Abstract:
A grinding head for a grinding machine is disclosed in which the axial bore of the head is provided with flutes of variable width. The dimension of the flutes from an upstream location to a downstream location of the head is variable to provide different effects in operation of the grinding machine. Flutes may be wider in areas of the head where greater shear is expected or may be narrower in width to decrease backpressure. Flutes may be primarily located adjacent to or along an increased diameter area of the head and may be constructed by casting.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/728,565, filed Oct. 20, 2005, the contents of which are hereby incorporated by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to a grinding head of a meat grinder, and more particularly, relates to improved design and function of parts of a grinding head that improve the meat grinding process in terms of ease of disassembly and reassembly, safety, increased quality and output, reduced cost of production of parts, and reduced need for replacement parts. 
   DISCUSSION OF THE RELATED ART 
   The general structure of grinding machines is well known. Typically, a grinding machine has a hopper into which the material to be ground is placed, a grinder portion, including a grinding head, a mounting ring, a bridge, and a collection tube. A feed screw is located within the grinding head to advance material in the hopper through the head. A knife assembly is mounted at the end of, and rotates with, the feed screw and, in combination with the orifice plate, serves to grind material that is advanced toward the orifice plate by the feed screw. The feed screw has a bore at its downstream end into which a center pin is inserted. The center pin extends through a central passage of the knife assembly, and through a bushing that is positioned in a central opening of the orifice plate. A collection cone is located downstream of the orifice plate and is secured to the bushing. The orifice plate is comprised of an outer section having a plurality of grinding apertures and an inner section having at least one collection passage. The collection passage or passages of the orifice plate lead to a collection structure defined by the collection cone, which generally includes a collection cavity and a discharge passage. An orifice plate guard is located downstream from the orifice plate and maintains the collection structure in place, and a mounting ring holds the guard against the orifice plate and mounts the intervening structures to the body of the grinding head. 
   BACKGROUND OF THE INVENTION 
   Improvements in grinding machines are generally directed at one of four goals: (1) improved separation of hard materials from useable materials and increased output of useable materials; (2) ease of disassembly and reassembly of the grinding head; (3) operator safety; and (4) reduction of costs in terms of production and replacement of parts. 
   The quality of meat produced by a grinding machine is limited by its ability to remove hard materials from the useable materials. Naturally, it is preferable if this can be done in a way that maximizes output of useable materials. Modifications of prior meat grinders that improve separation of hard materials while also improving output of useable materials are highly desirable. 
   Because grinding machines are intended for use with food products, frequent disassembly is required for maintaining sanitation. The various parts of the grinding machine must therefore be readily disassembled and accurately reassembled for maximum efficiency. Modifications of existing meat grinders that improve an operator&#39;s ability to disassemble the grinder parts and that assure proper reassembly of the parts are therefore also highly desirable. 
   Naturally, operator safety is also a concern for owners and operators of meat grinders alike. Modifications of present meat grinders that improve safety, especially when those improvements do not detract from overall cost or efficiency, are also desirable. 
   Finally, various parts of a grinding machine are subject to tremendous force and rotational stresses, and wear to these parts is expected. However, the overall cost of grinding machines and various replacement and wear parts is typically very high. Modifications that reduce the costs of producing various parts or that reduce wear, and thus frequency of the need for replacement parts, are therefore also desirable. 
   The present invention contemplates modifications to a meat grinding machine that maximizes the output of useable ground material without sacrificing quality, improves efficiency in disassembly and reassembly of the machine, improves operator safety, and reduces overall production costs and costs required for replacement parts. 
   SUMMARY OF THE INVENTION 
   In one aspect of the grinding machine of the present invention, a grinding head defines an axial bore, and the bore has a plurality of flutes. The width of the flutes is variable across the length of the bore, and is dimensioned to perform various functions. For example, the flutes may be dimensioned to generally decrease in width from the upstream end of the bore to the downstream end of the bore, or may be increased in size in areas of high shear, or may be adjusted across the angles of the bore, as the situation demands. Not only does the variable dimensioning of flutes within the bore of the grinding head control the flow of material through the head, the provisions of flutes in the head is also cost-effective since flutes can be cast along with head rather than being machined in the head or requiring additional parts, such as bars, to be welded to the head. 
   In another aspect of the grinding machine of the present invention, assembly of the grinding head is simplified and made consistent between grinder operators. Because the grinder head must be frequently disassembled and reassembled for cleaning, ease of assembly and consistent reassembly is desirable. One aspect of the grinding machine of the present invention includes provision of a stop portion within the bore of the grinder head so that the orifice plate can be inserted to the correct depth within the bore with each reassembly sequence. In another aspect of the grinding machine of the present invention, a tensioning device is mounted between the feed screw and knife assembly for application of constant pressure, urging the knife assembly against the orifice plate. This ensures that the knife assembly contacts the orifice plate with sufficient force to grind material as desired, but prevents premature wear of the grinder parts. 
   In an aspect of the grinding machine of the present invention that eases disassembly of the grinder head for cleaning, recesses such as slots are provided on the outer edge of the orifice plate, and corresponding removal recesses may be provided at the adjacent end of the grinder head. The combination of the orifice plate slots and the grinder head recesses allows an operator to insert a tool into one of the grinder head recesses to access an orifice plate slot and apply leverage to the orifice plate, thus removing it from the opening of the head despite any ground material that may have become lodged between the parts. Two or more corresponding orifice plate recesses and grinder head recesses are provided around the diameter of the orifice plate and adjacent grinding head for application of leverage at more than one location. 
   In yet another aspect of the grinding machine of the present invention, the grinding machine has improved ability to separate hard material, such as bone and gristle, from soft ground material because pieces of hard material are too large to pass through the grinding openings of the orifice plate. The knife inserts push these pieces of hard material toward the center of the plate by rotation of the knife assembly. It has been known to remove hard material from the primary stream of ground material through use of hard material collection passages located inwardly on the orifice plate relative to the grinding openings. Furthermore, providing the collection passages with ramped entryways opening onto the surface of the orifice plate to shear the hard material and to encourage movement of hard pieces through the collection passages has been effective. In a further improvement of this system, flutes are provided along the ramped entryway leading from the surface of the orifice plate to the collection passage. The raised areas of the flutes provide friction that helps keep pieces of hard material within the recessed area of the ramped entryway, while the grooved aspect of the flutes encourages migration of hard material toward the collection passages. In addition to increasing efficiency of hard material collection, the use of fluted entryways decreases production costs of the orifice plate, since a conventional end mill can be used to form the flutes rather than requiring machined entryways. 
   Another aspect of the orifice plate includes a secondary grinding section located inwardly on the orifice plate relative to the grinding openings, along with collection passages. Again, because hard material is pushed toward the inner section of the plate by the rotating motion of the knife assembly, but is carried in a substantial quantity of soft, usable material, further separation of soft, usable material is desirable. Providing a secondary grinding section at the intersection of the orifice plate allows additional soft material to be routed to the main ground material stream rather than being collected in the hard material collection passages for further processing or discard. 
   Alignment of the orifice plate within the opening of the grinding head has been discussed in relation to improving the ease of disassembly for cleaning. In addition, alignment of the orifice plate in a particular orientation with respect to the grinding head is required when secondary grinding sections are provided, since the downstream collection apparatus will necessarily have an irregular shape, allowing additionally acquired ground materials to enter the main stream of ground materials. In some embodiments, the collection apparatus downstream of the orifice plate also bears collection channels that must be aligned with the collection passages of the plate. In order to ease assembly of the grinder and ensure proper alignment of the orifice plate within the grinder head, a self-correcting installation feature is provided. The self-correcting feature preferably comprises a pair of lugs on the head portion and a corresponding pair of recesses on the orifice plate. One of the lugs is preferably larger than the other, and is preferably sufficiently larger than the other to allow a user to readily visually identify which lug corresponds to which recess. In any case, the orifice plate cannot be inserted if the operator misjudges the sizes of the lugs and recesses and the orifice plate is not correctly oriented. 
   In an aspect designed to improve safety for the operator without detracting from the ease of use of the machine, the invention contemplates a self-correcting plate guard mounting arrangement. Guards are typically used to ensure that a grinder operator cannot intentionally or inadvertently access the grinder head during use, yet allow the operator maximum visibility in order that he or she may monitor progress of the grinding operation. To that end, an orifice plate having small grinding openings, can be used with a guard having larger openings, while an orifice plate having larger grinding openings requires the use of a more closed guard. Each guard is provided with studs for mounting within apertures on an orifice plate, and the corresponding apertures of the orifice plate will accept only studs from guards rated safe for the particular orifice plate. As with the self-correcting installation of the orifice plate in the grinding head, this is accomplished through stud size. It is contemplated that a plate with relatively large grinding openings will only accept small studs of restricted guards. Less restrictive guards are available for orifice plates having smaller apertures, but the more highly restrictive guards can be used as well. In addition, the mounting ring is sized so that it cannot be tightened sufficiently without a guard present. This ensures maximum flexibility of use of guards while requiring appropriate guard use. 
   In yet another aspect of the present invention, a system is provided in order to extend the life of certain parts that are used in the machine. Wherever moving parts are employed, wear is to be expected. However, wear can be distributed over an assembly of parts by providing evenly spaced projections and recesses between any two parts in a rotating assembly. For example, the bushing held in place at the center bore of the orifice plate has traditionally been held in place by way of a single key-and-keyway arrangement. However, over time, the single key-and-keyway is subjected to wear and, despite the operability of the remainder of the part, would require replacement. In this aspect of the present invention, a plurality of evenly radially spaced projections and corresponding evenly radially spaced channels or recesses increases the life of the bushing despite consistent wear stresses in one location, since the bushing is randomly inserted into the plate in any number of different positions at each reassembly. Similarly, the pin inserted in the central bore of the feed screw has been improved by providing a plurality of radially evenly spaced recesses and corresponding keys or projections for the knife holder. The random installation of the knife holder on the pin extends the life expectancy of the pin. 
   After hard material is removed from the main ground material stream via the collection passages, it is still carried in a substantial quantity of soft, useable material. Another aspect of the grinding machine of the present invention contemplates a helical discharge passage provided in the collection structure downstream of the orifice plate that improves separation of hard material by providing a highly restricted flow toward the discharge passage. As a result, useable material tends to remain in the collection cavity of the collection structure, while primarily hard material is discharged. 
   The various features and aspects of the present invention as summarized above may be incorporated in a machine separately from each other, and each provides certain advantages in improving operation in terms of ease of disassembly and reassembly, safety, increased quality and output, reduced cost of production of parts, and reduced need for replacement parts. It is also understood that the various features and aspects may be incorporated in separate combinations or altogether. 
   Various other features, objects and advantages of the present invention will be made apparent from the following detailed description taken together with the drawings, which together disclose the best mode presently contemplated of carrying out the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings, in which like reference numerals represent like parts throughout, and in which: 
       FIG. 1  is an isometric view of a grinding machine incorporating the various aspects of the present invention; 
       FIG. 2  is an exploded view of the grinder head, showing each internal and external part (except the collection tube), with reference to line  2 - 2  of  FIG. 1 ; 
       FIG. 3  is a sectional side view showing a portion of the head taken along line  3 - 3  in  FIG. 2 ; 
       FIG. 4  is a close-up sectional side view of a portion of the orifice plate taken along line  4 - 4  of  FIG. 3 ; 
       FIG. 5  is a close-up sectional side view of a portion of the head and orifice plate, taken along line  5 - 5  of  FIG. 3 , and showing use of a tool to remove the orifice plate from the head; 
       FIG. 6  is a close-up sectional side view of a portion of the head, orifice plate, bridge, and mounting ring taken along line  6 - 6  of  FIG. 3 ; 
       FIG. 7  is section view, taken along line  7 - 7  of  FIG. 3 , showing the orifice plate mounted in the head; 
       FIG. 8  is a top plan view of the inner section of the orifice plate shown in  FIG. 7 ; 
       FIG. 9  is a partial isometric view of the orifice plate as shown in  FIG. 8 ; 
       FIG. 10  is a close-up isometric view of the edge of the orifice plate seated in the grinder head; 
       FIG. 10-A  is an alternate view of the grinder head and orifice plate showing use of a removal tool; 
       FIG. 10-B  is a view similar to  FIG. 10   a , shown with the orifice plate removed from the grinder head; 
     FIGS.  10 -C- 10 -J show alternate embodiments of the removal feature of the orifice plate as in  FIGS. 10-A  and  10 -B; 
       FIG. 11  is an isometric view of the grinder head of a preferred embodiment of the present invention, showing the variable flutes located in the bore of the head; 
       FIG. 12  is a longitudinal sectional view of the grinder head shown in  FIG. 11 ; 
       FIG. 13  is an alternate embodiment of the orifice plate of one aspect of the present invention showing a secondary grinding section; 
       FIG. 14  is a close-up detail view taken along line  14 - 14  in  FIG. 13 ; 
       FIG. 15  is an isometric view of a first orifice plate and plate guard in accordance with one aspect of the present invention; 
       FIG. 16  is an isometric view of a second orifice plate and plate guard; 
       FIG. 17  is a close-up sectional view of the connection between the orifice plate and orifice plate guard shown in  FIG. 15 ; 
       FIG. 18  is a close-up sectional view of the connection between the orifice plate and orifice plate guard shown in  FIG. 16 ; 
       FIG. 19  is a close-up sectional side view of a portion of the orifice plate shown in  FIG. 16  and a portion of the orifice plate guard shown in  FIG. 15 , showing that the orifice plate guard of  FIG. 15  cannot be installed on the orifice plate of  FIG. 16 ; 
       FIG. 20  is a close-up sectional side view of the orifice plate shown in  FIG. 15  and the orifice plate guard shown in  FIG. 16 , showing the mismatched connection; 
       FIG. 21  is a sectional side view of a preferred embodiment of the collection cone of the present invention; 
       FIG. 22  is an end view of the collection cone shown in  FIG. 21 , taken from the upstream end; and 
       FIG. 23  is a sectional view of the connection between the pin and the knife holder, taken along lines  23 - 23  of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   1. Resume 
   A grinding machine  50  is generally shown in  FIG. 1 . Grinding machine  50  has a hopper portion  52  and a grinder portion  54 . Grinder portion  54  includes a housing or head  56 , a mounting ring  58 , a bridge  60 , and a collection tube  62 . 
   Referring now to  FIG. 2 , head  56  is generally tubular and a feed screw  64  is rotatably mounted within head  56  so that, upon rotation of feed screw  64  within head  56 , meat or the like is advanced from hopper  52  through the interior of head  56 . A knife holder  68  is mounted at the end of, and rotates with, feed screw  64 . Knife holder  68  has six arms  70   a - f  and six knife inserts, one corresponding to each of arms  70   a - f , although it is understood that any number of arms and corresponding inserts may be employed. 
   Referring now to  FIG. 3 , knife holder  68  is located adjacent an inner grinding surface of an orifice plate  74 , which is secured in the open end of head  56  by mounting ring  58  and bridge  60 . The knife inserts bear against the inner grinding surface of orifice plate  74 . In accordance with known construction, the end of head  56  is provided with a series of external threads  76 , and mounting ring  58  includes a series of internal threads  78  adapted to engage external threads  76  of head  56 . Mounting ring  58  further includes an opening  80  defining an inner lip  82 . While a threaded connection between mounting ring  58  and head  56  is shown, it is understood that mounting ring  58  and head  56  may be secured together in any satisfactory manner. 
   Bridge  60  includes an outer, plate maintaining portion  84  and an inner, collection assembly maintaining portion  86  as shown in  FIG. 2 . Outer portion  84  of bridge  60 , which further includes an outwardly extending shoulder  88  adapted to fit within lip  82 , is held within ring  58  and shoulder  88  engages the outer peripheral portion of orifice plate  74  to maintain orifice plate  74  in position within the open end of head  56 , as most clearly seen in  FIG. 6 . Inner portion  86  of bridge  60  is generally tubular and retains a collection cone  90  at its upstream end and collection tube  62  at its downstream end. 
   A center pin  92  has its inner end located within a central bore  94  formed in the end of feed screw  64 , shown in  FIGS. 7 and 9 , and the outer end of center pin  92  extends through a central passage  96  formed in a central hub area of knife holder  68  and through the center of a bushing  98 . Bushing  98  supports center pin  92 , and thereby the outer end of feed screw  64 , and also functions to maintain collection cone  90  in position against the outer surface of orifice plate  74 . As best seen in  FIG. 23 , center pin  92  is keyed to feed screw  64  by means of recessed keyways  100  on center pin  92  that correspond to keys  102  on the hub of knife holder  68 . With this arrangement, center pin  92  rotates in response to rotation of feed screw  64 , driving knife assembly  66 . Bushing  98  and orifice plate  74  remain stationary, and rotatably support the end of center pin  92  to which an auger  108  is secured. As further seen in  FIGS. 21 and 22 , collection cone  90  includes a collection cavity  104  and a discharge passage  106 . Auger  108  is driven by feed screw  64 , and extends through collection cavity  104  and into and through discharge passage  106 . Discharge passage  106  empties into collection tube  62 . 
   2. Head Flute Profile Variation 
   Referring now to  FIGS. 3 ,  11  and  12 , head  56  is generally tubular and thus comprises an axial bore  109  in which feed screw  64  is rotatably mounted. Bore  109  is typically provided with flutes  110  for controlling the flow of material through head  56 , i.e. for preventing material from simply rotating with feed screw and for providing a downstream flow path to prevent backpressure from pushing material back into hopper  52 . 
   In a preferred embodiment of the present invention, the dimension of flutes  110  is varied along the flute length to produce different effects. For example, decreasing the size of flutes  110  in the direction of material flow can increase production rates while reducing the potential for material backflow between flutes  110 . Flutes  110  may also be increased in size in areas of high pressure in order to provide additional strength. Flutes  110  can also have an increased width in areas of high shear, where material slipping in feed screw  64  can destroy the material (such as by extracting fat) rather than merely grinding the material. In addition, flutes  110  could also vary in depth in either an upstream direction or a downstream direction. 
   Note that head  56  may have an increased diameter at its downstream end. Flutes  110  may be primarily located adjacent or along this increased diameter area. Flutes  110  may be dimensioned to move material more efficiently across the transition area between the main body of head  56  and the increased diameter area of head  56 . Other modifications to the dimensions of flutes  110  across their length or across the angles of bore  109  could match the requirements of specific functional areas. Advantageously, flutes  110  can be cast along with head  56 , which is an easier and less costly process than the current production method, which requires heads to have areas machined flat or have rolled bars welded therein. 
   3. Constant Force Assembly 
   Frequent disassembly and reassembly of grinder  54  is required for maintaining sanitary conditions. In the past, the force applied by knife assembly  66  against orifice plate  74  has been adjusted by screwing ring  58  onto head  56  during reassembly. Different operators have inevitably assembled the grinder differently after cleaning, which results in different operation since the force applied by the knife inserts  72  on the orifice plate  74  is determined by the position of the ring  58  on the head  56 . For example, when ring  58  is not advanced to at least a certain point, knife assembly  66  could fail to contact orifice plate  74  with sufficient force, and no (or unsatisfactory) cutting action would occur. On the opposite extreme, when ring  58  is tightened too far, knife inserts  72  and the grinding surface of orifice plate  74  wear prematurely. Variations between these extremes result in various degrees of sub-optimal operation and wear of grinder  54 . 
   To reduce the variations due to operator assembly, in the present invention, head  56  is provided with an interior shoulder or stop  111 , best seen in  FIGS. 3 and 6 , against which orifice plate  74  is seated when ring  58  is advanced onto head  56  during assembly. Stop  111  provides a positive stop for orifice plate  74  at a predetermined optimum position within head  56 , so that orifice plate  74  cannot be forced against knife assembly  66  by overtightening or other operator adjustment. In addition, an operator can know not to stop advancing orifice plate  74  until it engages stop  111 , which provides the operator with immediate feedback that orifice plate  74  is in the desired position within head  56 . 
   Referring to  FIG. 3 , a spring pack  112  is located between feed screw  64  and knife assembly  66  to provide a constant pressure between knife assembly  66  and orifice plate  74  when orifice plate  74  is seated against stop  111  upon advancement of ring  58 . Spring pack  112  preferably consists of a Belleville-type spring washer assembly, but could also use coil springs. A spacer washer  114  holds spring pack  112  in place on center pin  92  and out of contact with feed screw  64 . Alternately, a spring assembly may be mounted behind the center pin. 
   4. Orifice Plate Removal Slots 
   As noted above, frequent disassembly of the various parts of grinder  54  is required for cleaning. In operation, it is common for ground material to become lodged between the interior surfaces of head  56  and the annular outer surface  116  of orifice plate  74 , making removal of plate  74  from head  56  difficult. An operator would be required to tap or pound on plate  74  until it became dislodged, a practice which is time consuming and creates potential for damage to orifice plate  74 . 
   As seen in  FIGS. 5 ,  7 ,  10 ,  10 -A, and  10 -B, in the present invention, plate  74  is provided with removal recesses or other relief areas that enable plate  74  to be removed relatively easily from head  56 . The recesses or relief areas may be in the form of slots  118 , and head  56  may be provided with corresponding removal recesses or grooves  120 . When it is time to disassemble grinder  54  for cleaning, an operator can insert a simple removal tool  122  into one of grooves  120  to access one of slots  118  and apply leverage to orifice plate  74  against the surface of groove  120 , easily removing it from the opening of head  56 . Tool  122  is designed to fit grooves  120  and slots  118 , and may be in the form of a bar having a bent end although it is understood that any other suitable lever could also be used. 
   Head  56  is provided at its opening with lugs  124 , and orifice plate  74  is provided with corresponding recesses  126  within which lugs  124  are received, to ensure proper positioning of orifice plate  74  within the open end of head  56  such that slots  118   a ,  118   b  are aligned with grooves  120   a ,  120   b . Alternatively, it is contemplated that grooves  120   a ,  120   b  may be eliminated. In this embodiment, slots  118  in the side surface of orifice plate  74  are positioned so as to be exposed when mounting ring  58  is removed. That is to say, slots  118  have a sufficient width such that a portion of each slot  118  extends outwardly of the end of grinder head  56 , and can be accessed by tool  122  upon removal of mounting ring  58 . In this embodiment, tool  122  is levered against the end edge of grinder head  56  to apply an outward force on orifice plate  74 . 
   Further alternate embodiments of the plate removal slots  118  are shown in  FIGS. 10C-10-J , such as provision of a single slot  118  rather than a plurality of slots about the circumference of orifice plate  74 ; provision of a single slot  118  of varying dimensions; provision of a continuous slot  118  or multiple continuous slots  118  around the side edge of orifice plate  74 ; provision of a drilled hole serving as removal slot  118 ; and provision of a slot  118  that opens onto the grinding surface of orifice plate  74 . Each of these embodiments may have advantages and disadvantages that may dictate for or against use in a given circumstance. For example, the continuous slot(s)  118  shown in  FIGS. 10-D  and  10 -E are more expensive to produce than some of the other embodiments, but have the advantage of not requiring alignment with any corresponding structures, such as grooves  120 , of grinding head  56 . Conversely, the embodiment shown in  FIG. 10-I  is relatively inexpensive to produce, but may require greater care in reassembly to assure alignment with a corresponding structure of grinding head  56 , may require a non-standard tool  122  for removal, and may require additional effort for removal. 
   5. Fluted Collection Passages 
   Referring now to  FIG. 7 , orifice plate  74  has an outer section  128  that includes a large number of relatively small grinding openings  130 , and an inner section  132  that includes a series of radially spaced collection passages  134 . The size of grinding openings  130  varies according to the type of material being ground and the desired end characteristics of the ground material. In accordance with known grinding principles, material within head  56  is forced toward orifice plate  74  by rotation of feed screw  64  and through openings  130 , with rotating knife assembly  66  acting to sever the material against the inner grinding surface of orifice plate  74  prior to the material passing through openings  130 . 
   In some instances, pieces of hard material, such as bone or gristle, which are too large to pass through grinding openings  130 , will be present along with the useable material. These pieces, which are not readily cut by the action of knife inserts  72   a - f  against plate  74 , are pushed toward inner section  132  of plate  74  by the rotating action of knife assembly  66 , where the pieces of hard material can be removed from the primary ground material stream through collection passages  134 . Collection passages  134  are large relative to grinding openings  130 , and, as best seen in  FIGS. 7 and 8 , are preferably generally triangular, though other shapes are certainly possible. Each of collection passages  134  is provided with a ramped entryway  136  opening onto the surface of orifice plate  74 . 
   In the past, collection passages have been provided with smooth ramped entryways devised to encourage movement of hard pieces toward and through the collection passages. In order to encourage hard materials that migrate to inner section  132  to enter and move through collection passages  134 , the present invention includes a ramped entryway  136  having a series of axial flutes or grooves  138 , additionally shown in  FIGS. 8 and 9 . Flutes  138  provide a high friction surface that serves to maintain the pieces of hard material within the recessed area defined by the ramped entryway  136 , and also function to guide material in an axial direction along ramped entryway  136  toward collection passage  134 . In addition, flutes  138  can be formed in orifice plate  74  in a process using repetitive passes of a conventional end mill. This production process is relatively simple in comparison to the machining process required to form the smooth ramped entryways as used in the past, thus providing the additional advantage of lowering the cost of production of the orifice plate  74 . 
   Referring back to  FIG. 3 , collection passages  134  lead through plate  74  to a collection cone  90 , which keeps material that enters passages  134  separate from the primary ground material stream. Collected material accumulates in collection cone  90 , where it can be subjected to a secondary grinding and/or separation process to maximize ground material output. 
   Ramped entryways  136  are provided on both sides of plate  74 , which is double sided to double the lifetime of use of plate  74 , and plate  74  is provided with a wear indicator  140  on each side. Wear indicators  140  are shallow recesses located at the edge of plate  74  so that the operator can visualize when a particular plate is so worn that it should be turned or, if both wear indicators  140  indicate worn surfaces, the operator will be alerted to replace plate  74  altogether. 
   6. Alternate Orifice Plate Providing Secondary Grinding 
   Another embodiment of orifice plate  74  is shown at  74 ′ in  FIGS. 13 and 14 , and like parts are indicated by the same reference number with the addition of the prime symbol. In this embodiment, inner section  132 ′ of plate  74 ′ has additionally been provided with two secondary grinding sections  142 . Secondary grinding sections  142  have smaller grinding openings  144  than the primary grinding openings  130 ′ in outer section  128 ′, although it is understood that secondary grinding openings  144  may have any other size relative to the primary grinding openings  130 ′. To accommodate the placement of secondary grinding sections  142  in inner section  132 ′, preferably only one of the three collection passages  134 ′ is provided with a ramped entryway  136 ′. 
   Because hard material is carried in a substantial quantity of soft, usable material, in this embodiment, material that is pushed toward inner section  132 ′ has another opportunity to enter the primary material stream via secondary grinding sections  142 . While hard material is being routed toward and into collection passages  134 ′, knife inserts  72   a - f  continue to rotate and shear materials at inner section  132 ′ of plate  74 ′, processing the materials into smaller portions and further separating hard material from the soft material to which it is attached. Thus, during the process of separating and removing hard material, additional usable material is acquired. Such material is small enough to enter secondary grinding openings  144 , and is introduced into the main ground material stream rather than being collected in the collection cone such as  90  (not shown in  FIGS. 13 and 14 ) for subsequent separation from unusable material. In this embodiment, the collection cone (not shown) is modified to cover only the portion of inner section  132 ′ having collection passages  134 ′, and leaves the downstream surface of orifice plate  74 ′ exposed at secondary grinding sections  142  in order to allow material that passes through openings  144  to return to the usable material stream. 
   7. Self-Correcting Orifice Plate Installation 
   As previously discussed with reference to removal of orifice plate  74  from the opening of head  56 , head  56  is provided with lugs  124  and plate  74  is provided with recesses  126  so that on assembly, plate  74  will be oriented in head  56  to ensure that removal slots  118  and removal grooves  120  are aligned. In addition, when plate  74 ′ having secondary grinding sections  142  is used, the collection cone (not shown) has a shape that allows it to collect materials from collection passages  134 ′ but leaves secondary grinding sections  142  exposed. Orifice plate  74 ′ and the collection cone (not shown) must therefore also be aligned. 
   In order to ensure alignment of orifice plate  74 ′ and the collection cone (not shown) with each assembly of grinder  54 , each of lugs  124 ′ and each of recesses  126 ′ are also preferably of a different size. As seen in  FIG. 7 , a larger lug  124   a ′ corresponds with a larger recess  126   a ′ and a smaller lug  124   b ′ corresponds with a smaller recess  126   b ′ so that when an operator assembles grinder  54 , plate  74 ′ will only fit into head  56  in one way. The size difference between recesses  124   a ,  124   b  and lugs  126   a ,  126   b  is preferably large enough to allow a user to visualize the proper orientation of orifice plate  74 ′, and to position plate  74 ′ in head  56  properly on the first attempt. For example, in the illustrated embodiment, one recess is approximately 2 inches long and the other is approximately 1.5 inches long. However, if the operator should misjudge the sizes and attempt to replace plate  74 ′ in the wrong orientation, the operator will quickly realize that orifice plate  74 ′ is improperly oriented and will correct its orientation so that it fits properly within head  56 . 
   8. Self-Correcting Plate Guard Mounting 
   In a conceptually similar vein, the present invention provides a plate guard installation system that requires the operator to install a plate guard and further to install the correct guard for the orifice plate being used. As seen in  FIGS. 15 and 16 , plate guards  146  are carried on bridge  60  and have openings  148  and studs  150 . Guards  146  are used to ensure that an operator or other personnel cannot access the area of grinder head  56  adjacent the outer surface of orifice plate  74  when orifice plate  74  has grinding openings  130  that exceed a predetermined size, e.g. ¼ inch or more. It is generally advantageous to use a guard  146  that provides maximum visibility so that the operator can view the product as it is being ground, so an orifice plate  74  having small grinding openings  130  allows the use of a guard  146  with larger openings  148 , while an orifice plate  74  having larger grinding openings  130  requires the use of a guard  146  with smaller openings  148 . 
   Referring to  FIGS. 17-18 , studs  150  are designed to be received within a pair of apertures  152  located on orifice plate  74 . In order to ensure that an operator installs a plate guard  146 , mounting ring  58  is sized so that it cannot be tightened sufficiently into engagement with stop  111  without the presence of guard  146 . Furthermore, studs  150  and mounting apertures  152  are sized so that each guard  146  is matched to a particular orifice plate  74 . As illustrated in  FIGS. 15 and 16 , plates  74   a  having small grinding openings  130   a  thus have large apertures  152   a  matching the large studs  150   a  of relatively unrestricted guards  146   a , while plates  74   b  having larger grinding openings  130   b  have smaller apertures  152   b  matching the smaller studs  150   b  of relatively restricted guards  146   b . With this construction, the smaller studs  150   b  of a restricted guard can either be mounted to a plate with small grinding openings  130   a  (with large apertures  152   a ), as seen in  FIG. 18 , or a plate having larger grinding openings  130   b  (with small apertures  152   b ), as seen in  FIG. 20 . However, a plate  74  with larger grinding openings  130   b  (and small apertures  152   b ) can only accept the smaller studs  150   b  of the restricted guard  146   b . As a result, an operator cannot operate grinder  54  without a guard  146  in place, and if an operator tries to use a less restrictive guard than recommended for the size of grinding opening of the plate being employed, the studs of the guard will not fit in the apertures of the plate, as seen in  FIG. 19 , and the correct, more restrictive guard must be installed before grinder  54  can be assembled in an operative manner. 
   9. Wear-Reducing Bushing and Center Pin Design 
   At the interface between moving parts of grinder  54 , there are substantial forces and pressure between the parts that cause the parts to wear. For example, as previously discussed, the rotating action of knife assembly  66  against orifice plate  74  causes wear of knife inserts  72   a - f , which can be replaced, and also wear on plate  74 , which is two-sided to double its lifetime of use and which bears wear indicators  140  so an operator can visualize the degree of wear. 
   Wear also occurs between orifice plate  74  and bushing  98 , and between feed screw  64  and center pin  92 . In prior systems, the bushing was held in place within the center bore of the plate and the pin was held in place within the center bore of the feed screw by way of a single pin or key/keyway arrangement. Over time, pressure on the bushing and pin caused them to wear and, because of the single orientation of the parts, the wear pattern occurred primarily in one location due to the pressures and forces experienced during operation. Although only one location was worn, the entire part would have to be replaced. 
   In the present invention, the life of bushing  98  and pin  92  is extended by allowing alternate positions for each part, thus distributing wear more evenly and extending part life. As seen in  FIG. 9 , bushing  98  is preferably provided with a number of projections  154  and orifice plate  74  is provided with a corresponding number of recesses or channels  156 . In the illustrated embodiment, bushing  98  has three projections  154  and orifice plate  74  has three channels  156 , although it is understood that any number of projections and channels may be used. When grinder  54  is disassembled for cleaning and reassembled, bushing  98  is randomly inserted into plate  74  in any of three positions. Over the life of bushing  98 , the random insertion in one of three positions allows the part to wear evenly and triples its life expectancy. If desired, however, the operator may note the locations of the projections and channels prior to each disassembly, and take appropriate steps upon reassembly to ensure that bushing  98  is assembled to orifice plate  74  in a different orientation. 
   Likewise, as shown in  FIG. 23 , pin  92  is preferably provided with three recessed keyways  100  and knife holder  68  is provided with a corresponding number of keys  102 . Knife holder  68  is mounted in turn on feed screw  64  as shown in  FIGS. 2 and 3 . When grinder  54  is disassembled and reassembled, pin  92  is inserted in central bore  94  of feed screw  64 , and knife holder  68  is placed in position on pin  92  in any of three positions. Over the life of pin  92 , random installation of knife holder  68 , which rotates with feed screw  64 , in one of the three positions allows pin  92  to wear evenly and extends its life expectancy. If desired, however, the operator may note the locations of the keys and keyways prior to each disassembly, and take appropriate steps upon reassembly to ensure that knife holder  68  is placed in position on pin  92  in a different orientation. 
   This feature of the present invention contemplates the provision of a corresponding number of projections and recesses at evenly spaced radial and circumferential locations between any two parts in a rotating assembly that is capable of being disassembled and reassembled, in order to distribute wear due to forces and pressures between the parts during operation of the assembly. While this feature of the invention has been shown and described in connection with the interface between the bushing and the orifice plate, as well as between the center pin and the knife holder, it is contemplated that a similar arrangement may be provided between any two parts that are adapted to be non-rotatably assembled together in any assembly. 
   10. Helical Discharge Passage 
   As previously discussed, hard material is carried in a substantial quantity of soft, usable material. As a result, in prior hard material collection systems, this has resulted in collection cavity  104  of collection cone  90  containing a quantity of usable material that would preferably not be discharged into collection tube  62  via discharge passage  106 . To prevent as much usable material as possible from entering the discharge passage, the present invention includes a discharge passage  106  ( FIG. 21 ) having a single, helical discharge flute  158 . Flute  158  is helical in the direction of rotation of auger  108 , and defines a discharge path for material advanced by rotation of auger  108 . Helical flute  158  is formed in the peripheral wall that defines passage  106 , which is sized relative to auger  108  to cooperate with the outer edges of flights  160  of auger  108  to provide a highly restricted flow of material from cavity  104  to tube  62 . In this manner, the hard material is advanced through discharge passage  106  by rotation of auger  108  while the restriction provided by the size of the passage side wall and the outer edges of the flights of auger  108  provides sufficient backpressure to prevent soft material from entering collection cavity  104 . 
   In addition, in another embodiment of the present invention, collection cavity  104  is replaced by discrete channels  156  that lead from collection passages  134  to cone  90 . Channels  156  have side walls  162  so that hard material particles move directly toward auger  108 . Particles thus have another opportunity to be sheared by the revolution of auger  108  against walls  162  and reduce the size of the hard material particles lodged in channels  156  before the particles are supplied to helical discharge flute  158 .