RESISTANCE WELDING

Abstract: The present invention relates to a method for resistance welding of objects (39) by a spot welding gun och the spot welding gun itself, which gun comprises a C-formed arm (9), a front electrode (11) and a back electrode (13) and a cylinder (21), which is driven pneumatically or in another way. The back electrode (13) is axially impellable a limited distance by the cylinder (21). The spot welding gun is arranged to clamp the objects (39) between the electrode tips (15, 17) by impelling the cylinder (21) and providing a welding current to flow between said electrode tips (15, 17). According to the invention the back electrode (13) is arranged to be released from, moved axially in relation to, and fastened to the cylinder (21) to enable or simplify positioning of the spot welding gun so that the electrodes (11, 13) are on each side of the objects (39), removal of the spot welding gun from the welded objects and adjustment of the electrode distance (a) after the thickness (t) of the objects.

Description: RESISTANCE WELDING TECHNICAL FIELD The present invention relates to a method and a device for resistance welding by a spot welding gun, for instance for plate works, such as repairs of car bodies or the like. RELATED ART Resistance welding or spot welding is a method, wherein two or several welding objects, for instance by a spot welding gun, are joint together in one or several points. Resistance welding of this kind may be used for manufacture but more generally it is used for repairs, particularly of car bodies. Resistance welding is in this respect often the most suitable joining method, where it is applicable as regards accessibility. It is a fast and effective method, which provides for stronger welding joints with fewer welding spots than if a MIG-weld or the like is used. Resistance welding using a spot welding gun is furthermore harmless to the environment; for instance the flue gases are severely reduced. Furthermore, the method provides for a low heat release in the plate around the welding spot, which provides for a maintained corrosion resistance of zincified plate, and the thermal stresses in the structures are minimized. The so-called welding parameters that may be varied during the welding comprise electrode pressure, welding current strength and time, cooling effect, cooling time and hold time. There are two different kinds of spot welding guns, X- guns and C-guns. The X-gun has a pneumatic cylinder, which via a lever affects the electrode. The electrode pressure is thus dependent on the lever length, which in turn affects the rest of the welding parameters. The X-gun will not be closer described below. The spot gun, to which the present invention relates, is of the C-gun type, in which the pneumatic cylinder affects the electrode directly. The electrode pressure is constant, independent of the form of the arm. One of the problems of a conventional C-gun is that, when positioning the welding gun at the welding ob- jects, it is difficult to pass edges or other obstacles of the welding objects due to the relatively short stroke of the cylinder. Typically, this is solved in such a way that it will be possible for the arm to be opened or demounted. Thus, it is known to pivot a spot welding gun, called Prospot PR-10, laterally, while the arm and the front electrode are kept fixed, and thus create a larger space between the electrodes, whereby possible obstacles of the welding objects may be passed in order then to pivot the gun back to the original position, see for instance the information that was publicly available on 9 September 1998 on the Internet (wwe.prospot.com/uarms.htm) . It is also known to have the engagement of the arm in the gun radially displaced from the electrodes and turn/rotate the arm with the front electrode, whereby the space between the electrodes are created, see for instance the C-gun with article number 8652 of L0RS- TECNA. The arm is also demountable. In order to turn or demount the arm four screws must, however, be released. Furthermore, in all the instances described above, the electrodes must be centered afterwards. Another problem is that the electrode distance some- times has to be changed as a consequence of the fact that the appearance, i.e. the thickness, of the welding objects has changed. This is, according to the known arm, solved by the front electrode being exchangeable and/or movable, see for instance the guns described above. Thus, there are, particularly if both obstacles have to be passed and the electrode distance has to be adjusted, needed extensive working operations, which will severely reduce the capacity of the spot welding gun. SUMMARY OF THE INVENTION It is an object of the present invention to provide a spot welding gun lacking one or several of the problems which may appear using a spot welding gun according to the known art. Thus, it is an object to provide a spot welding gun, which through an uncomplicated process enables or simplifies the passage of obstacles during positioning of the spot gun at the welding objects and during removal of the gun therefrom, while maintaining the electrode centering. A further object of the invention is to provide a spot welding gun, which moreover allows adjustment of the electrode distance, while the number of working operations is reduced to a minimum. Yet a further object of the invention is to provide a spot welding gun of the kind mentioned above, which is compact and reliable. These and other objects are realized according to one aspect of the invention by a spot welding gun for resistance welding of objects comprising a C-formed arm, a front and a back electrode and a cylinder, which is driven pneumatically or in another way, the back electrode being axially impellable a limited distance by the cylinder. The spot welding gun is arranged to clamp the objects between the electrode tips by impelling the cylinder forwards and by letting a welding current flow between said electrode tips. According to the invention the back electrode is arranged to be releasable from, axially movable in relation to, and lockable to the cylinder. Thus, the inventive method comprises, before the weld- ing, releasing the back electrode from the cylinder, moving the back electrode axially backwards long enough in order to enable or simplify the positioning of the spot welding gun, positioning the spot welding gun so that the electrodes are on each side of the objects, moving the back electrode axially forwards to a position, where its electrode tip is at a distance from the front electrode less than the sum of the stroke of the cylinder, which is driven pneumatically or in another way, and the thickness of the objects, and fastening the back electrode to said cylinder. The method is hereby obtained that enables or simplifies the passage of obstacles during positioning of the spot welding gun and the method for adjustment of the electrode distance in one and the same operation, which will very considerably simplify the handling of the spot welding gun. Furthermore, the electrode cen- tering will be maintained during the entire working operation. The method after welding comprises releasing the back electrode from the cylinder, which is driven pneumatically or in another way, and moving the back electrode axially backwards long enough to enable or simplify the removal of the spot welding gun from the welded objects. The back electrode shall preferably be able to be moved axially backwards at least to a position, where its electrode tip is inside the cylinder, which is driven pneumatically or in another way, and forwards at least to a position, where its electrode tip is at a distance from the front electrode less than the sum of the stroke of the cylinder, which is driven pneumatically or in another way, and the thickness of the thinnest object, for which the gun is intended to be used. Preferably, the back electrode is arranged in order to be released manually and fastly from and fastened to the cylinder, which is driven pneumatically or in another way, via a locking lever, which will further simplify the working operation required by the gun. The C-formed arm with the front electrode may be radially turnable/rotatable around the cylinder, which is driven pneumatically or in another way, and/or demount- able. An advantage of the spot welding gun according to the present invention is, which has already been mentioned, its manageability, which provides for a high working capacity. A further advantage of the invention is that the centering of the electrodes is maintained. No advanced centering mechanisms or fastening mechanisms are thus needed. More advantages of the invention will be apparent in the description below. SHORT DESCRIPTION OF THE DRAWINGS The invention will be described closer below with reference to Figs. 1 and 2, which are only shown in order to illustrate the invention and shall therefore in no way limit the same. Fig. 1 shows an embodiment of the present invention as seen from one side. Fig. 2 shows the same embodiment as seen from behind. PREFERRED EMBODIMENTS In the following description, for the purpose of explaining and not limiting the invention, specific details are given, such as particular applications, techniques, etc., in order to give a thorough understanding of the invention. It shall, however, be apparent for a man skilled in the art that the invention may be practised in a manifold of other forms than these. In Fig. 1 there is shown a side view of an inventive embodiment of a device, or a so-called C-gun, 1 for resistance or spot welding. The C-gun comprises a substantially cylindrical house 3 with handles 5, 7 and a C-formed arm 9 , at the ends of which two electrodes 11, 13, particularly of copper, are facing each other. The front electrode 11 is preferably firmly mounted and has a rounded and possibly somewhat flattened electrode tip 15. Also, the back electrode has a preferably rounded and possibly somewhat flattened electrode tip 17 and also a thicker diameter 19 except at the part closest to the front electrode 11. According to the present invention the back electrode is axially movable, particularly stepless, whereby it may be released from och fastened to a pneumatic cylinder 21 by means of a locking lever 23. The pneumatic cylinder 21 is in turn by compressed air axially impellable a limited distance, the so-called stroke si of the cylinder, for instance 20 mm. Furthermore, the C-gun comprises a house 25 for connection of compressed air to the pneumatic cylinder 21, for electrical connection of the front electrode 11 and for connection of controlling electronics. The back end of the C-gun comprises an electrically insulating socket 29, which surrounds the back end of the back electrode, where also electrical connection 31 to said electrode exists. The handle 5 finally comprises a simple switch 33. The connection and regulation of pneumatics and welding current may be realized in different known ways and will not be closer described here. In Fig. 2, which shows the same embodiment from behind, all connections may be seen. As mentioned, the house 25 comprises a connection 27 for compressed air, an electrical connection 35 to the front electrode and a connection 37 for controlling electronics. The elec- trical connection 31 directly to the back electrode is also shown. The connection 35 is in electrical contact with the front electrode via electrical connectors inside the houses 25 and 3 and in the arm 9. According to the invention the back electrode 13 may consequently be released from the pneumatic cylinder 21 and moved axially to a desired position or completely removed without subsequent centerings or other adjustments being necessary. This construction allows passage of obstacles, such as saddle beam sills, at objects 39 during positioning and removal of the spot welding gun 1 and adjustment of the electrode distance a in the same operation. This together with an otherwise ergonomically designed construction provides a spot welding gun, which is very easy and handy to work with. Thus, the working operation for positioning the gun 1 and adjustment of the electrode distance a particularly comprises the following steps: 1. Releasing the back electrode 13 from the cylinder 21. 2. Movement of the back electrode 13 axially backwards sufficiently long to enable or simplify the positioning of the spot welding gun. In the present embodiment there is no limita- tion as regards how far the electrode may be moved in this direction, since it may even be demounted. 3. Positioning the spot welding gun so that the electrodes 11, 13 exist on each side of the objects 39. The electrodes shall be positioned at the point where it is desired to have the weld joint. 4. Movement of the back electrode 13 axially forwards to a position where its electrode tip 17 is at a distance a from the front electrode 11, which is shorter than the sum of the stroke si of the cylinder and the thickness t of the objects. The adjustment is not criti- cal - the principal thing is that the electrode is moved as far forwards that the stroke of the cylinder is "sufficient" to attain the necessary electrode pressure. 5. Fastening the back electrode 13 to the cylinder 21. In an analogous way the working operation for removal of the gun comprises particularly the following steps: 1. Releasing the back electrode 13 from the cylinder 21. 2. Movement of the back electrode 13 axially backwards sufficiently long to enable or simplify the removal of the welding gun from the welded objects 39. Another functionality of the inventive welding gun comprises that the C-formed arm 9 is radially turn- able/rotatable around the cylindrical house 3. Preferably, the C-formed arm has in its back portion a substantially ring-formed part, which is threaded onto the front portion of the house, which here is of a smaller diameter. The ring-formed part of the arm has a slit or a groove, particularly down in the lower part, so that it may be fastened to the house 3 by a simple screw. Possibly, if it should be needed, a fastening plate may be screwed from the front side. In the present embodiment the screw is integrated with the handle 7, whereby the arm 9 is turned by releasing the arm (the screw is released by turning the handle 7) , turning the arm 9 and fastening the arm (the screw is fastened by turning the handle 7) . Of course, the C- formed arm is then also demountable and exchangeable to another arm of a suitable size. The standard arm shown in Fig. 1 has a depth d of 80 mm and a length 1 of 160 mm, but the only limitation in this respect is the extent of ungainliness of the welding gun when using very large arms. If the C-formed part of the arm will substantially be in another direction than vertical, the slit or groove of the ring-formed part and the handle 7 with the screw may be located at another radial position, so that the welding gun achieves a suitable ergonomic design. Possibly, the handle 7 may be mounted directly at the house 3 and thus be fixed, while the fastening screw may be provided with a fastening lever similar to the one 23, which locks the back electrode 13. An inventive welding gun described above shall particularly be designed to satisfy different quality demands of car manufacturers, particularly for the so-called aftermarket. The welding gun must have a sufficient clamping force, current strength and cooling capacity so that it manages plate thicknesses and rust-proofing coatings existing at the car bodies. Welding time, cooling time and holding time shall have an exact and reliable regulation. If the different welding para- meters are optimized, after-treatment is normally unnecessary, but it is sufficient with just a slight grinding with grinding paper before painting. The present invention as herein described solves the problems, which are associated with known art. It is, of course, not limited to the embodiments described above and shown in the drawings, but may be modified within the scope of the appended claims.

Claims: CLAIMS 1. Spot welding gun for resistance welding of objects (39) comprising a C-formed arm (9) , a front electrode (11) and a back electrode (13) and a cylinder (21) , which is driven pneumatically or in another way, which back electrode (13) is axially impellable a limited distance by the cylinder (21) , the spot welding gun being arranged to clamp the objects (39) between the electrode tips (15, 17) by impelling the cylinder (21) and to provide a welding current to flow between said electrode tips (15, 17) , characterized in that the back electrode (13) is arranged to be releasable from, axially movable in relation to, and fastenable to the cylinder (21) to enable or simplify positioning of the spot welding gun so that the electrodes (11, 13) are at each side of the objects (39) , removal of the spot welding gun from the welded objects (39) and adjustment of the electrode distance (a) after the thickness (t) of the objects. 2. Spot welding gun according to claim 1, characterized in that the back electrode (13) is arranged, in order for enabling or simplifying the positioning and removal of the spot welding gun, to be movable axially backwards at least to a position, where its electrode tip (17) is inside the cylinder (21) , which is driven pneumatically or in another way. 3. Spot welding gun according to claim 1 or 2 , characterized in that the back electrode (13) is arranged, in order for facilitating or simplifying adjustment of the electrode distance (a) , to be movable axially forwards to a position, where its electrode tip (17) is at a distance from the front electrode, which is shorter than the sum of the stroke (si ) of the cylinder, which is driven pneumatically or in another way, and the thickness (t) of the objects. 4. Spot welding gun according to any of claims 1-3, characterized in that the back electrode (13) is arranged to be manually and fastly releasable from and fastenable to the cylinder, which is driven pneumatically or in another way, via a fastening lever. 5. Spot welding gun according to any of claims 1-4, characterized in that the C-formed arm (9) is radially turnable/rotatable around the cylinder (21) , which is driven pneumatically or in another way. 6. Spot welding gun according to any of claims 1-5, characterized in that the electrodes (11, 13) are of copper. 7. Method for resistance welding of objects (39) using a spot welding gun (1) having a C-formed arm (9) , a front electrode (11) and a back electrode (13) and a cylinder (21) , which is driven pneumatically or in another way, the back electrode (13) being axially impellable a limited distance by the cylinder (21) , comprising the steps of clamping objects (39) between the electrode tips (15, 17) by impelling the pneumati- cal cylinder and applying a welding current between said electrode tips (15, 17), characterized by, before the step of clamping the objects (39) , - releasing the back electrode (13) from the cylinder (21) , moving the back electrode (13) axially backwards to enable or simplify the positioning of the spot welding gun, - positioning the spot welding gun so that the electrodes (11, 13) are on each side of the objects, moving the back electrode (13) axially forwards to a position, where its electrode tip (17) is at a distance from the front electrode (11) , which is shorter than the sum of the stroke (si ) of the cylinder, which is driven pneumatically or in another way, and the thickness (t) of the objects, and fastening the back electrode (13) to said cylinder (21) . 8. Method according to claim 7, characterized by, after the step of applying a welding current, releasing the back electrode (13) from the cylinder (21) , which is driven pneumatically or in another way, and moving the back electrode (13) axially backwards to enable or simplify the removal of the spot welding gun from the welded objects (39) . 9. Method according to claim 7 or 8 , characterized in that the step of releasing and fastening the back electrode (13) are performed manually by means of a fastening lever (23) . 10. Method according to any of claims 7-9, characterized by turning the C-formed arm (9) radially around the cylinder (21) , which is driven pneumatically or in another way.

Inventors: STENKVIST SIVERT, STENKVIST, SIVERT
Applicants: CAR O LINER AB, STENKVIST SIVERT, CAR-O-LINER AB, STENKVIST, SIVERT
Application year: 1999
Main classification: B23K11
IPC codes: B23K11