Method

Abstract

In a method of milling a workpiece using a milling tool 10, the workpiece comprises glass. The tool includes a tool shank 12 having an axis of rotation (14, Figure 2) and a tool head 16 comprising su

Description

FIELD OF THE INVENTION This disclosure relates to a method of milling a glass workpiece using a milling tool. In particular, it relates to a milling method that uses an end milling tool comprising polycrystalline diamond (PCD). BACKGROUND Milling is a cutting process whereby a tool with multiple cutting surfaces is rotated to remove material from the surface of a work piece. Such tools, also known as cutters, come in all shapes and sizes, depending on the design of the workpiece. The tool has an elongate shank or handle, adjacent to a tool head which has the profded cutting surfaces. The shank is mounted in a milling tool holder that is then mounted in the tool spindle of the machine and rotated. End milling cutters are the most common form of milling cutter and they are available in a wide variety of heights, diameters and types. End milling cutters are used for machining the faces and sides of a workpiece. During a typical milling operation, the cutter moves perpendicularly to its axis of rotation, allowing it to remove material from the workpiece at the perimeter of the cutter. End milling cutters are used for slotting, profding, contouring, counterboring and reaming. The spiral-shaped cutting edges on the side of the end milling cutter are known as ‘flutes’ and they provide an empty path for the cutting chips to escape from when the end milling cutter is rotating in a workpiece. End milling cutters are commonly made out of high-speed steel (i.e. cobalt steel alloys) or from tungsten carbide in a cobalt lattice. Carbide is considerably harder, more rigid, and more wear resistant than high-speed steel. However, carbide is brittle and tends to chip instead of wear. The choice of material depends on the material to be cut as well as on the maximum spindle speed of the machine. For workpieces made of harder materials, diamond electroplated tool heads are often used. In electroplated cutters, hundreds of individual diamond grits are embedded into a bonding agent on the surface of the tool head to provide numerous cutting surfaces and edges. However, a problem with electroplated milling tools is that the diamond grits are prone to pull-outs from the bonding agent, rendering the workpiece vulnerable to unwanted scratches from the rogue grits. Another problem is that diamond electroplated tools have a limited tool life, necessitating regular tooling changes and increasing the cost of production with every tool required. In micro end milling cutters, the outer diameter of the tool head is usually no more than 15 mm, and is typically in the range of 6 to 10 mm. Micro end milling cutters are deployed in milling operations during the construction of, for example, mobile phone handset shells. Handset shells are typically made from aluminium, polycarbonate or ceramic. One of the incumbent technologies is diamond electroplated micro end milling cutters. A common problem associated with the use of diamond electroplated tools is that they can cause sub-surface damage to the handset shell (or other workpiece), which causes it to weaken and increases the risk of cracking in use. Furthermore, it is often found that the surface roughness of milled workpieces is not sufficiently small for the desired applications, and therefore an additional processing step of polishing is typically required after milling. It is an object of the invention to provide a method of milling a workpiece comprising glass which provides a sufficiently smooth finish such that subsequent polishing processes are not required after milling. SUMMARY OF THE INVENTION In accordance with the invention, there is provided a method of milling a workpiece using a milling tool, the workpiece comprising glass, the milling tool comprising a tool shank having an axis of rotation, and further comprising a tool head comprising superhard material at one end thereof, and operating the milling tool such that the Undeformed Chip Thickness, hm, of the workpiece is less than 0.05 pm. As an option, the Undeformed Chip Thickness, hm is less than or equal to 0.045 pm, 0.040 pm, 0.035 pm, 0.030 pm, 0.025 pm, 0.020 pm, 0.015 pm, 0.012 pm, 0.010 pm, 0.005 pm or 0.001 pm. As an option, the Undeformed Chip Thickness, hm is greater than or equal to 0.020 pm, 0.015 pm, 0.012 pm, 0.010 pm, 0.005 pm or 0.001 pm. As an option, the Undeformed Chip Thickness, hm is less than or equal to 0.025 pm. 5 As an option, the Undeformed Chip Thickness, hm, is less than or equal to 0.012 pm. As an option, the Undeformed Chip Thickness, hm, is greater than or equal to 0.001 pm. 10 As an option, the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.025 pm. As an option, the Undeformed Chip Thickness, hm, is in the range of 0.005 pm to 0.025 pm. As an option, the Undeformed Chip Thickness, hm, is in the range of 0.010 pm to 0.025 pm. 15 As an option, the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.015 pm. As an option, the Undeformed Chip Thickness, hm, is in the range of 0.005 pm to 0.015 pm. 20 As an option, the Undeformed Chip Thickness, hm, is in the range of 0.010 pm to 0.015 pm. As an option, the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.012 pm. As an option, the Undeformed Chip Thickness, hm, is in the range of 0.005 pm to 0.012 pm. 25 As an option, the Undeformed Chip Thickness, hm, is in the range of 0.010 pm to 0.012 pm. As an option, the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.049 pm. 30 As an option, operating the milling tool comprises controlling any one or more of the following: the depth of cut, the table feed, the spindle speed. As an option, the depth of cut is in the range of 5 to 100 pm. As an option, the table feed is 20 to 1500 mm / min. As an option, the spindle speed is in the range of 1000 to 30000 rpm. As an option, the milling tool has an outer diameter in the range of 1 to 15 mm, for example 2 to 15 mm, for example 3 to 15 mm, for example 4 to 15 mm, for example 4 to 10 mm, for example 6 to 8 mm. The outer diameter of the milling tool may be at least 1, 2, 3, 4, 5 or 6 mm. The outer diameter of the milling tool may be at most 8, 9, 10, 11, 12, 13, 14 or 15 mm. As an option, the surface roughness, Sa, of the workpiece after milling is less than or equal to 300 nm. As an option, the surface roughness, Sa, of the workpiece after milling is less than or equal to 100 nm. The surface roughness, Sa, of the workpiece is measured according to the method detailed in ISO 25178-2:2021 As an option, the tool head comprises a plurality of flutes arranged in a peripheral surface thereof. As an option, the quantity of flutes on the milling tool is in the range of 1 to 200. As an option, the milling tool is an end milling tool. As an option, the superhard material comprises any of high-pressure high-temperature polycrystalline diamond, chemical vapour deposition diamond, and polycrystalline cubic boron nitride. As an option, the superhard material comprises polycrystalline chemical vapour deposition diamond coated on a cemented carbide substrate. As an option, the superhard material is monolithic polycrystalline diamond. As an option, the superhard material is poly crystalline diamond adjoining a carbide backing portion. As an option, the tool head comprises at least two tiers, and the tiers are axially displaced from each other and separated by a non-cutting portion of the tool head. As an option, the method further comprises not polishing the milled workpiece. BRIEF DESCIPTION OF THE DRAWINGS The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a tool for use in accordance with the invention, with a first example of a tool head; Figure 2 is a front view of the tool of Figure 1; Figure 3 is an enlarged view of portion X from Figure 2; Figure 4 is a front view of a second example of a tool head; Figure 5 is a front view of a third example of a tool head; Figure 6 is a front view of a fourth example of a tool head; Figure 7 is a front view of a fifth example of a tool head; Figure 8 is a schematic view of a variation of the tool head shown in Figure 1; Figure 9 is an annotated version of the tool head of Figures 5 and / or 6; Figure 10 is another annotated version of the tool head of Figures 5 and / or 6; Figure 11 is a schematic indicating the lateral cross-section of the flutes in the tool head; Figure 12 is a schematic indicating the cutting action of the flutes during use; Figure 13 is a schematic used to define the Undeformed Chip Thickness (hm) during milling and also the Ductile-Brittle Transition Undeformed Chip Thickness (DBhm) of the workpiece material; Figure 14 is a schematic depiction of the test setup used in Example 1 and a sketch of the milling operation process; Figure 15 is a plot of surface roughness (Sa) vs undeformed chip thickness (hm); Figure 16 is SEM images showing surface finish under four different conditions of hm; Figure 17 is a table showing surface roughness Sa for a PCD tool as described herein and a prior art EP tool; Figure 18 is a plot of surface roughness (Sa) vs number of parts milled for a PCD tool as described herein; and Figure 19 is a table summarising the cost per part for both electroplated (EP) and PCD tools. Throughout the embodiments, similar parts are denoted by the same reference numeral and a further description is omitted for brevity. DETAILED DESCRIPTION The following description refers to a tool head comprising a superhard material. In the examples, polycrystalline diamond (PCD) is referred to, but this is by way of example only. For milling ferrous materials, polycrystalline cubic boron nitride is preferred. Furthermore, while PCD may be used, other forms of synthetic diamond may be used, such as chemical vapour deposition (CVD) diamond. Referring firstly to Figures 1 to 3, a tool for milling glass 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. The tool head 16 comprises at least one tier 18 (i.e. a stage or a level), the or each tier comprising a plurality of flutes 20 extending circumferentially around the tool head 16. In any one tier 18, all the flutes are in a band, i.e. they are in axial alignment with each other. 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. The tool head 16 in this example comprises poly crystalline diamond (PCD). Figure 3 shows a first example of a tool head 16. Tool head 16 comprises three tiers 18a, 18b, 18c and a notch element 22. 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 flutes. The flutes 20 are provided in an outer surface of the tool head 16. The flutes 20 extend around the entire circumference of the tool head 16. The flutes 20 are created in the outer surface using a laser which initially ablates unwanted material, thereby creating recesses between precursor flutes 20, and subsequently shapes the precursor flutes according to a desired profile into a final flute 20 configuration. More detail on the flutes 20 is provided later. Each tier may be separated from an adjacent tier by a non-cutting portion 17 of the tool head 16. The notch element 22 is configured to carve a correspondingly shaped notch into a workpiece, for example a microphone aperture in a mobile phone handset shell. As an example only, the notch element 22 may have a diameter of up to 1 mm and a height of up to 1 mm. The notch element 22 is entirely optional and may be omitted. In Figure 4, a second example of a tool head 24 is shown. In this example, a single tier 18a is provided. Turning now to Figure 5, a further example of a tool head 26 is shown. In this example, three tiers 18a, 18b, 18c are again provided. Each of the three tiers 18a, 18b and 18c is configured for finishing operations. However, the three tiers may all be configured for roughing, or alternatively they may all be configured for semi-finishing. The advantage of the configuration where all tiers are configured for the same milling operation is that it extends the service life of the tool by a factor of ‘n’ where ‘n’ is the quantity of tiers. As the first tier, whichever one it might be that is used first, wears out, then the spindle can be extended or retracted as appropriate, to move one of the other tiers into position. This is repeated as and when required, depending on the quantity of tiers 18 provided. Since the wear rate is the same for all three tiers, the operational life of the tool is maximised. In Figure 6, a further example of a tool head 28 is shown. In this embodiment, three tiers 18a, 18b, 18c are again provided. The first and second tiers 18a, 18b respectively, are configured for semi-finishing milling operations. Only the third tier 18c is configured for finishing milling operations. One of the advantages of this configuration is that, unlike the example given in Figure 5, it does not require the additional tool change between milling operations. The tool is multi-functional and can be used for more than one specific milling operation, thereby reducing machine downtime and maximising operational equipment effectiveness. A tool configured for more than one type of milling operation may be considered to be a ‘multi-tool’. Since a finishing operation produces half as much wear as a semi-finishing process, a tier configured for finishing will have a life that is approximately twice as long as a tier configured for semi-finishing. Having twice as many tiers for semi-finishing milling operations as tiers for finishing operations is therefore an optimum proportion. As an example, for a tool with six tiers in total, four of those tiers would be for semi-finishing and two of those tiers would be for finishing. To continue the example, a tool with twelve tiers in total, eight of those tiers would be for semi-finishing and four of those tiers would be for finishing. In another example, not shown, the tiers 18 may all be configured exclusively for roughing operations. Since a tier configured for roughing produces yet more wear than a tier configured for semi-finishing, the proportion of tiers configured for roughing will be at least double the quantity of tiers configured for semi-finishing, typically three to four times. For example, a single tool configured for all three milling operations may have nine tiers in total, may have six tiers for roughing, two tiers for semi-finishing, and one tier for finishing. Turning now to Figure 7, another example of a tool head 30 is shown. In this example, two tiers 18a and 18b are provided, each separated from the adjacent tiers by a non-cutting portion, and the tool head is provided with a notching element 22. The tool shank 12 comprises cemented metal carbide, for example tungsten carbide, although other suitable materials are envisaged. Optionally, the tool shank 12 comprises a conduit (not shown) for carrying compressed air to the tool head to eject waste milling media from the flutes. The tool head 16 is cylindrical and non-tubular. The tool head 16 in one example 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. In the examples shown above, a PCD portion 32 is sinter-joined to a carbide backing layer 34, though this need not be the case and the carbide backing layer 34 may be omitted. The tiers 18 are provided in the PCD portion 32 of the tool head, and not in the carbide backing layer 34. The carbide backing layer 34 facilitates attachment to the tool shank 12, which can be achieved using any reasonable means. Referring to Figure 8, it is not essential that the non-cutting portion 17 is continuous. Instead, as shown in Figure 8, the non-cutting portion 17 can be interrupted such that continuous channels running throughout the tiers are formed. Referring to Figure 9, an overall height of the tool head 16 is indicated at 36, and it is the sum of the height 38 of the PCD portion 32 and the height 40 of the carbide portion 34 if a carbide backing layer 34 is included (otherwise, it is only the height 38 of the PCD portion 32). Optionally, the height 36 of the tool head 16 is 0.5 mm to 12 mm. Optionally, the height 36 of the tool head 16 is 1 to 10 mm. Optionally, the height 36 of the tool head 16 is 6 mm. The height 38 of the PCD portion 32 may be in the range of 0.5 to 6 mm, for example 2.5 mm. It is envisaged that the height of the tool head may be in the order of nanometres (i.e. <100 nm), for example an overall height of 50 to 95 nm, or smaller. Optionally, the height 36 of the tool head 16 is no more than 12 mm. The outer diameter of the tool 10 is indicated at 42 and is the largest, outermost, diameter of any of the tiers 18 and the shank 12. Individual tiers 18 may have different diameters to each other, depending, for example on which milling operation they are configured for. Optionally, all tiers 18 have the same diameter. Preferably, the tool 10, 24, 26, 28, 30 is a micro end milling tool which has an outer diameter of no more than 15 mm. Optionally, the outer diameter 42 of the tool is 10 mm. In one example of a micro end milling tool, the overall height of the tool, including tool shank 12 and tool head 16 may be around 200 mm. The outer diameter of the milling tool may be in the range of 1 to 15 mm, for example 2 to 15 mm, for example 3 to 15 mm, for example 4 to 15 mm, for example 4 to 10 mm, for example 6 to 8 mm. The outer diameter of the milling tool may be at least 1, 2, 3, 4, 5 or 6 mm. The outer diameter of the milling tool may be at most 8, 9, 10, 11, 12, 13, 14 or 15 mm. The height 44 of each tier 18 (measured axially, the same as the previous height measurements) depends on the quantity of tiers 18 and the height 38 of the PCD, regardless of whether it is backed or unbacked with carbide backing layer 34. As an example, for a tool head 16 comprising a PCD portion 32 backed with a carbide layer 34 which has a tool head 36 height of 6 mm, the height 38 of the PCD portion is 2.5 mm, and for three tiers, the height 44 of each tier is 0.6 to 0.7 mm. Referring to Figures 10, 11 and 12, each flute 20 has a triangular lateral cross-section. Various flute parameters influence certain factors. The helix angle, a and the flute depth, d affect the amount of clogging with waste debris that occurs between flutes during milling, and therefore the cleaning of the tool head 16. The helix angle, a, also affects tool stability. The flute angle P, rake (cutting) angle 0, and the quantity of flutes, N, have a direct effect on the surface finish, subsurface damage, tool performance (cutting forces) and tool life. Figure 12 indicates schematically how each flute may cut the workpiece 46 as the tool advances laterally in the direction of the arrow during use. The aforementioned parameters, helix angle, a, flute angle P, rake (cutting) angle 0, quantity of flutes, N and flute depth, d, within the or each tier are optimised depending on whether the aim of the milling operation is for roughing, semi-finishing or finishing in the context of milling glass or other similar brittle material. A roughing milling operation is generally intended to prepare the surface of the workpiece before the finishing operation. The purpose is to bring the dimension to a “rough” size of the final dimension. How this looks may be of little importance since the main aim is to clear away relatively large amounts of material quickly. Roughing will likely require a greater flute angle P than the other operations in order to provide a more substantial flute body to deal with the higher forces. This will reduce the quantity of flutes that can be fitted into a finite space, and therefore the quantity of flutes in a tier. A semi-finishing milling operation is typically the next stage after roughing. The purpose is to achieve a dimension even closer to the final dimension. A finishing milling operation is the final stage of machining a workpiece. A minimal quantity of workpiece material is removed, the workpiece is machined to size, the final dimension is obtained and sometimes the surface is further refined too. The quantity of flutes on the milling tool may be in the range of 1 to 200, for example 2 to 200, for example 3 to 200, for example 4 to 200, for example 5 to 200, for example 6 to 200, for example 7 to 200, for example 8 to 200, for example 9 to 200, for example 10 to 200, for example 11 to 200, for example 12 to 200, for example 13 to 200, for example 14 to 200, for example 15 to 200, for example 16 to 200. The quantity of flutes may be at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16. The quantity of flutes may be at most 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200. 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 flutes is then formed in the precursor tool head using, e.g. laser ablation machining. The flutes are arranged in axially adjacent tiers. This latter step is then repeated as often as required, thereby forming a tool head comprising at least one tier, wherein the or each tier comprises a plurality of flutes extending circumferentially around the tool head. 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 flutes is formed in the precursor tool head using a laser. This step is repeated as required, to form a tool head comprising at least two tiers, each tier comprising a plurality of flutes extending circumferentially around 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 flutes using chemical vapour deposition. Typically, hot fdament 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 flutes. As mentioned above, the quantity of flutes is one of the factors affecting the level of subsurface damage on the workpiece. Minimising subsurface damage is essential in the process of shaping mobile phone handset shells. However, tool design is not the only way of minimising such damage; in fact, it is a combination of both milling process conditions and tool design that have to be manipulated to achieve this goal. For adverse surface damage not to occur, brittle materials must be machined in the ductile mode. Figure 13 shows a simplified schematic of the milling process, and identifies the Undeformed Chip Thickness hm, which in milling is defined as the distance between two consecutive cut surfaces. Also illustrated is the Ductile-Brittle Transition Undeformed Chip Thickness (DBhm) which is proportional to the product of the radii of lateral cracks (Ci) and the length of the medial cracks (Cm). To ensure that cracks generated at the cutting zone do not transition further into the workpiece and beyond, the Undeformed Chip Thickness hm, must be kept below a specific value, that of the Ductile-Brittle Transition Undeformed Chip Thickness DBhm. Table 1 below provides examples of DBhm for different workpiece materials. Material Ductile-Brittle Transition Undeformed Chip Thickness (DBhm) Zirconia 0.15 - 0.25 pm Glass 0.10 - 0.30 pm Sapphire 0.05 - 0.15 pm Table 1 DBhm can be estimated using Equation 1 below, with the parameters defined in Table 2. E DBhm = a — H Kc\2 Equation 1 Parameter Description (unit) H Hardness (Pa) E Young’s Modulus (Pa) Kc Fracture Toughness (Mpa m1 / 2) A Dimensionless tool factor [0-1, where 0 = sharp cutting edge, 1 = rounded or chamfered cutting edge] Table 2 5 Achieving the required hm is primarily a function of the cutting parameters as defined by Equation 2 below, however, in practice, the tool design must also be incorporated into the whole application design. h =2 f jz Equation 2 10 fz is provided by Equation 3. nZ. Equation 3 Parameter Description (unit) fz Feed per tooth (mm) Vr Table feed (mm / min) N Spindle speed (RPM) Zc Quantity of teeth ae Depth of cut (mm) D Tool diameter (mm) 15 Table 3 Using Equations 2 and 3, it can be found that utilizing a tool with only 21 cutting edges would mean a spindle speed of 30,000 RPM is required. Few manufacturing orientated milling machines are capable of 30,000 RPM. Therefore, a higher quantity of cutting edges are needed for lower RPMs, e.g.; at -16,000 RPM, 40 edges are needed for the same hm imposed. The present inventors have found that when the milling of glass is performed using the above-mentioned tool at an hm value of less than 0.05 pm, a surface roughness Sa of less than 400 nm, even as low as less than 100 nm, can be obtained by milling alone. This means that the requirement for a subsequent polishing step can be eliminated, thereby simplifying the manufacturing process. This also represents a significant improvement compared to electroplated tools, with which it is very difficult to achieve a surface roughness Sa of less than 400 nm. Operating the milling tool, for example the end milling tool, may comprise controlling any one or more of the following: the depth of cut, the table feed, and the spindle speed. The depth of cut may be in the range of 5 to 100 pm, for example 10 to 90 pm, for example 10 to 80 pm, for example 10 to 70 pm, for example 10 to 60 pm, for example 10 to 50 pm, for example 10 to 40 pm, for example 10 to 30 pm, for example 15 to 20 pm. The depth of cut may be at least 5, 10 or 15 pm. The depth of cut may be at most 20, 30, 40, 50, 60, 70, 80, 90 or 100 pm. The table feed may be in the range of 20 to 1500 mm / min, for example 50 to 1500 mm / min, for example 60 to 1500 mm / min, for example 70 to 1500 mm / min, for example 100 to 1500 mm / min, for example 200 to 1500 mm / min, for example 300 to 1500 mm / min, for example 400 to 1500 mm / min, for example 500 to 1500 mm / min, for example 600 to 1500 mm / min, for example 700 to 1500 mm / min, for example 800 to 1500 mm / min, for example 900 to 1500 mm / min, for example 1000 to 1500 mm / min, for example 1000 to 1400 mm / min. The table feed may be at least 20, 50, 60, 70, 100, 200, 300, 400, 500, 600, 700, 800, 900 or 1000 mm / min. The table feed may be at most 1400 or 1500 mm / min. The spindle speed may be in the range of 1000 to 30000 rpm, for example 2000 to 30000 rpm, for example 3000 to 30000 rpm, for example 4000 to 30000 rpm, for example 5000 to 30000 rpm, for example 6000 to 30000 rpm, for example 7000 to 30000 rpm, for example 8000 to 30000 rpm, for example 9000 to 30000 rpm, for example 10000 to 30000 rpm, for example 11000 to 30000 rpm, for example 12000 to 30000 rpm, for example 13000 to 30000 rpm, for example 14000 to 30000 rpm, for example 15000 to 30000 rpm, for example 15000 to 29000 rpm, for example 15000 to 28000 rpm, for example 15000 to 27000 rpm, for example 15000 to 26000 rpm, for example 15000 to 25000 rpm, for example 15000 to 24000 rpm, for example 15000 to 23000 rpm, for example 15000 to 22000 rpm, for example 15000 to 21000 rpm, for example 15000 to 20000 rpm, for example 15000 rpm to 19000 rpm, for example 15000 rpm to 18000 rpm, for example 15000 rpm to 17000 rpm. The spindle speed may be at least 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000 or 15000 rpm. The spindle speed may be at most 17000, 18000, 19000, 20000, 21000, 22000, 23000, 24000, 25000, 26000, 27000, 28000, 29000 or 30000 rpm. Advantageously, the present invention provides a method of milling a workpiece using a milling tool in which the need for a subsequent polishing step is eliminated as the required surface roughness is obtained by milling alone. The surface roughness Sa of the workpiece after milling may be less than or equal to 400 nm, for example less than or equal to 300 nm, for example less than or equal to 200 nm, for example less than or equal to 100 nm. The surface roughness Sa referred to herein is measured by an areal roughness measurement. The surface roughness Sa may be measured using a Digital 3-D focus-variation microscope, for example an InfmiteFocus G6 instrument manufactured by Bruker Alicona. The surface roughness Sa of the workpiece may be measured according to the method detailed in ISO 25178-2:2021. Example 1 Ductile-brittle transition assessment Initial tests were conducted with a commercially available 3-axis machining center (HURCO VM20i) to estimate the DBhm for Gorilla® glass in a peripheral milling operation using Element Six’s CTM302 PCD micro-milling tool. Based on the performance and productivity requirements, the PCD micro-milling tool was designed with 3 tiers and 50 flutes as described above. Figure 14 shows the experimental set up. Workpiece 46 is brought into contact with one of the tiers of the tool head 16. During the milling process, the tool rotates around its longitudinal axis and is moved around the periphery of the workpiece to provide a chamfered edge to the workpiece as shown in Figure 14. The Gorilla® glass surface roughness (Sa) analysis was acquired during the experiments using an Alicona COBOT optical roughness measurement system. In Figure 15, a plot of Sa versus hm is provided for a peripheral milling operation machining conducted with the micro-milling PCD tool (“D-B” in the Figure stands for “Ductile-Brittle”). The diagram reveals that Sa increases with hm, and the ductile regime is achieved for hm <0.025 pm, as indicated in the SEM images (Figure 16). It is worthwhile to highlight that the Sa value obtained for hm = 0.012 pm delivers a similar quality as a polished surface obtained during the post-machining polishing step (Sa <100 nm), suggesting that micro-milling tools with appropriate design and operating conditions can eliminate this requirement. Production testing in China The lessons learned from the internal testing were used to refine and improve the PCD tool used in a production environment in China. Considering the optimal values for hm determined at the E6 Global Innovation Center, the PCD micro-milling tool also showed an exceptional surface finish quality (Sa< 90 nm), which is at least 4 times superior to the surface machined with a conventional diamond electroplated (EP) tool (see Figure 17). Tool lifetime The tool life of the PCD micro-milling compared to the conventional EP tool was also investigated. The operating conditions used for the PCD micro-milling tool were: table feed = 70 mm / min, spindle speed = 24,000 RPM, Depth of cut (ae) = 0.01 mm (i.e., hm = 5 nm). The typical EP tool life is 150 pieces, considering a limit value for Sa ~ 400 nm. Using the same life criteria (Sa « 400 nm), the life of the PCD tool is « 3500 pieces / tier, or >10,000 for all 3 tiers, i.e., « 70 times more than the conventional diamond EP tool. Tooling Economics Figure 19 summarizes some of the economic aspects of the two different milling tool types. Note that the tool cost per workpiece is slightly lower for the PCD tool. The EP tools have an additional disadvantage of requiring frequent tool changes and thus decreasing productivity (i.e. OEE) with tool-change time. The PCD micro-milling tools have the potential significant additional advantage of eliminating the polishing step, thereby substantially lowering the overall manufacturing cost per part. The results show that defined cutting edge PCD tools (as detailed herein) perform more effectively than EP tools. With careful tool design and control of machining operating parameters, PCD tools perform more effectively than EP tools. With careful tool design and control of machining operating parameters, PCD tools can achieve surface roughness Sa <100 nm, demonstrating a 4-fold improvement compared to EP tools. Using a 3-tier tool design, PCD micro-milling tools can obtain lifetimes of 70 times longer than EP tools, resulting in a lower tool cost per part. In addition, defined PCD tools are able to replace the subsequent polishing process required to eliminate the sub-surface damage caused by the EP tools. In summary, the above-described results of the tests using PCD tools in peripheral milling operations for smartphone glass showed enhanced durability, extended tool life, and improved finish, compared to conventional EP tools. Customized tool design and operating conditions eliminate the need for polishing processes, typically linked to sub-surface damage caused by the tool. While the method has been shown and described with reference to various examples, 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.

Claims

1. A method of milling a workpiece using a milling tool,- the workpiece comprising glass,- the milling tool comprising a tool shank having an axis of rotation, and further comprising a tool head comprising superhard material at one end thereof, and- operating the milling tool such that the Undeformed Chip Thickness, hm, of the workpiece is less than 0.05 pm.

2. The method according to claim 1, wherein the Undeformed Chip Thickness, hm is less than or equal to 0.025 pm.

3. The method according to claim 1 or claim 2, wherein the Undeformed Chip Thickness, hm is less than or equal to 0.012 pm.

4. The method according to claim 1, wherein the Undeformed Chip Thickness, hm is greater than or equal to 0.001 pm.

5. The method according to claim 1, 2 or 4, wherein the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.025 pm.

6. The method according to claim 4 or claim 5, wherein the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.015 pm.

7. The method according to any one of the preceding claims, wherein the Undeformed Chip Thickness, hm, is in the range of 0.001 pm to 0.012 pm.

8. The method according to any one of the preceding claims, wherein operating the milling tool comprises controlling any one or more of the following: the depth of cut, the table feed, the spindle speed.

9. The method according to claim 8, wherein the depth of cut is in the range of 5 to 100 pm.

10. The method according to claim 8 or claim 9, wherein the table feed is 20 to 1500 mm / min.

11. The method according to claim 8, 9 or 10, wherein the spindle speed is in the range of 1000 to 30000 rpm.

12. The method according to any one of the preceding claims, wherein the milling tool has an outer diameter in the range of 1 to 15 mm.

13. The method according to any one of the preceding claims, wherein the surface roughness Sa of the workpiece after milling is less than or equal to 300 nm.

14. The method according to claim 13, wherein the surface roughness Sa of the workpiece after milling is less than or equal to 100 nm.

15. The method according to any one of the preceding claims, wherein the tool head comprises a plurality of flutes arranged in a peripheral surface thereof.

16. The method according to claim 14, wherein the quantity of flutes on the milling tool is in the range of 1 to 200.

17. The method according to any one of the preceding claims, wherein the milling tool is an end milling tool.

18. The method according to any one of the preceding claims, wherein the superhard material comprises any of high-pressure high-temperature polycrystalline diamond, chemical vapour deposition diamond, and poly crystalline cubic boron nitride.

19. The method of claim 18, wherein the superhard material comprises poly crystalline chemical vapour deposition diamond coated on a cemented carbide substrate.

20. The method of claim 18, wherein the superhard material is monolithic polycrystalline diamond.

21. The method of claim 18, wherein the superhard material is polycrystalline diamond adjoining a carbide backing portion.

22. The method according to any one of the preceding claims, wherein the tool head comprises 5 at least two tiers, and wherein the tiers are axially displaced from each other and separated by a non-cutting portion of the tool head.

23. The method according to any one of the preceding claims, wherein the method further comprises not polishing the milled workpiece.10T +44(0)30 0300 2000A

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