Abstract:
A machining center that includes a swivel bridge supported by a bearing wall and pivotable about a horizontal axis, at least one drive sprocket attached to a disc-type connector of the swivel bridge, and at least one drive system attached adjacent the bearing wall and operatively connected to the drive sprocket of the swivel bridge. The machining center is characterized by clamping of the swivel bridge being performed by a friction-locked clamping system which is arranged rotationally fixed about a bearing pin of the swivel bridge in a circular ring shape and which is operatively connected to the disc-type connector. A clamping force of the friction-locked clamping system is generated by an energy storage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 10 2008 021 416.7, filed Apr. 29, 2008, which is hereby incorporated by reference. 
     BACKGROUND AND SUMMARY 
     The invention relates to a machining center for drilling, milling, lathing or grinding, comprising a swivel bridge supported between two bearing walls to be pivoted about a horizontal axis, at least one drive sprocket attached to a disc-type connector of the swivel bridge, at least one drive system attached outside to the bearing wall and operatively connected to the drive sprocket of the swivel bridge. 
     Modern machining centers for the cutting processing of workpieces by drilling, milling, lathing or grinding are increasingly provided with rotatable and pivotable workpiece tables. When using such a machining center, the workpiece may be brought into nearly any possible position by rotating and pivoting the workpiece table, without the need for releasing the initial clamping of the workpiece on the workpiece table. Therewith, nearly any possible solid angle at the workpiece may be processed in one workpiece clamping e.g. by drilling or milling. 
     In general, the rotatable workpiece table is arranged on a so-called swivel bridge for receiving the workpiece to be machined. The swivel bridge is usually supported on both sides and may generally be pivoted about a horizontal axis. However, also constructions in which the swivel bridge is supported only on one side are familiar. 
     When starting a machining cycle, the swivel bridge is pivoted into a predetermined angular position and then securely fixed in this position by a retaining mechanism. Only then the corresponding drilling or milling processing is performed at the workpiece. During the machining by drilling or milling, partially very large processing forces or moments may occur. Said processing forces or moments must be received securely by the retaining mechanism of the swivel bridge. Otherwise, the swivel bridge would be pushed away from the predetermined position due to the processing forces or moments. In addition, the retaining mechanism of the swivel bridge also have to provide a fail safe functionality. It has to be guaranteed that the retaining mechanism will also work in case of an energy breakdown, such that an unregulated pivoting or coast down of the swivel bridge due to gravity is excluded. 
     In EP 1 262 275 B1, a generic rotatable and pivotable workpiece table including a swivel bridge is described. The swivel bridge disclosed in EP 1 262 275 B1 is supported on both sides and can be pivoted about a horizontally aligned pivot axis. The swivel bridge is driven directly through its drive sprocket by a drive motor arranged outside at the bearing wall, such that there results a short force flow and no torsional load on the bearing pins. Retaining mechanism for the swivel bridge having a fail safe functionality are not disclosed explicitly in EP 1 262 275 B1. However, it is state of the art that the drive motor of a drive assembly as disclosed in EP 1 262 275 B1 is generally provided with an integrated motor clamping. Said integrated motor clamping operates on the basis of the permanent magnet principle, i.e. it is clamped in the electroless state. The ventilation of the clamping is performed electro-magnetically. When switching off the machining center (main switch OFF and machining center galvanically separated from the energy supply) or in case of an unexpected energy breakdown, the clamping hub is attracted by the magnet and therewith the motor shaft of the drive motor is retained. The motor clamping is also configured for a specific number of emergency brakings, i.e. for brakings during motion in case of emergency OFF or energy breakdown. A fail safe functionality of the retaining mechanism for the swivel bridge is therefore secured. 
     However, the motor clamping is not adapted and suited to retain the swivel bridge during the processing step in an angularly exact processing position. For this purpose, the swivel bridge of the present case has to be retained in its processing position by the position control of the drive motor. This quasi-electronic clamping guarantees the angularly exact processing position of the swivel bridge. In the present case, the retaining mechanism of the swivel bridge consists of two members: a mechanical motor clamping for the fail safe functionality in case of an energy breakdown or emergency OFF (emergency clamping or emergency braking) and an “electronic” clamping for the angularly exact positioning of the swivel bridge during the processing step (operation clamping). 
     The retaining mechanism for the swivel bridge disclosed in EP 1 262 275 B1 has the following disadvantages: 
     In order to maintain the operation clamping for the swivel bridge, the “electronic clamping” and thus the drive control of the related rotary axis must be activated partially for hours. This results in a considerable consumption of electric current. 
     Based on this state of the art, it is an object underlying the present invention to enhance the known machining center in that the operation clamping of the swivel bridge can be realized without the drive control and that the operation clamping is substantially free of clearance and can receive also largest processing moments securely and exactly, while maintaining the advantages thereof. 
     According to the invention, the object is solved by the following features: 
     A machining center for drilling, milling or lathing or grinding including a swivel bridge supported between two bearing walls to be pivoted about a horizontal axis, at least one drive sprocket attached to a disc-type connector of the swivel bridge, at least one drive system attached outside at the bearing wall and operatively connected to the drive sprocket of the swivel bridge, wherein the operation clamping of the swivel bridge is performed by a friction-locked clamping system which is arranged rotationally fixed about the bearing pin of the swivel bridge in a circular ring shape and is operatively connected to the disc-type connector, and that the clamping force of the friction-locked clamping system is generated by an energy storage. 
     Since the clamping force for the operation clamping is generated by an energy storage according to the invention, the drive control of the swivel bridge is not required for the operation clamping. 
     Since the friction-locked clamping system is further provided externally about the bearing pin, the largest possible diameter can be chosen for the clamping system. Therewith, the clamping system has a lever arm as large as possible and thus the largest possible clamping force. 
     Since the clamping system is directly operatively connected to the disc-type connector of the swivel bridge, a short flow of force results when clamping the swivel bridge. This is very advantageous for the static and dynamic rigidity of the operation clamping. 
     Preferred further developments and embodiments result from the sub-claims. 
     According to a preferred further development, the energy storage for generating the clamping force for the operation clamping consists of a mechanical spring energy store. A mechanical spring energy store can be realized particularly simply and is also inexpensive, since mechanical spring energy stores as a common machine member are available in any design. In addition, a mechanical spring energy store also does not need a large control effort to generate the clamping force. 
     As an alternative, the energy storage for generating the clamping force for the operation clamping may also consist of a hydraulic or pneumatic pressure accumulator. 
     Preferably, the mechanical spring energy store consists of a plurality of annularly arranged springs or of an annularly arranged, disc-shaped diaphragm which can be radially elastically deformed. Due to such a design, a large-scale clamping can be generated, which in turn results in a large clamping force. 
     According to a preferred embodiment, the friction-locked clamping system is provided with a pressure piece which is loaded by the spring energy store toward the drive sprocket. Such a pressure piece can transmit a large clamping force to the drive sprocket and therewith guarantees a secure clamping. 
     According to a preferred embodiment, the friction-locked clamping system for the operation clamping comprises a fail safe functionality such that, in case of an energy breakdown, the clamping occurs inevitably by the energy storage. Due to this construction, a fail safe functionality is integrated into the friction-locked clamping system, without the need for any additional mechanical mechanism. The clamping system thus has a double function: it clamps the swivel bridge in the desired position on the one hand, and on the other hand works as a fail safe device. 
     In order to guarantee a symmetric force transmission of the clamping force into the swivel bridge, it is preferred to arrange the friction-locked clamping system for the operation clamping on both sides at bearing walls of the swivel bridge. 
     According to a preferred embodiment, the friction-locked clamping system for the operation clamping is further formed as a radial clamping system shaped as a circular ring. 
     Therein, the clamping preferably occurs radially to the outside or inside, such that the pressure piece is pressed to the outside or inside during clamping. 
     Preferably, the circular ring shaped radial clamping system acts on the drive sprocket of the swivel bridge. Therein, the circular ring shaped radial clamping system preferably clamps outwardly toward the drive sprocket of the swivel bridge. 
     As an alternative, the circular ring shaped radial clamping system may also act on the disc-type connector of the swivel bridge. 
     In this case, the circular ring shaped radial clamping system preferably clamps inwardly toward the disc-type connector of the swivel bridge. 
     According to a preferred further development, the swivel bridge of the inventive machining center includes two independently effective clamping systems with fail safe functionality, i.e. a first clamping system formed by a motor clamping with fail safe functionality at the drive motor of the swivel bridge and a second clamping system formed by the clamping system with fail safe functionality at the bearing pins of the swivel bridge. 
     Further details, features and advantages of the invention result from the following description based on the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a detailed view of a first embodiment of an inventive machining center including a released clamping system. 
         FIG. 2  shows a detailed view of a second embodiment of the inventive machining center including a released clamping system. 
         FIG. 3  shows a detailed view, analogous to  FIG. 1 , with a tightened clamping system. 
         FIG. 4  shows a detailed view of a third embodiment of the inventive machining center including a tightened clamping system. 
         FIG. 5  shows a comparison between the inventive machining center and a machining center according to the state of the art. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a first embodiment of a portion of an inventive machining center which is of interest here is shown. 
     The left portion of a swivel bridge  22  is discernible, which supports a rotatable workpiece table  24  (see  FIG. 2 ). On the workpiece table  24 , a workpiece can be clamped, which then protrudes into the working range  23  of the machining center located above the swivel bridge and can be processed by a tool spindle  28  which is movable in the x, y and z directions (see  FIG. 2 ). 
     The swivel bridge  22  has a cavity  21  serving to receive energy and signal lines and possibly driving members, e.g. tooth belts, for driving the rotatable workpiece table  24 . 
     The swivel bridge  22  has a circular connector  12  at one end thereof, in the center of which a bearing pin  13  is disposed. In the present case, the bearing pin  13  is hollow and supported in a ball bearing  14  seated on a bearing sleeve  16  which is in turn supported in a bearing wall  9  of the machining center (which is not shown). 
     At the free end of the bearing pin  13 , a shaft nut  20  is screwed, which guarantees a secure support of the swivel bridge  22 . 
     In order to enable access to the bearing pin  13 , for example for maintenance and/or repair purposes, the bearing pin  13  is accessible through a cover  15  disposed in the swivel bridge  22 . 
     At the outside, the connector  12  is surrounded by a sealing ring  10 . Further, a sliding seal or labyrinth seal  11  is provided between the connector  12  and the sealing ring  10 . 
     In the vicinity of the outer periphery of the connector  12 , a drive sprocket  8  is disposed, which protrudes from the connector  12  toward the bearing pin  13 . The drive sprocket  8  is provided with an external toothing which meshes with a pinion  7  which is provided with a drive motor  18  through a motor shaft  27  and an ancillary transmission  17 . The drive motor  18  is provided with a motor clamping  19  (motor brake) and a drive control  25  (see  FIG. 5 ). 
     By using the drive motor  18 , the swivel bridge  22  can be pivoted about the axis A. In addition, the integrated motor clamping  19  (motor brake) enables an emergency braking as well as a fail safe functionality. The integrated motor clamping  19  operates on the basis of the permanent magnet principle, i.e. it is clamped in the electroless state. The ventilation of the motor clamping  19  is performed electro-magnetically. When switching off the machining center (main switch OFF and machining center galvanically separated from the energy supply) or in case of an unexpected energy breakdown, the clamping hub is attracted by the magnet body and therewith the motor shaft of the drive motor  18  is retained. 
     In the bearing wall  9 , a circular ring shaped clamping system  1  is arranged about the bearing pin  13 , which serves to clamp the drive sprocket  8  and therewith the swivel bridge  22 . 
     The clamping system  1  includes a plurality of radially aligned pressure pieces  5 . Each pressure piece  5  is pressed outwardly toward the drive sprocket  8  by an energy storage  2  which is preferably formed as a spring. On the side of the pressure piece  2  facing away from the energy storage  2 , a pressure chamber  3  is provided, into which a fluid line  4  ends. The fluid line  4  is connected to an electro-magnetically operable control valve  6 . 
     While  FIG. 1  shows a one-sided support of the swivel bridge  22  in the bearing wall  9 ,  FIG. 2  shows an embodiment in which the swivel bridge  22  is supported in a left bearing wall  9  and a right bearing wall  26 , and is thus supported on both sides. The support of the swivel bridge  22  in the right bearing wall  26  is identical to that in the left bearing wall  9 , such that reference is made to the explanations concerning  FIG. 1 . 
       FIGS. 1 and 2  show a state in which the clamping system  1  is not active, i.e. no clamping of the swivel bridge  22  occurs.  FIG. 3  shows a state in which the clamping system  1  is active, i.e. a clamping of the swivel bridge  22  is performed. 
     In the positions shown in  FIGS. 1 and 2 , in which the pressure chambers  3  are pressurized with fluid, the control valve  6  is positioned such that the pressure pieces  5  and therewith also the energy storage  2  are pushed radially inwardly, in order to pivot the swivel bridge  22 . In this state, the pressure pieces  5  do not act on the drive sprocket  8 , such that the connector  12  and therewith the swivel bridge  22  can be pivoted by the drive motor  18 . 
     In order to clamp the swivel bridge  22 , the control valve is electro-magnetically displaced from the position shown in  FIGS. 1 and 2  to the position shown in  FIG. 3 . In this position, the pressure chambers  3  are not pressurized with fluid, such that the pressure pieces  5  are pushed radially outwardly by the energy storage  2 . In this state, the pressure pieces  5  act on the drive sprocket  8 , such that the connector  12  and therewith the pivot bridge  22  are clamped. 
     In addition, the clamping system  1  features a fail safe functionality. In the conducting state, the electro-magnetically operable control valve  6  is in the position shown in  FIGS. 1 and 2 . In this position, the pressure chamber  3  is pressurized with fluid and the pressure piece  5  is pushed radially inwardly against the force of the energy storage  2 , such that it cannot be operatively connected to the drive sprocket  8  and clamp it. 
     In case of an unexpected energy breakdown, the electro-magnetically operable control valve  6  is no longer supplied with current and then assumes the position shown in FIG.  3 —e.g. due to a spring load—in which the pressure chamber  3  is no longer pressurized with fluid. Since the pressure chamber  3  is now depressurised, the energy storage  2  pushes the pressure piece  5  radially inwardly, such that it acts on the drive sprocket  8  and clamps it. 
       FIG. 4  shows a further embodiment in which the clamping system  1  does not act on the drive sprocket  8  from the inside, as is shown in  FIGS. 1 to 3 , but from the outside. Apart from that, the structure and the function are identical to those explained in connection with  FIGS. 1 to 3 . 
       FIG. 5  shows a comparison between a machining center according to the state of the art (left side) and one according to the invention (right side). While the clamping of the swivel bridge  22  and also the fail safe function in the state of the art are only performed by the drive motor  19  alone, the clamping of the swivel bridge  22  according to the present invention is uncoupled from the drive motor  19 , such that the drive motor  19  only serves to pivot the swivel bridge  22 , whereas the clamping system  1  performs the clamping. Due to this construction, also the fail safe functionality becomes redundant, since the drive motor  19  as well as the clamping system  1  may perform a clamping of the swivel bridge  22  in case of an unexpected energy breakdown. 
     As an alternative, the clamping force for the operation clamping may be generated by a hydraulic or pneumatic pressure accumulator instead of a mechanical spring energy store, as it is shown in the figures and described above. 
     Instead of the energy storage  2  shown in the figures and formed by springs, also an annularly arranged, radially elastically deformable diaphragm in disc shape can be used. 
     Further, the clamping system  1  may also act on the connector  12  of the swivel bridge  22  instead of the drive sprocket  8 . In such a case, the pressure piece  5  would be axially displaceable, not radially. 
     The above description according the present invention only serves for illustration purposes and is not intended to restrict the invention. Within the scope of the invention, a plurality of amendments and modifications is possible without leaving the scope of the invention and its equivalents.