WO2001085405A1

WO2001085405A1 - Cutting machine with motor-driven knife

Info

Publication number: WO2001085405A1
Application number: PCT/IB2000/000594
Authority: WO
Inventors: Patrizio Caccia
Original assignee: Neolt S.P.A.
Priority date: 2000-05-08
Filing date: 2000-05-08
Publication date: 2001-11-15
Also published as: AU4138600A

Abstract

This cutting machine for boards of materials used in the manufacture of cardboard articles, posters and the like requiring large cutting force, has a knife (15, 115) optionally of adjustable cutting depth, motorized for its movement by a chain drive and optionally being able to also cut along its return stroke by its facility for selflocking swivelling. In order to help maintain the knife (15, 115) in its own place against the acting forces that can twist and bend it, the cutting machine has blade guide means (29a, 29b, 84).

Description

DESCRIPTION CUTTING MACHINE WITH MOTOR-DRIVEN KNIFE

TECHNICAL FIELD

This invention relates to a cutting machine with a motor-driven knife also cutting along its return stroke, for materials used in the manufacture of cardboard articles, posters and the like requiring large cutting force. BACKGROUND ART

Particularly in the manufacture of cardboard articles and in poster preparation, panels of cardboard and various other materials sometimes of considerable hardness have to be cut. These materials generally consist of cardboard of various thicknesses, boards of expanded material backed with paper or other sheets, or panels of semi-expanded PVC of the type marketed under the brand name of FOREX. When the force required for cutting boards of these materials is small, common fixed or rotary blade knives operated by hand are used. If however the required force can no longer be applied manually, or if the said common cutters are unable to make the required cut, toothed disc blades or circular saws are used. This latter type of cutting has however the drawback of forming dust or shavings, which are never welcome. Likewise unwelcome is the high noise level accompanying these machines. Their cut forms a line of various millimetres in thickness, created by the thickness of the saw and the width of its teeth, resulting in a material wastage not always negligible costwise. It is well known however that the use of bladed knives which cut by incision, hence without forming shavings, presents considerable difficulties. In this respect, the blades must be of small thickness in order not to deteriorate the edges of the cut material. However this small thickness means that the blade is unable to remain rigidly within its own plane against the acting forces, hence it tends to twist and bend, ie it assumes oblique positions which besides forming non-rectilinear cuts facilitates blade fracture. An object of this invention is to define a machine which can use the principle of residue-less incision, typical of knives, for cutting materials requiring a large cutting force which cannot be applied manually. A further object is to define a machine in which the cutting knife can be easily changed to adapt it to the type of material concerned. A further object is to define a machine using a knife which can operate during both its outward and return stroke. A further object is to define a machine with a cutting knife able to operate along the outward and return strokes at two different depths, the difference being automatically preset. A further object is to define a machine which enables a suitable knife to cut materials of the stated type having different thicknesses and hardnesses. A further object is to define a machine in which the means provided for retaining the board to be cut is able to withstand the large thrusts created by the cutting action of the moving knife. DISCLOSURE OF THE INVENTION These and further objects will be seen to have been attained from the ensuing detailed description of a cutting machine for cardboard industry materials requiring large cutting force, comprising a cutting knife of adjustable cutting depth motorized for its movement by a chain drive and optionally being able to also cut along its return stroke by its facility for self-locking swivelling.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated by way of non-limiting example on the accompanying drawings, on which: Figure 1 shows schematically the constituent parts of the machine;

Figure 2 is a cross-section through a carriage carrying a cutting knife of adjustable retraction;

Figure 3 is a perpendicular side view of the same machine region as Figure 2, but with the carriage carrying a cutting knife of fixed projection;

Figures 4, 5, 6 show respectively a cross-section through a support element for the cutting knife of fixed projection and internal views of its two constituent halves;

Figures 7, 8, 9 show respectively a cross-section through a support element for the cutting knife of adjustable retraction and internal views of its two constituent halves;

Figure 10 shows a device for clamping the boards against the fixed machine structure in the position in which they are to be cut;

Figure 11 shows a stabilizer device for containing the whole blade which can be adjusted to the thickness of the material to be cut;

Figure 12 is a perpendicular view of the device of Figure 11;

Figure 13 is a cross-section through a central region of the machi ne;

Figure 14 shows a method for fixing the cutting carriage to the drive chain;

Figure 15 shows a variant of the clamping device of Figure 10. BEST MODE OF CARRYING OUT THE INVENTION

With reference to the said Figure 1, the machine has a base structure 1 with a flat resting surface 2 for receiving a board 3 to be cut. The flat surface 2 is substantially vertical but is slightly inclined rearwards so that the board 3 can rest on it. This board rests with its lower edge 3A against an appropriate bar 4 rigid with the base structure 1. Parallel to the bar 4 there is upperly provided a second bar 5 which, in cooperation with the bar 4, supports a fixed bridging upright 6. Said fixed upright houses the devices for supporting and pressing a panel 7 which maintains the board 3 to be cut fixed by pressing it against the resting surface 2 of the machine structure 1. Above the bars 4 and 5 there are provided two long parallel profiled bars 8A, 8B also positioned in a bridging arrangement by being fixed to raised crosspieces 9 and 10 rigid with the machine structure 1. The two profiled bars 8A, 8B act as rails for the travel of a carriage 11 provided with two groups of four wheels 12A, 12B, 12C, 12D and 13A, 13B, 13C, 13D (not all of which are shown on the drawing) on its two sides, to act in a plane containing a knife 15. The reaction for carriage travel stabilization in the plane of the surface 2 is provided by at least one pair of wheels 14A-14B.

These pairs of wheels are a necessary requirement if the traction force exerted by a chain 17 does not lie in the plane 30 in which the knife 15 acts, as for example in Figure 1. However, if the driving traction is provided by a pair of chains 17 symmetrical about the plane 30, the said pairs of wheels 14 perform only a marginal role. The carriage 11 travels along its rails 8A, 8B with a precision deriving from the use of the method used for driving drawing machines on usual drawing boards. The said ten wheels enable the carriage 11 to travel with extreme stability sufficient to withstand reactive thrusts in all directions without altering its travel trajectory. This stability is additionally enhanced by using further pairs of wheels 14. Said carriage is therefore ideal for carrying the cutting knife 15. This knife is housed in a suitable blade holder 16 inserted into a rectangular hole 81 until its ledge 80 (Figures 4, 5, 6) rests on the carriage 11. This holder can be of various types, each carrying a specific knife for the various types of board to be cut. Alternatively it can be of universal type carrying a knife suitable for any type of cut, in the manner specified hereinafter. The carriage 11 is driven with downward outward movement and upward return movement regulated by usual limit icroswitches which reverse the direction of rotation of an electric motor 24. Maximum cutting force is exerted during the downward movement, the board being retained in this direction by its edge 3A resting on the bar 4. During its upward movement the carriage 11 can encounter two separate conditions: - either a condition in which no force acts because the cut has been completed during the downward movement; or a condition in which a certain force, less than the maximum, acts deriving from the fact that the board is fixed by the pressure of the board pressing panel 7. These two conditions derive from the manner in which the knife is installed on the carriage 11 using the specific blade holder, namely whether it is of fixed type with single direction cutting or of swivel type with two-direction cutting. These two types are shown in Figures 4-5-6 and 7-8-9 respectively. The carriage 11 is driven by the traction of the link chain 17 which passes endlessly about a fixed drive sprocket 18 and a movable reversal sprocket 19. Said chain is maintained taut by withdrawing the reversal sprocket 19 from the fixed sprocket 18 by the screw action of a nut 20 on the threaded shank of a fork 95 rotatably supporting the shaft of the sprocket 19. The chain 17 is closed to form an endless ring by fixing its two ends 21 and 22 to a suitable fixing piece 23 (Figure 14). The 15 drive sprocket 18 is driven by a usual electric motor 24 via an angular mechanical speed reducer 25. Figure 2 shows the aforedescribed details in their mounted configuration, these details being identified by the same numerals. A bolt 26 of a pair of bolts can be seen for fixing the blade holder 16 to the 20 carriage 11. Figure 2 also shows a section through the surface 2 on which the board 3 to be cut (not shown) rests. This cross- section shows two extrusions 27A, 27B which are in mutual contact but upperly form a slot 28 in which the tip of the knife 15 slides without contact so that its cutting edge is not lost. The slot 28 25 is also defined with more precision by two steel plates 29A, 29B provided to support the board in those regions closest to the knife 15, not only to improve the quality of the cut but also to maintain the knife always in its vertical cutting plane 30 (Figure 1). Figure 3 shows the heads 26A, 26B of the pair of bolts by which the blade holder 16 is fixed to the carriage 11. It also shows the positions of four screws 31A, 31B, 31C, 31D by which the two holder halves 16A, 16B (Figures 4, 5, 6) are joined together to form the holder 16 and clamp the knife 15 between them inside its half-seats provided inside them. Figures 4, 5, 6 show the interiors of the two holder halves 16A, 16B, showing the oblique arrangement of the housing seat for the knife 15, having a cutting edge 15A. The knife blade projects a large distance, presupposing it to be used on boards of large thickness, for example thirty millimetres, formed of soft material (for example foamed polystyrene). The considerable inclination of the blade makes it stable in the vertical plane 30. Figures 7, 8, 9 show the interiors of two holder halves 116A and 116B of a blade holder 116 which enables a knife 115 to freely vary its inclination about a pivot pin 32 inserted through a matching end hole thereof. Said inclination can vary through an angle defined upperly by an edge 33 and lowerly by an edge 34. Said edge 34 is positioned such that the tip 115A of the knife 115 passes beyond the resting surface 2 by about 1 mm. The upper edge 33 forms a fixed limit stop which intervenes only if a movable stop 35 is no longer engaged. Said movable stop is formed by a flattened part on a key 36 interposed between the rear 39 of the knife 115 and an end 37 of a bolt 38 screwed into the holder 116 to provide said rear 39 with a matching non-rotating stop region. The said ability of the knife 115 to swivel about the pivot 32 gives the machine exclusive cutting capacity. In this respect, assuming that a board 113 resting and clamped against the surface 2 is to be cut, the carriage 11 carrying the holder 116 is driven in the downward direction 40. The reaction exerted by the material of the board 113 on the projecting end of the knife 115 causes this latter to rise in a direction 41 until the rear 39 rests on the edge 33 (or 35). With this configuration, the knife 115 (shown by thin dashed lines) is positioned at a certain level 42 and incises the board 113 to a certain depth 43 as a result of its travel in the direction 40. On terminating its downward stroke, the carriage 11 is driven upwards in the opposite direction 44. During this movement, the reaction of the board against the tip of the knife 115 (maintained in contact with the board by its own weight or by specific springs) creates a moment 45 about the pivot 32. This moment lowers the knife until it rests against the lower edge 34, so causing the cutting edge 115A to descend below the level of the surface 2 on which the board 113 rests. This hence cuts through any remaining thickness 46 of the board 113. Advantageously, this swivel movement means that the machine can cut hard boards of large thickness by the two separate travel strokes of the carriage 11, ie its outward stroke and its return stroke. This is all achieved using a very economical and widely available knife blade sharpened to razor sharpness. This blade differs from those commonly available in that it also has a cutting edge along its end 79. With reference to Figure 10, the board pressing panel 7 is operated by an electric motor 47 (Figure 1) via an angular speed reducer 48 which by way of a universal joint 49, rotatable on axial bearings 82A, 82B, rotates a threaded tube 50. A threaded shank 51 is engaged in the tube 50 and is integrated into a quadrangular bar 52 which is prevented from rotating by a pin 53 passing through a slot 54 therein. Said pin rotates in its pivoting hole 55 provided in a quadrangular slider 56 axial ly slidable on a cylindrical stem 57 of the quadrangular bar 52 and resting against a suitable stop 58 present on the quadrangular bar 52, being urged thereagainst by the preload of a spring 59 positioned between a nut 60 screwed onto the end of the stem 57 and the edge of the slider 56. The pin 53 joins together a first pair of connecting rods 61 operating to the sides of the quadrangular slider 56 and held together by an appropriate screw 62 which fixes them onto a usual spacer 96 interposed between them. In addition to the pin 53, the first pair of connecting rods 61 carries a fixed pin 63 and a translating pin 64. The first pair of connecting rods 61 cooperates with a second identical pair of connecting rods 65 having their fixed pin 66 supported by a suitable support 67 fixed to the fixed upright 6. The pin 63 also operates on an identical support 67A. The two pairs of identical connecting rods 61 and 65 carry a respective pin 64, 68C on their free end which engage in two matching holes in a rectangular section bar 69. Said holes are spaced apart by a distance equal to the distance between the fixed pins 63 and 66. In this manner an articulated quadrilateral is defined which, when the threaded tube 50 driven by the motor 47 rotates in one or other direction to cause the threaded shank 51 to screw inwards or outwards within it and hence move the pin 53, raises or lowers the quadrangular section bar 69. With reference to Figure 10 and assuming that the pin 53 moves towards the left, the quadrangular section bar 69 moves upwards. As the section bar 69 is slender and hence subject to bending, the said movement is assisted by a third pair of connecting rods 70. These connecting rods are pivoted with a fixed pin 71 on a support 67B and are pivoted with a translating pin 68B on the quadrangular section bar 69. Screws 72 fix onto the quadrangular section bar 69 an aluminium alloy section piece 73 of U-shape to penetrate into the fixed upright 6, which is also of U-shape but reversed and wider (see Figure 13). Finally, the panel 7, which clamps the board to be cut against the surface 2 before the knife 15 commences its downward cutting movement, is fixed onto the section piece 73 by usual screws 97. As the board pressing panel 7 is driven by the electric motor 47 which provides both its outward and return movement, the travel strokes of the board pressing panel are defined by usual microswitches connected into a usual electrical circuit, to reverse the direction of rotation of the said motor 47. The microswitch which defines the position of maximum distance of the board pressing panel from the resting surface 2 presents no problem, whereas the microswitch 74 provided for halting the electric motor 47 when pressing has reached the required force must take account of the possible different thicknesses of the boards to be cut. For this reason the microswitch 74 is fixed onto the quadrangular bar 52 and has its feeler or sensor 74A positioned within a slot 75 in a bar 76 fixed to the slider 56 by a screw 77 and having a slot 78 for optimum position adjustment. With reference to Figure 10, it can be seen that the pulling action provided by the quadrangular bar 52 via the screw 51 is exerted on the first pair of connecting rods by the pin 53 pivoted on the slider 56. As this slider is made rigid with the quadrangular bar 52 by the spring 59, the quadrangular section bar 69 advances freely during its movement for clamping the board 3, until it senses the reaction created by the presence of the board. At this point, the quadrangular bar 52 no longer moves together with the slider 56, but instead moves alone as the slider 56 is retained by the pin 53 blocked by the first pair of connecting rods 61, themselves blocked by the quadrangular section bar 69, itself blocked by the section piece 73 rigid with the board pressing panel 7, which is blocked by the board 3. This relative movement between the quadrangular bar 52 and the slider 56 compresses the spring 59, which discharges its force onto the board 3. In this manner, the pressure exerted on the board is constant whatever its thickness, as the pressing force substantially derives from the travel of the slot 75 relative to the feeler or sensor 74A, it being this travel which compresses the spring 59 and loads the pin 53. That stated with regard to the board pressing device of Figure 6 assumes a board pressing panel 7 of which that surface to rest against the board 3 has a low friction coefficient (for example by virtue of a Nylon or Teflon coating) such as to enable said board pressing panel to slide on the board 3 resting with its edge 3A on the bar 4, as a result of the movement to which it is subjected by the quadrangular section bar 69. This method of achieving sliding between the board 3 and the board pressing panel 7 could be replaced by other methods which allow said sliding. One of these is to engage the board pressing panel 7 on the board 3 by friction and allow the section piece 73 to slide on the lower surface of the quadrangular section bar 69 along their resting plane 99. To achieve this, a clearance 98 is necessary between the head of the bolt 72A and the outer surface of the section piece 73 (Figure 13), both to enable the bolt 72A to be properly fixed in its dead threaded hole in the quadrangular section bar 69 and to ensure that there are no forces generating friction in the plane 99 between the inner surface of the section piece 73 and the base of the quadrangular section bar 69. In the plane 99 there must only exist those forces generated by the crossing of the connecting rods 61, 70, 65. Hence with reference to Figure 13, the section piece 73 is supplemented with flanges 73A, 73B (which perform the function of the board pressing panel 7, or rather replace it) resting on the board 3 to be cut, retained by the bar 4 (Figure 1), and remains fixed on it to press it. At the same time, the quadrangular section bar 69 urged by the connecting rods 61, 70, 65, slides relative to said section piece 73 so causing the shank of the bolt 72A to slide within a short longitudinal slot. A further method of solving said problem of pressing the board pressing panel against the board without inducing sliding forces on it is to use the said motorized constant load device not for driving oblique connecting rods 61, 70, 75 but for driving pairs of symmetrical connecting rods of toggle type, of which Figure 15 shows a simplified example. In Figure 15, those parts having the same function as the parts of the device of Figure 10 are indicated by the same numerals followed by an apostrophe. Hence 52' indicates the quadrangular bar, 6' the fixed upright, 59' the load-providing spring, 56' the slider, 66', 71', 63 the three fixed pins, and 73' the thrusting section piece. New in Figure 15 are double pairs of connecting rods 100A-101A, 100B-101B, 100C- 101C, pivoted to each other by respective pins 102A, 102B and 53'. Slots 103A, 103B, 103C are provided in the fixed upright 6', to guide pins 104A, 104B, 104C in a direction perpendicular to the plane of the clamped board 3' to move the board pressing panel 7 or the equivalent section piece 73' in said direction. Figures 11 and 12 show how a knife 115, of similar design to that shown in Figures 4, 5 6, can have its upward movement range defined by a screw 38A operated by a knob 83A (equivalent to the wing nut 83 of Figure 9). The maximum projection of the blade can be blocked by the action of the screw 38A by making it impossible for its contact edge 34A to rise within the holder 16Z. When in this locked state the blade 115 can cut only in one direction. However when in this locked state, the blade 115 has its maximum projection, for cutting soft materials of maximum thickness (for example thirty millimetres). Notwithstanding this, to enable it to cut relatively hard materials of small thickness in a single pass, the invention uses a device giving maximum stabilization or containment to the blade, in the sense that this device allows the blade to project only by that amount necessary to cut that particular small thickness, the remainder of the blade being locked within the holder 16Z. It should be noted that this containment is not provided by the interior of the holder 16Z but by a blade guide 84 supported by the structure of the carriage 11. This blade guide is substantially an angled lever which is pivoted on a pin 85 fixed to the carriage 11 and is operated by the linear movement of a nut screw 86 fixed to an arm 87 by a pivot pin 88. The nut screw 86 is driven by rotating a screw 89 and could therefore be automated by usual electric motor means. This could be achieved by a connector 90 of diametrical pin type which, when the carriage 11 reaches its lower end-of-t ravel position in the machine, automatically engages by the effect of its ability to slide axial ly under the elastic loading of a return spring 91. Said angled lever, comprising the arm 87 and the pivot 85, has a second arm 92 (the dashed line shows it in its state of maximum projection) with a central slot for containing the knife 115. To stabilize the vertical ity of the knife plane, said arm 92 can move as far as a position close to the resting surface 2 of the board 3. The arm 92 derives its stability, necessary for stabilizing the knife 115, by providing precise guiding for its end 93, which is arcuate concentric to the pivot 85 to utilize the stability deriving from a pin 94 fixed to the carriage 11.

Claims

1. Cutting machine for boards (3) of materials used in the manufacture of cardboard articles, posters and the like requiring large cutting force, characterised by using a very sharp usual knife (15, 115) with an inclined blade, said blade exerting considerable incision-cutting force by being fixed to a carriage (11) moved by at least one link chain (17) driven in both directions by an electric motor (24) via an angular mechanical speed reducer (25) controlled by usual reversal commands provided by two electrical limit switches, said blade (15, 115) deriving stability in the plane in which it lies, by the penetration of its tip (115A) through a rectilinear guide slot (28) formed by the mutual proximity of two plates (29A, 29B) of conveniently hard material, and by being contained with minimum or zero clearance in seats provided in its holder (16, 116) and/or in a blade guide (84) of adjustable projection.

2. Cutting machine as claimed in the preceding claim, characterised in that the knife (115) has two operative inclinations automatically deriving from its travel in the outward direction (40) and in the return direction (44), by being contained in a blade holder (116) such that it can swivel about a pivot pin (32) on which it is mounted and by the effect of a force ensuring that contact between the knife (115) and the board (3) to be cut remains continuous.

3. Cutting machine as claimed in the preceding claims, characterised by a swivelling blade (115) having its operative inclinations defined lowerly by it resting against a lower edge (34) present in the interior of holder halves (116A, 116B) and upperly by a movable stop (35) provided on a key (36) resting against one end (37) of a bolt (38) screwable into the holder (116), said operative inclinations resulting from the reaction against the cutting action offered by the material of the board (3), which creates opposing moments (45, 41) about the axis of the pin (32) consequent on the two opposite directions of advancement (40, 44), said inclinations enabling the knife (115) to form two consecutive incisions (43, 46) at different levels with two specific blade cutting edges, namely the end edge (79) and the bottom edge (15A).

4. Cutting machine as claimed in the preceding claims, characterised in that the bolt (38) can be screwed manually (83, 83A) until the knife (115) locks against the lower edge (34, 34A).

5. Cutting machine as claimed in the preceding claims, characterised in that the knife (15, 115) is fixed to the carriage (11) by means of a blade holder (16) which is interchangeable to ensure that the knife is of optimum type and projection for the type of board and material to be cut, said holder being fixed to the carriage by at least one pair of bolts.

6. Cutting machine as claimed in the preceding claims, characterised by a holder (16) in the form of two holder halves (16A-16B, 116A-116B) joined together by screws (31A, 31B, 31C, 31D) to define a housing seat for the cutting knife (15, 115).

7. Cutting machine as claimed in the preceding claims, characterised by a carriage (11) provided centrally with a longitudinally extending rectangular hole (81) for the insertion of the lower part of the blade holder (16, 116), said lower part having a projection determined by an edge (80) by which it rests on the upper surface of the carriage.

8. Cutting machine as claimed in the preceding claims, characterised by a carriage (11) provided with two groups of four wheels (12A-12B-12C-12D, 13A-13B-13C-13D) engaged on long profiled bars (8A, 8B) acting as rails and provided with projections matching grooves in the wheels, said groups providing the carriage with movement precision by the known method used by drawing machines of linear movement type.

9. Cutting machine as claimed in the preceding claims, characterised by at least one pair of wheels (14A, 14B) operating on at least one of the two long profiled bars (8A, 8B) in a position to the front and rear of the carriage (11) with reference to its direction of advancement, to react against the two opposing crossing couples created by the distance existing between the force on the cutting line (30) and the plane in which the resultant of the pull of the moving chain (17) lies when this pulls the carriage (11) in one or the other direction.

10. Cutting machine as claimed in the preceding claims, characterised by comprising a board pressing panel (7) able to automatically clamp the board (3) whatever its thickness, said clamping deriving from an articulated parallelogram (69-6-61-65) movement undergone by constituent connecting rods (61, 65, 70) of this parallelogram by virtue of a spring (59) acting in an intermediate position of said connecting rods (61), said connecting rods having one end pivoted to a fixed upright (6) of the machine and their other end pivoted on a quadrangular section bar (69) supporting a section piece (73) to which the board pressing panel (7) is fixed.

11. Cutting machine as^laimed in the preceding claim, characterised in that the spring (59) which actuates the articulated parallelogram movement is loaded by the compression deriving from the rotation of a shaft with a universal joint (49) axial ly fixed (82A, 82B) by axial bearings and rotating a threaded tube (50) engaging a shank (51) provided with anti rotation means (53-61-63) in order to be able to move axial ly and compress the spring (59) by a preloading nut (60), said spring acting on the end of a slider (56) pivoted (53) on a pair of connecting rods (61) to enable a feeler-sensor (74A) of an electrical microswitch (74), fixed to a quadrangular bar (52), to halt the rotation of an electric motor (47) driving the shaft with universal joint (4g) after travelling within a slot (75) in a bar (76) rigid with the slider (56) through a length corresponding to the required compression of the spring (59).

12. Cutting machine as claimed in the preceding claims, characterised by a carriage (11) provided with an angled lever acting as a blade guide (84) pivoted on a pin (85) fixed to the carriage (11) and operated by the linear movement of a nut screw (86) fixed to an arm (87) by a pivot pin (88), the movement of the nut screw (86) deriving from the rotation of a screw (89), said angled lever, comprising the said arm (87) and the said pivot

(85), having a second arm (92) with a central slot for containing the knife (115) and being able to move angularly and project from its holder (16Z) to stabilize correct verticality of the plane of the knife even in proximity to the surface (2) against which the board (92A).

12. Cutting machine as claimed in claim 11, characterised in that the board pressing element (7', 73') is driven by a plurality of pairs of connecting rods (100A-101A, 100B-101B, 100C-101C) pivoted to each other (102A, 102B, 53') and interposed between the fixed part (6') and the movable part (73') by pivoting their ends on these parts, the pins (104A, 104B, 104C) associated with the movable part (73') being guided by slots (103A, 103B, 103C) provided in the fixed part (6') perpendicular to the plane of the board (3') to be clamped.