Background of the invention

[0001] The invention relates to an inspection apparatus and method for inspecting a sheet, for example a metal sheet, in a press in which a punching unit, in particular a multi- punch, is suitable for receiving a sheet provided with a plurality of printed portions, for example lithographed or screen printed and/or embossed with raised portions, arranged in a regular manner according to a set printing pattern to cut the sheet, separating into several steps a plurality of printed discs and leaving a continuous reject grid that is removed from the punching unit by a conveying device. The conveying device supplies the sheet to the punching unit and further removes from the punching unit the reject grid, which is received from a supporting plane arranged downstream of the press in which the reject grids are stacked during production.

[0002] In specifically, but not exclusively, the invention can be applied in the field of closures forming for containers, in particular metal closures, such as, for example, crown caps, aluminium caps.

[0003] The prior art comprises a press for crown caps comprising a punching unit having two rows that are parallel and staggered to one another, of punch dies arranged for cutting circular discs from a metal sheet having the plurality of printed portions, which are also circular in shape and are arranged in a regular manner in parallel and staggered rows corresponding to the arrangement of the punch dies. The sheet advances stepwise, taking a step forward when the dies are open, whilst at each stop of the sheet the dies close to cut a double row of printed discs. The metal closures, caps or capsules, are obtained by forming the printed discs and are conveyed to a collecting hopper, arranged downstream of the press. The sheet advances to the punching unit along a rectilinear path and in a direction that is parallel to a longitudinal axis of the sheet.

[0004] In order to align with precision the metal sheet in the punching unit, the press comprises abutting means arranged upstream of the punching unit and in a fixed position with respect to the punching unit, suitable for receiving the printed metal sheet and adjusting the position thereof so as to advance the previously adjusted metal sheet successively to the punching unit. The abutting means comprises a top abutment that is manually drivable to abut on a top advancement edge of the sheet, a manually adjustable side positioning means arranged for engaging with a side edge of the sheet and side abutments, arranged for engaging with the other side edge, which are fixed to respective elastic supports to abut the sheet during driving of one or both side positioning means.

[0005] It is requested that each disc be cut in such a manner as to have the printed portion in a desired position inasmuch as the printed portion usually has a logo identifying a manufacturer of the container with which the closure is associable.

[0006] It is added that usually the printing pattern has been made in such a manner as to be able to obtain the largest number of caps from the metal sheet, nevertheless leaving the reject grid conveyable. During cutting of the printed discs it is in fact necessary to avoid small fragments of cutting waste at a top edge, at a bottom edge and at the side edges of the metal sheet, the so-called "slivers" in the shape of a circular segment or circular sector, which could move in an uncontrolled manner in the punching unit, damaging both the unit and the movement members thereof.

[0007] It is thus necessary to align in the punching unit the printed portions with respect to the punch dies, so as to ensure good-quality closures, in which the printed logo is in a desired position, and the creation of a continuous reject grid that is conveyable without the creation of reject fragments.

[0008] In order to adjust the position of the sheet, an operator drives the abutting means manually, when necessary.

[0009] If for example the printing pattern comprises printed circular portions of the same dimension as the punch dies, the axis of each punch has to be aligned on the centre of each circular portion whereas if the printing pattern has printed portions of lesser dimension and/or of different shape and/or in a lateral position compared with the dimension of the corresponding punch die, the punch die has to be aligned on the centre of each disc to be cut and the printed portion has to be in a desired position in relation to the axis of the punch.

[0010] The nominal arrangement of the printing pattern in the sheet is the arrangement that is decided a priori for the type of sheet and for the type of production batch and enables cutting fragments to be avoided when the sheet is advanced to the punching unit.

[001 1] Nevertheless, it can happen that the arrangement of the printing pattern in a sheet does not always match the nominal arrangement thereof. In fact, there may be small variations in the arrangement of the printing pattern that depend on various factors such as the geometrical precision of the metal sheet and the precision of the metal sheet printing process. The printing pattern can, for example, be translated or tilted in the sheet in relation to a nominal position.

[0012] During adjustment of the position of the sheet in the abutting means, it thus becomes necessary to "centre" the printing pattern, i.e. ensure that the printing pattern of the sheet is arranged in a nominal position to ensure that the punching unit receives a metal sheet in which each printed portion is aligned correctly with respect to the axis of the corresponding punch.

[0013] US4628578 discloses a press comprising a punching unit with a single punch in which each printed portion of the printing pattern of the sheet is arranged in a nominal position. In addition to the plurality of rows of printed portions, the sheet comprises a plurality of circular notches, each associated and arranged near a respective printed portion. The press comprises a linear digital camera fixed to the frame of the punching unit and able to frame and inspect the positioning notches, detecting and storing possible deviations in the positioning of each notch with respect to the supply directions X and Y of the sheet. In particular, a shift is calculated along the axis X (ΔΧ), a shift along the axis Y (ΔΥ) for each reference notch and the angle of tilt is further calculated of the reference notches of each row of printed portions as an average tilt value of all the shifts along the axis X (ΔΧ) and of all the shifts along the axis Y (ΔΥ).

[0014] Each printed portion is advanced through the punching unit configured as an open punch in a wait position to enable the positioning notches to be inspected and then after the inspection of an entire row, the sheet is retracted through the same unit but is configured in an operating stop to separate the printed discs. Each printed portion is separated from the rest of the sheet by subsequent retraction steps of the sheet, and each step is corrected by the deviation ΔΧ and ΔΥ of the reference notch associated with the portion to be separated. At the end of a row, a subsequent row of the sheet is inspected and subsequently supplied to the punch until punching of the printed zones of the sheet has been completed.

[0015] The inspection method shown by US4628578 has some drawbacks.

[0016] First of all, it is necessary for the metal sheet to be provided with both printed portions and with reference notches, each associated with a respective printed portion, inasmuch as the camera processes the notches in succession. Making as many additional notches as there are printed portions implies additional costs during the step of printing the metal sheet. [0017] Further, the inspection method and the corresponding correction of the position of each notch, in addition to being very complex, can be implemented only if the press is a single punch press. For presses in which the punching unit is a multi-punch unit and comprises a staggered double row of punches, the inspection method disclosed by US4628578 cannot apply because the shift along the axis X (ΔΧ) and the shift along the axis Y (ΔΥ) evaluated for a first reference notch would apply to all the printed discs of two rows that are punched simultaneously.

[0018] Evaluating the shift along the axis X (ΔΧ) and the shift along the axis Y (ΔΥ) of the first notch would not be sufficient to identify sheets in which the printed portions are tilted in the sheet. Closures of insufficient quality could nevertheless be produced that are destined to be rejected at the reject station.

[0019] In order to evaluate the printed portions tilted in the sheet, in US4628578 a row of the sheet is inspected with the punch open and punching occurs by subsequent retraction. To extract the printed disc, a suitable extracting device is provided that is arranged upstream of the press, which nevertheless makes the press complex and further decreases production efficiency.

[0020] Further to this, separating the disc printed upstream of the press is not applicable to the multi-punch punching unit of the prior art disclosed previously, which provides abutting means of the sheet arranged upstream of the punching unit and the formed cap collecting hopper arranged downstream of the press.

[0021] JP2001058230 shows a press for punching a rectangular sheet, the rectangular sheet during punching being retained in position by two grippers.

[0022] In order to control deviation of the retaining performed by the grippers and during punching, the press performs initial punching of a first figure (two squares inscribed in a larger reference square and arranged along a diagonal of the square) and a second punch of the same first figure.

[0023] Subsequently, the punching press performs the required processing of the sheet according to a preset processing pattern.

[0024] Subsequently, the press performs third punching of a second figure (two squares inscribed into the same reference square and arranged along the other diagonal of the reference square) and fourth punching of the same second figure.

[0025] In this manner, four squares are punched that are inscribed in a respective reference square and are arranged at the corners of the reference square. [0026] By analysing the different tilt between the first figure and the second figure in the first reference square and between the first figure and the second figure in the second reference square, the angular deviation is obtained of the sheet in two directions X-Y.

[0027] The inspection method of JP2001058230 has some drawbacks.

[0028] As the sheet is devoid of a printing pattern, it is necessary to make holes in the sheet for the purpose of inspection.

[0029] Further, if the inspection finds that the sheet has incorrectly positioned drilled portions the sheet is rejected.

Summary of the invention

[0030] One object of the invention is to solve one or more of the aforesaid limits and drawbacks of the prior art.

[0031] Another object is to make a multi-punch press in which high precision is guaranteed in cutting and separating discs for making quality closures, with high production efficiency, at the same time avoiding sheet advancement and retracting devices.

[0032] A still further object of the invention is to provide an inspection method in a multi-punch press that is able to solve one or more of the aforesaid limits and drawbacks of the prior art.

[0033] Still another object is to provide an inspection method that permits simple calculating of a shift along the axis X (ΔΧ), a shift along the axis Y (ΔΥ) and of a tilt of the printing pattern in the sheet with respect to a nominal position.

[0034] Such objects, and still others, are achieved by the apparatus and/or by the method according to one or more of the claims set out below.

Short description of the drawings

[0035] The invention can be better understood and implemented with reference to the attached drawings that illustrate some embodiments thereof by way of non-limiting example.

Figure 1 is a perspective view of an embodiment of a press made according to the present invention.

Figure 2 is a view of the press of figure 1 from another perspective.

Figure 3 is a view of the press of figure 1 , from still another perspective, in which certain parts of the press are removed for the sake of clarity.

Figure 4 is a perspective view of a punching unit of the press in figure 1, in an open configuration. Figure 5 is a schematic side view of the image acquisition means arranged for framing frontally a sheet advanced along an advancement path P.

Figure 6 is a schematic plan view of a sheet provided with a printing pattern, advanced along the path P.

Figure 7 is a schematic plan view of four sheets supplied stepwise to the punching unit in a direction Dl, which are framed by the image processing means of figure 5 during advancement of the sheets in a second direction D2.

Figure 8 shows the sheets of figure 6 at a subsequent advancement step.

Figure 9 shows the sheets of figure 7 at another subsequent advancement step.

Figure 10 shows an enlarged schematic plan view of the image acquisition means of figure 5 framing a top corner of the sheet.

Figure 11 shows an enlarged schematic plan view of the image acquisition means of figure 5 framing a bottom corner of the sheet.

Detailed description

[0036] In this description, same elements that are common to the various illustrated embodiments have been indicated by the same numbering.

[0037] As shown in figure 1, with 1 is overall a mechanical press indicated, for example a multi-punch press, for producing closures, in particular for producing crown caps.

[0038] The press 1 is suitable for processing sheets 2 of rectangular shape, in particular sheets of metal, such as, for example, metal sheets of a tin-plated band, chromium-plated band, aluminium, etc. The sheets 2 being processed (figure 6) are sheets printed by lithography or silk-screen printing and/or embossing of raised portions, which are provided with a plurality of printed portions 3 arranged in a regular manner according to a printing pattern in a face of the sheet. It is supposed that the printing pattern is known a priori and provides rows of printed portions, arranged for example staggered, as defined better below. In detail, the term "printed portions" means that the printing pattern can have graphic marks (for example a company trademark, an image, a logo) which, optionally can have raised embossed zones to highlight further the graphic signs. Alternatively, the "printed portions" can also be only raised embossed zones.

[0039] The press 1 comprises at least one multi-punch punching unit 4 (figure 2 and figure 4) suitable for receiving the sheet 2 and for cutting the latter in dwell steps separating into several steps a plurality of printed discs, from which the crown caps subsequently formed are conveyed to a collecting hopper (which is not illustrated).

[0040] The punching unit 4 (which is also commonly known as a mould) comprises, as in this embodiment, two rows of punches that are parallel and staggered in relation to one another, which are operationally associated with two corresponding rows of dies. In detail, the punching unit 4 comprises a movable upper plate 5, fixed to an upper carriage, to which are connected various forming punches (which are not shown), a central plate 6 fixed to a base that carries sliding seats for the forming punches, and a movable lower plate 7 that is fixed to a lower carriage and carries the corresponding cutting and forming dies. The movable lower plate 7 can further have extracting means, for example pneumatic and/or spring and/or mechanical drive extracting means.

[0041] Guiding means is provided in the punching unit 4 for guiding the movable lower plate 7 relative to the central plate 6. The upper and lower plates 5 and 7 are movable between a closed position in which the metal sheet 2 is cut and the crown caps are formed and an open position in which the caps can be detached from the forming punches by the extracting means and can be conveyed, for example by blown air, to the collecting hopper.

[0042] The face of the metal sheet 2 having the printed portions 3 faces the movable lower plate 7 that carries the cutting dies, in such a manner that, for example, each printed portion 2 is outside the printed caps.

[0043] The printed portions 3 are arranged in a regular manner in the sheet 2 in rows that are parallel and staggered in relation to one another in a manner corresponding to the punches of the punching unit 4.

[0044] The press 1 further comprises abutting means arranged upstream of the punching unit 4 in a first advancement direction Dl of the sheet to the punching unit 4, which is suitable for receiving the sheet 2 to be cut and for adjusting the position thereof, as will be seen better below.

[0045] The press 1 further comprises one conveying device, for stepwise advancing to the punching unit 4 along a rectilinear conveying path T in the first direction Dl the sheet 2 adjusted by the abutting means and removing from the punching unit 4 a continuous reject grid. Downstream of the punching unit 4, the press 1 comprises a supporting plane 8 suitable for receiving the suitably stacked reject grids. [0046] The conveying device comprises a closed-loop slidable conveyor, which comprises two flexible elements 9 arranged parallel alongside one another at a suitable distance, each of which has gripping means 10, for example a gripper, for grasping laterally both sides of sheet 2 at a zone of the sheet not affected by the punching, in particular at the bottom angles of the sheet 2. The gripping means 10 is arranged spaced along the flexible elements 9 and is movable integrally with the conveyor to retain, during advancement and cutting, the sheet 2 and releasing the sheet 2 subsequently as a reject grid in the supporting plane 8.

[0047] The press 1 can comprise at least one magazine 11 of stacked sheets 2 (figure 1 and figure 2). In this specific case the press 1 comprises two magazines 11 of stacked sheets 2, for example positioned on the two sides of an inlet of the press 1 , in which the printing pattern faces downwards and the face devoid of a printing pattern faces upwards.

[0048] A removing device 12, for example of the suction cup type, is suitable for grasping the sheets 2 from the magazines 11, one by one, by the face devoid of printing pattern to transfer the sheets 2 to an inlet zone leading to a transferring path P for transferring the sheets 2 to the inside of the press 1.

[0049] The press 1 comprises a lubricating device 13, for example a roller or spray lubricating device, which receives the sheets 2 from the removing device 12 and transfers a thin layer of lubricant onto the face of the sheet 2 in which the printing pattern is present.

[0050] The press further comprises a transferring device 14 suitable for receiving the sheet 2 from the lubricating device 13 and advancing the sheet 2 to the abutting means along the transferring path P in a second direction D2. The sheet 2 is advanced longitudinally, i.e. the second advancement direction D2 along the transferring path P (and also along the conveying path T) is at each point of the path P parallel to the longitudinal axis of the sheet 2.

[0051] The transferring device 14 comprises a linear conveying module, having translating elements and teeth 14a supporting the sheet 2 which are arranged two by two at a set distance from one another to grasp the sheet 2 and translate the sheet 2 to an upper shaft 15, the height of which from the ground is greater than the height from the ground of the lubricating device 13. The transferring device 14 is thus suitable for moving the sheet 2 upwards, the sheet 2 being subsequently guided around the upper shaft 15 owing to the help of guides and rollers as far as the abutting means of the press 1. Rotating around the upper shaft 15 the sheet is transferred from one side of the press 1 facing the magazine 11 to the opposite side thereof, facing the punching unit 2. Between the upper shaft 15 and the base of the press (not shown) there is a gap 15'.

[0052] Possible sheets 2 with faults, for example having an incorrect printing pattern or for which the printing pattern does not belong to the correct production batch, are diverted from the path P to a reject magazine (not shown) arranged between the lubricating device 13 and the upper shaft 15, more precisely near the upper shaft 15.

[0053] If for example the sheet has a printing pattern that provides both lithographed marks and embossing, it is checked that the embossing is positioned correctly in relation to the lithographed marks and that the embossing is consistent with the lithography.

[0054] A sheet 2 with embossing that is not consistent with the lithographed marks or in which the lithography is not consistent with the production batch is considered to be a fault to be rejected. A sheet 2 that has on the other hand graphic marks and/or embossing that are correct for the production batch and, when requested, consistent with one another, is advanced to the punching unit 4.

[0055] The press further comprises processing means 16 of an acquired image of the sheet 2 that is suitable for detecting an arrangement of the printing pattern in the sheet 2 and comparing the detected arrangement of the printing pattern with a nominal arrangement thereof, to thus obtain a shift of the printing pattern in the sheet 2 with respect to the nominal arrangement.

[0056] The abutting means is suitable for translating and/or rotating the sheet 2, as will be seen better below, as a function of the shift detected so as to position the detected printing pattern in the nominal position so that the punching unit separates printed discs in which each printed portion is arranged in a desired position.

[0057] This solution is particularly advantageous if the sheet has already been embossed, a condition in which deformations of the sheet are possible.

[0058] The image processed by the processing means 16 is acquired by image acquisition means 17 which is arranged upstream of the abutting means, in particular in the gap 15' between the translating elements of the transferring device 14, for acquiring at least one image of the sheet 2, during the advancement of the sheet 2 in the second direction D2 by means of the transferring device 14. As is seen in figure 3, and schematically in figures 5, 7 and 8, the image acquisition means 17 frames at least one portion of the sheet 2 during the advancement of the sheet 2 in the second direction D2 by means of the transferring device 14. The face of the sheet 2 in which the printing pattern is present faces the image acquisition means 17.

[0059] If the second advancement direction D2 of the sheet 2 in the path P is considered, the sheet has a top edge 2a, a bottom edge 2b, which is opposite the top edge and two opposite side edges, between which a side working edge 2c can be distinguished that is framed by the image acquisition means 17. The side edge 2d opposite the side working edge 2c can be optionally framed. A top corner 18 is defined between the top edge 2a and the side working edge 2c, and a bottom corner 19 is defined between the side working edge 2c and the bottom edge 2b.

[0060] In detail, the processing means 16 is suitable for identifying a top graphic mark 20 in the top corner 18 and a bottom graphic mark 21 in the bottom corner 19 of the sheet 2 for calculating a longitudinal shift and a top side shift of the top graphic mark 20 and a bottom side shift of the bottom graphic mark 21.

[0061] It should be noted that if the sheet 2 has printed portions that comprise both a graphic mark (lithography or silk-screen printing) and also embossing, the processing means 16 preferably identifies the graphic mark and not the embossing to adjust the position of the sheet in the abutting means. Alternatively, the processing means 16 and image acquisition means 17 can be selected that are specifically configured for identifying an embossing pattern, if the printed portions are exclusively embossed or if identification of the embossing pattern is preferable. Below, and without any loss of generality, reference will be made to identifying a top graphic mark 20 and a bottom graphic mark 21, it nevertheless being understood that if the graphic marks have embossing, also the latter could be identifiable if necessary.

[0062] As shown in figures 7 to 9, the abutting means comprises side positioning means, for translating and/or rotating the sheet 2 in a transverse direction to the first advancement direction Dl as a function of the top side shift and/or of the bottom side shift, and front positioning means, for translating the sheet 2 in the first advancement direction Dl as a function of the longitudinal shift. The abutting means is suitable for positioning the printing pattern in a nominal position so as to obtain in the punching unit printed discs in which each printed portion 3 is arranged in a desired position.

[0063] The front positioning means comprises a front positioner 22 that is driven by means of electric, pneumatic or hydraulic actuation, which is controlled and is drivable for translating the sheet 2 in the first advancement direction Dl . [0064] The side positioning means comprises a pair of lateral positioners 23, which are also driven and controlled by means of electric, pneumatic or hydraulic actuation, which are each drivable independently of one another, arranged for each engaging a side edge of the sheet 2, and a pair of non-controlled lateral abutments 24, each facing a respective lateral positioner 23, arranged for engaging the side edge of the sheet 2. Each lateral abutment 24 is fixed to a respective elastic support for abutting the sheet 2 during driving of one or both controlled lateral positioners 23.

[0065] The image acquisition means 17 comprises an optical acquisition apparatus and an illuminator associated with the optical acquisition apparatus (which are not shown in detail), to acquire images of the sheet 2 when the latter is advanced by the transferring device 14 in the second advancement direction D2.

[0066] The expression "optical image acquisition apparatus" means an optoelectric image acquisition apparatus that is able to acquire images of a metal sheet, and in particular process images so as to extract features of interest therefrom, for example geometrical and/or shape features of the sheet. The acquired images can be in colour or in black and white and from the images information can be extrapolated on the colour (tone, saturation, etc) or respectively on the level of grey, and on the luminous intensity.

[0067] The optical apparatus comprises a body on which is arranged an electronic sensor, for example an alignment or array of photosensitive elements of linear or two dimensional matrix type, for example of the CCD or CMOS type, and appropriate optical receiving means fixed to the body, for example an objective consisting of one or more lenses, by means of which the sensor is suitable for receiving the light diffused by the object to be acquired, in this case the sheet 2. The number of photosensitive elements of the electronic sensor is matched by the number of pixels or punctiform elements that make up the representation of a raster or bitmap image in the memory of the optical apparatus. It should be noted that an image with a resolution of n * m pixels can be obtained by a single acquisition with the help of a bidimensional or matrix sensor, of n * m photosensitive elements or by n successive acquisitions, also known as scans, using a linear sensor of m photosensitive elements. For example, linear sensor optical apparatuses can be advantageously used to acquire images of the sheet 2 advanced by the transferring device 14, in which case it is known to arrange the optical apparatus in a fixed reading position and perform subsequent scans, each of which constitutes a very thin "line", or "row" of the overall image, when the product advances at the fixed reading position. In this case, the complete image of the product is reconstructed by putting together and arranging alongside one another all the rows, which are stored after each scan.

[0068] A field of view is defined as a field for acquiring the optical apparatus, or a preset area inside which the sheet 2 is framed and the images of the sheet can be acquired, which is located inside a focusing range and for which, along an optical axis A of the optical means (shown in figure 5) it is possible to define a set depth of field. As the optical apparatus is generally located in relation to the sheet 2 in a position established a priori between 0.3 to 3 metres from the plane of the image to be acquired, the optical apparatus usually comprises manually adjusted and fixed focus optical means, in which focusing and adjusting the diaphragm apertures can be achieved only by an operator using respective adjusting ring nuts with which the optical means is equipped.

[0069] The processing means 16 can be integrated into the optical apparatus to command the acquisition of the image, switching on of the illuminator and to process the acquired image in order to extract the features of interest from the image, like the position of the graphic marks in the inspected sheet 2 with respect to the nominal position thereof and to determine the result of the optical inspection, i.e. whether the sheet 2, as will be seen better below, is to be considered to conform to the quality requirements or whether the sheet 2 is to be rejected. Optical apparatuses of this type are known as video cameras or matrix cameras of the "smart camera" type.

[0070] Alternatively, the processing means 16 can also be only connected and not integrated into the optical apparatus and receive the images acquired from the latter via communication means, for example via a high-speed Ethernet data transmission network.

[0071] The press 1 comprises control means 25 connected to the processing means 16 and to the abutting means, in particular to the front positioner 22 and to the lateral positioners 23, which are controlled and drivable by the control device 25, by communication means 29.

[0072] The communication means 29 is arranged between the processing means 16 and the control means 25 such that the processing means 16 communicates the result of the optical inspection to the control means 25 of the press 1, and can comprise an Ethernet high-speed communication network for example created by a set of digital input or output signals from the optical apparatus, connected to similar respectively output and input digital signals of the control system 25 of the press 1. The communication means 29 arranged between the abutting means and the control means 25 can on the other hand comprise fieldbuses or other typical connections for integrating sensors/actuators into the control means 25 of the press 1.

[0073] The control means 25 is thus able to actuate the abutting means, i.e. the front positioner 22 and the lateral positioners 23 as a function of the shift detected by the processing means 16.

[0074] Further, the control means 25 of the press 1 is also able to reject directly (or to send a defect message to an external device performing the rejection task), the inspected sheet 2 if the detected shift is greater than a threshold shift, beyond which the sheet is judged to be non-conformant to the requested quality requirements. The sheet 2 is rejected as soon as it reaches the rejection magazine, arranged downstream of the image acquisition means 17 along the transferring path P.

[0075] It should be noted that the image acquisition means 17 can frame entirely, or only partially, the metal sheet 2 in relation to the advancement mode of the sheet 2 in the second direction D2 and to the positioning of the image acquisition means in relation to a face of the sheet to be inspected.

[0076] As shown in figures 10 and 11 , the image acquisition means 17 is arranged near the sheet 2 to frame a first sheet portion 2 (figure 7 and figure 10) bounded by a portion of the working edge 2c and by a portion of the top edge 2a and to acquire a first image of the top corner 18 of the sheet in which to identify the top graphic mark 20 and subsequently to frame a second sheet portion 2 (figure 8 and figure 11) bounded by a portion of the working edge 2c and by a portion of the bottom edge 2b to acquire a second image of the bottom corner 19 of the sheet 2 in which to identify the bottom graphic mark 21.

[0077] The second image is suitable for being acquired subsequently to the first during the advancement of the sheet 2 by means of the transferring device 14 along the transferring path P in the second direction D2. The second advancement direction D2 of the sheet 2 at the image acquisition means 17 is rectilinear.

[0078] Alternatively, according to a version which is not illustrated, the image acquisition means 17 can be arranged at a greater distance from the sheet 2 to frame at least one strip of sheet 2 bounded by an edge of the sheet that is the side working edge 2, by a top edge 2a portion and by a bottom edge 2b portion of the sheet 2 to acquire simultaneously in a single image both the top corner 18 and the bottom corner 19 of the sheet 2. This single image is processed in order to identify therein both the top graphic mark 20 in the top corner 18 and the bottom graphic mark 21 in the bottom corner 19 of the sheet. For example the press 1 can comprise a punching unit arranged rotated by 180° in relation to what is illustrated in figure 4, in which the movable upper plate carries the cutting dies and the movable lower plate carries the forming punches, to which the sheets 2 are supplied with the face devoid of a printing pattern facing the transferring device 14. In this manner, the face provided with a printing pattern faces image acquisition means 17 which can be arranged on an additional support positioned above the transferring device 14, for f aming the sheet 2 frontally entirely during the advancement in the second rectilinear direction D2.

[0079] In use, the inspection method of a metal sheet 2 in a press 1 provides for the multi-punch punching unit receiving the sheet 2 provided with a plurality of printed portions 3, arranged in a regular manner according to the printing pattern, and cutting the sheet 2 in dwell steps, separating into several steps a plurality of printed discs and leaving a continuous and conveyable reject grid. The inspection method further provides for the abutting means 22, 23 and 24 being arranged upstream of the punching unit 4, in a first advancement direction Dl of the sheet 2 along a rectilinear path T to receive the sheet to be cut and adjust the position of the sheet, such that the conveying device advances the thus adjusted sheet, grasped by the grippers 10, to the punching unit 4 and the plurality of printed discs are obtained.

[0080] By processing at least one acquired image of the sheet 2 to detect an arrangement of the printing pattern in the sheet 2, the detected arrangement of the printing pattern is compared with a nominal arrangement thereof to obtain a shift of the printing pattern in the sheet 2 with respect to said nominal arrangement.

[0081] A transferring device 14 of the sheet 2 is provided upstream of the abutting means 22, 23 and 24 and at least one image of the sheet 2 is acquired when the sheet advances in a second rectilinear direction D2, along the path P to the abutting means. To detect the arrangement of the printing pattern in the sheet, it is provided to identify a top edge 2a, a bottom edge 2b and a side working edge 2c of the sheet 2 to identify a top graphic mark, arranged in a top corner 18 defined between the top edge 2a and the side working edge 2c, and a bottom graphic mark which is arranged in a bottom corner 19 between the side working edge 2c and the bottom edge 2b.

[0082] A longitudinal shift and a top side shift of the top graphic mark 20 and a bottom side shift of the bottom graphic mark 21 are calculated to adjust the position of the sheet 2 in the abutting means 22, 23 and 4 by translating and/or rotating the sheet 2 as a function of the longitudinal shift, and/or of the top side shift and/or of the bottom side shift, thus positioning the printing pattern in the nominal position so as to obtain in the punching unit 4 printed discs in which each printed portion 3 is arranged in a desired position.

[0083] The sheet is translated by means of the front positioner 22 as a function of the longitudinal shift, in the first rectilinear direction Dl, i.e. is advanced if the longitudinal shift is negative or is retracted if the longitudinal shift is positive.

[0084] If the top side shift and the bottom side shift are the same, the sheet is on the other hand translated transversely to the first direction Dl as a function of the side shift by means of the lateral positioners 23 driven for a stroke that is equal to the shift, to the right if the side shift is negative or to the left if the side shift is positive.

[0085] If on the other hand the top side shift and the bottom side shift are different, a composite shift is calculated that comprises an angular rotation shift with respect to the first direction Dl and a composite side shift that is transverse to the first direction Dl .

[0086] The sheet 2 is rotated as a function of the angular shift with respect to the first direction Dl clockwise if the angular shift is negative or anticlockwise if the angular shift is positive, by means of the lateral positioners 23 each driven with a different stroke so as to rotate the sheet 2 by the desired angular shift. The sheet 2 is further translated transversely to the first direction Dl as a function of the composite side shift, translating the sheet to the right if the shift is negative or translating the sheet to the left if the shift is positive, by means of the lateral positioners 23 driven for a stroke that is equal to the composite side shift.

[0087] To compare the detected arrangement of the printing pattern with the nominal arrangement of the printing pattern, a longitudinal distance 26 of the identified top graphic mark 20 from the top edge 2a is calculated and the calculated longitudinal distance 26 is compared with a nominal longitudinal distance to obtain the longitudinal shift of the top graphic mark 20.

[0088] A top side distance 27 of the identified top graphic mark 20 is further calculated from the side working edge 2c and the calculated top side distance 27 is compared with a nominal top side distance to obtain the top side shift of the top graphic mark. [0089] Lastly, a bottom side distance 28 of the identified bottom graphic mark 21 from the side working edge 2c is calculated and the calculated bottom side distance 28 is compared with a nominal bottom side distance to obtain the bottom side shift.

[0090] Typically, the comparison between the calculated longitudinal distance and the nominal longitudinal distance involves calculating a difference between what was obtained from the result of the inspection and the nominal distance and the longitudinal shift is the result of this distance.

[0091 ] If the longitudinal shift (considered as an absolute value without regard to its sign) is less than a minimum longitudinal shift threshold, the abutting means is not driven to translate the sheet 2.

[0092] If the longitudinal shift is comprised in an acceptability range between a minimum longitudinal shift threshold and a maximum longitudinal shift threshold the abutting means is driven to translate the sheet 2 in the first direction Dl according to what has been said previously.

[0093] If on the other hand the longitudinal shift exceeds the maximum longitudinal shift threshold, the sheet is deemed not to be suitable to be punched and thus has to be rejected.

[0094] The same considerations (difference between values, minimum shift threshold, maximum shift threshold, acceptability range, evaluation of the quality of the sheet and possible rejection) also apply to calculating the top side shift and the bottom side shift. If the top side shift exceeds a maximum threshold of the top side shift and/or the bottom side shift exceeds a maximum threshold of the bottom side shift, the sheet is deemed not to be suitable to be punched and thus has to be rejected.

[0095] It has been said that the printing pattern comprises a plurality of printed portions 3 that are usually of circular shape and arranged in rows that are parallel and staggered in relation to one another.

[0096] Preferably, the top graphic mark 20 and/or the bottom graphic mark 21 are selected as one of said printed circular portions 3. The top graphic mark 20 and the bottom graphic mark 21 can nevertheless also be additional to the printed portions, if, for example, the printed portions 3 are not circular.

[0097] The top graphic mark 20 can be a printed circular portion 3 of one of the first four rows from the top edge and, inside the row, can be arranged between the first four printed circular portions 3 from the side working edge 2c of the sheet. As shown in figure 10, the sheet 2 comprises a top graphic mark 20 that is the first printed circular portion 3 starting from the side working edge 2c of the second row from the top edge 2a.

[0098] The bottom graphic mark 21 can be a printed circular portion of one of the four last rows from the bottom edge 2b and, inside the row, be arranged between the first four printed circular portions from the side working edge 2c of the sheet 2. As shown in figure 11, the sheet 2 comprises a bottom graphic mark 21 that is the first printed circular portion starting from the side working edge of the penultimate row starting from the bottom edge 2b.

[0099] It is added that it is possible to analyse further the graphic mark of the printed circular portion by exploiting image processing algorithms for comparing the graphic mark of a printed portion 3 with a reference printed portion that is valid for a particular production batch. Errors could in fact occur in the supply of the metal sheets to the punching unit, if by mistake the printing pattern of the sheet does not correspond to the requested production batch. A sheet of a non-conformant production batch has to be rejected.

[00100] If the arrangement of the image acquisition means 17 with respect to the sheet 2 is such as to frame at least one strip of the sheet 2 bounded by the side working edge 2c, by a top edge 2a portion and by a bottom edge 2b portion of the sheet 2, in a single image of the sheet 2 both the top corner 18 and the bottom corner 19 of the sheet 2 can be acquired and it is thus sufficient to process the single acquired image to identify both the top graphic mark in the top corner 18 and the bottom graphic mark in the bottom corner 19 of the sheet 2.

[00101] If on the other hand the arrangement of the image acquisition means 17 with respect to the sheet 2 is such as to frame a first sheet portion 2 bounded by a portion of the side working edge 2c and by a portion of the top edge 2a, a first image of the top corner 18 of the sheet 2 is acquired initially. Subsequently, a second sheet portion 2 is framed that is bounded by a portion of the working edge 2c and by a portion of the bottom edge 2b and a second image of the bottom corner 19 of the sheet 2 is acquired, the second image being acquired after the first, during the advancement of the sheet 2 along the transferring path P in the second direction D2.

[00102] Owing to the invention, it is thus possible to provide an inspection method in which, by processing an image of the metal sheet acquired upstream of the abutting means, during an advancement in the second direction D2, and identifying a top graphic mark arranged in a top corner 18 and a bottom graphic mark arranged in a bottom corner 19 of the sheet 2, it is possible to calculate a longitudinal shift and a top side shift of the top graphic mark and a bottom side shift of the bottom graphic mark that enables the sheet to be translated and/or rotated in the abutting means as a function of the longitudinal shift and/or top side shift and/or bottom side shift. The abutting means can thus adjust with precision the sheet 2 upstream of the punching means.

[00103] The abutting means is thus drivable to enable the sheet 2 to be positioned correctly according to the indications given by the inspection method. It is added that no modification of the position of the cap collecting hopper or of the supporting plane of the reject grid of the sheet downstream of the punching unit is necessary, with consequent simplification of the press.

[00104] Owing to the positioning of the image acquisition means 17 upstream of the abutting means great flexibility in the manner of acquiring the image is also possible. As the sheet 2, adjusted by the abutting means, advances in the first direction Dl to the punching unit 4 whereas on the other hand the image acquisition means 17 frames the sheet 2 during the advancement of the sheet 2 in the second direction D2, the image acquisition means 17 can be arranged far from the punching unit 4 and from the abutting means.

[00105] The most suitable arrangement of the image acquisition means 17 in the press 1 can thus be chosen, for example the image acquisition means 17 can be arranged at a distance from the sheet 2, to frame an entire side strip of the sheet 2 bounded by the side working edge 2c and by portions of the top edge 2a and of the bottom edge 2b or alternatively, the image acquisition means can be arranged near the sheet 2, to frame only a portion of the sheet 2 during the advancement of the sheet in the transferring path P.