Background art In many situations, there is a need to provide indi- cative markings on a product, for example, traceability markings indicating the origin of the product, or pro- motional marks. Such indicative markings can be provided by non-mechanical, i. e. non-contact, marking techniques, such as laser engraving or ink jet printing. These non- contact techniques provide for flexibility in production and high production speeds. Often, the markings must be precisely located in a specific area of the product. To maintain a positional relationship between the markings and this specific area, the markings should be provided immediately before or after the processing step in which this area attains its final shape. Such a processing step could include stamping and/or punching of a target mate- rial to form an intermediate, or even a final product.

A conventional laser engraving system comprises a laser unit, which is fixedly mounted to a supporting unit and arranged to provide markings on a target in a marking area. This marking area is often, for reasons given above, located in the immediate vicinity of. a processing apparatus. Normally, to eliminate any movements of the laser unit during the marking operation, the supporting unit is of heavy-weight construction and firmly secured to the ground by bolts or the like. In such an arrange- ment, when any adjustment or maintenance of the process- ing apparatus should be made, the laser unit and support- ing unit must be disassembled and removed, and then

reassembled again before restarting the processing apparatus. This is a time-consuming operation.

Summary of the Invention The object of the invention is to find a solution to the above described problems.

This object is achieved by an arrangement according to appended claim 1, preferred embodiments being defined in the dependent claims.

By making the supporting unit slidable between the two positions, the arrangement of the invention allows for accurate alignment of the marking unit with the mark- ing area adjacent to the processing apparatus, during operation thereof, while also providing immediate and complete access to the marking unit and the processing apparatus since the marking and supporting units can be readily moved away from the processing apparatus. Thus, the time-consuming operation of disassembling the marking unit and the supporting unit, for example for maintenan- ce, is eliminated.

The arrangement according to the invention is parti- cularly useful in the manufacture of marked opening tabs to be attached to ends for cans. In this embodiment, the arrangement comprises a processing apparatus which is adapted to punch and stamp the strip to form, the opening tabs. A laser unit is arranged upstream the processing apparatus and is adapted to provide markings on the strip in the marking area adjacent to the processing apparatus, such that the opening tabs after being formed in the processing apparatus have markings on a tab surface. The laser unit is supported by a supporting member which is slidable between a first position in which the laser unit is aligned with the marking area, and a second position in which the laser and supporting units are spaced from the marking area and the processing apparatus. This arrangement allows for high-precision marking, while also providing the operator immediate and complete access to the processing apparatus and the laser unit for mainten-

ance or the like. Preferably, the laser unit is adapted to engrave the markings in the strip surface. Such an engraving operation provides for permanence of the mark- ings, high throughput, high precision in position and layout of the markings, as well as flexibility in the layout and position of the markings on the strip.

Brief Description of the Drawings A presently preferred embodiment of the invention will now be described in more detail, reference being made to the accompanying schematic drawings.

Fig. 1 is a side view of an arrangement according to an embodiment of the invention.

Fig. 2a is a top view of the arrangement in Fig. 1.

Fig. 2b is an end view of the arrangement in Fig. 1 taken in the feeding direction of the strip towards the supporting member and the processing apparatus.

Fig. 3 is a plan view of a portion of a metal strip provided with indicative markings.

Fig. 4 is a bottom view of a tab having markings on its bottom surface.

Description of Preferred Embodiments Fig. 1 shows part of an arrangement for manufacture of marked tabs or opening rings to be included in ends for beverage cans (not shown). A blank in the form of a thin, continuous metal strip S is fed from a supply 1 to a laser unit 2 supported by a supporting member 3. The laser unit 2 is of a high-power and high-speed type and is capable of providing engravings or markings in the surface of the strip S. The laser unit 2 is adapted to generate laser radiation at a suitable wavelength, and to focus and direct the generated radiation to an engraving area (indicated with dotted, thick lines in Fig. 2a) on the surface of the strip S. After the engraving operation in the engraving area, the strip S is fed to an adjacent tab forming apparatus 4 which is of a type known per se and which forms tabs by punching and stamping the strip S (see for instance the pamphlet"This is PLM Fosie"issued

by Applicant's company PLM Fosie AB in the mid nineties).

When passing the laser unit 2, the strip S is guided by a guiding unit 5 secured to the supporting member 3.

Further, a dust protection device 6 is connected to the laser unit 2 to protect the laser unit 2 from dust or debris produced during the engraving operation.

In the embodiment shown in Fig. 1, the arrangement includes a second laser unit 2'which is arranged oppo- site to the laser unit 2 and is adapted to provide mark- ings on an opposite side of the strip S. The following description of the laser unit 2, and its connection to the supporting member 3, is equally applicable to the second laser unit 2'and will therefore not be repeated.

The supporting member 3 comprises a horisontally extending base portion 7 and a vertical column 8, to which the laser unit 2 and the guiding unit 5 are connec- ted. A foundation 9 is secured to the ground G, for example by means of bolts or the like (not shown). The base portion 7 is connected to the foundation 9 by means of cooperating elongate elements forming a rail-like structure 10, so that the supporting member 3 is slidable with minimum friction between two limiting positions A, B. These limiting positions are, as indicated in the top view of Fig. 2a; an operating position A in which the laser unit 2 (not shown in Fig. 2a) is aligned with the engraving area adjacent to the tab forming apparatus 4; and a maintenance position B in which the laser unit 2 as well as the supporting member 3 are arranged at a distan- ce from the engraving area and the tab forming apparatus 4, so as to provide an operator complete access to the laser unit 2 as well as to the tab forming apparatus 4.

The rail-like structure 10 will guide the supporting unit 3 in a controlled path between the operating and mainten- ance positions A, B, and allows for displacement of the supporting unit 3, if desired by hand, even when carrying a heavy load. To this end, it is preferred that the rail- like structure is formed by two parallel linear bearings.

In the illustrated embodiment, the supporting unit 3 and the associated equipment weighs at least 900 kg.

Each limiting position A, B is defined by the base portion 7 engaging a stop plate 11 (not shown in Fig. 1) which projects above the foundation 9 at a respective end thereof. The operator can lock the supporting member 3 in the operating position A by engaging screws 11'with the base portion 7 via through-holes (not shown) in the asso- ciated stop plate 11. Although not shown, the associated stop plate 11 and the base portion 7 could have cooperat- ing elements, such as a V-shaped block and a correspond- ingly V-shaped recess, to accurately locate the support- ing member 3 in the desired operating position A. Prefe- rably, the supporting member 3 can be returned to the operating position A with an accuracy of at least 0,1 mm.

A hydraulic shock absorber 12 is mounted on the stop plate 11 and provided to suitably decelerate the support- ing member 3 before it engages with the stop plate 11 at the limiting positions A, B, to eliminate any undesired thrusts in the supporting member 3 on reaching the limiting positions A, B.

Since the tab forming apparatus 4 operates by stamp- ing and punching the strip S, it will generate signifi- cant vibrations, often of relatively low frequency. If such vibrations were transmitted from the tab forming apparatus 4 to the supporting member 3 they might lead to undesired variations or changes in the position or align- ment of the laser unit 2, the guiding unit 3, or any other associated equipment. To attenuate any such vibra- tions, the foundation 9 includes a vibration isolator 13, preferably of the type used in for example industrial measuring equipment. Further, the supporting member 3 is at least partly hollow and defines an interior space 14.

By filling this space to a given degree by a filler material, for example metal balls, the eigenfrequencies of the supporting member 3 and the associated equipment

can be adjusted away from the major frequencies of the vibrations generated by the tab forming apparatus 4.

The tab surface available for the markings is very small. Therefore, the laser radiation must be accurately positioned on the strip S, and the strip S must be also be accurately positioned during the laser engraving operation. Consequently, the laser unit 2 and the guiding unit 5 must be accurately positioned with respect to the engraving area adjacent to the tab forming apparatus 4.

To this end, there is provided for fine adjustment in the position of these units 2,5.

The guiding unit 5 is connected to the vertical column 8 via a two-dimensional positioner 15, which allows for fine adjustment of the local position of the guiding unit 5 in two orthogonal directions in the plane of the strip S. Thereby, the guiding unit 5 can be brought into perfect alignment with the tab forming apparatus 4, to minimize the forces acting on the strip S. Any slight mutual misalignment of the guiding unit 5 and the tab forming apparatus 4, especially in the late- ral direction of the strip S, might bring about jamming, or even deformation, of the strip S. In an alternative embodiment, not shown, the positioner 15 provides for adjustment of the guiding unit 5 also in the vertical direction, to compensate for any settlement in the supporting member 3 or foundation 9.

The laser unit 2 is connected to the vertical column 8 via a three-dimensional positioner 16, which allows for fine adjustment of the local position of the laser unit 2 in the lateral and longitudinal directions of the strip S as well as in the vertical direction. The adjustment in the vertical direction allows the focal region of the laser unit 2 to be brought on a level with the engraving area, i. e. the surface of the strip S.

For acceptable results in the illustrated arrange- ment for marking and shaping opening tabs, the position of the laser unit 2 and the guiding unit 5 should be

adjustable relative to the supporting member 3 with an accuracy of at least 0,02 mm, preferably at least 0,01 mm.

Fig 3 shows a portion of the strip S after the laser engraving operation at the laser unit 2, but before the punching and stamping operation in the tab forming apparatus 4. The laser unit 2 has provided the engraved markings WIN, A, 8,9 on one surface of the strip S. The approximate periphery of the tab T to be produced in the following tab forming steps in the apparatus 4 has been indicated with ghost lines in Fig. 3. In practice, the size of the laser engraved markings is about 1.4 x 1.2 mm which makes them easily readable. In Fig. 4, the final tab T is shown from below after processing in the tab forming apparatus 4. The markings WIN, A, 9,8 are now precisely located on a respective tab surface, in this case intermediate an opening T1 and peripheral edge portions T2, T3, T4 of the tab T.

It should be appreciated that the movement of the supporting unit 3 and/or the positioners 15,16 could be electrically controlled, by means of electric motors or stepper motors (not shown), and that the supporting unit 3 could be secured in the operating position A by, for example, electromagnetic attraction.

Although the invention is described in connection to laser engraving equipment, it may also be applicable in connection with equipment for any other type of non- mechanical marking of a target, such as ink jet printing.