Patent Application
20090148249
This invention relates to ultra-hard cutting tool components and more particularly PCBN cutting tool components.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (CBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blend form of boron nitride that has a similar structure to that of diamond. In the CBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds.
CBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
CBN may also be used in bonded form as a CBN compact, also known as PCBN (polycrystalline CBN). CBN compacts comprise sintered masses of CBN particles. When the CBN content exceeds 80 percent by volume of the compact, there is a considerable amount of CBN-to-CBN contact. When the CBN content is lower, e.g. in the region of 40 to 60 percent by volume of the compact, then the extent of direct CBN-to-CBN contact is limited.
CBN compacts will generally also contain a binder containing one or more ceramic phase(s) in compacts containing aluminium, cobalt, nickel, tungsten and titanium.
CBN compacts tend to have good abrasive wear, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in contact with a workpiece. The CBN compact, with or without substrate, is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.
When the CBN content of the compact is less than 70 percent by volume, the matrix phase, i.e. the non-CBN phase, will typically also comprise an additional or secondary hard phase, which may be ceramic in nature. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 (according to the new IUPAC format) transition metal aluminium oxide and mixtures thereof. The matrix phase constitutes all the ingredients in the composition excluding CBN.
CBN compacts may be bonded directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is bonded to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.
A known method for manufacturing the polycrystalline CBN compacts and supported compact structures involves subjecting an unsintered mass of CBN particles together with a powdered matrix phase, to high temperature and high pressure conditions, i.e. conditions at which the CBN is crystallographically or thermodynamically stable, for a suitable time period. Typical conditions of high temperature and pressure which are used are temperatures in the region of 1100° C. or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
CBN compacts with CBN content more than 70 volume percent are known as high CBN PCBN materials. They are employed widely in the manufacture of cutting tools for machining of grey cast irons, white cast irons, powder metallurgy steels, tool steels and high manganese steels. In addition to the conditions of use, such as cutting speed, feed and depth of cut, the performance of the PCBN tool is generally known to be dependent on the geometry of the workpiece and in particular, whether the tool is constantly engaged in the workpiece for prolonged periods of time, known in the art as “continuous cutting”, or whether the tool engages the workpiece in an intermittent manner, generally known in the art as “interrupted cutting”.
Commercially available PCBN cutting tools all have sintered PCBN layers with thicknesses above 0.2 mm. These thick PCBN layers are difficult and expensive to process. The cost of manufacture of a PCBN cutting tool has thus made it too expensive to compete successfully in the carbide cutting tool market. For PCBN to be considered for typical carbide applications, it has to be easier and cheaper to process and have higher chip resistance, while still outperforming carbide in terms of wear resistance.
U.S. Pat. No. 5,697,994 describes a cutting tool for woodworking applications comprising a layer of PCD or PCBN on a cemented carbide substrate. The PCD is generally provided with a corrosion resistant or oxidation resistant adjuvant alloying material in the bonding phase. An example is provided wherein the PCD layer is 0.3 mm in thickness. For PCBN the layer thickness is preferably 0.3 to 0.9 mm.
A cutting tool component of the invention comprises a body comprising a cemented carbide substrate and having at least one working surface, the at least one working surface presenting a cutting edge or area for the body, characterized in that the at least one working surface comprises PCBN adjacent the cutting edge or area and extending to a depth of no greater than 0.2 mm from the at least one working surface and wherein the substrate has a thickness of 1.0 to 40 mm.
In one preferred embodiment of the invention, the cutting tool component body comprises a cemented carbide substrate and an ultra-thin layer of PCBN bonded to a major surface of the substrate, the ultra-thin layer of PCBN having a thickness of no greater than, generally less than, 0.2 mm and the substrate has a thickness between 1.0 to 40 mm.
In an alternative preferred embodiment of the invention, one or more intermediate layers is/are located between the cemented carbide substrate and the layer of PCBN, preferably based on a ceramic, metal or ultra-hard material or combination thereof that is softer than the PCBN.
In another alternative preferred embodiment of the invention, the cutting tool component body comprises a cemented carbide substrate having a working surface presenting a cutting edge or area for the tool component and having a plurality of grooves or recesses extending into the substrate from the working surface, and a plurality of strips or pieces of ultra-hard material located in the respective grooves or recesses, the arrangement being such that the PCBN extends to a depth of no greater than 0.2 mm from the working surface and forms a part of the cutting edge or area of the tool component.
The thickness or depth of the PCBN layer or inserts is preferably from 0.001 to 0.15 mm.
The PCBN optionally contains a second phase comprising a metal or metal compound selected from the group comprising aluminium, cobalt, iron, nickel, platinum, titanium, chromium, tantalum, copper, tungsten or an alloy or mixture thereof.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:
The object of the present invention is to provide an engineered PCBN cutting tool with properties between cemented carbide and PCBN.
The object is addressed by providing a cutting tool component 10, as illustrated for example in
The ultra-thin hard layer together with the softer substrate results in a “self-sharpening” behaviour during cutting, which in turn reduces the forces and temperatures at the cutting edge. The hard layer is a high or low CBN content PCBN, of the type described above. The thickness of the hard layer preferably varies between 0.001-0.15 mm, depending on the required properties for specific applications.
Referring to the tool component 30 of
Alternatively, referring to the tool component 40 as illustrated in
The substrate material can be selected from tungsten carbides, ultra-fine grain tungsten carbides, titanium carbides, tantalum carbides and niobium carbides. Methods for producing cemented carbides are well known in the industry. Because cutting is done with both the PCBN and the carbide, the selection of the substrate is another variable which can be changed in order to alter the properties of the cutting element to suit different applications.
In some applications, it may be preferable to provide a substrate having a profiled or shaped surface, which results in an interface with a complimentary shape or profile.
From a processability perspective the critical feature of the invention is the ultra-thin hard layer which will reduce the processing cost of PCBN cutting tools.
In terms of performance the critical feature of the invention is to adjust the hard layer thickness so that the desired properties can be achieved and also to ensure that a “self-sharpening” effect takes place during cutting. This could mean adding a softer ceramic or metal intermediate layer just below the PCBN. This means that when the wear progresses through the hard layer at some stage during the cutting process, the cutting will be done by both the hard layer and the substrate and/or the intermediate layer. Conventional tools all have a hard layer thickness above 0.2 mm, and hence the substrate never comes in contact with the workpiece (since tool life criteria is VBBmax=0.2−0.3 mm) and the properties and behaviour of the tool is that of the hard layer only.
As illustrated in
A major benefit of cutting with both the ultra-thin hard layer 14 and the substrate 12 is the “self-sharpening” effect it has on the tool. As illustrated in
Another benefit of ultra-thin hard layers is the improved chip resistance it gives to the tool. Thicker layers have higher residual stresses and are more susceptible to chipping and fracture. Also, if chipping does occur, the carbide substrate will arrest the crack and stop it from getting bigger than the thickness of the top hard layer.
All processing (EDM, EDG, grinding) is easier and faster as the top hard layer becomes thinner. Having ultra-thin hard layers will shorten processing times.
As explained earlier conventional PCBN compacts are manufactured with PCBN layer thicknesses >0.2 mm in order for the cutting to be done by the hard layer only. However, during the synthesis of such thick layers, the compact often bows because of the thermal expansion differences between that of PCBN and the carbide substrate. This results in additional processing (mechanical grinding, EDG or lapping) to get the compact back to flatness. With ultra-thin hard layers, bending of the disc is minimised and additional processing is not required. This allows for the production of near-net shape PCBN compacts.
The invention will now further be discussed, by way of example only, with reference to the following non-limiting examples. These examples show the advantages of an ultra-thin PCBN cutting tool component. The PCBN cutting tool components used in the examples were made by PCBN manufacturing methods well known in the art and as described above.
The test is believed to be very representative of hard machining. Two PCBN cutting tool components of the type described above were used in the test. The one had an ultra-thin PCBN layer 0.1 mm in thickness and the other a PCBN layer of 0.5 mm thickness. The maximum chip size was recorded. The test conditions were as follows:
From the graph of
An interrupted milling operation was performed using the same two PCBN cutting tool components of Example 1 whereby the conditions and workpiece were chosen as to minimise any wear events and in return promote fracture. The feed per tooth was increased from 0.1 to 0.2 to 0.3 etc until catastrophic failure of the nose was observed. The feed per tooth represent the load on the cutting edge and is therefore a suitable fracture resistance indicator. The test conditions that were used are as follow:
From the Box-plot of
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-10083 | Dec 2005 | ZA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/003563 | 12/12/2006 | WO | 00 | 8/13/2008 |
