METHOD OF SEAM WELDING A METAL TUBE FORMED FROM A FOLDED STRIP

Abstract: A method for restarting the longitudinal welding seam when producing thin-walled tubes from a metal tape comprises the steps of starting welding with the tape in motion and the welding current ampere being gradually raised from zero to the steady state values steady state step in which the longitudinal welding seam is continuously produced and the welding current ampere is at its steady state value, a third step in which the welding is stopped and the welding current ampere is gradually decreased from its steady state value to nearly zero, and a restarting stage in which the welding seam is overcast partially behind the point at which it had been stopped and the welding current ampere is gradually raised from nearly zero to the steady state value once again. An apparatus for performing the method is also disclosed.

Description: This invention relates to a method of seam welding a metal tube formed from a folded strip. The manufacturing process of welded tubes obtained by shaping a strip does not involve any special problems as to stop and restart of the productive run if the thickness of the workpieces exceeds 0.8 mm to 0.9 mm as an average. In addition, the lengths usually required in the trade from such installations do not exceed, as a rule, ten metres. The installations for the production of thinwalled tubes, for example for making hypodermic needles, are capable of continuously producing the article with the limitations inherent in the welding electrodes or, anyway, in the correct operation of the welding torch. With the TIG process, such limitation is due to the evaporation of thorium as alloyed in the electrode, the formation of the melted drop and the resultant stop of the welding bath. Usually, this occurs after 2 or 3 hours of continuous operation. With the plasma welding process, the limitation is to be attributed, instead, to the formation of carbon deposits (between the electrode and the clamps) which actually impair the processing continuity. This usually takes places after 6-8 hours of continuous operation (a working shift). In the installations mentioned above, the stoppage of the welding run together with the impossibility of restarting same involves the acceptance of the as-produced length and the consequential start of another production cycle. It has been proposed (AT-BB-277 719) a method of seam welding a metal tube wherein, during a restarting stage, an auxiliary electrode is interposed between the welding electrode and the tube in order to avoid weld faults. This method, however, is not practical when thin tubes are to be welded. In fact, as known, the space between the welding electrode and the tube must be substantially equal to the thickness of the tube; it would be very difficult to interpose an auxiliary electrode in such a small space. An objective of the present invention is to provide a method of seam welding a metal formed from a folded strip, which makes it possible to produce thin-walled tubes having high length. Having this objective in view, according to the present invention, it has been envisaged to provide a method of seam welding a metal tube formed from a folded strip through welding means, the method comprising the step of moving, during a steady state stage, the tube with respect to the welding means to form a continuous longitudinal welded seam on the tube, the step of stopping, during a stop stage, the tube, and the step of moving again, during a restart stage, the tube with respect to the welding means, shifting the welding means from an inoperative position along the previously formed welded seam to an operative position upon the welded seam end, to continue the welded seam, characterized in that during said stop stage the welding means are shifted along the welded seam from said operative position to said inoperative position overwelding the previously welded seam and their welding power is gradually decreased from the steady state value to a zero value, and during said restart stage the welding means are operated starting from said inoperative position, overwelding the previously welded seam, and their welding power is gradually increased from the zero value to the steady stage value, the welding means being shifted together with means which shape and cool the tube. The structural and functional features and the advantages of the method according to the invention, will be more clearly understood from the ensuing nonlimiting description, aided by the accompanying drawings, wherein: Figure 1 is a diagrammatical perspective view of an apparatus used for performing a method according to the invention. Figures 2 and 3 are perspective views of a single detail of the welding area of said apparatus in different working stages, and Figure 4 is a diagram of the several working stages of the aforesaid method according to the invention. Figure 1 shows an automatic machine, 10, for welding a longitudinal seam 11 in a process for producing thin-walled tubes 12 as obtained from a strip 16, and more particularly adapted to contain, for example, electric wire 13. A frame 14, rigidly connected to a movable slide 22, bears a set of couples of rollers 15, of the pulley type, for holding the cable 17, so shaped and welded, within a cooling tub 26. A couple of shaping rollers 18, at the inlet end of the machine 10, completes the shaping of the strip 16 prior to carrying out the longitudinal seam 11, that which is effected by a specially provided member 19, to be selected consistently with the process used. Upstream of the couple of the shaping rollers 18 and the welding member 19, a blade 20 is provided, to guide and to position the electric wire 13 in the interior of the folded strip 16, said blade 20 being equipped with feeding means 21 which are adapted to introduce a protective gas (such as argon) into the interior of the shaped tube (Figures 2, 3). The welding member 19, the blade 20, the couples of rollers 15 and 18 and the cooling tub 26 are secured to the slide 22, which is equipped with driving means 23 which drive the slide on guideways 24 along a direction which is parallel to that of the tube 17, the forward motion being that of the arrow 25 and the backward motion taking place in the reverse direction. The operation of an apparatus so conceived is diagrammatically set forth in the scheme of Figure 4, namely: The rolled section, as obtained from the strip 16 and enclosing in its interior the wire 13, is brought into registry with the couple of shaping rollers 18. In said position, the blade 20 is slipped between the strip edges, which are still spaced apart, and acts so as to shift by a few millimetres the wire 13 away of the wall of the strip 16 to be welded, so as not to damage the wire, protective gas being meanwhile introduced as fed by the means 21. Once the folded strip 16 and the wire 13 are enclosed therein between the shaping rollers 18, the welding member 19 is started. According to a programme which is a function of the material to be processed, the welding current is raised up to the steady state value (as symbolized by 0 to 1) and the slide 22, driven by the driving means 23, sets the means 19 into the position where the welding operation begins. During this stage, called the "CYCLE START", the cable 17 so sheathed is fed forward along the direction of the arrow 25 as propelled by the shaping rollers of a shaping machine (not shown), while the welding means 19 are fed backwards and moved in a direction against that of the arrow 25. As the welding current has attained its maximum preset ampere value (1), the machine 10 is in its "steady" state, the cable 17 is being continually shaped and the carriage 22 holds the welding means in its operative position (Figure 2). Past the welding means 19, the cable 17 is immersed in the tub 26, an entity with the carriage 22, and is submerged for at least one half of its thickness so as to be cooled. It is now necessary to act so as to stop welding, such as for adjustments, control, or a stoppage due to the end of a workshop shift or for any other reason whatsoever To obtain this, the cable 17 is stopped, the carriage 22, which carries the blade 20, the welding means 19, the couples of rollers 15 and 18 and the cooling tub 26, is carried from the area 1 to 0 and the already welded seam 11 is travelled over again, while, simultaneously, the welding current ampere is gradually decreased from the value 1 to the value 0 (Figure 3). The entire stage as now described is called "CYCLE STOP" and makes it possible to effect a stoppage in the production of the cable 17 without piercing the wall of the tube 12 or jeopardizing the integrity of the wire 13. During a "CYCLE RESTART" stage, the operations performed during the "CYCLE START" tage are virtually repeated as such, that is, the carriage 22 brings the welding means 19, the roller couples 15 and 18 and the cooling tub 26 from the position 0 to the position 1 while the welding current ampere is gradually raised from the value 0 to the steady state value, as indicated by the reference numeral 1 herein. During the latter stage, however, the welding means 19 and the couples of shaping rollers 18 travel over an already welded section once again, whereby the longitudinal welding seam 11 is overcast and its continuity and integrity are thus warranted while the internally enclosed wire 13 remains unaffected It is apparent that such a welding process and machine can be used with advantage also in those cases in which the tube 12 is to contain in its interior, once it has been welded, optical fibres, wire bundles or other particular articles, so as to obtain cables which are outstandingly resistant, for example, to heat and corrosion. A cable 17 so constructed is then subjected, as a rule, to extrusion, so as to immobilize the wire or other article therein and, if necessary, also to correct the roundness of the tube. The longitudinal welding operation can be effected both as a butt-welding or with a partial edge overlapping, or according to any other suitable technological operations.

Claims: 1. A method of seam welding a metal tube (12) formed from a folded strip (16) through welding means (19), the method comprising the step of moving, during a steady state stage, the tube (12) with respect to the welding means (19) to form a continuous longitudinally welded seam (11) on the tube (12), the step of stopping, during a stop stage, the tube (12), and the step of moving again, during a restart stage, the tube (12) with respect to the welding means (19), shifting the welding means (19) from an inoperative position along the previously formed welded seam (11) to an oper- ative position upon the welded seam end, to continue the welded seam (11), characterized in that during said stop stage (Fig. 4; C) the welding means (19) are shifted along the welded seam (11) from said operative position to said inoperative position overwelding the previously welded seam (11) and their welding power is gradually decreased from the steady state value to a zero value, and during said restart stage (Fig. 4; D) the welding means (19) are operated starting from said inoperative position, overwelding the previously welded seam (11), and their welding power is gradually increased from the zero value to the steady state value, the welding means being shifted together with means which shape and cool the tube.

Inventors: PARMEGGIANI, ALDO
Applicants: CISE- CENTRO INFORMAZIONI STUDI ESPERIENZE S.P.A.
Application year: 1984
Main classification: B23K31
IPC codes: B23K31, B23K9