Tool inserts.

BACKGROUND OF THE INVENTION

This invention relates to tool inserts, and more particularly inserts for use i_ll cutting tools such as turning tools or milling cutters.

Cubic boron nitride (CBN) compacts are well known in the art and consist essentially of a mass of CBN particles, present in an amount of at least 70%, preferably 80 to 90%, by volume of the compact, bonded into a hard conglomerate. Compacts are polycrystalline masses and can replace single large crystals.

Cubic boron nitride compacts, may be self bonded without the aid of a metal or like bonding matrix. Alternatively, such compacts may be produced wherein there is a suitable second or bonding matrix present. The second or bonding phase may be essentially metallic or ceramic in nature.

The starting material for the compact may include a catalyst (also known as a solvent or sintering aid material) for CBN growth. Examples of suitable catalysts are aluminium or an alloy of aluminium with nickel, cobalt, iron, manganese or chromium. The presence of a solvent or catalyst in the starting material is desirable because then some intergrowth between the particles may occur during compact manufacture. As is known in the art, cubic boron nitride compacts are manufactured under conditions of temperature and pressure at which the abrasive particle is crystallographically stable.

Cubic boron nitride compacts may be used for a variety of abrading applications, but are particularly useful in the turning and milling of workpieces. For such applications they are mounted in the support body of a suitable cutting tool. In particular, cubic boron nitride compacts are recommended and used for the turning of Ni-hard alloys, white/chill cast iron, high speed steels, cold work tool steels, nickel/chromium irons, grey cast iron, sintered iron, cobalt-based hard facing alloys, nickel-based hard facing alloys, and for the milling of meehanite, Ni-hard alloys, cold work tool steels, and nickel-based hard facing alloys. Cubic boron nitride compacts, despite their hardness, have, however, been found not to be suitable in the cutting of ferrous materials having a Rockwell C Hardness of less than 45.

Cemented carbide inserts are also known and used for the turning and milling of ferrous materials. Cemented carbide tool inserts are sometimes provided with a thin, wear-resistant refractory layer behind the cutting edge. This refractory layer acts as a chip breaker face for the insert and has been found to increase the working life of the insert.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a tool insert comprising a cubic boron nitride compact comprising a polycrystalline mass of cubic boron nitride particles, present in an amount of at least 70 percent by volume, bonded into a hard conglomerate, and a thin wear-resistant layer of a refractory bonded to a surface thereof and presenting a cutting edge for the insert.

Further according to the invention, there is provided a cutting tool comprising a supporting body having secured thereto at least one tool insert as described above. The tool is typically a turning tool or a milling cutter.

Still further according to the invention, there is provided a method for cutting a workpiece made of a ferrous material having a Rockwell C Hardness of less than 45 including the steps of providing a cutting tool as described above, the insert of which presents its refractory cutting edge to the workpiece, causing the cutting edge to contact the workpiece, and moving the workpiece relative to the cutting edge to remove material from the workpiece.

DESCRIPTION OF THE DRAWINGS ......

<ul id="ul0001" list-style="none"><li>Figures 1 to 3 are perspective views of three embodiments of tool inserts bf the invention,</li><li>Figure 4 is a perspective view of a milling cutter of the invention,</li><li>Figure 5 is a perspective view of a turning tool of the invention, and</li><li>Figure 6 is a perspective view of the turning tool of Figure 5 acting on a workpiece.</li></ul>

DESCRIPTION OF PREFERRED EMBODIMENTS

The wear-resistant layer is a thin layer generally having a thickness of less than 50 micrometres, preferably less than 20 micrometres. It is this wear-resistant layer which provides the cutting edge for the insert.

The refractory is preferably a nitride, boride, carbide, carbonitride, silicide, phosphide, or oxide of a transition metal, aluminium or silicon. The preferred refractory is titanium nitride, titanium carbide, or mixtures thereof.

The wear-resistant layer may consist on one type of refractory only or may contain more than one type of refractory. Where more than one type of refractory is provided in the layer, each refractory may be provided in its own discrete layer or the various refractories may be mixed in the layer.

The refractory layer may be of a crystalline or amorphous nature.

The refractory layer may be applied to the compact surface by methods oell known in the art such as chemical vapour deposition, plasma activated vapour deposition, sputtering or vacuum plating. All these methods are well known and well documented in the art. The preferred method is chemical vapour deposition.

The cubic boron nitride compact may be any known in the art. Generalfy such compacts present a major surface on each of opposite sides thereof and to one of these major surfaces the refractory layer will be bonded. Preferred compact shapes are disc, triangular and rectangular. In the latter two shapes it is the corners which provide the cutting edge for the insert. As mentioned, the thin wear-resistant refractory layer will be bonded to one of the major surfaces of the compact. The other major surface may be polished, for example using a polishing scaife. The polished surface of the compact provides good heat-conducting contact between the compact and the support body of a tool when the tool insert is secured thereto. The cubic boron nitride compact may be bonded to a cemented carbide support. Such a supported compact is also known as a composite compact and is well known in the art. The cemented carbide is preferably cemented tungsten carbide.

The tool insert of the invention has particular use in a cutting tool such as a turning tool or milling cutter. The insert may be secured to the supporting body of such a tool in the conventional manner.

The tool insert'of the invention has been found to have particular application in the cutting of ferrous material having a Rockwell Hardness of less than 45. Indeed, it has surprisingly been found that very effective cutting of such workpieces can be achieved. For such cutting operations, the workpiece will be rotated against the cutting edge of the tool insert which will simultaneously traverse the workpiece. High peripheral speeds of the workpiece can be used, i.e. at least 50 metres/minute.

Figures 1 to 3 illustrate three embodiments of tool inserts of the invention. In each case the insert comprises a cubic boron nitride compact 10 to which is bonded a thin, wear-resistant refractory layer 12. In each case the refractory layer is bonded to one of the major surfaces of the compact. The insert of Figure 1 has a disc shape and it is the top edge 14 of the refractory layer 12 which provides the cutting of the insert. The insert of Figure 2 has a rectangular shape and it is the top edge 16 of each corner 18 which provides the cutting edge for the insert. The insert of Figure 3 is triangular in shape and it is the edge 20 of each corner 22 which provides the cutting edge for the tool.

In one particular example of the invention, the refractory layer for a tool insert consisted of three zones. The first zone in contact with the compact surface was an 0,5 micrometre thick zone of titanium carbide; the second zone on top of the titanium carbide zone was a 10 micrometre thick titanium carbide/titanium nitride transition zone; and the third zone on top of the transition layer was a 1 to 2 micrometre thick titanium nitride outer zone. The various zones for the layer were deposited on the compact substrate using standard chemical vapour deposition techniques. The refractory layer as a whole, i.e. comprising all three zones, adhered strongly to the compact substrate.

Figure 4 illustrates the cutting head of a milling cutter incorporating a plurality of tool inserts of the invention. The cutting head 24 has a shaft 26 adapted to be mounted for rotation in the milling machine and an enlarged cutting end 28. On the cutting end 28 are secured a plurality of tool inserts 30. Each insert 30 is clamped by means of clamping plate 32 to the side of a projection 34. The mounting is such that the surface 36 of each insert is the refractory layer and it is the edge thereof which provides the cutting edge: The cutting head is of conventional configuration, save for the provision of the inserts of the invention.

Figure 5 illustrates a tool holder for use in turning workpieces. - The tool holder is of conventional configuration, save for the provision of the insert of the invention. The tool holder has a shank 38 for mounting between appropriate clamping plates or arms. The end 40 of the shank has a recess 42 which accommodates a supporting shim 44 (usually of cemented carbide) and an insert 46 of the invention. The shim 44 and the insert 46 are held in the recess by means of clamp 48. The top surface 50 of the insert is the refractory layer and it is the peripheral edge of this surface which provides the cutting edge for the insert.

Figure 6 illustrates a typical turning operation using a turning tool of Figure 5. The tool shank 52 is mounted between clamping plates 54 and in this position presents an upper cutting edge 56 to a workpiece 58. The mounting is generally such that this cutting edge 56 is at a negative rake angle relative to the workpiece. The workpiece is rotated and the cutting edge fed in a certain depth. Typically, the depth of infeed of the cutting edge will be of the order of 0,5 to 2,5 mm. The cutting edge, during rotation of the workpiece, is caused to traverse the workpiece a predetermined distance per revolution of workpiece. The traverse rate will vary according to the nature of the workpiece being turned. The rotational speed of the workpiece will generally be such that the peripheral speed thereof will be at least 50 metres/minute. During this operation, material is removed from the workpiece.

It has surprisingly been found that soft ferrous materials, i.e. having a Rockwell C Hardness of less than 45, such as annealed D3 steel can be effectively turned and milled using the method and tools of the invention, as described above. Better removal rates compared with uncoated cubic boron nitride compacts and cemented carbide inserts are achieved, and the life of the insert of the invention is longer than either of these two prior art products.