Clamping fixture

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

InFIG. 1is represented a first illustrative embodiment of a mandrel1according to the invention, which can be used, for example, to clamp and center a workpiece14(illustrated in dash-dot representation) on the work spindle of a machine tool. The mandrel has a main body2, with which it can be screwed by fastening screws to the work spindle of a machine tool. Integrally connected to the main body2is a longitudinally slotted expansion bush3. The expansion bush3has in a middle region an annular projection, which serves as a clamping surface. The expansion bush3is 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 jaws4. The segmentally divided region between the clamping jaws4and the main body2constitutes elastic flexural webs5, which resiliently connect the clamping jaws4to the main body2. In this region, the expansion bush3tapers conically from the clamping jaws4toward the main body2, so that the flexural webs, in their slackened position (release position), are inclined radially outward starting from the main body2, 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 bush3which is remote from the main body2, the flexural webs5continue onward into segmentally divided extension webs6, which are likewise flexibly deformable. The extension webs6end at an annular end piece7, in the region of which the expansion bush3is unslotted. The segmented strips of the expansion bush3are thus rigidly connected to one another at the annular end piece7. As a result of the, at both ends, rigid connection of its segmented strips, the slotted expansion bush3acquires approximately the transverse rigidity of an unslotted tube.

The expansion bush3is hollow. Inside it is fitted an actuating element8, which continues also through an axial bore of the main body2. The actuating element8is configured as an internally drilled clamping bolt, which, by a tension screw9, can be clamped against a counter bearing10on the rear side of the main body2. The clamping bolt8bears inside the expansion bush3a sleeve11, which is supported against the main body2and serves as a spacer element for one or more RINGSPANN® disks12seated level with the clamping jaws4on the clamping bolt8. On the other side of the RINGSPANN® disks12, the clamping bolt8bears a ring13likewise serving as a spacer element. At its end facing away from the RINGSPANN® disks, the ring13is supported axially against a shoulder8aof the clamping bolt8.

If the tension screw9is now screwed into its counter bearing10, then the shoulder8aof the clamping bolt8axially compresses the RINGSPANN® disks12between the spacer elements11and13. 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® disks12is supported against the shank of the clamping bolt8, the external diameter thereof enlarges and spreads the clamping jaws4outward counter to the spring force of the flexural webs5and extension webs6. The workpiece14is hereby clamped by its internal diameter and centered to the axis of the mandrel1.

Since the length of the flexural webs5remains constant, the axial distance of the clamping jaws4to the main body2is shortened by virtue of the angle of inclination of the flexural webs5in relation to the longitudinal axis. Due to this distance shortening, the clamped workpiece14undergoes an axial retraction or encounters a pressing force against the axial contact surface2′ of the main body2. 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 webs5in 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 bolt8moves axially in the clamping operation, the annular end piece7is mounted slidingly on the clamping bolt8. The sliding mounting is attained by true-to-size production of the bore of the annular end piece7and of the external diameter of the clamping bolt8. In a groove in the inner periphery of the annular end piece7, moreover, is inserted a sealing ring7a, which seals the annular end piece7against the external diameter of the clamping bolt8. The sealing ring7acan be, for instance, an O-ring made of PTFE, rubber or a fabric ring.

InFIG. 2, the mandrel1is shown in an isometric representation. The slotted expansion bush3, with its annular clamping surface made up of clamping jaws4arranged in the shape of a circular segment, can clearly be seen. By the clamping bolt8located inside the expansion bush3, which clamping bolt is tensioned against the main body2by the tension screw9, the expansion bush3is expanded in order to clamp a workpiece (not shown). An internal locking ring9a, which lies in a corresponding groove in the end region of the axial bore of the clamping bolt8, ensures that the tension screw9can also apply to the clamping bolt8, in the unscrewing operation, a tensile force in order to pull this, counter to possible clamping forces, back out of the expansion bush3through the RINGSPANN® disks in locking position and thus re-release the clamping jaws4.

A second illustrative embodiment of a mandrel1according to the invention is shown in a longitudinal section inFIG. 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 plate2a, which forms the contact surface2′ for a workpiece14to be clamped, is screwed on the main body2. In the second illustrative embodiment, the expansion bush3is also not integrally connected to the main body2, but is configured as a separate part and screwed in a cylindrical recess2″ of the main body2from the rear side of this same.

The expansion bush3in the second illustrative embodiment has an annular foot region5a, from which the flexural webs5lead to the clamping jaws4. On the opposite side of the clamping jaws4, flexible extension webs6lead to the annular end piece7of the expansion bush3. An advantage with this design is that the expansion bush3can be continuously slotted, already in its foot region5ato shortly before the annular end piece7, which, from a production engineering viewpoint, is easier to realize than a merely sectional slotting as in the first illustrative embodiment. Since the foot region5aof the expansion bush3is secured in the cylindrical recess2″ and is screwed from the rear side of the main body2by appropriate screw connections respectively to the segments of the foot region, the slotting in the foot region5aof the expansion bush3is not detrimental.

Serving in this case as the actuating mechanism for the expansion of the clamping jaws4is a conically tapering clamping bolt8, which is bolted to the power-operated clamping device of the machine tool. Radially within the clamping jaws4, the clamping bolt8has a conically running sliding surface8′, which bears against correspondingly shaped contact surfaces4′ of the clamping jaws4. By virtue of the conical shape, the clamping jaws4, upon actuation of the power-operated clamping device, are forced apart and thus clamp a workpiece14in its internal bore.

At its free end, the clamping bolt8includes teeth for an appropriate tightening tool. As in the first illustrative embodiment, the annular end piece7is mounted by a sealing ring7aslidingly on the outer periphery of the clamping bolt8.

InFIG. 4is shown a third illustrative embodiment of a mandrel1according to the invention. As in the second illustrative embodiment, the expansion bush3is realized as a separate part and is screwed by its foot region5ain a cylindrical recess2″ of the main body2. The screwing of the annular foot part5aof the clamping bush3in the cylindrical recess2″ of the main body2is here effected from the front side, however, so that the clamping bush3, in the event of a change of workpiece, can be exchanged without the main body2of the mandrel having to be removed from the machine tool.

As in the second illustrative embodiment, the expansion of the clamping jaws4is effected by a conical clamping bolt8. The contact surface2′ for a workpiece to be clamped is formed by a contact ring2ascrewed onto the main body2.

As in the preceding illustrative embodiments, the annular end piece7of the clamping bush3is mounted slidingly in relation to the outer periphery of the clamping bolt8. The end region of the clamping bolt8and/or of the annular end piece7are here shaped such that they can not only receive the customary tightening tool, but also a tailstock center15. The mandrel1can 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 webs5and extension webs6of the expansion bush3is 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 mandrel1, the extension webs6in the first two illustrative embodiments run roughly axially parallel. This can lead to slight tilting of the clamping jaws4in 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 webs6, in relation to the longitudinal axis, are inclined oppositely to the flexural webs5. The course of the flexural webs5and extension webs6is here mirror-symmetrical in relation to a transverse plane through the center of the clamping jaws4. Since the bending forces of the flexural webs5and extension webs6are thus roughly equal in size and inversely equi-directional, no tilting or conicality of the clamping surfaces arises.

Finally, inFIGS. 5 and 6is shown an illustrative embodiment of a chuck1′ according to the invention, with which a workpiece14can be clamped by its external diameter. The chuck1′, once again, has a main body2, which is screwed onto the work spindle of a machine tool. The main body2has centrally on the front side a cylindrical recess2″, in which is inserted a longitudinally slotted clamping sleeve3′ which, given kinematic reversal of the working principle, corresponds to the expansion bush3of the preceding illustrative embodiments. The clamping sleeve3′ has an annular foot region5a, by which it is screwed to the main body2. To the foot region5a, clamping jaws4are connected by appropriate flexural webs5. On the opposite side, flexible extension webs6lead to an annular end part7, by which the individual extension webs6of the clamping sleeve3′ are connected to one another.

The clamping sleeve3′ is surrounded by an outer sleeve8, which is mounted axially displaceably on the chuck1′. The outer sleeve8is connected by a transverse bolt16to a central actuating element17, which is connected up to a power-operated clamping device of a machine tool. With a shoulder8aformed by a step in its internal diameter, the outer sleeve8presses, via a ring acting as a spacer13, onto one or more RINGSPANN® disks12, which are supported outwardly against the outer sleeve8. Axially, the RINGSPANN® disks12lie on the margin, serving as a stop11, of the main body2. If the central actuating element17is moved to the left by a clamping force, then the outer sleeve8presses the RINGSPANN® disks, via the shoulder8a, against the stop11of the main body2. Since the RINGSPANN® disks12are supported with their external diameter against the outer sleeve8, their internal diameter diminishes and thus presses the clamping jaws4against a workpiece14to be clamped.

The axial contact surface2′ for the workpiece14forms a contact part2abolted inside the clamping sleeve3′. As in the preceding illustrative embodiment, flexural webs5and extension webs6are inclined in relation to the axial direction, wherein the direction of inclination of the flexural webs5, due to the kinematically reversed clamping direction, here extends radially inward to the clamping jaws. Due to the inclination of the flexural webs5, in the clamping operation the distance between clamping jaws4and foot region5aof the clamping sleeve is shortened, so that the workpiece14, upon being clamped, undergoes an axial retraction and an axial contact pressing force against the contact surface2′.

The outer periphery of the annular end part7of the clamping sleeve3′ is mounted slidingly in the outer sleeve8, so that a high rigidity against transverse forces is obtained.

InFIG. 6, the chuck1′ is shown in a perspective, cut-open representation, without a workpiece to be clamped. The slotting of the clamping sleeve3′ and the fastening of clamping sleeve3′ and stop part2ato the main body2by 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 bush3or of the clamping sleeve3′ 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 ring7and the main body2. 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 bolt8, 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.