DESCRIPTION

The present invention relates to the field inherent in the production of rolls of sheet materials. More particularly, the present invention relates to a method for controlling the wear of components of cutting machines for cutting logs or sticks of paper, tissue paper and the like.

Background of the invention

Conventionally, the manufacture of rolls of sheet materials, such as toilet paper, tissue paper, kitchen paper and similar products, requires a plurality of process steps, comprising the unwinding of the sheet material from a large diameter roll, the rewinding, possibly on a core, to form logs or sticks of smaller diameter, the cutting of the logs into rolls of the desired length, based on their end use, and the packaging of the rolls.

The cutting of the logs to form shorter length rolls is carried out using special cutting machines, which have one or more metal or ceramic blades, of the disc type, which are made to rotate in an orbit around an axis in a cutting plane transverse to the direction of feeding of the logs, or of the belt type, which are cyclically lowered onto the log to be cut.

The logs are made to move forwards, normally in several parallel channels, towards the cutting blade, pushed by special pushers of a drive chain driven on idler wheels.

Another type of cutting machine involves a series of fixed flat blades against which the logs are dragged in a direction perpendicular to their axes, so as to be "sliced".

A problem that reduces, even drastically, the productivity of cutting machines is the maintaining of roll quality due to the edge loss of the cutting machine.

Since the cutting blade must have a high speed of rotation, during cutting a high level of friction is generated between the surface of the blade and that of the log to be cut, which in the long term causes loss of sharpness with jagged bevelled edges. Consequently, once a limit deformation has been reached, it is necessary to sharpen the blade in order not to have rolls in output from the cutting station with such a low quality as to be unusable. An excessively jagged blade, moreover, could be further deformed and damaged to the point of breaking.

In tissue paper production and packaging lines there are several stations whose components need to be replaced. Typical replacements of worn components are linked specifically to the maintenance of cutting machines, where the cutting blades have to be periodically replaced due to the edge loss caused by contact with both the logs to be cut and the sharpening wheels. The sharpening wheels themselves need to be replaced periodically.

Such replacements are costly and require time and the suspension of production, with consequent effects on productivity.

For this purpose, cutting machines are equipped with systems designed to check the wear of the blade by monitoring the deformation thereof and sharpening it when necessary. However, most of the systems currently used are inefficient. It is often the case, in fact, that the sharpening wheel does not do its job properly. In the industry, the use of glues and various substances during production can soil or generally affect the state of the sharpening wheel, leading to a deterioration in the quality of the sharpening.

These control systems are generally based on load cells that measure the contact pressure between the sharpening wheel and the cutting blade, and none of them takes into account the actual state of the wheel, without checking which part thereof is actually working and whether it is effectively performing its task.

Furthermore, in many embodiments, the control takes place with open loop, i.e. the sharpening wheel is moved forwards at time intervals or every predetermined number of cuts, without particularly taking into account the actual wear of the blade.

Summary of the invention

The object of the present invention is to provide a control method for controlling the process of sharpening the cutting blade of a cutting machine which eliminates, or at least reduces, the disadvantages of the prior art. In particular, it is an object of the present invention to provide a control method for performing optimal cutting blade sharpening while avoiding unwanted edge losses and deformations in order to ensure the quality of the rolls cut.

Another object of the present invention is to provide a control method for controlling the sharpening of the cutting blade which can be applied to a cutting machine of any of the known types in order to achieve the results foreseen.

A further object of the present invention is to provide a cutting machine in which the cutting blade does not require frequent sharpening operations.

These and other objects are achieved by a control method for controlling the process of sharpening the cutting blade of a cutting machine according to the invention having the features listed in the appended independent claim 1 and by a relative cutting machine as disclosed in claim 7.

Advantageous embodiments of the invention are disclosed by the dependent claims.

Substantially, the present invention relates to a control method for controlling the process of sharpening of the cutting blade of a cutting machine, in particular a cutting machine used for cutting logs of sheet material, in particular paper, comprising at least one sharpening wheel, wherein said cutting blade and said at least one sharpening wheel are formed of an electrically conductive material and said at least one sharpening wheel is electrically insulated from the rest of the structure forming the cutting machine and is controlled by a PLC controller, comprising the steps consisting of: bringing said at least one sharpening wheel close to the cutting blade so as to cause the closure of the circuit when the contact between the two components establishes an electrical connection; measuring a contact signal in said circuit which depends on the quality of the electrical connection generated between the sharpening wheel and the cutting blade, this contact signal having zero value when no electrical connection is established; comparing the contact signal with a reference signal; and, alternatively, sending an input signal to the PLC controller to launch a phase of sharpening of the cutting blade by acting on the sharpening wheel if the contact signal is greater than or equal to the value of the reference signal; launching a maintenance operation if the contact signal is lower than the value of the reference signal.

Further features of the invention will be made clearer by the following detailed description, referring to a purely illustrative, and therefore non-limiting, example thereof illustrated in the accompanying drawings, wherein:

Figure 1 is a schematic perspective view of a cutting machine with a cutting disc mounted on an orbiting arm according to the prior art; and

Figure 2 is a simplified schematic view of the electrical circuit created at the contact between the sharpening wheel and the cutting blade.

Detailed description of the invention

The control method for controlling the process of sharpening of the cutting blade according to the present invention can be applied to a standard cutting machine 10 such as, for example, that shown in Fig. 1.

Such a machine 10 substantially has a rotating disc cutting blade 11 mounted on an orbiting arm in such a way that the blade completes an orbit in a cutting plane perpendicular to the axis of the logs 13 to be cut. This drawing also shows a sharpening assembly composed in the illustrative example of a pair of sharpening wheels 14, even if a single sharpening wheel 14 can be provided. The functioning of the machine 10 is controlled by a PLC controller 20.

According to the present invention, in a cutting machine 10 the cutting blade 11 and the sharpening wheels 14 are made in an electrically conductive material, and the sharpening wheels 14 are also electrically insulated from the rest of the structure. When a sharpening process is launched, one or more sharpening wheels 14 (also individually) are brought close to the perimeter of the cutting blade 11. Given the conductive nature of the two components, on contact between the sharpening wheel 14 and the cutting blade 11 an electrical connection is generated capable of closing a circuit such as the one shown schematically in Fig. 2, in which the electrical contact between sharpening wheel and blade functions as a sort of switch.

The closure of the circuit following the electrical connection generates a contact signal I in the circuit which is directly dependent on the quality of the contact between sharpening wheel 14 and blade 11. If, for example, the sharpening wheel 14 shows signs of dirt or residues of substances such as glue, etc., due to the continuous cuts made on the logs, the contact established by the sharpening wheel 14 with the blade 11 is not perfect, in that the unwanted substances act as an insulating material, with a consequent decrease in the intensity of the contact signal I. On the contrary, a sharpening wheel 14 in excellent condition in contact with the blade 11 gives rise to a perfect electrical connection with consequent contact signal I at an ideal level. In extreme cases, the contact between sharpening wheel and blade may not even generate an electrical connection. In this case the contact signal I is clearly zero.

Once the sharpening process has been launched, after having therefore brought the sharpening wheels 14 close to the blade 11, the contact signal I is compared with a reference signal Letpoint which represents a threshold value below which the process must be interrupted. If the value of the detected signal I is low, it is in fact probable that the electrical connection has been influenced by the poor condition of the sharpening wheel 14. In such a situation, the PLC 20 notifies the operators that a maintenance operation has to be performed.

Among the various maintenance operations there is cleaning of the sharpening wheel 14. Cleaning operations on the sharpening wheel may be performed manually by the operators or in a "natural" way. In general, in fact, the plane of the sharpening wheels 14 is inclined by a few degrees with respect to the plane of the cutting blade 11. By making the sharpening wheel 14 move forwards gradually, calculated by the PLC 20, towards the blade 11, the working pressure is also gradually increased, which in turn contributes to cleaning the surface of the components.

In case of excessive wear verified by the operators, another maintenance operation is represented by the replacement of the components. A replacement operation is in itself always expensive and affects production. However, when it is carried out according to the indications supplied by the method according to the present invention, it represents a considerable saving from all points of view, given that it is performed at the ideal time. As already mentioned, it is in fact highly inefficient if not harmful to continue with sharpening processes in which the sharpening wheel does not perform its work in the best possible way.

When the contact signal I is greater than or equal to the reference signal Letpoint, the PLC 20 controller receives a signal in input In_PLC that starts the actual sharpening phase. The In_PLC input signal arrives at the PLC 20 for example via a relay 30 which is activated when the reference value L _etpoint is reached (Fig. 2).

The value of the input signal In_PLC can also be used as a parameter to optimise the sharpening phase. For example, it is possible to set the closure time of the sharpening assembly 14 proportional to the signal in input to the PLC. In addition, signals from two sharpening assemblies separate one from the other can be used to set feeds at different times, further reducing consumption.

The control system is also able to measure, again by means of the contact signal I, any vibrations existing between blade 11 and sharpening wheel 14 due to overheating or poor assembly and process the relative data.

According to a further aspect of the invention, a sharpening wheel 14 of a cutting machine 10 can be equipped with a control sensor that measures the distance from the outer perimeter of the blade 11 and based on this parameter measured the PLC 20 establishes the moment when it is appropriate to start a sharpening process, as well as the sharpening wheel feed distance and the sharpening times.

From what is stated above, the advantages of the invention appear clear, which, by applying a method for controlling the sharpening process of the cutting blade in a cutting machine of the prior art, makes it possible to monitor the state of wear of the components and intervene promptly when they are in a non-optimal condition, in particular by evaluating the conditions of the sharpening wheel, minimising the consumption of the cutting blade by moving the sharpening assembly forwards only when necessary.

Naturally the invention is not limited to the particular embodiments previously described and illustrated in the accompanying drawings, but numerous detailed changes may be made thereto within the reach of the person skilled in the art, without thereby departing from the scope of the invention itself, as defined by the appended claims.