WO1997012716A1 - Work table for joining flat products - Google Patents

Abstract

A work processing table for joining or welding of sheet material, especially for laser fusion welding of metal blanks into a tailored blank assembly, such as e.g. automotive parts or body panels, comprises a vacuum platen/table plate structure (1, 2) with at least two separate surface suction areas (6, 10) for holding a set of adjoining blanks. Said suction areas extend continuously along the opposite walls of a bond processing or a welding trench (5) and are spaced therefrom by blank edge supporting rim areas (8, 9). Advantageously, the welding table contains retractable positioning means (12, 22) mounted integral with the table structure. Said means allow quick and accurate positioning of the blanks relative to the bonding or welding track without interfering with the operative bonding means such as a welding beam and its access area, nor with the blank loading operation.

Description

WORK TABLE FOR JOINING FLAT PRODUCTS

The invention pertains generally to a work table for holding flat products, such as sheet material, in an adjusted position allowing to join the opposite edges of adjacent sheets in a reliable manner. Usually, but not exclusively, said edge joining operation involves thermal bonding, particularly welding of closely positioned metal blanks. More in particular, the invention relates to a work processing table which is connected to a vacuum source and especially adapted for positioning and holding metal blanks to be joined by laser welding into tailored panels, such as automotive body members. The present invention also pertains to a laser welding process and unit including such a welding table.

Because of the increasing emphasis on producing lower weight automobiles in order, among other things, to conserve energy, considerable effort has been directed toward developing weight saving structural members and body panels of rolled steel in car design and construction. Laser welding is a most suitable technology for producing tailored " panels combining steel blanks of predetermined shapes in possibly different thicrkness and strength grades, thereby allowing substantial weight reduction. Commonly the fabrication of car body panels, in particular when intricate shapes are needed, involves a series of stamping and pressing operations on heavy and bulky press machinery, which is an expensive investment that calls for large scale mass production. Making particular panel shapes by fusion weld joining of tailored blanks and preferably by laser welding, on the contrary, offers more flexibility in serial output, is less demanding in expensive equipment and space, and moreover allows to reduce the waste in sheet material. (scrap losses) to a considerable extent. Conventional work holding tables presently used in laser welding installations require cumbersome hydraulic, pneumatic and/or mechanical means for tightly clamping and accurately positioning the blanks. After each welding cycle a locating/positioning and clamping operation of a new set of blanks is needed which is rather time consuming compared to the duration of the actual welding cycle. Also, it is desirable that the weld lines are of excellent and uniform quality so as to allow the welded part to sustain subsequent forming operations (bending, stretching, drawing...) without cracking or tearing. In this connection accurate positioning of the blanks, and maintaining the facing edges to be joined in a equidistant relationship, also vertically, is most important. The latter requirements are difficult to fulfil with common work tables, in particular when large-sized blanks have to be supported and fixed.

Accordingly, it would be advantageous to provide an improved work table suitable for joining flat workpieces accurately and at a faster pace. More in particular it would be advantageous to provide a work table for weld bonding of metal blanks, especially by laser welding , which enables to overcome the aforementioned problems and restrictions of the prior art.

In accordance with the present invention the above problems are solved by a work table containing the technical features as defined in the appended claims. The novel table is particularly suitable for automated laser welding of tailored metal blanks and comprises a localizable base plate including at least one vacuum platen connected to a suction source, said vacuum platen being integral with said base plate or detachably disposed therein, whereby the upper planar surface of the platen features at least two sets of depressions connected each separately to said vacuum source. Located between but spaced apart from said depressions, a continuous groove (trench) of appropriate depth and width follows the length and curvature matching the geometrical form of a desired blank joining weld line. The platen surface not covered by the welding groove and the suction depressions form the actual blank support areas The opposed rim areas surrounding the welding groove sustain the blank edges hanging over the groove side wall. Preferably the platen and/or base plate is provided with positioning means, adapted to perform a fine positioning of the blank vis-a-vis the location of the welding groove. Most preferably, said positioning means are integral with said table plate and platen, wherein they define a retracted position which allows free access of the welding area by a laser beam. The work table usually includes a movable frame for supporting the base plate, which is generally in the form of a slide structure allowing to move the table base plate in X,

Y directions to adjust the desired location of the same relative to a laser welding head, and to a blank loading area serviced by a robot loader.

The vacuum platen may comprise a groove and depression array which can accommodate blanks of different cross section, for instance circular blanks of different diameter.

For mass production, however, a platen adapted to one given blank shape/welding job is more convenient, because it does not require complicated valve and interior closing means to shut off part of the active suction area depressions. Preferably the positioning means comprise positioning elements which are slidably received in the suction platen and actuatable by actuation means fixed preferably below the base plate or on a bearing frame thereof.

The invention will now be described in more detail with reference to the following drawings exemplifying embodiments of a work table adapted for circular weld lines. It is to be understood that the present invention is not restricted to the embodiments illustrated in the drawings. In fact various adaptations and modifications may be made therein without departing from the basic inventive concept and the scope of the invention as defined by the appended claims.

Fig.1 gives an overall pictorial view of a laser welding installation. Fig.2 illustrates a slide-supported work table according to the invention, including here two vacuum platens for circular welds. Fig.3 shows a radial cross section, perpendicular to the platen surface plane, of a platen according to the invention having therein incorporated positioning elements. . Fig.4 gives a view from above of a work table embodiment as in fig.2, including details of blank positioning means. Fig^'.5 shows the work table of fig.3 with suction line connections.

Fig.6a-b shows a detail of the suction/supporting surface areas of the platen according to the invention. Fig.7a-b shows details of the drive and positioning means of the work table.

Fig.8 illustrates a vacuum platen of the invention suitable for welding circular blanks of variable diameter.

Referring now to fig.1 , there is shown a schematic overall picture of a laser welding installation comprising usually the main components as depicted and defined in the drawing. The present invention concerns more particularly the part of the installation comprising the turntable and laser processing machine. Of course the invention is not restricted to the use of the illustrated lasing environment.

In the shown example the turntable will carry a welding table unit according to the invention at the locations denoted as fixtures A and B (in fig.1 ) which correspond to a welding location, respectively to a loading/unloading position in front of a work robot.

Fig.2 illustrates a work table ( 1 ) with two suction platens 2, supported on sliding support members 3 and 4. Reference numeral 5 indicates the welding groove area. An annular depression 6 of shallow depth in use is connected to a suction line. Circular central area 7 and interior and exterior annular bridging rim areas 8,9 are flush with the table upper surface plane and define therein supporting surfaces for the blanks, whereas depression zones such as 6 define vacuum holding/gripping areas of the platen 2. Outside the exterior rim zone 9 there is provided a depression area 1 0 which surrounds the circular platen section to a desired lateral extent. In the illustrated embodiment a rectangular metal blank, for example steel, sheet, having two circular openings is to be disposed on the welding table. Furthermore two circular blanks of a given steel grade and desired thickness, for example larger than the thickness of the rectangular blank, have been positioned over the centre platens 2. Their position should accurately fit within said openings such that the adjacent concentric edges of the mating blanks remain about equidistant circumferentially and close to one another above annular groove 54. In this adjusted position the blanks are very tightly suction held by actuating suitable valves in the vacuum source lines connected to suction zones 6 and 1 0. The laser welding cycle causes melt joining of the adjacent circular blank edges by means of a laser welding head following a corresponding circular path with controlled side oscillation of the welding beam, which oscillation amplitude covers the edges of the mating blanks to a desired extent, thereby producing a clean weld seam line and an intimate metallurgical bond. After finishing the laser welding cycle the suction pressure is released and the welded 'tailored' panel can be immediately unloaded and replaced by another set of blanks. In the installation according to fig.1 the turntable will comprise at least two of such welding tables 1 (fixture A and B in fig.1 ), and a robot will take care of unloading and loading after each half turn of the turn table. Thus the loading and accurate positioning of a new set of blanks may take place during the welding cycle of the set being on the operative part of the turn table under the lasing head.

Fig.4 shows a preferred embodiment of a twin-platen work table comprising therein integrally mounted positioning means to adjust both the location of a rectangular blank and of two circular blanks loaded onto the central suction platens

Fig.3 gives an enlarged view of a radial cross-section of platen 2 of fig.2. It shows an embodiment wherein the circular centre part of the suction platen is provided with positioning means 1 1 to precisely locate a circular blank on the platen surface such that the blank edges in welding position extend diametrically from about half the width of groove 5 to half of the radially opposed part 5' of said annular groove. In this embodiment a vacuum source line (not represented) is connected to a plenum chamber 1 8 provided on the lower side of the platen. The vacuum underpressure is transmitted to the upper surface suction depressions 6 by one or more conduits 1 3 arranged around the central support area 3. In a simplified version the plenum may be omitted and the vacuum suction line is then directly connected to conduit 1 3 by means of a suitable threaded seal connection.

Turning now to positioning means 1 1 , it is to be understood that said means are designed to adjust the position of a circular blank on a platen having a circular central zone delimited by a circular welding groove concentric with central support area 3. As can be seen in the drawing, said means are arranged below the platen and integral therewith, without interfering with the actual welding operation carried out from above the blanks covering the upper surface of the platen 2. In the shown embodiment the positioning means 1 1 comprise preferably three positioning pins 1 2 slidably guided in bore holes passing through the platen body, whereby the pin bodies reach into the annular welding groove 5,5' in a partly overlapping relationship.

The rodlike pinbodies are fixedly disposed onto a movable supporting bridge member 13 which itself is actuatably sustained by jack means 15 that is mounted on a fixed frame member 14 extending below platen 2. The positioning pins 1 2 are designed to have a truncated conical head portion 16. The base of said conical portion defines the desired limit position of the blank circumference in relation to the width of welding groove 5. The position means are operated as follows : first the jack is actuated to push bridge 1 3 upwards so that conical head 1 6 of pins 1 2 is moved to a start position above the upper surface of platen 2. Then a loading system, e.g. a robot, puts a circular blank on the platen. Since a loading robot has a limited load/positioning precision, the circular blank will not yet lie exactly concentric so as to cover about half the width of the welding groove 5. Hence the blank will hang above the platen between the conical head surfaces of the lifted positioning pins. Then the pins are lowered and preferably vibrated, e.g. by proper choice of jack conditions, in advance or simultaneously with retracting of the pins. As a result, the circular blank positions itself exactly toward the lower base line of the conical head 1 6 and is put in an accurately adjusted position upon the platen support areas 7 and 8 in relation to concentric groove 5. When the pins are lowered, a suction valve is actuated and the blank is tightly stuck to the platen in a precise position. Thereafter a second blank having circular holes is placed over the already positioned circular blanks such that the hole circumference comes into close encounter with the exterior edge of the circular blank at about half the width of welding groove 5. The loaded second blank may be adjusted in precise relationship to the circular blank position by means of a second positioning means as explained herein below with reference to fig.4. Illustrated therein is a preferred embodiment of a work table for laser welding of circular blanks into a rectangular blank having circular holes; said table embodying preferred means for positioning a rectangular blank precisely relative to circular platen sections.

In fig.4 a work table is provided with location abutments or studs 21 ,t which define a first limit position of the rectangular blank to be positioned thereagainst. Studs 21 ' define a second limit position.

The circumference of the blank is illustrated as broken line 20. Common pushing means 23,23' and 24 mounted on the work table will displace a loaded blank in its desired position. To make this possible with minor function and without interfering with the circular blanks already suction held on the platens 2, the lower side of the table is equipped with a movable suspension bridge 22 actuated by a jack means 25. The bridge frame comprises longitudinal elements 26 and cross-bars 27, which are guided in U- shaped members 28 fixed to the lower surface of the work table. Pin elements 29 (see fig.7 for more details) are spring seated in the work table body and slidably received in a bore thereof to be movable vertically. In rest (seated) position and during welding the heads of said pin elements are flush with or below the upper plane of the work table. For loading and positioning a second blank, the pins are pushed upwards by the action of cam surface portions 29' provided at suitable locations on the contact surface of the bridge members 26,27 (see also fig.7). Indeed, when jack 25 is actuated, it moves the bridge frame 22 (to the left in fig.4) such that cam element 29' come into registry with a foot portion of spring seated pin 29 which are thereby lifted. In the raised position the pin heads are extending sufficiently above the table surface so that they can receive the loaded blank. In this free floating position upon pins 29, the blank is moved by jack means 23 and 24 in two steps, respectively against position abutments 21 and against abutment 21 '. Thereafter the bridge frame 22 is returned to its initial position so that the cams will release the pins 29 which are spring retracted to below the table upper surface of the work table.

Then the vacuum line of the suction depression zones 10 is activated and a suction pressure transmitted via conduits 1 7 will tightly pull the blank onto the table contact surface areas. Hence these positioning steps definitively adjust the circular blank holes such that the edges thereof meet almost the facing edges of the circular blanks in a close concentric relationship, already located on the centre platens. A reliable laser welding operation can thus be carried out to metallurgically bond the adjoining edges, precisely positioned and firmly/uniformly suction held above the welding groove 5. Since the blank suction holding force is very strong; and uniform over the circumference, and the blank surface is furthermore evenly supported, bending of the blanks and vertical distortion of blank edges during laser fusion bonding is entirely prevented. As a consequence, the welding table according to the invention enables to perform superior weld line qualities in a consistent manner.

Fig.5 illustrates the vacuum line connections with the different platen suction zones. In the exemplified embodiment two separate lines 32 and 33 deliver vacuum underpressure to central annular suction zones 6 (circular blanks), respectively. to surrounding suction zones 10 (holding of rectangular blank with two holes). An automatic valve 36, resp. 37 in the supply line (38,39) of each suction circuit allows to actuate the suction circuit or to release the suction pressure according to a desired process sequence. In the shown example the main supply lines are connected to a collector plenum of a vacuum pump system (not shown). Here the collector 31 is mounted on the turntable 30 of a laser welding installation (cfr fig.1 ) as is also the case of the work table 1 . In this circumstance the collector will have a rotary connection with the conduit of the vacuum pumping system.

Fig.6-a shows a top plan view of the central platen area. The positioning pins 1 2 (see also fig.3) are located partially within the annular welding groove area 5. The welding groove, or generally sheet edge bonding/processing trench, may further include a separate bottom perforation communicating with the platen underside (not shown in the figure). This provision allows the release of possible gas overpressure in the trench cavity resulting from the bonding or welding operation and hence avoids bonding/welding defects which could otherwise occur by too high a pressure difference between lower and upper side of the bonding/welding seam.

Fig.6-b shows a plan view of a central platen seen from below the platen, illustrating a movable positioning bridge ( 1 3,40) and a mounting frame ( 14,41 ) of driving means

(1 5), e.g. a pneumatic cylinder which bears and displaces said bridge. Positioning pins 1 2 are slidably guided in a bushing chamber 42 between the platen underside and the bottom of the welding trench (5).

Fig.7-a and 7-b illustrate a circumferential portion of a work table 1 in a longitudinal cross section cutting through part of a preferred positioning means for adjusting the location of a second blank (relative to a positioned first blank)

Fig.7-a, to be seen in combination with fig.4, shows the working principle of the supporting pins 29, which in the drawing are actuated by cam element 29' forming a protruding part on traverse member 27 of bridge frame 22 (cfr fig.4). In fig.7-b a detail is shown of a blank supporting/lifting pin 29 of a longitudinal bridge frame member 26 in seated, respectively in lifted position.

Fig.8 shows a top plan view of a multi-groove suction platen comprising a concentric pattern of welding trenches in the centre of a rectangular table plate. Successive joining/welding trenches (5,5'5 " ,...) define concentric circular paths of joining/welding lines between round blanks and overlaying blanks with an opening of matching diameter. Outside the central platen rings there are provided successive suction zones d ό', 10" ,...) of a dimension in accordance with the actual size of an overlaying blank. As illustrated by the accompanying enlarged radial cross-section, the blank suction surfaces (6', 6" ,...) for tightly holding adjoining blank edge portions closely to one another, are confined to the concentric annular areas shaped as shallow surface slots between the side walls of deeper joining/welding trenches. The upper surface of facing sidewalls define annular contacting rims (8',8",...) for supporting the blank edge portions overlapping the trench cavities. Reference numerals (50', 50",...) denote conduits for communicating the suction rings to a vacuum source or a vacuum plenum chamber below the platen underside. Said conduits preferably have a threaded bore (52) ending into a larger upper cavity (51 ) formed in the bottom surface of said suction slots. This configuration allows selective sealing of suction zones by meas of screwable bolt elements. If desired, the welding trench cavities may be connected with the ambient by means of grooves (55) cutting radially through the concentric ring pattern. In this case the suction zones are divided into ring segments interrupted by said grooves 55.

It will be obvious that the exemplified work table and platen, according to the invention, may be modified and adapted to make them suitable for particular blank shapes and forms of the tailored weld line connections. Also the table can be constructed as a monolithic piece, or in the form of an assembly. The central platens, for instance, can be designed as exchangeable parts of the table, furthermore a platen may be provided in accordance with the invention to accommodate a plurality of blank dimensions. In this case the particular combinations of welding groove and vacuum suction, resp. support areas of such a multipurpose platen, e.g. for welding circular blanks of different diameter, will have to be provided with adaptable exchangeable interior seals and suitable conduits and valves and line circuitry. An example of such a platen is illustrated in fig.8 without the preferred integral positioning means.

Claims

1 . A work table for welding, especially for laser fusion welding of metal blanks into a tailored panel of desired configuration, comprising a generally positionable support structure carrying a table base plate ( 1 ) including at least one blank holding platen (2), characterized in that the holding platen is a vacuum platen incorporating therein the combination of :

(a) a trench (5) of suitable depth and width extending lengthwise in the upper surface of the platen along a curvilinear path corresponding with the path of a desired weld bonding line between adjacent blanks, said trench path defining a welding beam receiving groove or slot for co-fusion of blank edge portions adjoining one another in a closely facing relationship within the width area of said welding groove (5),

(b) blank holding areas by suction in the form of shallow surface depressions (6, 10) laterally spaced apart from the opposed trench edges by blank-supporting rim areas (8) and (9), said depression areas extending along the full length of trench (5) and covering a portion of the platen surface area which is sufficiently large to firmly hold a blank positioned thereupon, with the proviso that depression zones (6) and (10), belonging to platen surface areas along opposed sides of trench (5), are individually connected to a vacuum source by means of a distinct and separately actuatable suction line for each suction area.

2. A work table according to claim 1 , characterized in that it further comprises blank positioning means integrally linked with said base plate (1 ) and/or holding platen (2), said means including a plurality of pin elements slidably received in said holding platen, and adapted to be moved up and down between a raised position for receiving a loaded blank at a given height above the upper surface of the platen, and a lowered position wherein the blank is contacting the platen and the positioning pin is retracted to a position flush with or below the supporting surface plane of the platen.

3. A work table according to claims 1 or 2 , characterized in that suction pressure is transmitted to a blank holding depression zone (6) or (10) by means of either a plenum chamber attached to the lower side of the platen and one or more vertical channels drilled through the platen thickness from plenum level to depression bottom, or by means of a single channelling hole between depression bottom and platen bottom, the lower part of said single channel being adapted to form a threadable connection with a vacuum line.

4. A work table according to claims 1 to 3, characterized in that suction/depression zone (6, 10) comprises intermediate blank contacting elevations flush with the upper plane of the platen, and so disposed therein such that vacuum pressure transmission remains effective within the whole of a suction holding area.

5. A work table according to claims 1 to 4, characterized in that the holding platen comprises a welding trench forming a closed path of predetermined shape, such as a circle, a square, a triangle and the like, for welding a first blank into an accordingly shaped opening of a second blank.

6. A work table according to claim 2 and 5, for welding a first blank into a matching opening of an overlaying second blank, including first and second positioning means for separately adjusting said first inner blank and said second overlaying blank, wherein said means comprise first retractable pin elements for precisely adjusting the location of said first blank within an area surrounded by said pin elements, placed along a closed path of a welding trench, and said second positioning means for adjusting the location of said second overlaying blank, including second liftable pin elements received in an exterior region of the table plate to carry said second blank in a suspended position, and further abutment elements defining a positioning stop for the second blank while moving the same in the suspended state.

7. A work table according to claims 1 to 6, wherein said vacuum platen (2) contains a plurality of welding trenches (5), adapted for laser fusion bonding of blanks different in shape on a same platen.

8. A work table according to claim 7, wherein said trenches form an array of closed paths whereby adjacent paths are equidistantly disposed into one another such as e.g. concentric rings or other geometrical forms.

9. A work table according to claims 1 to 8, wherein at least the welding trench area is made from a metallic material having preferably an elevated thermal conductivity such as copper, copper allays, aluminium and aluminium alloys, optionally including forced cooling means.

10. A work table according to claim 9, wherein the table plate and platen constitute a monolithic piece made from aluminium or an aluminium alloy.

1 1 . A work table according to claim 1 adapted for receiving a generally rectangular blank with circular openings and blanks of circular cross-section fitting into said openings, and further for closely positioning said blanks relative to one another and tightly holding them so as to enable laser fusion welding the circular blanks in said openings, said table being composed of a monolithic planar table plate/vacuum platen structure with annexed, especially integral blank positioning means, said plate/platen structure comprising:

- one or more circular suction regions (6) separated from a surrounding outer suction region (10) by an U-shaped trench (5) with facing concentric blank edge supporting ring areas (8,9) of suitable width, whereby a central suction region (6) is in the form of an annular depression area enclosing a central support region (7), said suction regions (6) and (10) being shaped as shallow platen surface depressions having a planar bottom surface including each at least one opening (13), respectively (1 7), communicating with the lower platen surface to connect each of said suction regions with a separate suction line (32), respectively (33) for each region, said blank contacting surface areas (7,8,9) being flush with the upper plane of the remainder of the table plate (1 ) enclosing platen suction regions (10), and

- said blank positioning means including first positioning means (1 1 ) to precisely adjust the location of a circular blank, loaded onto a central platen region, relative to a welding trench (5), wherein said first means comprise pin elements ( 12) slidably received in a bushing (42) of plate (1 ) and mounted on a movable suspension bridge (1 3) which is supported and actuated by a lower jack means ( 1 5) fixed onto a lower frame (14) that is affixed to the lower side of the plate (1 ), said pin elements (12) penetrating into the welding trench (5) and being disposed circumferentially thereto such that a most inner edge line of the pin stem coincides about with a circle at half the width of the welding trench, and second positioning means to adjust the position of an overlaying blank so as to precisely locate the circular openings thereof relative to said positioned circular blanks.

1 2. A work table according to claim 1 1 , wherein said second positioning means comprise a plurality of pin elements (29) slidably received in table plate ( 1 ) and mounted therein with a lower spring seat for retracting the pin to a rest position wherein the pin head surface is flush with or below the upper surface of the table plate, said pins having a bottom foot part extending out of the lower plane of the table plate, which pin foot is displaced upwards when in registry with camlike projections (29') provided on transverse and longitudinal members of a frame (22,27,26) movably suspended below the table plate in suspension guides (28), and also jack means (25) to move said frame, and further comprising pushing means (23, 23', 24) to move said blank, when loaded onto said positioning pins (29), which are then in a raised position above the table plate, towards an accurate final position defined by abutment elements (21 ,21 ') mounted on the upper side of the table at precisely adjustable positions relative to said trenches (5).

1 3. A laser welding unit for laser fusion assembling metal blanks into a desired tailored panel, comprising a welding table as defined in any one of claims 1 to 1 2, said table being operatively connected to a master control panel of said welding unit to actuate positioning means and vacuum suction means of said table prior to laser fusion welding the^'vacuum held blanks.

14. A laser welding installation, including a laser processing machine with a laser focusing head which is movable along three mutually perpendicular axes (X,Y,Z), a laser oscillator, a loading/unloading device of the robot type, a control unit including a master operation panel, an oscillator control panel and a welding monitoring controller,and further a turntable at least partly turntable under said laser processing machine and partly extending into a region of said loading/unloading robot, wherein said turntable is provided with a work processing/welding table as defined in claims 1 to 12 for automated laser fusion joining of blanks loaded onto said table.

1 5. Use of a work processing table as defined in claims 1 to 1 2 in a method of manufacturing a tailored blank from separate blanks, wherein said method includes the steps of loading the blanks onto said table upon which said blanks adopt a first loading location precision, then actuating positioning means of said table capable of closely positioning said blanks relative to one another so that adjoining edge portions thereof meet each other above a welding trench defined in said table surface, actuating vacuum lines connected with corresponding blank holding areas of said table to firmly grip the positioned blank, subsequently fusion welding said adjoining blank edge portions by means of an energy beam such as a laser beam moved along said welding trench and thereafter releasing vacuum holding pressure and unloading the welded blank structure.

1 6. A work table for joining flat sheet materials comprising at least one vacuum platen adapted for suction holding at least two sheet pieces, characterized in that the platen surface comprises at least two distinct, spaced apart suction zones having a separate connection with a vacuum source, said suction zones forming shallow depressions in the platen surface extending at opposite sides of a dividing trench having a depth and cross section suitable for enabling a bonding operation to be carried out on adjoining and/or overlapping side edges of adjacent sheets, positioned and suction-held on the platen, said trench having a length and a form matching at least part of a bonding line between said sheets, said platen preferably further comprising, integral thereto, sheet positioning means in the form of rod or pin shaped supporting members, slidably received in the platen body and movable therein, between a position above and below the platen upper surface, by driving means attached to the lower side of the platen and/or table.