referring to fig1 , grinding machine 50 has a hopper section 52 and a grinder section 54 which are designed to receive and reduce material , which may be frozen blocks of an edible material such as frozen beef , pork , poultry , or fish . the frozen blocks are reduced by a feed screw assembly 56 , which includes a feed screw 58 , shown in fig2 , and which extends through the grinder section 54 . the feed screw assembly 56 includes a drive motor contained within a motor housing 60 that is designed to rotate the feed screw 58 . the grinding machine 50 also includes a bulkhead 62 into which the reduced material is fed and collected , as known in the art . it is understood that the grinding machine 50 illustrated is representative and that the present invention may be used with other types of grinding machines . referring now to fig2 , grinder section 54 includes a main housing section 64 and a feed section 66 . a grinding head section 68 extends forwardly from feed section 66 . feed screw 58 extends throughout the length of main housing section 64 , feed section 66 and grinding section 68 . feed screw 58 includes pressure righting 70 that advances the material through main housing section 64 and through feed section 66 and grinding section 68 upon rotation of feed screw 58 . an orifice plate 72 is secured to the end of grinding section 68 via a mounting ring 74 , in a manner as is known . a bridge 76 extends outwardly from mounting ring 74 . feed section 66 is generally tubular and extends forwardly from main housing section 64 . feed screw 58 and feed section 66 are configured such that the end of feed screw 58 extends outwardly from feed section 68 and through grinding section 68 , such that the end of feed screw 58 is located adjacent to the inner surface of orifice plate 72 . referring now to fig3 , a knife holder 78 is mounted at the end of , and rotates with , feed screw 58 . knife holder 78 may hold one or more knife blades or inserts 79 , in a manner as is known . knife holder 78 is located adjacent an inner grinding surface of orifice plate 72 , which is secured in the open end of head section 66 by mounting ring 74 and bridge 76 . the knife inserts 79 bear against the inner grinding surface of orifice plate 72 to shear material as the material is advanced by operation of feed screw 58 from grinding section 68 toward and through the orifices of orifice plate 72 . the end of grinding section 68 is provided with a series of external threads 80 , and mounting ring 74 includes a series of internal threads 82 adapted to engage external threads 80 of feed section 68 . mounting ring 74 further includes an opening 86 defined by an inner lip 88 . while a threaded connection between mounting ring 74 and feed section 68 is shown , it is understood that mounting ring 74 and feed section 68 may be secured together in any satisfactory manner . bridge 76 includes an outer plate maintaining portion 90 , which has an outwardly extending shoulder 92 adapted to fit within lip 88 so that bridge 76 is held within ring 74 . shoulder 92 engages the outer peripheral portion of orifice plate 72 to maintain orifice plate 72 in position within the open end of grinding section 68 . a center pin 94 has its inner end located within a central bore 96 formed in the end of feed screw 58 , and the outer end of center pin 94 extends through a central passage 98 formed in a central hub area of knife holder 78 and through the center of a bushing 100 . bushing 100 is received within an opening 101 in orifice plate 72 and supports center pin 94 , and thereby the outer end of feed screw 58 . center pin 94 is keyed to feed screw 58 by means of recessed keyways on center pin 94 that correspond to keys on the hub of knife holder 78 . an inner portion 102 of bridge 76 defines a pin support 103 within which the end of a center pin 94 is received . with this arrangement , center pin 94 rotates in response to rotation of feed screw 58 , driving knife assembly 78 . bushing 100 and orifice plate 72 remain stationary , and rotatably support the end of center pin 94 . as noted above , feed section 68 provides an internal chamber in which feed screw 58 rotates to shear the frozen block material . conventionally , the internal chamber is defined by a wall along which chunks of material , which are sheared from the frozen block of material , are moved through main section 64 . the sheared chunks of material typically rotate upon rotation of the feed screw 58 until discharged . referring now to fig4 - 9 , feed section 68 has a primary longitudinal shear edge 104 . the shear edge 104 runs along the length of the main section 64 , and is positioned generally along the backside 106 of an internal chamber 108 defined by main section 64 . as particularly illustrated in fig6 , the shear edge 104 is positioned below the inlet 105 into the chamber 108 . as the feed screw 58 rotates counter - clockwise within chamber 108 , sheared chunks of frozen material will be rotated along with the pressure flighting 70 of the feed screw 58 , similarly in a counter - clockwise direction . as the sheared chunks are rotated they will be forced against the primary shear edge 104 . the primary shear edge 104 thus effectively provides a pinch point against which the frozen blocks are forced and held . as such , the primary shear edge 104 provides a fulcrum point against which further shearing of the frozen blocks may take place , thereby reducing the side load on the feed screw 58 . primary shear edge 104 is also effective in holding the frozen chunks in internal chamber 108 , thereby avoiding the “ bouncing around ” allowed by conventional hopper and grinder assemblies in which the hopper wall is tangential to the housing wall . in addition , feed section 68 includes a secondary shear edge 112 at the forward end of main section 64 , which provides an additional fulcrum point against which a frozen block of material may be sheared as the material is advanced from main section 64 toward feed section 66 . while the primary shear edge 104 extends longitudinally along the length of the main section 64 , secondary shear edge 112 extends transversely relative to the longitudinal axis of the feed section 66 and , as shown in fig7 , extends to a plane that is below that of the shear edge 104 . the secondary shear edge 112 extends transversely across the internal chamber 108 , at the forward area of internal chamber 108 , upstream of feed section 68 . as such , in addition to providing an additional point against which frozen blocks may be held for improved shearing , the secondary transverse shear edge 112 prevents frozen blocks from being prematurely translated forward by the feed screw 58 , since the blocks of material must be reduced to a size that is less than the distance between the underside of the shear edge 112 and the exterior surface of the feed screw 58 . in yet a further aspect , head section 66 includes a tertiary shear edge 114 forward of the secondary shear edge 112 ( relative to the front of the feed screw 58 ) that provides an additional fulcrum point against which the frozen block material may be held . in addition , the tertiary shear edge 114 prevents frozen blocks from passing to the front of the head section 66 until they are reduced to a size that allows them to fit between the underside of the shear edge 114 and the exterior surface of the feed screw 58 . moreover , for blocks sized to fit between the tertiary shear edge 114 and the feed screw 58 , the underside of the shear edge 114 is angled to form an axially extending pinch point 116 , as shown particularly in fig8 - 9 , against which a block may be forced by the pressure flights 70 of the feed screw 58 for additional shearing . it is understood that the terms “ primary ”, “ secondary ”, and “ tertiary ” are not terms of relative importance , but simply terms to distinguish the shear edges from one another . additionally , it is contemplated that the head section 66 may be constructed to have one , all , or some combination of the primary , secondary , and tertiary edges . as particularly shown in fig6 , head section 66 includes an expansion or transition zone 118 defined at the front or discharge end . the expansion zone 118 provides a volume into which reduced blocks may be translated by the feed screw 58 until subsequently discharged by continued translation of the feed screw 58 . in addition , the expansion zone 118 is believed to improve material distribution in the head 66 and around the feed screw 58 . in one embodiment , the secondary shear edge 112 and the tertiary shear edge 114 are positioned in the expansion zone 118 . referring now to fig1 - 13 , according to another aspect of the invention , feed screw 58 has a knife holder reinforcement fin 120 preferably for each arm of the knife holder 78 . each fin 120 forms a wall that is recessed into the feed screw 58 such that a recess 122 is formed between the pair of fins . the recess is adapted and configured to receive the knife holder 78 . more particularly , each fin 120 includes a portion that is located behind a respective knife holder arm 124 to provide support for the knife holder arm 124 during the shearing process . this support helps to prevent material flow within the head 66 from forcing the knife holder 78 into orifice plate 72 , which otherwise may cause premature wear of the knife inserts . each fin 120 also includes a portion that is located alongside and parallel to a respective knife holder arm 124 , to reinforce the knife holder arm against side loads experienced during the shearing process . each knife holder arm 124 is slotted to receive a knife or blade 79 in a manner that allows the blades 79 to be easily replaced as needed . referring to fig1 , each fm 120 is specially configured to relieve side loads experienced by the knife holder arms 124 . the fighting 70 of auger 58 defines a pair of ramped end areas 130 , and each fin 120 is at the end of one of the ramped end areas 130 . on the leading side of knife arm 124 , the fin 120 extends radially outwardly to the outer edge of the auger fighting 70 so as to fully protect the leading side of the knife arm 124 . the ramped end area 130 at the end of the fighting 70 leads to the leading side of the fm 120 , so that only the portion of the knife insert 179 extending from the fin 120 and the knife holder arm 124 is exposed in order to shear the material against the orifice plate 72 . auger 58 also defines a pair of outwardly extending arm reinforcement sections 132 , each of which is spaced from one of the fins 120 . each arm reinforcement section 132 terminates at a location spaced inwardly from the outer edge of the auger flighting 70 . auger 58 also defines a discharge surface 134 that extends from each arm reinforcement section 132 . each discharge surface 134 is configured to as to route material from the flighting 70 past the portion of the fin 120 located behind the knife holder arm 124 , and toward the ramped end area 130 leading to the fin 120 adjacent the opposite knife holder arm 124 . each arm reinforcement section 132 functions to engage its respective knife holder arm 124 in order to rotate the knife holder arm 124 upon rotation of auger 58 . in addition , the arm reinforcement section 132 extends throughout a substantial portion of the length of the knife or arm 124 , to relieve lateral stresses that may be experienced by the knife holder arm 124 when the material is sheared by the knife inserts 79 against the orifice plate 72 . it can thus be appreciated that each arm reinforcement section 132 along the trailing side of the knife holder arm 124 , in combination with the portion of the fins 120 that extends the full length of the leading side of the knife holder arm 124 , function to form a pocket within which the knife holder arm 124 is received in order to reinforce and protect the knife holder arms 124 . each knife holder arm 124 extends outwardly from a central hub section 135 which , in the illustrated embodiment , is generally circular . the end of the auger 58 is formed with a generally circular recess 136 , which has a shape corresponding to that of hub section 135 . the walls defining the recess 136 , shown at 138 , are formed so as to extend between one of the fins 120 and the opposite reinforcement section 132 . with this construction , the hub section 135 is fully encased and protected by the end of auger 58 . various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention .