Abstract:
A cutting assembly for a grinder pump comprised of a rotary cutter rotatable against an opposing plate cutter. The cutting edges of the plate cutter include a plurality of V-slice cutting teeth, which create bridging spaces to pinch material which is being sucked in to ports and begin cutting along the V-slice and then for cut material to pass through and onward into the volute of the pump. The rotary cutter has a ground edge with a rake angle which shears the gathered material in cooperation with the cutting edges of the plate cutter. A grinder pump including the cutter assembly is also disclosed.

Description:
This invention relates in one embodiment to a grinder pump, and more particularly to such a pump with a grinding impeller and cutter assembly for grinding, cutting, shredding, and/or comminuting suspended solids in a liquid, and for simultaneously pumping a slurry of such suspended solids. 
   FIELD OF THE INVENTION 
   A grinding impeller and cutter assembly for a grinder pump for reducing the size of suspended solids in a liquid stream and simultaneously pumping such liquid stream. 
   BACKGROUND OF THE INVENTION 
   Grinder pumps are commonly used in liquid transfer applications that require the grinding of large solid or semisolid materials contained in a liquid, in order to grind, cut, or shred such materials. Ultimately, such solid or semisolid materials are reduced in size to the point where a slurry is formed, which is more easily pumped or otherwise transported, and which is more disposable than the solids themselves. Grinder pumps typically have an axial inlet connected to a pumping chamber, and a driven shaft extending through the pumping chamber and into the inlet. The shaft rotates a cutting cylinder in proximity to an annular ring, or a cutting disk in proximity to a plate cutter, thereby effecting the cutting action of the pump. Numerous other variations and configurations of grinder pumps are known, which are intended to provide shearing action between shearing parts operating cooperatively at close tolerances. 
   A number of patents have disclosed such grinder pumps, the relevant portions of which may be briefly summarized as follows: 
   U.S. Pat. No. 3,650,081 of Conery et al. discloses a grinder pump including an electric motor drive, a cooperating cutter blade, and grinder members, one of which is secured to the motor shaft at an inlet for the unit. 
   U.S. Pat. No. 3,961,758 of Morgan discloses a submersible pump for pumping liquids and liquid slurries, and for concurrently grinding and comminuting solid and semi-solid material contained in the liquid product to be pumped. The liquid product is initially shredded by a cutter bar and then drawn upwardly through a grinding and comminuting section where an abrasive drum mounted on a rotary drive shaft cooperates with an interior cylindrical stator surface to grind and comminute solids and semi-solids contained in the liquid product. From the grinding section, the resulting slurry is drawn into a centrifugal pump section with an impeller having a frusto-conical pumping face formed with symmetrical pumping cavities that are operable in both directions of rotation. 
   U.S. Pat. No. 4,108,386 of Conery et al. discloses a grinding pump including a comminutor located at the pump inlet to grind solid material as it passes therethrough and into a pumping chamber. The comminutor includes a stationary annular ring in the inlet having a plurality of grinding teeth which form the internal diameter of the ring. A cutting impeller is rotatable within the ring and has at least one blade which extends from one side of the impeller body axially outwardly beyond the ring to force the material between the teeth of the ring. The other side of the impeller body is provided with means to provide additional shearing of the material and prevent clogging of the material between the teeth of the ring. 
   U.S. Pat. No. 4,378,093 of Keener discloses a grinder pump cutter assembly specifically adapted to be useful in grinding rubber and other elastomeric substances. The grinder pump cutter assembly comprises a pair of cutting blades mounted on a cutter disk which extend at a hook angle to substantially the center of the disk, and a blade that is multi-surfaced, with the surfaces thereof being angled toward the periphery of the disk to break up centralized matter and disperse it into cutting engagement between the disk and a cutter ring. 
   U.S. Pat. No. 4,454,993 of Shibata et al., discloses a grinder pump which includes a motor driven impeller, a grinder ring fitted in the suction port at the bottom of a pump casing, the grinder ring being provided with a plurality of axially extending grinding grooves and edges on the inner surface thereof, and a grinder impeller fixedly screwed on the distal end of the pump shaft, the grinder impeller being provided with at least two axially extending grinding blades provided on the lower conical surface of the hub thereof so that any foreign material contained in the pumping liquid is ground or shredded into smaller pieces by cooperation of the grinding edges of the grinder ring with the grinding blades of the grinder impeller. 
   U.S. Pat. No. 4,640,666 of Sodergard discloses a grinder pump comprising a pump housing having an internal surface bounding a central inlet and provided with grooves and an impeller having at least a portion received in the inlet for rotation therein, the portion being substantially cylindrical and having a diameter which is smaller than that of the inlet, the portion having substantially axially extending cutting means which project radially from the portion to cooperate with the grooves to cut solid objects such as rags and other elongated objects. 
   U.S. Pat. No. 4,697,746 of Nishimori discloses a release type grinder pump that is capable of readily permitting the fitting and removal of a grinder ring with respect to a pump casing. The grinder pump includes an annular suction cover in which the grinder ring is fittedly held and which is detachably fitted in the pump casing so as to surround a suction port and allow the grinder ring not to have any portion directly held within the pump casing. 
   U.S. Pat. No. 4,842,479 of Dorsch discloses a high head centrifugal slicing slurry pump comprising a booster propeller that is located at the inlet of a flared funnel leading toward arcuate inlet apertures in an end plate of a centrifugal pump casing. The propeller has radially projecting blades of generally right triangular cross section with bases formed of the broad trailing sides and broad bottom sides remote from an impeller; the hypotenuse side of each blade being a broad upper surface inclined relative to a plane perpendicular to the axis for propelling slurry toward the inlet apertures of the pump. 
   U.S. Pat. No. 5,016,825 of Carpenter discloses a cutting assembly for a grinder pump comprising a disk member rotatable within an opposing annular ring. The inner circumferential surface of the annular ring carries a plurality of cutting teeth, which partially extend at an angle along the lateral dimension of the inner circumferential surface. The disk member has an annular edge which separates a side distal to the pump inlet and a side proximal to the inlet. At least one projection extends from the distal side and has a leading edge facing toward the direction of normal shaft rotation. At least one cutting member extends from the proximal side and also has a leading edge. The proximal side has a recess forming first and second cutting edges. Similarly, the distal side has a recess having first and second cutting edges. The proximal recess overlaps the distal recess along the width of the annular edge. 
   U.S. Pat. No. 5,044,566 of Mitsch discloses a grinder pump comprised of a housing; a shaft disposed within the housing, a moveable cutter carried on and rotationally coupled to the shaft. A stationary cutter is held within the housing and the moveable cutter cooperates with the stationary cutter to perform cutting along respective surfaces of the moveable cutter and the stationary cutter. The respective surfaces of the stationary and moveable cutters are substantially within a plane that is substantially perpendicular to the axis of the shaft. The pump includes an apparatus that resiliently biases the moveable cutter against the stationary cutter. 
   U.S. Pat. No. 5,256,032 of Dorsch discloses a centrifugal chopper pump comprising open impeller with vanes having cutting edges at both the intake side of the pump bowl and the closed side of the bowl. The cutting edges of the vanes cooperate with narrow anvil ribs projecting inward from both sides of the pump bowl such that solid matter in the material being pumped is sliced and chopped inside the bowl. 
   U.S. Pat. No. 6,010,086 of Earle et al. discloses a grinder pump comprised of a pump assembly, a grinder mechanism, and a motor disposed between the grinder mechanism and the pump assembly. A shaft of the motor is operably attached at one end thereof to the grinder mechanism and at the other end thereof to the pump assembly to provide small radial clearances between the cutting portions of the grinder mechanism. In one embodiment, vortex-type impeller vanes are associated with a grinding head of the grinder mechanism to assist flow of effluent from the grinder mechanism to the pump assembly via a passageway extending about, and/or in parallel with, a motor mounting unit. 
   U.S. Pat. No. 6,190,121 of Hayward et al. discloses a centrifugal pump with solids cutting capability comprising an impeller having a plurality of radially extending vanes connected to a hub and a partial back shroud with sharpened leading edges. The pump has a pump casing with a back plate adjacent to the back side of the impeller, wherein spiral grooves on the back plate interact with the sharpened edges on the back shroud to aid in protecting the area between the back plate and the impeller by cutting of solids and expulsion of solids through an output port. The leading edges on the back shroud are preferably also serrated and beveled and the spiral grooves are outward threaded. A disintegrator is preferably mounted on the end of the drive shaft in the conical intake of the pump. Cutting bars on the front plate of the casing project into the pump intake and interact with front edges of the vanes to cut incoming solids in a liquid mixture. 
   The disclosures of each of these United States patents are incorporated herein by reference. 
   Also known is the Shark® 820 grinder pump manufactured and sold by the Zoeller Co. of Louisville, Ky. This pump is comprised of a star-shaped cutter rotating against a stationary flat disc having inlet apertures, with radial grooves disposed between such apertures. 
   Also known is the Piranha® Grinder pump manufactured and sold by the ABS Corporation. This pump is comprised of an intake plate having spiral cutting grooves on the inside thereof cooperating with the edges of the pump impeller vanes, which are flat in profile. The cutter design of this pump is susceptible to binding problems because material is cut in large pieces and such material can become wedged either in the cutter or between the impeller edges and the intake spiral cutting plate. Also the discharge of this pump can become clogged under severe operating conditions. 
   It is known that that the configuration of the cutting disk and annular ring, or of the other shearing parts are of high importance in the operation of grinder pumps. The particular shearing parts must be capable of shearing a wide range of entrained solids in a liquid stream that is entering the pump. Such solids may have a wide variety of properties that are adverse to the operation of the pump, including high shear strength, abrasiveness, hardness, elasticity, and/or plasticity. Materials that are abrasive may gradually wear away the cutting edges of the grinding parts of the pump. Materials that have high shear strength and/or high hardness may shatter or deform the cutting edges of the grinding parts of the pump. Materials that undergo elastic and/or plastic deformation may be particular resistant to the cutting action by the grinding parts of the pump. Instead of being cleanly cut up into smaller parts, these materials may deform and “gum up” or otherwise clog the grinding pump. 
   Fibrous, string-like materials suspended in a liquid are particularly difficult to shred with a grinder pump. On a local scale on the order of the fiber diameter, these materials have a relatively small cross section and are more difficult to shear by the cutting edges. In addition, the relatively long length and elastic or plastic properties often cause them to wrap around the internal pump components parts such as a cutting disk, an annular ring and a pump shaft. It is therefore desirable that the grinding parts of a grinder pump are not only capable of efficient grinding/cutting of solids or semisolid materials, such as e.g. materials in a sewage stream, but also have the capability of grinding fibers and string-like materials that are suspended in such a stream. It is also desirable that the grinder pump be capable of rejecting any very hard objects that are not grindable, such as metal buttons, rivets and the like that may be on objects such as clothing items that are accidentally introduced into the stream. Numerous prior art grinder pumps are not capable of rejecting such hard objects, rendering them especially vulnerable to jamming at start up. 
   In addition, numerous prior art grinder pumps are not capable of grinding a sudden heavy load of fibrous material that may be encountered. Such grinder pumps will bog down and clog on the heavy fibrous load, or suddenly jam if a hard solid object is introduced into the cutting parts of the pump. The applicant&#39;s grinder pump comprised of his cutter assembly invention is capable of rejecting hard objects, and grinding a sudden heavy load of fibrous material. The applicant&#39;s grinder pump is also capable of grinding suspended solids into a sufficiently small size so as to produce a substantially homogeneous liquid. The applicant believes that such a capability enables his grinder pump to pump such a liquid at a higher pressure through a smaller diameter pipe. Because of these capabilities, the applicant&#39;s grinder pump including his cutter assembly invention is thus superior to prior art grinder pumps. Accordingly, embodiments of the present invention are provided that meet at least one or more of the following objects of the present invention. 
   It is an object of this invention to provide a grinder pump with a cutter assembly that is capable of grinding a sudden heavy load of solid fibrous material without stalling, or without becoming clogged and ceasing the pumping of liquid therethrough. 
   It is a further object of this invention to provide a grinder pump with a cutter assembly that is capable of grinding a substantial and continuous load of solid and semisolid material, including fibrous material, without stalling, or without becoming clogged and ceasing the pumping of liquid therethrough. 
   It is another object of this invention to provide a grinder pump with a cutter assembly that is capable of rejecting a piece of hard solid material, thereby preventing such material from jamming and stalling such pump, and ceasing the pumping of liquid therethrough. 
   It is a further object of this invention to provide a grinder pump comprised of a cutter assembly that renders such pump capable of pumping a homogenized liquid stream at a higher pressure and through a smaller diameter pipe. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a cutting assembly for size reduction of solids in a liquid to be pumped, the cutting assembly comprising a drive shaft rotatable in a first direction of rotation; a rotary cutter rotatably engaged with the drive shaft and comprised of at least a first cutting blade and a second cutting blade, each of the first and the second cutting blades including a leading cutting edge advanceable in the direction of rotation, the leading cutting edge formed at the junction of a leading side wall of the blade and a flat base of the blade; and a plate cutter comprised of an apertured wall including an inner discharge surface, and an outer cutter surface, the outer cutter surface including a plurality of cutting ports, each of the cutting ports comprising an entry opening, a first orifice passing though the apertured wall to the inner discharge surface, and a second orifice passing though the apertured wall to the inner discharge surface. The entry opening further comprises a first V-slice cutting edge and a second V-slice cutting edge intersecting at an angle directed opposite to the direction of rotation; a first cutting edge of the first orifice connected to the first V-slice cutting edge; and a second cutting edge of the second orifice connected to the second V-slice cutting edge. When the rotary cutter is rotated in the first direction of rotation, the cutting blades are rotationally advanced along the outer cutter surface of the plate cutter in a shearing action against the first V-slice cutting edge, the second V-slice cutting edge, the first cutting edge of the first orifice, and the second cutting edge of the second orifice of the cutting ports of the outer cutter surface of the plate cutter. 
   In accordance with the present invention, there is provided a grinder pump comprising a housing including a motor housing portion enclosing a pump motor, and a volute portion forming a pump volute, said pump volute enclosing a pump impeller; a volute cover; and a cutting assembly for size reduction of solids in a liquid to be pumped, said cutting assembly as recited in the immediately foregoing description. In one embodiment, the plate cutter of the cutting assembly is integrally formed into the volute cover of the grinder pump. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
       FIG. 1  is a side elevation view of a grinder pump that utilizes the rotary cutter and plate cutter assembly of the present invention; 
       FIG. 2  is a cross-sectional view of the grinder pump of  FIG. 1 , taken along line  2 — 2  thereof; 
       FIG. 3  is an enlarged cross-sectional view of the volute portion of the pump of  FIGS. 1 and 2 , including the pump volute and the rotary cutter and plate cutter assembly thereof; 
       FIG. 4A  is an upward perspective exploded view of the liquid/solid transporting components of the grinder pump, including the rotary cutter and plate cutter assembly; 
       FIG. 4B  is a downward perspective exploded view of the liquid/solid transporting components of the grinder pump, including the rotary cutter and plate cutter assembly; 
       FIG. 5A  is an exterior perspective view of a preferred rotary cutter of the grinder and cutter assembly of the grinder pump depicted in  FIG. 2 ; 
       FIG. 5B  is an interior view of the rotary cutter of  FIG. 5A ; 
       FIG. 6A  is a bottom view of a volute cover of the applicant&#39;s grinder pump that includes a plate cutter of the rotary cutter and plate cutter assembly; 
       FIG. 6B  is a cross-sectional view of the cover of  FIG. 6A , taken along the line  6 BCD— 6 BCD of  FIG. 6A ; 
       FIG. 6C  is an exterior cutaway perspective view of the cover of  FIG. 6A , cut along the line  6 BCD— 6 BCD of  FIG. 6A ; 
       FIG. 6D  is an interior cutaway perspective view of the cover of  FIG. 6A , cut along the line  6 BCD— 6 BCD of  FIG. 6A ; 
       FIG. 7  is a bottom or exterior view of the grinder pump of  FIG. 1  taken along line  7 — 7  of  FIG. 1 , and depicting the rotary cutter and plate cutter assembly of such grinder pump; 
       FIG. 8  is a magnified bottom or exterior view of the rotary cutter and plate cutter assembly depicted in  FIG. 7 ; and 
       FIG. 9  is a vertical downward cross-sectional view of a preferred volute of the applicant&#39;s grinder pump, taken along line  9 — 9  of  FIG. 1 . 
   

   The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. In describing grinding assemblies for size reduction of solids in liquids to be pumped, a variety of terms are commonly used in the description. A rotating blade device is commonly referred to as a grinder impeller or a rotary cutter. Herein, such terms may used interchangeably and are to be understood as meaning the same thing. It is also to be understood that a grinder impeller as used herein is distinct from a pump impeller, which is commonly understood to be a rotating vaned disc that draws a liquid into a central region thereof and centrifugally discharges such liquid outwardly in a pumping action. 
   An overall cutting assembly for size reduction of solids in liquids to be pumped is often referred to as a grinding impeller and cutter assembly, a cutting assembly, a cutter assembly, a grinder/cutter assembly, or a cutter assembly. Although the grinding apparatus herein will generally be referred to as a cutting assembly, it is to be understood that such terms where used are considered to be interchangeable. 
   The general construction of grinder pumps that utilize the applicant&#39;s cutter assembly may vary to some degree.  FIG. 1  is a side elevation view of one preferred embodiment of a grinder pump that utilizes the cutter assembly of the present invention; and  FIG. 2  is a cross-sectional view of the grinder pump of  FIG. 1 , taken along line  2 — 2  thereof. Referring to  FIGS. 1 and 2 , pump  100  comprises a housing  110  including a motor portion  120  and a volute portion  130 . Pump motor  170  is disposed within housing portion  120 , and comprises motor drive shaft  172 . Pump  100  may optionally comprise numerous other elements beneficial to the operation and control thereof, such as e.g., a float switch  182 , and solid state switch  184  for turning off the start winding when motor  170  is started up. Such solid state switch is generally more reliable that a relay device. Pump  100  preferably further comprises sealed cord fitting  112 , handle  114 , and support feet  131 ,  132 , and  133 . 
     FIG. 3  is an enlarged cross-sectional view of the volute portion of the pump of  FIGS. 1 and 2 , including the pump volute and the cutter assembly thereof. During the operation of pump  100 , the grinding and size reduction of solids (not shown) in a liquid stream occurs by the action of cutter assembly  190  in the flow regions indicated by arrows  199 . Ground up suspended/homogenized solids and liquid are drawn into pump  100  by spinning impeller  140  and are discharged into pump volute  134  as indicated by arrows  198 , and are subsequently discharged out of outlet  136  as indicated by arrow  197 . 
   The optimal construction and arrangement of certain pump components to form the pump volute, and the cooperative relationships of the various liquid/solid transporting components of the grinder pump are best understood with reference also to  FIG. 4A  and  FIG. 4B , which are upward and downward perspective exploded views, respectively, of the liquid/solid transporting components of the grinder pump, including the rotary cutter and plate cutter assembly. It is noted in particular that pump  100  comprises a minimum of parts and simple construction to form a housing for motor  170  and volute cavity  134 . 
   Referring to  FIGS. 3 ,  4 A, and  4 B, the upper face  135  of volute cavity  134  is formed by wall  152  of motor plate  150 . An upper receiving flange  137  is formed in volute portion  130  of housing  110 , and mounting flange  154  of motor plate  150  is secured thereto by suitable fastening means such as e.g., bolts (not shown). Motor plate  150  further comprises a central housing  156  for the fitting of pump seals and a motor bearing therein. The outer circumferential wall  138  of volute  134  is formed in a downwardly extending section of volute portion  130  of housing  110 , such section extending downwardly beyond receiving flange  137 . The lower face  139  of volute cavity  134  is formed by wall  202  of bottom volute cover  200 . A lower receiving flange  129  is formed in volute portion  130  of housing  110 , and mounting flange  204  of volute cover  200  is secured thereto by suitable fastening means such as e.g., bolts (not shown). 
   Impeller  140  is mounted on and operatively coupled to drive shaft  172  by suitable means such as e.g. a key engaged in key slots formed in impeller  140  and shaft  172 . Impeller  140  and rotary cutter  300  are rotated in the direction indicated by arrow  196  to effect the pumping of liquid through volute  134 , and the grinding of solids in such liquid by cutter assembly  190  as will be described presently. 
   The general arrangement of the components of cutter assembly is best understood with reference additionally to  FIG. 7  and  FIG. 8 . Referring to  FIGS. 3 ,  4 A,  4 B,  7 , and  8 , cutter assembly  190  is comprised of rotary cutter  300  and stationary plate cutter  250 . Rotary cutter  300  is mounted on and operatively coupled to drive shaft  172  by suitable means such as key  302 , bolt  304 , and washer  306 . In operation, rotary cutter  300  spins with the rotation of shaft  172 , and the leading cutting edges such as edge  312  of blade  310  are rotationally advanced along the outer cutter surface  260  of plate cutter  250  in a shearing action against the various cutting edges formed in cutter surface  260 . Such cutting edges will be described in detail subsequently herein. 
   In the preferred embodiment of grinder pump  100 , plate cutter  250  is formed as an integral part of bottom cover  200 . Such a construction minimizes the parts needed for grinder pump  100 , simplifies the assembly thereof, and reduces overall manufacturing cost. Plate cutter is formed as an annular boss  252  that extends downwardly from wall  202  of cover  200 . Cutter surface  260  is ground flat such that in the operation of cutter assembly  190 , surface  260  is perpendicular to drive shaft  172 , and parallel to the plane of rotation of the cutting edges of the blades of rotary cutter  300 . The running clearance between the cutting edges of the blades of rotary cutter  300  and cutter surface  260  may be selected during pump assembly or subsequently changed by the fitting of shims  174  between motor shaft bushing  176  and the base of rotary cutter  300 . 
   The magnitude of the running clearance between the cutting edges of the blades of rotary cutter  300  and cutter surface  260  is selected according to the properties of the suspended solids in the liquid to be processed and upon the materials of construction of plate cutter  250  and rotary cutter  300 . In one embodiment, cutter assembly  190  may be made self-sharpening, wherein there is zero running clearance between the cutting edges of the blades of rotary cutter  300  and cutter surface  260 . Preferably, there is provided between about 0.002 inches and about 0.10 inches of running clearance. In such an embodiment, plate cutter  250  may be made of a suitable metal alloy including but not limited to a martensitic stainless steel alloy and rotary cutter  300  may also be made of a suitable metal alloy including but not limited to a martensitic stainless steel alloy. In one preferred embodiment, plate cutter  250  and rotary cutter  300  are both made of 440 Stainless steel hardened to approximately 58 Rockwell C. 
   In other embodiments, the rotary cutter and/or the plate cutter may be made of technical ceramics that exhibit superior wear resistance and strength including but not limited to aluminum oxide (alumina) ceramic, chromium oxide (chromia) ceramic, titanium oxide (titania) ceramic, zirconium oxide (zirconia) ceramics, including fully and partially stabilized zirconia, and combinations of such metal oxides; and silicon nitride, silicon carbide, or tungsten carbide. In a further embodiment, the rotary cutter and/or the plate cutter may be made of a metal alloy coated with a suitable wear resistant thin film coating such as hard chrome, titanium nitride, or the technical ceramics recited herein. Processes such as e.g., electroplating are well known for applying a layer of a protective metal such as hard chrome, as are process such as e.g. plasma deposition for applying ceramics and other protective non-metallic thin films to metal substrates. 
   In the preferred embodiment, plate cutter  250  and rotary cutter  300  are each provided with certain unique features that enable superior cutting and grinding performance by cutter assembly  190 . Although not every particular feature is required to achieve solids grinding by cutter assembly  190 , the applicant believes that providing cutter assembly  190  with these features provides the best grinding performance, and superior performance to prior art grinder assemblies. The preferred features of rotary cutter  300  will now be described, then followed by the preferred features of plate cutter  250 , then followed by a summary of the cooperative working of rotary cutter  300  against plate cutter  250  to achieve solids grinding. 
   The preferred features of rotary cutter  300  are best understood with reference additionally to  FIG. 5A  and  FIG. 5B .  FIG. 5A  is an exterior perspective view of a preferred rotary cutter of the grinder and cutter assembly of the grinder pump depicted in  FIG. 2 ; and  FIG. 5B  is an interior view of the rotary cutter of  FIG. 5A . Referring to  FIGS. 5A and 5B , rotary cutter  300  is provided with a plurality of cutting blades. At minimum, rotary cutter  300  is provided with at least two cutting blades spaced at 180 degrees for proper balance. In one preferred embodiment depicted in  FIGS. 5A and 5B , rotary cutter  300  is provided with four cutting blades  310 ,  320 ,  330 , and  340  spaced at 90 degree intervals. It will be apparent that other suitable configurations of rotary cutter  300  may be provided, such as e.g. three blades spaced at 120 degree intervals. Rotary cutting blade  300  is further provided with a shaft bore  301  having a keyway  302  for mounting on shaft  172  (see  FIG. 4A ) as described previously. 
   Blades  310 ,  320 ,  330 , and  340  are provided with ground leading cutting edges  312 ,  322 ,  332 , and  342 , respectively. In the preferred embodiment, cutting edges  312 – 342  are provided with an arcuate shape such that these edges sweep outward from the central region of cutter  300  and backward, i.e. in a direction opposite the direction of rotation indicated by arrows  196 . This arcuate shape provides a superior shearing and scissoring action when such edges cut against the cutting edges of plate cutter  250  to be described subsequently herein. The applicant believes that such a shape also reduces liquid drag on the rotary cutter during operation, thereby reducing the torque load on the pump motor  170  (see  FIG. 2 ). 
   The details of the preferred construction of blade  310  of rotary cutter  300  will now be provided, with it being understood that in the preferred embodiment, each of blades  320 ,  330 , and  340  are identical to blade  310 . Referring again to  FIGS. 5A and 5B , cutting edge  312  of blade  310  is formed at the junction of leading side wall  314  and flat base  316 . In one embodiment, leading side wall  314  is perpendicular to flat base  316 . However, in the preferred embodiment, leading sidewall  314  is tilted backward opposite the direction of rotation, thereby forming an acute rake angle  315  with flat base  316 . Rake angle  315  is defined as the angle between vector  399  directed up the slope of leading sidewall  314  and vector  398  directed along base  316 , with both of such vectors lying in a plane perpendicular to a tangent to cutting edge  312  at the particular point on cutting edge  312 . Rake angle  315  is preferably between about 60 degrees and about 90 degrees. In one preferred embodiment depicted in  FIGS. 5A and 5B , rake angle  315  is 67.5 degrees. 
   The provision of blades  310 – 340  with such acute rake angles at cutting edges  312 – 342  provides such blades with better shearing action with plate cutter  250 , and with an effect that pushes shorn solids outward and away from the cutting edges, to help prevent clogging and/or jamming of cutter assembly  190 . The rake angle also begins a transition to angled surfaces  311 ,  313 , and  317 , which are formed to streamline blade  310 , thereby reducing liquid drag and the torque load on pump motor  170 . 
   In an alternate embodiment, blades  310 – 340  are provided with obtuse rake angles  315 . The use of obtuse rake angles provides a downward force on the solids impinging on leading sidewall  314 , thereby assisting such solids in being forced through the orifices  286  and  288  (see  FIG. 8 ) of plate cutter  250 . Such obtuse rake angle  315  is preferably between about 90 degrees and about 120 degrees. 
   It will be apparent that flat bases  316 ,  326 ,  336 , and  346  are all coplanar in order to provide cutting action with cutting surface  260  of plate cutter  250 . Cutting blade  310  (et seq.) is provided with a first recessed area  318  located adjacent to flat base  316  on the outer portion of such blade  310 , and a second recessed area located adjacent to flat base  316  proximate to the central region of rotary cutter  300 . These recessed areas work cooperatively with exclusion slots on cutting surface  260  of plate cutter  250  in a manner to be described subsequently herein. 
   Referring again to  FIGS. 3 ,  4 A, and  5 A, rotary cutter is preferably provided with a counterbore  308  which receives and provides a shroud for the head of bolt  304  or other suitable fastener when rotary cutter is fitted and secured to shaft  172 . This shrouding effect is made more beneficial by the selection of a bolt  304  having a radiused head (e.g. a pan-head or dome-head bolt or screw) as depicted in  FIG. 3  in particular. In this manner, there are no exposed sharp edges resulting from the fitment of bolt  304  to the assembly. The applicant has discovered that this is beneficial in that if a bolt with a head having exposed sharp-edges is used (such as a hex-head bolt), and such bolt head is exposed rather than countersunk into recess  308 , any stringy fibrous solid material present in the liquid to be processed will likely become wrapped around and entangled with such bolt head, thereby reducing liquid flow and further loading the pump motor  170 . 
   The preferred features of plate cutter  250  to be presently described are best understood with reference additionally to  FIGS. 6A–6D .  FIG. 6A  is a bottom view of the preferred volute cover  200  of the applicant&#39;s grinder pump that includes a plate cutter  250  of the cutter assembly  190 .  FIG. 6B  is a cross-sectional view of the cover of  FIG. 6A , taken along the line  6 BCD— 6 BCD of  FIG. 6A ;  FIG. 6C  is an exterior cutaway perspective view of the cover of  FIG. 6A , cut along the line  6 BCD— 6 BCD of  FIG. 6A ; and  FIG. 6D  is an interior cutaway perspective view of the cover of  FIG. 6A , cut along the line  6 BCD— 6 BCD of  FIG. 6A . 
   Referring to  FIGS. 6A–6D , bottom cover  200  is comprised of plate cutter  250 , which is formed as an integral part of. Plate cutter is formed as an annular boss  252  that extends downwardly from wall  202  of cover  200 . Wall  202  extends radially outward to an annular region  203 , to which is joined mounting flange  204 . Wall  202  preferably includes a plurality of reinforcement ribs  201  on the outer surface thereof. 
   Plate cutter  250  is further comprised of an apertured wall  254  with cutting ports  270  formed in cutter surface  260 , and apertures or orifices extending from the openings of such ports  270  through wall  254  to inner discharge surface  256 . In the operation of cutter assembly  190 , liquid and ground up/shorn solids flow through cutting ports  270  into cavity  258  as indicated by arrow  199 . In the preferred embodiment of cover  200  and plate cutter  250 , annular cavity  258  is provided with an impeller eye  251  that is formed at the inner radial terminus of wall  252 . Impeller eye  251 , together with motor shaft bushing  176  and/or annular collar  145  of pump impeller  140  (see  FIG. 4A ), forms an annular passageway into the volute cavity  134  of pump  100 . Liquid and homogenized solid material flows past impeller eye  251  as indicated by arrow  198  of  FIG. 3 . Impeller eye is dimensioned to throttle the amount of liquid allowed to be drawn by impeller  140  into volute cavity  134 , thereby regulating the maximum load placed on pump motor  170 . The optimum size of impeller eye  251  will vary with the properties of the liquid being pumped therethrough and the desired pump performance. 
     FIG. 8  is a magnified bottom or exterior view of the cutter assembly  190 , which depicts in particular certain unique features of cutting ports  270  that provide superior cutting and grinding of solids. Referring to  FIGS. 6A–6D  and  FIG. 8 , a plurality of cutting ports  270  are formed in cutting surface  260  of plate cutter  250 . Each of cutting ports  270  comprises a recessed entry opening  272  having a perimeter approximately the shape of a “FIG.  8 ,” and a recessed bridge area  274  that connects adjacent wide openings  276  and  278  in recessed opening  272 . 
   Without wishing to be bound to any particular theory, the applicant believes that in operation, when rotary cutter  300  is rotating and the cutting blades thereof are rotationally advancing as indicated by arrow  196  for cutting blade  320  and cutting edge  322 , the pump impeller sucks material into recessed bridge area  274 , enabling such material to at least partially “bridge” or fill the gap between wider openings  276  and  278 . At the recessed bridge area  274  (i.e. the “neck” of the “FIG.  8 ” of the perimeter of recessed opening  270 ), on the side that is toward the direction of rotation of rotary cutter  300 , there is provided a first V-slice cutting edge  282  and a second V-slice cutting edge  284  intersecting at an angle  280  directed opposite to said direction of rotation. When the cutting edge  322  of cutting blade  320  passes over these V-slice cutting edges  282  and  284 , any solid material (not shown) that has dropped into recessed bridge area is cut and parted in two directions. A first portion of the solid material is directed to open area  276  and into orifice  286 , and a second portion of the solid material is directed to open area  278  and orifice  288 . 
   Subsequently, as cutting edge  322  of blade  320  advances further, and encroaches upon orifices  286  and  288 , any solid material that is partially disposed in such orifices  286  and  288  is shorn off as cutting edge  322  passes the trailing edges  287  and  289  of such orifices. Shorn solid material and liquid that are disposed in orifices  286  and  288  are subsequently sucked into pump  100  by impeller  140  (see  FIG. 3 ), and solid shorn material on the exterior of cutting edge  322  are swept from the cutting surface  260  of plate cutter  250  and back into the bulk liquid. This unique construction of cutting ports  270  results in “three cuts per slice” form each passage of a cutting blade  310 – 340  over a cutting port  270 : a V-slice cut at V-slice cutting edges  282  and  284 , a cut at trialing edge  287  of orifice  286 , and a cut at trailing edge  289  at orifice  288 . The angle  280  of first V-slice cutting edge  282  and a second V-slice cutting edge  284  is preferably between about 60 and about 120 degrees. In one preferred embodiment depicted in  FIG. 8 , V-slice cutting angle  280  is about 90 degrees. 
   In one preferred embodiment of grinder pump  100  and cutter assembly  190 , rotary cutter  300  is provided with 4 blades, cutting surface  260  of plate cutter  250  is provided with 9 cutting ports, and pump  100  is provided with an AC motor that operates at 3540 revolutions per minute. Accordingly, in this embodiment of the applicant&#39;s cutter assembly  190 , such cutter assembly provides about 372,600 cuts per minute of operation, and is thus highly effective at grinding the solid material in a liquid stream. It will be apparent that numerous other configurations of rotary cutters and plate cutters may be provided that will provide rapid and effective size reduction and pumping of entrained solids in a liquid stream. 
   The applicant&#39;s cutter assembly is  190  is further provided with additional features to provide more effective cutting of such solids. Referring again to  FIG. 6B  and  FIG. 8 , orifices  286  and  288  are preferably conical orifices. Such conical orifices provide a sharper angle for the cutting and shearing action with the blades of rotary cutter  300 , with much the same effect as the rake angle  315  (see  FIG. 5A ) of the cutting edges of such blades. In addition, such conicity further serves to limit the amount of liquid inflow allowed to the pump impeller  140  in conjunction with impeller eye  251  of cover  200 , thereby limiting the maximum hydraulic load on the pump motor  170 . The angle of conicity of orifices  286  and  288  is preferably between about 10 degrees and about 20 degrees. In one preferred embodiment, the angle of conicity is about 15 degrees, and the entry diameter of conical orifices  286  and  288  is about 0.190 inch. 
   Referring again to  FIG. 8 , the cutting surface  260  of plate cutter  250  is preferably provided with a plurality of outer exclusion slots  292 . Such outer exclusion slots are preferably slightly arcuate in shape and are disposed in a direction that forms an acute angle with the cutting edges  312 – 342  of the blades  310 – 340  of rotary cutter  300 , as such cutting edges pass over exclusion slots  292 . Referring also to  FIG. 5B , these outer exclusion slots  292  work cooperatively with the outer recessed areas such as recessed area  318  of blade  310  to outwardly eject any cutting debris that has begun to accumulate under surface  316 . The provision of such recessed area  318  in blade  310  (et seq.) and outer exclusion slots  292  prevents the accumulation and binding of any material between rotary cutter  300  and plate cutter  260 . In the preferred embodiment, the number of outer exclusion slots  292  provided in cutting surface  260  is equal to the number of cutting ports  270  provided in cutting surface  260 . 
   Referring now to  FIG. 8 ,  FIG. 6A  and  FIG. 6D , the cutting surface  260  of plate cutter  250  is preferably also provided with a plurality of inner exclusion slots  294 . Such inner exclusion slots may also be slightly arcuate in shape and are disposed in a direction that forms an acute angle with the shaft bore  253  through wall  252  of cover  200 . Inner exclusion slots  292  form channels from shaft bore  253  to at least several of openings  276  of cutting ports  270 . Referring also to  FIG. 5B , these inner exclusion slots  292  work cooperatively with the inner recessed areas such as recessed area  319  of blade  310  to inwardly direct any cutting debris that has begun to accumulate in recessed area  319  into orifice  288 . The provision of such recessed area  319  in blade  310  (et seq.) and inner exclusion slots  294  also serves to prevent the accumulation and binding of any material between rotary cutter  300  and plate cutter  260 , making the assembly self cleaning. In one preferred embodiment depicted in  FIG. 6A , three exclusion slots  294  are provided in cutting surface  260 . 
   Inner exclusion slots  294  and outer exclusion slots  292  may be provided with a relatively small cross-section. In one preferred embodiment in which plate cutter  250  is 3.4 inches in diameter, such slots are provided with a width of 0.05 inch and a depth of 0.09 inch. 
   Grinder pump  100  is further provided with additional beneficial features to more effectively pump liquid containing homogenized solids produced by cutter assembly  190 . Referring now to  FIG. 3 , and  FIGS. 4A and 4B , inner surface of wall  202  of cover  200  is provided with a plurality of spiral grooves  259  formed therein. These spiral grooves  259  spiral in an outward direction with respect to the direction of rotation of pump impeller  140 , and thus these spiral grooves  259  work cooperatively with the vanes  142  of impeller  140  to outwardly eject any solid debris that begins to accumulate between impeller vanes  142  and wall  202 . 
   In like manner, the inner surface of wall  152  of cover motor plate  150  is provided with a spiral groove  159  formed therein. This spiral groove  159  also spirals in an outward direction with respect to the direction of rotation of pump impeller  140 , and thus the spiral groove  159  works cooperatively with ribs  144  (see  FIG. 4A  and  FIG. 9 ) formed on the upper surface of impeller  140  to outwardly eject any solid debris that begins to accumulate between such upper surface of impeller  140  and wall  152 . 
     FIG. 9  is a vertical downward cross-sectional view of a preferred volute of the applicant&#39;s grinder pump, taken along line  9 — 9  of  FIG. 1 . Referring to  FIG. 3  and  FIG. 9 , the applicant&#39;s grinder pump  100  is comprised of a volute  134  that has a simple annular shape, without complex pockets or barriers, or volute shape known as a “cutwater” where solid debris may accumulate. Liquid and homogenized solids are swept around the outer circumferential wall  138  of volute  134 , and are subsequently discharged out through outlet  136 . 
   It is, therefore, apparent that there has been provided, in accordance with the present invention, a cutting assembly for size reduction of solids in a liquid to be pumped, and a grinder pump comprising such a cutter assembly. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.