WO2000069580A1

WO2000069580A1 - Method and apparatus for bending workpieces

Info

Publication number: WO2000069580A1
Application number: PCT/NL2000/000323
Authority: WO
Inventors: Cornelis Hendricus Liet
Original assignee: Cornelis Hendricus Liet
Priority date: 1999-05-17
Filing date: 2000-05-16
Publication date: 2000-11-23
Also published as: EP1181116A1; AU4785700A; DE60009897T2; DE60009897D1; ATE264147T1; EP1181116B1

Abstract

The invention relates to a method and apparatus for bending workpieces, wherein the workpiece is bent at a desired angle with the aid of bending means, which can be moved under the control of a processing unit for the purpose of carrying out a bending operation. The desired bending angle and a desired tolerance are fed to the processing unit, after which the processing unit moves the bending means in successive bending steps i (i=1.....N), wherein each bending step i comprises the step of bending the workpiece at a desired bending angle minus an angular value, which increases with each step i. Then the bending means are moved back and the return movement is stopped before the force with which the workpiece is clamped between the bending means falls below a predetermined value, after which a next bending step is carried out until the bending angle that is measured upon the return movement remains smaller than the desired bending angle plus the desired tolerance.

Description

Method and apparatus for bending workpieces

The invention relates to a method for bending work- pieces, wherein the workpiece is bent at a desired angle with the aid of bending means, which can be moved under the control of a processing unit for the purpose of carrying out a bending operation, as well as to apparatus wherein said method can be used. O-A-9641690 discloses a method and apparatus for bending workpieces, wherein the spring-back of. the workpiece after the bending operation is measured. When the measured amount of spring-back exceeds a desired tolerance value, an additional bending operation is carried out.

The object of the invention is to provide an improved method and apparatus of the above kind.

In order to accomplish that objective the method ac- cording to the invention is characterized in that the desired bending angle and a desired tolerance are fed to the processing unit, with the processing unit moving the bending means in successive bending steps i (i=l N) , wherein each bending step i comprises the step of bending the workpiece at a desired bending angle minus an angular value, which increases with each step i, wherein subsequently the bending means are moved back and the return movement is stopped before the force with which the workpiece is clamped between the bending means falls below a predetermined value, after which a next bending step is car- ried out until the bending angle that is measured upon the return movement remains smaller than the desired bending angle plus the desired tolerance.

In this manner the bending means, such as the combination of a punch and a die (Fig. 1) , for example, or the combi- nation of a pair of bending fingers and a movable jaw (Fig. 11) , and the workpiece are prevented from moving relative to each other between the bending steps . Such movement may cause damage to the workpiece when a next bending step is carried out and even render it useless in some cases. In addition, the bending process can be carried out at a high speed.

In one preferred embodiment the bending angle is continuously measured, also during the return movement, therefore, and fed to the processing unit, and the return movement is stopped when a bending angle is measured which corresponds to the desired angle plus the desired tolerance. It is not the actual spring-back that is measured during the return movement, but it is only ascertained that the bending angle does not fall beyond the desired tolerance, and an additional bending opera- tion will be carried out as soon as this is found to be the case.

In another preferred embodiment the return movement is stopped approximately the moment when the force with which the workpiece is clamped between the bending means falls below a predetermined value or threatens to do so. This enables a further reduction of the number of bending steps. The aforesaid moment is preferably determined by means of a pressure sensor. A pressure sensor makes it possible to determine precisely whether the clamping force which is being exerted on the work- piece by the bending means is still sufficient to prevent movement of the workpiece. Besides, a pressure sensor can be easily calibrated.

The invention also provides apparatus by means of which this method can be carried out . The invention will be explained in more detail hereafter with reference to the drawings, which very schematically represent exemplary embodiments (of parts) of the apparatus for bending workpieces according to the invention. Fig. 1 is a perspective view of a part of an embodiment of the apparatus for bending workpieces according to the invention.

Fig. 2 is a perspective view of the apparatus of Fig. 1, seen from the other side of the punch.

Fig. 3 is a cross-sectional view of the measuring device used with the apparatus according to Fig. 1, at the location of a first sensor on the left-hand side of the bending line. Fig. 4 is a cross-sectional view corresponding to Fig.

3, at the location of a second sensor on the same side of the bending line as the sensor in the cross-sectional view of Fig. 3.

Fig. 5 is a partially represented cross-sectional view of the measuring device at the location of a first sensor on the right-hand side of the bending line of the device of Fig. 1.

Fig. 6 is a cross-sectional view corresponding to Fig. 3 of an alternative embodiment of the measuring device. Fig. 7 is a top plan view, partially in sectional view, of the measuring device of Fig. 4, along the line VII- VII.

Fig. 8 shows a unit for measuring the force with which a workpiece is clamped between the bending means of apparatus as shown in Fig. 1.

Fig. 9 is a perspective view of a part of another embodiment of the apparatus for bending workpieces according to the invention.

Fig. 10 schematically shows the manner in which the apparatus disclosed herein is controlled, using the method according to the invention.

Fig. 11 very schematically shows a part of a second embodiment of the apparatus according to the invention. Figs. 1 and 2 are perspective views of a part of an apparatus for bending workpieces, in particular for bending plate-shaped workpieces. In the drawing, a relatively small plate-shaped workpiece 1 is shown by way of example. The bend- ing apparatus comprises a die 2 provided with a groove 3 of V- shaped section, which die 2 is supported on a table 4 of the bending apparatus. The bending apparatus furthermore comprises a punch 5, which is schematically indicated by dashed lines. Aligned with punch 5 is a measuring device 6 for measuring the bending angle of workpiece 1. The measuring device comprises a housing 7, the shape of at least the end of which corresponds to the shape of the end of punch 5. Housing 7 is mounted in an upper beam 8 of the bending apparatus by means of a clamp 9, in the same manner as punch 5. Although the measuring device 6 is mounted on one end of the upper beam 8 in Figs. 1 and 2, it is also possible to mount the measuring device 6 elsewhere on the upper beam 8, if desired. Furthermore it is possible for the upper beam to include more than one measuring device .

The bending apparatus furthermore comprises a driving unit (not shown) for moving die 2 and punch 3 relative to each other in a bending direction so as to carry out a bending operation on workpiece 1. The bending direction is indicated by a broken line 10 in the sectional views of Figs. 3 - 6. The construction of the bending apparatus does not form part of the present invention and consequently it is not discussed in more detail herein. The construction may be of a design which is known per se .

As appears from Figs. 1 - 6, housing 7 as well as punch 5 include an end having sloping flanks 11, 12 which de- fine a bending line 13 extending in the longitudinal direction of punch 5 and housing 7. The sloping flanks 11, 12 furthermore define a V-shaped cross-section. The shapes of the ends of housing 7 and punch 5 correspond to each other to such an ex- tent that the bending line 13 and possibly the flanks 11, 12 of the housing and the punch are in alignment. Although the shape of the entire housing 7 corresponds to that of punch 5 in the embodiment described herein, this is not necessary. The measuring device 6 includes two pairs of sensors

14, 15 and 16, 17, wherein one pair of sensors 14, 15 is present on the left-hand side of bending line 13, seen in the perspective view of Figure 1, and the other pair of sensors 16, 17 is present on the right-hand side of said bending line. Each one of the sensors 14 - 17 is movably mounted in housing 7, preferably in the bending direction 10, and projects from a respective sloping flank 11, 12 of housing 7. The projecting ends of sensors 14 - 17 are in contact with workpiece 1 during a bending operation, wherein all sensors 14 - 17 project with re- spect to bending line 13 of punch 5 and housing 7 in the illustrated position of rest of sensors 14 - 17, seen in the bending direction 10 according to the views of the drawing.

Housing 7 is of massive construction, similar to punch 5, wherein a sensor chamber 18 is formed in housing 7 for each sensor 14 - 17, at least in the V-shaped end defined by the sloping flanks 11, 12. A guide dam 19 is thereby maintained at the location of bending line 13. As a result, bending line 13 does not exhibit an interruption, not even at the location of sensors 14 - 17. This enables a relatively large width of the sensor chambers 18 in the direction of bending line 13, as appears from Figs. 1 and 2. The sensor chambers 18 are open on the respective side of the bending line 13 in the bending direction 10 and in a direction transversely thereto. Each sensor 14 - 17 comprises a sensor body 20 which occupies the entire space in sensor chamber 18 to the left and to the right, respectively, of the guide dam 19. This gives the sensor body 20 a robust construction, with the sensor body being substantially confined within sensor chamber 18. As a result the sensor is capable of withstanding relatively rough working conditions during use of the bending apparatus .

The guide dam 19 in sensor chambers 18 includes a guide surface 21 for the of movement of the associated sensor with great accuracy, which direction of movement corresponds to the bending direction 10 in the preferred embodiment being described. The sensor body 20 of each sensor 14 - 17 includes a second guide surface 22, which abuts against guide surface 21 and which co-operates therewith. Since a small recess 23 is formed in sensor body 20, the dimension of the guide surface 22 in the bending direction is considerably smaller than the corresponding dimension of the guide surface 21 of guide dam 19. On the one hand this ensures accurate guidance of the sensor in the bending direction and on the other hand it prevents wear on the guide surfaces 21, 22 caused by dirt particles or the like. Each guide dam 19 furthermore includes a stop surface 24 for the associated sensor 14 - 17, which determines the position of rest of sensors 14 - 17 that is shown in the drawing. As already noted above, each sensor 14 - 17 projects with re- spect to bending line 13 in this position of rest. The sensor body 20 of each sensor 14 - 17 has a line of contact 25 outside sensor chamber 18, which line of contact extends parallel to the bending line 13. As appears from Figs. 3 and 4, the line of contact 25 of sensor 14 lies further away from bending line 13 than the line of contact 25 of sensor 15.

In Fig. 4 die 2 having the V-shaped groove 3 is schematically indicated, whereby it is indicated that the line of contact 25 of sensor 14 lies outside V-shaped groove 3, whilst the line of contact 25 of sensor 15 lies within said groove 3. As is apparent from said schematic indication, the sensors 14, 15 and 16, 17 are displaced over different distances with respect to the bending line 13 during the bending operation, wherein it is possible to derive from said difference in dis- placement the angle at which the respective part of the work- piece 1 has been bent. The manner in which this difference in displacement is measured will be described in more detail hereafter. Fig. 5 is a partial sectional view of sensor 16. It will be apparent that sensor 17 is substantially the mirror image of sensor 15, so that it is possible to measure the angle through which the respective part of the workpiece 1 has been bent on the right-hand side of bending line 13 as well. In Fig. 4 the positions of sensors 16, 17 are indicated by means of a dashed line. Since the angles are measured on either side of the bending line 13 , the angle through which the workpiece has been bent can be measured with great accuracy.

In the embodiment described herein sensor 14 is con- nected to a lower end of a sensor rod 26 which is movably mounted in housing 7. In the preferred embodiment as described herein the direction of movement of the sensor rod is the same as the bending direction 10. This is not absolutely necessary, however. The direction of movement of sensor rod 26 must be ex- actly parallel to the guide surfaces 21, 22, however. Numerals 27 and 28 indicate guide or slide bearings for the sensor rod 26. A spring 29 loads the sensor rod and thus sensor 14 such that the sensor is pressed against the stop surface of the guide dam 19 in the position of rest. At the upper end, sensor rod 26 is connected to a connecting piece 30 which is shown in Fig. 7, which connects the sensor rod 26 to a holder 31 of a displacement gauge 32. Said displacement gauge 32 is preferably in the form of a glass ruler. Holder 31 is likewise movably mounted in housing 7, to which end slide bearings 33 are pro- vided. Also as regards the direction of movement of holder 31 it applies that it preferably extends in the bending direction 10. Said direction of movement must be exactly parallel to the guide surfaces 21, 22, however. From the foregoing it will be understood that movement of sensor 14 in the bending direction 10 causes holder 31 of displacement gauge 32 to undergo a corresponding movement .

A measuring pin 34 of displacement gauge 32 is con- nected to sensor 15 by means of a connecting piece 35, as is shown in the sectional view of Fig. 4. Furthermore a sensor tube 36 engages connecting piece 35, which sensor tube is mounted in housing 7 in such manner as to be movable in the bending direction. To this end slide bearings 37 are provided. A spring 38 loads the sensor tube 36 such that sensor 15 is pressed against the stop surface 24 of the associated guide dam 19 in the illustrated position of rest. It will be understood that movement of sensor 15 in the bending direction 10 causes the measuring pin 34 of displacement gauge 32 to undergo a cor- responding movemen .

When a bending operation is carried out on workpiece 1, both sensors 14, 15 will undergo a movement in the bending direction 10, whereby the above-described connections between sensors 14, 15 and displacement gauge 32 enable the displace- ment gauge 32 to measure the difference in movement between the two sensors 14, 15. The measuring signal of displacement gauge 32 is fed, via a link 39 (schematically shown) , to a processing unit (not shown) which may include a microprocessor. The difference in movement between sensors 16, 17 on the right-hand side of the bending line is measured in a similar manner. The processing unit derives the bending angle of the workpiece 1 from the measuring signals of the two displacement gauges 32. This enables a highly accurate measurement of the bending angle of the workpiece . The construction including sensors and guide surfaces on guide dam 19 has the advantage that a force including a component directed towards the guide surface 21 of guide dam 19 is exerted on sensors 14 - 17 by workpiece 1 during bending. This means that an accurate guidance of sensors 14 - 17 by guide dam 19 is ensured. In accordance with a preferred embodiment of the measuring device 6 described herein, in order to have said guidance take place as accurately as possible, the connection between each sensor 14 - 17 and a respective sensor rod 26 and connecting piece 35 is adjustable in a direction transversely to the bending line 13. This embodiment is shown in sectional view in Fig. 6 for sensor 14. As is apparent from said sectional view, sensor 14 is provided with a slotted hole 40, so that sensor 14 is movable in a direction transversely to bending line 13 with respect to the bolt 41 used for effecting the connection. As a result, the mutual abutment of the guide surfaces 21, 22 is adjustable independently of any manufacturing tolerances of the various parts of measuring device 6. In the embodiment of the bending apparatus and the measuring device as shown in the drawing, sensors 14 - 17 are formed with a line of contact 25. Such a line of contact could possibly cause damage to the surface of the workpiece. In order to prevent this, it is possible to form sensor body 20 with a rounding having a small predetermined radius at the location of line of contact 25 rather than use a line of contact. This rounding can be taken into account upon calculation of the bending angle by the processing unit.

The measuring device 6 described herein is also used for measuring possible spring-back of workpiece 1. It is a well-known fact that when a workpiece is bent by means of the bending apparatus described herein, the workpiece not only undergoes plastic deformation, but also a slight elastic deformation. The resulting spring-back of workpiece 1 is measured by returning the punch 5 following the bending operation, wherein the return movement is stopped the moment punch 5 no longer exerts a pressure on the workpiece. This moment in the return movement can be detected by mounting a small sensor (not shown) in the bending line of housing 7, which sensor on the contrary projects with respect to bending line 13 in the position of rest . The moment this sensor starts to move with respect to housing 7 during the return movement of the punch, said return movement will be stopped and the difference between the bending angles at the beginning and at the end of the return movement of punch 5 can be determined.

Fig. 8 shows a cross-sectional view of a spring-back measuring unit 50, which can be used advantageously in the ap- paratus for bending workpieces which is described herein. As is apparent from the cross-sectional view, unit 50 comprises a housing 51, the end of which includes sloping flanks 52 and 53. A sensor chamber 54 is recessed in the sloping flank 53, into which chamber the head 55 of a spring-back sensor 56 projects. Spring-back sensor 56 comprises a sensor rod 57, which is guided in a bore of housing 51.

Spring-back sensor 56 is capable of up-and-down movement in housing 51. The sloping flank 52 of housing 51 includes an end edge 55 adjacent to head 55, which end edge is in align- ment with the bending line 13 of punch 5 and housing 7. Head 55 likewise includes a sloping flank having an end edge, which slightly projects with respect to the bending line 13 in the position of rest which is shown in Fig. 8, and which is in alignment with bending line 13 when a bending operation is be- ing carried out. Head 55 may also include two sloping flanks, which jointly define the end edge. A head of this kind is easier to form and experiences less friction from housing 51. Preferably, at least the material surrounding the end edge is hardened. Furthermore, the head is preferably positioned be- tween the end walls of housing 51, so that unintentional damage to spring-back sensor 56 is practically excluded.

A sensor 58, preferably a pressure sensor 58, is mounted above sensor rod 57 in housing 51 by means of screw thread 59. Pressure sensor 58 comprises a resilient body 60, such as a plastic knob, for example, which abuts against the upper end or shoulder of sensor rod 57. Mounted in the side of housing 51 is a conical bolt 57, which falls into a conical re- cess in sensor rod 57. Turning the conical bolt inwards will cause the sensor rod to move upwards .

The head 55 of sensor rod 57 is provided with a stop 62 on the side which abuts against the wall of sensor chamber 54. A stop shoulder 63 is formed in the wall of sensor chamber 54. When head 55 is pressed in the direction of pressure sensor 58, stop 62 will abut against stop shoulder 63, thus preventing further compression and overloading of resilient body 60 and pressure sensor 58. As a result, the pressure on pressure sensor 58 is limited to a maximum value, which is independent of the pressure being applied during bending, so that it is possible to use sensitive pressure sensors which are capable of measuring loads within a range of 0 - 100 kilograms, for example.

Pressure sensor 58 is adjusted by tightening the coni- cal bolt 61 until the shoulder of sensor rod 57 abuts against the resilient body 60. Then the pressure sensor 58 is turned, so that the resilient body 60 is further pressed against the shoulder of sensor rod 57 until the pressure sensor measures a high pressure, for example a value just below the maximum value, P_max, which can be measured by the pressure sensor. The unit, kilogram, Newton or Ohm, for example, and the magnitude of the measured value delivered by the sensor will vary with each type of sensor or even with every individual sensor, they are not essential for the invention for that matter. Then the pressure sensor 58 is locked in position by means of locking bolt 64, and the conical bolt 61 is turned back, for example until the pressure sensor 58 measures a value 30, and locked in position, for example by means of an adhesive. Now an angle of 90° ± 0.2°, for example, is input into the processing unit. The value which is measured by the pressure sensor can vary due to temperature and other ambient influences. It is preferable, therefore, to register the value, i.e. P_0/ which is measured by the pressure sensor 58 in unloaded condition, just before the operation is carried out. A workpiece 1, for example a metal sheet, is placed between punch 5 and die 2, and punch 5 is moved downwards. Almost immediately after punch 5 makes contact with workpiece 1, the value meas- ured by pressure sensor 58 rises to the maximum value, P_maχ, which can be measured by pressure sensor 58, and stop 62 comes into abutment with stop shoulder 63. The movement of punch 5 is continued until workpiece 1 has been bent at an angle of 90°. The punch is now moved back until pressure sensor 58 measures a predetermined threshold value between P_D and P_max, wherein the clamping force exerted on workpiece 1 by punch 5 and die 2 is still sufficient to prevent workpiece 1 from being moved. A suitable threshold value for example equals half the sum of P₀ and P_maχ. The moment a value lower than the threshold value is measured, punch 5 is stopped. Then the angle to which the sheet has been bent, for example 93°, is determined. The input angle is now reduced by a value equal to the difference between the measured angle (93°) and the originally input angle (90°) , i.e. by 3°, so that it is 87° now. These process steps are repeated until an angle is reached which is smaller than or equal to 90° ± 0.2° .

Fig. 9 shows in perspective view an embodiment of the apparatus described herein, wherein the spring-back measuring unit 50 and the measuring device described herein are formed with a common housing 70. The advantage of this is that the bending apparatus only needs to be fitted with one measuring device, by means of which the bending angle and the amount of spring-back can be measured.

In the bending apparatus described herein the measuring device 6 is calibrated by placing a calibrating plate on die 2 as the workpiece and moving punch 5 downwards with a small force, in such a manner that the calibrating plate cannot deform. When the downward movement of punch 5 is stopped, the measuring values of displacement gauge 32 are set to a predetermined value, for example zero, because the sensor pairs 14, 15 and 16, 17 lie in one plane in this position. In order to further enhance the accuracy, a second calibration step may be carried out, using a calibration gauge having a V-shaped groove with sloping flanks which include a very precisely defined angle of 90°. Said calibration gauge is placed under punch 5 and subsequently punch 5 is pressed into said V-shaped groove without deforming the calibration gauge.

The difference in displacement between sensors 14,15 and 16, 17 must correspond exactly to an angle of 45° at that point. Since the measured displacement equals the horizontal distance between the sensors at this angle, it is possible to accurately record said horizontal distance in the processing unit. The processing unit can use this horizontal distance for accurately converting the measuring values supplied by the displacement gauges into angle measuring values .

Since every material exhibits a certain amount of spring-back, this spring-back can be taken into account in advance by bending workpiece 1 beyond the desired bending angle. Fig. 10 very schematically shows the manner of controlling the apparatus described herein, which comprises a processing unit 80 including input means 81, such as a keypad, by means of which the desired bending angle and the desired tolerance can be input into the processing unit 80. The processing unit 80 controls the driving unit 85 of punch 5, which is schematically shown in Fig. 10, to carry out the bending operation, wherein the bending angle is continuously measured by means of the measuring device described herein. When an angle of 90° ± 0.3' is to be bent, for example, the processing unit 80 will move punch 5 until an angle of 89.7° (90° minus the tolerance times one) is measured. Then the processing unit 80 controls the driving unit so that punch 5 is moved back, with the bending angle being measured continuously again. The return movement is stopped by the processing unit as soon as a bending angle of 90.3° is measured. When this is the case, the processing unit 80 carries out a next bending step, wherein workpiece 1 is bent at an angle of 89.4° (90° minus the tolerance times two) . Then the punch 5 is moved back. Once an angle of 90,3° or more is measured, the next bending step is carried out, by bending at an angle of 89.1°, etc. With each following bending step the workpiece is bent further, preferably by an amount equal to the desired tolerance value. Once the processing unit 80 measures a bending angle which remains smaller than 90.3° during the return movement, the bending operation is stopped and the punch is moved completely upwards. In this manner a workpiece is bent at the desired angle with the desired tolerance at a great operating speed. The speed at which successive bending operations are carried out is high.

Preferably, the angle that is formed in the first step is not 89.7° but, depending on the type of material, a smaller angle of for example 89.5°, since there will some spring-back at all times, which can be taken into account directly. The required number of bending steps is thus reduced.

According to an advantageous embodiment the bending angle is measured in two or more places, which places are dis- tributed over the length of the bending line. An advantageous solution is provided by an embodiment wherein three measuring devices 6 are used, two of which are placed near the ends of the workpiece and one of which is placed approximately in the centre. With an embodiment of this kind the method according to the invention can also be used advantageously for curving the die. Generally, a so-called curving device 86 is disposed under die 2, which device is schematically indicated in the block diagram of Fig. 10, by means of which device the deflection of the bending means 2, 5 can be compensated. Said curve is adjusted in a usual manner in dependence on the differences between the bending angles being measured along the length of the bending line. With the prior art apparatuses the adjustment of the curving device is a relatively time-consuming process. With the device according to the invention as described herein a quick and accurate adjustment of the curving device can be achieved in the following manner.

During the first bending step the processing unit 80 can compare the bending angles by means of said two or more measuring devices 6, three in this embodiment, and establish whether there are any differences. When a difference is established, for example a difference greater than the desired tolerance value, the curve must be adjusted. The processing unit 80 will adjust the curving device as soon as the return movement has been stopped upon establishment of a bending angle of more than 90.3° by one of the three measuring devices 6. The processing unit 80 can then adjust the curving device without loading the die, in such a manner that the measured differences are eliminated. The adjustment of the curve can take place while the first bending operation is in progress, as a result of which the operating speed is enhanced.

The above-described spring-back measuring unit 50 can be used advantageously in the apparatus described herein for detecting the presence or absence of a workpiece 1. The processing unit 80 contains data on the distance which the upper beam 8 must cover before punch 5 reaches groove 3 of die 2. When punch 5 must have reached the mould according to said data, and measuring unit 50 does not deliver a signal, it is obvious that no workpiece 1 is present on die 2. This means that the workpiece has not been placed in the apparatus, at least not correctly. The processing unit will return the punch 5 to the starting position in that case.

It is noted that the method described herein can also be used with other types of bending apparatuses, for example with apparatus including a movable jaw. Fig. 11 is a very schematic view of a part of such apparatus, showing clamps 82, 83 which clamp down a workpiece 1. A jaw 84 is movable in a manner known per se for bending the workpiece 1. Fig. 11A shows the jaw in a starting position, and in Fig. 11B the desired angle has been achieved. The jaw 84 is provided with a spring-back measuring unit 50 in one or more places, which are capable of detecting the moment when the jaw 84 threatens to become disengaged from the workpiece. Since the processing unit is capable of measuring, in a usual manner, the angle through which the jaw 84 is turned, it is possible to establish by means of measuring unit 50 whether the jaw 84 threatens to become disengaged from workpiece 1 when the jaw is moved back over the desired tolerance value. The desired angle plus the tolerance value, that is, the maximum allowable amount of spring-back, is shown in Fig. 11C. If the jaw 84 has not become disengaged from the workpiece yet at that stage, a next bending step will be car- ried out, wherein the workpiece is bent further yet, as is shown in Fig. 11D. The bending steps will be repeated until a bending angle is measured upon the return movement which remains smaller than the desired bending angle plus the desired tolerance. When a particular angle to be bent in order to obtain the desired angle with the desired tolerance has been determined by means of the spring-back measuring unit when using the method described herein, said angle can be stored in a memory and be used for bending workpieces in a larger series of products, so that it will not be necessary to go through all bending steps. This is also possible for different bending operations on one and the same product. When a stored angle is used, sensors 14 - 17 can be made inoperative by fixing them in the highest position by means of a suitable locking arrangement. This prevents unnecessary wear on the sensors, thus prolonging their working life.

The invention is not restricted to the above-described exemplary embodiments, which can be varied in several ways without departing from the scope of the claims.

Claims

1. A method for bending workpieces, wherein the work- piece is bent at a desired angle with the aid of bending means, which can be moved under the control of a processing unit for the purpose of carrying out a bending operation, characterized in that the desired bending angle and a desired tolerance are fed to the processing unit, with the processing unit moving the bending means in successive bending steps i (i=l N) , wherein each bending step i comprises the step of bending the workpiece at a desired bending angle minus an angular value, which increases with each step i, wherein subsequently the bending means are moved back and the return movement is stopped before the force with which the workpiece is clamped between the bending means falls below a predetermined value, after which a next bending step is carried out until the bending an- gle that is measured upon the return movement remains smaller than the desired bending angle plus the desired tolerance.

2. A method according to claim 1, wherein said angular value depends on the desired tolerance.

3. A method according to claim 1 or 2 , wherein the bending angle is continuously measured, also during the return movement, therefore, and fed to the processing unit, and wherein the return movement is stopped when a bending angle is measured which corresponds to the desired angle plus the desired tolerance.

4. A method according to claim 3, wherein said angular value equals i times the desired tolerance.

5. A method according to claim 4, wherein said angular value is greater than the desired tolerance at the first bending step.

6. A method according to claim 1, wherein the return movement is stopped when the force with which the workpiece is clamped between the bending means falls below a predetermined value .

7. A method according to claim 6, wherein the force with which the workpiece is clamped between the bending means is measured by means of a pressure sensor.

8. A method according to claim 7, wherein the pressure sensor is calibrated during the first bending step.

9. A method preferably according to any one of the preceding claims, wherein the bending angle is measured in at least two measuring points which are distributed over the length of the bending line, wherein the bending means comprise curving means, wherein the processing unit compares the bending angles which are measured in the various measuring points, wherein the processing unit adjusts the curving means upon detection of a difference between the bending angles, in such a manner that said difference is at least substantially eliminated.

10. A method according to claim 9, wherein the processing unit adjusts the curving means as soon as the return movement is stopped during the first and possibly next bending steps .

11. A method according to any one of the preceding claims, wherein the processing unit detects contact between the bending means and the workpiece and measures the movement of the bending means, wherein the bending means are moved back if no contact with the workpiece is detected after a predetermined movement .

12. Apparatus for bending workpieces, comprising bending means, a driving unit for driving the bending means to carry out a bending operation, and a measuring device for measuring a bending angle of the workpiece, wherein a processing unit is provided which includes input means for inputting a desired bending angle, characterized in that the processing unit is capable of receiving the desired bending angle with a desired tolerance via the input means, wherein the processing unit is arranged for controlling the driving unit in successive bending steps i (i=l N) , wherein each bending step i comprises the step of bending the workpiece at a desired bending angle minus an angular value, which increases with each step i, wherein subsequently the bending means are moved back and the return movement is stopped before the force with which the workpiece is clamped between the bending means falls below a predetermined value, after which a next bending step is carried out until the bending angle that is measured upon the return movement remains smaller than the desired bending angle plus the desired tolerance.

13. Apparatus according to claim 12, wherein said processing unit controls the driving unit in dependence on the desired bending angle and receives bending angle information from the measuring device during a bending operation, and wherein the return movement is stopped when a bending angle is measured which corresponds to the desired angle plus the desired tolerance.

14. Apparatus according to claim 13 , wherein said angular value equals i times the desired tolerance.

15. Apparatus according to claim 14, wherein said angular value is greater than the desired tolerance at the first bending step .

16. Apparatus according to any one of the preceding claims 13 - 15, wherein the return movement is stopped when the force with which the workpiece is clamped between the bending means falls below a predetermined value.

17. Apparatus according to claim 16, wherein the bending means include a pressure sensor for determining the force with which the workpiece is clamped between the bending means or a derivative of said force.

18. Apparatus according to claim 17, wherein the pressure sensor incorporated in a spring-back measuring unit com- prising a spring-back sensor, which is positioned between the pressure sensor and the workpiece, and which co-operates with the housing of the spring-back measuring unit in such manner as to prevent overloading of the pressuring sensor during bending.

19. Apparatus according to claim 18, wherein a resil- ient body is present between said spring-back sensor and said pressure sensor.

20. Apparatus according to any one of the preceding claims 13 - 19, wherein at least two measuring devices and a curving device are provided, wherein the processing unit is ar- ranged for comparing the bending angles which are measured by the various measuring devices, wherein the processing unit is capable of adjusting the curving device upon detection of a difference between the bending angles, in such a manner that said difference is at least substantially eliminated, wherein the processing unit adjusts the curving means as soon as the return movement is stopped during the first and possibly next bending steps .