Abstract:
A clamping fixture for clamping a component, in particular a workpiece or tool, having a number of clamping jaws, distributed along a periphery, which are connected by appropriate elastic flexural webs to a main body, wherein the clamping jaws are adjustable by an axially movable actuating device in a radial direction from a release position into a holding position, which clamping fixture is characterized in that the flexural webs, in their release position, are inclined in relation to an axially parallel direction radially in a direction of their respective holding positions.

Description:
INCORPORATION BY REFERENCE 
     The following documents are incorporated herein by reference as if fully set forth: DE 102013217401.2, filed Sep. 2, 2014. 
     BACKGROUND 
     The present invention relates to a clamping fixture for clamping and centering a component, in particular a workpiece to be machined or a tool. 
     A clamping fixture of this type can be, for example, a mandrel or a chuck. A mandrel serves for the internal clamping of a hub-shaped component, in that clamping jaws arranged along a periphery are expanded in an internal bore of the component. Conversely, a chuck serves for the external clamping of a component, in that clamping jaws are clamped around an outer periphery of the component. In both cases, a clamping of the component is effected by radially acting closure force. 
     Expanding mandrels, for instance, in which a tubular, longitudinally multi-slotted expansion bush is expanded by an inner, longitudinally displaceable cone in order to clamp a component centrically in its bore, are known from the market place. Such expanding mandrels have the drawback that their rigidity is low compared to transversely acting machining forces. Nor is it ensured that the component bears in a defined manner against a contact surface of the clamping fixture. Each of these leads to inaccuracies in the machining. 
     SUMMARY 
     The object of the invention is therefore to define a clamping fixture, of the type stated in the introduction, which enables higher machining accuracy. 
     The object is achieved by a clamping fixture with one or more features of the invention. Advantageous refinements are described below and in the claims. 
     In a clamping fixture having a number of clamping jaws, distributed along a periphery, which are connected by appropriate elastic flexural webs to a main body and are adjustable by preferably movable actuating device in a substantially radial direction from a release position into a holding position, this is achieved by virtue of the fact that the flexural webs, in their release position, are inclined in relation to an axially parallel direction radially in the direction of their respective holding position. 
     In order to help the component to be clamped to be pressed against an axial contact surface, a so-called axial retraction takes place during the clamping operation. The pre-existing inclination of the flexural webs in the release position means that the clamping jaws do not perform a purely radial movement during the expansion, but instead simultaneously move axially onto the main body, on which the axial contact surface is found. The reason for this is that the clamping jaws, in the course of expansion, perform an approximately circular movement around the region at which they end on the main body. Since, in a mandrel, the center point of this circular arc lies on a smaller diameter than the clamping jaws, the clamping jaws, in their radial expansion movement, are simultaneously moved axially in the direction of the main body and thus press the part to be clamped against the axial contact surface. Hence the position of the component is clearly defined, and the clamping is effected with higher accuracy. 
     The angle at which the flexural webs, in the release position, are inclined in relation to the axially parallel direction is 3 to 12 degrees, more preferably 5 to 10 degrees, and most preferably about 6 degrees. 
     In one refinement of the invention, which can be realized irrespective of the inclination of the flexural webs, in the extension of the flexural webs which connect the clamping jaws to the main body of the clamping fixture, extension webs which protrude beyond the clamping jaws and at their ends are rigidly connected to one another by an annular end piece, are provided. As a result of these measures, the transverse rigidity of the clamping fixture is improved. In an arrangement comprising flexural webs which are connected at both ends, the areal moment of inertia, which is critical to the transverse rigidity of the clamping fixture, is several times greater than the sum of the areal moments of inertia of the individual flexural webs if these were not connected at the front end. 
     Following completion of the clamping operation, frictional engagement exists between the clamping jaws and the clamped component. The clamping jaws are thus held in an axially non-displaceable manner, which should produce the same stiffening effect as is achieved by the annular end piece. Surprisingly, tests by Applicant have revealed, however, that, by virtue of the extension webs rigidly connected to one another by the annular end piece, a higher clamping accuracy compared to known expanding mandrels is obtained. In particular, in tests by Applicant, concentricity errors of just a few μm were able to be attained, which was not possible with the previously used expanding mandrels. As the explanation for this, Applicant ultimately found that the cause can lie merely in the transverse rigidity of the clamping fixture, which in the course of, i.e. during, the clamping operation, compels a radial expansion of the clamping jaws which is uniform over the periphery and which is exactly equal from clamping operation to clamping operation. 
     In a preferred embodiment, the extension webs, in the release position, are inclined oppositely to the flexural webs. In particular, the extension webs and flexural webs can be arranged roughly in mirror symmetry to a mirror plane running radially through the clamping jaws. A conicality of the clamping surfaces of the clamping jaws in the expanded state can hereby be prevented. 
     The flexural webs can be formed, for instance, by segmented strips of an expansion bush or clamping sleeve slotted from the fastening end. An arrangement which is easy to realize from a production engineering viewpoint and achieves high flexural rigidity is hereby obtained. 
     The actuating device, with which the clamping jaws are shifted into the holding position, preferably comprises an actuating element, which, when moved under force in an axial actuating direction, applies a radial clamping force to the clamping jaws. A radial clamping force of this kind can be applied by an elastic element, which, upon axial compression by the actuating element, undergoes a radial change in dimension. Preferably, an elastic element of this kind is constituted by a plurality of flatly conical rings made of spring steel and having a characteristic slotting made up of alternating slots in the external and internal diameter of the ring, in particular by a plurality of RINGSPANN® disks, available from the assignee. 
     Alternatively, the actuating element can also have a conical sliding surface, which cooperates with correspondingly shaped contact surfaces of the clamping jaws in order to apply a radial clamping force to the clamping jaws. 
     It proves particularly advantageous if the annular end piece is mounted slidingly in relation to the actuating element. The rigidity against transversely acting machining forces can hereby be further enhanced, since the annular end piece, and with it the clamping jaws connected by the extension webs, in the event of transversely acting load, are supported on the actuating element by the mounting. 
     The clamping fixture can be configured as a mandrel for internal clamping in an appropriate bore of the component or as a chuck for external clamping on an appropriate outer periphery of the component. 
     The clamping fixture according to the invention has, in particular, an axial contact surface which is fixed in relation to the main body and against which the component, upon being clamped, is axially pressed. The contact surface against which the clamped component bears with frictional engagement defines a precise clamping position. 
     In addition, the invention relates to a machine tool having a clamping fixture according to the invention. A preferred application of the high-precision clamping fixture according to the invention is obtained, in particular, in the grinding of workpieces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention emerge from the following description of illustrative embodiments and from the drawings, wherein: 
         FIG. 1  shows an axial section of a mandrel according to the invention along a longitudinal axis in a first illustrative embodiment, 
         FIG. 2  shows an isometric representation of the mandrel from  FIG. 1 , 
         FIG. 3  shows a second illustrative embodiment of a mandrel according to the invention in an axial section, 
         FIG. 4  shows a third illustrative embodiment of a mandrel according to the invention in an axial section, 
         FIG. 5  shows an axial section of a chuck according to the invention, and 
         FIG. 6  shows an isometric representation of the chuck from  FIG. 5  (sectioned along the longitudinal axis perpendicular to the in  FIG. 5 ). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  is represented a first illustrative embodiment of a mandrel  1  according to the invention, which can be used, for example, to clamp and center a workpiece  14  (illustrated in dash-dot representation) on the work spindle of a machine tool. The mandrel has a main body  2 , with which it can be screwed by fastening screws to the work spindle of a machine tool. Integrally connected to the main body  2  is a longitudinally slotted expansion bush  3 . The expansion bush  3  has in a middle region an annular projection, which serves as a clamping surface. The expansion bush  3  is divided by the longitudinal slots into a multiplicity of segmented strips; in the illustrative embodiment, these are ten segmented strips. 
     The segmented portions of the annular protrusion serving as a clamping surface form individual clamping jaws  4 . The segmentally divided region between the clamping jaws  4  and the main body  2  constitutes elastic flexural webs  5 , which resiliently connect the clamping jaws  4  to the main body  2 . In this region, the expansion bush  3  tapers conically from the clamping jaws  4  toward the main body  2 , so that the flexural webs, in their slackened position (release position), are inclined radially outward starting from the main body  2 , to be precise, in the illustrative embodiment, by an angle of about 6 degrees in relation to the mandrel axis. 
     On that side of the expansion bush  3  which is remote from the main body  2 , the flexural webs  5  continue onward into segmentally divided extension webs  6 , which are likewise flexibly deformable. The extension webs  6  end at an annular end piece  7 , in the region of which the expansion bush  3  is unslotted. The segmented strips of the expansion bush  3  are thus rigidly connected to one another at the annular end piece  7 . As a result of the, at both ends, rigid connection of its segmented strips, the slotted expansion bush  3  acquires approximately the transverse rigidity of an unslotted tube. 
     The expansion bush  3  is hollow. Inside it is fitted an actuating element  8 , which continues also through an axial bore of the main body  2 . The actuating element  8  is configured as an internally drilled clamping bolt, which, by a tension screw  9 , can be clamped against a counter bearing  10  on the rear side of the main body  2 . The clamping bolt  8  bears inside the expansion bush  3  a sleeve  11 , which is supported against the main body  2  and serves as a spacer element for one or more RINGSPANN® disks  12  seated level with the clamping jaws  4  on the clamping bolt  8 . On the other side of the RINGSPANN® disks  12 , the clamping bolt  8  bears a ring  13  likewise serving as a spacer element. At its end facing away from the RINGSPANN® disks, the ring  13  is supported axially against a shoulder  8   a  of the clamping bolt  8 . 
     If the tension screw  9  is now screwed into its counter bearing  10 , then the shoulder  8   a  of the clamping bolt  8  axially compresses the RINGSPANN® disks  12  between the spacer elements  11  and  13 . This axial actuating force produces an elastic alteration of the cone angle and thus of the diameter of the RINGSPANN® disks. Since the internal diameter of the RINGSPANN® disks  12  is supported against the shank of the clamping bolt  8 , the external diameter thereof enlarges and spreads the clamping jaws  4  outward counter to the spring force of the flexural webs  5  and extension webs  6 . The workpiece  14  is hereby clamped by its internal diameter and centered to the axis of the mandrel  1 . 
     Since the length of the flexural webs  5  remains constant, the axial distance of the clamping jaws  4  to the main body  2  is shortened by virtue of the angle of inclination of the flexural webs  5  in relation to the longitudinal axis. Due to this distance shortening, the clamped workpiece  14  undergoes an axial retraction or encounters a pressing force against the axial contact surface  2 ′ of the main body  2 . The axial retraction here roughly corresponds (disregarding the sagging of the flexural webs) to the difference of the cosine-angle functions of the angles of the flexural webs  5  in the release position and holding position, multiplied by the length of the flexural webs. Thus the greater is the angle of inclination of the flexural webs in the release position, the greater is also the axial retraction which the clamping jaws undergo in the course of the expansion or apply to the workpiece. An angular range from 3 to 12 degrees in the release position has here proved advantageous. 
     Since the clamping bolt  8  moves axially in the clamping operation, the annular end piece  7  is mounted slidingly on the clamping bolt  8 . The sliding mounting is attained by true-to-size production of the bore of the annular end piece  7  and of the external diameter of the clamping bolt  8 . In a groove in the inner periphery of the annular end piece  7 , moreover, is inserted a sealing ring  7   a , which seals the annular end piece  7  against the external diameter of the clamping bolt  8 . The sealing ring  7   a  can be, for instance, an O-ring made of PTFE, rubber or a fabric ring. 
     In  FIG. 2 , the mandrel  1  is shown in an isometric representation. The slotted expansion bush  3 , with its annular clamping surface made up of clamping jaws  4  arranged in the shape of a circular segment, can clearly be seen. By the clamping bolt  8  located inside the expansion bush  3 , which clamping bolt is tensioned against the main body  2  by the tension screw  9 , the expansion bush  3  is expanded in order to clamp a workpiece (not shown). An internal locking ring  9   a , which lies in a corresponding groove in the end region of the axial bore of the clamping bolt  8 , ensures that the tension screw  9  can also apply to the clamping bolt  8 , in the unscrewing operation, a tensile force in order to pull this, counter to possible clamping forces, back out of the expansion bush  3  through the RINGSPANN® disks in locking position and thus re-release the clamping jaws  4 . 
     A second illustrative embodiment of a mandrel  1  according to the invention is shown in a longitudinal section in  FIG. 3 . Here, as also in the other Figures, same features and features having the same effect are provided with same reference symbols. In the second illustrative embodiment, a separate annular contact plate  2   a , which forms the contact surface  2 ′ for a workpiece  14  to be clamped, is screwed on the main body  2 . In the second illustrative embodiment, the expansion bush  3  is also not integrally connected to the main body  2 , but is configured as a separate part and screwed in a cylindrical recess  2 ″ of the main body  2  from the rear side of this same. 
     The expansion bush  3  in the second illustrative embodiment has an annular foot region  5   a , from which the flexural webs  5  lead to the clamping jaws  4 . On the opposite side of the clamping jaws  4 , flexible extension webs  6  lead to the annular end piece  7  of the expansion bush  3 . An advantage with this design is that the expansion bush  3  can be continuously slotted, already in its foot region  5   a  to shortly before the annular end piece  7 , which, from a production engineering viewpoint, is easier to realize than a merely sectional slotting as in the first illustrative embodiment. Since the foot region  5   a  of the expansion bush  3  is secured in the cylindrical recess  2 ″ and is screwed from the rear side of the main body  2  by appropriate screw connections respectively to the segments of the foot region, the slotting in the foot region  5   a  of the expansion bush  3  is not detrimental. 
     Serving in this case as the actuating mechanism for the expansion of the clamping jaws  4  is a conically tapering clamping bolt  8 , which is bolted to the power-operated clamping device of the machine tool. Radially within the clamping jaws  4 , the clamping bolt  8  has a conically running sliding surface  8 ′, which bears against correspondingly shaped contact surfaces  4 ′ of the clamping jaws  4 . By virtue of the conical shape, the clamping jaws  4 , upon actuation of the power-operated clamping device, are forced apart and thus clamp a workpiece  14  in its internal bore. 
     At its free end, the clamping bolt  8  includes teeth for an appropriate tightening tool. As in the first illustrative embodiment, the annular end piece  7  is mounted by a sealing ring  7   a  slidingly on the outer periphery of the clamping bolt  8 . 
     In  FIG. 4  is shown a third illustrative embodiment of a mandrel  1  according to the invention. As in the second illustrative embodiment, the expansion bush  3  is realized as a separate part and is screwed by its foot region  5   a  in a cylindrical recess  2 ″ of the main body  2 . The screwing of the annular foot part  5   a  of the clamping bush  3  in the cylindrical recess  2 ″ of the main body  2  is here effected from the front side, however, so that the clamping bush  3 , in the event of a change of workpiece, can be exchanged without the main body  2  of the mandrel having to be removed from the machine tool. 
     As in the second illustrative embodiment, the expansion of the clamping jaws  4  is effected by a conical clamping bolt  8 . The contact surface  2 ′ for a workpiece to be clamped is formed by a contact ring  2   a  screwed onto the main body  2 . 
     As in the preceding illustrative embodiments, the annular end piece  7  of the clamping bush  3  is mounted slidingly in relation to the outer periphery of the clamping bolt  8 . The end region of the clamping bolt  8  and/or of the annular end piece  7  are here shaped such that they can not only receive the customary tightening tool, but also a tailstock center  15 . The mandrel  1  can thus be clamped and centered by a tailstock in order to further enhance the stability against transversely acting machining forces and the centering accuracy. 
     In the third illustrative embodiment, the course of the flexural webs  5  and extension webs  6  of the expansion bush  3  is of further interest. While, according to the invention, the flexural webs, in the non-spread release position, are inclined radially outward by an angle of around 6 degrees in relation to the longitudinal axis of the mandrel  1 , the extension webs  6  in the first two illustrative embodiments run roughly axially parallel. This can lead to slight tilting of the clamping jaws  4  in the course of the expansion, and thus to a conicality of the clamping surface. In the third illustrative embodiment, this is very largely avoided in that the extension webs  6 , in relation to the longitudinal axis, are inclined oppositely to the flexural webs  5 . The course of the flexural webs  5  and extension webs  6  is here mirror-symmetrical in relation to a transverse plane through the center of the clamping jaws  4 . Since the bending forces of the flexural webs  5  and extension webs  6  are thus roughly equal in size and inversely equi-directional, no tilting or conicality of the clamping surfaces arises. 
     Finally, in  FIGS. 5 and 6  is shown an illustrative embodiment of a chuck  1 ′ according to the invention, with which a workpiece  14  can be clamped by its external diameter. The chuck  1 ′, once again, has a main body  2 , which is screwed onto the work spindle of a machine tool. The main body  2  has centrally on the front side a cylindrical recess  2 ″, in which is inserted a longitudinally slotted clamping sleeve  3 ′ which, given kinematic reversal of the working principle, corresponds to the expansion bush  3  of the preceding illustrative embodiments. The clamping sleeve  3 ′ has an annular foot region  5   a , by which it is screwed to the main body  2 . To the foot region  5   a , clamping jaws  4  are connected by appropriate flexural webs  5 . On the opposite side, flexible extension webs  6  lead to an annular end part  7 , by which the individual extension webs  6  of the clamping sleeve  3 ′ are connected to one another. 
     The clamping sleeve  3 ′ is surrounded by an outer sleeve  8 , which is mounted axially displaceably on the chuck  1 ′. The outer sleeve  8  is connected by a transverse bolt  16  to a central actuating element  17 , which is connected up to a power-operated clamping device of a machine tool. With a shoulder  8   a  formed by a step in its internal diameter, the outer sleeve  8  presses, via a ring acting as a spacer  13 , onto one or more RINGSPANN® disks  12 , which are supported outwardly against the outer sleeve  8 . Axially, the RINGSPANN® disks  12  lie on the margin, serving as a stop  11 , of the main body  2 . If the central actuating element  17  is moved to the left by a clamping force, then the outer sleeve  8  presses the RINGSPANN® disks, via the shoulder  8   a , against the stop  11  of the main body  2 . Since the RINGSPANN® disks  12  are supported with their external diameter against the outer sleeve  8 , their internal diameter diminishes and thus presses the clamping jaws  4  against a workpiece  14  to be clamped. 
     The axial contact surface  2 ′ for the workpiece  14  forms a contact part  2   a  bolted inside the clamping sleeve  3 ′. As in the preceding illustrative embodiment, flexural webs  5  and extension webs  6  are inclined in relation to the axial direction, wherein the direction of inclination of the flexural webs  5 , due to the kinematically reversed clamping direction, here extends radially inward to the clamping jaws. Due to the inclination of the flexural webs  5 , in the clamping operation the distance between clamping jaws  4  and foot region  5   a  of the clamping sleeve is shortened, so that the workpiece  14 , upon being clamped, undergoes an axial retraction and an axial contact pressing force against the contact surface  2 ′. 
     The outer periphery of the annular end part  7  of the clamping sleeve  3 ′ is mounted slidingly in the outer sleeve  8 , so that a high rigidity against transverse forces is obtained. 
     In  FIG. 6 , the chuck  1 ′ is shown in a perspective, cut-open representation, without a workpiece to be clamped. The slotting of the clamping sleeve  3 ′ and the fastening of clamping sleeve  3 ′ and stop part  2   a  to the main body  2  by appropriate connecting screws can clearly be seen. 
     Besides the actuating mechanisms, shown in the illustrative embodiments, for shifting the clamping jaws by RINGSPANN® disks or conical contact surfaces, other arrangements for expansion or clamping of the clamping jaws are also conceivable, such as, for instance, a hydraulic actuating mechanism. 
     In the described illustrative embodiments, moreover, the longitudinal slots of the expansion bush  3  or of the clamping sleeve  3 ′ can be filled with an elastic compound, for example rubber, in order to avoid penetration of dirt and chips. 
     In the mandrels shown, a further improvement can be achieved by the clamping bolt being connected in a rotationally fixed, yet axially displaceable manner to the end ring  7  and the main body  2 . This can be brought about by an appropriate anti-twist protection, such as, for instance, a longitudinal toothing or tongue and groove geometry of the clamping bolt  8 , on the one hand, and of the end ring and main body, on the other hand. It is hereby achieved that the workpiece or tool is held more torsionally rigid in the peripheral direction.