Abstract:
A boring tool has a receiving part implemented as a borer body and a front part implemented as a borer head, which extend along a central axis and can be removably fastened to one another via driver connection. The driver connection has at least two coupling pairs which are separate from one another and are disposed eccentrically relative to the central axis. Each coupling pair is formed by interlocking coupling elements, namely a receiving pocket and a driver pin. The coupling elements have an asymmetrical cross-sectional area and widen with increasing distance to the central axis. Through this design, a reliable transmission of high torques is achieved with lower strains of the borer body in the area of the driver connection. Simultaneously, an automatic centering of the two tool parts to one another is performed.

Description:
BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The invention relates to a rotating cutting tool, in particular a boring tool, comprising a receiving part and a front part which extend along a center axis and are detachably fastened to one another via a driver connection. 
     b) Description of the Related Art 
     Such a cutting tool designed as a boring tool can be seen from WO 2007/107294 A1. This boring tool is a modular cutting tool which has an interchangeable tool head which can be detachably connected to a fluted boring body via the driver connection. The driver connection in this case comprises a driver web arranged on the underside of the boring head and extending across a center longitudinal axis. Said driver web is inserted into a receiving pocket, corresponding to the shape of the driver web, of the boring body. The receiving pocket in this case encloses the driver web over the full circumference. 
     The driver connection generally serves to transmit torque between the two tool parts. In such driver connections, there is often a conflict between as robust a driver web as possible for the torque transmission and as little weakening of the boring body as possible. 
     The object of the invention is to specify a tool of the type mentioned at the beginning having an improved driver connection which is designed for the transmission of high torques and at the same time causes only slight weakening of the receiving part in order thus to ensure stable return of the tool. 
     SUMMARY OF THE INVENTION 
     The object is achieved according to the invention by a tool having the features of claim  1 . The tool is generally a rotary tool for machining a workpiece. The tool is of multi-piece, in particular of two-piece, design and comprises a receiving part and a front part. In the case of a boring tool, the receiving part is the boring body and the front part is the boring head. In the case of a milling tool, the receiving part would be a milling cutter shank and the front part would be a milling cutter head. These two parts extending along a center axis (axial direction) are detachably fastened to one another via a driver connection. The latter has two coupling pairs separate from one another and arranged eccentrically with respect to the center axis. Each of the coupling pairs is in this case formed by two interlocking coupling elements, namely, on the one hand, a driver pin and, on the other hand, a receiving pocket completely enclosing said driver pin. The coupling elements serve firstly to transmit the torque forces between the two parts. Secondly, the two coupling elements also serve to orient the two parts in alignment with one another; i.e. the two parts are centered relative to one another and with respect to the center axis via the coupling elements. With regard to this double function, the coupling elements are of asymmetrical design and widen—as viewed in the plane perpendicular to the center axis—with increasing distance from the center axis. Due to the asymmetrical configuration, automatic centering of the two parts is achieved, in particular when the two parts are being fitted together. At the same time, with the widening with increasing radial distance from the center axis, the driver connection is designed to be especially strong in the radially outer regions, such that high torque forces can be transmitted. 
     The expression “asymmetrical configuration” of the coupling elements refers in this case in particular to the fact that said coupling elements have a cross-sectional area which is oriented perpendicularly to the center axis and which does not have symmetry either with regard to an axis of rotation or with regard to a plane. 
     A special advantage of this configuration can be seen in the fact that the two coupling pairs are separate from one another and are each arranged eccentrically with respect to the center axis. In the region of the center axis itself, therefore, the driver connection does not alter the receiving part or the front part. The two parts preferably bear on one another in a planar manner in the region of the center axis without interlocking in this region. The individual coupling pairs are therefore shifted into a radially outer region. As a result, the core of the tool is unaffected by the driver connection. 
     Investigations have shown that this configuration having the two eccentric coupling pairs separate from one another, compared with the configuration as described in WO 2007/107294 A1, leads to a reduction in the stresses in the boring body, that is to say in the receiving part. The loading of the receiving part—at the same or improved torque transmission—is reduced by about 20%. Furthermore, due to the separate arrangement, the center region is free, in which coolant bores, clamping screws, etc., can now be introduced without any problems. In addition, the enlarged free space around the center axis permits greater design flexibility with regard to the configuration in particular of the flutes. The latter can now be brought much closer to the center axis. 
     According to an expedient configuration, the driver pin in this case is designed like a prism. In the same way, the receiving pocket is also designed as a prismatic receptacle in a manner adapted to the driver pins. 
     The expression “designed like a prism” refers to the fact that the two coupling elements each have a roughly polygonal base area with side walls preferably extending parallel to the center axis. In particular, a base area is provided with 4 corner regions. In an expedient manner, the corner regions in this case are designed to be rounded. The connection between the individual corner regions also need not inevitably be effected rectilinearly. 
     The special advantage of this prismatic configuration can be seen in the fact that adjacent side walls serve for the automatic fixing or centering of the two parts. For the purpose of as robust a design as possible, provision is preferably made in this case for the extent of the respective driver pin in the radial direction to preferably be more than 50% of the radius of the tool. The two coupling elements are designed to be preferably rotationally symmetrical to one another with respect to a rotation about the center axis. In the preferred configuration having two coupling pairs, the coupling elements are therefore designed symmetrically to one another with respect to a rotation about the center axis by 180°. Alternatively, it is in principle also possible to design the coupling elements asymmetrically to one another. 
     According to an expedient development, the coupling elements each comprise radially outer bearing surfaces, in particular curved outer bearing surfaces, and further driver surfaces. In the load case, that is to say during torque transmission, both the outer bearing surfaces and the driver surfaces of the two coupling elements fitted one inside the other are pressed against one another. The outer bearing surfaces and the driver surfaces are in this case separate from one another. They expediently form adjacent side faces of the prism, via which side faces the self-centering is effected. 
     The driver surfaces are in this case preferably oriented substantially in the radial direction in order to enable as optimum a force transmission as possible for the torque driving. The expression “oriented substantially in the radial direction” refers in this case in particular to the fact that the driver surfaces are oriented at most at an angle of +/−20°, preferably +/−10°, with respect to the radial. 
     The receiving pocket expediently has an outer web which comprises the outer bearing surface and widens in the direction of that region of the outer bearing surface which is loaded in the load case. In this case, the outer web, with its outer side, at the same time also forms the outer side of the tool. The outer bearing surface running substantially concentrically to the outer lateral surface of the tool therefore deviates, according to the preferred configuration, from the concentric arrangement to the effect that the width of the wall region increases toward the loaded regions. 
     The receiving pocket preferably comprises at least one web-like and elastic wall region. The latter is sufficiently thin and thus elastic, such that, in the load case, when the torque forces occur, an elastic deformation of the wall region can be effected, and therefore surface contact is formed between the elastic wall region and an associated surface section of the driver pin. On account of this configuration, automatic tolerance compensation is therefore effected between the two contact surfaces between the driver pin and the receiving pocket in order to achieve a desired planar bearing surface for the torque transmission. Such tolerance inaccuracies are caused, for example, by dimensional inaccuracies during the production of the receiving pocket and/or of the driver pin. It should be taken into account here that the receiving part (boring body) is designed for many front parts (borer heads), which constitute wearing parts. 
     In general, the wall regions of the receiving pocket which surround the driver pin are of web-like design having a small width in comparison with the driver pin. 
     With regard to the desired self-centering of the two tool parts relative to one another, the two coupling elements, for assembly, have clearance relative to one another and also different cross-sectional geometries in such a way that a slight relative rotation is made possible between the two coupling elements inserted one inside the other. During such a relative rotation, the coupling elements are clamped against one another. In this case, the curved outer bearing surfaces preferably come to bear against one another. Since the two coupling elements are supported against one another in the radial direction via the outer bearing surfaces, a force component directed radially inward is exerted on the respective driver pin by this measure. Since this is effected at each coupling pair, this leads to the desired automatic centering of the two tool parts. The outer bearing surfaces of the two coupling elements of a coupling pair are preferably arranged eccentrically to one another. This refers to the fact that the outer bearing surfaces run along a circular path with a defined radius of curvature, the centers of the circles being arranged offset from one another. Different radii of curvature for the outer bearing surfaces can also be additionally provided. 
     In this case, the clearance is preferably selected in such a way that the two coupling elements have a free rotation angle within the range of 1-5°; that is to say they can be rotated relative to one another within a limited angular range of at most 1-5°. 
     At least one substantially axially running bore which is in alignment with a corresponding bore in the receiving pocket extends through the driver pin. This bore is preferably a coolant bore or also a bore for receiving a fastening means, such as a screw for example. Two bores, namely one for a coolant and one for a fastening means, in particular a clamping screw, are expediently provided in the driver pin. Both bores are in alignment with corresponding bores in the receiving pocket. 
     The bore provided for receiving the clamping screw is in this case expediently oriented obliquely relative to the longitudinal direction, to be precise in such a way that the two coupling elements are clamped against one another when the clamping screw is tightened. The oblique position is in this case selected in such a way that clamping is preferably effected in both the circumferential direction and the axial direction. The longitudinal axis of the bore is in this case inclined approximately in the circumferential direction, to be more precise in the clamping direction in which the driver pin is clamped against the receiving pocket. The longitudinal axis of the bore therefore runs within approximately a tangential plane, to be more precise within the plane which is defined by the axial direction and the clamping direction. The bore longitudinal axis lying in this plane has an angle of inclination relative to the axial direction of greater than 1° within the range of between 3° and 20°, preferably within the region of about 10°. 
     According to an expedient configuration, a compensating element extending preferably in the axial direction parallel to the center axis is arranged between the receiving pocket and the driver pin, said compensating element, in the load case, bearing against both the driver pin and the receiving pocket in a planar manner. In the load case, a force exerted by the driver pin is therefore transmitted to the receiving pocket. The compensating element is in this case in particular freely rotatable about its longitudinal axis. This compensating element serves for tolerance compensation in order to make possible desired planar contact of the surfaces participating in the transmission of force. Dimensional inaccuracies caused during production or possibly also during operation are compensated for by the compensating element. 
     To this end, the compensating segment preferably has a circle-segment-like cross-sectional area. The compensating segment is therefore of roughly semi-cylindrical design and has a partly cylindrical bearing surface and a flat bearing surface. Furthermore, the compensating element has a cylindrical fastening shank, with which it is inserted into a shank receptacle, designed as a bore, in the receiving pocket, in which case rotatability of the compensating element inside the shank receptacle is made possible. The compensating element is in this case preferably arranged in a corner region of the prismatic coupling elements, to be precise in particular in the corner region which connects the outer bearing surfaces to the driver surfaces. Furthermore, in order to make possible the free rotatability, provision is made in an expedient configuration for the receiving pocket to have a corner hole and for the driver pin to be flattened in the corresponding corner region. A sufficient rotary movement of the compensating element is made possible on account of the corner hole. 
     In this case, the compensating element and at least one of the coupling elements are preferably made of materials of different hardness. The compensating element can be softer or also harder than the at least one coupling element. 
     Due to the different material hardness, additional adaptation and additional tolerance compensation are made possible by plastic deformation. 
     According to an expedient development, in order to ensure that the two parts are reliably fastened to one another in the desired position, a clamping device is provided which can be actuated from the side and via which the driver pin can be clamped together with the receiving pocket. The expression “to clamp” in this case refers in particular to clamping both in the circumferential direction and in the axial direction, such that the two coupling elements are brought into their desired position relative to one another. This clamping device is preferably used as an alternative to the already described clamping screw which is oriented substantially in the axial direction and is actuated from the end face. The lateral clamping device, which can therefore be actuated from the lateral surface of the lathe tool and is oriented in the radial direction and also in an approximately tangential direction in the lathe tool, has the advantage that no modifications to the front part have to be made. 
     For the design of the clamping device, a longitudinally extending clamping pin is expediently arranged on the driver pin, said clamping pin plunging into a pin receptacle of the receiving pocket. In addition, a clamping element is provided which acts laterally on the clamping pin in order to clamp the driver pin together with the receiving pocket. Due to the additional arrangement of the clamping pin, the clamping device is isolated from the driver pin. The driver pin therefore is not weakened. The clamping pin preferably lies free of stress in the pin receptacle, even in the clamped state between the receiving pocket and the driver pin. In the clamped state, too, the clamping pin therefore has clearance relative to the pin receptacle. The clamping pin in this case is expediently arranged asymmetrically and eccentrically on the underside of the driver pin. 
     As an alternative to the configuration having the clamping pin, the clamping device preferably has an element, for example a threaded element such as a headless setscrew or a bolt, which is adjustably mounted in one of the coupling elements, the element being supported with one of its end faces against the other coupling element for the clamping. In the configuration as a headless setscrew, said headless setscrew is therefore screwed into or out of the respective coupling element until the headless setscrew is supported against the other coupling element and therefore clamps the two coupling elements against one another. This clamping device is again designed in such a way that clamping is effected both in the circumferential direction and in the axial direction. 
     The element is expediently adjustably mounted in a through-hole of the one coupling element, such that the element can also be supported with its second end face against the other coupling element. This serves to release the driver connection when coupling elements are clamped together. 
     A development of the invention, this development being inventive on its own, consists in mounting a supporting element in the coupling region, that is to say in the region of the parting line between borer body and borer head. The supporting element can be produced from a specially adapted material and serves to specifically stabilize the tool in the coupling region. The supporting element serves to dampen any movements or vibrations of the borer body and of the borer head relative to one another. In particular, opposed vibrations of borer head and borer body are to be reduced or neutralized in order to reduce the vibration wear of the tool. In addition, the transmission of solid-borne sound in the region of the parting line between borer body and borer head is to be reduced or eliminated. This reduction in the transmission of solid-borne sound also leads to an improvement in the properties of the tool. 
     Such tools are especially suitable for transverse bores of considerable depth in workpieces. In addition, the tools are also suitable for bores having oblique bore exits. 
     In a first configuration, the supporting element is configured as a disk which is plane-parallel to the end faces, adjacent to one another, of borer body and borer head. This disk-shaped supporting element preferably projects like a sliding ring segment beyond the lateral surfaces of borer body and borer head and thus supports the boring tool relative to the bore wall. In this way, the course of the boring tool in the bore is stabilized, while the cutting edges in the borer head can machine the bore wall. 
     A further preferred embodiment of the supporting element has a ring-like integrally formed portion which overlaps the lateral surfaces either of the borer body or of the borer head or of borer body and borer head at the same time. This embodiment has the advantage that the supporting element bears like a sliding ring against a large area of the bore wall. In addition, in a bowl-like configuration of the supporting element having a basic body configured as a plane-parallel disk and an annular integrally formed portion, especially good mounting of the borer body and of the borer head in the coupling region is ensured. In a further configuration, recesses corresponding to the flutes formed in the borer body and in the borer head can be provided in the supporting element in order to optimize the chip removal. The comparatively large outer circumferential surface of the ring-like integrally formed portion on the supporting element enables the attachment of special guide elements for guiding the boring tool on the bore wall. These guide elements can be strip-shaped or bulged or can be designed in other geometries for improving the concentric running of the tool in the bore. These guide elements can be integrally embedded in the outer surface of the ring element. However, it is also possible to fix them in the ring element in a clamping manner. 
     In a further configuration, the supporting element can have a coating, in particular in the region of the ring-like integrally formed portion. This coating can serve to prevent the wear on the outer surface of the ring relative to the bore wall. The coating can also influence the other vibratory and transmission properties of the tool. 
     In a further embodiment of the invention, the ring-like integrally formed portion, in particular in the configuration as a receiving bowl, can at the same time be configured as a flexural spring. In this configuration, the ring element applies a spring force to the bore wall and thus counters drifting of the tool away from its centered position with the acting spring force. The tool is therefore resiliently guided on the bore wall. In this way, the supporting element forms a damping member for the tool relative to the workpiece to be machined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are explained below in more detail with reference to the figures. In the drawing, partly in schematic illustration: 
         FIG. 1  shows a perspective cutaway illustration of a modular boring tool, 
         FIG. 2  shows a perspective view of the rear side of a boring head of the boring tool according to  FIG. 1 , having driver pins of a first embodiment variant, 
         FIG. 3  shows a perspective plan view of the front end face of a boring body of the boring tool according to  FIG. 1 , having receiving pockets of the first embodiment variant, 
         FIGS. 4   a - c  show a schematic plan view of the driver connection between the boring body and the boring head ( FIG. 4   a ) of a second embodiment variant and cutaway sectional views along the section lines  4   b - 4   b  and  4   c - 4   c , respectively, in  FIG. 4   a,    
         FIG. 5  shows a schematic plan view of the driver connection of the first embodiment variant, 
         FIG. 6  shows a schematic plan view of the driver connection similar to  FIG. 4   a  and  FIG. 5  of a third embodiment variant, 
         FIG. 7  shows a perspective view of the driver region of the boring head according to the third embodiment variant, 
         FIG. 8  shows a perspective plan view of the driver region of the boring body according to the third embodiment variant, 
         FIG. 9   a, b  show a plan view ( FIG. 9   a ) of the end face of the boring body and a sectional view through the borer head ( FIG. 9   b ), the section plane being determined by the section line  9   b - 9   b  in  FIG. 9   a,    
         FIG. 10  shows a perspective view of the driver connection of the borer head according to a fourth embodiment variant, 
         FIG. 11  shows a schematic plan view of the driver connection according to a fifth embodiment variant which forms a combination of the third and fourth embodiment variants, 
         FIG. 12  shows a perspective view of the driver region of the borer head according to a sixth embodiment variant, 
         FIG. 13  shows a schematic plan view of the driver connection according to the sixth embodiment variant, 
         FIG. 14  shows a partial sectional view in the region of a clamping pin according to the sixth embodiment variant along section line  14 - 14  in  FIG. 13 , 
         FIG. 15  shows an overall view of a section of a first embodiment of the boring tool with supporting element, 
         FIG. 16  shows an exploded illustration of the boring tool shown in  FIG. 15 , 
         FIG. 17  shows a view of the coupling surface of the borer head according to  FIG. 15 , 
         FIG. 18  shows a view of a supporting element according to  FIG. 15  designed as a plane-parallel disk, 
         FIG. 19  shows a plan view of the coupling surface of the borer body from  FIG. 15 , 
         FIG. 20  shows the exploded illustration of a borer body and of a supporting element having a two-sided receiving bowl, 
         FIG. 21  shows the exploded illustration from  FIG. 20  having a supporting element with a receiving bowl effective in the direction of the borer body, 
         FIG. 22  shows an exemplary embodiment, modified compared with the exemplary embodiment shown in  FIG. 21 , of a supporting element with an enlarged receiving bowl, 
         FIG. 23  shows an embodiment of the supporting element having guide elements on the outer circumferential surface of the annular integrally formed portion, 
         FIG. 24  shows an embodiment of an annular integrally formed portion configured as a flexural spring and as a receiving bowl, and 
         FIG. 25  shows a section through the ring-like integrally formed portion shown in  FIG. 24  and configured as a flexural spring. 
     
    
    
     Parts having the same effect are provided with the same designation in the figures. The driver connection is described below with reference to a boring tool in various embodiment variants. The driver connection can also be generally applied to other cutting tools for coupling two parts of a tool. The individual features described below with respect to the various variants and design configurations, provided they are not mutually exclusive, can also be combined with one another. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The boring tool  2  shown in  FIG. 1  is of modular construction and comprises a borer head  4  which forms the front part and is interchangeably fastened to a borer body  6  forming the receiving part. The boring tool  2  extends in the axial or longitudinal direction along a center axis  8 . In the exemplary embodiment, the borer head  4  has a central borer point  10  designed as a cutting insert and two cutting tips  12  arranged radially on the outside. A total of four discharge openings  14  for coolant can be seen on the front end face of the borer head  4 . The cutting tips  12  are oriented with their free flat side toward a flute  15  which starts in the borer head  4  and is continued as a helical flute  15  in the borer body  6 . The borer head  4  can be screwed to the borer body  6  by means of clamping screws  16  which are passed through the borer head  4  from the end face. 
     The borer head  4  and the borer body  6  are detachably fastened to one another via a driver connection. 
     As can be seen from  FIGS. 2 and 3 , the driver connection comprises two driver pins  18  which are arranged at a distance from one another on the underside of the borer head  4  and eccentrically to the center axis  8 . In the assembled state, the driver pins engage in a respective, corresponding receiving pocket  20  of the borer body  6 . The driver pins  18  are designed as prism-like prominences starting from the bottom flat side of the borer head  4 . In the same manner, the receiving pockets  20  are designed as prism-like recesses starting from a flat end face of the borer body  6 . A coolant bore  22  and a fastening bore  24  pass through each driver pin  18  and are in alignment with respective associated bores  22 ′,  24 ′ in the flat bottom surface of the receiving pocket  20 . The coolant bores  22 ,  22 ′ are fed from a central coolant passage  26  in a manner not shown in any more detail here. 
     Each driver pin  18  is defined by an end face which lies in a plane perpendicular to the center axis  8  and by a circumferential or lateral surface which is oriented parallel to the center axis. The respective receiving pockets  20  are also designed complementary hereto. 
     The separate configuration of the two driver pins  18  and the complete, enclosing reception thereof on all sides in the receiving pockets  20  is especially important. The driver pins  18  and the receiving pockets have a special geometry explained in more detail below. The cross-sectional area—perpendicular to the center axis—of each coupling element  18 ,  20  is characterized by asymmetry. This ensures very good torque transmission via the driver connection with the lowest possible stress of the boring body  6  in the region of the receiving pockets  20 . At the same time, the driver pins  18  are formed and arranged in conjunction with the receiving pockets  20  in such a way that automatic self-centering of the two parts  4 ,  6  relative to one another is effected. Each of the driver pins  18  forms a coupling pair with the receiving pocket associated with it; the driver pins  18  and the receiving pockets  20  form coupling elements which are designed to be generally complementary to one another and which—except for the slight differences described below—have identical cross-sectional geometries. 
     As can be seen from  FIG. 4   a , each coupling element has an approximately polygonal cross-sectional contour (as viewed in a plane perpendicular to the center axis  8 ). In the exemplary embodiments, each coupling element  18 ,  20  has four corner regions which are each of rounded design. The coupling elements generally have a trapezoidal cross-sectional area in the exemplary embodiments. Respective wall sections are formed between the individual corner regions.  FIG. 4   a  shows the driver connection in the loaded state, that is to say when the individual coupling elements  18 ,  20  bear against one another for the torque transmission. As can be seen from  FIG. 4   a , two respective wall regions bear against one another in this case. The one wall region is the radially outer wall region in which the coupling elements  18 ,  20  bear against one another via outer bearing surfaces  28  to transmit forces only in the tangential direction. The latter are of curved design and run, in the exemplary embodiment in  FIG. 4   a , concentrically to the outer circumferential side of the boring tool  2 . Furthermore, the two coupling elements  18 ,  20  bear against one another with driver surfaces  30  adjacent to the bearing surfaces  28 . In the exemplary embodiment in  FIG. 4   a , said driver surfaces  30  are arranged substantially radially to the center axis  8 . The other two wall regions of the coupling elements each have clearance relative to one another, such that the driver pins  18  overall rest with clearance in the respective receiving pocket  20 . These other wall regions therefore do not have any function with respect to the torque transmission and also do not serve to center the two parts  4 ,  6  relative to one another. In the exemplary embodiment, these wall regions are oriented approximately perpendicularly to one another and run parallel to and at a distance from two respective planes which are likewise arranged at a right angle to one another and which each include the center axis  8 . As seen in  FIG. 2 , the driver pin wall regions  19  closest to the center axis  8  oppose one another and define a free space or gap G therebetween. 
     As can be seen from the sectional illustration of  FIG. 4   b , the two coupling elements  18 ,  20  bear directly against one another in the region of their driver surfaces  30 . At the same time, it can be seen that the borer head  4 , with its flat underside, rests flat on the flat end face of the borer body  6 . In contrast, the underside of the driver pin  18  is at a distance from the bottom surface of the receiving pocket  20 . Finally, it can also be seen from  FIG. 4   c  that the rear wall regions, not participating in the torque transmission, of the two coupling elements  18 ,  20  are at a distance from one another. 
     When the borer head  4  is being fitted onto the boring body  6 , first of all the driver pins  18  are inserted into the receiving pockets  20 . The borer head  4  is then rotated slightly relative to the borer body  6 , the borer head  4  and the borer body  6  being clamped together via the coupling elements  18 ,  20  on account of this relative rotation. In  FIG. 5 , to explain this action, the coupling pair is shown in the unclamped state in the left half of the figure and in the clamped state in the right half of the figure. Shown in  FIG. 5  is the embodiment variant as can also be seen in  FIGS. 2 and 3 . 
     The radii of curvature r 1 , r 2  of the outer bearing surfaces  8  of the driver pin  18  (r 1 ) and of the receiving pocket  20  (r 2 ), respectively, are depicted in  FIG. 5 . As can be seen, the centers of the radii of curvature are arranged offset from one another, such that the driver pins  18  overall are arranged eccentrically to the receiving pockets  20 . The outer bearing surfaces  28  corresponding to one another are radially clamped against one another during the rotary movement by this measure. On account of the 180° symmetry of the two coupling pairs in the exemplary embodiment, the automatic centering of the borer head  4  relative to the borer body  6  is effected during this rotary movement. In the process, the coupling elements  18 ,  20  can be rotated relative to one another by a free rotation angle α which is in the region of a few degrees, in the region of 2° in the exemplary embodiment. 
     Except for this eccentric configuration, the two coupling elements  18 ,  20  are identical, i.e. they are designed with the same cross-sectional geometry. To form the clearance, which can readily be seen from the left half of  FIG. 5 , the driver pins are merely designed to be somewhat smaller than the receiving pockets. 
     Whereas in the exemplary embodiment in  FIG. 4   a  the driver surfaces  30  run inward in a concavely arched manner toward the radially inner corner region, the driver surfaces  30  according to the exemplary embodiment in  FIG. 5  extend substantially rectilinearly, wherein they deviate at an angle of a few degrees from the radial line which runs through the center axis  8 . 
     The third embodiment variant shown in  FIG. 6  is based on the embodiment variant according to  FIG. 5 . Here, too, the left half of the figure shows the unclamped state and the right half of the figure shows the clamped state. In contrast to the exemplary embodiment in  FIG. 5 , a compensating element  32  is arranged in the corner region which connects the driver surfaces  30  to the outer bearing surfaces  28 . The compensating element is designed like a dowel pin, extends parallel to the center axis  8  and has a circle-segment-like area as viewed in cross section. On account of the compensating element  32 , that corner region of the driver pin  18  which relates thereto is of flattened design, as can best be seen from  FIG. 7 . At its rear end, the compensating element  32  has a cylindrical fastening shank  34 , with which it is inserted into a cylindrical shank receptacle  36  ( FIG. 8 ) in the bottom of the receiving pocket  20 . The compensating element  32  is rotatable about its longitudinal axis in the shank receptacle  36 . 
     The compensating element  32  is arranged in the loaded region of the coupling elements  18 ,  20 , namely between the driver surfaces  30  and the outer bearing surfaces  28 . The driver pin  18  is supported with its flattened corner region against the flat side of the compensating element  32  and the latter in turn is supported with its approximately semi-cylindrical, rounded lateral surface side against the corner region of the receiving pocket. In this case, the corner region has the same radius as the compensating element  32 . On account of the rotatability of the compensating element  32  and on account of the design flattened on the one side and rounded on the other, the compensating element adapts itself automatically to the flat side of the driver pin  18 , such that planar contact is formed between driver pin  18  and compensating element  32 . On account of the rounded design with which the compensating element  32  bears against the wall of the receiving pocket  20 , largely planar contact is formed here, too. Overall, therefore, the compensating element  32  serves for compensating for tolerances which are caused during production, for example, or also form in the course of operation. In order to prevent the compensating element  32  from falling out, said compensating element  32  can be fixed in the shank receptacle  36  by means of a fastening lacquer, for example. The fixing force is proportioned in such a way that, in the load case and during a requisite rotation for compensating for tolerances, the compensating element  32  rotates automatically into the optimum position. 
     The borer head  4  is fastened to the borer body  6  via an additional clamping mechanism, such that the borer head is clamped against the borer body  6  in a defined axial position and in a defined rotary position in the clamping or circumferential direction. 
     According to a first embodiment variant, which is explained with reference to  FIGS. 9   a ,  9   b , a clamping screw  16  is passed through the borer head  4  from the front end face of the borer head  4 , and this clamping screw  16  runs through the fastening bore  24  of the driver pin  18  and can be screwed into an associated fastening bore  24 ′, designed as a screw hole, in the receiving pocket  20 . The fastening bores  24 ,  24 ′ are not arranged parallel to the center axis  8  but rather are arranged in an inclined manner. In this case, the center axis of the fastening bore  24  runs within a plane. This plane is defined by the axial direction and a clamping direction which is indicated in  FIG. 9   a  by the arrow  37 . The clamping direction  37  is in this case defined by the direction in which the driver pin  18  is clamped against the receiving pocket  20 . The clamping direction is in this case preferably oriented perpendicularly to the driver surface  30 . With regard to a line running in this plane parallel to the center axis  8  (and thus with regard to the driver surfaces  30 ), the center axis of the fastening bore is inclined by an inclination angle β of &gt;1° within the range of 3°-20° and preferably within the region of about 10°. On account of this sloping orientation, the borer head  4  is clamped against the borer body  6  both in the axial direction and in the clamping direction  37 . 
     As an alternative to this clamping, running substantially in the longitudinal direction, through the tool head  4  by means of the clamping screw  16 , a lateral clamping device is provided according to a preferred alternative. Said clamping device can be actuated from the circumferential side of the borer body  6 . The special advantage can be seen in the fact that no through-bores, weakening the borer head  4 , for the clamping screw  16  have to be provided. This lateral clamping device is explained in more detail below in connection with  FIGS. 10 to 14  in two different embodiment variants. 
     In the first embodiment variant, which is explained with reference to  FIGS. 10 and 11 , the clamping device comprises a headless setscrew  38  which can be adjusted in a corresponding tapped hole in the respective driver pin  18  by means of a tool  40 . To this end, the respective driver pin  18  has a through-hole  42  which is provided with an internal thread (not shown in any more detail here) at least in one section. The headless setscrew  38  has a receptacle  44  for the tool  40  on its rear end face accessible from outside, said receptacle  44  being designed as a hexagon socket in the exemplary embodiment ( FIG. 10 ). A dog point  46  is integrally formed on its opposite end face. For clamping the two coupling elements, the headless setscrew  38  is unscrewed slightly from the through-hole  40 , such that the headless setscrew  38  is supported with a section of its rear end face against the wall region of the receiving pocket  20  and therefore clamps the driver pin  18  in the desired direction. The headless setscrew  38  is designed roughly in a spherical cap shape at its rear end face. Complementary hereto, the wall region of the receiving pocket  20  is also designed in a spherical cap shape, such that, in addition to the clamping approximately in the circumferential direction, clamping in the axial direction is also effected. 
     For a change of the borer head  4 , the headless setscrew is screwed in until the dog point  46  is supported against the opposite wall region of the receiving pocket  20  and thus the clamping between the coupling elements is released again, such that the borer head  4  can be removed. 
     A further feature concerning the compensating element  32  can additionally be seen from  FIG. 11 . To be precise—as can be seen from the left half of the figure—the corner region in which the compensating element  32  is arranged is formed by a corner hole in the receiving pocket  20  in such a way that there is as far as possible free rotatability of the compensating element in order to achieve the greatest possible planar contact between the flat sides of the driver pin  18  and the compensating element  32 . 
     The second embodiment variant of the clamping device will now be explained in more detail with reference to  FIGS. 12 to 14 . In  FIG. 13 , the unclamped state between the two coupling elements is again indicated in the left half of the figure and the clamped state is indicated in the right half. In this embodiment variant, the driver pin  18  additionally comprises a respective clamping pin  48  which extends in the axial direction starting from the base side. In the exemplary embodiment, the clamping pin  48  has a roughly rectangular to elliptical cross-sectional contour and is arranged eccentrically on a marginal side. 
     The clamping pin  48  has a frustoconical receiving opening  50  in which a clamping element  52  designed as a screw and having a likewise frustoconical point engages ( FIGS. 13 and 14 ). Due to the frustoconical, that is to say tapering, configuration of the receiving opening  50  and of the clamping element  52 , an axial force component is also generated at the same time, in addition to the clamping in the circumferential direction, for clamping the borer head  4  in the axial direction against the boring body  6 . 
     The generation of the force in the axial direction on account of the frustoconical configurations can be readily seen once again from  FIG. 14 . Furthermore, a slot-shaped recess  54  on the top end face of the fastening shank  34  of the compensating element  32  can be seen from this figure. This slot-shaped recess  54  permits the engagement of, for example, a screwdriver in order to be able to rotate the compensating element into the desired position during the initial assembly. 
     The borer head  4  in  FIG. 15  has, on its cutting side  62 , the boring point  10  and two cutting tips  12  opposite one another at the circumference. Furthermore, coolant passages  65  and fastening means  66  are provided in the region of the borer head  4 . The fastening means  66  serve, for example, to fasten the cutting tips  12  or to fasten cutting tip holders or the like. 
     The head coupling side  67  is remote from the cutting side  62  of the borer head  4 . The head coupling side  67  has that end face of the borer head  4  which faces the borer body  6 . From this end face on the head coupling side  67 , in the exemplary embodiment, two driver pins project from the head coupling side  67  in the direction of the borer body  6 . The driver pins  18  again also have the rear openings of the coolant passages  65 . 
     That side of the borer body  6  which faces the borer head  4  is the shank coupling side  70 . The shank coupling side  70  and the head coupling side  67  form the coupling region between borer body  6  and borer head  4 . Two receiving pockets  20  designed to be complementary to the driver pins  18  are formed in the surface of the shank coupling side  70 . Coolant passages  65  can again be seen in the receiving pockets  20 , said coolant passages  65  being in alignment with the coolant passages  65  in the borer head  4  in the final assembled state. The coolant passages  65  therefore pass through the entire boring tool. Finally, both the borer head  4  and the borer body  6  each have a centering bore  72 . Flanks  73  on the borer head  4  and on the borer body  6  and helical flutes  15  incorporated between the flanks  73  can also be seen. 
     The tool mounting end (not shown in the figures) of the borer body  6  is remote from the shank coupling side  70  on the borer body  6 . With the tool mounting end, the borer body  6  is clamped in place in the boring tool. 
     In the exemplary embodiment according to  FIG. 15  and  FIG. 16 , the supporting element  75  designed as a plane-parallel plate is arranged between the borer head  4  and the borer body  6 . The supporting element  75  has through-openings  76  corresponding to the outer contour of the driver pins  18 . By means of the through-openings  76 , the supporting element  75  is slipped onto the borer head  4  in a simple manner, the driver pins  18  passing through the supporting element  75  in the through-openings  76 . The driver pins  18  thus form a form fit with the supporting element  75 . For the final assembly, shown in  FIG. 15 , of the tool, the driver pins  18  first of all pass through the through-openings  76  in order to then engage in the receiving pockets  20  in the borer body  6  in a form-fitting manner. The supporting element also has a centering bore  72 . Furthermore, the supporting element  75  has recesses  77  corresponding with the flutes  15 . 
     It can be seen from the illustration in  FIG. 15  that the supporting element  75  projects beyond the lateral surface, formed by the flanks  73 , of the borer head  4  and of the borer body  6 . In the finally assembled boring tool, the supporting element  75  therefore forms a protruding annular region  78  which projects beyond the envelope surface of the boring tool, namely of the borer head  4  and of the borer body  6 . During the boring operation, the supporting element  75  bears with this annular region  78  against the bore wall and thus guides the tool relative to the bore wall. 
     The exploded illustration in  FIG. 20  shows a borer body identical to  FIGS. 15 to 19 . In the exemplary embodiment according to  FIG. 20 , the supporting element  75  has a ring-like integrally formed portion  79 . The integrally formed portion  79  overlaps both the head coupling side  67  of the borer head  4  and the shank coupling side  70  of the borer body  6  in the direction of the center axis  8  of the boring tool. The supporting element  75  shown in  FIG. 20  therefore forms a two-sided receiving bowl for receiving both the head coupling side  67  of the borer head  4  and the shank coupling side  70  of the borer body  6 . 
     In contrast thereto, the exemplary embodiment of the supporting element  75  shown in  FIG. 21  has only one annular integrally formed portion  79 , which extends in the direction of the center axis  8  of the boring tool toward the shank coupling side  70  of the borer body  6 . In other words, the annular integrally formed portion only overlaps the shank coupling side  70  of the borer body  6  and at the same time bears as a plane-parallel plate against the head coupling side  67  of the borer head  4 . 
     In contrast thereto, the annular integrally formed portion  79  in the exemplary embodiment shown in  FIG. 20  overlaps both the shank coupling side  70  of the borer body  6  and the head coupling side  67  of the borer head  4  and thus forms a double-sided receiving bowl for both the shank coupling side  70  of the borer body  6  and the head coupling side  67  of the borer head  4 . The exemplary embodiment according to  FIG. 21 , on the other hand, forms only one receiving bowl for the borer body  6 , namely the shank coupling side  70  of the borer body  6 . 
     The exemplary embodiment shown in  FIG. 22  again shows a supporting element  75  having a receiving bowl which is effective only with regard to the borer body  6  and a plane-parallel plate bearing against the borer head  4 . In contrast to the exemplary embodiment shown in  FIG. 21 , the annular integrally formed portion  79  overlaps the shank coupling side  70  of the borer body  6  by a considerably greater amount in this exemplary embodiment. When the tool is assembled, the extent of the integrally formed portion  79  in the direction of the center axis  8  is considerably greater than in the exemplary embodiment shown in  FIG. 21 . In this way, it is possible to arrange guide elements  81  on the annular integrally formed portion  79 . 
     The exemplary embodiment shown in  FIG. 23  shows, as an example of such guide elements  81 , guide studs attached in pairs to the annular integrally formed portion  79 . These guide elements  81  slide on the bore wall during the machining process. 
     The exemplary embodiment shown in  FIG. 24  again shows a supporting element  75  having a receiving bowl effective only in the direction of the borer body  6 . In this case, the annular integrally formed portion  79  is configured as a flexural spring. For this purpose, a spring slot  82  is made in the annular integrally formed portion  79 . Furthermore, it can be seen from the illustration in  FIG. 25  that the guide elements  81  are pushed from the borer body  6  into the annular integrally formed portion  79 . On account of its spring action, the annular integrally formed portion  79  is resiliently mounted on the boring tool in the transverse direction  83  running transversely to the center axis  8 . The supporting element  75  is thus designed as a flexural spring element. 
     It is of course possible for all the embodiments in  FIG. 21  to  FIG. 24 , with regard to the configuration of the supporting element  75  as a receiving bowl in relation to the borer body  6 , to also be equally applied to the borer head  4 . Configurations are also conceivable in which a receiving bowl acting on both sides overlaps not only the shank coupling side  70  of the borer body  6  but also a smaller region of the head coupling side  67  of the borer head  4 .