Cutting insert for a rotary cutting tool

Abstract: A cutting insert (18) for a rotary cutting tool (28) comprises a body (12) of tungsten carbide, with layers (14, 16) of polycrystalline diamond on opposite sides of the body. The diamond layers overlap across the centre-line of the insert. The cutting insert is shaped for retention at the end of the rotary cutting tool so that the first and second sides of the cutting insert are substantially parallel to the axis of the cutting tool, and the diamond layers are offset from one another across the width of the body so that they define the lips or major cutting edges (38, 40) of the cutting tool.

Description: BACKGROUND OF THE INVENTION This invention relates to a method of manufacturing a cutting insert for a rotary cutting tool and a cutting insert comprising the features of the preamble of claim 1 (see US-A-4 966 352). Rotary cutting tools such as boring heads are known which comprise a cylindrical body defining a pair of arms, one on either side of a line passing diametrally through the body. On each arm is mounted a cutting insert comprising a body of polycrystalline diamond (PCD) material, such as a PCD compact comprising a PCD layer on a tungsten carbide backing. Other cutting tools such as twist drills are known which have an approximately chisel-type bit, with a pair of cutting inserts mounted in seats oriented more or less diametrally in the end of a twist drill body. SUMMARY OF THE INVENTION According to the invention a cutting insert for a rotary cutting tool comprises a body of relatively soft material, with layers of relatively hard material on opposite first and second sides thereof, the cutting insert being shaped for retention at the end of the rotary cutting tool so that the first and second sides of the cutting insert are substantially parallel to the axis of the cutting tool, the layers of relatively hard material being formed in respective recesses in the opposite first and second sides of the body, and being offset from one another across the width of the body so that they define the lips or major cutting edges of the cutting tool. Preferably the cutting insert has a pair of cutting edges which define a generally wedge- or V-shape. The cutting insert may be gable-shaped or chevron-shaped, for example. Preferably, the layers of relatively hard material overlap across the centre-line of the insert. In one version of the cutting insert, the insert defines a chisel edge comprising portions of the relatively hard material on either side of a central portion of the relatively soft material. Typically, the relatively soft material comprises tungsten carbide and the relatively hard material comprises polycrystalline diamond. Further, a rotary cutting tool comprising an insert embodying the invention has a body which is shaped to retain a cutting insert as defined above at an end thereof, the body defining a pair of opposed seating surfaces to which the insert is fixed so that the insert is disposed diametrally at the end of the cutting tool. The body may have a pair of upstanding arms, each arm defining a seating surface to which a respective seating portion of the insert is fixed, for example, by brazing. The invention extends to a method of manufacturing a cutting insert for a rotary cutting tool including providing a blank which comprises a body of relatively soft material an which has first and second opposed major surfaces, with first and second layers of relatively hard material provided in respective recesses on opposite sides of the body, said first and second layers of relatively hard material defining portions of the respective major surfaces; and cutting the blank to define a cutting insert having a body with first and second sides corresponding to the opposed major surfaces of the blank, so that the first and second sides of the insert include offset cutting edges formed from the respective first and second layers of relatively hard material. Preferably, the first and second layers of relatively hard material in the blank are offset from one another across the thickness of the blank, the method comprising cutting the blank about a line equidistant from adjacent edges of the first and second layers. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a pictorial view of a composite diamond compact blank used to form a cutting insert embodying the invention; Figure 2 is a side view of the blank of Figure 1; Figures 3a and 3b are side views of alternative versions of the blank; Figure 4 is a top view of the blank of Figure 1, showing the pattern of cutting of the blank; Figure 4a is a pictorial view of a single insert cut from the blank; Figure 5 is a pictorial view of a further embodiment of a blank for forming cutting inserts embodying the invention; Figure 6 is a top view of the blank of Figure 5, showing a cutting pattern therefor; Figures 7a and 7b are exploded and assembled pictorial views of a boring head incorporating a cutting insert embodying the invention; Figure 8 is an end view of the boring head of Figure 7; and Figures 9 and 10 are side views of alternative cutting inserts. DESCRIPTION OF EMBODIMENTS Referring first to Figure 1, a polycrystalline diamond (PCD) composite blank 10 is disk shaped and comprises a body of tungsten carbide 12 with opposed generally flat faces. On each side of the disk, strip-shaped PCD layers 14 and 16 are formed in respective recesses in the flat faces. The layers 14 and 16 are generally semi-circular in plan. The layers 14 and 16 are formed on opposite sides of a diameter of the disk, but overlap slightly across the diameter, as seen clearly in Figure 2. Figures 3a and 3b show variations of the blank, in which the layers 14 and 16 do not overlap. Referring now to Figure 4, a cutting pattern is illustrated, superimposed on the blank of Figure 1. According to the cutting pattern, a number of chevron-shaped cutting inserts 18 are laser cut, nose-to-tail, from the central portion of the blank, symmetrically about the diameter of the blank. A single cutting insert is shown pictorially in Figure 4a. It can be seen that the insert has diamond layers 14 and 16 on opposite wings of the chevron which overlap across the centre-line of the insert. The insert has cutting edges 42 and 44. Figures 5 and 6 illustrate an alternative version of the PCD blank, with a pair of strips of PCD material 20 and 22 formed in slots or channels on its upper surface, and a staggered pair of strips 24 and 26 in slots or channels on its lower surface. As shown in Figure 6, two rows of cutting inserts can be cut from the single blank, about lines equidistant from adjacent overlapping edges of the strips of PCD material on opposite sides of the blab This results in less wastage than the cutting pattern of Figure 4. The chevron-shaped cutting inserts are relatively easy to manufacture and minimise wastage of the PCD material. Of course, the inserts could have other shapes, as shown in Figures 9 and 10, for example. The insert of Figure 9 is gable-shaped, while the insert of Figure 10 is generally gable-shaped but has a central slot or notch 46 formed in its working end. However, all the illustrated cutting inserts have cutting edges which define a generally wedge- or V-shape. Figure 7 is a pictorial view of a boring head body 28 which has a pair of upstanding arms 30 and 32 which define opposed, staggered mounting faces 34 and 36 which lie on either side of a diameter of the body 28. As shown in Figure 7b, when the cutting insert 18 is fitted into the seat defined by the arms 30 and 32 and their respective seating faces 34 and 36, the tungsten carbide faces of the inserts abut the seating surfaces 34 and 36, and the insert is brazed into position in the seat. A top view of the assembled boring head is shown in Figure 8. It can be seen that the major sides of the cutting insert are parallel to the axis of the boring head, and that the lips or major cutting edges 38 and 40 of the finished boring head are formed by the upper edges of the PCD layers 14 and 16 on either side of the body of the insert. In the case of the cutting insert of Figure 4a, the finished boring head has a chisel edge (48) which comprises portions of diamond material on either side of a central tungsten carbide portion, as shown. The illustrated cutting inserts have the particular advantage, compared to prior art cutting inserts, that they are formed as a single unit, and thus resist damage due to torsional stresses which can damage prior art rotary cutting tools employing twin inserts. It will be understood that the cutting inserts embodying the invention can be fitted to a variety of cutting tools, such as twist drills, roof and face boring heads, endmills, routers and milling cutters, and that the described embodiments are purely exemplary. It was found that a cutting tool comprising an insert embodying the present invention was extremely stable during collaring and drilling, generating little or no vibration compared to a two-wing prior art cutting tool. This is significant, particularly with respect to collaring, since it is believed that vibration and instability cause most of the chipping of the PCD cutting edges which occurs with prior art cutting tools. However, the prototype cutting tool comprising an insert embodying the present invention exhibited no evidence of chipping.

Claims: A cutting insert (18) for a rotary cutting tool (28) comprising a body (12) of relatively soft material, with layers (14, 16) of relatively hard material on opposite first and second sides thereof, the cutting insert being shaped for retention at the end of the rotary cutting tool so that the first and second sides of the cutting insert are substantially parallel to the axis of the cutting tool, the layers of relatively hard material being formed in respective recesses in the opposite first and second sides of the body, offset from one another across the width of the body so that they define the lips or major cutting edges (38, 40) of the cutting tool, characterised in that said layers (14, 16) of relatively hard material are formed in respective recesses in the opposite first and second sides of the body. A cutting insert according to claim 1 characterised in that the cutting insert has a pair of cutting edges (42, 44) which define a generally wedge- or V-shape. A cutting insert according to claim 2 characterised in that it is gable-shaped. A cutting insert according to claim 3 characterised in that a notch or slot (46) is formed centrally between the cutting edges (42, 44). A cutting insert according to claim 2 characterised in that it is chevron-shaped. A cutting insert according to any one of claims 1 to 5 characterised in that the layers of relatively hard material (14, 16) overlap across the centre-line of the insert. A cutting insert according to claim 6 characterised in that the cutting insert defines a chisel edge (48) comprising portions of the relatively hard material (14, 16) on either side of a central portion of the relatively soft material (12). A cutting insert according to any one of claims 1 to 7 characterised in that the relatively soft material (12) comprises tungsten carbide and the relatively hard material (14, 16) comprises polycrystalline diamond. A rotary cutting tool characterised in that it has a body (28) which is shaped to retain a cutting insert (18) according to any one of claims 1 to 8 at an end thereof, the body defining a pair of opposed seating surfaces (34, 36) to which the insert is fixed so that the insert is disposed diametrally at the end of the cutting tool. A rotary cutting tool according to claim 9 characterised in that the body (28) has a pair of upstanding arms (30, 32), each arm defining a seating surface (34, 36) to which a respective seating portion of the insert (18) is fixed. A rotary cutting tool according to claim 10 characterised in that each respective seating portion of the insert (18) is defined by the relatively soft material (12) of the insert body. A rotary cutting tool according to any one of claims 9 to 11 characterised in that the insert (18) is fixed to the seating surfaces (34, 36) of the cutting tool body (28) by brazing. A method of manufacturing a cutting insert for a rotary cutting tool characterised in that it includes providing a blank (10) which comprises a body of relatively soft material (12) and which has first and second opposed major surfaces, with first and second layers (14, 16) of relatively hard material provided in respective recesses on opposite sides of the body, said first and second layers (14, 16) defining portions of the respective major surfaces; and cutting the blank to define a cutting insert (18) having a body with first and second sides corresponding to the opposed major surfaces of the blank, so that the first and second sides of the insert include offset cutting edges (42, 44) formed from the respective first and second layers of relatively hard material. A method according to claim 13 characterised in that the first and second layers (14, 16) of relatively hard material in the blank are offset from one another across the thickness of the blank, the method comprising cutting the blank about a line equidistant from adjacent edges of the first and second layers. A method according to claim 13 or claim 14 characterised in that a plurality of chevron-shaped cutting inserts (18) are cut in a nose-to-tail pattern from the blank (10). A method according to claim 14 or claim 15 characterised in that the blank (10) has a plurality of first and second strips (20, 22; 24, 26) of the relatively hard material on opposite sides thereof, the first and second strips being offset from one another across the thickness of the blank, and a plurality of rows of cutting inserts (18) being cut from the blank about lines equidistant from adjacent edges of respective first and second strips.

Inventors: LLOYD ANDREW IAN, TANK KLAUS, LLOYD, ANDREW IAN, TANK, KLAUS
Applicants: DE BEERS IND DIAMOND, DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1991
Main classification: B23B51
IPC codes: B23B51, B23P15, E21B10