Abstract:
A machine for slitting a metal sheet has a number of knife holder assemblies each containing a rotary knife. The knife holder assemblies are mounted for movement along upper and lower rotating shafts which rotate the knives during a slitting operation. The position of the knife holder assemblies is programmably controlled for efficient and accurate positioning and adjustment of the knives along the rotating shafts for a variety of slitting configurations. Additionally, each of the upper and lower drive shafts contains a pair of drive shaft sections that are normally connected together during operation and can be disconnected for servicing of the knife holder assemblies on the shafts. The slitting machine includes upper and lower frames that are movable relative to each other by adjustment of a pair of jack screws to slit metal sheets of differing thicknesses without labor intensive adjustment or reconfiguration of the machine.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to slitter machines for slitting sheet metal into “mults” or strips and, more particularly, to slitter machines having adjustable knives for varying the size and/or number of mults processed by the slitter machine. 
   BACKGROUND OF THE INVENTION 
   Much of the steel produced by mills is in the form of coiled steel sheet, but rarely does the sheet correspond in width to the multitude of products that are stamped or otherwise formed from it. Accordingly, the steel sheet is usually slit longitudinally to sizes suitable for the particular products. Indeed, special slitting machines are made for this purpose. 
   The typical slitting machine has circular blades or knives arranged in pairs on two powered shafts or arbors, there being one knife of each pair on one of the arbors and the second knife of the pair on the other arbor. The arbors are connected to a drive system for counter-rotation. During operation, sheet metal is moved between the arbors and cut into mults by the knives counter-rotating on the arbors. Actually each knife is nothing more than a hardened steel disk having flat end faces and a cylindrical peripheral face which intersects the end faces at relatively sharp cutting or shearing edges. The disks of each pair are positioned on their respective arbors, often with a slight overlap. Overlap or not, the knives of each pair are positioned close enough to each other to enable them to cut or shear the metal sheet as it passes between those knives. In other words, the metal sheet is drawn between the two knives of a pair the disk-like knives shear the sheet along the opposite cutting edges, thus producing a clean longitudinal cut in the sheet. Not only are the disk-like knives arranged in pairs, but the pairs of knives are also usually organized into left and right hand configurations to prevent the longitudinal segments of the slit sheet from acquiring a twist or spiral upon emerging from the slitting machine. 
   The size of the mults is determined by the spacing of the knives on the arbors. The knives, while being fixed firmly on their respective arbors during the operation of the machine, nevertheless may be removed for sharpening or may be repositioned so that the width of the segments slit may be varied. Setting the knives on the arbors of a slitting machine however is a tedious and time-consuming procedure, requiring a high degree of skill, for the knives must be located with considerable precision, not only to acquire the proper width for the cut, but to also maintain a clean high quality cut as well. 
   In one type of slitting machine, the knives are carried on hubs that slide over the arbor and are secured with set screws in the desired positions. To set the knives of a pair in the proper position, the location of the cut desired from the pair of knives is usually located by measuring with a tape measure from reference point on the machine. One of the knives is then moved over its arbor to the point located with the tape measure and the set screw of its hub is turned down to secure the knife. Once the knife is so positioned, an indicator gage should be brought against it while the arbor is turned slowly. With the indicator gage the knife is checked for wobble and usually adjustments must be made by loosening the set screws and tapping the knife lightly to eliminate the wobble. The same procedure is then repeated with the other knife of the pair, only its location is determined from the location of the previous knife, there usually being an axial gap on the order of 7 to 10 percent of the thickness of the metal sheet between the opposite cutting edges of the two knives. To change the size and number of mults produced from the sheet metal, the hubs must be released from the arbors and moved to new locations. New hubs would be added, or existing hubs removed, as dictated by changes in the number of mults to be cut in the sheet metal. 
   In another type of slitting machine, spacers separate the knives. These spacers are large enough and are machined with enough precision to minimize the wobble inherent with conventional arbors, but present complexities in the selection of spacers and shims to properly locate the knives. The selection of spacers and shims requires a considerable amount of skill. Furthermore, the spacers must be handled carefully, to avoid nicks that will skew the knives and create a wobble as they rotate. 
   To change the size and number of mults produced from the sheet metal, the spacers must be removed from the arbor and replaced with a new set of spacers adapted to the new cutting pattern. 
   In the past, such replacements and adjustments were generally performed by hand. This use of manual labor was expensive and slowed the process of conversion from one cutting job to the next. The task of replacement and adjustment was difficult physically, often requiring workers to lift the heavy hubs or spacers to uncomfortable heights. Furthermore, where spacers were used, it was necessary to maintain a sizable inventory of such spacers to provide flexibility in cutting different sizes and numbers of mults. 
   One prior attempt to solve such problems is disclosed in U.S. Pat. No. 4,887,502 directed to a machine for slitting metal. The machine includes upper and lower powered arbors and also upper and lower storage arbors which align respectively with the upper and lower powered arbors. Each powered arbor supports and turns several knives which are mounted on hubs along those arbors, and these knives when not needed may be moved, along with their hubs, onto the aligned storage arbors. Each knife is captured in a carriage which moves along one of the beams. The knives are positioned through a lead screw which drives a carriage having stops against which knives on the upper and lower arbors are manually moved and set in position through contact with the stops. The carriage may also be provided with fingers which actually capture the knives of a pair and move them to the correct position. 
   To eliminate the need to reconfigure a slitting machine for a particular slitting operation, a slitting line may include multiple slitters having different knife configurations that can be moved into and out of the line. 
   There remains a need in the art for slitting machines which can be automatically set up and adjusted, including the replacement or servicing of knives on the arbors, with minimal labor on the part of the operator or user. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the foregoing and other shortcomings and drawbacks of slitting systems and methods of slitting heretofore known. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention. 
   These and other needs are addressed by a CNC slitting machine having an upper and a lower frame, an upper and a lower rotating shaft, and pairs of knife holder assemblies supported for movement along the respective shafts. Each knife holder assembly supports an arbor for mounting a knife. Preferably, the rotating upper and lower shafts are mounted respectively in the upper and lower frames. A drive motor is operably coupled to the shaft assemblies for rotation. 
   The slitting machine of one presently preferred embodiment includes a knife holder position adjustment system that is operably coupled to each of the knife holder assemblies for movement of the assemblies along the respective drive shafts. The machine advantageously includes a programmable logic controller that is electrically coupled to the knife holder position adjustment system associated with each knife holder assembly. The programmable logic controller, in combination with the knife holder position system associated with each knife holder assembly, positions the knife holder assemblies along the shafts and secures the knife holder assemblies in place for rotation of the knives with the shafts. A presently preferred machine is capable of cutting from one-to-five mults. To change jobs, the operator stands at an operating station and enters the number of desired mults, the desired individual mult widths, the material thickness, the desired percentage of horizontal gap between cooperating upper and lower knives, the desired relative vertical knife position, and the desired offset distance from centerline into a human-to-machine (HMI) interface coupled to the programmable logic controller. The slitting machine itself then sets up the machine automatically. 
   The programmable logic controller is preferably part of a closed-loop feedback control system which receives one or more signals from sensors monitoring the position or movement of the knives and which reacts to the sensed position or movement of the knives to properly position the knives on the shafts. 
   In alternative preferred embodiments of the slitting machine, the drive shaft assemblies each include a number of drive shaft sections releasably coupled to one another for rotation in the machine frame. In one embodiment, each drive shaft assembly includes a pair of drive shaft sections that are releasably coupled to each other to form a single elongated drive shaft assembly. The pair of adjacent drive shaft sections of each drive shaft are selectively uncoupled from one another for servicing the machine, such as repair or replacement of a knife in the knife holder positioned proximate the juncture between the drive shaft sections. In one presently preferred embodiment, each drive shaft section includes a spindle that projects axially from the section and a coupling releasably connects the spindles on the pair of adjacent drive shaft sections. A screw is connected to at least one of the drive shaft sections so that rotation of the screw axially withdraws the connected drive shaft section from the adjacent shaft section to thereby provide access to the knife holder assembly and associated knife proximate the juncture between the pair of drive shaft sections. 
   In another preferred embodiment of a slitting machine according to this invention, the upper and lower frames of the machine are pivotally coupled together. The upper frame is movable relative to the lower frame to adjust the relative vertical positioning of the knives supported in the upper and lower knife assemblies for slitting metal sheets of differing thicknesses. A frame adjustment mechanism in the form of a pair of jack screws is mounted between the upper and lower frames and an actuator coupled to each of the jack screws simultaneously adjusts the jack screws and moves the upper frame in a direction generally perpendicular to the drive shafts relative to the lower frame. Preferably, the upper frame remains generally parallel relative to the lower frame during movement. 
   As a result of the various embodiments of this invention, a slitting machine is easily and efficiently set up and reconfigured by an operator through the HMI interface, programmable logic controller and knife holder position adjustment system for slitting mults of various sizes without significant machine downtime and labor-intensive procedures. Furthermore, the machine is readily adjustable for slitting metal sheets of differing thicknesses by conveniently adjusting the upper frame relative to the lower frame. Moreover, in one embodiment of the machine, each of the drive shaft assemblies is split or segmented into sections which can be uncoupled from one another for convenient servicing and/or replacement of the knife and knife holder assemblies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
     The objectives and features of the invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a perspective view of a slitting machine according to a presently preferred embodiment with the metal sheet being slit and other components used in conjunction with the machine; 
       FIG. 2  is a side elevational view of the slitting machine according to this invention; 
       FIG. 3  is a cross-sectional view taken generally along line  3 — 3  of the slitting machine of  FIG. 2  showing a pair of knife holder assemblies on the upper and lower drive shaft assemblies, respectively; 
       FIG. 4  is a cross-sectional top view taken along line  4 — 4  of  FIG. 2  of the slitting machine; 
       FIG. 5  is a cross-sectional view taken along line  5 — 5  of  FIG. 4  of a knife holder assembly according to a presently preferred embodiment of this invention on the slitting machine; 
       FIG. 6  is a cross-sectional view taken along line  6 — 6  of  FIG. 3  of the knife holder assembly on the slitting machine; 
       FIGS. 7-8  are side elevational views partially broken away of a portion of the upper drive shaft assembly in coupled and uncoupled configurations, respectively; 
       FIG. 9  is a view seen on line  9 — 9  of  FIG. 2 ; 
       FIG. 10  is a cross-sectional view taken along line  10 — 10  of  FIG. 9 ; 
       FIG. 11  is a functional block diagram of a control system according to a presently preferred embodiment of this invention; and 
       FIGS. 12-19  are software flow diagrams of various routines performed by the control system of the present invention to control the position of the knife holder assemblies on the slitting machine. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a slitting machine  10  according to a presently preferred embodiment of this invention is used for shearing metal sheet  12 , such as sheet steel, into multiple segments or mults  14  of a desired width along slits  16 . The metal sheet  12  is normally provided from a mill or other supplier of mill products in a coil  18 . The coil  18  is supported on a spool  20 . The metal sheet  12  is withdrawn from the coil  18  and fed into the machine  10 . Typically, the metal sheet  12  passes through a straightening machine  22  to remove the coil set. The sheet  12  alternatively may be fed into the machine  10  in individual sections, preferably with the assistance of a skewed roller table (not shown) or the like. 
   Referring to  FIGS. 1 and 2 , a presently preferred embodiment of the slitting machine  10  includes an upper frame  26  movably coupled at spaced ends thereof to a lower frame  28 . The upper and lower frames  26 ,  28  of the machine  10  include upper and lower drive shaft assemblies  44 ,  46 , respectively, mounted therein for rotation. The drive shaft assemblies  44 ,  46  are supported in the respective frames  26 ,  28  by spaced pillow block bearings  48 . Corresponding ends of the upper and lower drive shaft assemblies  44 ,  46  are coupled to a gear box  50  by separate universal couplings  52 . A motor  54  is connected to the gear box  50  to provide rotational movement through the gear box  50  to the universal couplings  52  and ultimately the drive shaft assemblies  44 ,  46 . The drive shaft assemblies  44 ,  46  are rotated in opposite counter-rotating directions for pulling and slitting the metal sheet  12  passing therebetween. 
   A number of knife holder assemblies  56  are supported for movement along the upper and lower drive shaft assemblies  44 ,  46  as shown in  FIGS. 2 and 3 . The knife holder assemblies  56  are supported in cooperating pairs at spaced positions along the upper and lower drive shaft assemblies  44 ,  46  in the upper and lower frames  26 ,  28 , respectively. The metal sheet  12  to be slit passes between the knife holder assemblies  56  on the upper shaft  44  and the knife holder assemblies  56  on the lower shaft  46  along a pass line PA as indicated in FIG.  2 . Each upper knife holder assembly  56  includes a rotary knife  58  which cooperates with the rotary knife  58  in the corresponding lower knife holder assembly  56  of each cooperating pair to cut, shear or otherwise slit the metal sheet  12 . A total of twelve knife holder assemblies  56  (six pair) are shown in  FIG. 1  for producing five strips or mults  14 . Although, it should be readily understood that the exact number of knife holder assemblies  56  is dependent upon the desired width and configuration of the mults  14  and the metal sheet  12  being slit. 
   Each of the knife holder assemblies  56  is not only supported for movement along the respective drive shaft assembly  44 ,  46 , but is also operatively coupled to either a fixed upper threaded shaft  60  or a fixed lower threaded shaft  62 . As shown in  FIGS. 5 and 6 , the respective fixed or stationary threaded shafts  60 ,  62  pass through a ball nut  64  in each of the knife holder assemblies  56 . Each ball nut  64  is connected to a positioning motor  66  which is likewise electrically connected to a programmable logic controller  68  according to one aspect of this invention. The positioning motor  66  may be a servo motor, stepper motor, DC motor, AC vector motor, pneumatic motor, hydraulic motor, linear induction motor or any other type of drive motor. The programmable logic controller  68  is coupled to a human-to-machine (HMI) interface  70 , such as a touch screen or the like (FIG.  1 ), that receives data inputs from a user. The controller  68  is also coupled to a user input  72  (FIG.  1 ), such as user actuatable buttons (not shown), so that the controller  68  receives these user inputs as well to control operation of the slitting machine  10 . The threaded shafts  60 ,  62 , ball nuts  64 , positioning motors  66 , programmable logic controller  68  and associated components contribute to form a knife holder position adjustment system that moves the individual knife holder assemblies  56  along the respective drive shaft assembly  44 ,  46  for proper, efficient and accurate positioning prior to slitting the metal sheet  12  as described in detail below. 
   In another aspect of the slitting machine  10  according to this invention, each drive shaft assembly  44 ,  46  includes a pair of drive shaft sections  74  as shown in FIGS.  2  and  7 - 8 . The pair of drive shaft sections  74  for each drive shaft  44 ,  46  are adapted to be selectively uncoupled so that the pair of drive shaft sections  74  for each drive shaft  44 ,  46  can be separated. Each drive shaft section  74  preferably includes a spindle  76  projecting axially therefrom in opposition to the spindle  76  on the adjacent drive shaft section  74  of the respective pair. When the pair of sections  74  are coupled together, a tubular coupling  77  surrounds the spindles  76  to transfer the rotational movement of the drive shaft assembly  44 ,  46  along the length of the shafts. 
   To provide for convenient and efficient access to the knife blades  58  of the respective knife holder assemblies  56  for repair, replacement or servicing of the various components of the knife holder assemblies  56 , the drive shaft sections  74  can be uncoupled through rotation of an actuator in the form of a handle  80  as shown in  FIGS. 7 and 8 . Specifically, a user rotates the handle  80  and thereby a collar  82  threaded onto a screw  84 . The collar  82  is connected to the pillow block bearing  48  on the end of the drive shaft section  74  so that retraction of the collar  82  by rotation of the screw  84  likewise axially retracts the pillow block bearing  48  and connected drive shaft section  74  away from the adjacent drive shaft section  74  as shown in FIG.  8 . Retraction of the drive shaft section  74  allows increased access for a technician to the knife holder assembly  56  and associated components for servicing, repair, replacement or the like. 
   The appropriate knife holder assembly  56  can be conveniently and efficiently moved into location proximate the juncture between the drive shaft sections  74  for appropriate servicing. Once the servicing is completed, reverse rotation of the handle  80  likewise advances the retracted drive shaft section  74  toward the adjacent drive shaft section  74  for subsequent recoupling with the coupling  77  and operation of the slitting machine  10 . While one particular arrangement for coupling the drive shaft sections  74  together and movement thereof for uncoupling has been shown and described herein, it should be readily appreciated that alternative arrangements can be provided within the scope of this invention. For example, utilization of a servo motor or other automated process may be relied upon for movement of the drive shaft sections  74  relative to one another upon demand by a service technician. Likewise, various arrangements and schemes for coupling the drive shaft sections  74  together with or without a coupling  77  or the like may be utilized within the scope of this invention. 
   Referring to  FIGS. 2-3 , another feature of the slitting machine  10  according to presently preferred embodiments of this invention includes a pair of jack screws  86  positioned between spaced opposite ends of the upper and lower frames  26 ,  28  of the slitting machine  10 . The jack screws  86  are positioned between the upper and lower frames  26 ,  28  proximate a front of the machine  10 . The upper and lower frames  26 ,  28  are pivotally coupled together around a pivot shaft  88  proximate the back of the machine  10 . The lower frame  28  is stationary while the upper frame  26  is capable of pivotal movement relative to the lower frame  28  about the pivot shaft  88 . A pair of die springs (not shown) may be connected between the upper frame  26  and the lower frame  28  on respective opposite sides of the slitting machine  10  and close to the jack screws  86  to eliminate clearances between the upper and lower frames  26 ,  28  at their connection points. A jack screw motor  90  is mounted to provide a rotational input to one of the jack screws  86  and to a jack screw transfer shaft  92  that couples the two jack screws  86  together. Coupling sleeves  94  are mounted on each of the spaced ends of the transfer shaft  92  for joining the shaft  92  to the respective jack screw  86 . 
   In operation, the jack screw motor  90  provides a rotational input to the adjacent jack screw  86  and to the opposite jack screw  86  through the transfer shaft  92 . Jack screw motor  90  is electrically coupled to the programmable logic controller  68  and receives instructions from the controller  68  according to inputs entered by the operator through the HMI interface  70 . Rotation of the motor  90  simultaneously raises or lowers the jack screws  86  for pivotally moving the upper frame  26  relative to the stationary lower frame  28  about the pivot shaft  88 . As a result, the spacing between the upper and lower drive shaft assemblies  44 ,  46  is adjustable by rotation of the jack screw motor  90  that causes extension or retraction of the jack screws  86 . The movement of the upper and lower frames  26 ,  28  and the associated drive shaft assemblies  44 ,  46  relative to each other controls the relative vertical positioning of the rotary knives  58  supported in knife holder assemblies  56  to accommodate metal sheet  12  of different thicknesses passing between the knife holder assemblies  56  for slitting. The jack screw motor  90  coupled to each of the jack screws  86  allows for more precise adjustment of both jack screws  86  and the movement of the entire upper drive shaft assembly  44  relative to the lower drive shaft assembly  46  is in a generally parallel orientation throughout the movement. As a result, the vertical spacing between the knife holder assemblies  56  on the upper drive shaft assembly  44  relative to the knife holder assemblies  56  on the lower drive shaft assembly  46  is consistent and does not vary dependent upon the lateral position of the respective knife holder assemblies  56 . It will be appreciated that the jack screw motor  90  could be replaced with a hand wheel (not shown) or any other suitable device that is capable of moving the jack screws  86  as desired. Moreover, it will be appreciated that the jack screws  86  can be replaced with any other type of motor capable of moving the upper frame  26  relative to the lower frame  28  about the pivot shaft  88 . 
   As shown in  FIGS. 9 and 10 , precise alignment of the upper frame  26  relative to the lower frame  28  is provided by an alignment block  30  fixed to the upper frame  28  being captured within a clevis  32  fixed to the lower frame  28 . The tolerances of the alignment block  30  and clevis  32  are selected to assure proper registration of the upper and lower frames  26 ,  28  relative to each other. The engagement surfaces of the alignment block  30  and/or the clevis  32  may be hardened with a suitable material to reduce wear of the alignment components through repeated movement of the upper frame  26  relative to the fixed lower frame  28 . 
   Referring now to  FIGS. 2-6 , a presently preferred embodiment of the knife holder assembly  56  according to this invention and the manner in which the position of the knife holder assemblies  56  is adjusted in the CNC slitting machine  10  will now be described. The knife holder assemblies  56  are supported in cooperating pairs along the upper and lower drive shaft assemblies  44 ,  46  such that one knife holder assembly  56  of each pair is positioned along the upper drive shaft assembly  44  and the complimentary knife holder assembly  56  of each pair is positioned along the lower drive shaft assembly  46 . The knife holder assemblies  56  are generally identical with the exception of their orientation in the slitting machine  10 ; therefore, a knife holder assembly  56  positioned along the upper drive shaft assembly  44  will be described with respect to  FIGS. 3-6 . It should be readily understood that the same description applies to each of the other knife holder assemblies  56  positioned along the upper drive shaft assembly  44  as well as those positioned along the lower drive shaft assembly  46  in a reoriented position. 
   As shown in  FIGS. 3 ,  5  and  6 , each knife holder assembly  56  includes a retainer block  96  with an upper smaller hole  98  and a lower larger hole  100  passing between the front and back faces of the retainer block  96 . The retainer block  96  also includes a pair of anchor flanges  102  ( FIG. 5 ) spaced on the lateral sides of the retainer block  96  and positioned with an exposed face similarly oriented in the direction of the smaller hole  98 . A pair of linear bearing blocks are mounted in spaced relationship to each of the anchor flanges  102  in either an inboard or outboard position  106 ,  108  (FIG.  3 ). Each linear bearing block  104  is sized and configured to capture one of the rails  110  ( FIGS. 3 and 5 ) which extends lengthwise on the slitting machine  10  and which are provided in inner and outer rail pairs to support the knife holder assemblies  56 . More specifically, a pair of upper inner rails  110 , a pair of upper outer rails  110 , a pair of inner lower rails  110  and a pair of outer lower rails  110  are provided on the machine  10  for supporting the respective knife holder assemblies  56 . 
   Each knife holder assembly  56  is coupled through the linear bearing blocks  104  to each of the rails  110  in one of the inner or outer rail pairs. The inner and outer rails  110  on the upper and on the lower frame  26 ,  28  of the machine  10  advantageously allow for more intimate nesting of the adjacent knife holder assemblies  56  on the drive shaft assemblies  44 ,  46 . A first knife holder assembly  56  is coupled through the linear bearing blocks  104  to each of the rails  110  on the inner pair of the respective upper or lower machine frames  26 ,  28 . The knife holder assemblies  56  adjacent to the first are coupled through their respective linear bearing blocks  104  to the rails  110  of the outer pair to avoid interference with the first knife holder assembly  56  and allow for close pack nesting of the adjacent knife holder assemblies  56  and slitting of the metal sheet  12  for relatively narrow mults  14 . 
   As shown in  FIG. 6 , one of the fixed or stationary threaded shafts  60 ,  62  in the respective machine frame  26 ,  28  projects through the smaller hole  98  of each retainer block  96 . The ball nut  64  is inserted into a sleeve  112  positioned in the smaller hole  98  of each retainer block  96 . The ball nut  64  is threadably coupled to the threaded shaft  60  or  62  and is fastened to the sleeve  112  so that the ball nut  64  and sleeve  112  are free to rotate relative to the fixed or stationary threaded shafts  60 ,  62 . An opening  114  is provided in the sleeve  112  to accommodate the ball nut  64 . A presently preferred embodiment of the ball nut  64  is commercially available from Thomson-Saginow (www.thomsonind.com) as Catalog Part No. 5704271. 
   As shown particularly in  FIGS. 5 and 6 , the ball nut  64  is coupled by a gear belt  116  to the positioning motor  66  mounted by a pivot mount  118  to an upper arm  120  of the retainer block  96 . The positioning motor  66  is mounted by the pivot mount  118  on a tension plate  122  and a tension adjustment mechanism  124  allows for the accurate positioning of the positioning motor  66  and tension plate  122  on the retainer block  96 . Appropriate tension on the gear belt  116  coupled to the output shaft of the positioning motor  66  is maintained by the tension adjustment mechanism  124 . The orientation of the positioning motor  66  relative to the ball nut  64  on the retainer block  96  according to the presently preferred embodiment of this invention is correctly shown in its relative position in  FIGS. 3 and 5 ; however, in  FIG. 6  the positioning motor  66  is shown out of position for clarity and completeness without being blocked by other components of the knife holder assembly  56 . 
   Each positioning motor  66  of the knife holder assemblies  56  is electrically and operably coupled to the programmable logic controller  68 . The programmable logic controller  68  is likewise electrically and operably coupled to the HMI interface  70  (FIG.  1 ). Each positioning motor  66  receives instructions from the programmable logic controller  68  according to inputs entered by the operator through the HMI interface  70  and, upon actuation, the respective positioning motors  66  rotate the gear belt  116  trained around the output of the positioning motor  66  and the ball nut  64 . Rotation in the appropriate direction of the positioning motor  66  output shaft and likewise the ball nut  64  that is threadably coupled to the fixed or stationary threaded shaft  60  or  62  moves the knife holder assembly  56  relative to the threaded shaft  60  or  62  to the appropriate position. Likewise, the ball nut  64  and positioning motor  66  assembly lock the knife holder assembly  56  at the desired position during operation of the machine  10  through the torque of the motor  66 . 
   Each drive shaft section  74  of the upper and lower drive shaft assemblies  44 ,  46  includes a keyway  126  projecting radially inwardly from the outer circumference of the drive shaft sections  74 . The keyway  126  is sized and configured to receive a key  128  projecting radially inwardly from an arbor  130  seated within the large hole  100  in the retainer block  96 . The arbor  130  is therefore coupled to the drive shaft assembly  44 ,  46  for rotation with the drive shaft relative to the retainer block  96 . Likewise, the arbor  130  has the rotary disk-shaped knife  58  with a pair of stripper plates  132  mounted on the opposite faces of the knife  58  for rotation with the arbor  130 . The stripper plates  132  and knife  58  are mounted by bolts or otherwise to the arbor  130  for rotation with the drive shaft assembly  44 ,  46 . Ball bearings  134  are provided between sleeve  112  and retaining block  96  at the smaller upper hole  98  and bearings  140  are likewise provided between the inner surface of the larger hole  100  in the retainer block  96  and the arbor  130  as shown in FIG.  6 . Spacers  138  and ball bearings  140  are included to allow for the free rotational movement of the arbor  130  relative to the knife holder assembly retainer block  96 . As such, rotational input from the motor  54  through the gear box  50  and universal couplings  52  to the appropriate drive shaft assembly  44 ,  46  drives the arbor  130  and associated stripper plates  132  and knife  58  for slitting of the metal sheet  12 . 
   In accordance with the principles of the present invention, the knife holder assemblies  56 , arbors  130  and knives  58  are not supported by the upper and lower drive shaft assemblies  44 ,  46 . Rather, the knife holder assemblies  56  are supported for movement along the upper and lower drive shaft assemblies  44 ,  46  by the upper and lower frames  26 ,  28  through the rails  110  and the linear bearing blocks  104 . In this way, the upper and lower drive shaft assemblies are torsional members only to provide torque to the knives  58 . The drive shaft assemblies  44 ,  46 , therefore, do not take any separating load during the slitting operation since the load is transmitted from the knife holder assemblies to the upper and lower frames  28 ,  28 . 
   Referring now to  FIG. 11 , a control system  200  of the slitting machine  10  according to a presently preferred embodiment is shown. As described in detail above, the movement of each knife holder assembly  56  along the upper and lower drive shaft assemblies  44 ,  46  to a desired position is controlled through inputs applied to the positioning motors  66  from the programmable logic controller  68 . The control system  200  includes a pair of upper and lower linear encoders  202  ( FIGS. 3 and 12 ) associated with the upper and lower knife holder assemblies  56  that provide inputs to the programmable logic controller  68  to indicate the position of each knife holder assembly  56  along the respective upper and lower drive shaft assemblies  44 ,  46 . Each linear encoder  202  includes an elongated scale  204  that is supported by the upper and lower frames  26 ,  28  and scanning units  206  that are each mounted to one of the knife holder assemblies  56 . The scanning units  206  are electrically coupled to the controller  68  and are operable to read the scale  208  ( FIG. 3 ) housed within each linear encoder  202  and provide scale data to the programmable logic controller  68  so that the position of each knife holder assembly  56  is monitored and controlled by the controller  68  in a closed-loop feedback control. A suitable linear encoder  202  for use in the slitting machine  10  of the present invention is commercially available from Heidenhain Corporation of Schaumburg, Ill., although other linear encoders and other position detecting systems are possible as well. 
   The control system  200  of the present invention is also operable to adjust the relative vertical positioning of the upper and lower rotary knives  58 . As described in detail above, pivotal movement of the upper frame  26  relative to fixed lower frame  28  is controlled through actuation of the jack screws  86  by the jack screw motor  90 . To this end, the jack screw motor  90  receives an input from the programmable logic controller  68  to extend or retract the jack screws  86  according to inputs entered by the operator through the HMI interface  70 . A transducer  210  is mounted on the slitting machine  10  and is electrically coupled to the programmable logic controller  68  to provide an input to the controller  68  that indicates the relative vertical position of upper and lower rotary knives  58 . The programmable logic controller  68  uses the data generated by the transducer  210  to monitor and adjust the relative vertical positioning of the upper and lower knives  58  in a closed-loop feedback control as well. 
   Referring now to  FIG. 12 , a main “KNIFE POSITIONING ROUTINE”  300  is shown that is performed by the control system  200  of the present invention to control positioning of the upper and lower knife holder assemblies  56  along the upper and lower drive shaft assemblies  44 ,  46  and to adjust the relative vertical positioning of upper and lower knives  58  in accordance with the principles of the present invention. As will be described in greater detail below, “KNIFE POSITIONING ROUTINE”  300  generally includes five (5) routines, including the “ENTER VALUES ROUTINE”  302 , “AUTO START ROUTINE”  304 , “PERMISSIVE CHECK ROUTINE”  320 , “POSITION KNIVES ROUTINE”  306  and “KNIFE POSITION CHECK ROUTINE”  308 , that are performed by the programmable logic controller  68  or the HMI interface  70  to enable the slitting machine  10  to set up the machine automatically according to data input by a user through the HMI interface  70 . 
   More specifically, and referring to  FIG. 13 , an “ENTER VALUES ROUTINE”  302  is initially performed by controller  68  and HMI interface  70  that prompts the user at step  310  to input data or values through the HMI interface  70 . These values include the number of desired mults  14 , the desired width of each mult  14 , the material thickness of sheet  12 , the desired percentage of horizontal gap between cooperating upper and lower knives  58 , the desired relative vertical position of the upper and lower rotary knives  58  and the desired offset distance from centerline, although other inputs are possible as well without departing from the spirit and scope of the present invention. At step  312 , the HMI interface  70  determines whether these input values are within acceptable size limits previously defined and stored in the HMI interface  70 . If the values input by the user are acceptable, the input values received at step  310  are then stored in the programmable logic controller  68  at step  314 . Otherwise, an error message is displayed at step  316  to alert the operator that one or more of the entered values are out of the acceptable range. The user is then prompted at step  310  to continue inputting data through the HMI interface  70  that is within the acceptable range. 
   After the acceptable values are received and stored in the programmable logic controller  68  from the “ENTER VALUES ROUTINE”  302 , the controller  68  executes an “AUTO START ROUTINE”  304  that enables the slitting machine  10  to automatically position the knife holder assemblies  56  according to the data input by the user during the “ENTER VALUES ROUTINE”  302 . The “AUTO START ROUTINE”  304  is shown in FIG.  14  and includes a step  318  at which the programmable logic controller  68  determines whether an “Auto Start” push button (not shown) has been turned on or actuated by the user. The “Auto Start” push button is a user actuatable button located at the user interface  72  of the machine  10  that enables the slitting machine to automatically set itself up according to the data input by the user at step  310  when the “Auto Start” push button is enabled. If the “Auto Start” push button is enabled, the “AUTO START ROUTINE”  304  performs a “PERMISSIVE CHECK ROUTINE” at step  320  that checks various conditions of the slitting machine  10  to insure that the machine  10  is operating properly. The “PERMISSIVE CHECK ROUTINE”  320  is performed continuously during execution of the main “KNIFE POSITION ROUTINE”  300  of FIG.  1  and will be described in detail below in connection with FIG.  15 . Otherwise, if the “Auto Start” push button is not enabled as determined at step  318 , control passes back to the “ENTER VALUES ROUTINE”  302  of FIG.  13 . 
   If the “PERMISSIVE CHECK ROUTINE”  320  passes, indicating that the machine  10  is operating properly, the programmable logic controller  68  enables the “Auto Start” capability of the slitting machine  10  and turns on the “Auto Light” (not shown) located at the user interface  72  at step  322 . If the “PERMISSIVE CHECK ROUTINE”  320  fails, indicating that the machine  10  is not operating properly, the programmable logic controller  68  disables the “Auto Start” capability of the slitting machine  10  and turns off the “Auto Light” at step  324  and control passes back to the “ENTER VALUES ROUTINE”  302  of FIG.  13 . 
   Referring now to  FIG. 15 , the “PERMISSIVE CHECK ROUTINE”  320  performed continuously by the programmable logic controller  68  will now be described. The “PERMISSIVE CHECK ROUTINE”  320  performs various system checks at steps  326 - 336  to determine whether the slitting machine  10  is operating properly. In particular, the controller  68  determines at step  326  whether an “Emergency Stop” or “E-Stop” button (not shown) has been turned on or actuated by the user. The “E-Stop” push button is a user actuatable button located at the machine  10  that immediately stops all operation of the slitting machine  10  when the “E-Stop” push button is enabled, such as during an emergency. If the “E-Stop” push button is not enabled, the controller  68  performs a check at step  328  to determine if all communication systems of the slitting machine  10  are functioning properly. At step  330 , the programmable logic controller  68  determines whether an “Auto Stop” push button (not shown) has been turned on or actuated by the user. The “Auto Stop” push button is a user actuatable button located at the user interface  72  that disables the “Auto Start” operation of the slitting machine  10  and turns off the “Auto Light” when the “Auto Stop” push button is enabled. 
   Further referring to  FIG. 15 , the programmable logic controller  68  performs a “KNIFE MOVEMENT CRASH ROUTINE” at step  332  to determine whether movement of the knife holder assemblies  56  will cause any two or more of them to hit into each other, thereby possibly damaging the slitting machine  10 . The “KNIFE MOVEMENT CRASH ROUTINE”  332  will be described in detail below in connection with FIG.  16 . At step  334 , the programmable logic controller  68  determines whether the knives have moved to their desired positions along the upper and lower drive assemblies  44 ,  46  so that movement of the knives is complete. At the last step  336  of the “PERMISSIVE CHECK ROUTINE”  320 , the programmable logic controller  68  performs a “AUTO ON WATCH DOG ROUTINE” to determine whether the control system  200  is operating properly. The “AUTO ON WATCH DOG ROUTINE”  336  will be described in detail below in connection with FIG.  17 . Failure of any one of the permissive checks performed at steps  326 - 336  causes the programmable logic controller  68  to disable the “Auto Start” capability of the slitting machine  10  and turn off the “Auto Light” at step  338  and control passes back to the “ENTER VALUES ROUTINE”  302  of FIG.  13 . 
   Referring now to  FIG. 16 , the “KNIFE MOVEMENT CRASH ROUTINE”  332  performed by the programmable logic controller  68  will now be described. At step  340 , the programmable logic controller  68  monitors the movement and position of each knife holder assembly  56  through the scale data provided by the scanning units  206  of the linear encoders  202 . At step  342 , the programmable logic controller  68  determines whether movement of the knife holder assemblies  56  to the values entered at step  310  will cause any two or more of them to hit into each other. If this is the case, the programmable logic controller  68  at step  344  stops movement of all knife holder assemblies  56  that are going to hit each other while permitting all other knife holder assemblies  56  to continue movement to their desired positions. If all knife holder assemblies  56  are clear of each other as determined at step  342 , control passes back to step  340 . 
   Referring now to  FIG. 17 , the “AUTO ON WATCH DOG ROUTINE”  336  performed by the programmable logic controller  68  will now be described. At step  346 , the controller  68  monitors the movement and position of each knife holder assembly  56  through the scale data provided by the scanning units  206  of the linear encoders  202 . At step  348 , the controller  68  determines whether each knife holder assembly  56  has reached its desired position within a predetermined period of time stored in the controller  68 . If any one of the knife holder assemblies  56  does not reach its desired position within the predetermined period of time as determined at step  348 , the controller  68  indicates that the permissive check has failed at step  350  and control passes back to step  338  as described in detail above. 
   Referring now to  FIG. 18 , a “POSITION KNIVES ROUTINE”  306  performed by the programmable logic controller  68  during the main “KNIFE POSITIONING ROUTINE”  300  of  FIG. 12  will now be described. The “POSITION KNIVES ROUTINE”  306  is responsible for moving the knife holder assemblies  56  to their desired positions according to the data input by the user at step  310 . At step  352 , the controller  68  reads the values input by the user at step  310  and stored at step  314  of the “ENTER VALUES ROUTINE”  302  (FIG.  13 ). At step  320 , the controller  68  performs the “PERMISSIVE CHECK ROUTINE” described in connection with FIG.  15 . If the “PERMISSIVE CHECK ROUTINE”  320  passes, the controller  68  determines at step  354  whether the user has chosen to run the sheet  12  along the centerline of the slitting machine  10 . If the user entered an offset value during step  310 , the controller  68  reads the desired offset distance from the machine centerline at step  356 . At steps  358  and  360 , the controller  68  determines the necessary movement direction of the knife holder assemblies  56  and also compares all mult requests entered by the user at step  310  with the actual positions of the knife holder assemblies  56  as determined by the control system  200 . 
   Further referring to  FIG. 18 , at step  362  the controller  68  jogs or steps all upper and lower knife holder assemblies  56  along the upper and lower drive shaft assemblies  44 ,  46  at a generally rapid speed toward their desired positions. At step  320 , the controller  68  again performs the “PERMISSIVE CHECK ROUTINE” described in connection with FIG.  15 . If the “PERMISSIVE CHECK ROUTINE”  320  passes, the controller  68  determines at step  364  if the upper and lower knife holder assemblies  56  are nearing their desired positions along the upper and lower drive shaft assemblies  44 ,  46 . If not, the controller  68  continues at step  366  to jog or step the upper and lower knife holder assemblies  56  toward their desired positions at the generally rapid speed and control passes back to the “PERMISSIVE CHECK ROUTINE”  320 . If the controller  68  determines at step  364  that one or more of the knife holder assemblies  56  are nearing their desired positions along the upper and lower drive shaft assemblies  44 ,  46 , the controller  68  at step  368  jogs or steps those knife holder assemblies  56  nearing their desired positions at a lower speed to insure extremely accurate movement of the knife holder assemblies  56  to their desired positions. At step  320 , the controller  68  again performs the “PERMISSIVE CHECK ROUTINE” described in connection with FIG.  15 . If the “PERMISSIVE CHECK ROUTINE”  320  fails at any time during execution of the “POSITION KNIVES ROUTINE”  306 , the controller  68  disables the “Auto Start” capability of the slitting machine  10  and turns off the “Auto Light” at step  338  ( FIG. 15 ) and control passes back to the “ENTER VALUES ROUTINE”  302  of FIG.  13 . In this way, the control system  200  rapidly, accurately and safely moves the knife holder assemblies  56  to their desired positions along the upper and lower drive shaft assemblies  44 ,  46 . 
   Referring now to  FIG. 19 , a “KNIFE POSITION CHECK ROUTINE”  308  performed by the programmable logic controller  68  during the main “KNIFE POSITIONING ROUTINE”  300  of  FIG. 12  will now be described. At step  370 , the controller  68  determines whether each upper and lower knife holder assembly  56  has reached its desired position along the upper and lower drive shaft assemblies  44 ,  46 . If not, control passes back to the “POSITION KNIVES ROUTINE”  306  of  FIG. 18  so that each remaining knife holder assembly  56  is moved to its desired position as described in detail above in connection with the “POSITION KNIVES ROUTINE”  306  of FIG.  18 . At step  372 , the controller  68  determines if all knife holder assemblies  56  have reached their desired positions for the desired mults  14  entered by the user at step  310 . If not, control again passes back to the “POSITION KNIVES ROUTINE”  306  of  FIG. 18  so that each remaining knife holder assembly  56  is moved to its desired position for the desired mults  14  as described in detail above in connection with the “POSITION KNIVES ROUTINE”  306  of FIG.  18 . 
   Further referring to  FIG. 19 , if all knife holder assemblies  56  are properly positioned along the upper and lower drive shaft assemblies  56  for the desired mults  14  entered by the user at step  310 , the controller  68  at step  374  actuates the jack screw motor  90  to set the desired relative vertical position of the upper and lower knives  58  according to the vertical knife position data input by the user at step  310 . At step  376 , the controller  68  monitors movement of the jack screws  86  through the data generated by the transducer  210  ( FIG. 11 ) and determines if the desired relative vertical knife position of the upper and lower knives  58  has been achieved. If not, control passes back to step  374  so that the controller  68  actuates the jack screw motor  90  to set the desired relative vertical position of the upper and lower knives  58  according to the vertical knife position data input by the user at step  310 . When the desired relative vertical knife position of the upper and lower knives  58  is achieved, control passes back to step  310  of the “ENTER VALUES ROUTINE”  302  of FIG.  13 . 
   Accordingly, through execution of the main “KNIFE POSITIONING ROUTINE”  300  of  FIG. 12  by the programmable logic controller  68 , the knife holder assemblies  56  can be accurately, efficiently and safely positioned in the respective upper and lower machine frames  26 ,  28  by a user inputting appropriate data through the HMI interface  70 . The input data includes the number of desired mults  14 , the desired width of each mult  14 , the material thickness of sheet  12 , the desired percentage of horizontal gap between cooperating upper and lower knives  58 , the desired relative vertical position of the upper and lower knives  58  and the desired offset distance from centerline, although other inputs are possible as well without departing from the spirit and scope of the present invention. This information is then processed in the programmable logic controller  68  which sends appropriate instructions to each of the positioning motors  66  to cause rotation of the respective ball nuts  64  and thereby position the knife holder assemblies  56  along the drive shaft assemblies  44 ,  46  as appropriate. The programmable logic controller  68  also actuates the jack screw motor  90  to achieve the desired relative vertical position of the upper and lower knives  58 . Manual manipulation, dismantling and extended downtime of the slitting machine  10  is avoided with the CNC slitting machine  10  according to this invention. 
   From the above disclosure of the general principles of the present invention and the preceding detailed description of at least one preferred embodiment, those skilled in the art will readily comprehend the various modifications to which this invention is susceptible. Therefore, we desire to be limited only by the scope of the following claims and equivalents thereof.