Patent Publication Number: US-2006019118-A1

Title: Coated cutting tool with brazed-in superhard blank

Description:
RELATED APPLICATION  
      This application is a divisional patent application of co-pending U.S. patent application Ser. No. 10/463,379 filed on Jun. 17, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a coated cutting tool that uses a superhard blank. More specifically, the present invention pertains to a coated cutting tool that uses, for example, a brazed-in polycrystalline cubic boron nitride blank.  
      Heretofore, there have been known coated cutting tools that use a superhard blank as the cutting element that provides a cutting edge. These cutting tools comprise a cutting tool body that contains a notch or pocket. The superhard blank is brazed into the notch or pocket using a braze alloy so that a braze joint is formed between the superhard blank and the cutting tool body.  
      In one alternative, the superhard blank comprises a support (e.g. cobalt cemented tungsten carbide) on which there is a layer of superhard material (e.g. polycrystalline cubic boron nitride). As another alternative, the blank is comprised of entirely superhard material. During the material removal process, the superhard layer (or superhard material) defines a cutting edge that comes into contact with the workpiece material to remove workpiece material so as to function as a cutting element.  
      During the material removal operation there is generated heat (and sometimes considerable heat) at the point of contact between the cutting edge of the superhard material and the workpiece material. This is especially the case when the workpiece material is hard such as, for example, a D3 tool steel. Such a material has a hardness on the order of Rockwell C 60 (AISI D3) wherein the quenched hardness can range between about 64 to about 66 Rockwell C and the tempered hardness can range between about 54 and about 61 Rockwell C.  
      Because of the generation of excessive heat, the use of coated cutting tools with a brazed-in superhard blank have experienced the drawback of debrazing of the superhard blank from the cutting tool body. In other words, the heat generated at the point of contact between the superhard blank and the workpiece material passes through the superhard blank so as to cause the temperature at the braze joint to reach such a level that the braze alloy melts (or softens) thereby reducing the shear strength of the braze joint. A reduction in the shear strength of the braze joint weakens the braze joint so that the cutting forces exerted on the superhard blank can detach the superhard blank from the cutting tool body.  
      Dry cutting processes such as removing material by machining from carbon:carbon composite materials, abrasive-reinforced polymeric materials, and various types of wood materials through the use of cutting tools using a brazed-in superhard blank can also generate higher cutting temperatures. As mentioned earlier, these higher cutting temperatures result in a higher temperature at the braze joint. These higher temperatures at the braze joint can result in a softening or melting of the braze alloy thereby reducing the shear strength so as to cause the superhard blank to become detached or separated from the cutting tool body under the influence of the cutting forces exerted on the superhard blank.  
      The degree of the cutting forces exerted on the cutting tools taken in light of the cutting temperature, the temperature at the braze joint, the liquidus temperature of the braze alloy, and the shear strength of the braze alloy appear to influence the ability of the superhard blank to be retained in the pocket of the cutting tool during the material removal operation. When the temperature at the braze joint reaches a certain level, there begins a reduction in the shear strength of the braze joint. When the shear strength of the braze joint is less than necessary to maintain its integrity against the cutting forces exerted on the superhard blank, the superhard blank becomes detached from the cutting tool body. As one can appreciate, the premature (or catastrophic) separation or detachment of the superhard blank from the cutting tool body is an undesirable result.  
      It would thus be desirable to provide a coated cutting tool that uses a brazed-in superhard blank as the cutting element that presents a cutting edge wherein the braze joint between the cutting tool body and the superhard blank is able to withstand the heat generated during the material removal operation.  
      It would also be desirable to provide a coated cutting tool that uses a brazed-in superhard blank as the cutting element that presents a cutting edge wherein the braze joint between the cutting tool body and the superhard blank is able to withstand the heat generated during the material removal operation through the use of braze alloys that maintain an adequate shear strength at the temperatures that exist at the braze joint during the material removal operation (i.e., high temperature braze alloys) so as to maintain the integrity of the braze joint.  
      It would further be desirable to provide a coated cutting tool that uses a brazed-in superhard blank as the cutting element that presents a cutting edge wherein the braze joint between the cutting tool body and the superhard blank is able to withstand the heat generated during the material removal operation through the use of high temperature braze alloys along with the geometry and design of the superhard blank so as to reduce the exposure of the braze joint to excessive temperatures so that the integrity of the brazed joint is maintained during the material removal operation.  
      It would be additionally desirable to provide a coated cutting tool that uses a brazed-in superhard blank as the cutting element that presents a cutting edge wherein the braze joint between the cutting tool body and the superhard blank is able to withstand the heat generated during the material removal operation through the geometry and design of the superhard blank so as to reduce the exposure of the braze joint to excessive temperatures so that the integrity of the braze joint is maintained during the material removal operation.  
      Finally, it would be desirable to provide a coated cutting tool that uses a brazed-in superhard blank as the cutting element that presents a cutting edge wherein the braze joint between the cutting tool body and the superhard blank is able to withstand the heat generated during the material removal operation through the use of high temperature braze alloys as well as coating schemes that help protect the braze joint from exposure to excessive temperatures.  
     SUMMARY OF THE INVENTION  
      In one form thereof, the invention is a coated cutting tool that comprises a body containing a pocket. The tool further includes a polycrystalline cubic boron nitride blank that is brazed into the pocket using a braze alloy. The braze alloy has a liquidus temperature of at least about 900 degrees Centigrade. There is a coating applied to the cutting tool.  
      In still another form thereof, the invention is a coated cutting tool that comprises a body that contains a pocket wherein a superhard blank is brazed into the pocket using a braze alloy. The braze alloy has a liquidus temperature of at least about 900 degrees Centigrade wherein the braze alloy is selected from the group comprising a nickel-gold braze alloy containing nickel and gold, a copper-gold braze alloy containing copper and gold, and a silver-palladium braze alloy containing silver and palladium. There is a coating applied to the cutting tool.  
      In still another form thereof, the invention is a coated cutting tool for removing material from a workpiece in a material removal operation wherein the coated tool engages the workpiece at a selected depth of cut. The cutting tool comprises a body that contains a pocket. A polycrystalline cubic boron nitride blank has a rake surface, a cutting edge, and a leg extending away from the cutting edge. The blank is brazed into the pocket using a braze alloy whereby there is a braze joint between the body and the polycrystalline cubic boron nitride blank. The braze joint is located a distance away from the rake surface of the polycrystalline cubic boron nitride blank ranging between about 1.5 millimeters and about 4.9 millimeters and the leg having a length that is at least about 1.7 times as great as the depth of cut. A coating is applied to the cutting tool.  
      In still another form thereof, the invention is a coated cutting tool for removing material from a workpiece in a material removal operation. The coated cutting tool comprises a body that contains a pocket. A superhard blank has a rake surface and a cutting edge wherein the blank is brazed into the pocket using a braze alloy whereby there is a braze joint between the body and the blank. The braze alloy has a liquidus temperature. The superhard blank engages the workpiece during the material removal operation so that cutting forces are exerted on the superhard blank and heat is generated at the cutting edge of the superhard blank. The braze joint is located a selected distance away from the cutting edge so that the temperature at the braze joint is less than the liquidus temperature of the braze alloy whereby the braze joint has sufficient shear strength to retain the superhard blank in the pocket during the material removal operation. There is a coating on the cutting tool.  
      In yet another form thereof, the invention is a process for making a coated cutting tool comprising the steps of: providing a body containing a pocket; brazing a polycrystalline cubic boron nitride blank into the pocket using a braze alloy wherein the braze alloy having a liquidus temperature of at least about 900 degrees Centigrade; etching the polycrystalline cubic boron nitride blank with titanium tetrachloride and aluminum chloride gases; and coating the cutting tool by chemical vapor deposition.  
      In yet another form thereof, the invention is a method of removing material in a threading operation from a workpiece comprising a ferrous alloy having a hardness between about 50 Rockwell C and about 65 Rockwell C using a coated cutting tool, the process comprising: providing a coated cutting tool having body containing a pocket where a superhard blank is brazed using a braze alloy into the pocket to form a braze joint wherein the braze alloy having a liquidus temperature of at least about 900 degrees Centigrade wherein the braze alloy is selected from the group comprising a nickel-gold braze alloy containing nickel and gold, a copper-gold braze alloy containing copper and gold, a gold-copper-nickel braze alloy contains gold and copper and nickel, a silver-titanium-copper braze alloy containing silver and titanium and copper, and a silver-palladium braze alloy containing silver and palladium; and engaging the workpiece with the coated cutting tool on multiple passes wherein each pass removes a volume of material substantially equal to or less than the volume of material removed from the workpiece in the previous pass. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The following is a brief description of the drawings that form a part of this patent application:  
       FIG. 1  is an isometric view of one embodiment of a coated cutting tool that comprises a body with a pocket (or a notch) and a polycrystalline cubic boron nitride blank brazed in the pocket wherein the blank is comprised entirely of polycrystalline cubic boron nitride;  
       FIG. 2  is a cross-section view of the coated cutting tool of  FIG. 1  taken along section line  2 - 2  of  FIG. 1 ;  
       FIG. 3  is an isometric view of another embodiment of a coated cutting tool (i.e., a threading tool) that contains a pocket and a polycrystalline cubic boron nitride blank brazed in the pocket wherein the blank comprises a layer of polycrystalline cubic boron nitride attached to a layer of cobalt cemented tungsten carbide, and the coating is a multi-layer coating;  
       FIG. 4  is a cross-sectional view of the coated cutting tool of  FIG. 3  taken along section line  4 - 4  of  FIG. 3  showing, among other things, the multiple layers of the coating scheme and the braze joint;  
       FIG. 5A  is a side schematic view of the coated threading tool of  FIG. 4  that shows thermal gradients therein as would occur during a material removal operation;  
       FIG. 5  is a top schematic view of a portion of a coated threading tool showing selected dimensions of the polycrystalline cubic boron nitride blank;  
       FIG. 6  is a side schematic of the coated cutting tool of  FIG. 5  showing selected dimensions of the polycrystalline cubic boron nitride blank;  
       FIG. 7  is a cross-sectional view of a coated cutting tool wherein the cutting tool body contains a pocket and a polycrystalline cubic boron nitride blank is brazed in the pocket wherein the blank is comprised entirely of polycrystalline cubic boron nitride, and the coating is a multi-layer coating;  
       FIG. 8  is an isometric view of a coated grooving tool wherein the geometry of the tool is of a TOP NOTCH® (TOP NOTCH® is a registered trademark of Kennametal Inc.) design; and  
       FIG. 9  is an isometric view of a coated threading tool wherein the geometry of the tool is of a TOP NOTCH® (TOP NOTCH® is a registered trademark of Kennametal Inc.) design. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to the drawings,  FIGS. 1 and 2  illustrate one embodiment of the coated cutting tool generally designated as cutting tool  20 .  
      Cutting tool  20  has a body  22  that contains a notch or pocket  24 . The body  22  can be made from a number of materials, but one preferred material is cobalt cemented tungsten carbide wherein the cobalt comprises about 6 weight percent of the cemented tungsten carbide and the tungsten carbide comprises about 94 weight percent of the cemented tungsten carbide. The notch  24  is located at one corner of the body  22 , but it should be appreciated that the notch could be at opposite corners or at all four corners of the body  22 .  
      A polycrystalline cubic boron nitride (PcBN) blank  25  is brazed to the body  22  within the notch  24 . The PcBN blank  25  comprises a carbide support  26  on which there is a layer of polycrystalline cubic boron nitride  27 . Although specific compositions of PcBN will be set forth hereinafter, the typical PcBN insert is a mixture of cubic boron nitride and another material such as, for example, titanium carbide, or some other suitable binder material. A braze joint  28  is at the juncture between the body  22  and the PcBN blank  25 . Specific braze alloys will be described hereinafter. The preferred braze alloy for this application is a high temperature braze alloy in which the liquidus temperature (i.e., the lowest temperature at which the alloy is completely liquid) is at least about 900 degrees Centigrade, and even more preferably, the liquidus temperature is at least about 1000 degrees Centigrade. Braze alloys such as those listed in Table 1 are useful high temperature braze alloys. However, as will become more apparent from the discussion hereinafter, braze alloys with lower liquidus temperatures can be appropriate to use in light of the design and geometry of the polycrystalline cubic boron nitride blank that reduces the exposure of the braze joint to excessive temperatures during the material removal operation.  
      The cutting tool has a coating  30  applied thereto. The coating is also shown as a single layer. It is typical, however, that the coating may comprise more than one layer as will become apparent from the specific coating schemes described hereinafter.  
      Referring to  FIGS. 3 and 4 , there is shown another embodiment of a coated cutting tool generally designated as  40 . Cutting tool  40  is one example of a threading tool. Cutting tool  40  includes a body  42  that contains a pocket  44  and an aperture  45 . As discussed above with respect to the body  22  of cutting tool  20 , the body  42  may be made from a number of materials including cobalt cemented tungsten carbide. A polycrystalline cubic boron nitride blank  46  is brazed within the pocket  44 . The PcBN blank  46  comprises a PcBN layer  48  and a cobalt cemented tungsten carbide support  50 . The PcBN layer  48  is affixed to the support  50 . The PcBN blank  46  has a rake surface  52  and flank surfaces  54  wherein the rake surface  52  and the flank surfaces  54  intersect to form a cutting edge  56 . Like for the cutting tool  20  of  FIGS. 1 and 2 , the composition of the PcBN layer may vary depending upon the specific application. One specific composition for the PcBN layer is KD050 wherein KD050 has a composition that comprises about 50 volume percent cubic boron nitride and about 50 volume percent titanium carbide. The support  50  may also comprise any one of a variety of compositions depending upon the application.  
      The PcBN blank  46  has a multi-layer coating applied thereto. In the specific embodiment shown in  FIG. 3  the coating comprises six layers, and the coating scheme is applied in the following steps beginning with preparation of the substrate surface first and applying the coating layer to the substrate surface: (1) etching the substrate surface with titanium tetrachloride and aluminum chloride gases; (2) applying via CVD a coating of titanium nitride (coating layer  56 ) at a temperature of about 900 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 3 micrometers with a preferred thickness that equals about 1 micrometers; (3) applying via CVD a coating of titanium carbonitride (coating layer  58 ) at a temperature of about 880 degrees Centigrade to a thickness that ranges between about 1 micrometers and about 10 micrometers with a preferred thickness that equals about 3 micrometers; (4) applying via CVD a coating of titanium carbonitride (coating layer  60 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 5 micrometers with a preferred thickness that equals about 1 micrometers; (5) applying via CVD a bonding layer of titanium oxycarbonitride and titanium aluminum oxycarbonitride [graded] (coating layer  62 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.3 micrometers and about 5 micrometers with a preferred thickness that equals about 1 micrometer; (6) applying via CVD a coating of alumina (coating layer  64 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 1 micrometer and about 10 micrometers with a preferred thickness that equals about 4 micrometers; and (7) applying via CVD a coating of titanium nitride-titanium carbonitride (coating layer  66 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 6 micrometers with a preferred thickness that equals about 2 micrometers.  
      Referring to  FIGS. 5 and 6 , there is shown the arrangement of a PcBN blank  70  brazed into the pocket  72  of a body  74  for a threading tool generally designated as  76 . The PcBN blank  70  comprises a layer of PcBN  78  and a cobalt cemented tungsten carbide support  80 . There is a braze joint  82  between the PcBN blank  70  and surfaces of the cutting tool body that defines the pocket  72 . The PcBN blank  70  has a rake surface  84  and a flank surface  86 . The rake surface  84  and the flank surface  86  intersect to form a cutting edge  88 . For illustrative purposes the coating is removed from  FIGS. 5 and 6 .  
      A way to optimize the ability of the braze joint to withstand the temperatures that exist at the braze joint during a cutting operation is to locate the braze joint an optimum distance away from the point of contact between the PcBN blank (i.e., the cutting edge) and the workpiece material. The dimensions of the PcBN blank determine the distance between the point of contact and the braze joint. In the case of a superhard blank that comprises a PcBN layer on a cobalt cemented tungsten carbide support, the selection of these dimensions is based on balancing the cost of increasing the size of the layer of PcBN or increasing the size of the support (or a combination of increasing the size of the layer of PcBN and the size of the support) against the extent of thermal protection for the braze joint due to the distance between the point of contact and the braze joint. In the case of a superhard blank that is one piece of PcBN, the size (and hence the cost) of the PcBN piece is balanced against the extent of thermal protection of the braze joint due to the distance between the point of contact and the braze joint.  
       FIG. 5A  is a schematic view that shows thermal gradient lines T max  and T 1  through T 5  for the coated cutting tool  76  as would occur during a material removal operation. The maximum temperature is (T max ) is at the point of contact between the cutting edge of the PcBN blank  70  and the workpiece material. The temperature gradient lines T 1 , T 2 , T 3 , T 4 , and T 5  represent five different temperatures at various distances away from the point of contact. As one can appreciate, the temperature decreases as one moves farther away from the point of contact. The temperature gradient has the following relationship: T 1 &gt;T 2 &gt;T 3 &gt;T 4 &gt;T 5 .  
      The braze joint  82  comprises the joint between the PcBN blank  70  and the pocket  72  contained in the cutting tool body  74  wherein the braze joint  82  has two principal portions. One portion is between the PcBN blank and the seating (or horizontal in  FIG. 5A ) surface of the pocket and the other portion is between the PcBN blank and the backing (or vertical on  FIG. 5A ) surface. These temperature gradient lines show that the braze joint  82  is exposed to the higher temperatures at a location near the point of contact and that the temperature decreases as one moves farther away from the point of contact. By selecting certain dimensions of the PcBN blank that impact upon the location of the braze joint relative to the cutting edge, one can select the temperature that exists at the braze joint or at least provide some degree of thermal protection for the braze joint. In the case of a superhard blank that comprises a support and a PcBN layer, the thickness of the support is typically increased since it is the least expensive component of these two components of the superhard blank. The extent to which the thickness of the support is increased depends upon the cutting temperatures and the properties (e.g., the liquidus temperature and the shear strength) of the particular braze alloy. The thickness of the support should be sufficient so that the temperature that exists at the braze joint, which in  FIG. 5A  would be about T 5 , would be less than the liquidus temperature of the braze alloy and so that the braze alloy would possess sufficient shear strength so as to maintain the integrity of the braze joint so as to thereby retain the PcBN blank to the cutting tool body against the cutting forces that would be exerted on the PcBN blank.  
      In one embodiment as illustrated in  FIGS. 5 and 6 , the length “a” of the PcBN blank  70  is 0.190 inches (4.82 millimeters); the thickness “d” of the PcBN layer  78  is 0.030 inches (0.76 millimeters); the thickness “c” of the support  80  is 0.160 inches (4.1 millimeters); and the overall thickness “b” of the PcBN blank  70  is 0.190 inches (4.83 millimeters). The leg length of the PcBN blank is dimension “e”, and it equals about 0.220 inches (5.59 millimeters).  
      Referring to  FIG. 7 , there is shown a cross-sectional view of a coated cutting tool generally designated as  90  wherein the cutting tool  90  has a PcBN blank  92  brazed into the pocket  94  of the tool body  96 . The PcBN blank  92  is comprised entirely of polycrystalline cubic boron nitride. One exemplary PcBN blank  92  that is comprised entirely of polycrystalline cubic boron nitride is a PcBN material sold under the designation AMBORITE (Grade AMB 90) by Element Six wherein the AMBORITE (Grade AMB 90) PcBN material comprises about 90 volume percent cubic boron nitride and about 10 volume percent comprising the sum of aluminum nitride, titanium diboride and silicon carbide. A coating scheme for the AMBORITE (Grade AMB 90) PcBN material comprises: a chemical vapor deposition coating scheme applied at a temperature between about 970 degrees Centigrade and about 1000 degrees Centigrade having an initial layer of alumina, then a layer of titanium carbonitride, followed by a layer of titanium oxycarbonitride, followed by a layer of titanium aluminum oxycarbonitride, followed by a thick layer of alumina, then a layer of titanium nitride, then a layer of titanium carbonitride, and a final layer of titanium nitride. The thickness of the thick alumina layer was between about 5 and about 6 micrometers. The thickness of the layers beneath the thicker alumina layer was about 3 micrometers, and the thickness of the layers on top of the thicker alumina layer was about 2.5 micrometers.  
      Another coating scheme (shown by brackets  98 ) that is suitable for use with the cutting tool  90  is described as follows: the coating scheme is applied in the following steps: (1) etching the substrate surface with titanium tetrachloride and aluminum chloride gases; (2) applying via CVD a coating of aluminum oxide (coating layer  100 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 10 micrometers with a preferred thickness that equals about 1 micrometer; (3) applying via CVD a coating of titanium nitride (coating layer  102 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 6 micrometers with a preferred thickness that equals about 1 micrometers; (4) applying via CVD a coating of titanium carbonitride (coating layer  104 ) at a temperature of about 990 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 8 micrometers with a preferred thickness that equals about 2 micrometers; (5) applying via CVD a bonding layer of titanium oxycarbonitride and titanium aluminum oxycarbonitride (graded) (coating layer  106 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 6 micrometers with a preferred thickness that equals about 1 micrometers; (6) applying via CVD a coating of alumina (coating layer  108 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 12 micrometers with a preferred thickness that equals about 4 micrometers; and (7) applying via CVD a coating of titanium nitride-titanium carbonitride-titanium nitride (coating layer  110 ) at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 6 micrometers with a preferred thickness that equals about 2 micrometers.  
       FIG. 8  shows a coated cutting tool generally designated as  116 . Cutting tool  116  has a superhard blank  118 . Cutting tool  116  is a grooving tool that is designated as a TOP NOTCH® tool.  
       FIG. 9  shows a coated cutting tool generally designated as  120 . Cutting tool  120  has a superhard blank  122 . Cutting tool  120  is a threading tool that is designated as a TOP NOTCH® tool.  
      Set forth below in Table 1 are typical braze alloys useful for the brazing of the polycrystalline cubic boron nitride blanks to the body of the cutting tool.  
               TABLE 1                          Examples of Useful Braze Alloys                                                     Melting               Composition   Liquidus   Solidus   Point   Shear           (weight   (Degrees   (Degrees   (Degrees   Strength       Braze Alloy   percent)   Centigrade)   Centigrade)   Centigrade)   (pounds)                                             Gold-     50% Au   969   954   954   13,752       Copper     50% Cu       Gold-     35% Au   1028   973   973   12,844       Copper-     62% Cu       Nickel     3% Ni       Gold-   37.5% Au   1015   940   940   13,023       Copper   62.5% Cu       Bau-4     82% Au   948   948   948   21,686       Gold-Nickel     18% Ni       Bau-6 Gold-     70% Au   1045   1006   1006   26,670       Nickel-     22% Ni       Palladium     8% Pd       Silver-     95% Ag   995   970   970   Not Measured       Palladium     5% Pd                  
 
 In the Table 1 above, the shear strength is reported in pounds and is the result of a test in which a one-half inch square carbide is brazed to a carbide member and the braze joint between these carbide members (i.e., a carbide-to-carbide braze joint) is placed in pure shear. The braze joint is continually loaded until failure and the result is given in the pounds at which the braze joint failed. 
 
      As is apparent from the discussion below, Table 2 provides information that is necessary to fully understand the coating schemes that are set forth below in Tables 3 through 7. Tables 3 through 7 contain examples of coating schemes that are applicable to the polycrystalline cubic boron nitride cutting tools of the invention. In these examples, the cutting tools used the KD050 composition of polycrystalline cubic boron nitride. KD050 composition comprised about 50 volume percent cubic boron nitride and about 50 volume percent titanium carbide. Tables 3 through 7 each set forth the details regarding one series of the coating schemes.  
      Referring to Tables 3 through 7, the left hand column identifies the tool number and lists the composition of the polycrystalline cubic boron nitride layer. The composition of the PcBN is either KD050 or KD120 as set forth above.  
      Still referring to Tables 3 and 7, the column entitled “coating scheme” presents the various coating schemes (or arrangements) that were applied to the PcBN blank starting with the coating next to the substrate. The abbreviations have the meanings set out in Table 2 below.  
               TABLE 2                          Meanings of Abbreviations For       Coating Schemes Listed in Tables 3 and 4                     Abbreviation   Meaning               N   High temperature (970-1000° C.) CVD           application of titanium nitride       CN   High temperature (970-1000° C.) CVD           application of titanium carbonitride       MT   Moderate temperature (870-890° C.) CVD           application of titanium carbonitride       A   CVD application of alumina at a           temperature equal to 970-1000° C.       OCN   CVD application of a layer of TiOCN           which is the alpha-alumina modification           layer at a temperature equal to 970-1000° C.       TiAlOCN   CVD application of a layer of TiAlOCN at           a temperature equal to 970-1000° C.       MT-N   Moderate temperature (870-890° C.) CVD           application of titanium nitride                  
 
      In regard to the column entitled “Thickness”, this column sets forth three thickness measurements in micrometers. The first thickness measurement represents the total thickness of the layers of from the substrate to the alumina layer. The second thickness measurement represents the thickness of the alumina layer. The third thickness measurement represents the thickness of the layers on top of the alumina layer.  
      In regard to the column identified as “Braze Alloy”, this is the braze alloy that was used to braze the PcBN blank into the pocket of the body. These braze alloys correspond to those braze alloys listed in Table 1 hereinabove.  
               TABLE 3                          Coating Schemes for One series of Cutting Tools                             Tool No. (MCT8686)/       Thickness           PcBN Composition   Coating Scheme   (micrometers)   Braze Alloy               10 KD050   N/CN/OCN/A/N   4.0/7.6/2.7   Nickel-Gold                  
 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
               
               
                 Coating Schemes for a Second Series of Cutting Tools 
               
            
           
           
               
               
               
               
            
               
                 Tool No. (MCT8787)/ 
                   
                 Thickness 
                   
               
               
                 PcBN Composition 
                 Coating Scheme 
                 (micrometers) 
                 Braze Alloy 
               
               
                   
               
               
                  8 KD050 
                 CN/OCN/ 
                 3.3/7.0/2.4 
                 Nickel-Gold 
               
               
                   
                 TiAlOCN/A/N 
               
               
                 10 KD050 
                 C/CN/OCN/ 
                 5.2/7.0/4.0 
                 Nickel-Gold 
               
               
                   
                 TiAlOCN/A/N 
               
               
                 17 KD050 
                 C/CN/OCN 
                 3.5/4.4/3.4 
                 Nickel-Gold 
               
               
                   
                 TiAlOCN/A/N 
               
               
                 19 KD050 
                 MT-N/MT/CN/ 
                 3.4/9.2/2.2 
                 Nickel-Gold 
               
               
                   
                 OCN/TiAlOCN/ 
               
               
                   
                 A/N 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   
               
               
                 Coating Scheme for a Third Series of Cutting Tools 
               
            
           
           
               
               
               
               
            
               
                 Tool No. 
                   
                   
                   
               
               
                 (MCT9610) 
                 Coating 
                 Thickness 
               
               
                 PcBN Composition 
                 Scheme 
                 (micrometers) 
                 Braze Alloy 
               
               
                   
               
               
                 2/KD050 
                 MT-N/MT/CN/ 
                 5.9/4.0/3.1 
                 Silver-Palladium 
               
               
                   
                 OCN/ 
               
               
                   
                 TiAlONC/A/N 
               
               
                 3/KD050 
                 CN/OCN/ 
                 3.8/6.0/2.4 
                 Silver-Palladium 
               
               
                   
                 TiAlOCN/A/N/ 
               
               
                   
                 CN/N 
               
               
                 5/KD050 
                 MT-N/MT/CN/ 
                 9.4/4.4/3.1 
                 Copper-Gold 
               
               
                   
                 OCN/ 
               
               
                   
                 TiAlONC/A/N 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                   
               
               
                 Coating Scheme for a Fourth Series of Cutting Tools 
               
            
           
           
               
               
               
               
            
               
                 Tool No. 
                   
                   
                   
               
               
                 (MCT9319) 
                   
                 Thickness 
               
               
                 PcBN Composition 
                 Coating Scheme 
                 (micrometers) 
                 Braze Alloy 
               
               
                   
               
               
                  6/KD050 
                 MT-N/MT/CN/ 
                 2.9/5.0/3.8 
                 Copper-Gold 
               
               
                   
                 OCN/ 
               
               
                   
                 TiAlOCN/ 
               
               
                   
                 A/N 
               
               
                 14/KD050 
                 CN/OCN/ 
                 4.8/10.5/2.5 
                 Copper-Gold 
               
               
                   
                 TiAlOCN/A/N/ 
               
               
                   
                 CN/N 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                   
               
               
                 Coating Scheme for a Fifth Series of Cutting Tools 
               
            
           
           
               
               
               
               
            
               
                 Tool No. 
                   
                 Thickness 
                   
               
               
                 PcBN Composition 
                 Coating Scheme 
                 (micrometers) 
                 Braze Alloy 
               
               
                   
               
               
                 1/KD050 
                 MT-N/MT/CN/ 
                 3.4/3.8/2.4 
                 Silver- 
               
               
                   
                 OCN/TiAlOCN/A/N 
                   
                 Palladium 
               
               
                 2/KD050 
                 MT-N/MT/CN/ 
                 3.5/2.1 
                 Silver- 
               
               
                   
                 OCN/TiAlOCN/A 
                   
                 Palladium 
               
               
                   
               
            
           
         
       
     
      Another coating scheme comprises the following steps: (1) etching the blank surface with titanium tetrachloride and aluminum chloride gases; (2) applying via CVD a coating of titanium nitride at a temperature of about 900 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 3.0 micrometers with a preferred thickness that equals about 1 micrometers; (3) applying via CVD a coating of titanium carbonitride at a temperature of about 880 degrees Centigrade to a thickness that ranges between about 1.5 micrometers and about 10.0 micrometers with a preferred thickness that equals about 4 micrometers; (4) applying via CVD a coating of titanium carbonitride at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 5 micrometers with a preferred thickness that equals about 1 micrometers; (5) applying via CVD a bonding layer of titanium oxycarbonitride at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.1 micrometers and about 4 micrometers with a preferred thickness that equals about 0.3 micrometers; (6) applying via CVD a coating of titanium aluminum oxycarbonitride at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.1 micrometers and about 4.0 micrometers with a preferred thickness that equals about 0.5 micrometers; (7) applying via CVD a coating of alumina at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 1 micrometer and about 10 micrometers with a preferred thickness that equals about 4.0 micrometers; and (8) applying via CVD a coating of titanium nitride-titanium carbonitride at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.5 micrometers and about 8 micrometers with a preferred thickness that equals about 2 micrometers. The composition of the polycrystalline cubic boron nitride was KD050. The braze alloy that was used was copper-gold or silver-palladium.  
      Another coating scheme comprises the steps of: (1) applying by CVD (880 degrees Centigrade) a layer of titanium nitride to a thickness that ranges between about 0.5 micrometers and about 3 micrometers with a preferred thickness that equals about 1 micrometers; (2) applying by moderate temperature CVD (880 degrees Centigrade) a layer of titanium carbonitride to a thickness that ranges between about 1 micrometers and about 10 micrometers with a preferred thickness that equals about 2 micrometers; (3) applying by high temperature CVD (1000 degrees Centigrade) a layer of titanium carbonitride to a thickness that ranges between about 0.5 micrometers and about 6 micrometers with a preferred thickness that equals about 1 micrometers; (4) applying via CVD a bonding layer of titanium oxycarbonitride at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.1 micrometers and about 4.0 micrometers with a preferred thickness that equals about 0.3 micrometers; (5) applying via CVD a bonding layer of titanium aluminum oxycarbonitride at a temperature of about 1000 degrees Centigrade to a thickness that ranges between about 0.1 micrometers and about 4.0 micrometers with a preferred thickness that equals about 0.5 micrometers; and (6) applying by CVD (990 degrees Centigrade) a layer of alumina to a thickness that ranges between about 1 micrometers and about 10 micrometers with a preferred thickness that equals about 2 micrometers. The composition of the polycrystalline cubic boron nitride was KD050. The braze alloy that was used was silver-palladium.  
      Applicants further contemplate the use of a coating scheme that contains at least one layer comprising a nitride, carbide, carbonitride, oxynitride, oxycarbonitride, or oxycarbide of one or more of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, aluminum, and silicon. This coating scheme includes as a possible layer a layer of titanium aluminum nitride applied by physical vapor deposition.  
      In testing applicants have found that the coated cutting tools that use the high temperature braze alloys, especially when coupled with the coating schemes containing layers of MT-TiN/MT-TiCN/HT-TiCN/TiOCN/TiAlOCN/Al 2 O 3  and, MT-TN/MT-TiCN/HT-TiCN/TiOCN/TiAlOCN/Al 2 O 3 /HT-TiCN/HT-TiN, have shown excellent performance results. Along this line, applicants have found that the results are very good in the hard turning of hard steels such as D3 tool steel.  
      Applicants have also found that the coated cutting tools that have coatings such as, for example, titanium aluminum nitride or alumina, use the design and geometry of the superhard blanks disclosed herein, regardless of the braze alloy used to braze the superhard blank to the cutting tool body, also have experienced improved performance. It appears that the design and geometry of the superhard blank have resulted in the reduction of the exposure of the braze joint to the temperatures generated at the point of contact between the cutting tool and the workpiece material.  
      One specific method of threading that has provided excellent results when using these cutting tools (i.e., threading tools) is a constant volume threading method. In this method, the depth of the pass is constantly decreased so that the volume of material removed from the workpiece is constant for each pass. To achieve this constant volume one takes the infeed per pass formula: accumulated depth=initial depth of cut (doc)·(No. pass) 1/2  to arrive at the depth of cut for each pass. The Table 8 below sets forth an example of this method showing the first four passes. Additional passes determined per the calculation are necessary to obtain an external thread depth of 0.0789 inches.  
               TABLE 8                          Sample Calculations for a Constant Volume Threading Method       [8 pitch external thread had a depth of .0789 inches]                             Pass           Infeed depth of       No.   First Calculation   Second Calculation   cut for the pass               1   25% of .0798 = 0.197   NA   .0197       2   .0197 · 2 ½  = .0278   .0278 − .0197 = .0082   .0082       3   .0197 · 3 ½  = .0341   .0341 − .0278 = .0063   .0063       4   0197 · 4 ½  = .0394   .0394 − .0341 = .0053   .0053                  
 
 The constant volume threading method can be described as a method of removing material in a threading operation from a workpiece comprising a ferrous alloy having a hardness between about 50 Rockwell C and about 65 Rockwell C using a coated cutting tool. The method has the following steps: providing a coated cutting tool having body containing a pocket where a superhard blank is brazed using a braze alloy into the pocket to form a braze joint wherein the braze alloy having a liquidus temperature of at least about 900 degrees Centigrade wherein the braze alloy is selected from the group comprising a nickel-gold braze alloy containing nickel and gold, a copper-gold braze alloy containing copper and gold, a gold-copper-nickel braze alloy contains gold and copper and nickel, a silver-titanium-copper braze alloy containing silver and titanium and copper, and a silver-palladium braze alloy containing silver and palladium; and engaging the workpiece with the coated cutting tool on multiple passes wherein each pass removes a volume of material that is substantially equal to the volume of material removed from the workpiece in the previous pass. 
 
      While the constant volume threading method is the preferred method of threading, applicants contemplate that one or more threading passes may remove either a lower volume of material than calculated per the formula or a lower volume of material than removed in the previous pass. Thus, such a method includes the step of engaging the workpiece with the coated cutting tool on multiple passes wherein each pass removes a volume of material substantially equal to or less than the volume of material removed from the workpiece in the previous pass.  
      Applicants note that there is a United States patent application by the same inventors filed on the same day as this patent application and entitled UNCOATED CUTTING TOOL USING BRAZED-IN SUPERHARD BLANK.  
      All patents, patent applications, articles and other documents identified herein are hereby incorporated by reference herein. Other embodiments of the invention may be apparent to those skilled in the art from a consideration of the specification or the practice of the invention disclosed herein. It is intended that the specification and any examples set forth herein be considered as illustrative only, with the true spirit and scope of the invention being indicated by the following claims.