Abstract:
The invention relates to a turning center which comprises a machine frame, a first workpiece spindle unit having a first workpiece spindle, a second workpiece spindle unit, which is disposed on the machine frame and having a second workpiece spindle, at least one tool carrier unit, which is disposed on the machine frame and on which at least one tool for machining a workpiece held in the workpiece spindle units is disposed, and a working space, in which workpieces held in the workpiece spindle units can be machined with the tool. The aim of the invention is to provide a turning center which is compact as possible while allowing extensive turning and/or milling applications, and also ensures highest possible machining accuracy. For this purpose, the machine frame having a machine bed formed by two spaced-apart columns, by the first workpiece spindle unit being disposed on a front side of a first of the columns, facing the working space, by the second workpiece spindle unit being disposed on a front side of a second of the columns, facing the working space, and by the at least one tool carrier unit being disposed on a transverse side of one of the columns, facing the respective other column.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application is a continuation of International application No. PCT/EP2007/004243 filed on May 12, 2007. This patent application claims the benefit of International application No. PCT/EP2007/004243 of May 12, 2007 and German application No. 10 2006 026 184.4 of May 30, 2006, the teachings and disclosure of which are hereby incorporated in their entirety by reference thereto. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a turning center comprising a machine frame, a first workpiece spindle unit, which is disposed on the machine frame and has a first workpiece spindle that is rotatable about a first workpiece spindle axis, a second workpiece spindle unit, which is disposed on the machine frame and has a second workpiece spindle that is rotatable about a second workpiece spindle axis, at least one tool carrier unit, which is disposed on the machine frame and on which at least one tool for machining a workpiece held in the workpiece spindle units is disposed, and a working space, in which workpieces held in the workpiece spindle units can be machined with the tool. 
         [0003]    Turning centers of this type are known from the prior art, requiring a disadvantageously great amount of space. 
         [0004]    It is therefore an object of the invention to improve a turning center of the generic type in such a way that it is constructed to be as compact as possible while allowing extensive turning and/or milling applications, and also ensures highest possible machining accuracy. 
       SUMMARY OF THE INVENTION 
       [0005]    This object is achieved according to the invention in the case of a turning center of the type described at the beginning by the machine frame having a machine bed formed by two spaced-apart columns, by the first workpiece spindle unit being disposed on a front side of a first of the columns, facing the working space, by the second workpiece spindle unit being disposed on a front side of a second of the columns, facing the working space, and by the at least one tool carrier unit being disposed on a transverse side of one of the columns, facing the respective other column. 
         [0006]    The advantage of the solution for achieving the object as provided by the invention can be seen in the possibility of the turning center being constructed as compactly as possible, with high stability of the machine bed, as a result of two columns being used for constructing the machine bed. 
         [0007]    It is particularly advantageous here if the columns extend with their longitudinal axes transversely in relation to the standing area for the machine frame, so that the columns rise up over the standing area and consequently allow attachment of the workpiece spindle units and the tool carrier unit in a simple manner. 
         [0008]    In principle, it would be conceivable here for the columns to extend with their longitudinal axes in different directions, for example in a V-shaped manner in relation to one another. 
         [0009]    A particularly advantageous configuration provides, however, that the columns extend parallel to one another. 
         [0010]    Furthermore, within the scope of the solutions described so far, the columns could be formed in such a way that they extend obliquely with their longitudinal axes, that is to say at an acute angle in relation to the standing area. However, a suitable solution provides that the columns extend substantially perpendicularly in relation to the standing area. 
         [0011]    Furthermore, the columns could in principle also be of different lengths. 
         [0012]    A structurally advantageous solution provides, however, that the columns are of approximately the same length. 
         [0013]    The space that is obtained by the columns being spaced apart could optionally also be spanned by a wall or a connecting structure between the columns. 
         [0014]    A particularly advantageous solution, however, provides that at least one opening is provided between the columns, that is to say that a clearance that is freely accessible from both sides exists between the columns. 
         [0015]    When providing an opening, it would be conceivable in the simplest case for the columns to rise up over a frame base and not be connected to one another. 
         [0016]    In order, however, to be able to position the columns stably in relation to one another, it is preferably provided that the columns are connected by at least one cross bracing. 
         [0017]    In the case of an opening, it is provided here that the cross bracing lies outside the opening. 
         [0018]    A structurally advantageous solution provides that the columns are connected at the ends by at least one cross bracing. 
         [0019]    The at least one cross bracing may be provided here on a side opposite from the frame base. 
         [0020]    However, it is also conceivable for the columns to be connected to one another by a cross bracing at both of their ends. 
         [0021]    The cross bracing may be formed in a wide variety of ways. 
         [0022]    For example, the cross bracing may be a separate part that is to be connected to the columns. 
         [0023]    A solution that is structurally particularly suitable for stability provides that the at least one cross bracing is formed onto at least one of the columns and consequently forms a unit with at least one of the columns. In this case, the cross bracing may then be releasably connected to the other column. 
         [0024]    However, it is also conceivable to form the cross bracing integrally as a part formed onto both columns. 
         [0025]    With regard to the forming of a machine bed constructed according to the invention, no further details have been specified so far. Thus it would in principle be conceivable to form the machine bed as a one-piece part, but it is advantageous for reasons of simpler production to form the machine bed such that it is made up of two parts. 
         [0026]    The separation of the machine bed could take place here at a wide variety of points. An advantageous solution provides that the machine bed is made up of a first column with at least one crosspiece and a second column, the first column with the at least one crosspiece and the second column forming the two parts of the machine bed. 
         [0027]    A structural solution that is particularly suitable with regard to stability provides that the columns are connected to form a yoke-like body. Such a yoke-like body provides, in particular, that it encloses at least one opening. 
         [0028]    However, it also constitutes a yoke-like body if it encloses two openings. 
         [0029]    With regard to the forming of the front sides of the columns, no further details have been specified so far. 
         [0030]    Thus an advantageous solution provides that the columns have flat front sides, so that the workpiece spindle units can be mounted on them in a simple manner. 
         [0031]    Furthermore, it is provided with regard to the shape of the front sides that the respective front side runs transversely in relation to the standing area of the machine frame, it being provided in an advantageous solution that the two front sides lie in a common geometric plane. 
         [0032]    In addition, it is likewise provided in a suitable way that the columns have mutually facing flat transverse sides, it being possible for a tool carrier unit to be mounted on at least one of these transverse sides. 
         [0033]    The flat transverse sides are preferably disposed here on opposite sides of the opening. 
         [0034]    Furthermore, it is preferably also likewise provided that the transverse sides run transversely to a standing area of the machine frame. 
         [0035]    With regard to the forming of the columns themselves, no further details have been specified so far. 
         [0036]    Thus the columns could in principle have any cross-sectional form; such a cross-sectional form may be round or oval here, the front sides and the transverse sides optionally being flattened portions of a round or oval cross-sectional form. 
         [0037]    However, it is also conceivable for the columns to have polygonal cross-sectional forms. 
         [0038]    For example, a triangular cross-sectional form of the columns would be conceivable. 
         [0039]    A particularly advantageous solution provides that the columns have approximately rectangular cross-sectional forms. 
         [0040]    With regard to the way in which the workpiece spindle units are disposed on the columns, no further details have been specified so far. 
         [0041]    In principle, it would be conceivable to dispose the workpiece spindle units in a stationary manner on the front sides of the columns. 
         [0042]    A particularly advantageous solution provides that at least one of the workpiece spindle units is guided movably in relation to the machine bed parallel to the respective front side, in order to be able to realize, with the workpiece spindle unit, an axial movement that is required for the machining of the workpiece. 
         [0043]    However, it is suitable if each of the workpiece spindle units is guided movably in relation to the machine bed along defined movement axes parallel to the front side respectively carrying said units. 
         [0044]    In order to obtain an advantageous distribution of the axial movements in the case of the turning center according to the invention, it is preferably provided that at least one of the workpiece spindle units is guided movably in relation to the machine bed parallel to the respective workpiece spindle axis. 
         [0045]    It is preferably provided that both the workpiece spindle units are guided movably in relation to the machine frame parallel to their respective workpiece spindle axis. 
         [0046]    In order to obtain yet a further degree of freedom, it is preferably also provided that a workpiece spindle unit is guided in relation to the machine bed transversely to the respective workpiece spindle axis. 
         [0047]    In order to be able to machine a workpiece with the two workpiece spindle units as optimally as possible, it is preferably provided that both workpiece spindle units are movable with their workpiece spindle axes in a common geometric spindle guiding plane. 
         [0048]    With regard to the number of tool carrier units, no further details have been specified so far. It has merely been stated that at least one tool carrier unit is required. 
         [0049]    It is particularly advantageous, however, if two tool carrier units are provided. 
         [0050]    An advantageous distribution of the tool carrier units is obtained if each of the two tool carrier units is disposed on a transverse side of one of the columns of the machine bed. 
         [0051]    It would be conceivable in principle to dispose both tool carrier units on one column. 
         [0052]    However, it is particularly advantageous if a first tool carrier unit and a second tool carrier unit are disposed on different columns of the machine bed. 
         [0053]    A solution that is particularly advantageous with regard to precision during the machining provides that a first tool carrier unit is provided and that the first tool carrier unit is associated with the first workpiece spindle unit, so that machining of a workpiece held on the first workpiece spindle unit can be realized in a simple manner. 
         [0054]    It is advantageous here with regard to the machining precision if the first tool carrier unit is disposed on the column carrying the first workpiece spindle unit. 
         [0055]    Furthermore, a second tool carrier unit is preferably provided, and the second tool carrier unit is associated with the second workpiece spindle unit. 
         [0056]    In this case, it is likewise advantageous if the second tool carrier unit is disposed on the column carrying the second workpiece spindle unit. 
         [0057]    With regard to the way in which the respective tool carrier unit is disposed in relation to the opening, no further details have been specified so far. 
         [0058]    Thus an advantageous solution provides that the tool carrier unit extends into the opening. 
         [0059]    It is preferably even provided that the tool carrier unit extends through the opening. 
         [0060]    The tool carrier unit is preferably constructed in such a way that it has a tool carrier base and a tool carrier. 
         [0061]    In particular, the tool carrier base is formed in such a way that it is disposed at least partially in the opening. 
         [0062]    Furthermore, an advantageous embodiment provides that the tool carrier is movable in the working space outside the opening. 
         [0063]    In order to utilize the space optimally, it is preferably provided that the tool carrier unit has a drive unit, which is disposed on a side of the tool carrier base that is opposite from the tool carrier. 
         [0064]    The drive unit is suitably disposed on a side of the machine bed that is opposite from the tool carrier, so that it does not have any adverse influence with regard to the utilization of space in the region of the working space and lies on a side of the machine bed on which it has no disturbing influence with regard to its spatial extent. 
         [0065]    In particular, it is also provided in the case of this solution that the drive unit is disposed on the side of the opening that is opposite from the tool carrier. 
         [0066]    With regard to the movability of the tool carrier, no further details have been specified so far. Thus an advantageous embodiment provides that the tool carrier of the at least one tool carrier unit is movable parallel to a tool guiding plane which runs transversely to the workpiece spindle axis of the workpiece spindle associated with it. 
         [0067]    Furthermore, it is provided here that the tool carrier of the at least one tool carrier unit is movable in the tool guiding plane in a first direction, running parallel to the spindle guiding plane 
         [0068]    A further suitable embodiment provides that the tool carrier of the at least one tool carrier unit is movable in a second direction, running perpendicularly to the spindle guiding plane. 
         [0069]    Finally, in the case of an advantageous solution, it is provided that the tool carrier of the at least one tool carrier unit is rotatable about an axis running parallel to the tool guiding plane. 
         [0070]    With regard to the way in which the tool carrier base is disposed in relation to the machine bed, no further details have been specified so far. Thus a suitable solution provides that the tool carrier base is disposed on a tool slide. 
         [0071]    The tool slide is preferably movable here in a direction of slide advance, which runs approximately parallel to a longitudinal direction of the respective column. 
         [0072]    Furthermore, the tool slide is preferably movable in a direction of slide advance which runs approximately parallel to a transverse direction in relation to the respective workpiece spindle axis. 
         [0073]    In particular, the tool slide is movable in a direction of slide advance which runs approximately parallel to the respective transverse side on which the tool slide is disposed. 
         [0074]    Furthermore, it is provided that the tool slide is movable in an X direction of the turning center. 
         [0075]    With regard to the way in which a further direction of advance of the tool carrier is realized, it is provided that the tool carrier is movable in a direction of advance with respect to the tool carrier base. 
         [0076]    The direction of advance preferably runs approximately parallel to a transverse direction in relation to the respective workpiece spindle axis. Here, the direction of advance is preferably a Y direction of the turning center. 
         [0077]    In particular, in this case the direction of advance runs parallel to a transverse direction in relation to the direction of slide advance along which the tool carrier base is movable. 
         [0078]    In addition, the tool carrier is preferably rotatable about an axis of rotation with respect to the tool carrier base. 
         [0079]    In particular, the axis of rotation is aligned such that it runs approximately parallel to the direction of advance. 
         [0080]    The axis of rotation is preferably aligned such that it runs in a direction which is aligned parallel to a transverse direction in relation to the respective workpiece spindle axis. 
         [0081]    Furthermore, in particular, the axis of rotation runs parallel to a transverse direction in relation to the direction of slide advance. 
         [0082]    A preferred solution provides that the axis of rotation is a B axis of the turning center. 
         [0083]    In order to be able to realize the movability of the tool carrier in relation to the tool carrier base, it is preferably provided that the tool carrier is connected to the tool carrier base by a guiding arm, the guiding arm with the tool carrier suitably being movable in relation to the tool carrier base. 
         [0084]    With regard to the forming of the tool carrier, no further details have been specified in connection with the description so far of the individual exemplary embodiments. Thus an advantageous solution provides that the tool carrier has a tool spindle. 
         [0085]    For example, it is provided in the case of such a tool spindle that a tool spindle housing is held on the guiding arm of the tool carrier unit. 
         [0086]    Here, the tool spindle housing is preferably fixedly connected to a front end of the guiding arm. 
         [0087]    The tool spindle may be formed and disposed in relation to the axis of rotation of the tool carrier unit in various ways. An advantageous solution provides that the tool spindle has a tool spindle axis aligned transversely to the axis of rotation. 
         [0088]    The tool spindle is preferably likewise formed in such a way that it has a tool spindle axis aligned transversely to the direction of advance. Furthermore, a spindle shaft mounted in the tool spindle housing is provided with a tool receiving means at one end. 
         [0089]    Furthermore, the tool receiving means is disposed in relation to the axis of rotation on one side of the same. 
         [0090]    In the case of such a tool spindle, initially only one tool can be used. 
         [0091]    In order to increase the number of tools that can be used, it is preferably provided that the tool spindle housing has a supplementary tool carrier. 
         [0092]    Such a supplementary tool carrier could in principle be disposed anywhere on the tool spindle housing. 
         [0093]    For example, it would be conceivable to dispose the supplementary tool carrier on the tool spindle housing on a side opposite from the tool receiving means. 
         [0094]    However, a particularly advantageous solution provides that the supplementary tool carrier is disposed on the tool spindle housing on at least one side of the tool spindle axis. 
         [0095]    A further suitable solution provides that the tool spindle housing carries a supplementary tool carrier on more than one side, so that there is the possibility of using a number of tool carriers and in this way multiplying still further the number of tools that can be used. 
         [0096]    It is particularly suitable here if the tool spindle housing carries a supplementary tool carrier on mutually opposite sides. 
         [0097]    The supplementary tool carriers are preferably disposed here on the tool spindle housing on mutually opposite sides of the axis of rotation. 
         [0098]    In order to be able to increase the number of tools further, it is suitably provided that the supplementary tool carrier is formed as a multiple tool carrier. 
         [0099]    Such a multiple tool carrier could, for example, have tools disposed in a two-dimensional area. 
         [0100]    However, it is particularly suitable if the multiple tool carrier is formed as a linear tool carrier, since such a linear tool carrier has spatially suitable conditions for the disposition of the multiple tools while at the same time causing little disturbance of the other tools during the use of one of the tools on the workpiece. 
         [0101]    Such a linear tool carrier may be disposed in a wide variety of ways. 
         [0102]    For example, it would be conceivable to align the linear tool carrier parallel to the tool spindle axis. 
         [0103]    However, it is particularly suitable if the linear tool carrier has tool receiving means disposed in a direction of a row parallel to the direction of advance of the tool spindle, the direction of advance being the direction along which movement of the tool spindle takes place in relation to the tool carrier base. 
         [0104]    Such tool receiving means are usually tool holder receiving means, in which tool holders carrying tools can be inserted. However, it is also conceivable to form the tool receiving means in such a way that tools, such as drilling tools for example, can be inserted into them directly. 
         [0105]    In order to create particularly advantageous spatial possibilities for using the tools, it is preferably provided that all the tool receiving means of the linear tool carrier lie in one tool plane. 
         [0106]    In the case of a tool spindle with linear tool carriers, it is particularly advantageous if a tool plane of the tool spindle and the tool planes of the linear tool carriers that follow this tool spindle plane respectively form with one another an included angle that is greater than 90° and less than 180°. 
         [0107]    It is preferably provided that each two successive tool planes form with one another an included angle that is greater than 100°, still better greater than 110°, in particular approximately 120°. 
         [0108]    This provides the possibility of using the tools located in one of the tool planes in such a way that the tools located in the other tool planes require as little space as possible, and consequently also impede other machining operations as little as possible by their extent. 
         [0109]    A further advantageous solution of a tool carrier unit according to the invention provides that the tool carrier is formed as a multiple linear tool carrier, that is to say is made up of a multiplicity of linear tool carriers. 
         [0110]    It is preferably provided here that the linear tool carriers have tool receiving means lined up in a direction of a row parallel to the direction of advance of the multiple linear tool carrier, the direction of advance being the direction along which the multiple linear tool carrier is movable in relation to the tool carrier base. 
         [0111]    In particular, it is also the case with such a multiple linear tool carrier that all the tool receiving means of each of the linear tool carriers are disposed in one tool plane. 
         [0112]    It is suitably also the case with such a multiple linear tool carrier that the tool planes of the linear tool carriers are disposed such that each two successive tool planes form with one another an included angle that is greater than 90° and less than 180°. 
         [0113]    It is preferably also the case with such a multiple linear tool carrier that the included angle which successive tool planes of the linear tool carriers each form with one another is greater than 100°, still better greater than 110°, in particular approximately 120° 
         [0114]    In the case of a further advantageous exemplary embodiment, it is preferably provided that the tool carrier is formed as a tool turret. 
         [0115]    In the case of a tool turret, it is provided that the tool turret has a turret head, which is rotatable about a turret axis. 
         [0116]    It is preferably provided in the case of this solution that the turret axis intersects the axis of rotation about which the tool turret is rotatable with respect to the tool carrier base. 
         [0117]    In this case, a turret housing of the tool turret is preferably fixedly connected to the guiding arm. 
         [0118]    Such a turret head is developed for example in such a way that it has tool receiving means which are disposed in such a way that tool directions of tools inserted in it lie in one tool plane. 
         [0119]    Such a tool plane is preferably aligned such that it runs perpendicularly to the turret axis. 
         [0120]    Furthermore, it is particularly advantageous with regard to the acceptance of the forces acting on the tools if the axis of rotation about which the tool turret is rotatable with respect to the tool carrier base runs parallel to the tool plane. 
         [0121]    It is particularly advantageous if a point of intersection of the axis of rotation and the turret axis lies in the tool plane. 
         [0122]    Further features and advantages of the invention are the subject of the following description and the pictorial representation of a number of exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0123]      FIG. 1  shows a schematic front view of a first exemplary embodiment of a turning center according to the invention; 
           [0124]      FIG. 2  shows a section along line  2 - 2  in  FIG. 1 ; 
           [0125]      FIG. 3  shows a view similar to  FIG. 1  of a second exemplary embodiment of a turning center according to the invention; 
           [0126]      FIG. 4  shows a representation similar to  FIG. 3  in a transfer position of the workpiece spindle units; 
           [0127]      FIG. 5  shows a representation similar to  FIG. 3  in a machining position of the workpiece spindle units; 
           [0128]      FIG. 6  shows a view similar to  FIG. 1  of a third exemplary embodiment of a turning center according to the invention; 
           [0129]      FIG. 7  shows a view similar to  FIG. 1  of a fourth exemplary embodiment of a turning center according to the invention; 
           [0130]      FIG. 8  shows a section along line  8 - 8  in  FIG. 7 ; 
           [0131]      FIG. 9  shows a view similar to  FIG. 1  of a fifth exemplary embodiment of a turning center according to the invention; 
           [0132]      FIG. 10  shows an enlarged plan view similar to  FIG. 9  of one of the tool spindles of the fifth exemplary embodiment; 
           [0133]      FIG. 11  shows a plan view in the direction of the arrow X in  FIG. 10 ; 
           [0134]      FIG. 12  shows a view similar to  FIG. 9  when using tools of the linear tool carriers in a radial direction in relation to the workpiece spindle axis; 
           [0135]      FIG. 13  shows a representation similar to  FIG. 9  when using tools of the linear tool carriers parallel to the tool spindle axis; 
           [0136]      FIG. 14  shows a view similar to  FIG. 1  of a sixth exemplary embodiment of a turning center according to the invention; 
           [0137]      FIG. 15  shows a plan view in the direction of the arrow Z in  FIG. 14 ; 
           [0138]      FIG. 16  shows a view similar to  FIG. 1  of a seventh exemplary embodiment of a turning center according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0139]    A first exemplary embodiment of a turning center according to the invention comprises a machine frame, which is designated as a whole by  10  and has a frame base  12 , which rests with its underside  14  on a standing area  16 , and a machine bed  18 , which rises up above the frame base  12  and comprises a first column  20  and a second column  22 , which are spaced apart and extend away from the frame base  12  with longitudinal axes  24 ,  26  running approximately parallel to one another, the longitudinal axes  24 ,  26  preferably running transversely to the standing area  16 , preferably perpendicularly thereto. 
         [0140]    The two columns  20 ,  22  could in principle rise up above the frame base  12  independently of one another. However, a more stable construction of the machine bed  18  is one in which the two columns  20 ,  22  are connected to one another by a first cross bracing  28  and a second cross bracing  30 . 
         [0141]    Furthermore, the two columns  20 ,  22  are spaced apart in such a way that between them there is a sufficiently large opening  32 , which in the case of the cross bracings  28 ,  30  is enclosed on all sides by the machine bed, so that the machine bed  18  is formed overall in the manner of a yoke and consequently surrounds the opening  32  in a closed yoke-like manner. 
         [0142]    Furthermore, the first column  20  has a first front side  34  and the second column  22  has a second front side  36 , which are facing a working space  40 , which is disposed on one side of the machine bed  18  and lies in front of the two front sides  34 ,  36  and in front of the opening  32  and extends in front of the machine bed  18  transversely to the longitudinal axes  24 ,  26  and in a direction parallel to the longitudinal axes  26 . 
         [0143]    As further represented in  FIGS. 1 and 2 , disposed on the first front side  34  is a base body  42  with slide guides  44 ,  46 , which run parallel to one another, extend transversely to the longitudinal axis  24  of the first column  20  and on which a spindle slide  48  is guided, on which slide a first workpiece spindle unit designated as a whole by  50  is disposed, this unit comprising a first workpiece spindle housing  52  in which a first workpiece spindle  54  is mounted rotatably about a first workpiece spindle axis  56 . 
         [0144]    The workpiece spindle axis  56  preferably runs parallel to the slide guides  44 ,  46  and also substantially in a horizontal direction. 
         [0145]    The slide guides  44 ,  46  seated on the base body  42  consequently have the effect that the first workpiece spindle unit  50  is movable in a direction parallel to the first workpiece spindle axis  56 , and consequently in a so-called Z direction in relation to the first column  20 . 
         [0146]    Likewise provided on the second front side  36  of the second column  22  is a base body  62 , on which there are disposed slide guides  64 ,  66  that likewise run transversely to the longitudinal axis  26  and on which guides a spindle slide  68  is movably mounted. 
         [0147]    The spindle slide  68  also carries a second workpiece spindle unit  70  with a second workpiece spindle housing  72 , in which a second workpiece spindle  74  is mounted rotatably about a second workpiece spindle axis  76 . 
         [0148]    In particular, the first workpiece spindle axis  56  and the second workpiece spindle axis  76  lie in a spindle guiding plane which is designated as a whole by  80  and preferably runs parallel to the front sides  34  and  36  and at a distance from them. 
         [0149]    In principle, it would be conceivable to dispose the base body  62  on the front side  36  in a stationary manner, to be precise in such a way that the first and second workpiece spindle axes  56  and  76  are in line with one another. 
         [0150]    In the case of the first exemplary embodiment, as represented in  FIGS. 1 and 2 , provided on the front side  36  are slide guides  82 ,  84 , on which the base body  62  is guided by guiding bodies  86 ,  88 , the slide guides  82 ,  84  preferably running approximately parallel to the longitudinal axis  26  of the column  22 . 
         [0151]    Consequently, the second workpiece spindle unit  70  is movable on the one hand likewise in the Z direction by the spindle slide  68  and on the other hand also in a so-called X direction, which runs perpendicularly to the spindle axes  56  and  76 , by travel of the base body  62  on the slide guides  82  and  84  relative to the second column  22 . 
         [0152]    The first workpiece spindle  54  and the second workpiece spindle  74  are formed in such a way that a workpiece clamping device that is not pictorially represented is provided on the side facing the respective other workpiece spindle  74  or  54 , by which device a first workpiece W 1  can be fixed in the first workpiece spindle  54  and a second workpiece W 2  can be fixed in the second workpiece spindle  74  and, for machining the same, can be rotated about the respective workpiece spindle axis  56  or  76 . 
         [0153]    As represented in  FIGS. 1 and 2 , the turning center according to the invention is provided with a tool carrier unit, which is designated as a whole by  100  and has a tool carrier base  102 , extending partially in the opening  32 , and a tool carrier  104 , disposed in the working space  40 . 
         [0154]    The tool carrier  104  is seated here on a guiding arm  106 , which is mounted in the tool carrier base  102  displaceably in relation to the tool carrier base  102  in the direction of a longitudinal axis  108  of the guiding arm  106  and is also rotatable in the tool carrier base  102 , about the longitudinal axis  108  of the guiding arm  106 . 
         [0155]    For this purpose, the guiding arm  106  is preferably guided in a longitudinal and rotary guide  110  of the tool carrier base  102 . 
         [0156]    The longitudinal and rotary guide  110  of the tool carrier base  102  is seated here on a tool carrier slide  112 , which for its part is guided on slide guides  114 ,  116 . 
         [0157]    The slide guides  114 ,  116  are seated on a first transverse side  120  of the first column  20 , which is facing the opening  32  and the opposite second column  22 . 
         [0158]    The slide guides  114 ,  116  preferably extend here parallel to the longitudinal axis  24  of the first column  20 , so that the tool carrier slide  112  is movable in the opening  32  along the first column  20 , preferably in the X direction. 
         [0159]    There is consequently the possibility of feeding a tool WZ 1  that is held in the tool carrier  104  in the X direction in the direction of the workpiece W 1  that is held in the first workpiece spindle  54 . 
         [0160]    Furthermore, the longitudinal and rotary guide  110  for the guiding arm  106  is disposed on the tool carrier slide  112  in such a way that, as represented in particular in  FIG. 2 , the longitudinal axis  108  of the guiding arm  106  runs transversely to the slide guides  114 ,  116 , and consequently also transversely to the spindle guiding plane  80 . The longitudinal axis  108  preferably runs perpendicularly to the spindle guiding plane  80 . 
         [0161]    Consequently, the tool carrier  104  is movable not only by moving the tool carrier slide  112  in the X direction in relation to the workpiece W 1 , but also movable transversely to the X direction in the direction of the longitudinal axis  108 , and consequently in a Y direction, as well as rotatable about the longitudinal axis  108 , that is to say about a so-called B axis. 
         [0162]    Overall, the tool carrier  104  is consequently movable in a tool guiding plane  118 , which runs parallel to the X direction and parallel to the Y direction, the tool guiding plane  118  running transversely, preferably perpendicularly, to the spindle guiding plane  80 . 
         [0163]    Provided for the movements in the Y direction and the rotation about the B axis is a drive unit, which is designated as a whole by  122  and the housing  124  of which is fixed on the longitudinal and rotary guide  110 , and which is disposed on a side of the machine bed  18  that is opposite from the tool carrier  104 . 
         [0164]    In the case of the first exemplary embodiment, represented in  FIGS. 1 and 2 , the tool carrier  104  is formed as a tool turret  130  and comprises a turret housing  132 , on which a turret head  136  is disposed rotatably about a turret axis  134 . 
         [0165]    The turret axis  134  runs here transversely, preferably perpendicularly, to the longitudinal axis  108  and intersects the longitudinal axis  108  at a point of intersection  138 , which preferably lies in a central plane  140  of the turret head  136 . 
         [0166]    The tool turret  130  can preferably be used in such a way that the turret housing  132  is always on a side facing away from the respective workpiece. By turning the entire tool turret  130  about the longitudinal axis  108  as an axis of rotation, there is consequently the possibility of using the tools of the turret head  136  for machining the workpiece W 1 , which has been received in the first workpiece spindle  54 , or for machining the workpiece W 2 , which has been received in the second workpiece spindle  74 . 
         [0167]    In the case of the structural design of the machine bed  18  according to the invention, the machining in particular of the workpiece W 1  accommodated in the first workpiece spindle  54  by the tools of the tool turret  130  is very accurate, since both the first workpiece spindle unit  50  and the entire tool carrier unit  100  are held on the first column  20 , the first front side  34  and the transverse side  120  being disposed such that, though they lie on different sides of the column  20 , they follow one another in the circumferential direction of the column  20 , so that the effects of any thermal expansion of the column  20  on the machining accuracy are minor and thermal stresses otherwise have no significant effect on the relative position of the first front side  34  and the transverse side  120 . 
         [0168]    In the case of a second exemplary embodiment of a turning center according to the invention, represented in  FIG. 3 , those elements that are identical to those of the first exemplary embodiment are provided with the same reference numerals, so that, with regard to the description of the same, reference can be made to the statements made with respect to the first exemplary embodiment in their entirety. 
         [0169]    By contrast with the first exemplary embodiment, the opening  32  is made to extend in the direction of the longitudinal axes  24 ,  26  of the columns  20 ,  22  in such a way that not only the first tool carrier unit  100  can be provided in said opening but also a second tool carrier unit  150 , which in the same way as the first tool carrier unit  100  comprises a tool carrier base  152  and a tool carrier  154 , the tool carrier base  152  being formed in the same way as the tool base  102 , and in particular comprising a tool carrier slide  162 , which is held on slide guides  164 ,  166 , which are disposed on a transverse side  170  of the second column  22 , the transverse side  170  delimiting the opening  32  and facing the transverse side  120  of the first column  20 . 
         [0170]    Furthermore, the slide guides  164 ,  166  are formed in such a way that the second tool carrier unit  150  is movable, substantially in such a way as to avoid any collisions, in relation to the first tool carrier unit  100 , guided on the slide guides  114 ,  116 , in this case the second tool carrier unit  150  being disposed between the frame base  12  and the first tool carrier unit  100 . 
         [0171]    In the case of the second exemplary embodiment, the second workpiece spindle unit  70  can preferably be made to move along the slide guides  82 ,  84  to such an extent that it can, on the one hand, be brought into a transfer position which is represented in  FIG. 4  and in which the second workpiece spindle axis  76  is aligned coaxially in relation to the first workpiece spindle axis  56 , so that, by travel of the two workpiece spindle units  50 ,  70  in the Z direction parallel to their workpiece spindle axes  56 ,  76 , the workpiece W 1  can be transferred for example from the first workpiece spindle  54  into the second workpiece spindle  74 . 
         [0172]    Then, by travel of the base body  62  along the slide guides  82 ,  84 , the second workpiece spindle unit  70  can travel from this transfer position into a machining position, which is represented in  FIG. 5  and in which the workpiece W 2  that is held in the second workpiece spindle  74  can be machined by means of a tool of the second tool carrier unit  150 . 
         [0173]    Here, either the second tool carrier unit  150  can travel in the X direction along the slide guides  164 ,  166  or the second workpiece spindle unit  70  can travel in the X direction along the slide guides  82 ,  84 . 
         [0174]    However, it is also conceivable to make both the second tool carrier unit  150  and the second workpiece spindle unit  70  move simultaneously in the X direction. 
         [0175]    In addition, the machining of a workpiece W 1 , which has been received in the first workpiece spindle unit  50 , by a tool of the first tool carrier unit  100  can be carried out at the same time. 
         [0176]    In the case of the turning center according to the invention, in the machining position, the workpiece spindle units  50 ,  70  have been displaced in relation to one another to such an extent that, though their workpiece spindle axes  56 ,  76  run parallel to one another and in the spindle plane  80 , they are offset in relation to one another in the spindle plane  80  in a direction running transversely to the workpiece spindle axes  56 ,  76 , in particular the X direction. 
         [0177]    Furthermore, in the machining position, the first and second tool carrier units  100 ,  150  are preferably disposed in such a way that the workpiece spindle axes  56 ,  76  lie between them, so that no tool is disposed in the space between the mutually offset workpiece spindle axes  56 ,  76 , but instead the tools of the tool carrier units  100 ,  150  can be respectively fed in the X direction toward the respective workpiece W 1  or W 2  in the respective workpiece spindle  54  or  74  from a side facing away from the other respective spindle axis  76  or  56 . 
         [0178]    In the case of the second exemplary embodiment, the tool carrier units  100 ,  150  are preferably disposed in such a way that the points of intersection  138  of the longitudinal axes  108  and the turret axes  134  lie in a common movement plane  180 , the longitudinal axes  108  of the guiding arms  106  also preferably being movable in this movement plane  180 . 
         [0179]    Here, the movement plane  180  runs in particular centrally through the opening  32  between the columns  20 ,  22 . 
         [0180]    In the case of a third exemplary embodiment, represented in  FIG. 6 , the second tool carrier unit  150  is disposed in a stationary manner on the transverse side  170  of the second column  22 , and is consequently not displaceable. 
         [0181]    Consequently, to machine workpieces W 2  that are held in the second workpiece spindle unit  70 , it is necessary to produce the movement in the X direction by moving the second workpiece spindle unit  70  in the X direction, to be precise by displacing the base body  62  on the slide guides  82 ,  84 . 
         [0182]    Otherwise, the tool carrier  154  is movable in relation to the tool carrier base  152  in the same way as described in connection with the second exemplary embodiment, so that the tools of the turret head  136  can also be used for machining the workpiece W 2  that is held in the second workpiece spindle unit  70 . 
         [0183]    In the case of a fourth exemplary embodiment, represented in  FIG. 7 , those elements that are identical to those of the first exemplary embodiment are likewise provided with the same reference numerals, so that, with regard to the description of the same, reference can likewise be made to the statements made with respect to the first exemplary embodiment in their entirety. 
         [0184]    By contrast with the first exemplary embodiment, in the opening  32  between the columns  20 ,  22  only the first tool carrier unit  100  is disposed movably in the X direction, as described in detail in connection with the first exemplary embodiment. 
         [0185]    Furthermore, the second tool carrier unit  150  is disposed in an opening  182 , provided specifically for this purpose, between the columns  20 ,  22 , the openings  182 ,  32  being separated from one another by a cross bracing  184 , which additionally contributes to improving the stiffness of the machine bed  18 . 
         [0186]    As represented in  FIG. 8 , in the case of this exemplary embodiment the tool carrier base  152  likewise comprises the longitudinal and rotary guide  110 , which for its part however is held in a stationary manner directly by a further transverse side  186  of the second column  22  that is adjacent the opening  182 . 
         [0187]    In the case of a fifth exemplary embodiment, represented in  FIG. 9 , those elements that are identical to those of the first exemplary embodiment are provided with the same reference numerals, so that, with regard to the description of the same, reference can be made to the statements made with respect to the first exemplary embodiment or the second and third exemplary embodiments in their entirety. 
         [0188]    By contrast with the previous exemplary embodiments, the tool carrier  104  is formed as a tool spindle  200 , as additionally represented in  FIG. 10 , which has a tool spindle housing  202 , in which there is mounted a spindle motor  204 , which drives a spindle shaft  206 , into which a tool holder  210  can be inserted into a tool holder receiving means  209  in the region of a front end  208 . Consequently, a tool  212 , which is held in the tool holder  210  and can be driven in a rotating manner by the spindle shaft  206 , is seated in the spindle shaft  206 , a tool axis  214  running coaxially in relation to a tool spindle axis  216 , which represents the axis of rotation of the spindle shaft  206 . 
         [0189]    The tool spindle  200  has, for example, a power output of at least 8 kW and reaches rotational speeds greater than 10,000 rpm. 
         [0190]    The tool axis  214  corresponds to a tool direction, defined by the advance and/or feed directions in the machining of the workpiece, and lies here in a tool plane  250 , which runs through the longitudinal axis  108 . 
         [0191]    As likewise represented in  FIGS. 10 and 11 , disposed as supplementary tool carriers on the tool spindle housing  202 , to be precise on opposite sides of the longitudinal axis  108 , are linear tool carriers  220 ,  222 , which have tool holder receiving means  232 ,  234  that are disposed one after the other in the direction of a row  226 ,  228  and into which tool holders  236 ,  238  can be inserted. 
         [0192]    The tool holders  236 ,  238  are all formed such that each insert  242 ,  244  that can be inserted into the tool holder receiving means  232 ,  234  of the linear tool carriers  220 ,  222  can be inserted in a direction of insertion  246 ,  248 , which, as represented in particular in  FIG. 10 , lie in further tool planes  252 ,  254 , which also run through the longitudinal axis  108 , so that the tool planes  250 ,  252 ,  254  intersect in the longitudinal axis  108 . 
         [0193]    Furthermore, the directions of the rows  226 ,  228  of tool holder receiving means  232 ,  234  also run parallel to the tool planes  252 ,  254 . 
         [0194]    All the tool holders  236 ,  238  are loaded with tools  256  or  258 , which have at least one tool direction  262 ,  264 , which runs parallel to the respective tool plane  252 ,  254  and is defined by the advance and/or feed directions in the machining of the workpiece, the tool directions  262 ,  264  preferably lying in the tool planes  252 ,  254 . 
         [0195]    Furthermore, the tool directions  262 ,  264  extend transversely, in particular perpendicularly, to the directions of the rows  226 ,  228 . 
         [0196]    The tool planes  252 ,  254  are preferably not perpendicular to the tool spindle axis  216  and the tool plane  250 , but respectively form therewith an included angle of approximately 120°, so that, with respect to the longitudinal axis  108 , an angle of 120° exists between the tool plane  250  and the tool plane  252  as well as between the tool plane  250  and the tool plane  254 , and, on a side facing away from the tool  212 , the tool planes  252 ,  254  likewise form with one another an included angle of approximately 120°. 
         [0197]    Furthermore, the tools disposed one after the other in the respective direction of the row  226 ,  228  can be brought into use by displacing the tool spindle  200  in the direction of the longitudinal axis  108 . 
         [0198]    Furthermore, as represented in  FIG. 11 , the tools  256 ,  258  respectively disposed on opposite sides of the longitudinal axis  108  lie in alignment planes  266 ,  268 ,  270  running perpendicularly to the longitudinal axis  108 , so that, depending on the position of the tool spindle  200 , the tools  256 ,  258  that lie in one of the alignment planes  266 ,  268 ,  270  can be brought into use on the workpiece, a change of tool being possible just by turning the entire workpiece spindle  200  about the longitudinal axis  108 . 
         [0199]    Furthermore, the alignment plane  268  is preferably disposed such that not only the directions of advance  262 ,  264  of the tools  256 ,  258  of the linear tool carriers  220 ,  222  lie in it, but so too does the tool axis  214  of the tool  212 . 
         [0200]    The changing use of the different tools is represented in  FIGS. 12 and 13 , the advantage of this solution being that the tools that are not in use on the workpiece are at such an angular distance from one another, with respect to the longitudinal axis  108 , that they do not hinder the machining of the workpiece in the respective workpiece spindle unit  50 ,  70 . 
         [0201]    In the case of a sixth exemplary embodiment, represented in  FIGS. 14 and 15 , those elements that are identical to those of the previous exemplary embodiments are provided with the same reference numerals, so that, with regard to the description of the same, reference can be made to the statements made with respect to the previous exemplary embodiments in their entirety. 
         [0202]    In the case of the sixth exemplary embodiment, the tool carrier  104  is formed as a multiple linear tool carrier  280 , such that, in a way similar to in the case of the fourth exemplary embodiment, it has three tool planes  250 ,  252 ,  254 , which all form between them an included angle of 120°, and in which tools are respectively disposed one after the other in the direction of the corresponding row, the tools also in this case lying in successive alignment planes  266 ,  268 ,  270 . 
         [0203]    By contrast with the fourth exemplary embodiment, however, all the tools are, for example, non-driven tools. They are merely used as stationary tools in connection with one of the workpiece spindle units  50 ,  70 . 
         [0204]    A change of the respective tool is possible in a simple manner, by a base  282  of the multiple linear tool carrier  280 , carrying all the tool holder receiving means, being rotatable about the longitudinal axis  108  and displaceable in the direction of the longitudinal axis  108 . 
         [0205]    In the case of a seventh exemplary embodiment, represented in  FIG. 16 , the machine bed  18 ′ is, for example, made up of two parts. Thus, one part of the machine bed  18 ′ is formed by the column  20  and the cross bracings  28  and  30 , and the other part is formed by the column  22 , which can be separated from the cross bracings  28 ,  30  along a separating plane  290 , so that the machine bed  18  can be easily produced and transported. 
         [0206]    The tool carriers  104  described in the context of the exemplary embodiments according to the invention merely represent examples. Depending on the machining to be carried out, tool turrets  130 , tool spindles  200  and multiple linear tool carriers  280  may be combined with one another within the scope of the present invention. 
         [0207]    Instead of the examples of tool carriers  104  described, it is also possible to use other kinds of tool carriers, such as for example grinding units.