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CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application relies upon U.S. Provisional Patent Application No. 60/604,201, filed on Aug. 24, 2004, and entitled “High Chrome/Nickel Milling with PDC Cutters.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     This invention is in the field of mills used for downhole milling of metal objects, in an oil or gas well.  
         [0005]     2. Background Art  
         [0006]     Metals with over 12% chrome, or over 6% nickel, or over 12% chrome in combination with over 6% nickel, present unique cutting challenges. Specifically, such metals require substantially more energy to cut, and produce significantly greater cutter wear than conventional metals. They exhibit unusually high friction between the cutter and the base material being cut, and between the cutter and the chip being formed. Previously known methods of cutting these materials involved the use of tungsten carbide cutters. When these materials are being cut with these conventional tungsten carbide cutters, the cutters suffer heavy breakage, wear quickly, require considerably more load and torque, and tend to vibrate or chatter severely. These tungsten carbide cutters exhibit a relatively high coefficient of friction with the high chrome and high nickel materials. The high coefficient of friction increases torque by increasing the sliding force between the cutter and the base material or substrate.  
         [0007]     The high friction also increases the force required to make the chip flow up the tungsten carbide cutter face. This increased force requirement directly increases the cutting load, as the force required to hold the tungsten carbide cutter down into the cut is increased. This increased force requirement also indirectly increases the cutting load, as the chip tends to form a hard “ball” of the material being cut around the outer edge of the tungsten carbide cutter, which effectively blunts the cutter edge.  
         [0008]     An important result of the higher friction, higher load, and duller edge is that considerable heat is generated at the cut by the tungsten carbide cutters. This heat increases the wear rate of the conventional tungsten carbide cutters, and the elevated temperature generally increases the strain-to-failure characteristics of the high chrome/high nickel materials, which further increases the cutting energy required.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention comprises the downhole cutting of high chrome/high nickel materials with a cutting tool which is dressed with polycrystalline diamond (PDC) cutters as the primary cutting elements.  
         [0010]     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]      FIG. 1  is a side elevation view of a first embodiment of a mill used according to the present invention;  
         [0012]      FIG. 2  is a partial side elevation view of a second embodiment of a mill used according to the present invention;  
         [0013]      FIG. 3  is a lower end elevation view of the mill shown in  FIG. 2 ; and  
         [0014]      FIG. 4  is a third embodiment of a mill used according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The PDC material of the cutters used according to the present invention, being composed largely of diamond, exhibits less wear by virtue of its hardness, but more importantly has a much lower coefficient of friction with the high chrome/high nickel materials. The low coefficient of friction reduces torque by reducing the sliding force between the cutter and the base material or substrate. The low friction also reduces the force required to make the chip flow up the cutter face. This low friction directly reduces the cutting load, as the force to hold the cutter down into the cut is reduced, and it indirectly reduces the cutting load, as the chip does not tend to form a hard “ball” of the material being cut around the outer edge. This effectively leaves a sharper cutter edge. An important result of the lower friction, the lower load, and a sharper edge is that considerably less heat is generated at the cut, by the PDC cutters. This lower heat generation decreases the wear rate of the PDC cutters, and the lower temperature generally decreases the cutting energy required.  
         [0016]     As shown in  FIG. 1 , a first embodiment of a mill  10  for use in the present invention has a mill body  12 , on the periphery of which are arranged and mounted a plurality of PDC cutting elements  14 . The PDC cutting elements  14  can be arranged in a plurality of spiral rows  16 , arranged generally along the axial dimension of the mill body  12 . The mill  10  is designed to be rotated in a selected direction, clockwise when viewed from the top end  18 , for the mill shown. The cutting face  20  of each cutting element  14  is oriented toward the direction of rotation. The distance  22  between any two adjacent cutting elements  14  in any given row  16 , measured along a line parallel to the longitudinal axis of the mill body  12 , is a minimum of 20% of the diameter of the cutting face  20  of either of the two adjacent cutting elements  14 .  
         [0017]     As shown in  FIG. 2 , the spiral rows  16  of cutting elements  14  can be paired, with each such pair having a leading row  24  and a trailing row  26 , with the leading row  24  being in front of the trailing row  26 , relative to the selected direction of rotation. The cutting elements  14  in the trailing row  26  can be aligned to follow in the cutting paths established by respective cutting elements  14  in the leading row  24 . The cutting elements  14  can be substantially cylindrical, with circular cutting faces  20  as shown, or they can have other shapes, without departing from the present invention. The cutting elements  14  can be mounted directly on the mill body  12  as shown, or they can be mounted on cutting blades (not shown) on the mill body, as is known in the art. The cutting elements  14  can be mounted on the lateral periphery  28  of the mill body  12 , and they can be mounted on the lower end face  30  of the mill body  12 , as shown in  FIG. 3 . The mill body  12  can be generally tapered from a larger diameter at the upper end  18  thereof to a smaller diameter at the lower end  30  thereof, or it can be cylindrical or any other shape known in the art.  
         [0018]     As shown in  FIG. 4 , a fourth embodiment of the mill used in the present invention can have a mill body  32  with an extended tapered shape, with an even greater axial separation between any two adjacent cutting elements  14  in a given row  16 .  
         [0019]     In operation, according to the present invention, the mill  10  is rotated while contacting the item to be milled, which is made of a material having a high chrome/high nickel content. The PDC cutting elements  14  remove chips or cuttings of metal from the substrate of the item being milled. Because of the greater hardness and lower coefficient of friction exhibited by the PDC cutting elements, as compared to the prior art tungsten carbide cutting elements, the required torque to turn the mill is less, the required vertical force applied is less, and the cutting edges of the cutting elements  14  remain sharper. In addition, less heat is generated than with the prior art tungsten carbide cutting elements, and the rate of cutting element wear is less than with the prior art tungsten carbide cutting elements.

Summary:
A method of downhole milling of an item having high chrome/high nickel content, using polycrystalline diamond cutting elements, exhibiting less wear, requiring less torque and weight, and producing less heat, than prior art tungsten carbide elements.