SINGLE SIDED RESISTANCE WELDING

Abstract: Single sided resistance welding apparatus comprising an electrically operated servo motor (4) connected to drive a welding electrode a predetermined distance into engagement with a surface of a first metallic member (1) to be welded to a second metallic member (2) to which access is limited and which is in engagement with the first metallic member. Means are provided for dynamically controlling the position of the welding electrode with respect to the surface of the first metallic member during the welding operation by controlled activation of the electrically operated servo motor. The first and second metallic members may comprise a metal sheet (1) and metal tube (2) respectively. The metal tube may be produced by a hydroforming process.

Description: SINGLE SIDED RESISTANCE WELDING This invention relates to a method of and apparatus for single sided resistance welding of metallic members. Single sided resistance welding is a process used for welding two superimposed metal members together .where access to and support for the surface of one of the members is restricted or impossible. In this process a welding electrode is driven downwardly into engagement with a surface of the accessible member by means of a pneumatically operated piston disposed in a cylinder, air being selectively admitted or removed from the cylinder to control the position of the piston and consequently the position of the electrode relative to the surface of the member- Movement of the electrode is continuous until the force applied by the electrode tip onto the accessible member reaches a predetermined value. In the case of a thin section the continuous application of force can cause mechanical collapse of the member. Another disadvantage of such a system is that as electricity flows from the electrode to the superimposed members and the temperature of the weld site increases, so the tendency of the respective member surfaces to yield under the force imposed by the electrode increases. In other words, when the welding force is applied and the welding current is activated, portions of the members tend to yield under the applied pressure and generated heat thereby causing excessive indentation of both surfaces at the weld site. This excessive indentation is undesirable aesthetically and from a standpoint of producing a weld of acceptable quality. One example of an occasion where access is limited is the manufacture of vehicle bodies where it is required to resistance weld a generally flat metallic member to a closed section rectangular tubular steel member, typically a tube formed by a hydroforming process. Restricted access to the interior of the tubular member prevents the use of conventional welding using two opposed electrodes and the lack of any direct support on the inside of the tubular member increases the tendency for the member surfaces to yield. Hydroformed components are formed by expanding a steel tube using fluid pressure into a die to give the final component shape. Single sided welding is well suited for welding hydroformed components to other components in automotive assembly. To alleviate the problem of excessive indentation US-PS-5115113 discloses apparatus in which a pneumatically driven welding electrode is locked against further movement as soon as welding current is applied. Whereas this apparatus may assist to alleviate the problem of excessive indentation, it also removes the ability for dynamic control of the position of the electrode relative to the weld zone during the actual welding operation. Dynamic control is important if pressure and movement are to be controlled at the weld site during the entire welding process. It is an object of the present invention to provide a method and apparatus for single sided resistance welding which overcomes, or at least alleviates, this undesirable excessive indentation of members during welding while permitting dynamic control of the electrode position during the welding process. According to the present invention in one aspect, there is provided single sided resistance welding apparatus which comprises an electrically operated servo motor connected to drive a welding electrode a preαetermined distance into engagement with a surface of a first metallic member to be welded to a second metallic member to which access is limited and which is in engagement with the first metallic member, and means for dynamically controlling the position of the welding electrode with respect to the surface of the first metallic member during the welding operation by controlled activation of the electrically operated servo motor. By controlling the position of the welding electrode during the welding operation, excessive indentation and mechanical collapse of the metallic members and can be avoided. Preferably, the means for dynamically controlling the position of the welding electrode is set for a predetermined maximum movement after engagement of the electrode with the surface of the first metalhc member. In this way, the maximum indentation of the member is limited to the predetermined maximum movement. According to a preferred embodiment, the means for dynamically controlling the position of the welding electrode comprises a weld time controller. Thus, the welding time can be controlled. Preferably, the means for dynamically controlling the position of the welding electrode exerts a predetermined dynamic force on the surface of the first metallic member during welding. By controlling the force and/or movement during welding, indentation can be controlled and better welds can be made. According to a preferred embodiment, the welding electrode has a shaped tip, preferably a tip in the form of a truncated cone or a tip having a domed or radiused end. These latter forms can give good welds when the welding force is relatively low. The tip profile may be chosen to achieve the desired weld size. Preferably the welding electrode is connected to a source of high frequency direct current (HF-DC). Experiments have shown that using HF-DC gives the best results. According to a preferred embodiment an assembly of the servo motor and the welding electrode is supported on a robotic arm of an electronically programmed robotic device. Using a robotic device makes it easy to make a number of welds besides each other in a short time. The current return path from the weld site may be via an electrode pad positioned close to and connected by a damper to the servo motor. The damper may be spring mounted. Alternatively, the current return path may be via a fixture which supports one or both metallic members. The welding electrode may take the form of a rotatable wheel connected and supported by the servo motor. In another aspect, the invention provides a method for single side resistance welding of the superimposed metal members, the method comprising the steps of operating an electrically powered servo motor to drive an electrode a predeteπnined distance into engagement with a surface of one of the superimposed members, passing current through the electrode to create a weld between the superimposed members, and dynamically controlling the position of the electrode relative to the member surface during the welding process. Preferably, in this method an apparatus according to the first aspect is used. The method is especially suited to weld a metal sheet and a metal tube, for instance a hydroformed metal tube. The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which:- Figure 1 is a schematic side view of a first embodiment of single sided welding apparatus in accordance with the invention; Figure 2 is a perspective view of a second embodiment of single sided welding apparatus in accordance with the invention; Figures 3a, 3b and 3c illustrate three operating positions of the apparatus illustrated in Figure 2; Figure 4 is a perspective view of a third embodiment of single sided welding apparatus in accordance with the invention; and Figure 5 is a perspective view of a fourth embodiment of single sided welding apparatus in accordance with the invention. The apparatus illustrated in Figure 1 illustrates a first embodiment of single sided welding apparatus in accordance with the invention positioned to produce a weld between a metal sheet 1 and a metal tube 2. The single sided welding apparatus comprises a welding electrode 3 movable into and out of contact with the exposed surface of the sheet 1 by an electrically operated integrated servo motor 4. The welding electrode is connected to high frequency direct current (HF-DC), direct current (DC) or alternating current (AC) received from a transformer 5, the respective housings of the motor 4 and transformer 5 being secured one to the other. Thus, when assembled, the welding electrode 3, the servo motor 4 and the transformer 5 comprise an integral unit. The transformer 5 is connected by cabling 6 to a weld time controller 7 incorporating integrated dynamic force and/or displacement control features. The weld time controller 7 operates to control the position of the servo and thus the position of the electrode 3. The assembly of the servo motor 4 and transformer 5 is carried by an electrically operated robot 8. Thus, the assembly can readily and automatically be moved between required weld sites. Alternatively, a hydraulic robot could be used. The current return paths from the weld sites may, for example, be achieved by use of a clamp (or series of clamps) which supports the sheet 1 anάVor the tube 2, or a suitably positioned support cradle. Other fixtures for accommodating the return of current from the weld sites may be adopted, these being dictated by the geometry of the members to be welded and their location within, for example, a vehicle shell. Typically, 50Hz AC, 300Hz DC or 800Hz HF-DC welding systems are employed. Experiments have shown that HF-DC in many cases gives best results. Movement of the electrode 3 is controlled by the controller 7 to enable the electrode 3 initially to be moved downwardly into contact with the metal sheet to impose sufficient pressure on the sheet to ensure close contact with the adjoining surface of the tube. Once contact is made between the electrode 3 and the metal sheet high frequency direct current or alternating current is passed through the electrode 3 to effect welding of the two members 1, 2 together, the heat required being generated by the electrical resistance of the members. The controller 7 operates to impart secondary movements to the electrode daring the welding operation to ensure that the pressure imposed by the electrode tip at the weld site does not create unwanted indentations in the sheet and tube surfaces in the weld site. The position of the electrode 3 is, therefore, dynamically controlled during the welding cycle. Control may be effected by reference to a preset programme of electrode displacement profile, or a force profile, or a combination of both such profiles, After completion of the required weld the electrode is lifted by the servo motor and then indexed to the next position at which a spot weld is to be made. The process then continues until the required welds have been created. The single sided welding apparatus illustrated i Figure 2 is similar to that illustrated in Figure 1 and like integers have been given the same reference numerals. In this embodiment, the electrode 3 is again supplied with high frequency DC, DC or AC current from a transformer (not shown in Figure 2). In the Figure 2 embodiment, the current return assembly comprises an electrode pad 11 connected through a swivel joint 12 to a spring mounted carrier 14 which is positioned close to the servo motor 4 and is connected thereto by a spring mounted damper. As for Figure 1, the welding apparatus is movable robotically to selected locations on the sheet surface to create the required pattern of welds between superimposed metal sheet 1 and tube 2. The relative positions of the electrode 3 and return path electrode pad 11 are shown in Figure 3. Figure 3a shows the relative positions before the electrode is moved downwardly into contact with the sheet 1; Figure 3b shows the relative positions immediately before initiation of the welding process; and Figure 3c shows the relative positions during the welding operation. The return current path is shown in dotted line. In the embodiment illustrated in Figure 4, an electric motor driven integrated high frequency - direct current (HF-DC) servo gun 15 carrying an electrode 16 is illustrated. The servo motor is driven robotically to create a number of spaced welds 17. Operation of the servo motor is as described above in relation to Figure I. In this embodiment, the current return comprises a clamp on fixture 18. Alternatively, a series of clamp on fixtures or a conductive cradle on which the metal tube is supported could be provided. Thus, current flows from or between the welding electrode 16 and the fixture 18 through the metal plate and the metal tube. In the embodiment illustrated in Figure 5 of the drawings, the electrode takes the form of a rotatable wheel 19 mounted on a stem 20 supplied with an HF-DC, DC or AC welding current. The electrode position is controlled by a servo motor 21. In this embodiment the wheel electrode 19 travels along the metal sheet to define a strip- weld 23 between the sheet and the tube positioned below the sheet. In this embodiment, the current return path is via a clamp 22, on which the tube is supported, or a series of clamps or a conductive cradle on which the metal tube is supported. For each of the embodiments of the invention, the servo motor which carries the welding electrode is initially driven downwardly by an electric motor into firm contact with the upper surface of the steel sheet. The required HF-DC, DC or AC welding current is then passed via the electrode tip of the appropriate geometry through the superimposed steel sheet and tube to create, by electrical resistance, the temperature necessary to effect welding between the superimposed members. As the temperature at the weld site increases so does the tendency of the steel at the weld site to yield. To counteract this tendency, the electric motor is operated via the weld time controller 7 to alter the electrode position from the sheet surface after a predetermined interval of time or in response to a measured pressure change. The electrode position is, therefore, dynamically controlled during the entire welding process. The tendency of the metal to yield is, therefore, significandy reduced. The tip of the welding electrode can have a special shape, for instance the form of a truncated cone, or have a domed or radiused end. Using lower forces, the latter tip forms can give better results than the usual tip ends. It will be appreciated that the foregoing is merely exemplary of welding apparatus in accordance with the invention and that various modifications can readily be made thereto without departing from the true scope of the invention.

Claims: CLAIMS 1. Single sided resistance welding apparatus which comprises an electrically operated servo motor connected to drive a welding electrode a predetermined distance into engagement with a surface of a first metallic member to be welded to a second metallic member to which access is limited and which is in engagement with the first metallic member, and means for dynamically controlling the position of the welding electrode with respect to the surface of the first metallic member during the welding operation by controlled activation of the electrically operated servo motor. 2. Apparatus as claimed in claim 1 wherein the means for dynamically controlling the position of the welding electrode is set for a predetermined maximum movement after engagement of the electrode with the surface of the first metallic member. 3. Apparatus as claimed in claim 1 or 2 wherein the means for dynamically controlling the position of the welding electrode comprises a weld time controller. 4. Apparatus as claimed in any one of claims 1 to 3 wherein the means for dynamically controlling the position of the welding electrode exerts a predetermined dynamic force on the surface of the first metallic member during welding. 5. Apparatus as claimed in any one of claims 1 to 4 wherein the welding electrode has a shaped tip, preferably a tip in the form of a truncated cone or a tip having a domed or radiused end, the latter for low welding forces. 6. Apparatus as claimed in any one of claims 1 to 5 wherein the welding electrode is connected to a source of high frequency direct current, direct current or alternating current, preferably a source of high frequency direct current. 7. Apparatus as claimed in any one of the preceding claims wherein the welding electrode and the servo motor have been assembled as an integral unit. 8. Apparatus as claimed in claim 7 wherein the assembly of the servo motor and the welding electrode is supported on a robotic arm of an electronically programmed robotic device. 9. Apparatus as claimed in any of the preceding claims wherein an electrode pad has been positioned close to and connected by a damper, for instance a spring mounted damper, to the servo motor to create a current path from the welding electrode to the electrode pad during use. 10. Apparatus as claimed in any one of the claims 1 to 8 wherein a second electrode has been connected to a fixture which supports one or both metallic members to create a current path between the welding electrode and the second electrode during use. 11. Apparatus as claimed in any one of the claims 1 to 8 wherein the welding electrode takes the form of a rotatable wheel connected and supported by the servo motor. 12. A method for single side resistance welding of the superimposed metal members, the method comprising the steps of operating an electrically powered servo motor to drive an electrode a predetermined distance into engagement with a surface of one of the superimposed members, passing current through the electrode to create a weld between the superimposed members, and dynamically controlling the position of the electrode relative to the member surface and/or dynamically controlling the electrode force during the welding process. 13. Method as claimed in claim 12, wherein the apparatus according to any one of claims 1 to 11 is used. 4. Method as claimed in claim 12 or 13, wherein the superimposed metal members comprise a metal sheet and a metal tube, for instance a hydroformed metal tube.

Inventors: GLEAVE MATTHEW, LEWIS COLIN, GLEAVE, MATTHEW, LEWIS, COLIN
Applicants: CORUS UK LTD, GLEAVE MATTHEW, LEWIS COLIN, CORUS UK LIMITED, GLEAVE, MATTHEW, LEWIS, COLIN
Application year: 2001
Main classification: B23K11
IPC codes: B23K11