Abstract:
A shredding machine for domestic or office use having a feed passage  3  leading to a cutting mechanism  10, 11  powered by an electric motor, has a thickness measuring device  15  for measuring the thickness of bundles of paper fed through the feed passage and the machine is controlled by a microprocessor which receives signals from the thickness measuring device and prevents the cutting mechanism from being energised if the thickness measured is above a threshold determined by the microprocessor. The microprocessor varies the threshold in accordance with electrical supply voltage, the electric motor temperature and the electric current drawn by the motor during a previous shredding operation, so that the maximum thickness the shredder will accept can be reduced automatically when motor temperature increases or as the effectiveness of the machine deteriorates throughout its working life.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/438,572, filed on Apr. 3, 2012, which is a continuation of U.S. patent application Ser. No. 13/082,657, filed on Apr. 8, 2011, which is a continuation of U.S. patent application Ser. No. 12/182,488, filed on Jul. 30, 2008, which claims the benefit of priority of UK Patent Application No. GB 0715074.1, filed Aug. 2, 2007. The entire contents of each of these prior applications are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    THE PRESENT INVENTION relates to a shredding machine for shredding sheet material. The present invention relates particularly, but not exclusively, to a shredding machine in the form of a paper-shredder suitable for home or office use. 
         [0003]    Over recent years it has been customary to provide shredding machines in domestic homes or work places such as offices, in order to provide a convenient method of securely disposing of confidential documentation or other sensitive papers. 
         [0004]    Conventional paper shredders of the type mentioned above are provided with a paper feed-aperture, particularly in the form of a feed-slot of elongate form, through which a plurality of paper sheets or the like can be fed towards a pair or rotating cutters located below the feed-slot which serve to shred the paper sheets into a plurality of strips having a width of only a few millimetres, the resulting strips of paper being collected in a basket or bin located below the cutters. For reasons of space and economy, the cutting mechanisms used in conventional paper shredders of this type are only effective in shredding stacks of paper or card up to a relatively small predetermined thickness. If a stack of papers or cards exceeding this predetermined thickness is inserted into the feed-slot, for example by being force-fed into the slot by an over-enthusiastic user, it is possible to present the shredding mechanism with such a bulk of material so as to overload the mechanism and stall the driving motor or otherwise jam the mechanism. Not only can paper-jams of this type represent an annoyance to a person using the paper shredder, but they can serve to damage the cutting mechanism, for example by distorting the shafts of the cutters or damaging the cutting blades. 
         [0005]    In co-pending International Patent Application PCT/GB06/004286, the applicants have disclosed an anti-jam mechanism to prevent overloading of a paper shredder by inserting sheet material of too great a thickness in the manner described above. The shredding machine of PCT/GB06/004286 comprises a feed passage extending from a feed aperture and further comprises a cutting mechanism driven by an electric motor, the feed aperture and feed passage being configured to receive multiple sheets and to direct said sheets towards the cutting mechanism for shredding. This machine is provided with an actuating element part of which extends into the feed passage and which is movable from a first position in which the actuating element permits energisation of the cutting mechanism, past a second position beyond which the actuating element prevents energisation of the cutting mechanism. The actuating element is biased towards its first position and is arranged to actuate a switch when moved past said second position, to break the electrical circuit providing power to the cutting mechanism. The shredding machine of PCT/GB06/004286 thus has a threshold thickness of superimposed sheets such that the machine will not attempt to shred a stack of superimposed sheets if the stack has a thickness above that threshold, herein referred to as the anti-jam threshold. 
         [0006]    The applicants have found, however, that the machine of PCT/GB06/004286 suffers from the following problems, in common with prior art shredders without the anti-jam system of PCT/GB06/004286, namely: 
         [0007]    Where the shredder is powered from a main supply, there is the difficulty that mains supply voltage is variable, within a certain tolerance, with the result that the maximum sheet capacity, in practice, of the mains driven electrical shredder will be less when the mains voltage is at the lower end of its tolerance range than when the voltage is at the higher end of that range. 
         [0008]    The temperature of the electric motor driving the shredder rises during use, causing the motor to be less efficient after a period of use, producing a drop in output power and hence a drop in sheet capacity. 
         [0009]    During the life of the shredder, the cutting unit and transmission system wear and become less efficient, the cutting mechanism clogs with paper dust and lubrication dries out or wears off, all of which place a greater load on the motor, again resulting in a drop in sheet capacity. 
         [0010]    In view of the above factors, the applicants found it necessary to set the anti-jam threshold, i.e. the thickness threshold at which the actuating mechanism operated to prevent energisation of the cutting mechanism, at a “worst-case” level and thus significantly below the actual cutting capacity of the cutting mechanism under conditions better than the “worst case” set of conditions. 
       SUMMARY 
       [0011]    The present invention provides an improved shredding machine in which the above difficulty is avoided. 
         [0012]    According to one aspect of the invention there is provided a shredding machine for shredding sheet material, the machine comprising a feed aperture and a cutting mechanism powered by an electric motor, the feed aperture being arranged to receive sheets for shredding and to direct such sheets to the cutting mechanism for shredding, the machine having means for measuring the thickness of sheet material passed into said feed aperture for shredding which sheet material may comprise a plurality of superimposed sheets which together provide such thickness, said measuring means controlling said cutting mechanism so as to permit energisation of the cutting mechanism where the thickness of sheet material measured thereby is below a controlling threshold, (herein referred to as the optimal sheet capacity threshold), and to prevent such energisation where the thickness of sheet material measured is above said controlling threshold, characterised in that the machine includes at least one sensor sensing a variable parameter relevant to such shredding and means operable to adjust said controlling threshold automatically in dependence upon the value of the parameter sensed. 
         [0013]    According to a further aspect of the invention there is provided a machine for processing sheet material, fed through a feed passage, the machine being characterised by means for measuring the thickness of sheet material fed through said passage, said measuring means including an actuating element which is movable from a first limiting position, engaging or relatively close to, one major wall of said passage, away from said major wall, against a biasing force acting on said element, and means for measuring displacement of said actuating element from said limiting position. 
         [0014]    Preferably, said means for measuring displacement of said actuating element comprises a member provided with a series of markers of alternately high and low light transmissivity or of alternatively high and low light reflectivity and optical sensing means sensitive to the passage of said markers through a measuring zone, said member being part of, or mechanically coupled with, said element so that the displacement of said actuating element will cause said markers to traverse said measuring zone, the apparatus including counting means operable to count displacement of said markers through said measuring zone. 
         [0015]    In a preferred embodiment of the present invention, a shredding machine incorporates a microprocessor receiving signals from various sensors, the microprocessor being arranged to vary the optimal sheet capacity threshold setting according to the signals from the various sensors, which may include a mains supply voltage sensor, whereby the system microprocessor will adjust the optimal sheet capacity threshold so as to allow larger quantities of paper to be shredded per pass than when the mains supply voltage is low and a temperature sensor fitted to the electric motor powering the shredder to monitor motor temperature, whereby the system processor can vary the threshold setting depending on motor temperature so that when the motor is cold, the system will allow a greater thickness of paper to be passed at the same time through the shredding mechanism than when the motor is hot. Furthermore, in the preferred embodiment, a current sensor is incorporated in the electric motor circuit, to monitor increase in the motor current drawn by the motor as the shredder wears and to lower the optimal sheet capacity threshold setting as the motor current drawn increases, so that the shredder will allow a greater thickness of paper to pass through the cutting mechanism when the machine is new than when the cutting mechanism has worn and the average motor current drawn has increased. 
         [0016]    In order to deal with a variable “optimal sheet capacity” thickness threshold or trigger point for the optimal sheet capacity mechanism, the movement of the actuator in the feed passage, due to deflection by the thickness of a stack of paper for shredding, must be measured quantitatively. In the preferred embodiment of the present invention, such movement is converted into an electronic digital count, using infrared sensors and a slotted disc operating in a manner similar to the sensing arrangement conventionally employed in a tracker-ball computer mouse. Thus, the actuator will measure the actual thickness of paper presented and the system microprocessor will calculate whether the cutting head will be capable of shredding that thickness, taking into account the voltage, temperature and current sensed by the respective sensors. Based on this calculation, the system will either start the shredder in a forward direction allowing the inserted paper to be shredded or, if the thickness of paper inserted is too great for the shredder to deal with, then the shredder will not start and a warning signal will be given to the operator. 
         [0017]    Conveniently, the shredding machine comprises at least one pair of rollers positioned in between the feed aperture and the cutting mechanism such that sheets being directed towards the cutting mechanism pass between the rollers, upstream of the cutting mechanism. 
         [0018]    Conveniently, a pair of said rollers is located adjacent the feed aperture. 
         [0019]    Conveniently, the shredding machine is further provided with indicating means to provide a visual indication to a user of the machine that energisation of the cutting mechanism is prevented by the optimal sheet capacity facility. 
         [0020]    Preferably, the shredding machine is provided in the form of a paper-shredder suitable for home or office use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    So that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0022]      FIG. 1  is a perspective view from above of a shredding machine in accordance with the present invention, taking the form of a paper-shredder for home or office use; 
           [0023]      FIG. 2  is a perspective view from above of the paper-shredder of  FIG. 1 , illustrating the arrangement with a top cover of the machine removed; 
           [0024]      FIG. 3  is a transverse cross-sectional view taken through the middle of the paper-shredder illustrated in  FIG. 1 , viewed from the right-hand end of the machine as illustrated in  FIG. 1 ; 
           [0025]      FIG. 4  is a sectional view which shows to a larger scale and somewhat schematically part of  FIG. 3  including a device for measuring the thickness of a bundle of papers passed into the shredder for shredding, 
           [0026]      FIG. 5  is a perspective view from above of an alternative form of thickness measuring device, 
           [0027]      FIG. 6  is a perspective view corresponding to  FIG. 5  but with part of the casing of the device removed, 
           [0028]      FIG. 7  is a perspective view from below of the device of  FIGS. 5 and 6 , but with the whole of the casing removed for purposes of illustration, and 
           [0029]      FIGS. 8   a  through  8   c  are a logic diagram related to the shredder of the present invention, 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Referring initially to  FIG. 1 , there is illustrated a shredding machine in accordance with the present invention, provided in the form of a domestic or office paper-shredder.  FIG. 1  illustrates the paper-shredder from above. 
         [0031]    The shredding machine comprises a relatively large plastic container or bin  1 , on top of which sits a housing  2  inside which the operative parts of the paper shredder are located, as will be described in more detail hereinafter. The housing  2  is provided with a feed slot or passage  3  which provides an elongate entrance aperture having a length sufficient to accommodate sheets of appropriate size to be shredded by the machine. During operation, sheet material to be shredded, such as sheets of paper or card or the like, is inserted into the paper feed slot to pass into the feed passage or chute, where the sheets are drawn into the shredding mechanism in a manner known per se and shredded into a plurality of strips which then exit the shredding mechanism from the bottom of the housing  2  so as to fall from the housing and be collected in the bin  1  located therebelow. 
         [0032]      FIG. 1  also illustrates an operating switch  4  which, in the embodiment illustrated, takes the form of a simple sliding switch. The switch  4  is operable by a person using the shredding machine in order to switch the machine on and off. 
         [0033]    The features of the shredding machine described above with reference to  FIG. 1  are conventional. 
         [0034]      FIG. 2  illustrates the internal workings of the shredding machine in more detail, with the upper part of the housing  2  having been removed. 
         [0035]    The feed slot or feed passage  3  is defined, in the absence of the top part of the housing  2 , by a pair of substantially parallel upstanding feed walls  5 ,  6 . As can be seen from  FIG. 2 , in the embodiment illustrated, the upper edge of the front feed wall  5  is located below the level of the upper edge of the rear feed wall  6 . The two feed walls  5 ,  6  are spaced apart from one another by a distance slightly greater than the maximum thickness of sheet material which the shredding machine is capable of shredding, as will be described in more detail hereinafter. 
         [0036]    As will be appreciated from a comparison of  FIGS. 1 and 2 , when the top part of the housing  2  is placed over the inner workings of the shredding machine, the region of the housing  2  defining the opening to the feed slot  3  is aligned with and overlies the space defined between the feed walls  5 ,  6 . In fact, this region of the upper housing  2  is preferably moulded from the plastics material in such a manner that inwardly-directed lips  7 ,  8  extend part-way down the inwardly-directed face of respective feed walls  5 ,  6  so as to define a smooth and uninterrupted opening into the feed slot. This is also illustrated more clearly in  FIG. 3 . 
         [0037]      FIG. 2  also illustrates part of an electric motor  9  which is mounted to the rear of the feed slot  3 . The motor  9  is connected, via a gear arrangement, to a pair of elongate rotatable cutters  10 ,  11  which are arranged for counter-rotation relative to one another in a region below the feed slot  3 , as illustrated most clearly in  FIG. 3 . Each cutter  10 , 11  is generally cylindrical in form and is provided with a plurality of spaced-apart cutting discs  12  along its length, the cutting discs of one cutter being interposed between those of the other cutter. Hence, in  FIG. 3 , which is a sectional view taken through the central region of the shredding machine, only one cutting disc  12  is visible. However, it will be seen that this cutting disc is provided with a number of cutting teeth  13  at spaced apart positions around its periphery. 
         [0038]    Upon energisation of the electric motor  9 , the two cutters  10 ,  11  are caused to rotate, such that the forwardmost cutter  10  rotates in a clockwise sense as viewed in  FIG. 3 , whilst the rearmost cutter  11  rotates in a counter-clockwise sense as viewed in  FIG. 3 . In this manner, the two cutters  10 ,  11  are arranged to pull sheet material passing through the feed slot  3 , through the nip  14  defined between the two cutters  10 ,  11 . 
         [0039]    As also illustrated in  FIGS. 2 and 3 , a thickness gauging device  15  is provided which includes a member having an actuating element in the form of an arm  17 , which extends into the feed passage  3  and has an upper surface  18  which, in the orientation of the actuating arm  17  illustrated in  FIG. 3 , slopes forwardly and downwardly. The arm  17  extends through a vertically-oriented slot  22  through the rear feed wall  6  and into the feed slot  3  defined between the rear feed wall  6  and the front feed wall  5 . 
         [0040]    The actuating arm  17  is spring biased into the feed passage  3  and is free to extend, under the spring bias, so far into the feed passage  3  as to engage the opposing wall  5  of the feed passage in the absence of any paper sheets to be shredded. This makes possible a self-calibrating function as described below. 
         [0041]    Although not essential to the operation of the present invention, it will be seen from the accompanying drawings that the shredding machine is also provided with a pair of photo-sensors, indicated generally at  38  and  39  in  FIG. 2 , which are arranged on either side of the actuating arm  17  so as to direct a beam of light such as infra-red light across the feed slot from one side and detect its arrival on the other side. In the arrangement illustrated, the first photo-sensor  38  is arranged so as to be operative across the feed slot at a level below the vertical slot  22  through which the actuating arm  17  projects into the feed slot  3 . The other photo-sensor  39  is arranged so as to be operative across the feed slot at a level above the vertical slot  22  through which the actuating arm projects into the feed slot. The function of the two photo-sensors  38 ,  39  can be varied at the manufacturing stage of the paper shredder, depending upon the desired functionality of the shredder. In one proposed arrangement, the higher level photo-sensor  39  is arranged so as to simply detect the presence of paper in the feed slot, whilst the lower level photo-sensor provides a signal on the basis of which the electric motor  9  may be energised to set the cutting mechanism in motion as the leading edge of a sheet of paper or stack of papers passes the photo sensor, and to detect the passage of the trailing edge of the sheet or stack upon shredding. (The machine is arranged to stop the electric motor after a predetermined time has elapsed following movement of such trailing edge past the lower level sensor  38 . 
         [0042]    In the embodiment of the present invention under discussion, the shredding machine incorporates a microprocessor which controls energisation of the electric motor driving the cutting mechanism and the feed mechanism and which, on the basis of various sensors (see below) establishes, as an optimal sheet capacity threshold, a maximum thickness of a stack or bundle of paper sheets or the like which, for prevailing conditions, the machine can comfortably deal with. Measuring the thickness of a stack of paper sheets inserted is effected by the device  15  and associated circuitry which provides corresponding information to the microprocessor. 
         [0043]    A stack of paper sheets or the like can be inserted into the feed slot to pass between the walls  5  and  6  for engagement by the cutting mechanism therebelow, the cutting mechanism being switched on and off in response to signals from the lower level photo sensor  38 , (which signals are also sent to the microprocessor). If the thickness of the stack of papers inserted into the feed slot is less than the currently determined optimal sheet capacity threshold, then the cutting mechanism will be switched on and the stack of sheets shredded. However, should a stack of papers be inserted into the feed slot which stack has a thickness greater than the currently determined optimal sheet capacity threshold, as determined by displacement of the actuating arm  17 , then the microprocessor will terminate supply of electricity to the motor driving the cutting mechanism and will activate an alarm signal to alert the operator to the fact that too thick a stack of paper sheets had been inserted. 
         [0044]    The stack of paper sheets inserted into the feed slot will pass between the wall  5  and the surface  18  of the actuating arm  17  thereby urging the actuating arm to move against its spring and so to generate signals to the microprocessor from which the latter can determine how far the actuating arm has moved and thus determine the thickness of the stack of sheets inserted. As noted above, the microprocessor thus prevents operation of the cutting mechanism located below the feed slot, even when the leading edge of the stack passes the lower level photo sensor  38  which would, if the stack of papers was not of excessive thickness, trigger operation of the cutting mechanism. 
         [0045]    In one form of the thickness measuring device  15  shown schematically in  FIG. 4 , the actuating arm  17  is part of an element  200  including a gear segment  202 . The element  200  is mounted in a casing  210  indicated in broken lines, for rotation about the axis of a shaft  220 . The element  200  is biased, e.g. by a spring (not shown), in a clockwise sense in  FIG. 4  so as to extend the arm  17  through the slot  22  and across the passage  3  to abut the wall  5  of the passage  3  of the shredder in the absence of any sheet in the passage  3  to be shredded. In this position, the upper surface  18  of the arm  17  extends at an angle downwardly from the slot  22  so as to be readily displaceable anti-clockwise in  FIG. 4  by paper sheets P passed into the passage between the wall  5  and arm  17 . The gear segment  202  meshes with a pinion  226  of relatively small radius which is fixed to a sensing wheel or disc  228  coaxial with pinion  226  and rotatable in housing  210  about an axis parallel with that of the shaft  220 . It will be understood that the disc  228  lies in a plane slightly behind that of the element  200  furthest from the viewer in  FIG. 4 , so that the element  200  overlaps the disc  228  which extends behind the element  200  in  FIG. 4 . 
         [0046]    With the arrangement illustrated in  FIG. 4 , a relatively slight angular rotational movement of element  200  about the axis of shaft  220  will produce a significant rotational movement of the disc  228 . The disc  228  is provided with an annular track comprising a plurality of equally spaced radially extending slots around the disc. Two optical signal sensors  230 ,  232 , straddle the disc to detect passage of the slots as the disc  228  rotates. Each sensor  230 ,  232  comprises a light source such as a LED and a photo detector such as a photodiode, on opposite sides of the disc so that as the disc rotates light passes periodically through the slots in the disc from the respective LED to the respective photo detector. The arrangement used is similar to that used in a conventional tracker ball computer mouse and, as in such a mouse, the sensors  230 ,  232  are positioned relative to one another and to the disc in such a way that, as the disc rotates, the signals from one sensor due to sensing the passage of the slots are somewhat out of phase with the signals from the other sensor, whereby the processor can determine the direction of rotation of the disc as well as the extent of rotation (by counting the signals). 
         [0047]      FIGS. 5 ,  6  and  7  show an alternative, and currently preferred, form of thickness measuring device  15  for the shredder. In this device, the pivotable element  200  of  FIG. 4  is replaced by an actuating element in the form of a probe member  300  which is guided in a casing  302  for longitudinal rectilinear displacement. The member  300  is urged longitudinally outwards from the casing  302 , through the slot  22  and into the passage  3  by a light spring  304 , (see  FIG. 7 ). The spring biased probe member  300  carries at its outer end a roller  301  for engagement with paper fed through the feed passage  3  or for engagement with the opposing passage wall  5  when no paper is present. Part of the probe member  300  is formed as a rack providing a series of gear teeth  306  along one side of the member  300  which mesh with gear teeth of a pinion  308 . The pinion  308  is fixed to a co-axial gearwheel  310  of much larger diameter than pinion  308 , which gearwheel  310  overlaps a slotted disc  314 , corresponding to the disc  228  in  FIG. 4 , and meshes with a small diameter pinion  312  fixed to that disc and co-axial therewith, the gearwheel  310  and disc  314  being rotatable about their respective parallel axes in the casing  302 . As with the arrangement of  FIG. 4 , the disc  314  is provided with a series or track of equally spaced radial slots therearound and two optical sensors  230 ,  232  are provided straddling the annular track of slots around the disc  314 , each sensor comprising a respective photo detector on one side of the disc and a respective LED on the opposite side of the disc, the optical detectors again being positioned somewhat out of phase with each other in the same manner as described with respect to  FIG. 4  so that the shredder microprocessor, or ancillary circuitry dedicated to the sensor disc  314 , can determine not only the extent of rotation of the disc but can determine the direction of displacement of the probe  300  in addition to the extent of such displacement. 
         [0048]    The thickness gauging devices described with reference to  FIG. 4  and  FIGS. 5 to 7  allow the thickness measuring facility in the shredder to be self-zeroing. Thus, for example, the microprocessor can be arranged, when the shredder is switched on and before any paper or the like is inserted for shredding, to take the rest position of the thickness measuring mechanism, in which the arm  17  or the probe  300  is in engagement with the opposing wall  5  of the shredder passageway  3 , as corresponding to the zero thickness position. In a currently preferred embodiment of the shredding machine, the aforementioned self-zeroing function is performed as a continual process throughout the life of the product, each time that the arm  17  or the probe  300  engages with the opposing wall  5  of the shredder passageway  3 , (i.e. whenever there is no paper sheets or the like present within the feed-slot). Providing this self-zeroing function as a continual process in this manner allows the machine to re-calculate the zero thickness position for the arm  17  or probe  300  in order to account for wear to certain parts of the mechanism, such as the arm  17  or the probe  300  itself, the opposing walls of the feed-slot, or any of the trigger gears. This continual self-zeroing function also accounts for changes in ambient temperature and possible distortion of the opposing walls of the feed-slot. This arrangement thus allows the zero thickness position of the arm  17  or the probe  300  to be continuously re-calibrated to suit the current conditions during the life of the product, and also offers a significant advantage in that it eliminates the need for accurate setting of the option sheet capacity threshold during assembly of the product at the manufacturing stage. 
         [0049]    If a stack of paper sheets or the like is inserted into the feed slot  3  so as to pass between the wall  5  and the arm  17  or probe roller  301  and that stack of papers has a thickness, (sensed by displacement of the arm  17  or probe roller  301 ), less than the optimal sheet capacity threshold thickness determined for the time being by the shredder processor, then the electric motor powering the cutting mechanism will be switched on in response to signals from the lower level photo-sensor  38  and the paper will be shredded, with the motor being switched off again once the paper has cleared the sensor  38 . However, should a stack of papers be inserted into the feed slot which has a thickness, (sensed by displacement of the arm  17  or probe roller  301 ), greater than the optional sheet capacity threshold thickness, the shredder microprocessor will prevent energisation of the cutter motor and thus prevent operation of the cutting mechanism located below the feed slot, even when the leading edge of the stack passes the lower level photo-sensor  38 . The microprocessor will also light a warning lamp to signal that the paper bundle inserted is too thick. 
         [0050]      FIGS. 8   a  through  8   c  show a logic diagram or flow chart for the shredder microprocessor. Considering the portion of the diagram which is of relevance to the present invention, at stage  400 , the processor is initialised and, assuming that the shredder has been set to shred automatically sheets fed into passage  3 , the microprocessor at  402  checks that motor temperature (signalled from stage  404 ) is not excessive, that the shredder is properly closed and that the bin for shredded material is not full. If any of these conditions is present, a warning light is illuminated at  404  and the shredder will not proceed further until the deficiency is remedied. If none of these conditions is present, the processor proceeds via stage  406  to stage  408  where the optimal capacity mode of operation is enabled. The processor then, at stage  410 , calibrates the thickness-sensing mechanism to zero, illuminates (at  412 ) a light to signal that the optimal capacity feature is operational, then checks (stage  413 ) the sensed motor current (stored at  415  from the previous use of the shredder), the mains voltage (box  414 ,  416 ) and motor temperature (boxes  418 ,  419 ) and determines at stage  422 , (using a predetermined scheme or algorithm which takes into account the sensed motor current from store  415 , the sensed mains voltage, and the sensed motor temperature), the appropriate optimal sheet capacity thickness threshold. 
         [0051]    When paper is inserted, as sensed by sensor  38 , (see above), the shredder motor runs, feeding the inserted sheets past the sensing arm  17  or probe  300 . At stage  426 , the processor determines whether the thickness actually sensed is below or at or above the optimal capacity threshold and if the sensed thickness is below or at the threshold allows shredding to proceed (stages  428 ,  430 ). If the processor determines (stages  432 ,  434 ) that the thickness of the paper bundle fed into passage  3  is excessive, the processor does not energise the shredder motor but actuates a warning light at  435  to inform the operator that too much paper has been inserted and once the paper has been removed from the passage  3 , the processor returns to stage  410 . If the optimal capacity threshold is not reached or exceeded, the inserted paper is shredded (stage  430 ), whilst the motor current is monitored at  440  and stored at  415 . The optimal sheet capacity thickness-measuring facility is deactivated (stage  441 ) during shredding until the inserted paper clears the sensor  38  (stage  443 ). The reason for this is that when paper is shredded it ripples and flaps within the feed passage  3 , which can cause the arm  17  or probe  300  to be constantly moved and can cause false readings as to the amount of paper inserted. 
         [0052]    Once the inserted paper has been shredded and has passed the sensor  38  (stage  443 ), the processor returns to stage  408  once again, re-activating the optimal sheet capacity thickness-measuring facility. 
         [0053]    If, during shredding, the shredder jams, despite the thickness monitoring, this condition is sensed at  450 , a warning light is lit (stage  452 ) and the shredder motor and hence the shredder mechanism is reversed, either automatically or by operation of a manual switch (stage  454 ), to free the jam. The processor then returns to the initial stage  400 . 
         [0054]    The preferred embodiment of the invention is also operable to break up CDs, or credit cards. When used for this purpose, the thickness measuring optimal sheet capacity facility is by-passed (stages  401 , 403 , 405 ) whilst the CD or credit card is being broken up. A manual switch or optical detector may be used to inform the processor that the optimal capacity facility is to be by-passed. 
         [0055]    When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. 
         [0056]    The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.