TECHNICAL FIELD  
       [0001]     The invention relates to a machine tool with a machine tower, which is linearly displaceable along a straight track, with a tool head, which is arranged on the machine tower and comprises a tool for machining workpieces, and with at least two work-holding devices for at least two workpieces. The invention also relates to a method for machining at least two workpieces with a machine tool which is displaceable along a straight track.  
       PRIOR ART  
       [0002]     As a rule, a number of workpieces are machined one after another with a machine tool. After all the machining steps have been carried out on a first workpiece, it is removed from a work-holding device which holds the workpieces in such a way that they can be machined by the machine tool. A new workpiece to be machined is then clamped into the work-holding device, after which its machining can begin. Exchanging the workpieces leads to a certain idle time in which the machine tool cannot carry out machining. Especially in the case of large or heavy workpieces and also with types of machining which necessitate complicated clamping, the idle time can be considerable and greatly reduce the capacity of the machine tool.  
         [0003]     U.S. Pat. No. 4,984,351 (Brother) discloses a machine tool, which is linearly displaceable along a straight track, with a vertically movable spindle. At least two tables, which are horizontally displaceable at right angles to the track independently of one another, are arranged next to one another along one side of the track. One large workpiece can be fastened on the tables together or a number of small workpieces can be fastened on the tables individually. If smaller workpieces are being machined, the tables allow one table to be loaded while a workpiece is already being machined on another table. In this way, the idle time can be minimized.  
         [0004]     If large workpieces, for example long workpieces which have to be clamped with their greatest extent parallel to the straight track, are to be machined, all the tables are used for fastening such a workpiece. In this case, the usual idle times occur when the current workpiece is exchanged for the next.  
         [0005]     DE 298 01 236 U (DS Technologie Werkzeugmaschinen GmbH) describes a machine tool for cutting machining of large (in particular long) components, for example of aircraft integral components made of aluminum alloys. The machine comprises a column which is linearly movable between a machine bed and an upper guide along a straight track, on which column a machining head, which is movable along a number of axes, is arranged at the side. At least one foldable clamping table is arranged along the track. For machining by means of the clamping table, the workpieces are brought out of a horizontal preparation position into a vertical machining position. If a number of clamping tables independent of one another are provided next to one another, one workpiece can be machined while a finished machined workpiece is exchanged for a new workpiece in the horizontal workpiece change position of the clamping table. Dead times can thus be avoided.  
         [0006]     If a long workpiece is being machined, all the clamping tables are occupied by it. In this case too, workpiece change therefore again takes place in the conventional manner while the machine tool is idle. A reduction of the idle times and thus an increase in the capacity is not possible with large workpieces.  
       DESCRIPTION OF THE INVENTION  
       [0007]     It is an object of the invention to provide a machine tool belonging to the technical field referred to in the introduction with increased capacity.  
         [0008]     The solution for achieving the object is defined by the features of claim  1 . According to the invention, a first of the work-holding devices is arranged on a first side of the straight track, and a second of the work-holding devices is arranged on a second side of the straight track, which lies opposite the first side. The tool head can be positioned in such a way that it can alternatively machine a first of the at least two workpieces in the first work-holding device or a second of the at least two workpieces in the second work-holding device.  
         [0009]     The arrangement of two work-holding devices on the two sides of the straight track and the corresponding positionability of the tool head afford a possibility of arranging two long workpieces in machining positions independently of one another. As long as one of the workpieces is being machined on one side of the track, the workpiece on the other side of the track can be exchanged or repositioned for further machining steps. As soon as the machining of the first workpiece has been carried out, the tool head can be repositioned and the machining of the second workpiece can begin immediately. The idle time is limited to the repositioning of the tool head and is negligible compared with the dead time when a large workpiece is exchanged.  
         [0010]     The machine tower is advantageously rotatable by at least 180° about a vertical axis. By virtue of this, a tool head arranged on the machine tower changes its position and orientation in such a way that the workpiece on in each case the other side can be machined. Both workpieces can be reached symmetrically by the same tool head in the same way. This design makes machining of an opposite workpiece possible independently of the movability of the tool head and guarantees good support of the tool head. It is advantageous especially when the machine head is guided along a considerable part of the machine tower, for example in order that it can be displaced vertically. If machining steps, such as drilling or cutting for example, are advantageously carried out in the axial direction of the tool head, the machine tool can also be designed in such a way that the machining can take place when the machine tower occupies an intermediate position between 0° and 180° or is rotated by more than 180°.  
         [0011]     Otherwise, it is also possible for only an upper part of the machine tower, on which the tool head is arranged and guided, to be rotatable by 180° about a vertical axis.  
         [0012]     Alternatively, the tool head can be pivotable by at least 180° about a horizontal axis. If the tool head works along a horizontal axis or is movable symmetrically in relation to a horizontal axis, such a pivoting movement about a horizontal axis parallel to the straight track likewise makes the machining of workpieces on both sides of the straight track possible. Since the entire machine tower does not have to be rotated during repositioning of the tool head, the mass to be moved is greatly reduced. If machining steps, such as drilling or cutting for example, are advantageously carried out in the axial direction of the tool head, the machine tool can also be designed in such a way that the machining can take place when the tool head occupies an intermediate position between 0° and 180° or is pivoted by more than 180°.  
         [0013]     The tool head advantageously has cartesian kinematics and is in particular arranged on the machine tower in such a way that it is rotatable about three rotational axes perpendicular to one another. Such a tool head affords maximum possible flexibility because it can occupy any orientation in relation to the workpiece. The positionability of the tool head is brought about by displacement along the straight track, which defines an X axis, and also if appropriate by vertical displacement (Z direction) along the machine tower and by horizontal displacement transversely to the X axis (Y direction) if the tool head is arranged on a correspondingly movable bracket.  
         [0014]     If lighter work is to be carried out, the machine tower is advantageously mounted in a free-standing manner on a guide which corresponds to the straight track. Lighter work includes in particular drilling, broaching, riveting, screwing, hardening or welding, during which the torque arising owing to the force between tool head and workpiece can easily be transmitted downward from the machine tower to the guide. The guide is a rail, for example, on which the machine tower is mounted movably via rollers.  
         [0015]     For carrying out heavier work, the machine tower is preferably mounted between a lower guide which corresponds to the straight track and an upper counterguide which extends parallel to the lower guide. The considerable torques which arise during milling, deburring or cutting, for example, can thus be carried off to the two guides via the machine tower. Guidance on both sides can also be advantageous for carrying out lighter work, as the machine tower can thus be of narrower and consequently lighter design than in the case of unilateral guidance, which makes the displacement of the tower along the straight track and the rotation of the tower about its vertical axis easier. The improved dynamics result in smaller delays during rotation or displacement of the machine tower.  
         [0016]     A method for machining at least two workpieces, in particular with a machine tool as described above which is displaceable along a straight track, comprises the following steps: 
    a) clamping a first of the at least two workpieces into a first work-holding device on a first side of the straight track;     b) positioning a tool head of the machine tool so that the first workpiece can be machined;     c) machining the first workpiece;     d) at the same time, clamping a second of the at least two workpieces into a second work-holding device on a second side of the straight track, which lies opposite the first side;     e) after completion of the machining of the first workpiece, positioning the tool head so that the second workpiece can be machined;     f) machining the second workpiece.    
 
         [0023]     Provided that the removal of a finished machined workpiece and the clamping of a new workpiece requires no more time than the entire machining of a workpiece, the idle time of the machine tool is reduced to step e), which can take place very quickly. The repositioning takes place in particular by rotating the machine tower by 180° about a vertical axis or by pivoting the tool head by 180° about a horizontal axis.  
         [0024]     A machine tool of the type described above, which comprises a machine tower, which is linearly displaceable along a first straight track, and also a tool head, which is arranged on the machine tower and comprises a tool head for machining workpieces, and with a work-holding device for a workpiece at the side of the first straight track, can comprise means for moving the machine tower onto a second straight track on an opposite side of the work-holding device in relation to the first straight track. The tool head can be positioned in such a way that it can machine the workpiece on both sides. By virtue of the fact that the workpiece can be machined on both sides with the same machine tower, the same tool head and the same tool, no second machine tower on the second side of the workpiece is required. The corresponding machine tool is therefore of more simple construction and more cost-effective. Such a machine tool is advantageous especially in the case of long workpieces, which cannot be rotated in relation to the machine tower in such a way that both longitudinal sides can be reached by the tool.  
         [0025]     The machine tool advantageously comprises a slide for moving the machine tower from the first straight track to the second straight track. Such a slide takes the machine tower at one end of the first straight track, for example, transports it transversely to the track direction, past the workpiece at the side, to the second straight track and delivers it to the second straight track. This leads to the tool head being directed away from the workpiece for the time being. By means of a machine tower which is rotatable by 180° about a vertical axis or a tool head which is pivotable by 180° about a horizontal axis as indicated above, it is made possible in a simple way to align the tool head in such a way that the second side of the workpiece can be machined. In comparison with guiding the machine tower around the workpiece in a U shape, for example, a considerable space-saving is achieved especially in the case of narrow workpieces.  
         [0026]     Further advantageous embodiments and feature combinations of the invention emerge from the detailed description below and the entirety of the patent claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     In the drawings used for explanation of the illustrative embodiment:  
         [0028]      FIG. 1  shows a perspective view of a machine tool according to the invention with a rotatable machine tower;  
         [0029]      FIGS. 2A, 2B  show diagrammatic illustrations of the machine tool with a rotatable machine tower and the work-holding devices;  
         [0030]      FIGS. 3A, 3B  show diagrammatic illustrations of another machine tool with a pivotable tool, and  
         [0031]      FIG. 4  shows a diagrammatic illustration of a machine tool according to the invention for machining a workpiece on both sides. 
     
    
       [0032]     In principle, identical parts are provided with the same reference numbers in the figures.  
       WAYS OF EMBODYING THE INVENTION  
       [0033]      FIG. 1  shows a machine tool according to the invention. The machine  1  comprises a vertical machine tower  2  which has an essentially rectangular cross section which tapers upward, the rear side  2 . 1  extending at an angle in the upper part. A tool slide  3 , which bears at one of its ends a tool  4  with a drilling head  5 , is arranged on the front side  2 . 2  of the machine tower  2 . The drill of the drilling head  5  is oriented vertically downward and can consequently produce vertical bores. The tool slide  3  is mounted horizontally displaceably along an axis Y on a carrier plate  6  which in turn is mounted vertically displaceably along an axis Z 1  on the front side  2 . 2  of the machine tower  2 . To this end, the carrier plate  6  comprises rails on its rear side which are guided in corresponding grooves on the front side  2 . 2  of the machine tower  2 .  
         [0034]     The tool  4  is vertically movable on rails  7  along the axis Z 2  in relation to the tool slide  3 . By virtue of this, movement of the entire tool slide  3  along the machine tower is avoided when small positional changes take place. The drilling head  5  moreover comprises a sprung pressing-on element, which likewise acts vertically and presses the drilling head onto the workpiece.  
         [0035]     The machine tower  2  stands on a base plate  8 , which also carries in a box  9  a hydraulic unit and also a vacuum sucker. The base plate  8  is in turn connected firmly to a rotary table  10 , which is arranged on a slide  11  rotatably about a vertical rotational axis C. The slide  11  comprises guide elements  12  on the outside on its lower side, which interact with rails  13  on the upper side of a machine guide  14 . This makes movement of the slide  11  and thus of the entire machine tool possible along the straight track (axis X) defined by the machine guide  14 . The machine guide  14  is formed by a hollow profile, on which a flexible energy chain  15  is guided at the side in a channel  16  which extends along the entire length of the machine guide  14 .  
         [0036]     The energy chain  15  is connected to an appropriate connection arranged on the slide  11 . During movement along the straight track, depending on direction, a longer or shorter section of the energy chain  15  is lifted out of the channel  16  or returned into the channel  16 . The energy chain  15  contains in particular hydraulic hoses, power cables and control cables for supplying and controlling the movement of the machine tower  2 , the tool slide  3  and the tool  4 . Energy is transmitted on from the slide  11  to the machine tower  2  via a second energy chain  17 , which is connected to hoses and cables in the interior of the machine tower  2 . By virtue of its flexibility, the second energy chain  17  allows the rotary movement of the machine tower  2  about its vertical rotational axis C as the energy chain  17  can corotate accordingly and permits at least a rotation by 180°.  
         [0037]     A further energy chain  18  forms the connection between the machine tower  2  and the carrier plate  6  of the tool slide  3  and allows vertical movement of the plate along the axis Z 1 . Still another energy chain  19  connects the carrier plate  6  and the tool slide  3  and is flexible along the axis Y. A final energy chain  20  connects the tool slide  3  to the tool  4  movably along the axis Z 2 .  
         [0038]     A drive  21 , which makes movement of the slide  11  along the machine guide  14  possible by, for example, interacting with a rack on the machine guide  14  via a gearwheel, is arranged on the slide  11 . The rotary movement of the machine tower  2  about its vertical axis C takes place hydraulically. The horizontal and vertical movements of the tool slide  3  are controlled via NC axes. For load compensation, a pressure cylinder  22 , which is connected to the carrier plate  6  for the tool slide  3  via a push rod  23 , is arranged on the outer side of the machine tower  2 . The tool  4  on the other hand comprises a separate drive with electric motor for the drill  5 .  
         [0039]     A tool change device  24 , which allows automatic exchange of the drilling head  5  for replacement drills held available in the tool change device  24 , is arranged on the lower side of the tool slide  3 .  
         [0040]      FIGS. 2A and 2B  are diagrammatic illustrations of the machine tool and the work-holding devices, seen from above. The work-holding devices  25 ,  26  extend along the rails  13  (or the machine guide) on both sides. Their length is slightly shorter than the length of the rails  13  and selected in such a way that the tool  4  can reach the entire area of in each case one of the work-holding devices  25 ,  26 . In  FIG. 2A , the machine tower  2  is rotated in such a way that a workpiece which is clamped in the first work-holding device  25  can be machined. For machining, the tool can be raised and lowered by raising the tool slide  3  with its carrier plate  6 , moved toward the workpiece and away from the workpiece by displacing the tool slide  3  in relation to the carrier plate  6  along the axis Y, and displaced along the workpiece (axis X) by displacing the entire machine tool  1  along the straight track on the rails  13 .  
         [0041]     If a workpiece is then to be machined on the second work-holding device  26 , the machine tower  2  is rotated by 180° about its vertical rotational axis C by means of the rotary table  10 , so that it occupies the position shown in  FIG. 2B . To this end, the machine tower  2  is raised hydraulically from a lower part of the rotary table  10 , rotated by 180° and set down again. Ring gears on the lower and upper part of the rotary table  10  mesh and bring about reliable fixing and rotational locking of the machine tower  2  in relation to its vertical axis.  
         [0042]     Workpieces in both work-holding devices  25 ,  26  can be machined in exactly the same way; the work-holding devices are entirely equivalent. All known devices which are suitable for long workpieces, for example fixed or pivotable or movable clamping tables, pallet supports, clamping jaws etc., can be used as work-holding devices  25 ,  26 . Their clamping surface can be arranged both vertically and horizontally or at an angle. It is also possible for different work-holding devices for receiving different workpieces to be provided on the two sides of the track.  
         [0043]      FIGS. 3A and 3B  show diagrammatic illustrations of another machine tool with a pivotable tool. The machine  101  comprises a machine tower  102 , which is arranged on a base plate  108 . The base plate  108  comprises guide elements  112  on its lower side which interact with rails  113  arranged on the upper side of a machine guide  114  which defines a straight track. By virtue of this, the base plate  108  of the machine tool  101  can be displaced horizontally along the straight track together with the machine tower  102 .  
         [0044]     Two vertical rails  127 , on which a carrier plate  106  is vertically displaceable along the axis Z 1 , are arranged on the front side  102 . 2  of the machine tower  102 . A rotary table  128 , on which a tool slide  103  is arranged linearly displaceably (axis Y), is mounted rotatably on the carrier plate  106 . The rotary table  128  is pivotable about a horizontal rotational axis D 1  in relation to the carrier plate  106 . A tool  104  is arranged pivotably about a horizontal axis D 2  at one end of the tool slide  103 , so that the orientation of the drilling head  105  can be varied in order that, for example, bores with different alignments are possible. The tool  104  can also be mounted on the tool slide  103  via a ball joint, so that the orientation of the drilling head  105  can be selected freely within the half-space facing the workpiece.  
         [0045]     In  FIG. 3A , a workpiece  129  which is clamped in a work-holding device  125  is being machined by the tool  104 . The work-holding device  125  comprises a base plate  125 . 1 , the upper surface of which lies roughly at the height of the machine guide  114 . A vertical carrier  125 . 3  is arranged thereon about a horizontal rotational axis  125 . 2 . The workpiece  129  is clamped vertically onto this carrier by means of various fastening elements  125 . 4 .  
         [0046]     The second work-holding device  126  is illustrated in its folded-down position in  FIG. 3A . This position allows simple loading and unloading of a workpiece  130  into and from the work-holding device  126  and is brought about by folding the carrier  126 . 3  down about the rotational axis  126 . 2 . In the horizontal position, the carrier  126 . 3  is laid onto a support  126 . 5  for stabilization. The foldable work-holding devices  125 ,  126  can be designed in such a way that they can also be fixed in an inclined position between horizontal and vertical position, so that corresponding workpieces can be machined better by the machine tool  101 .  
         [0047]      FIG. 3B  shows the machining of the other workpiece  130 . To this end, the second work-holding device  126  is folded up, and the tool slide  103  is pivoted by 180° about the rotational axis D 1  by means of the rotary table  128 . By virtue of the movability of the tool  104  with the drilling head  105 , it is likewise possible on the second working side to machine the second workpiece  130  while the first workpiece  129  can be removed from the first work-holding device  125  and a new workpiece clamped there.  
         [0048]     This second machine tool  101  with pivotable tool can also be combined with a large number of different work-holding devices.  
         [0049]     The method according to the invention is also illustrated by  FIGS. 3A and 3B . For machining workpieces  129 ,  130  with the machine tool  101  according to the invention, a first workpiece  129  is first clamped into the first work-holding device  125 . As soon as this has been done, the tool  104  can be positioned in such a way that the first workpiece  129  can be machined. While the latter is being machined, the second workpiece  130  is clamped into the second work-holding device  126  on the opposite side of the straight track. When machining of the first workpiece  129  is finished, the tool  104  is pivoted by 180° on the rotary table  128  together with the tool slide  103 . This brings the tool  104  into a position in which the second workpiece  130  can be machined with it. The workpiece  129  in the first work-holding device  125  can be exchanged for a new workpiece.  
         [0050]      FIG. 4  is a diagrammatic illustration of a machine tool according to the invention for machining a workpiece on both sides. The machine tower  2 , which is carried on the base plate  8  and comprises a tool slide  3  with a tool  4 , corresponds to the machine tower which is rotatable about its vertical axis illustrated above. The machine tool is linearly displaceable along the workpiece  25 ′ along a first straight track on rails  13   a.  At one end of the first track, the machine tower  2  can together with the base plate  8  be moved further onto a slide  201 . To this end, the slide  201  comprises rails  202  which with regard to their shape and arrangement correspond to the rails  13   a.    
         [0051]     As soon as the machine tower  2  has been moved onto the slide  201 , the slide  201  can be displaced on rails  203  transversely to the first track  13   a,  past the workpiece  25 ′, to one of the ends of a second straight track, which is formed by rails  13   b.  The second straight track is parallel to the first track, but lies on the side opposite the workpiece. After the machine tower  2  has been received by the rails  13   b  of the second track, the tool slide  3  with the tool  4  faces away from the workpiece for the time being. By means of a 180° rotation of the machine tower  2  about its vertical axis, the tool  4  is therefore positioned in such a way that the second side of the workpiece  25 ′ can be machined.  
         [0052]     Instead of a machine tower which is rotatable by 180° about its vertical axis, the machine tool can on the other hand—as illustrated above—comprise a tool which is pivotable by 180° about a horizontal axis.  
         [0053]     It can be seen clearly from  FIG. 4  that the device illustrated requires considerably less space than a U-shaped guide for the machine tower, which leads around the (narrow) workpiece. This is because a U-shaped guide necessitates a certain minimum radius of the rails on which the machine tower is moved. This leads to a certain minimum extent in prolongation of the workpiece, which is not desired especially in the case of workpieces which are long anyway. Guiding the machine tower along a curved section is moreover constructionally more involved as it necessitates a considerably more complicated bearing arrangement of the machine tower on the rails.  
         [0054]     The tool for machining the workpieces can have various degrees of freedom. In particular, it can be mounted on the machine tower in such a way that it is displaceable in the X, Y and Z directions and rotatable about three rotational axes perpendicular to one another. A universal head mounted in such a way can occupy all positions and orientations in relation to the machine tower within a certain range. The tool can be a broaching, milling, deburring, riveting, screwing, hardening, soldering or welding tool, laser sources or electron-beam sources also being possible, for example, for cutting, hardening or welding.  
         [0055]     If the machine tower is to be continuously adjustable in relation to its vertical axis, an NC axis, for example, can also be provided for rotating the machine tower instead of hydraulics.  
         [0056]     For heavy work, in which great force is transmitted between tool and workpiece, a second guide, arranged above the machine, can be provided parallel to the first machine guide. In the case of a rotatable machine tower, this is also mounted rotatably in the upper guide; in the case of a pivotable tool, this is moved, for the pivoting movement, along the machine tower into a vertical area in which a collision with neither the lower nor the upper guide occurs.  
         [0057]     The machine tool can be arranged together with the two work-holding devices, or with the two track guides, on a common bedplate or on a common frame, so that the stability of both the machine guide and the work-holding devices is further improved.  
         [0058]     In summary, it may be stated that the invention provides a machine tool and also a method for machining at least two workpieces with increased capacity.
The invention relates to a machine tool ( 1) comprising a machine tower ( 2) which can be displaced in a linear manner along a straight track ( 13), a tool head ( 4) which is arranged on the machine tower ( 2) and a tool ( 5) which is used to process work pieces. A first mounting device ( 25) is arranged on a first side of the straight track ( 13) and a second mounting device ( 26) is arranged on the other side of the straight track ( 13) which is opposite the first side. The tool head ( 4) can be positioned in such a manner that it can selectively process a first work piece in the first mounting device ( 25) or a second work piece in the second mounting device ( 26). Initially, the first work piece is mounted in the first mounting device for processing, and the tool head is positioned in such a manner that the first work piece can be processed. During processing, the second work piece is mounted in the second mounting device. When the first work piece has been processed, the tool head is positioned in such a manner that the second work piece can be processed. As a result, it is possible to arrange two long work pieces independently from each other in a processing position. The stoppage time when the work pieces are changed reduces due to the new position of the tool head and is significantly reduced in relation to the stoppage time in common tool machines.