in the illustrated embodiment , cutter arbor 10 comprises an elongated shaft extended through rotational journals or bearing units 12 and 14 respectively mounted in alignment on opposite upright end panels 16a and 16b of machine frame 16 . cutter 30 , comprising a series of cutter disks 32 separated by spacer disks 34 on arbor 10 , is accommodated between the frame panels 16a and 16b . these panels form opposite ends of what amounts to a feed hopper 35 in which the rotary cutter 30 operates on tires fed successively down into the hopper . frame plates 16a and 16b are maintained in spaced - apart , upright positions by longitudinal parallel frame members 18 , 20 , 22 and 24 . frame members 18 and 20 support an upright wall panel 36 adjacent and parallel to one side of the rotary cutter 30 . panel 36 extends both above and below the cutter as shown in fig1 . above the cutter , panel 36 terminates in a section 36a angled away from the opposing vertical hopper wall 37 . just above the cutter 30 , wall 37 terminates in a section 37a angled downwardly and outwardly away from the cutter 30 . a two - part plate 38 projects above the level of the cutter , angling outwardly from the cutter to the lower edge of panel 37a . plate 38 includes a lower segment 38a aligned with a stationary upper segment 38b . the latter is fixedly mounted on the frame member 24 . the two segments are joined together by a splint plate 60 and bolts 62 . the plate assembly 38 is structurally combined with a multitoothed rack assembly 40 . the plate assembly 38 and underlying rack assembly 40 , specifically rack bar 40a , are pivotally mounted as a unit by pivotal supports 42 on horizontally aligned lugs 44 carried by the frame member 22 . a hydraulic actuator piston and cylinder unit 26 pivotally connected to a stiffener rib 48 on the backside of the assembly is operable to advance and retract the assembly about the common axis of pivots 42 between its normal operating position illustrated in fig1 and its broken - line retracted position illustrated in the same figure . when the connecting bolts 62 are removed and the assembly is retracted from the cutter 30 , the rack teeth 50 may be repaired or replaced individually as need be . however , with plate assembly 38 in its normal operating position , it cooperates with the panels 37a , 37 and with the cutter 30 and rack assembly 40 to form a hopper base lobe or holding compartment t into which tires dropping onto the cutter 30 are thrown and tumbled about with successors until shredded as generally depicted in the sequence of views , fig8 - 10 . holding compartment t has a height greater than the width and less than the diameter of tire carcasses to be shredded . the hopper base portion has a throat i entering compartment t wider than the widest tire to be shredded . the rack assembly 40 includes a horizontal row of successively spaced teeth 50 that are mounted to project into and largely occupy the spaces between successive cutter disks 22 . the upper faces of the rack teeth lie substantially in the horizontal plane of the axis of the cutter . the teeth in operative position of the rack project into close proximity with the peripheral surfaces of the spacer disks 34 that separate the cutter disks . the parallel flat side faces of the cutter disks lie close to the adjacent parallel side faces of the rack teeth 50 . clearance distance between the side faces should be of the order of a few hundredths of an inch . the rack teeth are formed and mounted to be removable from their support bar structure 40a , 40b . individual pairs of bolts 54 threaded into the backside shank portions of the teeth draw the teeth against the rims of associated apertures in the rack bar 40b , the bolt heads of each pair reacting against a backing plate 56 positioned against the backside of the rack bar 40b . each of the disk - like rotary cutters 32 has two flat sided cutting bits 320 removably mounted in obliquely formed notches 32a at 180 ° spacing in the peripheral surface 32b of the cutter disk . from the base of each disk notch 32a , the disk &# 39 ; s outer peripheral surface 32b spirals gradually outward at increasing radius to the reversely angled face 32c of the other notch which turns abruptly inward at a sharp acute angle . face 32c forms one side of a shallow cutter bit holder slot 32d formed in the base of notch 32a . each cutter bit 320 is held fast in its holder slot in the base of the associated cutter disk notch 32a by an allan head machine screw s threaded and tapped into the cutter bit 320 . the base end of cutter bit 320 has a chamfered face 320e so as to assure firm face to face seating of the lagging face of the cutter bit against the backing wall 32c . this makes less critical the tolerance requirements in machining the cutter bit and notch slot walls . as shown best in fig4 - 7 , each cutter bit 320 has a peripheral surface 320a contoured such that when mounted it forms a smooth continuation of the cutter disk periphery . it has a surface 320b facing in the direction of rotation , but angled outwardly and forwardly to form an acute angle relative to the general arcuate extent of its peripheral surface . the radially inner portion 320b1 of surface 320b turns into a concave fillet ( as a machining convenience ) and the radially outer portion 320b2 thereof approaching the cutting edge 320b3 is even more sharply angled to form a sharp acute angle with the bit &# 39 ; s peripheral surface where the two intersect . the cutting edge 320b3 extends , from a sharply pointed tip 320b3 &# 39 ;, obliquely rearward from cutter bit side face 320c to a location near the opposite side face 320d , where it again sweeps forwardly to form a secondary cutter edge tip 320b3 &# 34 ;. thus , with the cutter rotating , the two points 320b3 &# 39 ; and 320b3 &# 34 ;, and particularly the former , gouge into the carcass wall by impact thereon and , due to the angular set of the subjoining face 320b - 329b2 , immediately draw the carcass material toward the cutter . this helps complete the cutter bit penetration and starts the formation of a cutter - wide flap - like formation shredding cut . the gouging and drawing action of the cutters also provide a self - feeding action of the carcass , assisting gravity in maintaining progressive feeding of the tire against the cutter . the system thereby avoids any need for complex of cumbersome feed mechanisms in order to achieve high shredding production rates . side edges 320c1 and 320d1 serve as shearing edges working cooperatively with the respectively adjacent rack bar teeth edges to shear off the sides of the flap - like shredding cut as cutter rotation continues . finally , the root portions of the cutter bits cooperate with the two adjoining bar teeth tips to tear the flap or &# 34 ; shred &# 34 ; free of the carcass wall . along the length of cutter shaft 10 , the successive cutters 32 , interdigitated with the rack bar teeth 50 , are rotationally offset from each other circumferentially by uniform angular increments , there being seven such cutters in the example . the aligned cutters , each with two opposed cutting bits , thereby divide the 360 ° of circumference into fourteen equal increments . in this way , the work impact reaction forces of the cutter bits impinging against the tire are spread over the rotation cycles , minimizing heavy frame loading stresses and avoiding heavy pulsations of drive power demand . the smoothness of action and the uniformity of loading along the length of the cutter is still further improved by alternately reversing the angular &# 34 ; set &# 34 ; of the oblique cutting edges of successive cutter bits 320 in the series , such that any lateral component of cutting forces on a tire carcass or portion thereof attributed to oblique slant of the cutter bit edges tend to average out as the successive cutter bits in turn impinge the material . each of the cutter bit disks 32 and spacers 34 is rotationally locked to arbor 10 by key 70 and is slidable longitudinally on the arbor 10 for individual removal and replacement as need requires . the angular locations of the cutter disk key slots in the successive disks , of course , determine their respective angular positionings about the shaft . barrier teeth 41 project from back wall 36 into the spaces between cutters . a geared drive motor unit ( not shown ) is connected to the arbor shaft 10 to drive the cutter at desired speed . in practice , a speed of the order of 50 revolutions per minute is considered about optimum with the outer diameter of the cutter bits of the order of twenty inches and the width of the individual cutters and spacers 32 and 34 , respectively , of the order of three inches . at this slow speed , heat generation in the rubber is minimized and attendant energy losses are kept within acceptable limits . with this set of dimensions and rotative speed of the cutter , tires are shredded efficiently into pieces averaging three inches and smaller across the face in any direction . the total length of the cutter assembly is in excess of the diameter of the largest tire to be processed by the unit ; in the illustration something over three feet in length . as shown in fig8 - 10 , tire carcasses to be shredded , relieved of rim reinforcement cables , as dropped off the infeed conveyor c into hopper 35 and down upon the rotating cutter 30 . in the process of initiating the shredding action on the carcass , it is thrown toward compartment t and followed by other carcasses similarly treated . the result is a cramming and tumbling of the diminishing carcasses as the process continues , with the compartment t adding to the volume - handling capacity of the cutter over what is found to be possible with a confined or narrow hopper base , allowing only the edge of one tire at a time to be presented to the cutter . due to the smoothly averaged reaction forces on the cutter during each rotation cycle , drive power required by the cutter , hence the cost of the drive unit , is kept at a minimum . capability of the cutters to produce chunks or shreds generally of the same order of size indicated , with relatively few &# 34 ; fines &# 34 ; produced in so doing , also contributes greatly to saving of energy . the self - feeding action of the cutter also expedites the process and simplifies the apparatus . the severed pieces are driven downwardly immediately away from the field of cutting and onto an offbear conveyor 0 , so they do not linger and cause unnecessary abrasive heating or interference with oncoming material to be cut . tires are shredded virtually as fast as they can be delivered to the shredder . the action being something other than slicing , the cutter bit edges need not be or remain knife sharp to be effective and , therefore , need not be removed and sharpened frequently . the machine is thus capable of sustained efficient operation over long periods with little or no down time . accordingly , the invention has been disclosed in its preferred embodiment with the understanding that minor modifications and changes with respect to details may be made , but that the novel concepts are as set forth and as defined in the accompanying claims . in such claims the invention is defined in terms of its application to shredding of tire carcasses as disclosed with the understanding that alternative embodiments for this and other shredding applications are intended to be covered as well under principles of equivalency .