Technical Field The invention relates to an improved machine for cutting a material into predefined shapes, and in particular for cutting rubber, foam rubber, expanded materials and the like. More specifically, the improvements of the invention relate to a control system for the cutting edge of the blade in these machines, and the sharpening of the blade itself.

Background Art The prior art has for some time taught cutting machines for cutting blocks of rubber, foam rubber, expanded materials or similar materials into two-and three- dimensional shapes. These machines comprise a portal frame bearing a second, mobile frame to which a band saw blade is associated. At least one active tract of the blade is arranged horizontally or vertically between the two uprights of the portal frame and makes the cut in the material, which is arranged on a conveyor plane; the conveyor plane and the blade have relation motion (the conveyor plane mobile with respect to the blade or vice versa) which enables the blade to penetrate the material to be cut. In order to position the blade in accordance with the direction of the cut, the tract arranged between the two uprights is supported at ends thereof by two supports which can rotate with respect to the second mobile frame, about the axis of the blade. The cut is achieved by a combination of the movement of the material with respect to the band saw and the rotation of the active tract of the blade about the axis thereof.

In order to obtain a precise cut the blade must be kept constantly very sharp. For

this purpose the machines are provided with means for sharpening associated to the vertically-slidable frame. These means for sharpening sharpen the blade even while the blade is actually in operation, and are activated at predetermined times, based on empirical calculations giving a precise regularity necessary. Basically, when it is decided that the cutting edge of the blade is no longer suitable for cutting, the blade is subjected to a sharpening cycle.

The prior-art machines exhibit some drawbacks. The sharpening cycles are determined on the basis of empirical measurements and not on the basis of the actual conditions of the band saw-which change according to the quality of the material to be cut, the speed of cut and the characteristics of the material the blade is made of. This means that it is improbable that the blade will be sharpened when actually necessary, and that it will therefore operate for some periods of time in non-optimal conditions, resulting in an imperfect cut.

There is also the risk that it will be sharpened too often at other times, which, given the high costs of saws, will reduce its working life unnecessarily.

The main aim of the present invention is to provide a machine for cutting a material into predefined shapes, in which the above-cited drawbacks in the prior art are obviated.

An advantage of the present invention is that the band saw is optimally used, avoiding useless sharpening cycles while guaranteeing an excellent cut quality.

Disclosure of Invention Further characteristics and advantages of the present invention will better emerge from the detailed description that follows of a preferred but non-exclusive embodiment of the invention, illustrated purely by way of a non-limiting example in the accompanying figures of the drawings, in which: figure 1 is a schematic frontal view of the machine of the present invention; figure 2 is a detail of figure 1 in perspective view;

figure 3 is a detail, seen from above, of figure 2; figure 4 is a detail of figure 3, in a frontal view.

With reference to the figures of the drawings, 1 denotes in its entirety the machine of the invention. It comprises a frame 2 for supporting and moving a band saw 3 having a cutting edge 3a; the frame 2 comprising a portal structure 21 defined by two uprights 22 connected by a crossbar 23. A mobile frame 24 is vertically slidable on the uprights 22; the mobile frame 24 has peripheral guides 25, equipped with rollers or discs, which support the band saw 3; the band saw 3 exhibits at least one active tract 4 predisposed to make the cut in a material which is moving with respect to the band saw 3 itself. This material is fed to the blade 3 by a mobile conveyor plane 26 arranged horizontally between the two uprights 22. The relative motion between the material and the blade can however be obtained using different means from those described, for example by moving the frame supporting the blade, or indeed the whole frame of the machine.

Actuators 15 are associated to the frame 2, in particular the mobile frame 24 ; the actuators 15 impart the cutting motion on the band saw 3. Also associated to the frame 2 are means for sharpening the band saw 3 and two orientable supports 5, which support the active tract 4 at both ends and can rotate about a same horizontal axis x. The actuators 15, the means for sharpening 16 and the orientable supports 5 can be independently activated by respective electric gear reducers controlled via a control unit. The means 16 for sharpening, of known type and normally present in these machines, comprise in particular at least two grinding wheels which are normally distanced from the band saw 3 and on command can be brought into contact with opposite sides of the cutting edge of the saw and be activated to rotate. In the present embodiment the sharpening is done by the cutting motion of the blade and the rotation of the grinding wheels which is in contrast with the direction of motion of the saw blade. The orientable

supports 5 are controlled by gear reducers mechanically connected thereto.

The cut is achieved on the basis of a combination of the movement of the material on the mobile plane 26, the motion of the band saw 3, due to the sliding of the mobile frame 24 on the uprights 22, and the rotation of the active tract 4 of the band saw 3 about the horizontal axis x by the orientable supports 5. The rotation of the active tract 4 is effectively a torsion of a portion of the band saw 3, achieved thanks to the rotation of the orientable supports 5. The active tract 4 is in reality an intermediate portion of a longer tract of the band saw 3, so that the torsion of the saw does not induce excessive tensions in the saw itself.

The orientable supports 5 each comprise first guide means 7 predisposed for guiding the band saw 3 in the direction of the cutting force; the guide means are arranged and conformed in such a way as to contrast the cutting thrust that the material exerts on the blade. The machine of the invention further comprises at least one sensor 6 for measuring the force acting on the active tract 4 of the band saw 3 due to the cutting action and for generating a signal which is proportional to the force of the thrust. In other words, the sensor 6 generates a signal which is proportional to the"cutting capacity"of the blade. The sensor 6, in the illustrated conformation, is associated to the first guide means 7.

The first guide means 7 comprise at least two first wheels 8a, 8b associated to a first wall 9 and arranged on opposite sides of the band saw 3 and having rotation axes which are perpendicular with respect to the direction of the cutting thrust.

The wheel 8a acts contrastingly to the cutting force exerted on the blade and is mounted on a support 10 which is mobile and can make small displacements in a parallel direction to the direction of the cutting thrust. The first wheels 8a, 8b guide the band saw 3 in its sliding. In particular, a channel is grooved on the wheel 8b in which the band saw 3 slides so that the cutting edge of the saw does not touch the wheel 8b.

The sensor 6 can be for example a load cell sensor and can be associated to the support 10: in this way the cutting force is unloaded on the sensor 6 which detects the intensity, generates a signal which is proportional thereto and sends the signal to the machine control unit where the value of the signal is compared with a reference signal. If the signal generated by the detected cutting force exceeds the reference value, a sharpening cycle is initiated. Alternatively to the load cell sensor, the sensor 6 could be optical or magnetic or inductive, and be directed onto the support 10 or the band saw 3 so as directly to measure the blade flexion due to the cutting force. The measurement of the flexion is then compared to a reference value which decides the moment at which a sharpening cycle should be initiated. Apart from starting up a sharpening cycle, the signal generated by the sensor 6 can be visualized, continuously and using instruments of known type, to provide a visual indication of the"cutting capacity"of the blade.

The orientable supports 5 comprise a cylindrical support organ 11, coaxial to the horizontal axis x and rotatably constrained to the frame 2 about the horizontal axis x. The support organ 11 is associated to the first wall 9 bearing the first guide means 7. The support organ 11 further comprises second guide means 12 predisposed for guiding the band saw 3 in a perpendicular direction to the cutting force, which second guide means 12 comprise at least two second wheels 13a, 13b associated to a second wall 14 which is perpendicular to the first wall 9, and arranged on opposite sides of the band saw with rotation axes parallel to the cutting force. The second wheels 13a, 13b are arranged with their lateral surface in contact with the surfaces of the band saw 3 so as to transmit the necessary momentum to the blade for the required torsion.

The machine of the present invention offers significant advantages. The presence of the sensor detecting the reaction force acting, following the cutting action, on the band saw means that real-time control is possible of the state of sharpness of

the saw blade. When the blade is blunt the reaction force acting on it increases progressively and therefore causes the signal detected by the sensor 6 to increase; when the reaction force, i. e. the signal relating thereto, reaches a predetermined level, corresponding to an unacceptable degree of bluntness of the blade, the device activates the blade sharpening means which restore the blade to the desired degree of sharpness. The sharpening operation can be terminated either after a predetermined time lapse, or when the reaction on the blade returns to below a second predetermined value. This is possible because the sharpening operation is carried out with the machine operating, i. e. providing a constant reading of the reaction on the blade following the cutting action.

This means that the cutting of the material is always done in the best circumstances, avoiding blunt blades and a consequent drop in the quality of the work and increase in energy consumption-or indeed the opposite in cases where the blade is"too"sharp and is sharpened when there is no need to do so, resulting in a shorter working life. The machine optimizes the relationship between cut quality, blade working life and energy consumption for the cutting operation.