Pressing machine

Abstract

The present invention relates to a press machine 10 for the plastic deformation of a tubular workpiece, in particular a fitting, wherein the press machine 10 comprises a motor 20; press jaws driven by the motor 20 and capable of applying a force to the workpiece during operation; a force transmission unit coupled to the motor 20 and the press jaws to transmit a force from the motor 20 to the press jaws; at least one sensor unit 42 for measuring at least one press parameter P; and a control unit 40 which receives the currently measured value of the press parameter P from the sensor unit 42 and uses this value to determine the slope 44 of a press parameter curve K of the press parameter P; wherein the control unit 40 terminates a press operation before reaching a maximum possible press force Pmax if the slope 44 of the press parameter curve K of the press parameter P meets a shutdown criterion.The present invention further relates to a corresponding method for operating a press machine 10.

Description

Technical area

[0001] The present invention relates to a pressing machine, in particular a hand-held pressing machine, for pressing tubular workpieces, as well as a method for operating a pressing machine. State of the art

[0002] Methods for pressing tubular workpieces, particularly pipes in plumbing technology, are known in the prior art. In one known method, two pipes are permanently connected using a press fitting. For this purpose, the pipes are inserted into openings of a press fitting, which has polymer seals to seal them. After the pipes to be connected are inserted, the press fitting is pressed using a suitable pressing machine, i.e., plastically deformed to such an extent that the inserted pipes can no longer be pulled out, and the seal securely seals.

[0003] Crimping is performed using a hand-held, motor-driven crimping tool that can feature interchangeable tools, such as crimping jaws of different sizes and geometries. Crimping tools are also known for other tasks, such as pressing, crimping, or cutting workpieces, for example, in the electrical industry.

[0004] With a handheld press, the pressing jaws are arranged around the press fitting for pressing. When the pressing jaws are closed, a force is exerted on the surface of the press fitting, compressing the fitting and causing it to plastically deform, thus securely joining the workpieces. The internal pipes may also undergo plastic deformation.

[0005] In state-of-the-art press machines, the pressing process is usually terminated by opening a pressure relief valve when a certain maximum pressure is reached. This specified maximum pressure ensures that a sufficiently high pressing force is applied to the workpiece to ensure sufficient compression.

[0006] For example, EP 2 501 523 B1 discloses a handheld pressing device for pressing a press fitting in plumbing technology and for pressing cable lugs. To generate the required high pressing forces, the pressing tool is connected to an electro-hydraulic conversion device. A brushless electric motor serves as the drive motor. As soon as the required pressing force is reached, a pressure relief valve opens and the motor speed increases rapidly. This is detected by a control system of the pressing device, and the electric motor is subsequently shut down.

[0007] However, stopping the pressing process with a fixed pressure relief valve has the disadvantage that when the maximum pressure is reached, the pressing jaws of the pressing tool are already in contact with each other, preventing any further deformation of the workpiece. The fitting has already been compressed to its maximum during the pressing process and can no longer undergo any further plastic deformation. Pressing the pressing jaws directly against each other naturally leads to significant wear on the tool, the power transmission components, and the drive motor. Furthermore, unnecessary electrical energy is consumed.

[0008] The publication WO 2016 / 005838 A1 describes a method and a hydrodynamic compression tool that enables adaptation of the maximum compression force, the electrical power consumption and the duration of the hydrodynamic pressure to the size and pressure resistance of an object to be compressed.

[0009] The document DE 10 2008 024018 A1 describes a method for operating a motor-operated hand-held pressing device in which, upon actuation of a switch, one or more pressing jaws are moved from an open start position into a closed pressing position until a predetermined pressing force is reached or a predetermined time has elapsed, after which a pressing jaw release occurs automatically, for example by the return of a press piston acting on the pressing jaws, but this release can be interrupted in an intermediate position before the start position is reached in order to begin a next pressing process from such an intermediate position.

[0010] It is therefore the object of the present invention to provide a press machine that overcomes the above-mentioned problems and achieves a secure and durable press connection between tubular workpieces without causing unnecessary wear 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

[0011] At least one of the above-mentioned 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 9.

[0012] 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, as well as pressing jaws that are driven by the motor and can apply a force to the workpiece during operation, a force transmission unit that 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 that receives the currently measured value of the pressing parameter from the sensor unit and uses this to determine the gradient of a pressing parameter curve of the pressing parameter, wherein the controller ends a pressing process before a maximum possible pressing force is reached if the gradient of the pressing parameter curve of the pressing parameter fulfills a switch-off criterion.

[0013] In the present invention, the gradient, i.e. the increase in a pressing parameter curve that 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 finally achieved maximum deformation and the pressing jaws of the pressing machine are fully closed, a further increase in pressure only results in elastic deformation on the pressing jaws. A purely linear increase in the values ​​of a pressing parameter can be detected, e.g. in the case of 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 control system to switch off the motor of the pressing machine before the maximum possible pressing force is reached, thus ending the pressing process.

[0014] The automatic termination of the pressing process before the maximum possible pressing force of the machine is reached, based on the evaluation of the gradient of at least one pressing parameter curve, prevents further unnecessary pressing of the pressing jaws, which reduces stress and wear on the pressing jaws and other machine parts and saves energy and time. No additional user interaction is required to terminate the pressing process.

[0015] Comparing the slope of the pressing parameter curve of the measured values ​​of at least one pressing parameter with a shutdown criterion automatically takes into account the properties of the workpiece to be pressed, such as the material, size, design, etc., without these properties having to be communicated to the pressing machine in detail before the respective pressing process. The shutdown criterion is independent of the individual properties of the workpiece to be pressed. By monitoring the slope of the pressing parameter curve, the control system can reliably terminate the pressing process after the workpiece has plastically deformed, regardless of the pressing force level at which plastic deformation of the workpiece no longer occurs.

[0016] During the evaluation, one or more compression parameters can be considered in any combination. Considering multiple compression parameters simultaneously increases the robustness of the analysis against so-called outliers, i.e., random deviations, due to redundancy.

[0017] Different sensors can be used to measure the same or different pressing parameters for evaluation with this control system.

[0018] The respective switch-off criterion can be adapted to the respective process parameter or to a combination of the process parameters.

[0019] Preferably, the shutdown criterion indicates that, if the pressing process continues, only elastic deformation will occur in the pressing jaws. This is the case when the pressing jaws are fully closed and no further plastic deformation of the workpiece occurs. The shutdown criteria are preferably predetermined values ​​of the gradient for a specific pressing parameter curve, based on which the control system can automatically decide when to terminate the current pressing process.

[0020] Preferably, the force transmission unit is a hydraulic system, and the sensor unit is a pressure sensor that measures the pressure in the hydraulic system as a pressing parameter. This allows for easy adaptation of the control system according to the invention to existing electro-hydraulic pressing machines. Using a pressure sensor, the hydraulic pressure can be easily and reliably measured as a process parameter and provided to the control system. The hydraulic pressure is directly proportional to the pressing force applied to the tool, so that the hydraulic pressure can be used to reliably determine the force curve on the pressing jaws.

[0021] 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 force transmission. In mechanical pressing machines, a force is transmitted from a motor to the pressing jaws via one or more force transmission units. In this case, the force occurring 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 generally also directly proportional to the pressing force applied to the tool, so that the force can be used to reliably determine the force curve on the pressing jaws.

[0022] A preferred pressing parameter is the current flowing through the motor. This current can be measured by the control system, which then preferably also takes over the function of the sensor unit. The force curve at the pressing jaws can also be derived from the current flowing through the motor.

[0023] According to the invention, the control system only terminates the pressing process when, in addition to the shutdown criterion, a predetermined minimum value for a pressing parameter (P) has been exceeded. This disregards start-up effects at the beginning of the pressing process or areas of the pressing process in which plastic deformation of the workpiece still regularly occurs. This reduces the risk of incorrect measurements and an unintentional premature termination of the pressing process. Reliable pressing of a workpiece generally requires a minimum pressing force to be reached, so the pressing process should not automatically terminate below this minimum pressing force.

[0024] The gradient of the pressing parameter curve preferably indicates the temporal progression of the values ​​of a pressing parameter and is preferably formed from the current and the previous value of the pressing parameter. The temporal progression of a pressing parameter is easily possible in the form of a time series, in particular if the gradient of the process parameter curve is calculated from successive measured values. The consideration of the current and previous value to determine the increase is simple and can be implemented quickly in terms of calculations, so that results can be available in real time. This enables immediate, timely control in response to the evaluation. The significantly consistent, linear progression of the pressing parameter curve when the pressing jaws are pressed directly together at the end of the pressing process enables robust and automatic detection of this pressing state.

[0025] Preferably, the controller can also have a database in which shutdown criteria for specific press jaws and / or workpieces are stored. The shutdown criteria are preferably digital values ​​of the gradient of the process parameter curve at which automatic shutdown is to occur. The database can contain values ​​for shutdown criteria that can depend on the properties of the press jaws used, such as material, size, type, etc.

[0026] The press is preferably an electrically driven hydraulic or mechanical hand-held press for pressing tubular workpieces. A hand-held press allows for flexible use at various locations, such as a construction site. Electrically driven hand-held presses can generate high pressing forces, ensuring reliable pressing. In a hydraulic hand-held press, 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 press jaws.

[0027] 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 specified order: a. Engaging the workpiece with the pressing jaws of the pressing machine; b. Starting a motor of the pressing machine to apply a force to the surface of the engaged workpiece through the pressing jaws; c. Measuring a value of a pressing parameter; d. Receiving the currently measured value of the pressing parameter by the controller and thereby determining the slope of a pressing parameter curve of the pressing parameter; and e. Stopping the motor by the controller before the maximum possible pressing force of the pressing machine is reached if the controller detects that the slope of the pressing parameter curve of the pressing parameter fulfills a shutdown criterion.

[0028] 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 of the pressing pressure of the pressing machine.

[0029] According to the invention, the motor is only stopped when, in addition to the shutdown criterion, a predetermined minimum value for a pressing parameter has been exceeded. The method preferably includes the step of reading at least one shutdown criterion from a database of the control system. The shutdown criterion can be stored in a database in the control system of the pressing machine and, for example, can be read out according to the pressing jaws used and used in the control system. Short description of the characters

[0030] Preferred embodiments of the present invention are illustrated below with reference to the accompanying figures. Herein: Fig. 1 is a schematic representation of an embodiment of the pressing machine as a hydraulic hand-held pressing device according to the present invention; Fig. 2 is a diagram of pressing parameter curves for different pressing jaws and materials to be pressed in a pressing machine according to the prior art; and Fig. 3 is a schematic representation of the evaluation of a pressing parameter curve according to the present invention. Detailed description of preferred embodiments

[0031] In the following, preferred embodiments of the present invention will be described in detail with reference to the accompanying figures.

[0032] The Fig. 1shows an embodiment of a hydraulic hand-held pressing device 10 with a hydraulic power transmission unit. In this hydraulic hand-held pressing device, a motor 20 drives an eccentric 24 connected to it via a gear 22. Preferably, the motor 20 is a brushless motor supplied by a controller 40 with appropriately modulated power from a battery or a wired power supply (not shown). The gear 22 reduces the speed of the motor 20 and increases the torque. The eccentric 24 connected to the gear converts the rotary motion of the output shaft of the gear 22 into a one-dimensional oscillating motion to drive a piston pump 27 of the hydraulic system 26.

[0033] Due to its movement, the piston pump 27 pumps a hydraulic fluid from a reservoir into a working cylinder 25, whereby the hydraulic pressure in the working cylinder 25 increases. The increasing hydraulic pressure pushes a piston 28, which is movably guided in the cylinder, in the illustration of the Fig. 1 to the left, toward the mounting area for interchangeable 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.

[0034] 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 manner between the inclined ends of the pressing jaws 30, which are thus closed and can plastically deform the workpiece with high force. During operation, the hydraulic pressure is thus transferred directly proportionally to the connected pressing jaw 30, generating a pressing force F on the workpiece that is directly proportional to the hydraulic pressure.

[0035] Due to the increasing hydraulic pressure P during pressing and the resulting increasing pressing force F on the workpiece or fitting, the workpiece is pressed and plastically deformed. The pressing force F on the tool can be determined by measuring the hydraulic pressure P.

[0036] The hydraulic pressure P in the hydraulic system 26 can be easily measured using a pressure sensor 42. The pressure sensor 42 transmits the measured pressure signal to the controller 40 via signal lines or wirelessly using a corresponding radio transmission. Wireless signal transmission means, such as common digital wireless connections such as 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.

[0037] For this purpose, the controller 40 comprises at least one digital processing unit, such as a microcontroller, DSP, FPGA, ASIC, or the like. Additionally, the control unit may have a database (not shown) stored on data storage devices, from which predetermined values ​​required for the evaluation can be retrieved.

[0038] 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 by these control signals to operate at a specific controlled speed and to stop it at the end of the pressing process.

[0039] 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 motion, which is transmitted via a gear to at least one mechanical power transmission unit, for example a lever or a screw drive. The mechanical power transmission unit converts the rotary motion into a linear motion, which, corresponding to the hydraulic hand-pressing device 10 described above, Fig. 1, rollers are moved with high force, which move the press jaws. The increasing force of the press jaws causes a workpiece, such as a fitting, that is located between the press jaws to undergo plastic deformation.

[0040] Force sensors for measuring the force F transmitted from the motor to the tool can be arranged at various points in the mechanical hand press device in order to measure a force proportional to the pressing force F and signal it to the control system.

[0041] 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.

[0042] The Fig. 2shows a graphical representation of measured pressing parameter curves K1, K2, and K3 from state-of-the-art pressing machines, representing hydraulic pressure values ​​P over time. Press parameter curve K1 was recorded during pressing without an inserted fitting, i.e., with "empty" pressing jaws. Press parameter curve K2 was recorded during pressing of a fitting made of a first material using the same pressing jaws as in curve K1. The fitting material was a comparatively soft material, such as copper.

[0043] The pressing parameters curve K3 was recorded during the pressing of a fitting made of a second material. The material of this fitting exhibited greater strength and hardness than curve K2 and, in one example, is stainless steel.

[0044] In Fig. 2it can be seen that in curve K1 the pressure is relatively constant at the beginning and almost zero, since there is no fitting in between when the press jaw closes. The press jaws can close unhindered at a minimum hydraulic pressure that overcomes the friction and spring forces in the system. From time T1 K1 the press jaws are completely closed and rest against each other. From this point in time they press directly against each other, 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 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 when fully closed.The present invention utilizes this property of the press jaws to automatically detect the end of the pressing process and then shut off the motor 20 before the maximum pressure Pmax K1 is reached. This protects the entire press machine and reduces the energy required.

[0045] The curve of the pressing parameter curve K2 shows an earlier hydraulic pressure increase than the pressing parameter curve K1, since a fitting made of a soft material is located between the pressing jaws. From time T1 K2, the pressing parameter curve K2 again increases linearly, since at this point the actual pressing process is complete, the fitting has been completely plastically deformed, and the pressing jaws are completely closed and resting against each other. The linear increase ΔT K2 occurs with essentially the same gradient as that of curve K1. When the maximum pressure Pmax K2 is reached, the pressure relief valve opens again at time T2 K2, and the curve K2 drops very quickly to a minimum pressure.

[0046] The pressing parameter curve K3 rises more sharply than the pressing parameter curve K2 due to the harder material of the fitting to be pressed. Furthermore, a higher overall pressure must be applied to curve K3 than to curve K2 in order to close the pressing jaws and completely plastically deform the fitting. At time T1 K3 the curve of K3 becomes linear, which indicates the end of the actual pressing process and signals that the pressing jaws are completely closed and in contact with one another. As can be seen, the gradient of the linear range ΔT K3 essentially corresponds to the gradient of the linear ranges ΔT K1 and ΔT K2 . This gradient of the pressing parameter curve is therefore a characteristic measure of the end of the actual pressing process on the workpiece.

[0047] As from Fig. 2As can be seen further, the time period ΔT K1 is significantly greater or longer than the time period ΔT K2 of curve K2 or the time period ΔT K3 of curve K3. The longer the time period ΔT, the longer the pressing jaws of the hand-held pressing device are in direct contact with one another and are pressing directly against one another. During the time period ΔT, no further plastic deformation of the workpiece or fitting occurs, so that the pressing force F is unnecessarily increased during this period.

[0048] Curves K1, K2, and K3 would result in a similar manner if, instead of the hydraulic pressure over time, a mechanical force acting in the system or the current through motor 20 were plotted over time. Here, too, the end of the pressing process and the contact of the pressing jaws can be identified from a linear rise in the curve with a characteristic gradient.

[0049] In Fig. 3shows, by way of example, the curve progression of the pressing parameter curve K2 when a controller 40 according to the invention is used. The controller 40 determines the gradient of the pressing parameter curve K2 and can use this to determine whether the actual pressing process on the workpiece has ended. The controller 40 compares the gradient of the pressing parameter curve K2 with a switch-off criterion, in this case a characteristic gradient of the pressing jaws used in the fully closed state. If the gradient meets the switch-off criterion, which the controller determines or calculates, it ends the pressing process by switching off the motor 20 or by no longer supplying it with power. The controller 40 can therefore reliably end the pressing process before a maximum possible pressing force Pmax is unnecessarily reached. This significantly reduces wear in the pressing machine, as well as the energy required and the time required.The savings are greater when pressing softer fittings than when pressing harder fittings.

[0050] The controller 40 will also make the shutdown of motor 20 dependent on further conditions, for example, to detect and exclude outliers in the pressing parameter curve. In addition to the shutdown criterion, for the motor 20 to be shut down, it will be necessary that a previously defined minimum value for a pressing parameter P has been exceeded, and it may be necessary that the duration of the pressing process has exceeded a minimum time Tmin.

[0051] In the example of curve K2 of the Fig. 3The minimum value is a minimum pressure Pmin K2 , which defines a 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 be generally fixed or can be variable, e.g., depending on the pressing jaws used or the workpiece. Equivalently, for example, a minimum current at the motor 20 can also be used as a criterion for the time To.

[0052] However, a minimum time Tmin can also be specified, which is the minimum time the pressing process must last before the control system can switch off the motor 20.

[0053] From time T1 K2 onward, the gradient value determined by the controller 40 corresponds to a predetermined gradient value stored in the database of the controller 40 as a shutdown criterion. From this time T1 K2 onward, the controller 40 then preferably determines the number of gradient values ​​that are equal within a time range.

[0054] At the later time point T3 K2, a predetermined number of climbs with identical climb values ​​were counted. The counted climb values ​​essentially correspond to the predetermined climb value stored in the database. Permissible tolerances for the climb values ​​can be determined empirically and stored in the controller 40.

[0055] In the example of Fig. 3For example, a certain predetermined number of a gradient value of the pressing parameter P of the curve K2 serves as the 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, the motor 20 is at a standstill and the hydraulic pressure Pstopp K2 remains constant. After a further reaction time period between the times T2 K2 and T4 K2, the controller 40 opens the pressure relief valve or a return valve 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.

[0056] In Fig. 3For comparison, the curve progression of the pressing parameter curve K2 according to the prior art without the control system according to the invention is shown by means of a dashed line K2'. Without the control system according to the invention, as shown in Fig. 2 As shown, the pressing process would continue 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 would only end when the pressure relief valve opens.

[0057] Out of Fig. 3Furthermore, a pressure difference ΔP K2 is visible, 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 be generated by the motor 20. At the same time, the pressure difference ΔP K2 also represents the energy saved by the present invention, since the motor no longer has to perform any work after it is switched off at time T2. The greater the pressure difference ΔP, the more effective the saving by the present invention. It will be greater for soft workpieces than for harder workpieces, as can be seen from Fig. 2 visible. List of reference symbols

[0058] 10 Hydraulic hand press 20 Motor 22 Gearbox 24 Eccentric 25 Working cylinder 26 Hydraulic system 27 Piston pump 28 Piston 29 Rollers 30 Mounting area for interchangeable tool 40 Control system 42 Pressure sensor 44 Determined rise (shown as a rise 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 the pressing process is stopped ΔP (K2) Pressure difference in K2 Tmin Minimum time for shutdown To (K2) Time To: Start of the test for switch-off criterion T1 (K1, K2, K3) Time T1: Start of the constant linear gradient 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 constantlinear gradient).

Claims

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.

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).

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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