GB2220884A

GB2220884A - Improvements in or relating to sawing machines and the like

Info

Publication number: GB2220884A
Application number: GB8914360A
Authority: GB
Inventors: James Paul Delbeke
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-06-22
Filing date: 1989-06-22
Publication date: 1990-01-24
Anticipated expiration: 2009-06-22
Also published as: FR2633551A1; GB8914360D0; FR2633551B3; GB2220884B

Abstract

A sawing machine 1 comprising a frame 16 containing one or more circular saws 2 and one or more workpiece holders 3, is provided with a guard (or protection device) comprising a switch mounted in a circuit for supplying the energy necessary for moving one or more saws 2 with respect to the holder 3 or frame 16, the guard being characterised in that it comprises a proximity detector for detecting the proximity of one or more circular saws 2 with respect to the holder 3 or frame 16, the proximity detector actuating the switch so as to prevent one or more circular saws 2 from damaging the holder 3 or the frame 16 or themselves becoming damaged. The proximity detector may be a magnetic, electric or light beam device. In a preferred embodiment, the proximity detector comprises a potential difference detector responsive to contact of a saw blade with a conductor (47, Fig 2). In another embodiment, a member (55, Fig 4) is moved by the saw blade to actuate a switch (71). The saws 2 are horizontally, vertically and pivotally adjustable. The holder 3 for workpiece 12 comprises conveyor chains 9 and pressers 5. The circular saws 2 could be replaced by a polishing disc, or sandpapering disc or milling tool etc. <IMAGE>

Description

A guard for a sawing machine. and a machine equi#oed with an aforementioned guard The invention relates to a guard for a sawing machine comprising a frame containing one or more circular saws and one or more workpiece holders, the guard comprising a switch mounted in a circuit for supplying the energy necessary for moving one or more saws with respect to the support(s) or frame.

The invention also relates to a machine for sawing pieces of wood and comprising one or more saws and one or more holders of pieces to be sawn, the machine being equipped with a guard on one or more saws.

The aforementioned sawing machines comprise a holder for pieces to be sawn or cut up, and also comprise means for actuating the saw with respect to the holder or with respect to a saw frame.

Known machines have a disadvantage in that an operator adjusting the position of the saw with respect to the holder of the piece to be cut or shaped has difficulty in checking whether the saw is coming into contact with the holder or frame of the machine.

In these machines, the controls for adjusting the position of the saws are placed so that the operator can check that the saws are not coming into contact with the holder or the machine frame.

As stated hereinbefore, sawing machines usually comprise a number of saws, some situated to the right and some to the left of the ends or extremities of the pieces to be sawn or shaped. The operator therefore has to move in order to change over from one machining station to another.

To solve this problem, it is has been proposed to place one or more cameras near each saw so that the position of each saw can be continuously monitored with respect to the holder or the saw frame. A system of this kind, however, has a number of disadvantages, such as rapid soiling of the camera lenses.

It has also been proposed to control the saw by means of a computer, using a suitable programme or software. This system has the disadvantage of being expensive.

Also, the information for replacing a saw of given dimensions by another saw or saw blade, or for replacing any other component of the holder and/or frame, has to be inserted into the computer at the risk of either damaging the saw or holder or frame or of excessively limiting the positioning of the saw with respect to the holder or frame.

The invention relates to a guard which does not have these disadvantages.

The guard according to the invention, of the kind described in the first paragraph of the present description, is mainly characterised in that it comprises a means for detecting the proximity of one or more circular saws with respect to the support(s) or frame, the detector actuating the switch so as to prevent one or more circular saws from damaging the support(s) or the frame or themselves becoming damaged by the support(s) or frame.

According to another feature of the invented guard, the proximity detector comprises one or more transmitters of a signal and one or more receivers of the signal, the signal being adapted to determine the proximity of one or more circular saws with respect to the support(s) or frame, so that when the signal reaches a value corresponding to the minimum proximity of one or more circular saws with respect to the support(s) or frame, the receiver actuates the switch.

The receiver can be mounted on the holder or on the frame or on a component secured to the holder or frame, and the transmitter is connected to the saw or the motor driving the saw or saws during adjustment thereof. In that case the transmitter is the saw.

According to one feature, the signal is an electric signal, preferably a dc or ac potential difference.

In an embodiment of the guard according to the invention, the receiver comprises at least a first element conducting electricity, the first conductive element being insulated from earth and brought to an electric potential with respect to earth, and the transmitter comprises a second conductive element brought to an electric potential different from that of the first conductive element, the potential difference between the first and the second conductive element producing a signal actuating the switch when the two conductors are placed in contact with one another.

The invention also relates to a guard comprising a means adapted to move the receiver and/or the transmitter so as to move them away from the element or part on which it or they are mounted during an operation for positioning the saw or saws with respect to the frame and/or support(s), and/or to move it or them towards the element or part on which it or they are mounted during a sawing operation.

According to a feature of the guard, the aforementioned means is a hydraulic or pneumatic or electric or magnetic means acting on the receiver or transmitter so as to move it away from and/or towards the element or part.

According to another feature of the guard, in which the receiver is acted upon by a resilient means adapted to return the receiver to a given position, the means for bringing about movement is a jack having a rod acting on the resilient means or on the receiver so as to move it away from or towards the element or part.

In an embodiment of the guard, in which the receiver has a fin attached to the element or part by at least one resilient strip, the rod of the jack acts on the fin so as to move it away from the element or part on which it is mounted during an operation of positioning one or more saws with respect to the support(s) or frame.

In an another embodiment of the guard, the receiver is a fin attached to the rod of a jack by at least one resilient strip, and the element or part of the holder or frame on which the jack is mounted is provided with a slope on which a part of the fin or a component secured to the fin can slide so as to move the fin away from or towards the element or part of the holder or frame.

Other features and details of the guard according to the invention will be clear from the following detailed description in which reference is made to the accompanying drawings. In the drawings: Fig. 1 is a diagrammatic perspective view of a sawing machine comprising two circular saws, one situated to the left of a frame and the other situated to the right of the frame; Figs. 2 and 3 are perspective views of details of the machine shown in Fig. 1, the machine comprising a guard according to the invention; Fig. 4 is a view in elevation of a holder provided with a guard according to the invention; Figs. 5 and 6 are perspective views of a holder provided with a guard according to the invention; the guard being associated with an electric and/or electronic circuit.

Fig. 7 is a perspective view of an embodiment of a guard according to the invention; Figs. 8 and 9 are views in section of the guard shown in Fig. 7, and Figs. 10 and 11 are views in section of another embodiment of a guard according to the invention, similar to that shown in Fig. 7.

In the drawings, like reference numbers denote like or similar components.

Fig. 1 is a diagram of a sawing machine 1 comprising a number of circular saws 2 disposed on either side of a holder 3 for workpieces to be cut up.

Holder 3 comprises a pair of conveyors 4 and pressers 5. Conveyor 4 and presser 5 comprise a frame 6 provided at each end 7 with a toothed wheel 8 and a chain 9 extending between two wheels of a single frame 6. The conveyor chain 9 bears a number of fingers 10.

At least one wheel 8 on each frame 6 is driven by a motor 11 so as to drive the chain 9 (arrow R). Chain 9 of conveyor 4 can thus move workpieces 12 (shown by broken lines), one side 13 of each workpiece bearing against a finger 10 on chain 9.

The circular saws 2 are mounted on a shaft of a motor 14 associated with a plate 15 movable with respect to the holder 3 and with respect to a frame 16 on which the holder 3 rests.

Plate 15 can be vertically moved with respect to frame 16 by a jack 17 secured to frame 16 and having a rod 18 secured to plate 15.

Finally, plate 15 bears a jack 28 acting on a platform 29 sliding in guides 30 of plate 15.

Of course, plate 15 and platform 29 could be moved by other means such as an endless screw or plate or platform.

Motor 14 is secured to a small plate 19 having the shape of a sector of a circle, plate 19 being jointed to platform 29 (arrow A).

One edge of plate 19 has teeth 20 engaging with teeth 21 of a wheel 22 mounted on a shaft of a motor 24. Motor 24 can thus be used for positioning the saw 2 at an angle with respect to the holder 3.

The presser 5 can be moved towards conveyor 4 by jacks 25 bearing on frame 6 and having a rod 26 secured to presser 5.

Frames 6 can be moved towards or away from one another (arrow B) by jacks 31 which bear on the frame 16. In that case frames 16 will have rollers 36 guided in rails 37 on frame 16.

Fig. 2 shows part of the sawing machine 1 shown in Fig. 1. The sawing machine 1 has a guard 27 for saw 2 with respect to holder 3 or frame 16.

Guard 27 comprises a switch 32 mounted in a circuit 33 supplying the energy for actuating the saw 2 with respect to holder 3 and frame 16. Circuit 33 comprises an energy supply 34 to the motor 14 of saw 2 for driving it in rotation, and an energy supply 35 to motor 24 and jacks 17, 28 for positioning the saw 2 with respect to holder 3 and frame 16.

Guard 27 also comprises a means 38 for detecting the proximity of saw 2 with respect to holder 3 and frame 16. Detector 38 actuates the switch 32 so as prevent the saw 2 from damaging the holder 3 or frame 16 or being damaged thereby.

The proximity detector 38 comprises a transmitter 39 of a signal and a receiver 40 of the aforementioned signal, the signal being used to determine how near the saw 2 is to the holder 3 or frame 16. When the signal reaches a value corresponding to the minimum proximity allowed for saw 2 with respect to holder 3 or frame 16, the receiver 40 actuates the switch 32.

The guard comprises a receiver 40 comprising at least one first conductive element 47, and a transmitter 39 comprising a second conductive element.

The first conductive element 47 is mounted on bases 39 of electrically-insulating material, the bases 49 being secured to holder 3.

The first conductive element 47, which is insulated from earth, is brought to a potential below e.g. 110V and preferably 12V, by a conductive wire 50 connecting the conductive element 47 to a source of current 51, e.g. a dc or ac current source.

The second conductive element is brought to a different electric potential from the first conductive element 47 so that the potential difference between the first conductor 47 and the second conductor produces a signal which actuates the switch 42 when the two conductors are placed in contact with one another.

Advantageously,the second conductive element is connected to earth. The second conductive element, which can be e.g. a component secured to the motor 14 or to saws 2 or to plate 19 (Fig. 1),is preferably one of the saws 2.

If the machine comprises a number of saws 2, each saw is advantageously a second conductive element.

An electric resistor 52 is mounted on the conductive wire 50 connecting the current source 51 to the first conductive element 47.

A potential difference detector 53 is mounted on wire 50 between resistor 52 and conductor 47. The potential difference appears when the saw or saws 2 touch one or more first conductive elements 47, i.e.

when elements 47 are earthed and a current travels through resistors 52.

The first conductive element 47 has a fin 55 which advantageously extends near an edge or extremity 54 of holder 3 or of the saw or saws 2 or of a component secured to the saw or saws 2 on which the element 47 is secured.

Fin 55 is mounted at one end 58 of a strip 56 having another end 59 which bears on base 49.

Strip 56 has a central part 57 which is inclined with respect to its ends 58 and 59, which are secured to fin 55 and to base 49 respectively. Fin 55 is thus kept apart from holder 3 or from any other component on which the element 47 is secured.

Fig. 3 is a view of a guard similar to that shown in Fig. 2 but differing in that a single resistor 52 is mounted on wire 50 near the current source 51, and also differing with respect to the shape of the conductive elements 47. The receiver or first conductive element 47 comprises a means 77, preferably a resilient means, for keeping the fin 55 away from holder 3. Advantageously, fin 55 intimately follows the shape of holder 3 so as to protect it.

Means 77 comprises a loop 78 having ends 60, 61 secured to bases 49 firmly attached to holder 3.

Fig. 4 is a view in elevation of part of a holder 3 of the machine shown in Fig. 1, the holder comprising a receiver 40. Receiver 40 has a fin 55 articulated to an end 62 of two parallel arms 63 and 64. The other end 65 of arms 63, 64 is articulated by a pivot to holder 3.

Thus, when arms 63, 64 pivot in the direction of arrow P with respect to holder 3, the fin 55 moves towards the holder 3.

Fin 55 has a part 66 between the ends 62 of arms 63 and 64 and acted upon by a spring 67 having an end 68 which bears on a surface 69 of holder 3. Spring 67 is adapted to keep fin 55 away from holder 3 when no pressure is exerted on fin 55 by saw 2.

A part 70 of fin 55 situated in the prolongation of part 66 or between the two arms 63, 64 is adapted to act on a switch or circuit-breaker 71 comprising a push rod 72, the switch 71 actuating the switch 32.

However, switch 71 can be the switch 32 itself, or part thereof.

Fig. 5 is a view of a guard 27 similar to that shown in Fig. 3. Guard 27 comprises a receiver 40 made up of a set of conductive elements 47 connected -in parallel, the elements being secured to holder 3 but electrically insulated therefrom by bases 49. Elements 47 are in the form of a loop having a central part, e.g. a fin 55, for bringing into contact with saw 2, and ends 60, 61 for keeping the fin 55 away from holder 3.

The saw or saws 2 (not shown in Fig. 5) are connected to earth, whereas each conductive element 47 is connected by a wire 50 to a dc electric current source 51. A resistor 52 is mounted on wire 50 near the current source 51.

A means 53 for detecting a potential difference is mounted at end 73 of resistor 52, the end being adjacent the conductive elements 47. Detector 53 comprises an electronic circuit 74 having an input 75 for a cable 76 connected to the aforementioned end 73.

The electronic circuit 74 actuates a switch 32 as soon as a potential difference is detected, the switch 32 being mounted in a circuit adapted to supply the energy necessary for actuating the saw or saws 2 with respect to the support(s) 3 and/or the support 3 with respect to the saw or saws 2.

Fig. 6 is a view of a guard similar to that shown in Fig. 5. Guard 27 differs from that shown in Fig. 5 in that each conductive element 47 is connected by a separate wire 50 comprising an electric resistor 52.

The electronic circuit 74 has a number of inputs, preferably one input 75 per cable 76, for identifying the saw or saws 2 which are in contact with the frames 6, holder 3 or frame 16.

The electronic circuit 74 is e.g. a logic gate having the following response routine Potential difference for Response of each receiver electronic circuit 1 1 0 1 0 0 0 1 0 0 0 1 1 = potential 0 = stoppage difference of saw detected As the table shows, the response "1" is obtained when no potential difference "O" is detected. In other cases there is a risk that holder 3 may be damaged by saw 2 and the machine therefore has to be stopped.

The guard 27 preferably comprises a magnetic or electric or light-beam proximity detector, but preferably comprises an electric detector such as those shown in Figs. 2, 3, 5 and 6. Guards 27 have one or more fins 55 adapted to be brought into contact with the saw or saws 2. In practice, contact between the saw or saws 2 and the fin or fins 55 occurs near the teeth on the saw or saws, more specifically near the cutting part of the teeth. Consequently, even when wood particles or shaving have settled on the fin or fins, good electric contact between the fin or fins and the saw or saws is possible, since the cutting part of the teeth is capable of removing the particles or shavings from the fin or fins.

Also, the devices are easy to construct and do not require skilled staff for installing them.

The guard described hereinbefore is not of use only for guarding one or more saws 2 from a holder or workpiece for cutting up or shaping. Of course, the circular saws 2 shown in Fig. 1 could be replaced by a polishing disc or sandpapering disc or milling tool, etc.

Finally, in the case where a voltage is applied to a saw and a voltage drop, if any, is detected by a measuring device, the guard according to the invention can stop a sawing operation (movement of the conveyor and/or rotation of the saw) if a metal object such as a tool has been entrained by the conveyor.

Fig. 7 is a perspective view of a guard 27 for a sawing machine as shown in Fig. 1. The guard comprises a switch 32 mounted in a circuit 33 (not shown) for supplying the energy required for moving one or more saws 2, and also comprises a means 38 for detecting the proximity of one or more saws 2 with respect to the holders or frame. The detector 38 is e.g. a potential difference detector mounted in the circuit connecting a fin 55 of the receiver to a source of current 51. A resistor 52 is placed between the source 51 and detector 38. The fin 55 (conductive element) is mounted at one end of a resilient strip 56, the other end of which rests on a base 49 secured to a component or part 80 of the holder or frame. Base 49 is made of electrically-insulating material.

When saw 2, which is brought to a different electric potential from the fin or receiver 55, comes into contact with the fin, a potential difference is detected by detector 38, which then actuates the switch supplying the energy for moving the saws. The saw 2 functions as a transmitter.

Guard 27 also comprises a means 81 adapted to move the fin or receiver 55 away from the component or part 80 on which it is mounted during an operation for positioning a saw 2.

The means 81 for moving the fin is a jack having a rod 82, one end of which acts on fin 55 so as to move it a distance d away from the component or part 80 during an operation for positioning a saw (Fig. 9).

When all the saws are properly positioned, the rod no longer acts on fin 55, so that the resilient strip 56 moves the fin back to a position where the fin 55 extends near the part or component 80.

The distance d corresponds to a safety distance preventing a saw damaging the guard during a sawing operation.

The end 90 of rod 82 is provided with a cap 91 made of insulating material.

Of course, instead of moving the receiver 55, the transmitter could be moved. As another alternative, a means could be used for moving the transmitter or receiver towards the component or part on which it is mounted during a sawing operation.

Means 81 can be hydraulic, pneumatic, electric or magnetic, but is preferably a pneumatic jack.

Figs. 10 and 11 are views in section of another embodiment of a guard according to the invention similar to that shown in Fig. 7. In this embodiment, fin 55 is attached to a plate 83 by two resilient strips 56, plate 83 being secured to a rod 82 of a jack 81.

The part or component 80 is formed with a slope 84 on which a part of fin 55 or a component 85 secured to fin 55 can slide.

When the piston 86 of jack 81 moves in the direction of arrow X, a surface 87 of the component 85 secured to fin 55 slides on slope 84 so as to move fin 55 away from the component or part 80 (see Fig. 10).

This position of fin 55 is the one for positioning the saws. When the saws have been positioned, fin 55 is brought near the element 80, so as to ensure a minimum distance d between saws 2 and fin 55.

Component 85 secured to fin 55 preferably comprises an inclined surface.

The slope 84 is made from an electricallyinsulating material or is insulated from the component or part 80. Similarly, the rod 82 of jack 81 is made of insulating material or is provided with an insulating cap 81 so as to insulate it from the fin.

The guard according to the invention has a number of important advantages, i.e.: - the position of particles does not prevent the guard from operating efficiently; - the guard acts as a buffer or shock-absorber between the saw and the frame, holder, sector or angle member of the saw holder; - the saw, more particularly the saw teeth, ensure good contact even if a layer of particles is present on the guard; - the guard can be actuated when pieces of wood are being sawn; - the resilient component can be used to reduce vibration of the guard during sawing, and - the position of the saw or saws with respect to the frame or holder can be widely varied, since protection is provided only in the neighbourhood of the frame or holder.

Claims

1. A guard (27) for a sawing machine (1) comprising a frame (16) containing one or more circular saws (2) and one or more workpiece holders (3), the guard (27) comprising a switch (32) mounted in a circuit (33) for supplying the energy necessary for moving one or more saws (2) with respect to the support(s) (3) or frame (16), the guard being characterised in that it comprises a means (38) for detecting the proximity of one or more circular saws (2) with respect to the support(s) (3) or frame (16), the detector (38) actuating the switch (32) so as to prevent one or more circular saws (2) from damaging the support(s) (3) or the frame (16) or themselves becoming damaged by the support(s) (3) or frame (16).

2. A guard according to claim 1, characterised in that the proximity detector (38) comprises one or more transmitters (39) of a signal and one or more receivers (40) of the signal, the signal being adapted to determine the proximity of one or more circular saws (2) with respect to the support(s) (3) or frame (16), so that when the signal reaches a value corresponding to the minimum proximity of one or more circular saws (2) with respect to the support(s) (3) or frame (16), the receiver (40) actuates the switch (32).

3. A guard according to claim 2 for a sawing machine in which the support(s) (3) comprise a conveyor (4) and a presser (5), characterised in that the or each transmitter (39) or receiver (40) is mounted on the support(s) (3) whereas the or each receiver (40) or transmitter (39) is mounted on the saw or saws (2).

4. A guard according to claim 2 or 3, characterised in that the signal is an electric signal.

5. A guard according to claim 4, characterised in that the signal is a dc or ac potential difference.

6. A guard according to claim 5, characterised in that the potential difference is below llOV, preferably 24V.

7. A guard according to any of claims 4 to 6, characterised in that the receiver (40) comprises at least a first element (47) conducting electricity, the first conductive element (47) being insulated from earth and brought to an electric potential with respect to earth, and the transmitter (39) comprises a second conductive element brought to an electric potential different from that of the first conductive element (47), the potential difference between the first (47) and the second conductive element producing a signal actuating the switch (32) when the two conductors are placed in contact with one another.

8. A guard according to claim 7, characterised in that the receiver (40) comprises a set of conductive elements (47) connected in parallel, the elements being secured to the support(s) (3) and/or frame (16) but electrically insulated therefrom.

9. A guard according to claim 7 or 8, characterised in that at least one electric resistor (52) is mounted on a conductive wire (50) connecting a source of electric current (51) to the first (47) or the second conductive element, and a detector (56) of an electric potential difference between the first (47) and the second element is mounted on the wire (50) between the resistor (52) and the first or second element.

10. A guard according to any of claims 6 to 9, characterised in that the second conductive element is connected to earth.

11. A guard according to claim 10, characterised in that the second conductive element is the saw itself (2).

12. A guard according to claim 2, characterised in that the transmitter (39) is a circuit-breaker (71) whereas the receiver (40) is connected at one end to one or more supports (3) or to a circular saw (2) or to a component secured to a saw (2) or to a motor actuating one or more saws (2), the receiver (40) acting on the circuit-breaker (71) when the minimum proximity of one or more saws (2) with respect to the support(s) (3) or frame (16) is reached.

13. A guard according to claim 12, characterised in that the circuit-breaker (71) is the switch (32) for preventing the saw or saws (2) from damaging the support(s) or frame (16) or being damaged by the support(s) (3) or frame (16).

14. A guard according to any of claims 7 to 13, characterised in that the receiver (40) comprises at least one fin (55) adapted to be brought into contact with the transmitter (39) when the minimum proximity of one or more saws (2) with respect to the support(s) (3) or frame (16) is reached.

15. A guard according to claim 2 and any of claims 4 to 15, characterised in that the receiver (40) comprises a means (77) adapted to move the fin (55) away from the support or supports (3) or from the saw or saws (2) or from a component secured to a saw (2) or from a motor actuating a saw (2) on which the receiver (40) is fixed.

16. A guard according to claim 15, characterised in that the means (77) comprises a resilient means having one end (59, 60) which bears on the support(s) or on the saw or saws (2) or on a component secured to a saw (2) or on the motor actuating the saw (2) on which the receiver (40) is fixed, whereas the other end of the resilient means acts on the fin (55).

17. A guard according to claim 16, characterised in that the resilient means is a spring strip (56).

18. A guard according to any of claims 14 to 17, characterised in that the fin (55) of the receiver (40) extends near at least one extremity or edge (54) of one or more supports (3) or a saw (2) or a component secured to a saw (2) or a motor actuating one or more saws (2) on which the receiver (40) is fixed.

19. A guard according to claim 18, characterised in that the fin (55) intimately follows the shape of the edge (54).

20. A guard according to claim 16, characterised in that means (77) comprises a loop (78), at least a part (60, 61) of which is secured to the support or supports (3) or a saw (2) or a component secured to a saw (2) or the motor actuating one or more saws (2) on which the receiver (40) is fixed.

21. A guard according to claim 20, characterised in that the fin (55) bears on the loop (78).

22. A guard according to claim 21, characterised in that a part of the loop (28) comprises the aforementioned fin (55).

23. A guard according to claim 14, characterised in that the fin of the receiver (40) is pivoted to arms (63, 64) each having an end (65) connected by a pivot to a holder (3) or a saw (2) or a component secured to a saw (2) or to a motor actuating a saw (2) on which the receiver (40) is fixed.

24. A guard according to any of the preceding claims, characterised in that it comprises a means adapted to move the receiver (55) and/or the transmitter (2) so as to move it or them away from the element or part (80) on which it or they are mounted during an operation for positioning one or more saws with respect to the frame and/or support(s), and/or so as to move it or them towards the element or part (80) on which it or they are mounted during a sawing operation.

25. A guard according to claim 24, characterised in that the aforementioned means (81) is a hydraulic or pneumatic or electric or magnetic means acting on the receiver (55) or transmitter (2) so as to move it away from and/or towards the element or part (80).

26. A guard according to claim 24 or 25, in which the receiver (55) is acted upon by a resilient means (56) adapted to return the receiver (55) to a given position, characterised in that the means (81) for bringing about movement is a jack having a rod (82) acting on the resilient means (56) or on the receiver (55) so as to move it away from or towards the element or part (80).

27. A guard according to claim 26, in which the receiver (55) has a fin attached to the element or part by at least one resilient strip (56), characterised in that the rod (82) of the jack (81) acts on the fin (55) so as to move it away from the element or part (80) on which it is mounted during an operation of positioning one or more saws (2) with respect to the support(s) (3) or frame (16).

28. A guard according to claim 26, characterised in that the receiver (55) is a fin attached to the rod (82) of a jack (81) by at least one resilient strip (56), and the element or part (80).of the holder (3) or frame (16) on which the jack (80) is mounted is provided with a slope (84) on which a part of the fin or a component (85) secured to the fin (55) can slide so as to move the fin (55) away from or towards the element or part (80) of the holder (3) at the frame (16).

29. A guard according to claims 8 and 9, characterised in that the receiver (40) comprises a set of conductive elements (47) connected in parallel, each conductive element (47) being connected to an electric current source (51) by a conductive wire (50) and a resistor (52), and the means (53) for detecting an electric potential difference comprises an electronic circuit (74) having at least one input (75) for a cable (76) connecting the circuit (74) to one end (73) of the resistor (52), the end (73) being adjacent one of the aforementioned elements, and the electronic circuit (74) being used to actuate the switch (32) as soon as an electric potential difference is detected.

30. A machine (1) for sawing workpieces, comprising at least one saw (2) or a milling tool or a sandpapering disc and at least one workpiece holder (3) characterised in that it comprises a guard (27) according to any of claims 1 to 29.

31. A machine according to claim 30, in which the workpiece holder(s) (13) comprise a conveyor (4) and/or a presser (5).

32. A machine according to claim 31, characterised in that the conveyor (4) and presser (5) comprise a guard (27) according to any of claims 1 to 29.