A mill cutting structure is differently configured in three zones. Those zones are the center, the outer edge and in between. At the center has highly wear resistant material that has good temperature bond strength and high impact resistance. The outer periphery can have a material that is highly resistant to wear and impact. In between can be inserts such as used in the Metal Muncher® mills using sintered carbide shapes that resist tracking and create a chipping rather than a grinding action. The shapes should have high edge retention capability and shapes such as a double sided pyramid can be used. The wear patterns of prior designs are addressed to allow longer and faster milling of the fish.

FIELD OF THE INVENTION 
     The field of the invention is subterranean milling tools and more particularly those mills that employ discrete cutting structures at different locations on the mill body to address the expected type of wear unique to that location so that overall milling effectiveness is improved. 
     BACKGROUND OF THE INVENTION 
     There are occasions where downhole devices such as packers or bridge plugs or cement shoes are milled out. Other times there is a tubing string or portion of a tubing string that needs to be cut so that subsequent operations can continue. Over time the design of such mills has evolved to address the need for greater speed and cutting efficiency. In the 1980s Baker Hughes came out with a line of mills known as Metal Muncher® as illustrated in U.S. Pat. Nos. 5,038,859 or 5,086,838; 4,796,709 and 5,456,312. 
     One example of this design is shown in  FIGS. 1 and 2 . The typical mill of this type had a body  10  with a central flow passage  12  that lead to a plurality of outlets  14  shown on the bottom face view of  FIG. 2 . A series of spaced apart vertical blades  16  had their leading face covered with a nested array of round inserts  18  made of a hardened cutting material such as tungsten carbide. These inserts were arranged in rows such as  20  and  22  and as one row would wear away with the blade that supported it the next row would take over the cutting task. Fluid such as drilling mud would be pumped through the outlets  14  located ahead of the inserts  18  on a given blade  16  in the direction of rotation. A matrix material  24  is deployed behind the blades  16  for structural support and for some limited cutting capability. The cuttings made by the inserts have to clear the outside edge of the mill and are carried off by the circulating fluid that also removes some of the heat generated from the milling operation. 
     Another prior mill design in three styles is illustrated in  FIGS. 3-5 . Here there are no blades and the matrix material with the crushed carbide particles in seen in a symmetrical array of pie shapes  26  about a center where there is no matrix or carbide particles. Each of the pie shapes has the identical formulation as the others. In some applications there are fluid outlets  28  or  30  illustrated to carry off cuttings and heat generated from the milling operation. 
     The common theme to these prior designs is symmetry about a center of the mill and uniformity of the cutting structure regardless of the position on the mill. While there was some intuitive rationale behind symmetry, the demands on different locations of a mill are not symmetrical and in certain cutting applications the limitations of such prior designs were made apparent. 
     The center of the mill has very low relative speed to the surface being cut and is a region where there is high abrasion and heat generation. In the previous designs this region tended to core badly as the matrix softened from heat and abrasion and then sloughed off to create the coring effect. As the core formed the cutting around the middle of the mill body deteriorated and as a result of that the ability of the mill to advance into the fish being milled was also impeded. The fish itself developed a peak which was the negative of the shape of the core that formed in the center bottom of the mill where the matrix was abraded off. It should be noted that in some milling applications such as when a packer with a hollow mandrel is milled there is little wear in the center of the mill as the packer mandrel is hollow. However, as the packer slips release their grip during milling the orientation of the packer can shift and the coring effect can be seen. 
     When chunks of the packer break off such as broken pieces of slips and the circulating fluid has to carry the cuttings to the edge of the body and then up the sides through recesses or water courses so that the cuttings can be recovered at the surface what results is high impact loading at the transition between the bottom and side of the mill such that the edge gets rounded off. This removal of the cutting structure from the periphery impedes the cutting ability of the mill. This effect can also require a trip in the hole for mill replacement which, particularly in offshore locations, can be a very expensive proposition. 
     The present invention focuses on tailoring the cutting structure to the nature of the expected wear on different parts of a mill. Thus the center of the mill uses a more abrasion resistant material to combat coring but the shapes of the cutting structure can be more rounded and less aggressive as most of the serious cutting occurs further away from the mill center. The outer periphery is made more impact resistant with a somewhat more aggressive cutting structure than the center of the mill. This is designed to control the rounding at the edges and associated loss of cutting structure adjacent the outer periphery. In between where the bulk of the cutting takes place the cutting structure is configured to be more robust and more highly resistant to having chunks of carbide to break off. As a result the carbide shapes in the matrix have more blunt cutting edges as opposed to the carbide at the periphery where there are longer extending segments and sharper angles so that there is a greater impact resistance with a decreased emphasis on cutting ability. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated figures while recognizing that the full scope of the invention is to be found in the appended claims. 
     SUMMARY OF THE INVENTION 
     A mill cutting structure is differently configured in three zones. Those zones are the center, the outer edge and in between. At the center has highly wear resistant material that has good temperature bond strength and high impact resistance. The outer periphery can have a material that is highly resistant to wear and impact. In between can be inserts such as used in the Metal Muncher® mills using sintered carbide shapes that resist tracking and create a chipping rather than a grinding action. The shapes should have high edge retention capability and shapes such as a double sided pyramid can be used. The wear patterns of prior designs are addressed to allow longer and faster milling of the fish. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a Metal Muncher® prior art mill; 
         FIG. 2  is a bottom view of  FIG. 1 ; 
         FIG. 3  is a bottom view of a prior art junk mill; 
         FIG. 4  is a bottom view of a prior art junk mill; 
         FIG. 5  is a bottom view of a prior art junk mill; 
         FIG. 6  is a bottom view of a mill of the present invention showing the locations of the different cutting structures; 
         FIG. 7  is a detailed view of an insert shape that is best used at the mill center or the mill periphery; 
         FIG. 8  is a detailed view of an insert best used between the center and the periphery and preferably on the leading cutting surface; 
         FIG. 9  is a detailed view of an insert shape best used in moderate wear areas between the center and the periphery and in a trailing location to the shape of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 6  shows the bottom view of a mill  32  that has a central zone  34  and a plurality of leading peripheral spaced apart zones  36  as determined by the direction of rotation represented by the arrow  38 . In between are leading  40  and trailing  42  cutting regions where most of the cutting takes place and the chips off the fish are formed. The trailing region is also disposed behind the peripheral zones  36  in the direction of rotation. A series of ports  44  border the trailing region  42  and are there to allow pumped fluid to drive the cuttings to the edges where they can make a turn uphole through gaps such as  46 . The cutting elements  48  in zone  34  are illustrated in detail in  FIG. 7 . This shape is formed from a starting shape of a sphere and cut with a cylindrical drill that passes through the shape at four perpendicular orientations. This leaves end rounded shapes  50  and  52  that are spherical and are separated by four part cylindrical walls of which three are visible in  FIG. 7  namely  54 ,  56  and  58 . The idea, regardless of the fabrication technique is to create some cutting edges in an otherwise fairly rounded shape so that the impact resistance is high while the penetration into the fish from such shapes dispersed in a matrix in zone  34  is fairly low. The main objective in zone  34  being to withstand the impact loads and heat generated and to remain in position long enough to prevent coring the center zone  34  by having parts of the carbide shapes break off. In past designs the inserts that were best suited for cutting in zones  40  or  42  were also used in the central zone where the loading was different so that the performance of those inserts such as  18  in  FIG. 2  when in the center of the mill did not optimally prevent coring. In the present invention the inserts  48  are fairly rounded but still have some cutting edges such as  60  and  62  in generally parallel planes and other cutting edges such as  64 ,  66  and  68  that are generally parallel to each other and in planes approximately perpendicular to edges  60  and  62 . Insert  48  can also be used in the peripheral zones  36  where the ability to resist high impact is more significant than cutting ability. Thus with a modest amount of cutting edges and a rounded overall shape coupled with placement at the bottom center zone  34  and the peripheral locations  36  the problems associated with the prior designs and their uniform insert distribution are avoided. At the center zone  34  the rounded shape tolerates high impacts such as can occur when milling a string inside another string where the inner string being milled is laying up against the outer string so that the mill center is over a wall portion of the fish trying to mill it out. The center zone  34  can receive impact loads as the mill bounces against the pipe wall of the fish. Heat is also generated there and the material for the matrix as well as the insert is selected to tolerate this expected heat load. The insert material can be tungsten carbide, cubic boron, polycrystline diamond compacts and other hard material cutting elements and the matrix in which it is embedded can be brazing materials like nickel bronze, silver solder, copper bronze and other braze materials having a significant wetting ability and high yield strengths. 
     The leading zone for creating the chips off the fish is  40 . Here it is desirable to have a robust structure that can hold an edge or generate an edge as the milling progresses. The insert  70  shown in  FIG. 8  is preferred for this service. Also suitable are cylindrically shaped inserts and inserts where cutting edges are defined by surfaces that meet at 90 or more degrees. In the illustrated embodiment it features opposed truncated pyramid shapes  72  and  74  on opposed sides of a rectangular block  76 . Cutting edges abound in this shape and its limited protrusion distance from the block  76  makes the pyramid shape edges such as  78 ,  80   82  and  84  more likely to remain in position rather than be chipped off. What characterizes the optimal shape for zone  40  is a massive core structure regardless of shape with protruding and opposed extending shapes that have multiple cutting edges that are generally shorter than the edge dimensions of the core. The extending shapes tend to have a height in the order of magnitude of the smaller core dimension and long and thin shapes are avoided in favor of short bulky shapes with a center of mass closer to the core periphery than the smallest dimension of the core. 
       FIG. 9  illustrates a design of lesser strength than in  FIG. 8  and one that is best suited for the zone marked  42  in  FIG. 6 . These designs are characterized by cutting edges where surfaces meet at an angle of under 90 degrees. The bulk of the cutting is taking place in zone  40  where more robust shape with blunter angles such as shown in  FIG. 8  is used. The trailing zone  42  does not require the same strength because the cuttings are already for the most part initiated in zone  40 . Another advantage of putting a less aggressive shape of  FIG. 9  behind the shape of  FIG. 8  is when the demand for what is being milled changes. For example, when milling a packer body the need for strength initially is there as the mandrel and body are milled away. When the seal and slips are reached the cutting demand is different. The slips are typically cast iron that breaks more easily. Very aggressive cutting shapes such as in  FIG. 8  work far less well in trying to cut a rubber shape. A less aggressive structure minimizes balling of the rubber or composite on the cutting structure. The less aggressive structure is also suitable for milling cast iron slips. 
     In the present invention the various zones  34 ,  36 ,  40  and  42  use the described shapes randomly disposed in a matrix that acts as a binder. Over time different inserts oriented randomly extend from the binder as the binder wears away and as pieces of the inserts wear or get broken off. The present invention seeks to address the different needs of different portions of a mill at a given time by presenting shapes in discrete zones that differ from each other and at the same time meet the cutting and durability needs of the specific zones. Thus the edges  86  and  88  that come to a sharp angled point  90  are more suited to a backup zone such as  42  where strength is less important as criteria for longevity than in primary cutting areas  40 . Additionally, if the wear rate of zone  40  is carefully matched to the penetration such as through a packer so that much of zone  40  has worn by the time the slips and the sealing element are being milled then it is more advantageous to have zone  42  evolve into a primary role when the cutting demand for that specific mill location evolves with time. 
     The contrast to be drawn with the prior mill designs is stark. Instead of selecting a single shape or different sizes of the same shape to deploy on a mill face as an overall compromise decision for the anticipated application, the present invention seeks to tailor specific zones on a mill to their discrete loading issues as the milling progresses. This concept applies to a specific point in time during a milling operation as well as taking into account how the needs of those discrete zones evolve as milling changes from a packer body to packer slips or a sealing element, for example. In that sense, different shapes are disposed to back each other up in the direction of rotation whether the cutting structure is on the bottom of a mill or on a blade. In each zone the shapes are randomly integrated into a binder matrix so that their orientations in the matrix are varied. Yet the less aggressive and more rounded shapes such as in  FIG. 7  best serve the expected demands of the center and periphery of the mill where impact loads are high and cutting demand is fairly low. As previously stated there is minimal relative rotation at the center and in most cases little cutting demand such as when there is a central hollow mandrel that remains near the mill center during milling. At the edges, the cuttings need to make the turn into the fluid courses up the side of the bit and high impact loads there are best resisted by a milder cutting structure that can tolerate the impact load while providing some cutting ability. By putting the milder cutting structure at the center the tendency to core the mill at the center is reduced. At the periphery, loss of the edge near the mill bottom due to impact loading is reduced while some limited cutting action can still take place. In the region in between the primary cutting chore is handled by a more blunt cutting shape that has higher strength by virtue of a more compact shape that avoids long and narrow edges and small angle sharp points such as 30-55 degrees. In a backup function behind the primary zone  40  a less aggressive shape with angles in the 30-55 degree range disposed in zone  42  can be useful due to the reduced cutting demand determined by the location. Additionally by being located behind the primary and more aggressive shape of the zone  40 , the backup shape of zone  42  can be counted on to take up a more primary load after some milling has worn away zone  40  shapes and the needs for the mill have changed for the zones  40  and  42  as slips or rubber seals need to be milled up as opposed to a steel body that started off the operation. 
     The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below.