline diamond (PCD). In one embodiment, the tool head 16 comprises a solid, monolithic PCD block. In this context, ‘monolithic’ means that the PCD has been sintered in a single piece in a single sintering operation. The PCD may be sinter-joined to a carbide backing layer, though this need not be the case and the carbide backing layer may be omitted. The carbide backing layer can facilitate attachment of the PCD to the tool shank 12, which can be achieved using any reasonable means. Instead of PCD, polycrystalline boron nitride (PCBN), either in the form of a solid, monolithic block or backed with carbide as described above, can be used in the tool head 16. Alternatively, the tool head 16 may be formed of a cemented carbide substrate onto which polycrystalline diamond has been deposited by chemical vapour deposition. As a further option, the tool head 16 may comprise two or more PCD segments stacked side by side adjacent to each other, each segment forming one or more of said tiers. In such an arrangement, the PCD segments may be annular, aligned coaxially with the axis of rotation, and mounted about a hub extending from the tool shank 12. A close-up view of the tool head 16 is shown in Figure 2. The tool head 16 comprises at least two tiers. In this embodiment, the tool head 16 comprises two tiers 18a, 18b (i.e. a stage or a level) extending around the outer surface of the tool head 16. Each tier 18a, 18b comprises a plurality of axially extending cutting surfaces 19a, 19b. Specifically, first tier 18a comprises a plurality of axially extending first cutting surfaces 19a, each first cutting surface 19a having a first profile, and second tier 18b comprises a plurality of axially extending second cutting surfaces 19b, each second cutting surface 19b having a second profile. The first profile differs (i.e. in size) from the second profile; in other words, each of the first and second tiers 18a, 18b has a unique cutting profile. Within each tier, the plurality of cutting surfaces extend circumferentially around the tool head 16. In any one tier, all the cutting surfaces are in a band, i.e. they are in axial alignment with each other. Tiers 18a and 18b are separated from each other by a non-cutting portion 17 of the tool head 16.

[0060] The cutting surfaces may be created in the outer surface of the tool head 16 using a laser which initially ablates unwanted material, thereby creating recesses between precursor cutting surfaces, and subsequently shapes the precursor cutting surfaces according to a desired profile (i.e. first profile and second profile) into a final cutting surface configuration.

[0061] In an embodiment, the axial extension of the cutting surfaces is inclined from the longitudinal 14 axis by up to 60 degrees, for example by from 5 to 60 degrees, for example by from 10 to 60 degrees, for example by from 15 to 60 degrees, for example by from 20 to 60 degrees, for example by from 25 to 60 degrees, for example by from 5 to 40 degrees, for example by from 10 to 40 degrees, by for example from 15 to 40 degrees, by for example from 20 to 35 degrees, by for example from 25 to 35 degrees. Herein, this angle of inclination of the axial extension of the cutting surface from the longitudinal axis is termed the lean angle 9. The lean angle 9 is measured as depicted schematically in Figure 2, i.e. it is the included angle between the axial extension of the cutting surface and the longitudinal axis of rotation of the tool. In an embodiment, all of the cutting surfaces (i.e. first and second cutting surfaces and any subsequent cutting surfaces) have the same lean angle 9.

[0062] As noted above, the tool head comprises at least two tiers. Additional tiers are axially displaced with regards to the initial tier. A tool with multiple tiers therefore has tiers that are co-axially aligned and adjacent to each other. Each tier is separated from an adjacent tier by a non-cutting portion 17 of the tool head 16. Any description herein of the first and second tiers is applicable to any additional tiers. In an embodiment, any additional tiers, like the first and second tiers, all have a unique cutting profile, i.e. the cutting surface of each tier has a profile which differs in size from that of each other tier.

[0063] The profile of the first and second cutting surfaces 19a, 19b is shown in Figure 3, which shows a longitudinal cross-section through the cutting surface. As depicted in Figure 3, when the first profile and the second profile are superimposed, the first profile nests within the second profile. In this embodiment, when the first and second cutting surfaces 19a, 19b are viewed in longitudinal cross-section (as in Figure 3), the first profile comprises a cut-out portion with a substantially planar base, and the second profile comprises a cut-out portion with a substantially planar base, wherein the depth of the cut-out portion of the first profile is less than that of the cut-out portion of the second profile. The cut-out portion of the first and second profiles further comprises symmetrical angled sides inclined from the outer periphery of the first and second cutting surfaces, respectively, and towards the substantially planar base. The angle of inclination of the first profile is less than that of the second profile. The angle of inclination is adapted depending on the final profile which it is desired to achieve, and the workpiece being cut. For example, the approximate increment of the angle of inclination for the profile of each tier of the tool can be calculated by dividing the desired chamfer angle by the number of tiers. So, if the desired chamfer angle is 30° and the number of tiers is 2, then the increment between profiles in adjacent tiers would be approximately 15°. In this particular embodiment, the angle of inclination of the first profile is 15° and the angle of inclination of the second profile is 30°.

[0064] An alternative way to estimate the suitable increment between the angles of inclination of the profiles is to use the percentage difference between the angle of inclination of the profiles of adjacent tiers. For example, the angle of inclination of the first profile could be 20-60% less than the angle of inclination of the second profile. In the above example, the angle of inclination of the first profile is 50% of that of the second profile.

[0065] When used in the machining of a profile onto a workpiece such as a chamfer or external radius (a method for which is detailed below), this tool design, specifically the design of the profiles of the cutting surfaces of the first and second tiers, significant advantages over prior art tools are provided. In this embodiment, instead of directly machining a chamfer with an angle of 30° onto the workpiece edge, the chamfer angle is incrementally increased using sequential roughing and finishing passes using the first and second profiles of the tool head, respectively. This ensures that the local uncut chip thickness at the edge of the workpiece can be controlled and minimised during machining to reduce the machining stresses imparted on the edge of the workpiece and prevent fracture. Additionally, by reducing the stresses concentrated at workpiece edges through optimisation of the cutting profile geometry, higher and more productive feed rates can be achieved during the roughing and finishing passes, whilst continuing to reduce the risk of workpiece fracture. The same principle can be applied to whatever desired complex form is intended to be formed on the workpiece.

[0066] A second embodiment of the tool is depicted in Figure 4, where the tool head 16 comprises three tiers 18a, 18b, 18c. Tier 18a corresponds to the tier closest to the shank, tier 18c corresponds to the tier furthest away from the shank, and tier 18b corresponds to the tier axially intermediate tiers 18a and 18c. Each tier 18a, 18b, 18c comprises a plurality of axially extending cutting surfaces 19a, 19b, 19c. The cutting surfaces 19a, 19b, 19c are provided in an outer surface of the tool head. Specifically, first tier 18a comprises a plurality of axially extending first cutting surfaces 19a, each first cutting surface 19a having a first profile, second tier 18b comprises a plurality of axially extending second cutting surfaces 19b, each second cutting surface 19b having a second profile, and third tier 18c comprises a plurality of axially extending third cutting surfaces 19c, each third cutting surface 19c having a third profile. Within each tier, the plurality of cutting surfaces extend circumferentially around the tool head 16. The cutting surfaces can be formed as detailed above in the context of the two-tier embodiment.

[0067] The first and second tiers 18a, 18b are the same as those described above in the context of the first embodiment. The profile of the third cutting surface 19c is shown in Figure 5, which shows a longitudinal cross-section through the cutting surface. As depicted in Figure 5, when the first profile and the second profile are superimposed, the first profile nests within the second profile, and the second profile nest within the third profile. In this embodiment, when the third cutting surface is viewed in longitudinal cross-section (as in Figure 5), the third profile comprises a cut-out portion with a substantially planar base, wherein the depth of the cut-out portion of the third profile is greater than that of the cut-out portion of the first and second profiles. The cut-out portion of the third profile further comprises symmetrical angled sides inclined from the outer periphery of the third cutting surface and towards the substantially planar base. The angle of inclination of the third profile is greater than that of the second profile. As noted above, the angle of inclination is adapted depending on the final profile which it is desired to achieve, and the workpiece being cut. In this example, the desired chamfer angle is 45° and the number of tiers is 3, and so the increment between adjacent tiers is 15°. In this particular embodiment, the angle of inclination of the first profile is 15°, the angle of inclination of the second profile is 30°, and the angle of inclination of the third profile is 45°. The angle of inclination of the first profile is 50% less than the angle of inclination of the second profile and the angle of inclination of the second profile is 33.3% less than the angle of the third profile.

[0068] While in the embodiments described above, a particular shape of cutting profile is shown, the nature of the cutting profile is not particularly limited, so long as the profiles nest within each other such that, when a workpiece is machined by sequential passes across the adjacent tiers, material is removal from the workpiece in an incremental fashion. For example, the cutting profiles may, when viewed in longitudinal cross section, be curved, with varying radiuses of curvature, or they may have a polygonal profile, for example triangular, rectangular, square, semi-circular, pentagonal, hexagonal, heptagonal or octagonal profile. The triangular profile may be selected from an equilateral triangle, an isosceles triangle, and a scalene triangle. The cutting profiles of the embodiments described above are symmetrical when viewed in longitudinal cross-section, however the cutting profiles may instead be asymmetrical.

[0069] This addition of a third tier further enhances the reduction of stresses detailed above. The three-tier tool design depicted in Figures 4 and 5 provides roughing, semi-finishing, and finishing passes using the first, second and third tiers, respectively.

[0070] The outer diameter of the tool 10 is the largest, outermost, diameter of any of the tiers 18 and the shank 12. The tool 10 may be a milling cutter, for example a micro end mill tool, which has an outer diameter of no more than 15 mm, for example in the range of from 5 mm to 15 mm, for example in the range of from 8 mm to 13 mm, for example in the range of from 10 mm to 12 mm. The outer diameter of the tool head 16 is the largest, outermost diameter of any of the tiers 18. The tool head 16 may have an outer diameter of no more than 15 mm, for example in the range of from 5 mm to 15 mm, for example in the range of from 8 mm to 13 mm, for example in the range of from 10 mm to 12 mm. Optionally, the outer diameter of the tool 10 or the tool head 16 is 10 mm or 12 mm. In one example of a milling cutter, for example a micro end mill tool, the overall height of the tool, including tool shank 12 and tool head 16 may be up to and including 200 mm, for example up to and including 150 mm, for example up to and including 100 mm, for example up to and including 75 mm, for example up to and including 50 mm.

[0071] The height of each tier 18 is measured axially along or parallel to the longitudinal axis of rotation 14 of the tool. An example of how the height Hi of the first tier 18a is measured is provided in Figure 2. Heights of further tiers are measured in an analogous way. The tiers may have a height of up to and including 1 mm, for example from 0.1 mm to 1 mm, for example from 0.2 mm to 1 mm, for example from 0.3 mm to 1 mm, for example from 0.4 mm to 1 mm, for example from 0.5 mm to 1 mm, for example from 0.5 mm to 0.9 mm, for example from 0.5 mm to 0.8 mm, for example from 0.5 mm to 0.7 mm. The tiers may have a height of at least 0.1 mm, for example at least 0.2 mm, for example at least 0.3 mm, for example at least 0.4 mm, for example at least 0.5 mm. The tiers may all be of equal height, or their heights may vary. In an embodiment, each of the tiers has the same height.

[0072] The number of cutting surfaces in each tier can be varied depending on the desired application and the diameter of the tool. The number of cutting surfaces in each tier may be from 5 to 100, for example from 5 to 75, for example from 10 to 75, for example from 25 to 75, for example from 35 to 75, for example from 45 to 55. In an embodiment, the cutting surfaces in each tier are connected to one another via a non-cutting portion so as to form continuous, axially extending channels which run across all of the tiers in the tool head. An example of this is shown in Figure 6, where the three labelled cutting surfaces 19a, 19b and 19c are connected to one another via a non-cutting portion so as to form channels 20. These channels allow debris from the cutting process to escape. In this embodiment, each tier therefore has the same number of cutting surfaces.

[0073] One exemplary way to make one of the tool heads described is as follows: a typically circular blank shaped like a disc comprising superhard material such as PCD or PCBN is provided. At least one precursor tool head is machined from the disc. The quantity of precursor tool heads available depends on the diameter of the blank, the useable area devoid of defects and the outer diameter of the tool. The blank may be backed with a carbide backing layer or alternatively unbacked, or ‘freestanding’. The depth of the blank determines the depth of the tool head 16. A plurality of cutting surfaces is then formed in the precursor tool head using a laser. The cutting surfaces are arranged in axially adjacent tiers. This latter step is then repeated for the second tier and thereafter as often as required, thereby forming a tool head comprising at least first and second tiers extending around an outer surface of the tool head.

[0074] An alternative way to make one of the tool heads described is as follows: a cemented carbide disc blank is provided and a precursor tool head is machined from the disc. A tier containing a plurality of cutting surfaces is formed in the precursor tool head using a laser. This step is repeated to form the second tier and thereafter as required, thereby forming a tool head comprising at least first and second tiers extending around an outer surface of the tool head, and wherein the tool head comprises the superhard material, and wherein the tiers are axially displaced from each other and separated by a non-cutting portion of the tool head. Finally, polycrystalline diamond is deposited on the plurality of cutting surfaces using chemical vapour deposition. Typically, hot filament CVD is used, but other forms of CVD such as microwave plasma CVD may be used. A final finishing operating may be required on the deposited diamond layer on the cutting surfaces.

[0075] The above-described cutting tool can be used in a method of cutting a workpiece. Figure 6 depicts schematically a three-tier tool. In each of drawings 1 , 2 and 3 in Figure 6, a different set of cutting profiles are highlighted. In drawing 1 , cutting surfaces 19a of the first tier 18a are highlighted, in drawing 2, cutting surfaces 19b of the second tier 18b are highlighted, and in drawing 3, cutting surfaces 19c of the third tier 18c are highlighted. As shown in Figure 6, cutting surface 19a has the shallowest profile of the three cutting surfaces of the three-tier tool, and cutting surface 19c has the deepest profile of the three cutting surfaces of the three- tier tool. In the first pass of the tool, the workpiece is contacted with the plurality of first cutting surfaces 19a (highlighted in drawing 1 of Figure 6) to form a first cut workpiece surface. The workpiece is contacted by rotating the cutting tool about its longitudinal axis of rotation and bringing the workpiece into contact with the plurality of first cutting surfaces 19a as the tool rotates. This is a roughing operation which forms an initial chamfer on the workpiece. Next, in the second pass of the tool, the first cut workpiece surface is contacted with the plurality of second cutting surfaces 19b (highlighted in drawing 2 of Figure 6) to form a second cut workpiece surface. This is a semi-finishing operation which forms a second chamfer on the workpiece. Then, in the third pass of the tool, the second cut workpiece surface is contacted with the plurality of third cutting surfaces 19c (highlighted in drawing 3 of Figure 6) to form a third cut workpiece surface. Thus, using this method, the tool can be used to increase the degree of chamfering through multiple, consecutive cutting passes performed on the same feature of the workpiece. The third cut workpiece surface can then be subjected to any necessary further processes, such as polishing, to arrive at the final shaped product.

[0076] An example of the evolution of a pair of chamfers on the workpiece through the above- mentioned method of cutting a workpiece is depicted in Figures 7 and 8. In Figure 7, an exemplary workpiece 21 is shown. This workpiece is square. However, the workpiece can be any suitable shape, so long as it is configured to be cut by the above-mentioned cutting tool.

[0077] In the first pass of the tool, workpiece 21 is contacted with the plurality of first cutting surfaces 19a (highlighted in drawing 1 of Figure 6) to form a first cut workpiece surface 22. The area labelled as 22a is the material which is removed during the contact of the workpiece 21 with the plurality of first cutting surfaces 19a. The amount of material to be removed can be adjusted depending on the nature of the material and the desired size and / or shape of the final shaped product. First cut workpiece surface 22 has two chamfers connected by a substantially straight edge. The chamfers have a chamfer angle A.

[0078] In the second pass of the tool, first cut workpiece surface 22 is contacted with the plurality of second cutting surfaces 19b (highlighted in drawing 2 of Figure 6) to form a second cut workpiece surface 23. The area labelled as 23a is the material which is removed during the contact of the first cut workpiece surface 22 with the plurality of second cutting surfaces 19b. In this example, the purpose of the second pass is to increase the chamfer angle B such that chamfer angle B is larger than chamfer angle A.

[0079] In the third pass of the tool, second cut workpiece surface 23 is contacted with the plurality of third cutting surface 19c (highlighted in drawing 3 of Figure 6) to form a third cut workpiece surface 24. The area labelled as 24a is the material which is removed during the contact of the second cut workpiece surface 23 with the plurality of second cutting surfaces 19c. In this example, the purpose of the third pass is to further increase the chamfer angle C such that chamfer angle C is larger than chamfer angle B, or, in other words, to form the final desired chamfer angle for the workpiece.

[0080] As noted above, it is particularly advantageous to form chamfers on the workpiece by means of at least two sequential passes as detailed above as doing so ensures that the local uncut chip thickness at the edge of the workpiece can be controlled and minimised during machining to reduce the machining stresses imparted on the edge of the workpiece and prevent fracture. If chamfers with the final desired chamfer angle were formed in one pass, the likelihood of the workpiece fracturing would be significantly increased, resulting in poor production yields. By reducing the stresses concentrated at workpiece edges through optimisation of the cutting profile geometry, higher and more productive feed rates can be achieved during the roughing and finishing passes, whilst continuing to reduce the risk of workpiece fracture.

[0081] The method using a two-tier tool would be performed in an analogous way, without the third pass. Additional tiers of the tool would provide additional passes. This could be useful, for example, if machining a particularly fragile brittle material where it was desired to remove smaller amounts of material per pass in order to mitigate against fracture.

[0082] The material to be cut is typically a brittle material. For example, the brittle material may be any of glass, ceramic, polymer, composites, metallic materials and metal-ceramic composites. While this invention has been particularly shown and described with reference to embodiments, 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.

[0083] Certain embodiments of the present invention include:

[0084] 1 . A cutting tool comprising: a tool shank having a longitudinal axis of rotation; and a tool head attached to one end of the tool shank, the tool head comprising first and second tiers extending around an outer surface of the tool head; wherein the tiers are axially displaced from each other along the longitudinal axis; wherein the first tier of the tool head comprises a plurality of axially extending first cutting surfaces, each first cutting surface having a first profile; and the second tier of the tool head comprises a plurality of axially extending second cutting surfaces, each second cutting surface having a second profile; wherein the first profile differs from the second profile.

[0085] 2. The cutting tool of embodiment 1 , wherein, when the first profile and the second profile are superimposed, the first profile nests within the second profile.

[0086] 3. The cutting tool of embodiment 1 or claim 2, wherein the first profile and the second profile comprise cut-out portions, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the cut-out portion of the first profile has a smaller area than the cut-out portion of the second profile.

[0087] 4. The cutting tool of any one of the preceding embodiments, wherein the first tier is closer to the tool shank than the second tier.

[0088] 5. The cutting tool of any one of the preceding embodiments, wherein the axial extension of the first cutting surface is inclined from the longitudinal axis by up to 60 degrees and / or wherein the axial extension of the second cutting surface is inclined from the longitudinal axis by up to 60 degrees.

[0089] 6. The cutting tool of any one of the preceding embodiments, wherein the tiers are separated by a non-cutting portion of the tool head.

[0090] 7. The cutting tool of any one of embodiments 1 to 6, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the first profile comprises a cut-out portion with a substantially planar base, and the second profile comprises a cut-out portion with a substantially planar base, wherein the depth of the cut-out portion of the first profile is less than that of the cut-out portion of the second profile.

[0091] 8. The cutting tool of embodiment 7, wherein, when viewed in longitudinal cross-section, the cut-out portion of the first and / or second profile further comprises angled sides inclined from the outer periphery of the first and / or second cutting surfaces and towards the substantially planar base.

[0092] 9. The cutting tool of embodiment 8, wherein the angle of inclination of the first profile is less than that of the second profile.

[0093] 10. The cutting tool of embodiment 8 or embodiment 9, wherein the angled sides are symmetrical when viewed in longitudinal cross-section.

[0094] 11. The cutting tool of embodiment 8 or embodiment 9, wherein the angled sides are asymmetrical when viewed in longitudinal cross-section.

[0095] 12. The cutting tool of any one of embodiments 1 to 6, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the first profile comprises a first curved portion with a radius of curvature, and the second profile comprises a second curved portion with a larger radius of curvature than that of the first curved portion.

[0096] 13. The cutting tool of any one of embodiments 1 to 6, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the first profile and / or the second cut-out portions comprise cut-out portions, wherein the cut-out portion has a triangular, rectangular, square, semi-circular, pentagonal, hexagonal, heptagonal or octagonal profile.

[0097] 14. The cutting tool of embodiment 13, wherein the cut-out portion has a triangular profile selected from an equilateral triangle, an isosceles triangle, and a scalene triangle.

[0098] 15. The cutting tool of any one of the preceding embodiments, wherein the tool head further comprises a third tier, wherein the third tier of the tool head comprises a plurality of axially extending third cutting surfaces having a third profile which differs from the first profile and the second profile.

[0099] 16. The cutting tool of embodiment 15, wherein, when the first, second and third profiles are superimposed, the first profile nests within the second profile, and the second profile nests within the third profile.

[0100] 17. The cutting tool of embodiment 15 or embodiment 16, wherein the third tier is further from the tool shank than the first tier and the second tier.

[0101] 18. The cutting tool of any one of embodiments 15 to 17, wherein, when the third cutting surface is viewed in longitudinal cross-section, the third profile comprises a cut-out portion with a substantially planar base and angled sides inclined from the outer periphery of the third cutting surface and towards the substantially planar base.

[0102] 19. The cutting tool of any one of the preceding embodiments, wherein the cutting tool is a micro end mill cutter.

[0103] 20. The cutting tool of any one of the preceding embodiments, wherein the first, second and / or third cutting surfaces comprise superhard material. 21 . A method of cutting a workpiece comprising: providing a cutting tool comprising a tool shank having a longitudinal axis of rotation; and a tool head attached to one end of the tool shank, the tool head comprising first and second tiers extending around an outer surface of the tool head; wherein the tiers are axially displaced from each other along the longitudinal axis; wherein the first tier of the tool head comprises a plurality of axially extending first cutting surfaces, each first cutting surface having a first profile; and the second tier of the tool head comprises a plurality of axially extending second cutting surfaces, each second cutting surface having a second profile; wherein the first profile differs from the second profile; contacting the workpiece with the plurality of first cutting surfaces to form a first cut workpiece surface; and contacting the first cut workpiece surface with the plurality of second cutting surfaces to form a second cut workpiece surface.

[0104] 22. The method of embodiment 21 , wherein the tool head further comprises a third tier, wherein the third tier of the tool head comprises a plurality of axially extending third cutting surfaces having a third profile which differs from the first profile and the second profile; and wherein the method further comprises contacting the second cut workpiece surface with the plurality of third cutting surfaces to form a third cut workpiece surface.

[0105] 23. The method of embodiment 21 or embodiment 22, wherein the workpiece is contacted by rotating the cutting tool about its longitudinal axis of rotation and bringing the workpiece into contact with, sequentially, the plurality of first cutting surfaces, followed by the plurality of second cutting surfaces, and, when present, followed by the plurality of third cutting surfaces by axial displacement of the cutting tool and / or the workpiece.

[0106] 24. Use of the cutting tool of any one of embodiments 1 to 23 in a method of cutting a brittle material.

[0107] 25. Use of the cutting tool of embodiment 24, wherein the brittle material is any of glass, ceramic, polymer, composites, metallic materials and metal-ceramic composites.

Claims

CLAIMS1 . A milling cutter comprising: a tool shank having a longitudinal axis of rotation; and a tool head attached to one end of the tool shank, the tool head comprising first, second and third tiers extending around an outer surface of the tool head; wherein the tiers are axially displaced from each other along the longitudinal axis; wherein the first tier of the tool head comprises a plurality of axially extending first cutting surfaces, each first cutting surface having a first profile; the second tier of the tool head comprises a plurality of axially extending second cutting surfaces, each second cutting surface having a second profile; and the third tier of the tool head comprises a plurality of axially extending third cutting surfaces, each third cutting surface having a third profile; wherein the first tier is closer to the tool shank than the second tier; and wherein the third tier is further from the tool shank than the first tier and the second tier; wherein, when the first, second and third profiles are superimposed, the first profile nests within the second profile, and the second profile nests within the third profile; wherein the first profile differs from the second profile, and wherein the third profile differs from the first profile and the second profile; and wherein the first, second and third cutting surfaces comprise superhard material.

2. The milling cutter of claim 1 , wherein the first profile and the second profile comprise cut-out portions, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the cut-out portion of the first profile has a smaller area than the cut-out portion of the second profile.

3. The milling cutter of claim 1 or claim 2, wherein the axial extension of the first cutting surface is inclined from the longitudinal axis by up to 60 degrees and / or wherein the axial extension of the second cutting surface is inclined from the longitudinal axis by up to 60 degrees.

4. The milling cutter of any one of the preceding claims, wherein the tiers are separated by a non-cutting portion of the tool head.

5. The milling cutter of any one of claims 1 to 4, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the first profile comprises a cut-out portion with a substantially planar base, and the second profile comprises a cut-outportion with a substantially planar base, wherein the depth of the cut-out portion of the first profile is less than that of the cut-out portion of the second profile.

6. The milling cutter of claim 5, wherein, when viewed in longitudinal cross-section, the cut-out portion of the first and / or second profile further comprises angled sides inclined from the outer periphery of the first and / or second cutting surfaces and towards the substantially planar base.

7. The milling cutter of claim 6, wherein the angle of inclination of the first profile is less than that of the second profile.

8. The milling cutter of claim 6 or claim 7, wherein the angled sides are symmetrical when viewed in longitudinal cross-section.

9. The milling cutter of claim 6 or claim 7, wherein the angled sides are asymmetrical when viewed in longitudinal cross-section.

10. The milling cutter of any one of claims 1 to 4, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the first profile comprises a first curved portion with a radius of curvature, and the second profile comprises a second curved portion with a larger radius of curvature than that of the first curved portion.11 . The milling cutter of any one of claims 1 to 4, wherein, when the first and second cutting surfaces are viewed in longitudinal cross-section, the first profile and / or the second cut-out portions comprise cut-out portions, wherein the cut-out portion has a triangular, rectangular, square, semi-circular, pentagonal, hexagonal, heptagonal or octagonal profile.

12. The milling cutter of claim 11 , wherein the cut-out portion has a triangular profile selected from an equilateral triangle, an isosceles triangle, and a scalene triangle.

13. The milling cutter of any one of the preceding claims, wherein, when the third cutting surface is viewed in longitudinal cross-section, the third profile comprises a cut-out portion with a substantially planar base and angled sides inclined from the outer periphery of the third cutting surface and towards the substantially planar base.

14. The milling cutter of any one of the preceding claims, wherein the milling cutter is a micro end mill cutter.

15. The milling cutter of claim 14, wherein the micro end mill cutter has an outer diameter of no more than 15 mm.

16. The milling cutter of any one of the preceding claims, wherein the superhard material is monolithic polycrystalline diamond.

17. The milling cutter of any one of the preceding claims, wherein the superhard material comprises any of high-pressure high-temperature polycrystalline diamond, chemical vapour deposition diamond, and polycrystalline cubic boron nitride.

18. The milling cutter of any one of the preceding claims, wherein the height of each of the tiers is up to and including 1 mm.

19. The milling cutter of any one of the preceding claims, wherein the height of each of the tiers is at least 0.2 mm.

20. The milling cutter of any one of the preceding claims, wherein the height of each of the tiers is from 0.2 mm to 1 mm.

21. The milling cutter of any one of the preceding claims, where the number of cutting surfaces in each tier is from 5 to 100.

22. The milling cutter of any one of the preceding claims, wherein each tier has the same number of cutting surfaces.

23. The milling cutter of any one of the preceding claims, wherein the cutting surfaces in each tier are connected to one another via a non-cutting portion so as to form continuous, axially extending channels which run across all of the tiers in the tool head.

24. The milling cutter of any one of the preceding claims, wherein the difference in the profiles is in the sizes of the profiles.

25. A method of cutting a workpiece comprising: providing a milling cutter as defined in any one of claims 1 to 24; contacting the workpiece with the plurality of first cutting surfaces to form a first cut workpiece surface; contacting the first cut workpiece surface with the plurality of second cutting surfaces to form a second cut workpiece surface; and contacting the second cut workpiece surface with the plurality of third cutting surfaces to form a third cut workpiece surface.

26. The method of claim 25, wherein the workpiece is contacted by rotating the cutting tool about its longitudinal axis of rotation and bringing the workpiece into contact with, sequentially, the plurality of first cutting surfaces, followed by the plurality of second cutting surfaces, followed by the plurality of third cutting surfaces by axial displacement of the cutting tool and / or the workpiece.

27. Use of the milling cutter of any one of claims 1 to 24 in a method of cutting a brittle material.

28. The use of claim 27, wherein the brittle material is any of glass, ceramic, polymer, composites, metallic materials and metal-ceramic composites.

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