Apparatus for bending workpieces and measuring device for such an apparatus The invention relates to an apparatus for bending work- pieces, in particular plate-shaped workpieces, comprising a die, a punch having a punch end with sloping flanks, which de- fine a bending line extending in the longitudinal direction of the punch and a V-shaped cross-section, a driving unit for mov- ing the die and the punch relative to each other in a bending operation direction for carrying out a bending operation, and a measuring device for measuring a bending angle of the work- piece, which measuring device comprises a housing having an end whose shape corresponds to that of the end of the punch, such that at least the bending line of the housing is in line with that of the punch, wherein two movable feelers are mounted in the housing, on one side of the bending line, which feelers project from the respective sloping flank of the housing and whose projecting ends are in contact with the workpiece during a bending operation, wherein a processing unit is provided, which is capable of deriving the bending angle of the workpiece from the difference in displacement between the two feelers during a bending operation.

An apparatus of this kind is known from GB-A-2 072 551. In this prior art apparatus, the two feelers are mounted side by side in a direction transversely to the bending line, as a re- sult of which each feeler is only small, because of the limited amount of space that is available. This makes the feelers of the prior art apparatus relatively vulnerable. With this prior art apparatus it is furthermore assumed that the angle which is derived from the difference in displacement amounts to half the bending angle. This is only correct, however, if the folding line of the workpiece and the bending line of the punch are in alignment with the direction of the bending operation, which is not the case under all circumstances. This may lead to measur- ing errors when using the prior art device. GB-A-2 072 551 also discloses an embodiment wherein a feeler is mounted on either side of the bending line of the punch, so that the angle can be

measured on either side of the bending line, so that the angle can be measured on both sides of the bending line. The angle is thereby derived from the displacement of one feeler, which likewise leads to a less accurate measurement of the bending angle.

WO-A-9641690 discloses an apparatus for bending workpieces comprising a measuring device fitted with two feelers, which are in contact with the workpiece on either side of the bending line, wherein the possible shift of the folding line of the workpiece relative to the bending line of the punch can be de- rived from a displacement of the feelers in a direction trans- versely to the bending line. In this prior art apparatus, slots are formed in the housing for the feelers, which slots inter- sect the bending line, so that said slots need to be narrow.

This makes the feelers relatively vulnerable.

The object of the invention is to provide an improved ap- paratus of the kind referred to in the introduction.

In order to accomplish that objective, the apparatus ac- cording to the invention is characterized in that two movable feelers are mounted on the other side of the bending line as well, which feelers project from the respective sloping flank of the housing and whose projecting ends are in contact with the workpiece during a bending operation, wherein the two feel- ers present on one side of the bending line are spaced apart in the direction of the bending line, and wherein the processing unit is capable of deriving the bending angle from the differ- ence in displacement between the two feelers of each pair of feelers positioned on one side of the bending line.

Thus, an apparatus is obtained wherein the bending angle is derived from the angles which are measured by two feelers on either side of the bending line of the punch, which makes it possible to determine the bending angle with great accuracy.

Since the feelers are spaced apart in the direction of the bending line, the feelers can be of relatively robust construc- tion. Moreover, the feelers only project from the sloping flanks of the housing of the measuring device, so that the bending line of the housing is not interrupted at the location of the feelers.

The invention furthermore provides a measuring device of the above-described kind.

Finally, the invention provides a method for calibrating the measuring device, wherein according to the invention a flat calibration plate is placed on the die, and the punch and the die are moved together with a force which is not capable of de- forming the calibration plate, wherein, in the position in which the punch presses on the calibration plate, the measuring signals from the displacement gauges are set at a predetermined value.

The invention will be explained in more detail hereafter with reference to the drawing, which highly schematically rep- resents an embodiment 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 inven- tion.

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

Fig. 3 is a cross-sectional view of the measuring device which is used with the apparatus according to Fig. 1, at the location of a first feeler 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 feeler on the same side of the bending line as the feeler shown in the cross-sectional view of Fig. 3.

Fig. 5 is a partial cross-sectional view of the measuring device at the location of a first feeler on the right-hand side of the bending line of the apparatus 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. 3, along line VII-VII.

Figs. 8A-8C are a sectional view, a top plan view and a side view, respectively, of a unit for measuring the spring- back.

Fig. 9 is a perspective view of a part of another embodi-

ment of the apparatus for bending workpieces according to the invention.

Figs. 1 and 2 show in perspective view a part of an appa- ratus for bending workpieces, in particular plate-shaped work- pieces. In the drawing, a relatively small plate-shaped work- piece 1 is shown by way of example. The bending apparatus com- prises a die 2 provided with a groove 3 of V-shaped cross- 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 the workpiece 1. The measuring device includes a hous- ing 7, whereby the shape of at least the end thereof corre- sponds 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 a similar manner as punch 5. Although the measuring de- vice 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 at an- other location in upper beam 8, if desired. Furthermore it is possible to accommodate more than one measuring device in the upper beam.

The bending apparatus further comprises a driving unit (not shown) for moving die 2 and punch 3 relative to each other in a bending operation direction for carrying out a bending op- eration on workpiece 1. The direction of the bending operation 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 will not be described in more detail herein. The construction may be of a kind which is known per se.

As is apparent from Figs. 1-6, housing 7 as well as punch 5 have an end with sloping flanks 11,12, which define a bending line 13 extending in the longitudinal direction of punch 5 and housing 7. Said 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 flanks 11,12 and the bending line 13 of the housing and the punch are in line with each other. Although the shape

of the entire housing 7 corresponds to that of punch 5 in the embodiment as described herein, this is not necessary. Impor- tant is that the flanks which are operative during the bending operation and the bending line are in alignment with each other.

Measuring device 6 includes two pairs of feelers 14,15 and 16,17, respectively, wherein one pair of feelers 14,15 is mounted on the left-hand side and the other pair of feelers 16, 17 is mounted on the right-hand side of bending line 13 in the view according to Fig. 1. Each feeler 14-17 is movably mounted in housing 7, preferably in the direction of the bend- ing operation 10, and projects from the sloping flank 11 or 12 of housing 7. The projecting ends of the feelers 14-17 are in contact with the workpiece 1 during a bending operation, wherein all feelers 14-17 project relative to the bending line 13 of punch 5 and housing 7 in the illustrated position of rest of feelers 14-17, seen in the direction 10 of the bend- ing operation as indicated in the views illustrated in the drawing.

Housing 7 is of massive construction, substantially simi- lar to punch 5, wherein a feeler chamber 18 for each feeler 14 -17 is formed in housing 7, at least in the V-shaped end de- fined by sloping flanks 11,12. A guide dam 19 is thereby main- tained at the location of bending line 13. As a result of this, bending line 13 is not interrupted, not even at the location of feelers 14-17. This makes it possible to form the feeler chambers 18 with a relatively large width in the direction of bending line 13, as is apparent from Figs. 1 and 2. Feeler chambers 18 are open on the respective side of bending line 13, in the direction 10 of the bending operation and in a direction transversely thereto. Each feeler 14-17 comprises a feeler body 20, which occupies the entire space in the feeler chamber 18 to the left and to the right of the guide dam 19. The feeler body 20 is thus of robust construction, whereby the feeler body is substantially retained within feeler chamber 18. The feeler is thus capable of withstanding comparatively rough operating conditions during use of The bending apparatus.

The guide dam 19 in feeler chambers 18 includes a guide

surface 21 for the associated feeler 14-17, which guide sur- face extends parallel to the direction of movement of the asso- ciated feeler with great accuracy, which direction corresponds to the direction 10 of the bending operation in the preferred embodiment as described herein. The feeler body 20 of each feeler 14-17 includes a second guide surface 22, which mates with guide surface 21 and which abuts against said guide sur- face 21. Since a small recession 23 is formed in feeler body 20, the dimension of the guide surface 22 is considerably smaller, seen in the direction of the bending operation, than the corresponding dimension of the guide surface 21 of guide dam 19. One the one hand, an accurate guidance of the feeler in the direction 10 of the bending operation is thus ensured, whilst on the other hand wear on the guide surfaces 21,22 caused by the presence of dirt particles or the like is pre- vented.

The feeler body 20 of each feeler 14-17 is furthermore accurately guided in the associated feeler chamber 18 so as to prevent rotation of the feeler. To this end, guide means (not shown) extending transversely to the bending line may be pres- ent on the walls of each feeler chamber 18.

Each guide dam 19 furthermore includes a stop surface 24 for the associated feeler 14-17, which determines the posi- tion of rest of feelers 14-17 as illustrated in the drawing.

As already noted above, each feeler 14-17 projects relative to the bending line 13 in this position of rest. The feeler body 20 of each feeler 14-17 comprises a contact line 25 lo- cated outside feeler chamber 18, which contact line extends parallel to bending line 13. As is apparent from Figs. 3 and 4, the contact line 25 of feeler 14 is located at a larger dis- tance from bending line 13 than the contact line 25 of feeler 15.

In Fig. 4, the die 2 including the V-shaped groove 3 is schematically represented, whereby it is indicated that the contact line 25 of feeler 14 is located outside V-shaped groove 3, whilst the contact line 25 of feeler 15 is located within said groove 3. As is apparent from this schematic representa- tion, feelers 14,15 and 16,17, respectively, are displaced

over different distances relative to bending line 13 during the bending operation, whereby the angle through which the respec- tive part of the workpiece 1 has been bent can be derived from this difference in displacement. The manner in which said dif- ference in displacement is measured will be described yet here- after.

Fig. 5 is partial sectional view of feeler 16. It will be apparent that feeler 17 is essentially the mirror image of feeler 15, so that the angle through which the respective part of workpiece 1 has been bent can be measured on the right-hand side of bending line 13 as well. In Fig. 4 the positions of feelers 16,17 are indicated by means of a dashed line. Since the angles are measured on either side of bending line 13, a highly accurate measurement of the angle through which the workpiece has been bent is possible.

In the embodiment as described herein, the feeler 14 is connected to a lower end of a feeler rod 26, which is movably mounted in housing 7. In the preferred embodiment as described herein, the direction of movement of the feeler rod is the same as the direction 10 of the bending operation. This is not abso- lutely necessary, however. What is necessary is that the direc- tion of movement of the feeler rod 26 is precisely parallel to guide surfaces 21,22. Numerals 27 and 28 indicate guides or slide bearings for feeler rod 26. A spring 29 loads the feeler rod, and thus feeler 14, in such a manner that the feeler is pressed on the stop surface 14 of guide dam 19 in the position of rest. At its upper end, feeler rod 26 is connected to a con- necting piece 30 as illustrated in Fig. 7, which connects feeler 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, for which purpose slide bearings 33 are provided. Also as regards the.. direction of movement of holder 31 it applies that it is preferably the same as the direction 10 of the bending opera- tion. Said direction of movement must be precisely parallel to guide surfaces 21,22, however. It will be apparent from the foregoing that a displacement of feeler 14 in the direction 10 of the bending operation will result in a corresponding dis-

placement of holder 31 of displacement gauge 32.

A measuring pin 34 of displacement gauge 32 is connected to feeler 15 by means of a connecting piece 35, as is shown in the sectional view of Fig. 4. Connecting piece 35 is also en- gaged by a feeler tube 36, which is mounted in housing 7 in such manner as to be movable in the direction 10 of the bending operation. Slide bearings 37 are provided for this purpose. A spring 38 loads the feeler tube 36 such that feeler 15 is pressed on the stop surface 24 of the associated guide dam 19 in the illustrated position of rest. It will be apparent that displacement of feeler 15 in the direction 10 of the bending operation will result in a corresponding displacement of meas- uring pin 34 of displacement gauge 32.

When a bending operation is carried out on workpiece 1, the two feelers 14,15 will undergo a displacement in the di- rection 10 of the bending operation, whereby the above- described connections between feelers 14,15 and displacement gauge 32 enable displacement gauge 32 to measure the difference in displacement between the two feelers 14,15. The measuring signal from displacement gauge 32 is supplied, via a connection 39 (schematically indicated), to a processing unit (not shown), which may include a microprocessor. The difference in displace- ment between feelers 16,17 on the right-hand side of bending line 13 is measured in a similar manner. The processing unit derives the bending angle of workpiece 1 from the measuring signals of the two displacement gauges 32. This makes it possi- ble to measure the bending angle of the workpiece with great accuracy.

In the illustrated embodiment, the displacement gauge 32 is connected to feeler 14 via connecting piece 30. Preferably, displacement gauge 32 is connected to feeler 15 and measuring pin 34 is connected to feeler 14. As a result of this, the dis- tance over which displacement gauge 32 moves during a bending operation will be smaller and a more compact construction of housing 7 is possible.

It is noted that measuring device 6 may include a guide element for each connecting piece 30, which guide element only allows up-and-down movement of the connecting piece. This pre-

vents any rotary movements which may lead to measurement inac- curacies and which may cause wear. Such a guide element may for example consist of a pin mounted on the housing, which engages in a slot of the connecting piece.

The above-described construction of the feelers and guides on guide dam 19 has the advantage that a force including a com- ponent directed at the guide surface 21 of guide dam 19 is ex- erted on feelers 14-17 by the workpiece 1 during bending.

This means that an accurate guidance of feelers 14-17 by guide dam 19 is ensured. In accordance with a preferred embodi- ment of the measuring device 6 as described herein, in order to enable an optimally accurate guiding, the connection between each feeler 14-17 and feeler rod 26 and connecting piece 35 is adjustable in a direction transversely to bending line 13.

This embodiment is shown in sectional view for feeler 14 in Fig. 6. As is apparent from this sectional view, feeler 14 is provided with a slotted hole 40, so that feeler 14 is movable in a direction transversely to the bending line 13 relative to the bolt 41 used for said connection. As a result of this, the mutual abutment of guide surfaces 21,22 can be adjusted inde- pendently of any manufacturing tolerances of the various compo- nents of measuring device 6.

In the embodiment of The bending apparatus and the measur- ing device as illustrated in the drawing, feelers 14-17 have a contact line 25. Such a contact line might cause damage to the surface of the workpiece. In order to prevent this, it is possible to form the feeler body 20 with a rounded portion hav- ing a small, predetermined radius rather than use a contact line 25. This rounding can be taken into account upon calcula- tion of the bending angle by the processing unit.

The measuring device 6 described herein is also used for measuring any rebound of workpiece 1. It is a known fact that when a workpiece is bent by means of The bending apparatus dis- closed herein, the workpiece is not only deformed plastically, but also undergoes a slight elastic deformation. The resulting rebound of workpiece 1 is measured by moving back the punch 5 after the bending operation, whereby the return movement is stopped the moment punch 5 no longer exerts a pressure on the

workpiece. This moment during the return movement can be de- tected by mounting a small feeler (not shown) in the bending line of housing 7, which feeler slightly projects beyond bend- ing line 13. As soon as this feeler starts to move with respect to housing 7 during the return movement of the punch, the re- turn movement will be stopped and the difference between the bending angles at, respectively, the beginning and the end of the return movement of punch 5 can be determined.

Figs. 8A-8C show in cross-sectional view, top plan view and side view, respectively, a rebound measuring unit 50, which can be used advantageously in the apparatus for bending work- pieces as disclosed herein. As is apparent from the cross- sectional view, unit 50 comprises a housing 51 whose end has sloping flanks 52 and 53, which are in line with the sloping flanks 11 of the punch and of the housing 7, respectively. A feeler chamber 54 is recessed in the sloping flank 53, into which the end 55 of a rebound feeler 56 extends. Rebound feeler 56 comprises a feeler rod 57, which is guided in a bore of housing 51 and which is pressed down by plate springs on the side remote from end 55. Plates springs 58 are confined between a nut 59 mounted in the bore of housing 51 and a shoulder of feeler rod 57, with a nut 60 being provided on the end of feeler rod 57 that is passed through nut 59, by means of which nut 60 the spring pressure of plate springs 58 can be adjusted.

This spring pressure can thus be selected in dependence on the workpiece that is to be bent.

Rebound feeler 56 is capable of upward and downward move- ment in housing 51, whereby the end 55 projects from feeler chamber 54 when no bending operation is being carried out. The sloping flank 52 of housing 51 has an end edge bounding feeler end 55, which is in line with the bending line 13 of the punch and of housing 7. Feeler end 55 likewise comprises a sloping flank 61 with an end edge which projects with respect to bend- ing line 13 in the position of rest as shown in Fig. 8, and which is in line with bending line 13 when a bending operation is being carried out. As is apparent from Fig. 8C, the feeler end 55 is positioned between end walls 63 of housing 51, so that unintentional damage to spring-back feeler 56 is practi-

cally excluded. The sloping flank 61 of feeler end 55 comprises an inwardly staggered portion 64, so that any unevennesses of the workpiece, or dirt particles or the like, cannot load the spring-back feeler 56 against the action of plate springs 58.

Provided in housing 51 is a detector 65, which is in the form of a proximity switch in the embodiment described herein.

When the spring-back feeler 56 is pressed inwards during a bending operation, so that end edge 62 is aligned with bending line 13, a stop 66 of the feeler will strike against a stop 67 of the housing, so that the spring-back feeler cannot move in- wards any further. In this position, the detector 65 will de- tect the pressed-down position of the return feeler 56, in that a detection surface 68 is detected by the proximity feeler. As soon as the spring-back feeler 56 is moved downwards by the plate springs 57 during the return movement of punch 5, this will be signalled by detector 65 and the return movement will be stopped. Then the difference between the bending angles at the beginning and the end, respectively, of the turn movement of punch 5 can be determined.

As is apparent from the cross-sectional view of Fig. 8B, housing 51 and spring-back feeler 56 comprise mating guide sur- faces 69, which join the end edges of sloping flanks 52 and 61, respectively.

Fig. 9 is a perspective view of an embodiment of the appa- ratus disclosed herein, wherein the spring-back measuring unit 50 and the measuring device as disclosed have 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 spring-back can be measured.

The measuring device 6 which is used in The bending appa- ratus that is disclosed herein is calibrated by placing a cali- bration plate on die 2 as the workpiece, and by moving down punch with a small force, in such a manner that the calibration plate cannot deform. When punch 5 does not move downward any further, the measuring values of displacement gauge 32 are set at a predetermined value, for example zero, because the feelers pairs 14,15 and 16,17 lie in one plane in this position.

In order to further increase the accuracy, a second cali-

bration step can be carried out by means of a calibration gauge having a V-shaped groove with sloping flanks, which include an angle of 90° with great accuracy. Said calibration gauge is placed under punch 5, and subsequently the punch is pressed into the V-shaped groove without deforming the calibration gauge. The difference in displacement between feelers 14,15 and 16,17 must then exactly correspond to an angle of 45°.

Since the measured displacement equals the horizontal distance between the feelers at this angle, it is possible to record this horizontal distance accurately in the processing unit. The processing unit can then use this information for accurately converting the measuring values obtained from the displacement gauges into angle measuring values.

The invention is not restricted to the above-described em- bodiments, which can be varied in several ways without depart- ing from the scope of the claims. In the above-described em- bodiments, for example, one displacement gauge is used for measuring the difference in displacement between the two feel- ers positioned on one side of the bending line. It is also pos- sible, however, to use one displacement gauge for each feeler and to use the measuring results for calculating the differ- ences.

It is furthermore noted that in the above-described em- bodiments of the apparatus the sloping flanks of housing 7 and 51 are indeed in line with the sloping flanks 11,12 of punch 5, but that this is not necessary. If desired, the sloping flanks of housing 7 and 51 may also include a more acute angle with the direction of the bending operation.