EP0747148B1 - Unit for prehemming and final hemming sheet edges - Google Patents

Description

The present invention relates to a pre-crimping and crimping the edge of a sheet which is supported by a fixed die, according to the preamble of claim 1.

It also relates to a pre-crimping and crimping assembly which includes several units of this kind, juxtaposed.

A well-known method for joining two sheets is to crimp the edge of one of them against that of the other.

At the start, the edge to be crimped is folded 90 ° from the rest sheet metal. The crimping operation consists, in a first step, in folding it down partially - generally at an angle of 45 ° - towards the other sheet. The crimping proper, carried out in a second step, corresponds to complete folding of the sheet metal edge, which comes to rest against the edge of the other by pinching it.

This kind of operation is traditionally done in press, on the table open with hydraulic movable matrix, or on fixed matrix using trolleys tilting blade holder.

Known devices have the drawback that they generate during the pre-crimping and / or crimping of the parasitic forces which cause a excessive winding of the sheet metal edge during its folding and tend to move the part on its support.

In addition, the amplitude of movement of the tools at the end of pressing is poorly controlled, which leads to crimping of irregular quality, not constant from one room to another in a production.

This is why an objective of the invention is to control the movement of the tools, while minimizing parasitic forces, and thus improving the quality and consistency of the setting.

Another objective of the invention is to propose a unit of pre-crimping and crimping of simple design, of a low cost price, simple to implement and maintain while working at high speed.

The state of the art in this area can be illustrated by the documents EP-A-0 592 848 and WO-A-93 05902.

These documents each have as their object a unit for pre-pressing and crimping the edge of a sheet whose plane is substantially horizontal and which is supported by a fixed matrix.

The unit includes two movable heads, one for pre-crimping and the other of crimping, which are moved synchronously to come one after the other press the edge of the sheet and release it.

According to EP-A-0 592 848 the synchronized movement of the two heads is controlled by a pneumatic cylinder or, alternatively, by a servomotor via a screw-nut system.

According to WO-A-93 05902, their movement is controlled by a common motor shaft.

The unit described in EP-A-0 592 848 has a crimping 24 which is mounted in the upper part of a vertical oscillating arm 38 of which the lower end carries a first pin 138 by which it is articulated on a rod 136 which can pivot freely around a fixed axis 142. In its portion median, this arm 38 carries a second pin 138, by which it is articulated on a another link 136 also pivotally mounted around a fixed axis 142.

The second pin 138, as well as the link 136, are positioned on the outer edge of the lever, in an area far removed from the matrix and edge of the sheet to be worked (see figure 8).

As a result of this arrangement, the crimping head describes a curvilinear trajectory, but the radius of this trajectory is relatively large relative to the width of the edge of the sheet.

As can be seen from a simple comparison of Figures 9 and 10, the crimping head describes a practically straight trajectory, of direction vertical, during the actual pressing phase, during which it is in contact with the sheet. The trajectory is only truly curvilinear during the approach phase.

This is why this known device does not make it possible to eliminate completely the phenomenon of winding the sheet edge mentioned above.

Like the device of EP-A-0 592 848, the unit according to the invention, which is defined in claim 1, is intended for pre-crimping and crimping the edge of a sheet whose plane is substantially horizontal, which is supported by a fixed matrix. It includes two movable heads, one for pre-crimping and the other for crimping, which are moved in synchronism to come one after the other press the edge of the sheet and release it, and the crimping head describes, during pressing, a curvilinear trajectory, the tangent at the end of crimping is substantially orthogonal to the plane of the sheet, its face active remaining substantially parallel to itself during this displacement, this crimping head being mounted in the upper part of a swing arm approximately vertical, the lower end of which carries a first pin by which it is articulated on a rod being able to pivot freely around an axis fixed, while its middle portion carries a second pin.

In accordance with the invention, this second journal is located below and substantially in line with the active face of the crimping head and is articulated on a control lever, the latter itself being articulated to one of its two ends on said second pin, at the other end on a rod - called end - can pivot freely around a fixed axis, and in the middle part on a rod - called central - can also pivot freely around a fixed axis, so that the curvilinear path described by the crimping head at during pressing has an instantaneous center of rotation located in the area where finds the bending angle of the sheet.

This particular movement of the crimping tool allows you to work with a regular and controlled quality, avoiding the formation of a ripple on the outer panel of the sheet, matrix side.

Thus, parasitic forces, likely to cause winding from the edge of the sheet and inadvertently moving the part on the matrix, are eliminated, or practically eliminated.

According to an additional advantageous characteristic of the invention, the movement of the presetting head is done during pressing - and in particular at the end of pressing - in an orthogonal or substantially orthogonal direction in terms of the sheet.

Preferably, as in the embodiment according to the document WO-A-93 05902, the two heads are driven by a common drive shaft. However, unlike this known embodiment, in which the movement of the motor shaft is oscillating (rotation in one direction, then in the other), the shaft drive the unit according to the invention always turns in the same direction. His movement is transmitted to the heads via a mechanism to eccentric.

Thus, the pre-crimping and crimping cycle corresponds to one revolution complete of the motor shaft, and the stop precision at the end of the cycle did not of importance, any interference between the transmission mechanisms of movement and moving heads being avoided.

Preferably, the motor shaft is rotated by a motor electric with incorporated speed reducer (gearmotor).

• l'axe d'articulation du levier de commande à la biellette d'extrémité est porté par un tirant actionné par un palonnier oscillant ;
• ce palonnier est articulé à l'extrémité de la bielle d'entraínement ;
• l'axe d'articulation reliant la bielle d'entraínement au palonnier est une barrette frangible, à seuil de rupture calibré.

According to a certain number of additional advantageous characteristics:

• the axis of articulation of the control lever to the end link is carried by a tie rod actuated by an oscillating lifter;
• this spreader is articulated at the end of the drive rod;
• the hinge axis connecting the drive rod to the lifter is a frangible bar, with calibrated breaking threshold.

Still according to a preferred embodiment, the plane of the sheet being substantially horizontal, the presetting head is carried by an oscillating arm, approximately vertical, the lower end of which carries a first pin by which it is articulated on a rod being able to pivot freely around an axis fixed, while its upper part carries a second pin hinged on a control lever.

• ce levier de commande pivote autour d'un axe fixe, et s'articule sur une biellette de commande ;
• cette dernière est articulée au palonnier oscillant.

According to a certain number of additional advantageous characteristics:

• this control lever pivots about a fixed axis, and is articulated on a control rod;
• the latter is articulated to the swinging beam.

Preferably, the plane of the sheet being substantially horizontal and the arms carrying the presetting and crimping heads being both approximately vertical, the crimping head is located below the pre-pressing.

The subject of the invention is also a pre-setting assembly and crimping tool for sheet metal, and in particular for very long sheet metal, this set being composed of several juxtaposed units such as that described above.

In an embodiment more particularly intended for working on a curved sheet, at least some of said units have a positioning angular offset from that of the others, the arrangement being such that the directions at the end of the travel of the different pre-crimping and crimping heads are in all zones substantially orthogonal to the surface of the sheet.

Other preferred features and advantages of the invention will become apparent from the description and the attached drawings which represent an embodiment thereof preferential.

• la figure 1 est une vue partielle et schématique de l'unité, qui représente la pièce à sertir montée sur une matrice, ainsi que la partie d'outillage servant au présertissage, avant l'opération ;
• la figure 2 est une vue similaire à la figure 1, représentant ces mêmes éléments en fin de présertissage ;
• les figures 3 et 4 sont des vues schématiques similaires aux figures 1 et 2 respectivement, montrant la partie d'outillage réalisant le sertissage ;
• les figures 5A et 5B sont des vues de détail montrant une opération de présertissage réalisée respectivement avec un outillage classique (figure 5A) et avec un outillage conforme à l'invention (figure 5B);
• les figures 6A et 6B sont des vues similaires aux figures 5A et 5B permettant de comparer l'opération de sertissage avec un outillage classique (figure 6A) et avec un outillage conforme à l'invention (figure 6B) ;
• la figure 7 est une vue de face d'une unité conforme à l'invention, représentée en position ouverte (têtes de présertissage et de sertissage dégagées), en phase de déchargement et de chargement d'une pièce ;
• les figures 8 et 9 sont des vues similaires à la figure 7, représentant l'unité respectivement à la fin de la phase de présertissage et à la fin de la phase de sertissage.

In the drawings:

• Figure 1 is a partial and schematic view of the unit, which shows the part to be crimped mounted on a die, as well as the tooling part used for pre-crimping, before the operation;
• Figure 2 is a view similar to Figure 1, showing these same elements at the end of preserting;
• Figures 3 and 4 are schematic views similar to Figures 1 and 2 respectively, showing the tooling part carrying out the crimping;
• FIGS. 5A and 5B are detailed views showing a presetting operation carried out respectively with conventional tooling (FIG. 5A) and with a tool according to the invention (FIG. 5B);
• Figures 6A and 6B are views similar to Figures 5A and 5B for comparing the crimping operation with conventional tooling (Figure 6A) and with a tool according to the invention (Figure 6B);
• Figure 7 is a front view of a unit according to the invention, shown in the open position (pre-crimping and crimping heads cleared), in the unloading and loading phase of a part;
• Figures 8 and 9 are views similar to Figure 7, showing the unit respectively at the end of the pre-crimping phase and at the end of the crimping phase.

It will be noted that in the schematic views of FIGS. 1 to 4, on the one hand, and on the more elaborate views of FIGS. 7 to 9, on the other hand, the same reference signs have been used to designate the same parts.

In Figures 1 to 4, we have artificially given the tools pre-crimping and, respectively crimping, simple shapes, and these two tools have been considered independently of each other, so as to allow better understanding of how they work. The fixed elements are shown hatched.

However, as shown in Figures 7 to 9, these two tools are nested one inside the other, which requires parts of more elaborate shape, avoiding interference of the different components of the tools during a cycle complete pre-crimping, crimping and unloading / loading of parts.

In addition, the drive mechanism with eccentric rod of each of these tools have not been shown in FIGS. 1 to 4.

Finally, in these same figures, the piece to be crimped and the active faces of the heads of work (which come to press against the sheet) have been artificially represented to scale significantly larger than that of the rest of the elements, in order to better illustrate the operations. The part to be crimped is not shown in FIGS. 7 to 9.

In Figures 1 to 4, there is designated by the reference 1 a piece to be crimped composed of two sheets 10 and 12, only the border area of this part being the subject of the crimping operation having been shown.

The part 1 is supported by a fixed matrix 14 whose support plane 15 is horizontal. This horizontal support plan will be conventionally called in the rest of the description and in the claims "sheet plan". The lower sheet, which rests on the matrix 14 has a width greater than that of the other sheet 12; she presents a flange 11 folded at right angles to the rest of the sheet, and therefore disposed vertically. The initial folding of the flange 11 was carried out using a separate tool, by example in press. It is this rim 11 which must be folded back against the border zone 13 of the other sheet 12 to perform the assembly by crimping the two sheets.

The pre-crimping tool comprises a head 2 having a working face - or active face - flat 20, carried by a moving part formed by an arm 22 and a upper intermediate part 21. The arm 22 stands in a position close to the vertical, slightly tilted outwards from this vertical direction.

At its lower end, the arm 21 carries a first pin 24, by which it is articulated to a link 4 articulated on a fixed axis 40, that is to say carried by the frame of the unit (not shown in Figures 1 to 4). Part 21 carries a second pin 23 by which it is articulated on a control lever 5, of approximately shape triangular. The latter pivots freely around a fixed axis 50, and carries a journal 51 by which it is articulated on a control rod 52. The rod 52 is curved, and its opposite end carries a pin 53 by which it is connected to the eccentric and connecting rod control, which will be described later. Trunnion 53 is subjected to a back and forth movement along a circular arc path, with center 530. The low 4 and high 52 links are arranged towards the outside (relative to the matrix 14). The points corresponding to axes 23, 50, 40 and 24 constitute the angles of a quadrilateral fictitious deformable of which one side is materialized by the part 21, 22 carrying the head 2.

The different axes are positioned, and the lever arms chosen, such so that the movement of the active face 20 during the operation is done according to a substantially vertical direction, perpendicular to the plane 15.

This face 20 is in a plane inclined at 45 ° relative to the plane 15, when it is at the end of the working stroke (Figure 2).

The preserting operation results from the upward movement of the pin 53 around the point 530 (arrow I ₁ , FIG. 2), which causes the lever 5 to tilt downwards, around the point 50 (arrow I ₂ ), and the deformation of the quadrilateral mentioned above.

At the end of the race, the lines L1 and L2 which respectively connect the points 23 and 50 on the one hand, and 24 and 40 on the other hand, are both parallel to the bearing face 15 (sheet metal plane), c 'that is to say horizontal (Figure 2).

The flange 11, once folded at 45 °, is referenced 11 '.

FIG. 5A illustrates a presetting operation using a working head similar to that which has just been described, but whose working direction is not perpendicular to the plane of the sheet. This direction, symbolized by the arrow E , is oblique to the vertical, and breaks down into a vertical direction E ₁ (orthogonal to the sheet metal plane) and horizontal E ₂ (tangent to the sheet metal plane).

The component E ₂ has a parasitic effect, since it tends to excessively wind the rim 11 during its folding, the amplitude of the total winding being referenced i in FIG. 5A.

With the unit according to the invention, and as illustrated in FIG. 5B, the displacement of the head 2 takes place on the contrary in a purely vertical direction, perpendicular to the plane of the sheet, which reduces the amplitude of winding, the latter corresponding to a value j very substantially less than i .

The crimping tool shown schematically in Figures 3 and 4 comprises a working head 3 which is carried by an arm 31 approximately vertical and has an active lower face - working face - 30 whose function is to press the partially folded edge 11 'to fold it completely and bring it in contact with the edge zone 13 of the sheet 12, so as to ensure complete crimping from room 1.

The arm 31 has, on the inner side, a middle portion 32 lying in overhang below the active face 30, substantially perpendicular to the latter.

Like the arm 21, the arm 31 is provided at its lower end with a first pin 33, by which it is articulated on a link 6 which can pivot freely around a fixed axis 60.

Part 32 carries a second pin 34 by which it is articulated the end of a lever 7. It is a slightly bent lever, the opposite end (outer side) carries a pin 70. The latter is articulated on two rods 73, 74. The rod 73, approximately horizontal, can pivot freely around a fixed axis 730. The rod - or tie rod - 74, approximately vertical, extends upwards, and its opposite end (upper end) carries a pin 75.

This ensures its connection to the eccentric drive system and connecting rod, which will be described later. Trunnion 75 can follow a back-and-forth trajectory in an arc around the point 530 already mentioned with reference to Figures 1 and 2.

In its central (angled) zone, the lever 7 carries a pivot 71 hinged on a link 72 able to pivot freely around a fixed axis 720.

The lever 7 and the link 6 are approximately horizontal, directed towards outside.

The rod 72 extends downward (relative to the lever 7), and it has a approximately vertical direction.

It will be understood, on examining FIGS. 3 and 4, that the crimping tool 3 describes a complex movement, the trajectory of which is imposed by the semi-circular trajectories pins 70, 71 and 33 around axes 730, 720 and 60 respectively.

Before the operation, the pin 75 is in the low position, which has the effect to keep the head 3 tilted outwards, free with respect to the part 1 (figure 3).

The upward movement of the pin 75 (arrow J ₁ , FIG. 4), deforms the assembly of the connecting rods and levers mechanism and causes the head 3 to tilt towards the part 1, which ensures crimping (arrow J ₂ ).

According to an essential characteristic of the invention, this mechanism is arranged, and the various lever arms determined, so that the active face 30 follows, as soon as it comes into contact with the sheet edge 11 ', a trajectory curvilinear whose established center of rotation (in mobile position) is very close to the area folding (top of the folding angle) of the sheet, this face 30 moving further parallel to itself from the moment it came into contact with the sheet, until the end of crimping. The tangent to this trajectory is, at the end of the race, normal to the sheet metal (perpendicular to face 15). At the start of crimping, points 730, 70 and 71 are almost aligned; point 71 progresses slower and slower, while points 34 and 33 go down. The linkage produces a knee brace effect which develops an effort of very high pressing during work.

FIG. 6A represents a crimping head the pressing direction G of which is rectilinear and oblique with respect to the plane 15 of the sheet; it breaks down into an orthogonal component G ₁ and a tangential component G ₂ . This generates, during the operation, a parasitic force which tends to move the part, which compromises the quality of the crimping.

On the contrary, as shown in FIG. 6B, the crimping head 2 according to the invention follows a curvilinear trajectory H centered at a point C (with variable position) located in the zone Z where the bending angle of prison. At the end of crimping, this trajectory is perpendicular to the surface 15.

The crimped edge is referenced 11 ".

As seen in Figure 7, the different fixed axes are carried by a frame 8, which also supports a motor shaft 90, rotated - in a single direction symbolized by the arrow R - by means of an electric geared motor 9 .

The shaft 90 is connected, via an eccentric pin, to the axis 910 of a connecting rod 91. The opposite end of the latter carries a journal 92 by which it is articulated to a spreader 93. This is pivotally mounted around the fixed axis 530 already mentioned.

This lifter carries the pins 53 and 75 for driving the connecting rod control 52 (of the presetting tool) and of the tie rod 74 (of actuation of the crimping), respectively.

In the cycle start position, favorable for removing the crimped part and when a new part is put in place, the lifter 93 is in a position average angular ensuring the clearances of the two heads 2 and 3 (Figure 7).

This change of part takes place by suitable means not shown, manual or robotic.

As a result of the rotation R of the motor shaft 90, we first observe an elevation of the eccentric point 910, causing the pivoting of the lifter 93 along I ₁ (FIG. 8).

Thus operates the pre-setting ( I ₂ ).

This movement of the spreader bar has practically no influence on the crimping control mechanism, the head 3 remaining apart from the part.

The rotation of the shaft 90 continuing, the point 910 is lowered, which causes the lifter 93 to pivot in the opposite direction J ₁ . Thus the crimping J _{2 takes place} , while the head 2 is again moved away from the part (FIG. 9).

At the end of crimping, the lines M1 and M2 which connect, respectively, the points 34 and 71 on the one hand, and 33 and 60 on the other hand, are both parallel to the sheet metal plane 15, that is to say say horizontal (see Figure 4).

The gearmotor and eccentric control enables the pre-crimping and crimping in one complete turn of the motor shaft. Contrary to oscillating shaft systems (such as, in particular, that described in WO-93 05902), the stop precision at end of cycle therefore no longer matters, and has no effect on the quality and precision of work.

Advantageously, the journal 92 which connects the eccentric rod 91 to the lifter 93 is a frangible bar, capable of breaking (by shearing) if the effort of transmission exceeds a determined threshold. It is therefore calibrated to break if an effort exaggerated (and abnormal) is requested at the crankshaft. This is the case, for example, if a body stranger inadvertently interposed between work heads and the part to be crimped. This barrette therefore has a safety role.

Advantageously, there are provision for backlash systems, of the type known per se, for each of the pre-crimping and crimping mechanisms.

For pre-pressing, one can for example provide a system of eccentric play catch mounted at the pin 53 which connects the lifter to the lever 52. For crimping, a similar system can be provided at the pin level 70 for example.

To simplify the adjustment of the unit, a mark can be provided on the shaft of the gearmotor, and two marks on the frame (one for pre-setting, the other for crimping). Two reference faces are provided on the preserting mechanism, and two reference faces are provided on the crimping mechanism.

When the motor shaft is placed in correspondence with the reference mark pre-tightening, we adjust the backlash system so that the distance measured between the reference faces of the preserting mechanism reach the desired value.

The procedure is similar for crimping.

It will be noted, in particular by observing FIGS. 7 to 9, that the head of crimping 3 is located below the pre-crimping head 2. In addition, in the release (Figure 7), the head 3 remains close to the part 1, while the head 2 is frankly dismissed.

The overhang of the crimping head 3 on its arm 31 is therefore reduced, and its working stroke has a relatively small amplitude, which makes it possible to develop a high compression force during the crimping operation.

On the other hand, the presetting head 2 has a higher overhang on its arm 21, and its working stroke is of greater amplitude. It acts on the part with a lower pressure than the crimping head.

Such an arrangement is justified by the fact that the effort required to pre-crimping is significantly lower than that required for crimping.

Note that according to the teachings of WO-93 05902 already cited, the relative arrangement of crimping and pre-crimping tools is reversed compared to that provided here, which does not allow to operate in optimal conditions.

It goes without saying that such a unit can only work over a length limited sheet metal. To treat a very long sheet, and in particular its entire periphery, it is possible to juxtapose a set of units such as the one described above, which will be operated jointly and synchronously, to ensure crimping on a great length, for example over the entire perimeter of the sheet. Furthermore, when we have case of a curved sheet, it is possible to orient all or part of the angularly of these units in a different way, so that each working head comes to attack orthogonally the sheet in the area assigned to it. For this purpose the units are adjustable mounting on a frame.

Claims (15)

1. A unit for pre-hemming and hemming the edge (11) of a metal sheet (10) whose plane is substantially horizontal and which is supported by a stationary matrix (14), the unit comprising two moving heads, a pre-hemming head (2) and a hemming head (3), which heads are moved synchronously to come one after the other to press against the edge (11) of the metal sheet and to disengage therefrom, wherein the hemming head (3) during its pressing stroke describes a curvilinear path whose tangent at the end of hemming is substantially orthogonal to the plane of the metal sheet (10), its active face (30) remaining substantially parallel to itself during this displacement, the hemming head (3) being mounted at the top portion of an approximately vertical oscillating arm (31) whose bottom end carries a first pin (33) whereby it is hinged to a link (6) mounted to pivot freely about a fixed axis (60), and whose middle portion (32) carries a second pin (34), the unit being characterized by the fact that said second pin (34) is situated beneath and substantially vertically beneath the active face (30) of the hemming head and is hinged to a control lever (7), the control lever itself being hinged at one of its two ends to said second pin (34), at its other end to an "end" link (73) mounted to pivot freely about a fixed axis (730), and in a middle portion to a "central" link (72) likewise free to pivot about a fixed axis (720) such that the curvilinear trajectory described by the hemming head (3) during pressing possesses an instantaneous center of rotation (C) situated in the zone (Z) containing the fold angle of the metal sheet.
2. A unit according to claim 1, characterized by the fact that the displacement of the pre-hemming head (2) takes place during pressing along a direction that is approximately rectilinear, substantially orthogonal to the plane of the metal sheet (10).
3. A unit according to claim 1 or claim 2, characterized by the fact that both heads (2, 3) are driven by a common rotary shaft (90) rotating in a single direction (R) and via a crank and connecting rod mechanism (910, 91).
4. A unit according to claim 3, characterized by the fact that said rotary shaft (90) is driven by an electric motor (9).
5. A unit according to any preceding claim, characterized by the fact that at the end of hemming, the line M1 connecting the second pin (34) to the hinge axis (71) between the central link (72) and the control lever (7), and the line M2 connecting the first pin (33) to the fixed pivot axis (60) of the bottom link (6) are both parallel to the plane (15) of the metal sheet, i.e. horizontal.
6. A unit according to any one of claims 1 to 5, characterized by the fact that the hinge axis (70) between said control lever (7) and said end link (73) is carried by a pull rod (74) which is actuated by an oscillating rocker (93).
7. A unit according to claim 6 taken in combination with claim 3 or 4, characterized by the fact that said oscillating rocker (93) is hinged to the end of said driving connecting rod (91).
8. A unit according to claim 7, characterized by the fact that the hinge axis (92) connecting the driving connecting rod (91) to the rocker (93) is a frangible bar having a calibrated break threshold.
9. A unit according to any preceding claim, characterized by the fact that the pre-hemming head (2) is carried by an approximately vertical oscillating arm (22) whose bottom end carries a first pin (24) whereby it is hinged to a link (4) mounted to pivot freely about a fixed axis (40), while its top portion (21) carries a second pin (23) hinged to a control lever (5).
10. A unit according to claim 9, characterized by the fact that said control lever (5) pivots about a fixed axis (50) and is hinged to a control link (52).
11. A unit according to claim 10, characterized by the fact that at the end of pre-hemming, the line L1 connecting said second pin (23) to the fixed axis (50) of the control link (5), and the line L2 connecting the first pin (24) to the fixed pivot axis (40) of the bottom link (4) are both parallel to the plane (15) of the metal sheet, i.e. horizontal.
12. A unit according to claim 10 taken in combination with claim 6, 7, or 8, characterized by the fact that said control link (52) is hinged to said oscillating rocker (93).
13. A unit according to any one of claims 9 to 12, characterized by the fact that said arms (22, 31) are disposed more or less vertically, and that the hemming head (3) is situated beneath the pre-hemming head (2).
14. A pre-hemming and hemming assembly for long pieces of metal sheet, the assembly comprising a plurality of juxtaposed units according to any preceding claim.
15. An assembly according to claim 14, for curved metal sheet, comprising units according to claim 2, at least some of which are in angular positions that are offset from those of the others so that the travel directions of the various pre-hemming and hemming heads at the end of the operation are all substantially orthogonal to the surface of the metal sheet.

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