EP3513911B1 - Pressing machine - Google Patents

Description

Technical area

The present invention relates to a press machine, in particular a hand-operated press machine, for pressing tubular workpieces, as well as a method for operating a press machine.

State of the art

In the prior art, methods for pressing tubular workpieces, in particular pipes in installation technology, are known. In a known method, two pipes are permanently connected to one another by means of a press fitting. For this purpose, the pipes are inserted into openings of a press fitting which has polymer seals for sealing with the pipes. After the pipes to be connected have been inserted, the press fitting is pressed using a suitable press machine, i.e. so plastically deformed that the inserted pipes can no longer be pulled out and the seal securely seals.

The pressing is carried out with a hand-held and motor-driven pressing tool, which can have interchangeable tools, such as pressing jaws with different sizes and geometries. In addition, pressing tools are also known for other tasks, for example pressing tools are used for pressing, crimping or cutting workpieces, for example in the electrical industry.

In the case of a hand-held press machine, the press jaws are arranged around the press fitting for pressing. When the press jaws are closed, a force is exerted on the surface of the press fitting, so that the fitting is compressed and plastically deformed, whereby the workpieces are securely connected to one another. The inner tubes can also experience plastic deformation.

In prior art press machines, the pressing process is usually ended by opening a pressure relief valve when a certain maximum pressure is reached. The specified maximum pressure ensures that a suitably high pressing force has been exerted on the workpiece in order to ensure sufficient pressing.

So is from the pamphlet EP 2 501 523 B1 a hand-held pressing device for pressing a press fitting in installation technology and for pressing cable lugs is known. In order to generate the required high pressing forces, the pressing tool is connected to an electro-hydraulic conversion device. A brushless electric motor is used as the drive motor. As soon as the required pressing force is reached, a pressure relief valve opens and the motor speed increases by leaps and bounds. This is recognized by a control of the pressing device and the electric motor is subsequently switched off.

Ending the pressing process by means of a fixed pressure relief valve, however, has the disadvantage that when the maximum pressure is reached, the pressing jaws of the pressing tool are already in contact with one another and the workpiece can no longer be deformed. The fitting was previously compressed as much as possible during the pressing process and can no longer experience any further plastic deformation. The direct pressing of the pressing jaws naturally leads to heavy wear on the tool, on the power transmission parts and on the drive motor. In addition, electrical energy is used unnecessarily.

The pamphlet WO 2016/005838 A1 describes a method and a hydrodynamic compression tool that enables the maximum compression force, the electrical power consumption and the duration of the hydrodynamic pressure to be adapted to the size and pressure resistance of an object to be compressed.

The pamphlet DE 10 2008 024018 A1 describes a method for operating a motor-operated hand pressing device, in which, upon actuation of a switch, one or more pressing jaws are moved from an open starting position into a closed pressing position until a predetermined pressing force is reached is reached or a predetermined time has elapsed, after which a press jaw release occurs automatically, for example by the return of a press piston acting on the press jaws, but this release can be interrupted in an intermediate position before reaching the starting position in order to start the next pressing process from such an intermediate position to start.

It is therefore the object of the present invention to provide a press machine which overcomes the above-mentioned problems and achieves a secure and durable press connection between tubular workpieces without unnecessary wear and tear on the press machine and unnecessary energy consumption. Furthermore, a corresponding method for operating a press machine is to be provided.

Summary of the invention

At least one of the above-mentioned problems is solved according to the invention by a press machine according to claim 1 and a method for operating a press machine according to claim 9.

In particular, at least one problem is solved by a press machine for the plastic deformation of a tubular workpiece, in particular a fitting, the press machine having a motor and press jaws that are driven by the motor and can apply a force to the workpiece during operation, a power transmission unit, which is coupled to the motor and the pressing jaws in order to transmit a force from the motor to the pressing jaws, at least one sensor unit for measuring at least one pressing parameter, and a controller which receives the currently measured value of the pressing parameter from the sensor unit and with this the Determines the slope of a press parameter curve of the press parameter, the control ending a pressing process before a maximum possible press force is reached if the slope of the press parameter curve of the press parameter fulfills a switch-off criterion.

In the present invention, the gradient, that is to say the rise of a pressing parameter curve, which results during the pressing process, is monitored. The increase is characteristic of the increase in the pressing force on the workpiece as the pressing process progresses. When the workpiece has reached the maximum deformation at the end and the press jaws of the press machine are completely closed, only an elastic deformation takes place on the press jaws with a further increase in pressure. A purely linear increase in the values of a press parameter can be recognized, for example, in the case of a hydraulic press device, a purely linear increase in the pressure-time curve or current-time curve. This characteristic increase at the end of the pressing process is used by the control to switch off the motor of the pressing machine before a maximum possible pressing force is reached and thus to end the pressing process.

The automatic termination of the pressing process before reaching the maximum possible pressing force of the machine based on the evaluation of the slope of at least one pressing parameter curve prevents further unnecessary pressing of the pressing jaws, which leads to a reduction in the load and wear of the pressing jaws and other parts of the machine and to one of the Saves energy and time. An additional interaction of a user to end the pressing process is not necessary.

The comparison of the slope of the press parameter curve of the measured values of the at least one press parameter with a switch-off criterion automatically takes into account the properties of the workpiece to be pressed, for example the material, the size, the design, etc. Pressing process would have to be made known. The switch-off criterion is independent of the individual properties of the workpiece to be pressed. Due to the monitoring of the slope of the pressing parameter curve, the control can reliably end the pressing process after the plastic deformation of the workpiece, regardless of the level of the pressing force beyond which plastic deformation of the workpiece no longer occurs.

During the evaluation, one or more pressing parameters can be taken into account in any combination. The consideration of several pressing parameters at the same time strengthens the robustness of the analysis against so-called outliers, ie random deviations, due to redundancy.

Different sensors can be used to measure the same or different pressing parameters for the evaluation with the present control.

The respective switch-off criterion can be adapted to the respective process parameter or to a combination of the process parameters.

The switch-off criterion preferably indicates that if the pressing process is continued, only elastic deformation occurs on the pressing jaws. This is the case when the press jaws are completely closed and there is no more plastic deformation of the workpiece. The switch-off criteria are preferably predetermined values of the slope for a specific pressing parameter curve, on the basis of which the control can automatically decide when the current pressing process is to be ended.

The force transmission unit is preferably a hydraulic system and the sensor unit is a pressure sensor which measures the pressure in the hydraulic system as a pressing parameter. This enables the control according to the invention to be easily adapted to existing electro-hydraulic press machines. The hydraulic pressure can be measured simply and reliably as a process parameter via a pressure sensor and made available to the control system. The hydraulic pressure is directly proportional to the pressing force applied to the tool, so that a reliable determination of the force profile on the pressing jaws is possible via the hydraulic pressure.

If the force transmission unit is preferably a mechanical system, the sensor unit is a force sensor which measures the force at a point in the mechanical system as a pressing parameter. The control of the present invention can also be applied to pressing devices with a purely mechanical power transmission. In mechanical press machines, a power is transmitted from a motor to the press jaws via one or more power transmission units. In this case, the force that occurs can be measured at various points in the mechanical system. Conventional load cells, strain gauges or similar sensors can be used for this purpose. The measured force is usually also direct proportional to the pressing force applied to the tool, so that the force can be used to reliably determine the force profile on the pressing jaws.

A current flowing through the motor can also be a preferred pressing parameter. This current can be measured by means of the controller, which then preferably also takes on the task of the sensor unit. The force profile on the press jaws can also be derived from the current flowing through the motor.

The control preferably only ends the pressing process when, in addition to the switch-off criterion, a previously defined minimum value for a pressing parameter (P) has been exceeded and / or when the duration of the pressing process has exceeded a minimum time. This means that start-up effects at the beginning of the pressing process or areas of the pressing process in which plastic deformation of the workpiece regularly still takes place are not taken into account. This reduces the risk of incorrect measurements and an unwanted premature termination of the pressing process. Reliable pressing of a workpiece generally requires that the minimum pressing force has been reached, so that the pressing process should not be terminated automatically below this minimum pressing force.

The slope of the pressing parameter curve preferably indicates the course over time of the values of a pressing parameter and is preferably formed from the current and the temporally preceding value of the pressing parameter. The time course of a pressing parameter can be viewed in a simple manner in the form of a time series, in particular if the slope of the process parameter curve is calculated from measured values that follow one another over time. The consideration of the current and the previous value for the determination of the increase is simple and can be implemented computationally quickly, so that results can be available in real time. This enables immediate, real-time control to be achieved in response to the evaluation. The significantly constant, linear course of the pressing parameter curve when the pressing jaws are pressed directly onto one another at the end of the pressing process enables robust and automatic detection of this pressing state.

The control can preferably also have a database in which switch-off criteria for specific press jaws and / or workpieces are stored. The switch-off criteria here are preferably digital values of the slope of the process parameter curve for which automatic switch-off is to take place. The database can contain values for switch-off criteria, which can depend on the properties of the press jaws used, such as material, size, type, etc.

The pressing machine is preferably an electrically driven hydraulic or mechanical hand pressing device for pressing tubular workpieces. With the help of a hand crimping device, crimps can be used flexibly at different locations, such as a construction site. Electrically driven hand pressing devices can apply high pressing forces, which ensure reliable pressing. In a hydraulic hand pressing device, for example, a hydraulic pressure of up to approx. 550 bar can be applied during operation, which acts directly on the workpiece enclosed by the pressing jaws.

1. a. Eingreifen des Werkstücks mit Pressbacken der Pressmaschine;
2. b. Starten eines Motors der Pressmaschine, um eine Kraft durch die Pressbacken auf die Oberfläche des eingegriffenen Werkstücks aufzubringen;
3. c. Messen eines Werts eines Pressparameters;
4. d. Empfangen des aktuell gemessenen Wertes des Pressparameters durch die Steuerung, und damit Bestimmen der Steigung einer Pressparameterkurve des Pressparameters; und
5. e. Stoppen des Motors vor dem Erreichen einer maximal möglichen Presskraft der Pressmaschine durch die Steuerung, wenn die Steuerung erkennt, dass die Steigung der Pressparameterkurve des Pressparameters ein Abschaltkriterium erfüllt.

At least one of the above-mentioned problems is also solved by a method for operating a press machine for plastic deformation of a tubular workpiece, in particular a fitting, the method having the following steps in the order given:

1. a. Engaging the workpiece with the press jaws of the press machine;
2. b. Starting a motor of the press machine to apply a force through the press jaws to the surface of the engaged workpiece;
3. c. Measuring a value of a pressing parameter;
4. d. Receiving the currently measured value of the pressing parameter by the controller, and thus determining the slope of a pressing parameter curve of the pressing parameter; and
5. e. The control system stops the motor before the maximum possible press force of the press machine is reached, if the control system detects that the slope of the press parameter curve of the press parameter fulfills a switch-off criterion.

The pressing parameter is preferably a pressure, a force, or a current through the motor or any combination of these parameters. These parameters are characteristic of the press pressure of the press machine.

According to the invention, the motor is only stopped when, in addition to the switch-off criterion, a predetermined minimum value for a pressing parameter has been exceeded and / or when the duration of the pressing process has exceeded a minimum time. The method preferably has the step of reading out at least one switch-off criterion from a database of the controller. The switch-off criterion can be stored in a database in the control of the press machine and, for example, read out to match the press jaws used and used in the control.

Brief description of the figures

Fig. 1

eine schematische Darstellung einer Ausführungsform der Pressmaschine als hydraulisches Handpressgerät gemäß der vorliegenden Erfindung;

Fig. 2

ein Diagramm von Pressparameterkurven für unterschiedliche Pressbacken und zu verpressende Werkstoffe bei einer Pressmaschine nach dem Stand der Technik; und

Fig. 3

eine chematische Darstellung der Auswertung einer Pressparameterkurve gemäß der vorliegenden Erfindung.

In the following, preferred embodiments of the present invention are illustrated with reference to the accompanying figures. It shows:

Fig. 1

a schematic representation of an embodiment of the pressing machine as a hydraulic hand pressing device according to the present invention;

Fig. 2

a diagram of press parameter curves for different press jaws and materials to be pressed in a press machine according to the prior art; and

Fig. 3

a chemical representation of the evaluation of a pressing parameter curve according to the present invention.

Detailed description of preferred embodiments

In the following, preferred embodiments of the present invention are described in detail with reference to the accompanying figures.

The Fig. 1 shows an embodiment of a hydraulic hand pressing device 10 with a hydraulic power transmission unit. With this hydraulic hand press a motor 20 drives an eccentric 24 connected to it via a gear 22. The motor 20 is preferably a brushless motor which is supplied by a controller 40 with correspondingly modulated current from a rechargeable battery or a wired power supply (not shown). The transmission 22 reduces the speed of the motor 20 and increases the torque. The eccentric 24 connected to the transmission converts the rotary movement of the output shaft of the transmission 22 into a one-dimensional oscillating movement in order to drive a piston pump 27 of the hydraulic system 26.

Due to its movement, the piston pump 27 pumps hydraulic fluid from a reservoir into a working cylinder 25, as a result of which the hydraulic pressure in the working cylinder 25 increases. The increasing hydraulic pressure pushes a piston 28, which is movably guided in the cylinder, as shown in FIG Fig. 1 to the left, in the direction of the fastening area for exchangeable pressing jaws 30 (not shown in detail). By using a large piston diameter, the piston 28 can transmit very high pressures to the pressing jaws.

The piston 28 is mechanically connected to rollers 29, which move with the movement of the piston 28. The rollers 29 move in the usual way between inclined ends of pressing jaws 30, which are thus closed and can plastically deform the workpiece with great force. During operation, the hydraulic pressure is thereby transmitted directly proportionally to the connected pressing jaw 30, and generates a pressing force F which is directly proportional to the hydraulic pressure and which is directly proportional to the workpiece.

Due to the increasing hydraulic pressure P during the pressing and the consequent increasing pressing force F on the workpiece or the fitting, the workpiece is pressed and plastically deformed. The pressing force F on the tool can be determined by measuring the hydraulic pressure P.

The hydraulic pressure P in the hydraulic system 26 can be measured in a simple manner by means of a pressure sensor 42. The pressure sensor 42 transmits the measured pressure signal to the controller 40 via signal lines or wirelessly with the aid of a corresponding radio transmission Signal transmission means, such as, for example, common digital wireless connections such as, for example, Bluetooth, NFC or the like, can be used. Analog signals from the pressure sensor 42 can be converted into digital signals in an A / D converter so that they can be evaluated by the digital controller 40.

For this purpose, the controller 40 has at least one digital processing unit, such as a microcontroller, DSP, FPGA, ASIC or the like. In addition, the control unit can have a database (not shown) stored on data storage devices, in which predetermined values that are required for the evaluation can be called up.

Depending on the evaluation results, the controller 40 outputs corresponding control signals to the motor 20 via power electronics (not shown). The motor 20 is controlled with the aid of these control signals in order to operate it at a certain regulated speed and to stop it at the end of the pressing process.

In another embodiment, not shown, the pressing machine can also be designed as a purely mechanical hand pressing device with a mechanical power transmission unit. In a mechanical hand pressing device, a motor generates a rotary movement which is transmitted via a gear to at least one mechanical power transmission unit, for example a lever drive or a screw drive. The mechanical power transmission unit converts the rotary movement into a linear movement, which, corresponding to the above-described hydraulic hand pressing device 10 of FIG Fig. 1 , Moves rollers with great force that move the press jaws. As a result of the increasing force of the press jaws, a workpiece, for example a fitting that is located between the press jaws, is plastically deformed.

Force sensors for measuring the force F transmitted from the motor to the tool can be arranged at various points in the mechanical hand pressing device in order to measure a force proportional to the pressing force F and to signal it to the controller.

Furthermore, the current consumed by the motor 40 is also proportional to the motor torque and thus to the pressing force F on the pressing jaws.

The Fig. 2 shows a graphic representation of measured press parameter curves K1, K2 and K3 of press machines according to the prior art, which represent hydraulic pressure values P over time. The pressing parameter curve K1 was recorded during pressing without a fitting inserted in between, ie with "empty" pressing jaws. The pressing parameter curve K2 was recorded when pressing a fitting made of a first material with the same pressing jaws as in curve K1. The material of the fitting was a comparatively soft material, such as copper.

The pressing parameter curve K3 was recorded when pressing a fitting made of a second material. The material of this fitting had a greater strength or hardness than in curve K2 and is stainless steel in one embodiment.

In Fig. 2 it can be seen that with curve K1 the pressure is relatively constant and almost zero at the beginning, since there is no fitting in between when the press jaw is closed. The press jaws can close unhindered with a minimum hydraulic pressure that overcomes the friction and spring forces in the system. From the point in time T1 _K1 , the press jaws are completely closed and rest on one another. From this point in time they immediately press against one another, as a result of which the pressing parameter curve K1 shows a constant linear increase ΔT _K1 until the maximum pressure Pmax _K1 is reached. When the maximum pressure Pmax _K1 is reached, a pressure relief valve opens, causing the hydraulic pressure to drop suddenly to a minimum pressure. As can be seen from the press parameter curve K1, the press jaws therefore show a linear elastic deformation in the completely closed state. The present invention uses this property of the pressing jaws to automatically recognize the end of the pressing process and then to switch off the motor 20 before the maximum pressure Pmax _{K1 is} reached. This protects the entire press machine and reduces the energy required.

The course of the curve of the pressing parameter curve K2 shows an earlier increase in hydraulic pressure than the pressing parameter curve K1, since there is a fitting made of a soft material between the pressing jaws. From the point in time T1 _{K2 the} increases Pressing parameter curve K2 again increases linearly, since at this point in time the actual pressing process has ended, the fitting has been completely plastically deformed and the press jaws are completely closed and rest on one another. The linear increase .DELTA.T _K2 takes place with a gradient that is essentially the same as that of the curve K1. When the maximum pressure Pmax _K2 is reached, the overpressure valve again opens at time T2 _K2 and curve K2 drops very quickly to a minimum pressure.

The press parameter curve K3 increases more than the press parameter curve K2 due to the harder material of the fitting to be pressed. Furthermore, a higher pressure has to be applied overall with curve K3 than with curve K2 in order to close the pressing jaws and to completely plastically deform the fitting. At the point in time T1 _K3 , the curve profile of K3 becomes linear, which indicates the end of the actual pressing process and signals that the pressing jaws are completely closed and are resting on one another. As can be seen, the slope of the linear area ΔT _K3 essentially corresponds to the slope of the linear areas ΔT _K1 and ΔT _K2 . This slope of the pressing parameter curve is therefore a characteristic measure for the end of the actual pressing process on the workpiece.

How out Fig. 2 It can also be seen that the time span ΔT _{K1 is} significantly greater or longer than the time span ΔT _{K2 of} the curve K2 or the time span ΔT _{K3 of} the curve K ₃ . The longer the time period ΔT, the longer the pressing jaws of the hand pressing device lie directly on top of one another and press directly against one another. During the period of time ΔT, there is no plastic deformation of the workpiece or fitting. more, so that the pressing force F is unnecessarily increased in this period of time.

The curves K1, K2 and K3 would result in a corresponding manner if, instead of the hydraulic pressure over time, a force acting mechanically in the system or the current through the motor 20 were plotted over time. Here, too, the end of the pressing process and the fact that the pressing jaws are lying on top of one another can be recognized from a linear increase in the curve with a characteristic gradient.

In Fig. 3 shows an example of the curve shape of the pressing parameter curve K2 if a control 40 according to the invention is now used. The control determines this 40 shows the slope of the pressing parameter curve K2 and can use this to determine whether the actual pressing process on the workpiece has ended. In this case, the controller 40 compares the slope of the pressing parameter curve K2 with a switch-off criterion, that is to say in the present case a characteristic slope of the press jaws used in the completely closed state. If the slope fulfills the switch-off criterion, which the control determines or calculates, it ends the pressing process in that it switches off the motor 20 or no longer supplies it with current. In this way, the controller 40 can reliably terminate the pressing process before a maximum possible pressing force Pmax is unnecessarily reached. This significantly reduces the wear in the press machine, the energy required and the time required. The savings are greater when pressing softer fittings than when pressing harder fittings.

The shutdown of the motor 20 can also be made dependent on further conditions by the controller 40, for example in order to recognize and exclude outliers in the pressing parameter curve. In addition to the switch-off criterion, it may be necessary for the motor 20 to switch off that a previously defined minimum value for a pressing parameter P has been exceeded and / or it may be necessary that the duration of the pressing process has exceeded a minimum time Tmin.

In the example of curve K2 the Fig. 3 the minimum value is a minimum pressure Pmin _K2 , which defines a point in time To _K2 , from which the controller 40 determines an increase (represented by a gradient triangle 44) between the current and the previous value for each measured pressing parameter value and compares it with the switch-off criterion. The minimum pressure Pmin can generally be fixed, or it can be variable, for example as a function of the press jaws used or the workpiece. Equivalently, for example, a minimum current at the motor 20 can also be used as a criterion for the point in time To.

However, a minimum time Tmin can also be specified which the pressing process must at least last before the control can switch off the motor 20.

From the point in time T1 _K2 , the gradient value determined by the controller 40 corresponds to a predetermined gradient value stored in the database of the controller 40 as a switch-off criterion. From this point in time T1 _K2 on, the controller 40 then preferably determines the number of increase values that are the same in a time range.

At the later point in time T3 _K2 , a predetermined number of increases with the same increase values was then counted. The counted increase values essentially agree with the predetermined increase value stored in the database. Permissible tolerances of the gradient values can be determined empirically and stored in the controller 40.

In the example of the Fig. 3 For example, a certain predetermined number of a slope value of the pressing parameter P of the curve K2 serves as a switch-off criterion. In the present exemplary embodiment, this switch-off criterion is met _{at time T3 K2.} The pressing process is then stopped by the controller 40. For this, however, the controller 40 and the motor require a reaction time between the times T3 _K2 and T2 _K2 . At time T2 _K2 , engine 20 is at a standstill and hydraulic pressure Pstopp _K2 remains constant. After a further reaction time span between times T2 _K2 and T4 _K2 , the control 40 opens the pressure relief valve or a return valve at time T4 _K2 , whereby the hydraulic pressure in the hydraulic system of the pressing device drops to a minimum or nominal value.

In Fig. 3 is shown by the dashed line K2 'for comparison of the curve profile of the press parameter curve K2 according to the prior art without the control according to the invention. Without the control according to the invention, as in Fig. 2 shown, the pressing process can be carried out _{until a maximum pressure Pmax K2 is reached.} Only when this maximum pressure Pmax _{K2 is reached} would the pressure relief valve open and the hydraulic pressure would drop to a minimum value, the pressing process not being ended until the pressure relief valve opened.

Out Fig. 3 a pressure difference ΔP _K2 can also be seen, which extends between the pressure when the motor is switched off according to the invention Pstopp _K2 and the set maximum pressure Pmax _K2 (overpressure protection). This pressure difference ΔP _K2 represents the saved pressure that would otherwise have to be applied by the motor 20. At the same time, the pressure difference ΔP _K2 also represents the energy saved with the present invention, since the motor no longer has to perform any work after it has been switched off at time T2. The greater the pressure difference .DELTA.P, the more effective the savings through the present invention. It will be greater with soft workpieces than with harder workpieces, such as from Fig. 2 evident.

List of reference symbols

10

hydraulic hand press

20th

engine

22nd

transmission

24

eccentric

25th

Working cylinder

26th

hydraulic system

27

Piston pump

28

piston

29

roll

30th

Attachment area for interchangeable tools

40

control

42

Pressure sensor

44

determined increase (shown as gradient triangle)

K1, K2, K3

Parameter curves

K2 '

non-inventive course of the parameter curve K2

Pmin (K2)

Minimum pressure in K2 for shutdown

Pmax (K1, K2, K3)

maximum pressure that can be applied in K1, K2, K3

P-T1 (K2)

Pressure in K2 at time T1

P-T3 (K2)

Pressure at time T3

Pstop (K2)

Pressure at the time when pressing is finished

ΔP (K2)

Pressure difference in K2

Tmin

Minimum time for shutdown

To (K2)

Time To: Start of the check for the switch-off criterion

T1 (K1, K2, K3)

Time T1: start of the constant linear slope

T2 (K1, K2, K3)

Time T2: end of the pressing process

T3 (K2)

Time T3: switch-off criterion met

T4 (K2)

Time T4: opening of the pressure relief valve or return

ΔT (K1, K2, K3)

Time difference between T1 and T2 (duration of the constant linear gradient)

Claims (11)

1. A pressing machine (10) for plastically deforming a tubular workpiece, in particular a fitting, wherein the pressing machine (10) comprises:

   a. a motor (20);

   b. pressing jaws which are driven by the motor (20) and can apply a force to the workpiece during operation;

   c. a power transmission unit coupled to the motor (20) and the pressing jaws for transmitting power from the motor (20) to the pressing jaws;

   d. at least one sensor unit (42) for measuring at least one pressing parameter (P); and

   e. a controller (40), which receives a currently measured value of the press parameter (P) from the sensor unit (42) and determines therewith a slope (44) of a press parameter curve (K) of the press parameter (P); wherein

   f. the controller (40) terminates a pressing operation before reaching a maximum possible pressing force (Pmax) when the slope (44) of the press parameter curve (K) of the pressing parameter (P) fulfills a switch-off criterion,
   wherein the controller (40) terminates the pressing process only when in addition to the switch-off criterion a predetermined minimum value (Pmin) for a pressing parameter (P) has been exceeded and/or if the duration of the pressing process a minimum time (Tmin) has been exceeded.
2. The press machine according to claim 1, wherein the switch-off criterion indicates that only an elastic deformation on the pressing jaws occurs in a continuation of the pressing operation.
3. The press machine according to claims 1 or 2, wherein the power transmission unit is a hydraulic system (26), and the sensor unit (42) is a pressure sensor, which measures a pressure in the hydraulic system (26) as the pressing parameter (P).
4. The pressing machine according to claims 1 or 2, wherein the power transmission unit is a mechanical system, and the sensor unit (42) is a force sensor which measures a force at a location in the mechanical system as the pressing parameter (P).
5. The pressing machine according to any one of claims 1 to 4, wherein the pressing parameter (P) is a current flowing through the motor (20).
6. The press machine according to any one of claims 1 to 5, wherein the slope (44) of the press parameter curve (K) indicates the time course of the values of the press parameter (P) and is preferably formed from the current and the temporally preceding press parameter value.
7. The pressing machine according to any one of claims 1 to 6, wherein the controller (40) has a database in which at least one shutdown criteria for certain pressing jaws and/or workpieces is stored.
8. The press machine according to any one of claims 1 to 6, wherein the pressing machine (10) is an electrically driven hydraulic or mechanical hand pressing device for pressing of the tubular workpieces.
9. A method of operating a pressing machine (10) for plastically deforming a tubular workpiece, in particular fittings, the method comprising the following steps in the given order:

   a. engaging the workpiece with pressing jaws of the pressing machine (10);

   b. starting a motor (20) of the pressing machine (10) to apply a force through the pressing jaws to the surface of the engaged workpiece;

   c. measuring a value of a press parameter (P);

   d. receiving a currently measured value of the press parameter (P) by the controller (40), and thereby determining a slope (44) of a press parameter curve (K) of the press parameter (P); and

   e. stopping the motor (20) from reaching a maximum possible pressing force (Pmax) of the pressing machine by the controller (40) when the controller (40) detects that the slope (44) of the press parameter curve (K) of the pressing parameter (P) fulfills a switch-off criterion,
   wherein the motor (20) is stopped only when in addition to the shutdown criterion, a predetermined minimum value (Pmin) is exceeded for a pressing parameter (P) and/or if the duration of the pressing operation a minimum time (Tmin) is exceeded.
10. The method according to claim 9, wherein the pressing parameter (P) is a force, a current through the motor (20) or any combination of these parameters.
11. The method according to claim 9 to 10, further comprising the following step:
    reading at least one switch-off criterion from a database of the controller (40).

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