BACKGROUND OF THE INVENTION 
     Various processes and apparatuses are known for the production of straight bead welded tubes from strips and sheet metal blanks, but none of them make possible the economic manufacture in medium size runs of tubes having a relatively short length (for example, 1=3000 mm), a small diameter (for-example, d=50 mm) and a relatively large wall thickness (for example, t=2.5 mm). It is therefore an object of the invention to provide a process and an apparatus for the production of such tubes. 
     In the prior art process of roller shaping (U.S. Pat. No. 2,110,378) a strip is shaped in a number of successively arranged stages by driven profiled rollers to give a slotted tube which is then welded. The investment cost of an installation suitable for this purpose is very high, so that it is unsuitable for the production of tubes in medium size runs. 
     In the prior art 3-roller bending, a flat sheet metal blank with two supporting rollers is bent around a working roller. Such a process enables tubes to be produced with a wall thickness of, for example, 1.0 mm and a diameter of 50 mm only with a length of less than 2000 mm, since the supporting rollers sag due to the heavy supporting forces to be applied. There is also the aspect that the resulting slotted tube must be removed from the apparatus and straight bead welded at another place. 
     A clamping and retaining apparatus for relatively short slotted tubes is also known (DE 44 32 674 C1) in which the slotted tube is retained by bands, which are partially looped around the tube, in a suitable welding position for a welding device which can be driven along the joint gap. To obtain a straight bead welded tube, therefore, a sheet metal blank must be shaped into a slotted tube in a separate apparatus. 
     In another prior art apparatus (DE-PS 966 111) for the production of straight bead welded tubes from flat sheet metal blanks with parallel longitudinal edges, the sheet metal blank is shaped into a slotted tube in the same apparatus, being retained by the means shaping the tube with the longitudinal edges to be welded in the welding position. Two form tools disposed laterally inverted in relation to one another, which are borne by a tool support, can be driven towards one another, and have outer cylindrical half shells received in their opened receiving position the sheet metal blank at its two longitudinal edges. When the form tool halves are moved together, the sheet metal blank is retained at its two ends fixed in the center, so that the sheet metal blank slides along the cylindrical half shells on both sides until its longitudinal edges impinge on one another at the top point. The sheet metal blank shaped into a slotted tube is retained in this position. Then, to weld the longitudinal edges to one another, the top ends of the form tool halves can be hinged upwards, so that the joint gap is opened up. An important disadvantage of such an apparatus is that there is the risk that the sheet metal blank may bend outwards for lack of internal and external guiding. This risk is particularly great in the case of thin-walled sheet metal blanks. 
     In a very similar prior art apparatus for the shaping of sheet metal blanks into tubes, which are then welded (DE-PS 593 622) the two form tool halves are formed not by cylindrical half shells, but by axially offset discs with cutaway portions in the shape of arcs of a circle. The apparatus is to be used to form a conical tube from a sheet metal blank. To this end the disc cutaway portions in the shape of arcs of a circle increase in radius in the axial direction. However, in distinction from the other aforedescribed prior art apparatus, in this prior art apparatus an internal conical mandrel is associated with the outer parts of the form tool halves. However, the mandrel is not operative during the entire shaping operation, but only at the end thereof, since it is applied to the sheet metal in the center between the form tool halves. Due to the absence of internal and external guiding during the shaping operation, even the use of such a mandrel does not obviate the risk that the sheet metal blank will bend outwards during the shaping operation. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a process and an apparatus which enable straight bead welded tubes to be produced from sheet metal blanks, more particularly blanks with a thin wall thickness. More particularly, the process and the apparatus are suitable for the processing of sheet metal blanks of different thicknesses, for example, so-called tailored blanks. 
     The invention therefore starts from a process for the production of a straight bead welded tube from a flat sheet metal blank having parallel longitudinal edges, wherein the sheet metal blank is shaped into a slotted tube by means of two form tool halves having outer cylindrical half shells and disposed laterally inverted in relation to one another which can be moved towards one another, whereafter the longitudinal edges are welded to one another at the top of the form tool, being retained in position by the form tool halves. In such a process according to the invention during shaping the sheet metal blank is borne on the inside by internal cylindrical mandrel halves co-operating with the outer half shells to produce form gaps, the longitudinal edges emerging at the top retaining exposed for welding (sic). 
     The invention also relates to an apparatus for the production of straight bead welded tubes from flat sheet metal blanks with parallel longitudinal edges having: form tool halves having outer cylindrical half shells and disposed laterally inverted in relation to one another which are borne by a tool support and can be moved towards one another and can be moved out of an opened receiving position for the sheet metal blank into a closed position, in which the longitudinal edges to be welded to one another are held together by the closed tool halves at their top point, the device also having a welding device which can be moved over the form tool halves along the longitudinal edges retained in the welding position. In such an apparatus the invention is characterized in that the form tool halves have cylindrical internal mandrel halves which are associatedly fixed with the outer half shells and which cooperate with the outer half shells to produce form gaps for the sheet metal blank to be inserted, while in the closed position of the form tool halves the form gaps retain the longitudinal edges emerging therefrom at the top exposed for welding. 
     The process according to the invention and the apparatus according to the invention enable sheet metal blanks of different thicknesses to be formed into a cylindrical tube without any risk that the blank will bend outwards during the shaping operation. More particularly, the invention enables short tubes to be economically produced in medium size runs from sheet metal blanks. Both tubes having a constant wall thickness can be produced, and also tubes which have differential wall thickness over their length or periphery. The special advantage of the invention is that the form tool halves themselves retain the shaped slotted tube with the joint gap in an optimum welding position, to produce the weld by the welding device movable along the joint gap. This means that two separate devices are no longer required for shaping and for retaining the slotted tube in the welding position. This also eliminates the laborious transfer of the slotted tube, with the necessary alignment and clamping. 
     In a first embodiment of the invention the sheet metal blank is first pushed by one half completely into one of the form gaps and then by its other half into the other form gap. One of the two form tool halves, more particularly the movable one, can have adjacent the entry to the form gap an abutment against which one longitudinal edge of the sheet metal blank can bear when the form tool halves are moved together. This gives the sheet metal blanks satisfactory guiding, making it impossible for the blank to be introduced at an angle into the form gap. 
     To improve the geometry of the joint gap, during or after its emergence from the form gap the sheet metal blank can be so after-shaped in narrow strips adjoining the longitudinal edges that they merge substantially tangentially into one another. With the apparatus this can be effected in two ways. Either a tool acting on the longitudinal edges in the sense of moving them together is associated with the top zone, or the form gaps terminate at the top in a common horizontal plane. 
     The required spatial fixation of the internal mandrel halves in the outer half shells with the possibility of being able to remove the shaped tube can, according to a further feature of the invention, be effected by the features that the outer half shell and the internal mandrel half of each form tool half are connected to one another at one of their ends, and the internal mandrel half is releasably located by its other end directly on the tool support, the internal mandrel half being otherwise retained in position by a number of supporting members which extend through the outer half shell via recesses and can be uncoupled via sliding couplings on the internal mandrel half in the direction of the outer half shell. 
     Constructionally this can be put into effect by the feature that the outer half shell and the internal mandrel half connected thereto at one end bear against one another without a gap in this connecting portion and are held together by releasable clamping elements. The result is a highly precise association between the half shell and the internal mandrel half with a very simple assembly. The releasable clamping elements enable the form gap to be somewhat opened, to pull the finished tube out of the form tool or even to pull the internal mandrel halves out of the finished tube. 
     To enable the tube to be pulled out of the form tool with the front end of the internal mandrel half released, without the internal mandrel half making it difficult to pull out the tube by bearing thereagainst, according to a feature of the invention the internal mandrel half projects in relation to the outer half shell at the connected end, and associated with the projecting portion is a pressure element via which a pivoting force around a horizontal transverse axis in the sense of relieving the front end of the internal mandrel half can be applied to the internal mandrel half. 
     To prevent the sheet metal blank from bending outwards when the blank is introduced into the form gaps, according to the invention a vertically adjustable supporting construction for the sheet metal blank to be introduced into the form gaps is provided immediately below and in the receiving position between the form tool halves. The supporting construction preferably has at the entrance of each form gap a deflecting plate extending over the entire length of the form gap and is borne resiliently in the direction in which the form tool halves move. The deflecting plates yield when on completion of shaping the form tool halves arrive in the closed position. 
     The precise positioning of the longitudinal edges at the top point of the form tool halves for straight bead welding can be ensured using simple means. According to one feature of the invention associated with one of the two form tool halves, more particularly the fixed one, is a retractable stop at the top for the longitudinal edge of the portion of the sheet metal blank shaped in said form tool half. The stop also then acts as a support when the sheet metal blank is inserted into the form gap of the other form tool half. Preferably the stop has a tip so asymmetrically constructed that when the other longitudinal edge impinges on said stop the stop yields, releasing one longitudinal edge, and both longitudinal edges impinge on one another, thus forming a butt joint. 
     To prevent dirt occurring during welding, such as splashes of melt or smoke (for example, evaporated zinc in the case of galvanized sheets) from dirtying the form tool, according to one feature of the invention an intercepting bowl is disposed in the zone of the top of the internal mandrel halves. Such an intercepting bowl can be cleaned or interchanged after each use of the welding device. 
     Preferably the intercepting bowl is disposedly fixed and extends over the entire length of the form tool halves. With such a construction also according to a possible feature of the invention the intercepting shell is connected tightly to the internal form halves and co-operates with the exposed longitudinal edge zones of the tube formed from the sheet metal blank to form a channel. Such a channel is suitable for protective gas flushing or for removing by suction the vapors occurring during welding. 
     Conveniently the intercepting bowl is made of a flexible material and has a V-shaped cross-section. This construction is particularly suitable, since it can readily adjoin the internal mandrel halves and does not impede the moving together of the form tool halves, since during this it folds together. 
     However, alternatively the intercepting bowl can be constructed to move together with the welding device. Constructionally this can readily be effected if according to the invention the intercepting bowl is borne at the end face by a tappet for the ejection of the internal mandrel halves. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     An embodiment of the invention will now be explained in greater detail with reference to the drawings, which show: 
     FIG. 1 a perspective view of an apparatus for the production of straight bead welded tubes from flat sheet metal blanks, 
     FIG. 2 a perspective view to an enlarged scale of a detail of the apparatus shown in FIG. 1, viewed from the front side, 
     FIG. 3 a front view of the apparatus shown in FIG. 1, 
     FIG. 4 a perspective view to an enlarged scale of a detail of the apparatus shown in FIG. 1, viewed from the rear side, 
     FIG. 5 a perspective view to an enlarged scale, and from a different perspective from FIG. 4, of a detail of the apparatus shown in FIG. 1, viewed from the rear side, 
     FIG. 6 a simplified front view of two form tool halves of the apparatus shown in FIG. 1, and 
     FIG. 7 a front view of one of the form tool halves shown in FIG. 6, with additional details. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Built up on a tool support  1  is a form tool consisting of two form tool halves  2 ,  3 . A welding device  5  for straight bead welding can be driven by means of a carriage  4  over the form tool halves  2 ,  3 . While the form tool half  2  is disposedly fixed on the tool support  1 , the form tool half  3  is mounted on linear guides  6  and can be moved by means of adjusting cylinders  7  in the direction of the other form tool half  2 . 
     The form tool halves  2 ,  3  are of substantially identical construction. They consist of an outer cylindrical half shell  8  (see FIG. 6) which is made up of individual portions in the longitudinal direction, and an internal mandrel half  10  fixed therein with the formation of a form gap  9 . At the front end (cf. FIG. 2) the internal mandrel half  10  is retained by means of a pivoting arm  11  engaging with an end face pin  11   a  of the internal mandrel half  10 . The internal mandrel half  10  has at the rear end (cf. FIGS. 4,  5 ) a portion  10   a  of enlarged external diameter which is identical to the internal diameter of the half shell  8 . Via the portion  10   a  the internal mandrel half  10  bears firmly against the half shell  8 . The portion  10   a  projects by an amount  10   b  axially in relation to the half shell  8 . Engaging with the portion  10   a  in this portion  10   b  are releasable pulling elements  12  which can be pulled radially outwards and therefore against the half shell  8  by means of couplable clamping pins  13 . A pivoting force around a horizontal axis extending transversely of the longitudinal direction can be exerted on the projecting portion  10   b  by means of a pivoting arm  14  and an adjusting cylinder  15 . Operation will be further discussed hereinafter in connection with the removal of a finished tube. 
     To enable the internal mandrel halves  10  to be supported in the radial direction in the zone between their ends, they are engaged by supporting members  16  (see FIG. 7) forming part of a comb  17 . The supporting members  16  engage through recesses  8   a  in the form of slots in the half shell  8  and are coupled to the internal mandrel half  10  via sliding couplings consisting of a cylindrical attachment  16   a  and a corresponding recess  10   e  in the internal mandrel half  10 . They can be uncoupled from the internal mandrel half  10  by displacement in the direction indicated by arrow P 1 . This is necessary to ensure that the supporting members  16  do not block the form gap  9  during the insertion of a sheet metal blank B. Alternatively, the fixation of the internal mandrel half  10  can also be effected by fixing mandrels which engage substantially radially through the half shell  8  and can be moved transversely of the form tool longitudinal axis and engage in recesses provided in the internal mandrel half. As a result, the internal mandrel half  10  is supported and fixed axially and radially. Preferably the fixing mandrels are adjusted at an angle of approximately 45° to the vertical plane of symmetry. 
     Provided below the two form tool halves  2 ,  3  and in the brought-up condition therebetween is a supporting construction  18  for the sheet metal blank to be shaped. The supporting construction  18  receives the sheet metal blank B and prevents it from bending downwards during introduction into the form gap  9 . The supporting construction  18  consists of a number of parallel beams  19  disposedly fixed on the tool support  1  transversely over the longitudinal direction of the apparatus, and deflecting plates  20 ,  21  which are disposed at each form gap and are supported by resiliently borne guides  22 ,  23 . When the form tool halves  2 ,  3  are moved together, the deflecting plates  20 ,  21  boost the introduction of the sheet metal blank B into the form gap  9 , outward bending of the sheet metal blank B lying by its own weight on beams  19  being prevented thereby. Due to the resilient bearing of the deflecting plates, they yield at the end of this shaping process, so that the form tool halves  2 ,  3  can be moved completely together. 
     The internal mandrel half  10  (cf. FIGS. 6,  7 ) of one form tool half  3  has at the lower end a projection  24  and thereabove an abutment  24   a . During its introduction into the form gap  9  of the other form tool half  2  one longitudinal edge of the sheet blank B is borne thereon and thereagainst. 
     Associated with the form tool half  2  at its top is a stop  25  which can move in the direction of the arrows P 2 , P 3  in such a way that the sheet metal blank B inserted into the form gap  9  abuts by its longitudinal edge the vertical flank  26  of the stop  25 —i.e., above a short chamfer  26   a . An opposite cutting edge  26   b  is substantially longer and lies in the zone of emergence of the other form gap  9 , so that the sheet metal blank emerging at this place impinges on the chamfer  26   a  by its longitudinal edge. 
     The half shell  8  and the internal mandrel half  10  can terminate at the top in horizontally extending portions  8   b ,  10   c . This configuration serves to bring the longitudinal edges into an even better position for welding. However, alternatively the carriage  4  can also have a contact pressure roller  27  which precedes the welding device  5  and forces the longitudinal edges downwards. 
     As shown in FIG. 6, disposed at the top point of the form tool halves  2 ,  3  is a fixed intercepting bowl  30  taking the form of a V-shaped profile with outwardly bent ends via which it bears against the portions  10   c . The intercepting bowl  30  is flexible, being more particularly made of sheet metal, so that it can be folded together when the form tool halves  2 ,  3  are moved together. It extends over the entire length of the form tool halves  2 ,  3  and serves for collecting waste materials deposited during welding. It collaborates with the brought-together ends of the tube formed from the sheet metal blank B to form channel  30   a  for a protective gas flushing, or it can act as a sectional removal channel. 
     There is an alternative construction for the intercepting bowl (not shown). In the alternative construction a bowl is provided which travels together with the welding device  5 . The bowl can be disposed at the end of a tappet by means of which the internal mandrels  10  can be ejected in accordance with the progress of the weld. 
     To shape sheet metal blanks of different thicknesses (tailored blanks), more particularly sheet metal blanks consisting of welded-together sheets of different thicknesses, the form gap has a different width, corresponding to the different thickness of the sheet, either in the peripheral or the longitudinal direction. In the case of sheet metal blanks with small differences in thickness up to approximately 0.1 mm, the form gap can have a constant width, since the small differences in thickness lie within the range of the overdimensioning of the form gap which must be provided in any case. 
     The apparatus according to the invention operates as follows: 
     As shown in FIG. 3, with the form tool halves  2 ,  3  moved up, a sheet metal blank B with parallel longitudinal edges is laid on the supporting construction  18 . The sheet metal blank B is threaded by the longitudinal edge shown on the right in the drawing into the form gap  9  of the form tool half  2 . The left-hand longitudinal edge is laid on the projection  24  of the internal mandrel half  10  of the other form tool half  3 , so that the sheet metal blank B bears via said longitudinal edge against the abutment. The abutment  24   a  gives the sheet metal blank B precise guiding, so that the sheet metal blank B cannot tilt in the form gap  9 . The form tool half  3  is then driven in the direction of the form tool half  2 . The sheet metal blank B is inserted into the form gap  9  until its right-hand longitudinal edge is situated adjacent the supporting members  16 . Then the supporting members  16  are pulled over the sliding couplings, so that the form gap  9  is completely opened up. Then the sheet metal blank B is further advanced, until the longitudinal edge abuts the stop  25 , namely at a vertical flank, as shown clearly in FIGS. 6 and 7. Since, there is no force operating in the direction of arrow P 2 , the stop  25  remains in the position shown. 
     Then the form tool half  3  is retracted a little until the left-hand longitudinal edge no longer bears against the projection  24 . The sheet metal blank B is then threaded by its left hand longitudinal edge into the form gap  9  of the form tool half  3 , and the form tool half  3  is driven in the direction of the form tool half  2 . The supporting members of the internal mandrel half are removed in the same manner as in the case of the right-hand internal mandrel half. As soon as the sheet metal blank with its left hand longitudinal edge leaves the form gap  9  and impinges on the stop  25 , namely on the chamfer  26   b , the stop  25  is moved upwards by the longitudinal edge in the direction of arrow P 2 . With this movement the right-hand longitudinal edge also arrives on the chamfer  26   a  and is released, so that with further pushing-together the longitudinal edges impinge on one another in precisely the required welding position. Then the stop  25  is removed from the zone of the top by means which are not shown and the joint gap for the straight bead welding is opened up. If necessary, the sheet metal edges can be after-shaped by contact pressure means prior to welding, to compensate for the springing up of the sheet metal edges due to the elasticity of the material and to obtain a parallel joint gap. 
     However, before moving together takes place, the intercepting bowl  30  taking the form of a V-shaped profile is laid by its outwardly pointing angled edges on the horizontal zones  10   c  and thus supported by the internal mandrel halves  10 . After further movement together, the intercepting bowl  30  becomes further folded and co-operates with the freely projecting edge zones of the tube formed from the sheet metal blank B to form a channel through which protective gas can be conveyed or via which vapour can be guided. 
     There are two possible ways of removing the straight bead welded tube from the apparatus. However, in any case the first thing is that the pivoting arms  11  at the front end are released. The tensioning at the rear end is then also released by pulling the pins  13  upwards. Then the form tool half  3  is moved back a little. In the first alternative, after the pulling members  12  have been removed the internal mandrel halves  10  can be pulled out. This is possible since they have clearance between them in the radial direction in the horizontal plane. In the second alternative the internal mandrel halves  10  remain in situ. However, to prevent the front end of the internal mandrel halves  10  from bearing against the tube and impeding its withdrawal, it can be somewhat lifted at the front end by a pivoting force being exerted on the projecting portion  10   b  by means of the pivoting arm  14  and the adjusting cylinder  15 . In both cases the tube can then be pushed out of the half shells  8  by means of an entraining member  28  borne by the carriage  4 . 
     The special advantages of the invention are that, using a comparatively simply constructed apparatus, it enables sheet metal blanks to be shaped into short tubes and to be straight welded in the clamping system provided for the sheet metal blanks by the form tool halves.
An apparatus to produce straight bead welded tubes from flat sheet metal blanks with two parallel longitudinal edges includes two form tool halves. The two form tool halves have outer cylindrical half shells borne by a tool support, and are disposed laterally inverted in relation to one another. They are moved towards one another and out of an opened receiving position for the sheet metal blanks into a closed position, in which the two longitudinal edges are held together by the two form tool halves. The apparatus also includes a welding device, which moves over the two form tool halves along the two longitudinal edges retained in a welding position. The two form tool halves have cylindrical internal mandrel halves which are fixedly associated with the outer cylindrical half shells and which cooperate with the outer cylindrical half shells to produce form gaps for insertion of the sheet metal blanks.