EP1452302A1 - Process for correcting a bending operation and press brake - Google Patents

Abstract

The process involves pre-registering gauging abacus in a lower apron containing compensating jacks. A correspondence is established by the abacus between forces measured at level of mounting and pressures applied to the jack for maintaining the lower apron in right position. Pressures resulting from the abacus are applied to the jack according to the measured forces, during a subsequent folding operation. An independent claim is also included for a folding press.

Description

The present invention relates to a correction method a folding operation carried out by a press brake of the type comprising a fixed deck, a movable deck, means movement of the movable platform supported by two uprights integral with the fixed bulkhead, sensors, associated respectively to the two uprights, measuring the forces exerted by said means of movement on said uprights, deformation compensation cylinders associated with one of the two aprons, and an electronic commanding device the movement of the movable platform between a top dead center and a bottom dead center.

The invention also relates to a press brake of this type.

Applicant's patent CH 653289 describes a press hydraulic comprising a fixed deck and a movable deck and which includes, inside the fixed deck, jacks for compensate for deformations occurring during the work of the hurry. Central control unit receives information means for measuring deformations and controlling the cylinders so that, during the work phase, the two tools remain parallel.

Document WO 91/03371 describes means of measurement suitable for this type of hydraulic press, consisting of two longitudinal bars assigned respectively to each of upper and lower aprons. One end of each of the bars is securely attached to the associated deck, while the other free end acts on a sensor inductive so as to compare the respective flexions of the two aprons. The control unit actuates the actuators compensation until compensation for bending difference of the upper and lower apron, so that the tools remain parallel.

The implementation of such a correction method in a such press can significantly reduce the difference bending angle between the middle and the ends of the pieces long. On the other hand, the two aprons and the tools being certainly parallel, but with an arrow, this arrow is transmitted to the part to be bent, so that its edge is not more perfectly straight, but curved. The process is wrong suitable for folding short pieces, i.e. pieces whose length is much less than the distance between the amounts.

Document CH 653289 also describes another type of hydraulic press, in which both the fixed deck that the movable deck are equipped with compensation cylinders. In such a machine, it is in principle possible, by means compensation cylinders, not only to make the two parallel aprons but again to bring back the die holder as the punch holder each in a straight line, parallel to each other. However, such a machine is more expensive to realize, since it must have two antagonistic series of compensating cylinders, one for each apron. In addition, the programming of an implementation effective antagonist of the two series of compensating cylinders is very difficult and the operation of such machines is not reliable. They have not met with success in convenient.

The object of the invention is therefore to propose a method of correction of simple and efficient folding operation (s), which can be automatically implemented by the command digital press equipped with a single row of cylinders compensation.

This goal is achieved by using, in a press brake of the defined input type, a method comprising pre-registration a calibration chart in memory of electronic control device, by means of parts very short calibration times, the abacus establishing a correspondence between the forces measured by the sensors associated with the amounts and pressures applicable to the cylinders compensation of the deck carrying them, to maintain said substantially straight deck, and in which process, during a subsequent folding operation, applied to said pressure compensation cylinders resulting from said abacus, as a function of the forces measured at the level of said sensors.

Preferably, to compensate for the deformation of the aprons by means of compensating cylinders, the process of correction according to the invention adds a correction to the penetration depth of the punch in the die, in recalculating the bottom dead center according to the characteristics of the workpiece and the values measured by the sensors associated with the amounts.

The method of calculating the correction of the bottom dead center preferably takes into account that the part to be bent is a long piece or a short piece. By "long" piece, there must hear a piece whose length is substantially equal to the distance between the two amounts of the press. Through "short" piece, you have to hear a piece whose length does not not exceed one third of the distance between the two uprights.

F est la charge locale sur la pièce courte (en Newton)

l est la distance entre les montants

l_a et l_b sont les distances respectives du centre de la pièce aux montants

E est le module d'élasticité du tablier supérieur (en N/mm²)

I est le moment d'inertie axiale du tablier (en mm⁴)

For a short part, the correction for bottom dead center ΔZ can be calculated from the formula ΔZ = Δf max = (Fl 2 at .l 2 b ) / (3.EIl) in which :

F is the local load on the short part (in Newton)

l is the distance between the uprights

l _a and l _b are the respective distances from the center of the room to the uprights

E is the modulus of elasticity of the upper deck (in N / mm ² )

I is the moment of axial inertia of the deck (in mm ⁴ )

The deformation Δf of the deck which does not include compensation cylinders increases during the elastic deformation of the folded part but varies little during the plastic deformation phase.

The bottom dead center is corrected by the value of the maximum deformation Δf _max of the deck.

For a long part, we can apply a correction of the bottom dead center to the stroke of the movable apron calculated by the formula ΔZ '= Δf max = (5.QI 4 ) / (384000.EI) where Q is the load per unit length of the part (in N / m)

If the exact length of the workpiece is known, the type of bottom dead center correction can be chosen by the operator of the machine, and the value of the length of the part entered in memory of the control electronics. If the length of the part is not perfectly known, in particular if it varies from one piece to another in a series, it can be determined during operation, by reference to a first reference folding operation, and the correction will be determined automatically by the electronics of ordered.

• La Fig. 1A est une vue schématique en perspective d'une presse hydraulique, montrant l'action de vérins hydrauliques de compensation sur le tablier inférieur;
• La Fig. 1B est une vue schématique en coupe transversale du tablier inférieur de la presse de la figure 1A;

Other features and advantages of the invention will appear to those skilled in the art from the description below of an embodiment of the invention, with reference to the figures, in which:

• Fig. 1A is a schematic perspective view of a hydraulic press, showing the action of hydraulic compensating jacks on the lower deck;
• Fig. 1B is a schematic cross-sectional view of the lower apron of the press of FIG. 1A;

• La Fig. 2a est une représentation du pliage d'une pièce longue;
• La Fig. 2b est une représentation du pliage d'une pièce courte centrée par rapport aux montants de la presse;
• La Fig. 2c est une représentation du pliage d'une pièce courte, décentrée par rapport aux montants de la presse.

Figs 2a, 2b and 2c are schematic representations illustrating the results of the stresses and deformations of the decks, namely:

• Fig. 2a is a representation of the folding of a long piece;
• Fig. 2b is a representation of the folding of a short piece centered with respect to the amounts of the press;
• Fig. 2c is a representation of the folding of a short piece, offset from the amounts of the press.

• La Fig. 3a est une représentation d'un pliage d'une pièce longue;
• La Fig. 3b est une représentation d'un pliage d'une pièce très courte;
• La Fig. 3c est une représentation d'un pliage d'une pièce courte.

Figs 3a, 3b and 3c are schematic representations illustrating the deformations of the decks in the case of a folding operation without any correction of the bending of the decks, namely:

• Fig. 3a is a representation of a folding of a long piece;
• Fig. 3b is a representation of a folding of a very short piece;
• Fig. 3c is a representation of a folding of a short piece.

• La Fig. 4a est une représentation d'un pliage d'une pièce longue;
• La Fig. 4b est une représentation d'un pliage d'une pièce très courte centrée;
• La Fig. 4c est une représentation d'un pliage d'une pièce courte centrée.
• La Fig. 5b est une représentation d'un pliage d'une pièce très courte décentrée.
• La Fig. 5c est une représentation d'un pliage d'une pièce courte décentrée.

Figs 4a, 4b, 4c, 5b and 5c are schematic representations illustrating the deformations of the decks during a folding operation, where only the bending of the decks is compensated, namely:

• Fig. 4a is a representation of a folding of a long piece;
• Fig. 4b is a representation of a folding of a very short piece centered;
• Fig. 4c is a representation of a folding of a short centered piece.
• Fig. 5b is a representation of a folding of a very short off-center piece.
• Fig. 5c is a representation of a folding of a short off-center piece.

Figs 6a, 6b, 6c, 7b and 7c are representations diagrams illustrating the deformations of the aprons during a folding operation, of the same parts as in the case of Figs 4a to 5c, where simultaneously the bending of the deck and the bottom dead center are fixed.

Fig. 1A shows a hydraulic press 1 with a upper movable deck 5, which is moved under the action of pistons and cylinders 6, 6 'associated with lateral uprights 10, 10 '. During the folding of a part, the movable deck 5 tends to bend under the action of these pistons, the middle of the movable deck 5 then being more higher than both ends. Conversely, in the absence of compensation device, the fixed lower deck 2 would have tends to bend in such a way that the middle of it fixed apron would be lower than the two ends. Under these conditions, the working surfaces of the two decks 2 and 5 and, consequently, the surfaces of the two tool holders, namely the die holder 8 and the punch holder 9, would not more parallel.

As shown in Figures 1A and 1B, the deck lower 2 comprises a central plate 3, which carries the matrix holder 8. The central plate 3 is surrounded on the side and on the other side of two reaction plates 4 and 7. The ends sides of the central plate 3 and the reaction plates 4 and 7 are secured respectively to the uprights 10 and 10 '.

The lower apron 2 of the press shown in the FIG. 1A has three reaction holes 13, 13 ', 13' ', right through plates 3, 4 and 7. Each hole reaction houses a hydraulic compensation cylinder 14.14 ', 14' ', which rests on reaction plates 4 and 7 and whose piston 11 presses from below at 12 on the plate central 3, as schematically illustrated in FIG. 1B, to provide a surge of compensation to the party upper of the central plate 3 of the lower deck, of so as to compensate for the deformation mentioned above. As illustrated in FIGS. 1A and 1B, the plates of reaction 4 and 7 undergo a downward reaction. The action hydraulic compensating cylinders 14, 14 ', 14' 'east ordered, just like that of the pistons and cylinders of work 6 and 6 'by an electronic control unit (not shown by drawing). On large presses length, the number of reaction holes fitted with cylinders compensation is higher.

The invention is equally applicable to this type of machine having more reaction holes than those with a single compensation slot, described for example in

CH 653 289.

Fig. 2c schematically illustrates the curvature of the upper deck during a folding operation of a part whose length L is relatively short compared to the distance 1 between the two uprights of a press. During this operation, the compensation cylinders act on the lower bulkhead in such a way that its upper edge remains substantially straight. In Fig. 2c, l _a and l _b respectively denote the distances from the center of the part being bent to each of the two uprights. The resultant F of reactions from the part to the upper apron, which corresponds to the load on the part, is applied substantially to the center of the part to be bent. The sensors associated with the two uprights respectively measure forces F _a and F _b , such that F = F at + F b and F at = Fl b / l and F b = Fl at / l

In the hypothetical case where the center of the piece to be bent is practically below the amount a, F _a would be worth almost 100% of F, and F _b ≃ 0.

In the case illustrated by FIG. 2b, where the part is perfectly centered on the lower apron, l _a = l _b = 1/2 and F _a = F _b = 50% of F.

In a first process step, a valid calibration is carried out for a pair of aprons, a pair of tool holders and tools. The calibration operation is carried out using calibration pieces of very short lengths, that is to say of which the length is less than 10% of the length between two jacks, placed in several successive positions between both amounts. The very short part is pressurized between the two aprons and, for a succession of values of F _a and F _b according to l _a / l _b , the cylinders of the lower apron are adjusted so that its upper edge is right. The set of values of F _a , F _b and the values of the pressures of the compensation cylinders thus measured constitutes a calibration chart, which is prerecorded in the memory of the electronic control device.

During an actual folding operation of a piece of relatively short length relative to the distance l between the two uprights, the sensors of the two uprights measure forces F _a and F _b during bending and the electronic control device actuates the compensation cylinders so that their pressures correspond to the corresponding values in the chart.

Those skilled in the art will readily understand that by operating the compensation cylinders in the above-mentioned manner, the lower apron remains substantially straight during the operation for bending short pieces, but has a some residual curvature when bending long pieces.

As can be seen in Fig. 2c, the area of the upper deck in contact with the part being folded is not at the same height as the ends of the deck, at the level of the two uprights; the height difference, i.e. the deformation Δf, is given by the expression: Δf = fl at 2 .l b 2 3.EIl

In which E is the modulus of elasticity (in Nn / mm ² ) of the upper deck and I denotes the moment of axial inertia (in mm ⁴ ) of the deck; the values of E and I are determined during the manufacture of the bulkhead and are stored in the memory of the control electronics.

If the part is centered in the press, this formula is simplified by: Δf = fl 3 48.EI

Fig. 2a illustrates the folding operation of a long piece. Under these conditions, the compensation cylinders being actuated as indicated above, the upper deck undergoes a reaction Q, during the folding operation, the distribution of which is substantially homogeneous, as illustrated in FIG. 2a. The deformation Δf of the upper deck is given by the relation Δf = 5.Ql 4 384'000.EI with the previously defined ratings.

After calculating the deformation Δf _max of the upper apron, the penetration depth of the tool in the matrix is corrected by correcting the position of the bottom dead center by an amount corresponding to the maximum deformation.

When the force sensors have detected a position off-center of the part, as illustrated in FIG. 2c, the correction applied may be different for the two amounts.

As a variant, the corrections of ΔZ can be entirely determined using digital charts, prerecorded in the memory of the electronic control device: for each angle of folding of the settings and for each ratio l _a / l _b , the abacus contains corrections of the ΔZ values for each amount, values which can vary from a few hundredths of a millimeter to around 2 mm. The values of the ΔZ corrections applied to the two amounts are pre-calculated using formulas such as the formulas above. The values of the applicable corrections are chosen by the electronic control device from the values sensed by the pressure sensors associated with each of the two uprights. This method has the advantage of being much faster to implement during a folding operation than if the electronic system had to recalculate the ΔZ corrections in real time.

Les Figs 3a, 3b et 3c illustrent des pliages sans aucune compensation de la flexion des tabliers :

• La Fig. 3a montre le pliage d'une pièce dont la longueur est environ égale à celle de la presse plieuse : l'angle de pliage au milieu de la pièce est supérieur à l'angle de pliage aux deux extrémités.
• La Fig. 3b montre le pliage d'une pièce très courte : l'angle est beaucoup plus ouvert que l'angle de consigne, à cause de la déformation quasi triangulaire des montants supérieur et inférieur.
• La Fig. 3c montre le pliage d'une pièce dont la longueur est environ le tiers de la longueur de la machine : l'angle de pliage est relativement constant sur la longueur de la pièce, mais il est nettement plus ouvert que l'angle de consigne.

Les Figs 4a, 4b, 4c ainsi que 5b et 5c illustrent des pliages dans lesquels seule la flexion des tabliers est compensée de telle façon à ce que ces tabliers restent parallèles lors du pliage d'une pièce longue :

• La Fig. 4a représente le pliage d'une pièce longue : l'angle est constant sur toute la longueur de la pièce et il est égal à la valeur de consigne;
• La Fig. 4b représente le pliage d'une pièce très courte centrée : l'angle est nettement plus ouvert que l'angle estimé à cause de la déformation quasi triangulaire du tablier supérieur.
• La Fig. 5b représente le pliage de la même pièce, mais excentrée : l'angle de pliage est également plus ouvert que la valeur de consigne, mais de plus, sa valeur est variable en fonction de la position de la pièce sur la machine, de sorte qu'une correction de cet angle peut difficilement être effectuée de manière reproductible.
• La Fig. 4c représente le pliage d'une pièce courte, dont la longueur est environ le tiers de la longueur de la machine : l'angle est relativement constant le long de la pièce, mais plus ouvert que la valeur de consigne;
• La Fig. 5c illustre le pliage de la même pièce fortement excentrée : l'angle de pliage n'est pas constant, il est plus ouvert que la valeur de consigne et des corrections sont très difficiles à estimer.

Les Figs 6a à 7c illustrent des pliages dans lesquels une correction des points morts bas des montants du tablier supérieur est superposée à une compensation de la flexion du tablier inférieur :

• La Fig. 6a montre le pliage d'une pièce longue : l'angle est constant sur toute la longueur de la pièce et égal à la valeur de consigne.
• La Fig. 6b montre le pliage d'une pièce très courte centrée : les deux points morts bas des deux extrémités du tablier supérieur sont corrigés de la déformation quasi triangulaire de celui-ci et la valeur de l'angle de pliage est correcte.
• La Fig. 7b montre le pliage de la même pièce excentrée : les deux points morts bas des deux extrémités du tablier supérieur ont subi deux corrections différentes adaptées pour corriger la déformation triangulaire asymétrique de ce tablier et l'angle de pliage de la pièce très courte est correct.
• La Fig. 6c montre le pliage d'une pièce dont la longueur est environ le tiers de la longueur de la machine, centrée : les deux points morts bas des deux extrémités du tablier supérieur ont reçu la même correction et l'angle de pliage présente une valeur sensiblement constante sur la longueur de la pièce et égale à la valeur de consigne.
• La Fig. 7c illustre le pliage de la même pièce excentrée dans la machine : les deux points morts bas des extrémités du tablier supérieur ont reçu des corrections différentes et adaptées, de sorte que l'angle de pliage est approximativement constant sur la longueur de la pièce et égal à la valeur de consigne.

Figs 3a to 7c illustrate the advantages of the invention over the state of the art:

Figs 3a, 3b and 3c illustrate bending without any compensation for the bending of the decks:

• Fig. 3a shows the folding of a part whose length is approximately equal to that of the press brake: the folding angle in the middle of the part is greater than the folding angle at the two ends.
• Fig. 3b shows the folding of a very short part: the angle is much more open than the set angle, due to the almost triangular deformation of the upper and lower uprights.
• Fig. 3c shows the folding of a part whose length is approximately one third of the length of the machine: the folding angle is relatively constant over the length of the part, but it is clearly more open than the set angle.

Figs 4a, 4b, 4c as well as 5b and 5c illustrate folds in which only the bending of the aprons is compensated so that these aprons remain parallel when folding a long piece:

• Fig. 4a shows the folding of a long piece: the angle is constant over the entire length of the piece and it is equal to the set value;
• Fig. 4b represents the folding of a very short piece centered: the angle is much more open than the estimated angle because of the almost triangular deformation of the upper deck.
• Fig. 5b shows the folding of the same piece, but offset: the folding angle is also more open than the set value, but moreover, its value is variable depending on the position of the piece on the machine, so that It is difficult to correct this angle in a reproducible manner.
• Fig. 4c represents the folding of a short part, the length of which is approximately one third of the length of the machine: the angle is relatively constant along the part, but more open than the set value;
• Fig. 5c illustrates the folding of the same highly eccentric part: the folding angle is not constant, it is more open than the set value and corrections are very difficult to estimate.

Figs 6a to 7c illustrate folds in which a correction of the bottom dead centers of the uprights of the upper deck is superimposed on a compensation for the bending of the lower deck:

• Fig. 6a shows the folding of a long piece: the angle is constant over the entire length of the piece and equal to the set value.
• Fig. 6b shows the folding of a very short piece centered: the two bottom dead centers of the two ends of the upper deck are corrected for the almost triangular deformation thereof and the value of the folding angle is correct.
• Fig. 7b shows the folding of the same eccentric part: the two bottom dead centers of the two ends of the upper deck have undergone two different corrections adapted to correct the asymmetrical triangular deformation of this deck and the folding angle of the very short piece is correct.
• Fig. 6c shows the bending of a part whose length is approximately one third of the length of the machine, centered: the two bottom dead centers of the two ends of the upper deck have received the same correction and the bending angle has a value substantially constant over the length of the part and equal to the set value.
• Fig. 7c illustrates the folding of the same eccentric part in the machine: the two bottom dead centers of the ends of the upper deck have received different and adapted corrections, so that the folding angle is approximately constant over the length of the part and equal at the setpoint.

Several other phenomena may require corrections to the bottom dead center of the machine, corrections that are added to the correction due to the deformations of the deck described above.

Thus, when bending a sheet, the force undergone by the amounts under the thrust of the cylinders causes a bending of these amounts, which can result in a deformation of the frame of the order of 1 to 2 mm, which changes the depth of penetration of the punch into the die. Several methods of correcting the deformation of amounts are known in the state of the art. We can, for example, use that described in patent CH 680619 of the depositor: it is determined, using the pressure sensors associated with the working cylinders, the force undergone by each of the amounts and we compare the values obtained with an abacus establishing the relationship between force experienced by each of the amounts and bending of the amount, this abacus being obtained at during an initial press calibration operation.

Another parameter likely to generate an error of the bending angle is the variability of the thickness of the pieces processed. Indeed, the steel sheets supplied by the manufacturers may have thickness variations up to ± 10% of the nominal value. An operation of precise folding must take into account the difference between the actual thickness of the part and the nominal thickness. Several methods have been proposed for this in the state of technique. We can, for example, use the one described in patent No. EP 1120176 of the applicant, according to which this difference is calculated by comparing the position of the movement of the movable platform, at which a predetermined variation of the pressure recorded by the sensors associated with the working cylinders, with the position theoretical of the deck where this variation should occur if the thickness of the piece was strictly equal to its nominal thickness. The position of the bottom dead center is corrected during the folding operation by the electronic device when this measurement has been made.

Another problem that arises during a process of folding is the compensation for the spring effect, i.e. elastic return of the bent part at a bending angle slightly lower, when the punch pressure is released. Because of this effect, the maximum value of the angle instantaneous folding under load must be greater than the setpoint of the desired bending angle, after loosening of the folded part. Several methods of correction of the elastic return effect have been proposed in the state of the art. We can, for example, use the processes proposed by US patents 4,408,471 or US 4,511,976 which determine the real elastic modulus of the piece based on the data recorded during the elastic deformation of the folding process and which determine a bending angle correction by extrapolating the process based on modeling.

We can also calculate the compensation for the effect of comes out without doing a possibly inadequate modeling using the process proposed by the applicant in his Patent application no PCT / CH 02/00154, which determines the correction by comparing the data recorded during the plastic phase of the deformation of the part to the data collected during a first folding test which serves as reference. In this method, the compensation for the effect of spring is deducted from the difference between the measured data during the folding operation and during the reference, without extrapolation and without modeling.

Finally, we can make a correction to take into account variations in the length of the pieces to be bent and, to do this, we can first perform a folding operation calibration with a part whose exact length is known, while measuring the actual thickness, as indicated above. During the calibration folding operation, for a given angle, for example 150 °, the pressing force is measured necessary for this folding. The exact length of this piece being known, the control unit can calculate the force by length unit, for example in T / m. For folding subsequent to the series, the pressing force is measured at this same angle, for example 150 °, and this force is compared to that recorded during the first calibration operation. The actual length of the successive pieces can then be determined by applying a simple proportionality rule, with an approximation of ± 10 mm, which is sufficient in convenient.

   dans laquelle :

e désigne l'épaisseur mesurée

désigne la résistance à la rupture par traction

V est l'angle d'ouverture

F est la force en tonnes

L est la longueur de la pièce

According to another variant, the actual length of the part can be determined by assuming that the tensile strength is constant and corresponds to the nominal value. The length of the part can be deduced from the relation

in which :

e denotes the thickness measured

denotes the tensile strength

V is the opening angle

F is the force in tonnes

L is the length of the part

All of the above corrections allow recalculation the bottom dead center of the top apron travel during that a folding operation is in progress.

Claims (9)

Method for correcting a folding operation carried out by a press brake of the type comprising a fixed deck, a movable deck, means for moving the movable deck based on two uprights integral with the fixed deck, sensors, respectively associated with two uprights, measuring the forces exerted by said displacement means on said uprights, deformation compensation cylinders associated with one of the two decks, and an electronic control device controlling the movement of the movable deck between a top dead center and a bottom dead center, characterized in that a calibration chart is pre-recorded in the memory of the electronic control device, by means of very short calibration pieces, said chart establishing a correspondence between the forces measured by the sensors associated with the uprights and the pressures applicable to said compensation cylinders of said apron in order to maintain the apron carrying them substantially straight, and in that , during a subsequent folding operation, pressures resulting from said abacus are applied to said compensation cylinders, as a function of the forces measured at the level of said sensors. Method according to claim 1, characterized in that a correction of the penetration depth of the punch in the die is carried out by recalculating the bottom dead center of each upright according to the characteristics of the workpiece and the values measured by the sensors associated with the amounts. Method according to claim 2, applied to a short bending piece, characterized in that a low dead center correction calculated using the formula is applied ΔZ = (Fl 2 at .l 2 b ) / (3.EIl) in which : F is the local load on the short part (in Newton) l is the distance between the uprights l _a and l _b are the respective distances from the center of the room to the uprights E is the modulus of elasticity of the upper deck (in N / mm ² ) I is the moment of axial inertia of the deck (in mm ⁴ ) Method according to claim 2, applied to a long bending piece, characterized in that a correction ΔZ 'of the bottom dead center is applied to the travel of the movable apron calculated by the formula ΔZ '= (5.QI 4 ) / (384000. E. I) in which : Q denotes the load per unit length of the part (in N / m) E is the modulus of elasticity of the upper deck (in N / mm ² ) I is the moment of axial inertia of the deck (in mm ⁴ ) Method according to one of claims 2 to 4, characterized in that the actual length of the part is determined by comparing the force measured by said force sensors with the corresponding data of a reference bending operation. Method according to one of claims 2 to 5, characterized in that the corrections of the bottom dead centers of the movable apron are predetermined as a function of the length of the part and of the forces measured by said force sensors, and prerecorded in memory of said device control electronics. Method according to one of Claims 2 to 6, characterized in that an additional correction for the bottom dead center is calculated after determining the actual thickness of the workpiece. Method according to any one of Claims 2 to 7, characterized in that an additional correction for the bottom dead center is calculated to compensate for the spring effect. Press brake comprising a fixed deck, a movable deck, means for moving the movable deck based on two uprights integral with the fixed deck, sensors, associated respectively with the two uprights, measuring the forces exerted by said displacement means on said uprights, deformation compensation cylinders associated with the lower deck, and an electronic control device controlling the movement of the movable deck between a top dead center and a bottom dead center, characterized in that said electronic control device is programmed to perform a method according to one of claims 1 to 8.

Images