Abstract:
A compensation chuck for centrically clamping workpieces includes a housing, two pairs of mutually diametrally opposite base jaws for receiving corresponding clamping jaws, and a drive unit which is moveable preferably by power clamping having two pairs of drivable entrainment members. Two adjacent entrainment members are respectively coupled together by a first rocker and two further adjacent entrainment members are coupled together by a second rocker. There is also provided a coupling ring which guides a first entrainment member and a diametrally opposite second entrainment member in the peripheral direction tangentially relative to the clamping axis, such that a motion play is provided between the entrainment members and the coupling ring in the radial direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase entry of International Patent Application No. PCT/EP2016/053416, filed on Feb. 18, 2016, which claims priority to German Application No. 102015204502.1, which was filed Mar. 12, 2015. These prior applications are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention concerns a compensation chuck for centrically clamping workpieces. In particular the invention concerns a compensation chuck comprising a housing, two pairs of mutually diametrally opposite base jaws for receiving corresponding clamping jaws, a drive unit which is moveable preferably by power clamping having two pairs of drivable entrainment members, wherein two adjacent entrainment members are respectively coupled together by means of a first rocker and two further adjacent entrainment members are coupled together by means of a second rocker. 
       BACKGROUND 
       [0003]    The term compensation chuck is used to denote a clamping chuck which by virtue of an unequal movement of its clamping jaws is capable of clamping even workpieces which are not perfectly cylindrical or symmetrical but deviate from a strictly symmetrical shape. The state of the art discloses for example three jaw chucks or six jaw chucks which can provide for limited compensation between the clamping jaws by way of pendulum bridges and similar mechanisms. Basically it is to be noted that any workpiece is elastically deformed by being clamped in a clamping chuck. The elastic deformation means that a workpiece which is machined in a clamped condition, after being removed from the chuck, no longer exactly corresponds in its shape, by virtue of the elastic return deformation, to the state in which it was during the clamped condition. 
         [0004]    That deformation is correspondingly more pronounced, the fewer clamping jaws are used for clamping the workpiece. For that reason four-jaw chucks which the present invention involves are viewed as being advantageous over three jaw chucks because they permit more uniform clamping of the workpiece. Centric clamping of workpieces is a central aspect in the case of four-jaw chucks, by virtue of the static over-determination with four jaws, and that aspect is to be taken into consideration in design. To ensure a sufficiently high level of reproduction accuracy and reliability in terms of clamping it is a matter of substantial significance that the chuck always clamps the workpieces in relation to the same center point—and thus centrically. There are various approaches for affording centricity. For example a compensation chuck is known from the publication DE 10 2013 201 231 B3, which ensures centric clamping by means of a guidingly supported drive slider. 
         [0005]    The publication WO 2011/137884 A1 discloses for example a compensation chuck. The compensation chuck of the foregoing kind as referred to in the opening of this specification endeavors to ensure centricity of the clamping operation by means of a coupling swing drive. That approach is disadvantageously viewed as being susceptible to trouble by virtue of its complexity. In addition unfavorable lever ratios involve adverse effects on the efficiency in terms of force transmission. 
         [0006]    Therefore the object of the invention was to improve a compensation chuck of the kind set forth in the opening part of this specification in such a way that centricity in clamping is improved while chuck assembly complication alleviated as far as possible. 
       SUMMARY 
       [0007]    In a compensation chuck of the kind set forth in the opening part of this specification the invention attains that object. In particular the invention proposes that the compensation chuck has a coupling ring which guides a first entrainment member and a diametrally opposite second entrainment member in the peripheral direction tangentially relative to the clamping axis, wherein a motion play is provided between the entrainment members and the coupling ring in the radial direction. The invention makes use of the realization that synchronization of the pivotal movement of the first and second rockers primarily contributes to ensuring centricity. That purely rotational synchronization task is isolated from radial guidance of the entrainment members by means of the coupling ring according to the invention so that that guidance effect can be ensured by other parts while the coupling ring according to the invention is concerned exclusively with guiding the rockers (by means of the diametrally opposite entrainment members) in the direction of rotation. By virtue of the fact that a motion play is afforded between the entrainment members and the coupling ring in the radial direction with respect to the clamping axis of the coupling ring, the occurrence of static over-determination and redundancy is at least restricted and assembly is markedly facilitated by virtue of reduced component complexity. 
         [0008]    An advantageous development of the invention provides that the coupling ring is arranged in the housing rotatable about the clamping axis. Preferably the coupling ring has a first preferably radially oriented arm engaging into an opening in the first entrainment member and a second preferably radially oriented arm engaging into an opening in the second entrainment member. The opening in the respective entrainment member is preferably in the form of a pocket, within which the entrainment member can slide in the radial direction with as little play as possible or without play. Preferably the opening is of a cross-section which in particular (radially inwardly) becomes larger towards the opening and which permits a pivotal movement in the angle between the entrainment member and the arm of the coupling ring. Tilting as between the entrainment member and the coupling ring is effectively avoided by virtue of that widened opening. 
         [0009]    In a further preferred configuration the coupling ring has an at least substantially central through opening. By virtue of the central through opening the applicant is capable of passing material through the chuck. 
         [0010]    In a further preferred configuration the first rocker and the second rocker are arranged rotatably on a respective slider. Preferably the sliders are respectively coupled to a piston or a traction guide and are moveable synchronously, in particular radially, towards and away from each other. The traction guide provides for positive synchronization of the sliders in the clamping movement. In the case of a clamping movement without contact in relation to the workpiece or other obstacles, the rocker in this configuration is not pivoted, which leads to a uniform movement of all entrainment members and thus all clamping jaws. It is only when a first obstacle like for example the workpiece is encountered that a pivotal movement of the rocker relative to the slider moves the entrainment member associated therewith with different motion components from the other entrainment member arranged at same, which results in an unequal movement of the clamping jaws. A synchronous counterpart movement of the respective diametrally opposite entrainment members on the other rocker is implemented by means of the coupling ring. As a result at any moment in time in compensation movements or in a clamping movement of the chuck, that is not a compensation movement, centricity is ensured relative to the clamping axis. 
         [0011]    The first and second rockers are preferably respectively pivotable about an axis of rotation parallel to the clamping axis. 
         [0012]    The entrainment members are respectively preferably guided linearly moveably in the housing. 
         [0013]    In a preferred embodiment respective mutually diametrally opposite entrainment members are guided parallel to each other while further preferably respectively adjacently arranged entrainment members are guided at a right angle to each other. The configuration of the entrainment members which are guided in opposite parallel relationship, in combination with the entrainment guide which are oriented in adjacent right-angled relationship with each other provides for a favorable kinematics of the chuck with at the same time minimized structural space. 
         [0014]    In a preferred embodiment the entrainment members on the rocker respectively associated therewith are respectively coupled by means of a rotor mounted rotatably on the rocker. The respective rotor is preferably moveable linearly guided relative to the entrainment member associated therewith. In a particularly preferred configuration the respective rotor and the entrainment member associated therewith are displaceable relative to each other at a right angle to the guidance direction of the entrainment member in the housing. 
         [0015]    In a further preferred configuration of the compensation chuck the first entrainment member has a first preferably radially oriented arm engaging into a first opening in the coupling ring and wherein the second entrainment member has a preferably radially oriented arm engaging into a second opening in the coupling ring. In that alternative configuration therefore the entrainment members do not have openings into which the coupling ring extends, but the coupling ring has corresponding openings into which projections from the entrainment members extend. In this embodiment the compensation chuck can be combined in the same manner with the other preferred embodiments described hereinbefore, like the embodiment in which the arms of the coupling ring engage into corresponding openings in the entrainment members. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention is described in greater detail hereinafter by means of the preferred embodiments by way of example and with reference to the accompanying Figures in which: 
           [0017]      FIG. 1  shows a compensation chuck according to a preferred embodiment, 
           [0018]      FIG. 2  shows an exploded or part-assembled state of the compensation chuck of  FIG. 1 , with the chuck body lifted off the cover plate, 
           [0019]      FIG. 3  shows a perspective view with further details of the cover plate of the compensation chuck of  FIG. 1 , and 
           [0020]      FIGS. 4 a  through 4 e    show various diagrammatic partial views of the compensation chuck of  FIGS. 1 through 3  in different operating states, more particularly: 
           [0021]      FIG. 4 a    illustrates a workpiece to be clamped located on the compensation chuck, 
           [0022]      FIG. 4 b    illustrates initial movements for a clamping operation at the compensation chuck, 
           [0023]      FIG. 4 c    illustrates a state in which one or more clamping jaws contact the workpiece while at least one other clamping jaw remains at a gap from the workpiece, 
           [0024]      FIG. 4 d    illustrates a further traction movement of the piston in the compensation chuck to pivot the rockers from the position shown in  FIG. 4 c   , and 
           [0025]      FIG. 4 e    illustrates a final clamped state at the compensation chuck. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The compensation chuck  1  according to the invention is shown in the assembled state in  FIG. 1 . The chuck  1  has a housing  3  which is subdivided into a chuck body  5  and a cover plate  7 . A total of four base jaws  11   a - d  are received in linearly guided relationship in the chuck body. The base jaws  11   a - d  are preferably oriented in radially guided relationship moveably with respect to a clamping axis A. The chuck body  1  further has a sealing tube  13  which is introduced substantially centrally into the chuck body  5 . The chuck body  5  is connected to the cover plate  7  by means of a plurality of screws  9 . 
         [0027]    In the view in  FIG. 2  the chuck body  5  is lifted off the cover plate  7  and therewith also the sealing tube  13  and the base jaws  11   a - d.  That reveals the constituent parts of a drive unit  10  of the chuck  1 . In its interior the chuck  1  has a piston  15  which cooperates with a traction tube of a power clamping device for clamping and unclamping purposes. 
         [0028]    The piston  15  is disposed in engagement with two diametrally oppositely arranged sliders  21   a,    21   b  (see also  FIG. 3 ) which respectively bear against each other with a slide surface inclined relative to the clamping axis A. 
         [0029]    Arranged rotatably on the first slider  21   a  is a first rocker  17 , also referred to as a bridge member. A second rocker  19  is arranged rotatably on the second slider  21   b  (see  FIG. 3 ). The axes of rotation of the first and second rockers  17 ,  19  are oriented parallel to the clamping axis A. 
         [0030]    Two entrainment members  23   a,    23   b  are arranged on the first rocker. Two entrainment members  25   a,    25   b  are arranged on the second rocker  19 . 
         [0031]    The chuck  1  has a coupling ring  27  which is arranged centrally rotatably about the clamping axis A and which is operatively connected to a first entrainment member  23   a  of the first rocker  17 , and to a second entrainment member  25   a  of the rocker  19 , that is arranged in diametrally opposite thereto. The coupling ring  27  is adapted to synchronize the entrainment members  23   a,    25   a  in the direction of rotation, to guide them, but has a motion play in relation to the entrainment members  23   a,    25   a  in the radial direction relative to the clamping axis A. As a consequence of the synchronized rotary movement about the clamping axis A the first and second rockers  17 ,  19  also move in positively synchronized relationship in the same direction. 
         [0032]    The first and second entrainment members  23   a,    25   a  each have a respective opening  29 , within which a respective arm  31  of the coupling ring  27  is accommodated displaceably, in particular slidingly. 
         [0033]    The entrainment members  23   a,    23   b  and  25   a,    25   b  are respectively coupled to the first and second rockers  17 ,  19  by means of a rotor  33   a - d,  wherein the rotor is respectively mounted slidingly in the entrainment member  23   a,    23   b,    25   a,    25   b  associated therewith. 
         [0034]    The coupling ring  27  has a central opening  35  which preferably extends coaxially with respect to the clamping axis A and is adapted to receive the sealing tube  13 . 
         [0035]    The entrainment members  23   a,    23   b,    25   a,    25   b  each have respective projections  37   a - d  which are adapted to engage into corresponding sliding guides in the base jaws  11   a - d  in order to convert a thrust movement produced by the entrainment members  23   a,    23   b,    25   a,    25   b  into the radial movement of the base jaws  11   a - d.    
         [0036]    More detailed information relating to the motion relationships of the compensation chuck according to the invention in accordance with the preferred embodiment will be apparent from  FIGS. 4 a    through  4   e.    
         [0037]      FIG. 4 a    firstly shows a workpiece  100  which is to be clamped centrically between four clamping jaws  39   a - d  with the chuck  1 . The clamping jaws  39   a - d  are coupled to the base jaws  11   a - d  in generally known fashion. 
         [0038]    If a clamping operation is to be initiated starting from the state shown in  FIG. 1  then the drive unit must be initiated by moving a traction tube in the direction of the arrow B ( FIG. 4 b   ). As a consequence of the movement in the direction of the arrow B the piston  15  is pulled down in the direction of the clamping axis A whereby the first and second sliders  21   a,    21   b  are moved towards each other in the direction of the arrows C. With the sliders  21   a,    21   b  the rockers  17 ,  19  are also moved towards each other in the direction of the arrows D, or a force is transmitted to the rockers  17 ,  19  in the direction of the arrows D. By means of the rockers, a force is transmitted to the entrainment members  23   a,    23   b,    25   a,    25   b,  whereby in turn transmission of force can be effected in the radial direction towards the clamping axis A (arrows E) on the base jaws  11   a - d  and thus on the clamping jaws  39   a - d  and the workpiece  100 . 
         [0039]    As a consequence of the transmission of force the clamping jaws  39   a - d  are moved towards the workpiece  100  until one or more clamping jaws contact the workpiece.  FIG. 4 c    shows a state in which the clamping jaws  39   a,    39   c  and  39   d  bear against the workpiece  100 , but a gap F still remains between the clamping jaw  39   b  and the workpiece  100 . Upon a continued traction movement of the piston  15 , and this is indicated in  FIG. 4 d   , the first and second rockers  17 ,  19  are pivoted. The entrainment members  23   a  and  23   b  are guided at a right angle to each other in the housing  3 . In the illustrated orientation the entrainment member  23   a  is guided vertically while the entrainment member  23   b  is moveably guided horizontally. The rotors are respectively guided at a right angle thereto in an opening  41   a,    41   b.  The entrainment members  23   a,    23   b  are moved relative to each other as a consequence of the pivotal movement of the first rocker. 
         [0040]    The symmetrically mutually oppositely arranged entrainment members  25   a,    25   b  are respectively linearly moveably guided parallel to the entrainment members of the first rocker, arranged in diametrally opposite relationship thereto, in the housing. The entrainment members  25   a,    25   b  each have an opening arranged at a right angle relative to their guidance for guiding the opening  43   a,    43   b  to guide the rotors associated therewith. By virtue of the coupling of the entrainment member  23   a  to the entrainment member  25   a  by means of the coupling ring  27  the second rocker  19  is pivoted in the same direction to the first rocker  17 . 
         [0041]    By virtue of the paired dependency of the entrainment members  23   a,    23   b  and  25   a,    25   b,  starting from the state shown in  FIG. 4 c   , the two clamping jaws  39   b,    39   d  are moved further towards each other because the gap F still has to be compensated. The base jaws  39   a,    39   b  which were already previously in contact ensure that while this is happening the workpiece is held centrically with respect to one of the two main axes while the two base jaws  39   b,    39   d  bring about centricities of the other main axis. By virtue of the guiding arrangement of the entrainment members  23   a,    23   b,    25   a,    25   b  relative to the rockers  17 ,  19  and thus the sliders  21   a,    21   b,  they can be still further moved towards each other, in spite of the base jaws  39   a,    39   c  bearing against the workpiece  100 , when the traction movement is continued. 
         [0042]    In that way the clamping operation is centrically continued until reaching the final state shown in  FIG. 4 e   , in which the workpiece  100  is held in the chuck  1  on all four sides centrically by the clamping jaws  39   a - d.    
         [0043]    As can be clearly seen from the foregoing description the compensation chuck according to the invention provides a mechanically robust and easily implementable option for providing a compensation function in relation to four-jaw chucks. 
         [0044]    The forgoing is merely preferred embodiments of the present invention and not intended to limit the patent scope of the present invention, and any equivalent structures or equivalent flow variations made by using the description and accompanying drawings of the present invention, no matter whether it is directly or indirectly used in any other related technical field, should be included within the protection scope of the present invention.