GB2501001A

GB2501001A - Drill with continuous cutting edges for composites

Info

Publication number: GB2501001A
Application number: GB1305675.9A
Authority: GB
Inventors: Warwick Spearing
Original assignee: Technicut Ltd
Current assignee: Technicut Ltd
Priority date: 2012-04-02
Filing date: 2013-03-28
Publication date: 2013-10-09
Anticipated expiration: 2033-03-28
Also published as: GB201205886D0; GB2501001B; GB201305675D0

Abstract

The drill 1 for producing holes in composite materials such as carbon fibre reinforced plastics includes an elongate drill body 2 having a shank portion 3 at one end and a tip portion 4 at another end. The tip portion has a maximum outside diameter D. End cutting edges (306, 307, 317, 318, Figure 3) each extend between a main central axis of the drill body and a circumferential periphery of the body. Circumferential cutting edges 11, 12 each extend along a length of the drill. Each end cutting edge forms a continuous cutting edge with a corresponding respective circumferential cutting edge. Each end cutting edge has a minimum radius of curvature R wherein R=D/12 to R=D/2. R may be in the range of 9 to 46 percent of D or 0.75mm to 3.2 mm. The tip may be of solid diamond or have a coating of polycrystalline diamond. The drill also includes flutes 5, 6 and end facets (300 to 304, 310, 311, Figure 3).

Description

DRILL FOR COMPOSITE MATERIALS

Field of the Invention

[0001] The present invention relates to drills, particularly but not exclusively for the drilling of materials such as carbon fibre composites (CEC5) and CFC/aluminium composites.

Background of the Invention

[0002] Diamond tipped drills, and specifically polycrystalline diamond tipped drills, are normally employed for the drilling of composite materials such as carbon fibre composites, or carbon fibre/aluminium composites. Such materials are used extensively in the aeronautical and aerospace industries.

[0003] Inherently, no two composites are the same. Their marbled internal texture is different from the uniformity of aluminum or steel, which can be expected to remain consistent from a drill's entry point to its exit.

[0004] Carbon fiber reinforced plastic (CFRF) comprises a matrix base material, reinforced with a mix of very strong carbon fibers. These fibers can be laid out either in a consistent direction or opposed weave pattern. Composite layers are stacked upon others to form composite blocks, boards and moulded shapes.

[0005] To add mass, foam or honeycombed core structures may be inserted between composite layers or attached to them. Layered and molded configurations are designed according to the specific need and use of the part and may be several centimeters in thickness. While core layers can strengthen a composite form, composite material strength is primarily due to its ratio of extreme strength to very light weight.

[0006] Typically in aerospace applications, holes must be drilled with high accuracy, since the hole size specification has a strict tolerance so as to avoid the risk of crack formation around drilled holes. Typically holes must be drilled within an accuracy of 50 pm.

[0007] It is known to drill carbon fibre composites using carbide cutting tools having a chemical vapour deposition (CVD) diamond or polycrystalline diamond (PCD) coating. A widely recognised problem in drilling such composites is the propensity for delamination of the work piece upon the breakout of a drill.

Delamination is a problem since delaminated surfaces around a hole increases the risk of crack formation, which in aerospace applications needs to be avoided for safety reasons.

[0008] With a view to mitigating delamination, various geometrical changes to drills have been proposed such as disclosed in US 7575401, which teaches the use of first and second cutting lips at various specified angles, resulting in what may be termed a circumferential high point around the junction between the first and second lips. Whilst drills of such geometry offer improved cutting of composites compared with drills of more conventional geometry, their life is relatively short, being capable of cutting for some 5 to 7 minutes.

[0009] Carbon fibre composite material may also be layered with aluminium layers, so that the resultant material has alternating layers of carbon fibre composite and aluminium. This means that a drill will encounter a non homogenous layer of Carbon fibre composite material, followed by an homogenous layer of aluminium, followed by another non homogenous layer of carbon fibre composite material.

[0010] An object of the invention is the provision of an improved diamond tipped, fluted drill for drilling composites which minimises delamination of carbon fibre composite materials.

Summary of the Invention

[0011] According to one aspect of the invention there is provided a drill for producing holes in composite materials, said drill comprising: an elongate drill body (2) having a shank portion (3) at one end, and a tip portion (4) at another end; said tip portion having a maximum outside diameter D; a plurality of end cutting edges (306, 317, 307, 318) each extending between a main central axis of the drill body and a circumferential periphery of the body; and a plurality circumferential cutting edges (11, 12) each extending along a length of said drill; characterised in that: each said end cutting edge forms a continuous cutting edge with a corresponding respective said circumferential cutting edge; and each said end cutting edge has a minimum radius of curvature R, wherein said minimum radius R is in the rangeR = D/12 to R = D12.

[0012] According to a second aspect there is provided a drill for producing holes in composite materials, said drill comprising: an elongate drill body having a shank portion (3) at one end, and a tip portion (4) at another end; said body having a main central axis; a plurality of end facets (300 -304, 310, 311) extending between said central axis and a circumferential periphery of the body; a plurality of circumferential facets (9, 10) extending along a length of said body; a plurality circumferential cutting edges (11, 12) each extending along a length of said body; a plurality of flutes (5, 6) separating said plurality of circumferential cutting facets; a plurality of end cutting edges (306, 307, 317, 318) each extending between said central axis and a corresponding respective said circumferential cutting edge; characterised in that each said end cutting edge forms a continuous cutting edge with a corresponding respective circumferential cutting edge; and each said end cutting edge joins a corresponding respective circumferential cutting edge with a minimum radius of curvature in the range 0.75mm to 3.2mm.

[0013] The invention includes a diamond tipped, fluted drill for the drilling of composites, comprising an elongated body member provided at one end, with a shank for location in a chuck or arbour, and at least one flute extending from the shank to a cutting tip with a centre point and multiple cutting facets, characterised in that a partial or full corner radius clearance is provided over a transition zone from the circumferential periphery of the body member to the cutting facets.

[0014] The provision of a stepless, curved transition between the circumference and the cutting facets may provide the required clearance to mitigate delamination, eliminates any high point, and as a result provides for longer drill life.

[0015] Other aspects are as set out in the claims herein.

Brief Description of the Drawings

[0016] For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 shows a drill bit according to a first specific embodiment in perspective view; Figure 2 herein shows one end of the drill bit of figure 1 in view from one side; Figure 3 herein shows the drill bit of figure 1 in end on view from the tip; Figure 4 herein shows in detail a point of the drill bit in view along a central axis direction, and showing the inner central portion of the tip; Figure 5 herein shows the tip of the drill in various views; Figure 6 herein shows the tip in a first side view, as seen in a direction perpendicular to a main length axis of the drill, showing the point angle C'; Figure 7 herein shows the tip in the first side view as seen in a direction perpendicular to a main length axis of the drill, and showing an outer radius of the tip; Figure 8 herein shows a detail of the tip as viewed in a direction perpendicular to a main central axis of the drill, showing a transition between an end cutting edge and a circumferential cutting edge; Figure 9 herein shows a detail of a variation embodiment of the tip as viewed in a direction perpendicular to a main central axis of the drill, showing a transition between an end cutting edge and a circumferential cutting edge; Figure 10 shows the drill tip in second side view, as seen in a direction perpendicular to a main length axis of the drill; Figure 11 shows a portion of the drill tip shown in the view of figure 10 herein; Figure 12 herein shows another view of the drill tip expanded from figure 5 herein; Figure 13 shows a further view of the drill tip, expanded from figure 5 herein, showing a notch angle and secondary cutting edge angle; and Figure 14 herein shows a plot of samples of variation in hole size tolerance for a drill according to the embodiments shown herein.

Detailed Description of the Embodiments

[0017] There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to

unnecessarily obscure the description.

[0018] In this specification, the term "leading" is used to describe an edge or face which leads, and/or encounters a work piece first in the intended direction of rotation of the drill, relative to a trailing edge or surface as appropriate.

[0019] The term "trailing" when used in relation to an edge or surface is used to denote a surface or edge which follows a leading edge or face in the intended direction of rotation of the drill.

[0020] All references to leading and following in this specification are intended to mean leading or following in the direction of intended rotation of the drill.

[0021] The specific embodiments herein disclose a geometry for a diamond drill for use in carbon fibre reinforced polymers (CFRP), carbon fibre reinforced plastics, carbon fibre composites (CFC) and carbon fibre composite with aluminium.

[0022] The embodiment shown in figures 1 to 11 herein is a four end facet drill bit of outside diameter 6.5mm, made of tungsten carbide, having a polycrystalline diamond coated tip.

[0023] Referring to figures 1 to 4 herein, the drill 1 comprises a drill body 2 of overall substantially cylindrical shape, having a shank portion 3 for holding the drill in a chuck of a drilling machine; a tip portion 4 at an opposite end of the body to the shank; extending between the shank and the tip, a plurality of helical flutes 5, 6; a plurality of lands 7, 8 between the flutes; and a plurality of circumferential helical outer surfaces 9, 10 each providing an outer facing surface to a corresponding respective land, the helical outer surfaces alternating between the plurality of helical flutes. Each helical outer surface has a corresponding respective leading cutting edge 11, 12.

[0024] Referring to figure 2 herein, there is shown the drill flutes and tip in view from one side in a direction perpendicular to a main central length axis of the drill. The tip comprises a central point 200 being the intersection of a plurality of curved end facets which meet at the main central axis of the drill. The tangents of each end facet extend in a direction transverse to the main central axis of the drill.

[0025] Referring to figure 3 herein, there is shown the first embodiment drill in view along a main central axis of the drill showing the tip. In use the embodiment shown rotates in a clockwise direction as viewed from behind the shank, which means that as viewed in figure 3, the direction of rotation is anticlockwise.

[0026] The tip comprises first and second leading facets 300, 301 and first and second trailing facets 302, 303, all of which meet substantially at a central point 305 which lies on a main central axis along the drill, which is also the axis about which the drill rotates in use. The leading facets each have a respective leading cutting edge 306, 307. The trailing facets 302, 303 each have a corresponding respective trailing edge 308, 309. Following the first and second trailing facets 302, 303 in the direction of rotation there are respective first and second gash facets 310, 311 each of which intersects with the adjacent respective trailing facet 302, 303 at the corresponding trailing edge 308, 309. The first leading facet is angled towards the direction of rotation, and the first trailing facet is angled away form the direction of rotation. The first leading and trailing facets form a first ridge 313 at the line where they meet. Similarly, the second leading edge and second trailing edge form a second ridge 314 at their line of intersection.

[0027] The leading cutting edges 306, 307 of the leading facets merge into the respective leading cutting edges of the corresponding helical cutting edges, and transition with a relatively large radius of curvature, to avoid the cutting edges delaminating the regions around a drilled hole on removal of the drill from a workpiece.

[0028] The circumferentially outer portions 315, 316 respectively of the leading facets merge with a gradual smooth curved transition with the helical outer surfaces 9, 10 of the flutes so as to avoid any abrupt angles at the circumference of the tip, so as to minimize the effect of the circumferential portions of the leading facets in delamination of the work piece during entry or exit of the drill to the work piece.

[0029] Referring to figure 4 herein, there is shown in enlarged detail the central portion of the tip around the point 305 in view along a main central axis looking end on towards the tip. As viewed in figure 4 the direction of rotation is anticlockwise. The view shows the central point 305 at which the leading and trailing facets meet; the secondary cutting edge alignment 0', edge to edge; a primary clearance N' behind centre; and a centre width M'.

[0030] In the example shown, the primary clearance N' is 0.0mm to 0.075mm behind centre; and the secondary cutting edge alignment (edge to edge) is 0.0mm to 0.025mm. The centre width M' is 0.0mm to 0.08mm [0031] Figure 5 herein shows the geometry of the drill of figure 1, in a plurality of views in various orientations. The shaded areas represent the gash faces ground during a split point operation of manufacture. The gash faces are cut to define the trailing edges of the trailing facets and part of the leading end cutting edges of an adjacent land.

Point Angle [0032] Referring to figure 6 herein there is shown in view from one side, the tip region, illustrating the point angle C'. The point angle is the angle of a conical surface which the forward most parts of the tip, in this case the ridges 313, 314 between the leading and trailing facets, lie on. Also shown in figure 6 is the helix angle J', being the angle between a tangent to the helical cutting edge, and the main central axis of the drill.

Radius of Curvature of the Ridge [0033] Referring to figure 7 herein, there is shown the tip in side view in a direction perpendicular to the main central axis and perpendicular to a line parallel to the outer parts of the leading cutting edges. The first ridge 313 and second ridge 314 are seen in side view, and also the first trailing facet 310, the first trailing edge 308 and the first gash face.

[0034] The forward ridges 313, 314 are shown in side view. The forward ridges transition into the corresponding respective helical outer surfaces 9, 10.

with a smooth radius r' as shown in figure 7 herein.

Radius of Curvature of Leading Cutting Edge [0035] Shown in figure 8 herein, is a curved transition R' of relatively large radius between the leading cutting edge 306, 307 of each leading facet, and the leading cutting edge of a corresponding helical facet at the outer circumference of the tip. Shown in figure 8 is just one end cutting edge 306 leading onto one circumferential cutting edge 11 on one side of the drill. Other leading cutting edges correspond.

[0036] In this example, the end cutting edge 306 consists of two substantially straight portions (see figure 3 herein) each of which are substantially straight, and the circumferential cutting edge 11 which is substantially helical. The end cutting edge and the circumferential culling edge join to from a continuous cutting edge which extends from the main central axis to the side of the drill, and having a minimum radius along the direction of the edge of R', where R' is the radius of a circle having centre point T' as shown. The part of the cutting edge of figure 8 at the periphery of the corresponding land is formed of a substantially straight end cutting edge 306, a circular segment cutting edge corner 800, and a helical circumferential cutting edge 11.

[0037] The radius R' should be no less than 9% of the diameter D of a hollow cylindrical surface which the drill end would fit into and touch, this being the same diameter as the distance between opposing helical cutting edges on a plane perpendicular to the main central length axis of the drill.

Manufacture of the Radial Cutting Edge Corner [0038] Referring to figure 9 herein, there is shown a detail of a variation of the continuous cutting edge of figure 8 herein. It may not always be possible to manufacture the cutting edge of figure 8 with an exact transition from a straight edge to a part circular edge to a helical edge. Rather it may be necessary to approximate the edge of figure 8 by inclusion of a straight portion 900 between the circular corner radius 800 and the substantially helical circumferential side cutting edge 11. Figure 9 shows a substantially straight end cutting edge 306 which transitions into a circular corner portion 800, which transitions into a straight side portion 900, which leads on to a substantially helical circumferential side portion 11.

[0039] The minimum radius R along the leading cutting edge can be defined relative to the maximum outer diameter of the drill end in a plane perpendicular to the main central length axis of the drill. The radius R' in the extreme case of a semicircular end cutting edge (when viewed from the side) would be R=D12, being equivalent to a "ball end", that is an end cutting edge which fits the inside of a concave hemispherical surface. Other minimum radii R' of the end leading cutting edge at the circumference of the drill tip for other embodiments of the drill may be expressed as R =D16; R = D112, for example.

[0040] Referring to figure 10 herein, there is shown the tip in another view from the side, with the drill rotated relative to figures 6 or 7 herein, showing the end of a flute, and showing the primary and radial clearance angle A'. Also shown in figure 9 is the secondary clearance angle B'.

[0041] Referring to figure 11 herein, there is shown a detail of a segment of the tip in end view, showing a trailing facet in end view.

End Facets [0042] At the tip, at the end of each land, there is provided a leading end facet and a trailing end facet, so for a two flute, two land drill as shown, there are six end facets (three facets per land) comprising first leading end facet 300, first trailing end facet 302, second leading end facet 301, a second trailing end facet 303; a first gash facet 310 and a second gash facet 311, as shown in figure 3 herein.

[0043] At the intersection between the first leading end facet and the first trailing end facet, is formed a ridge or edge 313. Similarly there is another ridge or edge 314 at the intersection between the second leading facet 301 and the second trailing facet. 303. The effect is the leading facet and the trailing facet are angled with respect to each other, in the form of a pitched roof with the leading facet facing into the direction of rotation of the drill and the trailing facet facing in a direction away for the direction of rotation of the drill.

[0044] The leading facets 300, 301 are angled such that the leading edges of the facets are at a more acute angle relative to the central axis of the drill as the leading cutting edges extend out to the circumference of the drill, than the trailing edges of the leading facets, so that in the direction of the length of the drill, the leading cutting edges 306, 307 do not project as far forward in the direction of drilling as the ridge 313 which connects the leading end facet 300 to the trailing end facet. 302.

Flutes [0045] Each flute 5, 6 comprise a helical substantially "U" shaped channel extending from the tip 4 to the shank 3. The flutes extend in to the body of the drill relative to an overall cylindrical outer projection of the outside surface of the shank portion. A leading edge of each flute (leading in the direction of intended rotation of the drill) forms a cutting edge 11, 12 with a trailing edge of a corresponding respective helical surface which leads in front of the flute. A trailing edge of each flute forms a cutting edge 12, 11 at the intersection with a following helical surface. Lands

[0046] Each land 7, 8 comprises a helical strip of metal which separates adjacent leading and trailing flutes. The outer helical surfaces 9, 10 of the lands face radially outwards and are diamond coated for hardness. A leading edge of each land intersects with a trailing edge of a preceding flute to form a said helical cutting edge 11, 12 extending along a length of the drill between the tip and the shank portion. For each helical cutting edge, the portion of the helical cutting edge towards the tip end smoothly merges into, and is continuous with, a corresponding leading cutting edge of a corresponding end facet on a same land as the helical cutting edge.

End Cutting Edges [0047] The primary cutting edges 306, 307 of the drill are the leading edges of the leading end facets 300, 301, and the leading edges 11, 12 of the helical outer facing surfaces of the lands. A part radius struck tangentially from the point, creating a partial radius at the diameter acts to give an improved exit burr on Carbon fibre composites. Considering a single leading end cutting edge 306, following the leading end cutting edge from the central point of the tip, towards the outer circumference of the tool, a portion 317 of the edge initially extends in a direction substantially radially outwards, and then after a distance X as shown in figure 3 herein follows a path which in view along the central axis of the drill sweeps around in a curve to a direction towards a central diameter of the drill, until it reaches the outer circumference of the drill, where it merges with a corresponding helical cutting edge 11. Hence each end cutting edge 306, 307 consists of two edges, one inner edge 317, 318 and one outer edge.

Transition Between End Facets and Circumferential Facets [0048] The outer circumferential facets 9, 10 (or back offs) transition into the leading end facets 300, 301 and the trailing end facets 302, 303. The circumferential facets are substantially helical, and face substantially radially outwards from the central axis of the drill. The leading end facets 300, 301 face outwards in directions divergent to the direction of linear forward movement of the drill i.e. to the central axis, and in directions between the forward direction and the radial directions. The transition between the circumferential surfaces/facets 9, 10 and the end facets form smooth curved surfaces having a maximum radius of curvature which is equal or greater than the minimum radius of curvature R at the transition between the end cutting edges and the circumferential cutting edges.

Transition Between the Trailing Facets and the Helical Outer Surfaces [0049] The trailing facets 302, 303 transition into the helical outer surfaces 9, 10 respectively over a smooth transition area 200 having a relatively large radius of curvature, as shown in figures 2 and 7 herein.

Ground Gash Facets [0050] At the forward ends of the flutes, the flute channels are ground away to form a pair of gash facets 310, 311 which face outwardly in a direction forward of a plane perpendicular to the main central axis. The gash facets intersect at their forwards ends with the trailing facets to form trailing edges to the trailing facets, which extend outwardly in a direction towards the outer circumference of the drill. Each ground facet intersects with one trailing facet so as to create the inner leading cutting edge 317, 318 of the following land. The ground facet intersects the trailing edge of outwardly facing circumferential surface in the tip region, at the trailing edge of the land.

Diamond Tip [0051] In the preferred embodiment, the whole tip 4 is a single solid diamond, brazed on to the tungsten carbide drill end, so that the end facets, the end cutting edges, the forward parts of the circumferential cutting edges and the radial portions joining the end cutting edges and the circumferential cuffing edges are all formed of the same piece of diamond.

Diamond Coating [0052] The outer circumferential helical surfaces 9, 10 are coated with diamond. The diamond coating may be a polycrystalline diamond coating, or a chemical vapour deposition diamond coating.

[0053] As an alternative to a solid diamond tip, the tip 4 may be formed of tungsten carbide, and have a diamond coating. The diamond coating may be a polycrystalline diamond coating, applied to the first and second leading facets 400,401, and to the first and second trailing facets 402, 403. The diamond coating is also applied to the outer helical surfaces.

[0054] The coating may be a veined diamond coating. The coating may be applied by chemical vapour deposition (CVD) or as polycrystalline diamond.

[0055] In a best mode embodiment, for a 6.5 mm drill, the tip has dimensions as follows: Point angle: 140 degrees Primary clearance: 12 degrees Secondary clearance angle: 24 degrees Secondary cutting edge angle: 35 degrees plus or minus 1 degree Alignment of secondary cuffing edges: 0.0cm to 0.025 cm Corner Radius size R': 2mm [0056] In various other embodiments, having different diameters, the following dimensions and angles have been found to give improved performance in avoiding delamination. The radius R is in the range 9% to 46% of the diameter of the drill body: Diameter Primary & Secondary Point angle Corner Radius R mm Radial Clearance C radius size as a % clearance angle B' R' mm of angle A' diameter 1.60 to 2.5 20 40 140 0.75 30-46 2.51 to3.2 17 34 140 1 31-40 3.21 to 6.3 14 28 140 125 19.8-39 6.31 to 12 24 140 2 15-31 12.71 12.71 to 12 24 140 2.5 13.1 -19.0 19.7 19.01 to 11 22 140 3 11.8-25.4 15.8 24.41 to 11 20 140 3.2 9.1-35.0 13.1 [0057] Referring to figure 14 herein, there is illustrated schematically a series of hole tolerances produced by a drill as described herein. For a nominal pm tolerance on a 6.5mm diameter hole size, the range of acceptable variations may be in the range 45pm to 55pm. Holes which are undersize are not necessarily a problem, since the hole can be re drilled. However, holes which are oversize, for example 6Opm too large, mean that the hole must be re drilled at a larger size using a different drill, and a different size bolt or screw being used, to avoid the component being scrapped. The specific embodiment disclosed herein may produce holes within tolerance, and avoiding delamination of the carbon composite material.

[0058] In the embodiment shown there are two helical flutes and two outer helical surfaces, although other embodiments may have between three and six flutes, and the corresponding number of helical outer surfaces, end facets and cutting edges.

[0059] In this specification a drill bit intended to rotate in a clockwise direction is described. The skilled person will appreciate that in other variations, a drill having a mirror image of all features described herein, and intended to rotate in an anticlockwise direction is also possible and is within the scope of the invention as claimed.

Claims

Claims 1. A drill for producing holes in composite materials, said drill comprising: an elongate drill body (2) having a shank portion (3) at one end, and a tip portion (4) at another end; said tip portion having a maximum outside diameter D; a plurality of end cutting edges (306, 317, 307, 318) each extending between a main central axis of the drill body and a circumferential periphery of the body; and a plurality circumferential cutting edges (11, 12) each extending along a length of said drill characterised in that: each said end cutting edge forms a continuous cutting edge with a corresponding respective said circumferential cutting edge; and each said end cutting edge has a minimum radius of curvature R, wherein said minimum radius R is in the rangeR = D/12 to R = D12.
2. The drill as claimed in claim 1, wherein the radius R is in the range 9% to 46% of the diameter of the drill body.
3. The drill as claimed in any one of the preceding claims, wherein each said end cutting edge joins a corresponding respective circumferential cutting edge with a minimum radius of curvature in the range 0.75mm to 3.2mm.
4. The drill as claimed in any one of the preceding claims, wherein said tip comprises a plurality of lands, each said land terminating in a leading end facet (300, 301), and a trailing end facet (302, 303), and said leading end facet and said trailing end facet intersect at a ridge (313, 314) extending between said central axis and an outer circumference of said drill, wherein a minimum radius of curvature r between said ridge and an outer periphery of said drill is in the range r = D/12 to r = D/2.
5. The drill as claimed in claim 4, wherein the radius r is greater than or equal to radius R.
6. The drill as claimed in any one of the preceding claims, wherein said tip comprises a plurality of lands, each said land terminating in a leading end facet (300, 301), and a trailing end facet (302, 303), and said leading end facet and said trailing end facet intersect at a circumferential outer surface of said land, such that a minimum radius of curvature between said leading end facet and said circumferential outer surface is greater than or equal to said radius R.
7. The drill as claimed in any one of the preceding claims, wherein said tip comprises a plurality of lands, each said land terminating in a leading end facet (300, 301), and a trailing end facet (302, 303) , and a further trailing facet (310, 311) which trails said trailing end facet.
8. The drill as claimed in any one of the preceding claims, wherein the minimum radius of curvature R of a continuous cutting edge, along the direction of said cutting edge, formed by a leading end cutting edge and a corresponding helical cutting edge at the transition between those edges, is less than the radius of curvature of said leading cutting edge, and less than the radius of curvature of said helical cuffing edge.
9. The drill as claimed in any one of the preceding claims, wherein a said end cutting edge comprises an inner portion (317, 318) which extends in a direction from said central axis of the drill towards an outer periphery of the drill; and an outer portion which extends in a direction from said central axis of the drill towards an outer periphery of the drill; wherein the outer portion extends at an angle to the radial direction greater than an angle relative to the radial direction, at which the inner portion extends.
10. The drill as claimed in any one of the preceding claims, wherein the minimum radius of curvature R of a continuous cutting edge formed by a leading end cutting edge and a corresponding helical cutting edge at the transition between those edges, is greater than the radial distance between the radially outermost tip of said cutting edge and the radial distance of a line in a direction perpendicular to the main central axis joining the trailing and leading edges of successive helical outer surfaces.
11. The drill as claimed in any one of the preceding claims, wherein a peripheral portion of each said end cutting edge is formed of diamond.
12. The drill as claimed in any one of the preceding claims, wherein said tip (4) comprises a solid diamond.
13. The drill as claimed in any one of claims 1 to 10, having a polycrystalline diamond coated tip.
14. The drill as claimed in any one of claims ito 10, having a chemical vapour deposition diamond coating.
15. A drill for producing holes in composite materials, said drill comprising: an elongate drill body having a shank portion (3) at one end, and a tip portion (4) at another end; said body having a main central axis; a plurality of end facets (300 -304, 310, 311) extending between said central axis and a circumferential periphery of the body; a plurality of circumferential facets (9, 10) extending along a length of said body; a plurality circumferential cutting edges (11, 12) each extending along a length of said body; a plurality of flutes (5, 6) separating said plurality of circumferential cutting facets; a plurality of end cutting edges (306, 307, 317, 318) each extending between said central axis and a corresponding respective said circumferential cutting edge; characterised in that each said end cutting edge forms a continuous cutting edge with a corresponding respective circumferential cutting edge; and each said end cutting edge joins a corresponding respective circumferential cutting edge with a minimum radius of curvature in the range 0.75mm to 3.2mm.