EP3513911A1 - Pressing machine - Google Patents

Abstract

The present invention relates to a press machine 10 for plastically deforming a tubular workpiece, in particular a fitting, the press machine 10 having a motor 20; as well as press jaws which are driven by the motor 20 and can apply a force to the workpiece during operation; a power transmission unit coupled to the motor 20 and the press jaws to transmit power from the motor 20 to the press jaws; at least one sensor unit 42 for measuring at least one pressing parameter P; and a controller 40, which receives the currently measured value of the pressing parameter P from the sensor unit 42 and uses it to determine the slope 44 of a pressing parameter curve K of the pressing parameter P; the controller 40 ends a pressing process before reaching a maximum possible pressing force Pmax if the slope 44 of the pressing parameter curve K of the pressing parameter P fulfills a switch-off criterion. The present invention further relates to a corresponding method for operating a press machine 10.

Description

Technical area

The present invention relates to a pressing machine, in particular a hand-held pressing machine, for pressing tubular workpieces, and a method for operating a pressing 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 non-releasably connected to one another by means of a press fitting. For this purpose, the tubes are inserted into openings of a press fitting which has polymer seals for sealing with the tubes. After insertion of the pipes to be joined, the press fitting is pressed by means of a suitable pressing machine, so plastically deformed that the plugged pipes can not be pulled out and the seal seals securely.

The pressing is carried out with a hand-held and motor-driven pressing tool, which can have interchangeable tools, such as press jaws of 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 a hand-held press machine, the press jaws are arranged around the press fitting for pressing. When closing the pressing jaws, a force is exerted on the surface of the press fitting, so that the fitting is compressed and thereby plastically deformed, whereby the workpieces are securely joined together. Here, the inner tubes can undergo plastic deformation.

The pressing process is terminated in press machines of the prior art usually characterized in that upon reaching a certain maximum pressure on Relief valve is opened. The specified maximum pressure ensures that a suitably high pressing force has been exerted on the workpiece to ensure adequate compression.

This is from the publication EP 2 501 523 B1 a hand-held pressing device for pressing a press fitting in the installation technology and for pressing cable lugs known. To generate the required high pressing forces, the pressing tool is connected to an electro-hydraulic conversion device. The drive motor is a brushless electric motor. As soon as the required pressing force is reached, a pressure relief valve opens and the engine speed increases abruptly. This is detected by a control of the pressing device and the electric motor subsequently switched off.

However, the termination of the pressing operation by means of a fixed overpressure valve has the disadvantage that upon reaching the maximum pressure, the pressing jaws of the pressing tool already abut each other and no deformation on the workpiece can take place more. The fitting was previously compressed as much as possible during the pressing process and can no longer experience further plastic deformation. The direct pressing together of the dies leads naturally to a strong wear on the tool, on the power transmission parts and on the drive motor. In addition, unnecessary electrical energy is consumed.

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

Summary of the invention

At least one of the above problems is solved according to the invention by a pressing machine according to claim 1 and a method for operating a pressing machine according to claim 10.

In particular, at least one problem is solved by a pressing machine for plastically deforming a tubular workpiece, in particular a fitting, wherein the pressing machine has a motor, and pressing jaws, which are driven by the motor and can apply a force to the workpiece during operation, a power transmission unit, coupled to the motor and the press jaws for transmitting a force from the motor to the press jaws, at least one sensor unit for measuring at least one press parameter, and a controller receiving the currently measured value of the press parameter from the sensor unit and with the latter Slope of a press parameter curve of the press parameter determined, the controller terminates a pressing operation before reaching a maximum possible pressing force when the slope of the press parameter curve of the press parameter meets a shutdown criterion.

In the present invention, the slope, ie the rise of a press parameter curve is monitored, which results during the pressing process. The increase is characteristic of the increase in the pressing force on the workpiece with progressive pressing process. If the workpiece has reached the maximum deformation at the end and the pressing jaws of the pressing machine are completely closed, is carried out with further pressure increase only an elastic deformation of the pressing jaws. In this case, a purely linear increase in the values of a pressing parameter can be detected, e.g. in a hydraulic pressing 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 controller to turn off the engine of the pressing machine before reaching a maximum possible pressing force and thus to end the pressing process.

The automatic termination of the pressing process before reaching the maximum possible pressing force of the machine due to the evaluation of the slope of at least one Pressparameterkurve prevents further unnecessary pressing together of the pressing jaws, resulting in a reduction of the load and wear of the pressing jaws and other parts of the machine and 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 shutdown criterion automatically takes into account the properties of the workpiece to be pressed, for example the material, the size, the design, etc., without that these properties in detail of the press machine before the respective Pressing process would have to be announced. 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 press parameter curve, the controller can reliably finish the pressing operation after the plastic deformation of the workpiece regardless of the level of the pressing force from which no plastic deformation of the workpiece occurs any more.

During 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 compared to so-called leaks, due to redundancy. random deviations.

Various sensors for measuring the same or different pressing parameters can be used 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, with a continuation of the pressing process, only an elastic deformation on the pressing jaws occurs. This is the case when the pressing jaws are completely closed and no plastic deformation of the workpiece occurs more. The shutdown criteria are preferably predetermined values of the slope for a particular press parameter curve, on the basis of which the controller can automatically decide when the current press operation is terminated.

Preferably, the power transmission unit is a hydraulic system and the sensor unit is a pressure sensor which measures the pressure in the hydraulic system as a press parameter. This makes it easy to adapt the control according to the invention to existing electro-hydraulic press machines. Via a pressure sensor, the hydraulic pressure can be measured simply and reliably as a process parameter and provided to the controller. The hydraulic pressure is in this case directly proportional to the pressing force applied to the tool, so that a reliable determination of the force curve on the pressing jaws is possible via the hydraulic pressure.

If the power transmission unit is preferably a mechanical system, the sensor unit is a force sensor which measures the force at a location in the mechanical system as a press 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 force is transmitted from an engine to the press jaws via one or more power transmission units. In this case, the force occurring at different points in the mechanical system can be measured. For this purpose, conventional load cells, strain gauges or similar sensors can be used. As a rule, the measured force is also directly proportional to the pressing force applied to the tool, so that a reliable determination of the force curve on the pressing jaws is possible via the force.

Preference may continue as a press parameter, a current flowing through the motor. This current can be measured by means of the controller, which then preferably also assumes the task of the sensor unit. From the current flowing through the motor can also be derived from the force curve on the pressing jaws.

Preferably, the control terminates the pressing process only when, in addition to the switch-off criterion, a predetermined 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 does not take into account start-up effects at the beginning of the pressing process or areas of the pressing process in which a plastic deformation of the workpiece regularly takes place. This reduces the risk of incorrect measurements and an unwanted premature termination of the pressing process. A Reliable compression of a workpiece, as a rule, requires an achieved minimum pressing force, so that under this minimum pressing force no automatic termination of the pressing process should take place.

The slope of the press parameter curve preferably indicates the time profile of the values of a press parameter and is preferably formed from the current and the time-preceding value of the press parameter. The consideration of the time profile of a press parameter is simply possible in the form of a time series, in particular if the slope of the process parameter curve is calculated from temporally successive measured values. Considering the current and previous value to determine the increase is simple and can be quickly implemented in a computational manner so that results can be real-time. Thereby, an immediate, timely control in response to the evaluation can be achieved. The significantly constant, linear course of the press parameter curve during the direct pressing together of the pressing jaws at the end of the pressing process, enables a robust and automatic detection of this pressing state.

Preferably, the controller may also have a database in which shutdown criteria for certain pressing jaws and / or workpieces are stored. The shutdown criteria here are preferably digital values of the slope of the process parameter curve in which an automatic shutdown is to take place. The database may contain values for shutdown criteria, which may depend on the properties of the press jaws used, such as material, size, type, etc.

Preferably, the pressing machine is an electrically driven hydraulic or mechanical hand pressing device for pressing tubular workpieces. With the help of a hand pressing device, pressings can be used flexibly at various locations, such as a construction site. In this case, electrically driven hand presses can apply high pressing forces, which ensure reliable pressing. In a hydraulic manual press device, for example, a hydraulic pressure of up to approximately 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 pressing machine, for plastically deforming a tubular workpiece, in particular a fitting, the method comprising the following steps in the order given:

1. a. Engaging the workpiece with pressing jaws of the pressing machine;
2. b. Starting a motor of the pressing machine to apply a force through the pressing jaws to the surface of the engaged workpiece;
3. c. Measuring a value of a press parameter;
4. d. Receiving the currently measured value of the press parameter by the controller, and thereby determining the slope of a press parameter curve of the press parameter; and
5. e. Stop the engine before reaching a maximum possible pressing force of the pressing machine by the controller, when the controller detects that the slope of the press parameter curve of the pressing parameter meets a shutdown criterion.

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

The motor is preferably stopped only when, in addition to the switch-off criterion, a previously defined minimum value for a press parameter has been exceeded and / or when the duration of the press process has exceeded a minimum time.

Preferably, the method comprises the step of reading 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 read, for example, suitable for 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.

Hereinafter, preferred embodiments of the present invention will be illustrated with reference to the accompanying drawings. Showing:

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 Pressparameterkurven for different pressing 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 press parameter curve according to the present invention.

Detailed description of preferred embodiments

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The Fig. 1 FIG. 1 shows an embodiment of a hydraulic manual press apparatus 10 with a hydraulic power transmission unit. In this hydraulic hand pressure device drives a motor 20 via a gear 22 an eccentric 24 connected thereto. Preferably, the motor 20 is a brushless motor that is powered by a controller 40 with appropriately modulated power from a 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 rotational movement of the output shaft of the transmission 22 into a one-dimensional oscillating motion to drive a piston pump 27 of the hydraulic system 26.

The piston pump 27 pumps due to their movement a hydraulic fluid from a reservoir in a working cylinder 25, whereby the hydraulic pressure in the working cylinder 25 increases. The increasing hydraulic pressure pushes a piston 28 movably guided in the cylinder in the representation of Fig. 1 to the left, towards the mounting area for replaceable dies 30 (not shown in detail). By using a large piston diameter, the piston 28 can transfer very high pressures to the dies.

The piston 28 is mechanically connected to rollers 29, which move with the movement of the piston 28. The rollers 29 move in a conventional manner between inclined ends of pressing jaws 30, which are thus closed and can plastically deform the workpiece with high force. In operation, this transfers the hydraulic pressure directly proportional to the connected pressing jaw 30, and generates a directly proportional to the hydraulic pressure pressing force F on the workpiece, which is directly proportional to the.

As a result of the increasing hydraulic pressure P during pressing and the thus increasing pressing force F on the workpiece or the fitting, the workpiece is pressed and plastically deformed. By measuring the hydraulic pressure P, the pressing force F on the tool can be determined.

The hydraulic pressure P in the hydraulic system 26 can be easily measured by means of a pressure sensor 42. The pressure sensor 42 communicates the measured pressure signal via signal lines or wirelessly to the controller 40 by means of a corresponding radio transmission. Wireless signal transmission means, such as common digital wireless connections such as e.g. 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.

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

Depending on the evaluation results, the controller 40 via control electronics (not shown) corresponding control signals to the motor 20 from. The motor 20 is controlled by means of these control signals to operate at a certain controlled speed and stop it at the end of the pressing process.

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

Force sensors for measuring the force F transmitted to the tool by the motor may be located at various locations in the mechanical hand press to measure a force proportional to the pressing force F and to signal it to the controller.

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

The Fig. 2 shows a graphical representation of measured Pressparameterkurven K1, K2 and K3 of prior art presses, which represent hydraulic pressure values P over time. The press parameter curve K1 was recorded during pressing without a fitting inserted between them, ie with "empty" press jaws. The press parameter curve K2 was recorded when pressing a fitting made of a first material with the same pressing jaws as curve K1. The material of the fitting was a comparatively soft material, such as copper.
The press parameter curve K3 was recorded during the compression of a fitting made of a second material. The material of this fitting had a greater strength than in curve K2 and is stainless steel in one embodiment.

In Fig. 2 It can be seen that in curve K1, the pressure at the beginning is relatively constant and almost zero, since when closing the pressing jaw there is no fitting in between located. The press jaws can close unhindered at a minimum hydraulic pressure, which overcomes the friction and spring forces in the system. From time T1 _K1 , the pressing jaws are completely closed and rest on each other. From this point on they press directly against one another, whereby the press 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, an overpressure valve opens, causing the hydraulic pressure to abruptly drop 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 fully closed state. This feature of the die jaws utilizes the present invention to automatically detect the end of the pressing operation and then shut off the engine 20 before the maximum pressure Pmax _{K1 is} reached. This protects the entire pressing machine and reduces the energy required.

The curve of the press parameter curve K2 shows an earlier increase in hydraulic pressure than the press parameter curve K1, since there is a fitting made of a soft material between the press jaws. From the time T1 _K2 the press parameter curve K2 again increases linearly, since at this time the actual pressing process is completed, the fitting has been completely plastically deformed and the pressing jaws are completely closed and rest on one another. The linear increase .DELTA.T _K2 occurs with a substantially same slope as that of the curve K1. When the maximum pressure Pmax _K2 is reached, the overpressure valve opens again 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, in the curve K3 a higher pressure must be applied overall than in the case of the curve K2 in order to close the pressing jaws and completely plastically deform the fitting. At time T1 _K3 , the curve of K ₃ becomes linear, which indicates the end of the actual pressing operation and signals that the pressing jaws are completely closed and abut one another. As can be seen, the slope of the linear range ΔT _K3 substantially corresponds to the slope of the linear ranges ΔT _K1 and ΔT _K2 .

This slope of the press parameter curve is therefore a characteristic measure of the end of the actual pressing operation on the workpiece.

How out Fig. 2 can be seen further, the time period .DELTA.T _{K1 is} significantly greater or longer than the time period .DELTA.T _{K2 of} the curve K2 or time period .DELTA.T _{K3 of} the curve K3. The longer the time interval .DELTA.T, the longer the pressing jaws of the hand press device lie directly on each other and press directly against each other. During the period .DELTA.T there is no plastic deformation of the workpiece or fitting. more so that the pressing force F is unnecessarily increased in this period.

The curves K1, K2 and K3 would be correspondingly given 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. Again, from a linear increase of the curve with a characteristic slope, the end of the pressing process and the juxtaposition of the pressing jaws can be seen.

In Fig. 3 shows an example of the curve of the press parameter curve K2, if now a controller 40 according to the invention is used. In this case, the controller 40 determines the slope of the press parameter curve K2 and can recognize from this whether the actual pressing operation is completed on the workpiece. Here, the controller 40 compares the slope of the press parameter curve K2 with a switch-off criterion, ie in the present case a characteristic pitch of the press jaws used in the fully closed state. If the slope meets the switch-off criterion, which the control determines or calculates, it terminates the pressing process by switching off the motor 20 or no longer supplying it with power. Thus, the controller 40 can end the pressing operation reliably, before unnecessarily a maximum possible pressing force Pmax is reached. This significantly reduces the wear in the press, 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 be made dependent on the controller 40 in addition to other conditions, such as to detect outliers of Pressparameterkurve and exclude. So in addition to Abschaltkriterium it be necessary for a shutdown of the motor 20 that a predetermined 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 the curve K2 the Fig. 3 For example, the minimum value is a minimum pressure Pmin _K2 which defines a time T0 _K2 at which the controller 40 determines, for each measured press parameter value, an increase (represented by a slope triangle 44) between the current and previous values and compares it with the shutdown criterion. The minimum pressure Pmin can be generally fixed, or can be variable, for example, depending on the press jaws or the workpiece used. Equivalent, for example, a minimum current at the motor 20 can be used as a criterion for the time To.

However, it is also possible to predefine a minimum time Tmin which the pressing process must at least take before the control unit can switch off the motor 20.

From time T1 _K2 , the slope value determined by the controller 40 coincides with a predetermined slope value stored in the database of the controller 40 as the switch-off criterion. From this point in time T1 _K2 on, the controller 40 then preferably determines the number of rise values which are the same in a time range.

At the later time T3 _K2 , a predetermined number of ascents with equal slope values were then counted. The counted rise values are essentially the same as the predefined rise value stored in the database. Permissible tolerances of the slope values can be determined empirically and stored in the controller 40.

In the example of Fig. 3 For example, a specific predetermined number of a slope value of the pressing parameter P of the curve K2 serves as a switch-off criterion. Wherein this switch-off criterion is met in the present embodiment at the time T3 _K2 . The pressing operation is then stopped by the controller 40. For this purpose, the controller 40 and the engine, however, requires a reaction time between the Time points T3 _K2 and T2 _K2 . At time T2 _K2 is the engine 20 and the hydraulic pressure Pstopp _K2 remains constant. After a further reaction period between the times T2 _K2 and T4 _K2 , the pressure relief valve or a return valve is opened by the controller 40 at time T4 _K2 , as a result of which the hydraulic pressure in the hydraulic system of the pressing device drops to a minimum or nominal value.

In Fig. 3 is shown by dashed line K2 'for comparison of the curve 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 until reaching a maximum pressure Pmax _K2 done. Only when this maximum pressure Pmax _{K2 was reached} would the overpressure valve open and the hydraulic pressure would drop to a minimum value, whereby the pressing process would not be completed until the overpressure valve was opened.

Out Fig. 3 is still a pressure difference .DELTA.P _K2 can be seen, extending between the pressure when switching off the motor Pstopp _K2 invention and the set maximum pressure Pmax _K2 (overpressure protection). This pressure difference .DELTA.P _K2 represents the saved, otherwise applied by the engine 20 pressure. At the same time, the pressure difference .DELTA.P _K2 also represents the energy saved with the present invention, since the engine does not have to do any work after it is switched off at time T2. The greater the pressure difference .DELTA.P, the more effective the saving is by the present invention. It will be larger on soft workpieces than on harder workpieces such as off Fig. 2 seen.

LIST OF REFERENCE NUMBERS

10

hydraulic hand press

20

engine

22

transmission

24

eccentric

25

working cylinder

26

hydraulic system

27

piston pump

28

piston

29

roll

30

Mounting area for replaceable tools

40

control

42

pressure sensor

44

ascertained 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 applicable pressure in K1, K2, K3

P-T1 (K2)

Pressure in K2 at time T1

P-T3 (K2)

Pressure at time T3

Pstopp (K2)

Pressure at the moment when pressing is stopped

ΔP (K2)

Pressure difference in K2

Tmin

Minimum time for shutdown

To (K2)

Time To: Start of test for 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 fulfilled

T4 (K2)

Time T4: opening of pressure relief valve or return

ΔT (K1, K2, K3)

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

Claims (13)

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 press jaws for transmitting power from the motor (20) to the press jaws; d. at least one sensor unit (42) for measuring at least one pressing parameter (P); and e. a controller (40) which receives the currently measured value of the press parameter (P) from the sensor unit (42) and determines therewith the slope (44) of a press parameter curve (K) of the press parameter (P); in which f. the controller (40) terminates a pressing operation before reaching a maximum possible pressing force (Pmax) if the pitch (44) of the press parameter curve (K) of the pressing parameter (P) fulfills a switch-off criterion. Pressing machine according to claim 1, wherein the switch-off criterion indicates that, with a continuation of the pressing operation, only an elastic deformation on the pressing jaws occurs. A pressing machine according to claim 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 a pressing parameter (P). A pressing machine according to claim 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 a pressing parameter (P). Pressing machine according to one of claims 1 to 4, wherein the pressing parameter (P) is a current flowing through the motor (20). Pressing machine according to one of claims 1 to 5, 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 operation is a minimum time (Tmin) has exceeded. Pressing machine according to one of claims 1 to 6, wherein the slope (44) of the press parameter curve (K) indicates the time course of the values of a pressing parameter (P) and is preferably formed from the current and the temporally preceding Pressparameterwert. Pressing machine according to one of claims 1 to 7, wherein the controller (40) has a database are stored in the shutdown criteria for certain pressing jaws and / or workpieces. Pressing machine according to one of claims 1 to 7, wherein the pressing machine (10) is an electrically driven hydraulic or mechanical hand pressing device for pressing tubular workpieces. Method for operating a pressing machine (10) for plastically deforming a tubular workpiece, in particular a fitting, the method comprising the following steps in the order indicated: 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 the currently measured value of the press parameter (P) by the controller (40), and thereby determining the 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 pressing parameter curve (K) of the pressing parameter (P) is on Switch-off criterion fulfilled. The method of claim 10, wherein the pressing parameter (P) is a pressure, a force, or a current through the motor (20) or any combination of these parameters. Method according to one of claims 10 or 11, wherein the motor (20) is stopped 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 has a minimum time ( Tmin) has exceeded. A method according to any one of claims 10 to 12, further comprising the following step: Reading at least one switch-off criterion from a database of the controller (40).

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