Abstract:
A rotary cutting apparatus in which the cutting of thin material is possible without the use of a curved blade. The fly and bed knives are positioned according to the formula 
     
       C=R-√R.sup.2 -(tangent S×L).sup.2 
     
     where C is bed knife camber, R is cutting radius, S is shear angle and L is rotor length. As a result of this geometry, uniform fly knife to bed knife clearance is maintained across the face of the rotor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to material size reduction machinery and in particular to rotary knife cutters. 
     BRIEF DESCRIPTION OF THE PRIOR ART 
     It is known in the art to mount longitudinal knives on a central rotating assembly and to reduce material between those fly knives and adjacent stationary knives known as bed knives. An apparatus embodying this concept is shown, for example, in U.S. Pat. No. 2,399,529. As is also known in that patent, it is also known to position the fly and bed knives in separate intersecting horizontal planes so that a scissor like action is achieved by their cooperative cutting action. In such an arrangement it has, however, conventionally been necessary to fabricate one of the interacting blades with a slight curve when they are positioned at close distances. Closely spaced blades are used to cut thin materials such as plastic film. Such a curved blade is shown in FIG. 3 of said U.S. Pat. No. 2,399,529. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a rotary knife cutter without curved blades which is suitable for cutting thin material such as plastic film without ripping or tearing. In the apparatus of this invention the fly blade extends at an equal distance from the longitudinal axis of the rotor. The fly and bed knives intersect with the plane of the horizontal center lines of the rotor at equal but opposite angles. The fly and bed knives are arranged according to the formula 
     
         C=R-√R.sup.2 -(tangent S×L).sup.2 
    
     In this formula C is camber, R is the cutting radius and L is the length of the rotor and S is the angle between the bed knife and the plane defined by the horizontal centerline of the rotor. Camber is the difference in the perpendicular lateral distance between the vertical center plane of the rotor at the two ends of the bed knife. As a result of this geometry, a uniform clearance between the fly knife and the bed knife is maintained across the face of the rotor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The apparatus of the present invention is further described with reference to the accompanying drawings in which: 
     FIG. 1 is a cut away end view of a preferred embodiment of the rotary cutter apparatus of the present invention; 
     FIG. 2 is a cut away side view of the apparatus shown in FIG. 2; 
     FIG. 3 is a perspective view of a rotor and bed knife arrangement which might be suitably incorporated in the rotary cutting apparatus of the present invention; 
     FIGS. 4a-c are plan views of the rotor and bed knife elements of the apparatus of the present invention showing the relative positions of these elements as the rotor progressively rotates through three sequential positions; 
     FIGS. 5a-c are end views of the rotor and bed knife elements of the apparatus of the present invention corresponding respectively, to FIGS. 4a-c; and 
     FIG. 6 is a side view of the rotor and bed knife elements of the apparatus of the present invention taken from VI--VI on FIG. 5b. 
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, a rotary cutter is shown generally at numeral 10. This cutter includes a feed mechanism through which material to be reduced is fed before being introduced into a cutting chamber shown generally at numeral 14. Below the cutting chamber there is a concave screen 16 which has holes which will alow material which has been reduced to a sufficiently small size to pass through to a generally concave, inclined collector 18 positioned below the screen for receiving and collecting reduced material. Such material slides downwardly on the inclined collector and is removed through transition chute 20. 
     Mounted on the side walls of the cutting chamber there are, respectively, bed knives 22 and 24. Positioned between these bed knives is a rotor shown generally at number 26. At one end this rotor is mounted at rotor bearing 28. At its opposite end it passes through bushing 30 and has a flywheel 32 fixed to its end. 
     It also will be seen that the rotor has a plurality of longitudinal pockets or cut-out portions as at 34 which are arranged at spaced intervals on its periphery. Each of the pockets forms a seat as at 36 on the rotor surface. Fixed to each of said seats is a fly knife as at 38 which engages in a cooperative cutting relationship with the bed knives as the rotor is rotated about its longitudinal axis by an electric motor (not shown). 
     It should be understood that the above described features are well known and conventional and do not in themselves describe the invention herein. Other equivalent arrangements to accomplish the above described functions are also possible. 
     The present invention lies in the relative arrangement of the fly and bed knives. This arrangement is shown in detail in FIG. 3 which for the sake of simplicity shows an arrangement in which there is only one bed knife and one fly knife on the rotor. It will be understood, however, that this arrangement will also be applicable to rotary cutters like the one shown in FIGS. 1 and 2 in which multiple fly and bed knives are used. Referring to FIG. 3, the rotor is shown at numeral 40 and the fly and bed knives are shown respectively at numerals 42 and 44. A rotor vertical center plane is defined by vertical center lines Vcl and Vcl&#39; on each end of the rotor. Similarly a horizontal center plane is defined by horizontal center lines Hcl and Hcl&#39; on each end of the rotor. The bed knife is elevated above the horizontal center plane of the rotor by a shear angle S. The fly knife is also oriented so that when the rotor is approximately positioned, it will intersect this horizontal center plane of the rotor from the opposite direction by angle S&#39; which is equal to angle S. The two ends 46 and 48 of the bed knife are laterally displaced from the center plane of the rotor at different positions. In FIG. 3 the perpendicular lateral distance of end 46 from the vertical center plane of the rotor (distance x) is greater than the perpendicular lateral distance of end 48 from the vertical center plane of the rotor (distance y). Thus in the case illustrated camber (C) equals y minus x. Rotor length is shown as L. R is the cutting radius of the distance between the longitudinal axis of the rotor and the terminal edge of the fly knife. The overall relationship between the above described factors in the cutter of the present invention is defined by the formula 
     
         C=R-√R.sup.2 -(tangent S×L).sup.2 
    
     Referring to FIGS. 4a-c, 5a-c and 6, there is shown another rotor 50 which has multiple fly knives 52, 54, 56 and 58 that sequentially engage bed knife 60. FIGS. 4a and 5a show fly knife 52 initially engaging bed knife 60. FIGS. 4b and 5b show the relative positions of these knives after the rotor has rotated slightly in a clockwise direction. FIGS. 4c and 5c show the relative positions of these knives as the rotor has rotated still further in a clockwise direction. As is shown particularly in FIG. 6, at any one time the point of engagement between these two knives is limited to a small area. As the rotor turns, this point of contact moves from one side to the other of the knives. As a result of the above described action, uniform fly knife to bed knife clearance is maintained across the face of the rotor. As the rotor turns still further, fly knives 54, 56 and 58 will be sequentially engaged by the bed knife in the same way that fly knife 52 was engaged. 
     It will be appreciated that a rotary cutter without curved blades has been described in which the cutting elements are arranged to allow efficient cutting of even of thin materials. Although the invention has been described with a certain degree of particularity, it is to be understood that the present disclosure has been made only as an example and that the scope of the invention is defined by what is hereafter claimed.