Abstract:
An apparatus for inductively heating a sleeve section, having a central holding opening for a shank of a rotary tool, of a tool holder that holds the shank in the holding opening in a press fit and releases it upon heating. The apparatus includes an induction coil arrangement with at least one induction coil, a generator that feeds the induction coil arrangement with electric current of periodically varying amplitude, and a yoke arrangement of magnetizable material, which concentrates the magnetic flux of the induction coil onto the sleeve section in a fashion distributed all around. The induction coil is axially offset from the tool holder axis of rotation, in particular substantially radially next to the axis of rotation, and does not wrap around the section. Furthermore, the induction coil includes a coil core, of magnetizable material, that is connected in a magnetically conducting fashion to the yoke arrangement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus for inductively heating a sleeve section, having a central holding opening for a shank of a rotary tool, for example a drill, milling cutter or reaming tool, of a tool holder that holds the tool shank in the holding opening in a press fit and releases it upon heating. 
     2. Background of the Related Art 
     Particularly in the case of rapidly rotating tools that run, for example, at speeds of 10 000-20 000 rpm or even higher, it is known to shrink the tool shank into a sleeve section of a tool holder. For this purpose, the sleeve section is heated, usually to a few 100° C., for example 250° C.-350° C., such that the holding opening of the sleeve section widens and the tool can be inserted with its shank into the holding opening. The nominal diameter of the holding opening is somewhat smaller than the outside diameter of the tool shank. After the cooling of the sleeve section, the tool shank is therefore held securely in terms of rotation in a press fit in the tool holder. This shrinking technique permits the tool to be clamped extremely accurately for true running and thus with little unbalance. 
     A gas flame was firstly proposed as heat source for heating the sleeve section, but thought has also been given to heating collars that are to be brought into bearing contact with the tool holder. Because they permit the heating up phase to be kept very short, attention has more recently been concentrated on inductive heating devices. Such inductive heating devices have so far regularly had an induction coil that is fed from a generator with alternating current or a pulsed direct current and can be mounted centrally on the sleeve section in order to heat it. The magnetic field of the induction coil induces in the electrically conducting material of the tool holder eddy currents that directly heat the sleeve section. In order to be able to use one and the same induction coil in the case of tool holders of different outside diameter of the sleeve section, the induction coil surrounds the sleeve section at an axial spacing in this case. 
     Usually, the induction coil is surrounded on its outer circumference by a yoke shell of magnetizable material. In addition, there are usually arranged at the axial end faces of the induction coil annular elements that likewise consist of magnetizable material and serve as pole shoes which are situated closely adjacent to the end of the sleeve section on the tool side and the end remote from the tool, in particular to even bear against the sleeve section. Because of their high magnetic conductivity relative to air, the yoke shell and the pole shoes ensure a concentration of the magnetic flux, which is directed, thus focused, onto the sleeve section. 
     SUMMARY OF THE INVENTION 
     The invention proceeds from an apparatus for inductively heating a sleeve section, having a central holding opening for a shank of a rotary tool, of a tool holder that holds the tool shank in the holding opening in a press fit and releases it upon heating, comprising
         an induction coil arrangement with at least one induction coil,   a generator that feeds the induction coil arrangement with electric current of periodically varying amplitude, and   a yoke arrangement, of magnetizable material, which concentrates the magnetic flux of the induction coil arrangement onto the sleeve section in a fashion distributed all around.       

     By contrast with the previous shrinking concepts with an induction coil that is to be mounted on the sleeve section in order to heat it, according to the invention, for the purpose of heating the sleeve section, the induction coil is arranged with reference to an axis of rotation of the tool holder in a fashion offset eccentrically from the sleeve section and without wrapping around said section, for example approximately radially next to the latter, and includes a coil core of magnetizable material, that is connected in a magnetically conducting fashion to the yoke arrangement. 
     In the case of the solution according to the invention, the entire tool holder can remain entirely outside the induction coil for the purpose of heating the sleeve section. Consequently, the axial mounting of the induction coil on the sleeve section or the axial insertion of the sleeve section into the induction coil is eliminated. 
     It has emerged that, even in the case of a tool holder arranged outside the induction coil, the magnetic flux can be directed onto the sleeve section in a sufficiently strongly focused fashion with the aid of the coil core and the yoke arrangement of magnetizable, that is to say ferromagnetic or ferrimagnetic material, in order to achieve the desired rapid heating of the sleeve section. It is advantageous in this case that the coil core can essentially completely fill up the coil interior and scattering losses in the coil interior can thus largely be reduced, otherwise than in the case of the conventional solutions, in which there regularly remains between the induction coil and the sleeve section inserted into the coil an empty radial space that renders scattering losses caused by the air in the coil interior unavoidable. 
     The offset arrangement of the induction coil additionally permits a better thermal insulation of the latter from the parts of the tool holder that are to be heated, and this facilitates the cooling of the coil. 
     Furthermore, in the solution according to the invention, there is no need for a yoke shell surrounding the induction coil at its outer circumference, and this can lead to design simplifications. Finally, the eccentrically offset arrangement of the induction coil provides a large degree of freedom in shaping the arrangement of the induction coil and that of the yoke. An example of these freedoms is the number of induction coils used. Thus, the induction coil arrangement can comprise only a single induction coil. However, it can also comprise a plurality of induction coils, with one coil core each, arranged distributed uniformly in the circumferential direction of the tool holder. Depending on the application, these coils can be connected at least partially in parallel or in series. If at least a fraction of the coils are connected in parallel, it is conceivable, in particular, to be able to activate a different number of coils depending on requirement. 
     A further example of the proffered freedoms of shaping relates to the direction in which the magnetic flux permeates the sleeve section. Thus, the induction coil arrangement and the yoke arrangement can be designed in such a way that, in order to heat the sleeve section, at least a portion of the magnetic flux, in particular substantially the entire magnetic flux, enters the sleeve section at points situated at an axial spacing from one another, and exits again from said section, preferably substantially without a circumferential offset between the entry and exit points. The induction coil arrangement and the yoke arrangement can, however, also be designed in such a way that, in order to heat the sleeve section, at least a portion of the magnetic flux, in particular substantially the entire magnetic flux, enters the sleeve section at points situated in the circumferential direction at a spacing from one another and emerges again from said section, if desired substantially without an axial offset between the entry and exit points. The flux of the sleeve section with a predominant or even exclusive component transverse to the axial direction provides a simple possibility of varying the strength of the magnetic flux axially along the sleeve section, and thus of influencing the expansion behavior of the sleeve section in axial terms. 
     The solution according to the invention also provides large degrees of freedom as regards the shape and orientation of the induction coil(s). At least one induction coil can, for example, be designed approximately as a cylindrical coil with a rectilinear coil axis. For the purpose of heating the sleeve section, the cylindrical coil can then be arranged with its coil axis substantially parallel to the tool holder axis, although another orientation can also be selected relative to the sleeve section, for example along a plane normal to the tool holder axis. 
     At least one induction coil can also be designed approximately as a cylindrical coil with a coil axis running in a curve, in particular as a toroidal coil. In this case, for the purpose of heating the sleeve section, the toroidal coil can be situated in a three-dimensional disk at least approximately orthogonal to the tool holder axis, or in a three-dimensional disk extending along the tool holder axis and including the latter. 
     Furthermore, at least one induction coil can be designed as a flat coil whose coil axis is situated, in order to heat the sleeve section, at least approximately orthogonal to the tool holder axis, in particular cuts the latter. 
     Given the presence of a plurality of induction coils, the yoke arrangement can comprise yoke elements that closely adjoin the sleeve section in order to heat it and are each connected in a magnetically conducting fashion only to the coil core of a single induction coil. However, the yoke arrangement can also comprise at least one yoke element that closely adjoins the sleeve section in order to heat it and which is connected in a magnetically conducting fashion to the coil cores of a plurality of, in particular all the, induction coils. 
     In a preferred embodiment, the yoke arrangement has a yoke clamp that is connected in a magnetically conducting fashion to the coil core of each induction coil and whose half clamps can be swiveled relative to one another about a swiveling axis running parallel eccentrically to the tool holder axis, between a clamp closed position, in which they hold the sleeve section between them in a closely adjacent fashion, and a clamp open position in which at least one of the clamp halves is swiveled away from the sleeve section. Such a yoke clamp permits the simple shrinking and outshrinking even of tools that have a substantially larger diameter in their operating region remote from the shank than the outside diameter of the sleeve region. 
     In order to reduce scattering losses caused by air as far as possible, for the purpose of heating the sleeve section, the yoke arrangement can reach up to near said section, or even bear against it. In order in the case of tool holders with a different diameter of the sleeve section not to have to exchange parts of the yoke arrangement or even the entire yoke arrangement, the radial position of at least one yoke element, which is closely adjacent to the sleeve section in order to heat it, of the yoke arrangement can be changed operationally relative to the sleeve section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below with the aid of the attached schematic drawings, in which: 
         FIG. 1  shows an axial longitudinal section through a first exemplary embodiment of an inductive heating unit for shrinking and outshrinking of a rotary tool in a tool holder, 
         FIG. 2   a  shows an axial section through a second exemplary embodiment of an inductive heating unit, 
         FIG. 2   b  shows an axial cross section, taken along line II—II in  FIG. 2   a , of the heating unit of  FIG. 2   a,    
         FIG. 3   a  shows an axial section through a third exemplary embodiment of an inductive heating unit, 
         FIG. 3   b  shows an axial cross section through the heating unit of  FIG. 3   a , with yoke clamp opened, 
         FIG. 4   a  shows an axial longitudinal section through a fourth exemplary embodiment of an inductive heating unit, 
         FIG. 4   b  shows an axial cross section through the heating unit of  FIG. 4   a,    
         FIG. 5   a  shows an axial longitudinal section through a fifth exemplary embodiment of an inductive heating unit, 
         FIG. 5   b  shows an axial cross section along the line V—V in  FIG. 5   a,    
         FIG. 6  shows an axial cross section through a sixth exemplary embodiment of an inductive heating unit, 
         FIG. 7   a  shows an axial section through a seventh exemplary embodiment of an inductive heating unit, and 
         FIG. 7   b  shows the heating unit of  FIG. 7   a  in axial cross section. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a tool holder  10 , one which is unipartite here, but could also be multipartite, of an electrically conducting, in particular also magnetizable material, for example steel. In the region of its axial end referred to an axis of rotation  12 , the tool bolder  10  has a standard coupling piece  14 , for example in the form of a steep-taper cone or hollow steep-taper cone, with the aid of which the tool holder  10  can be clamped in a machine tool (not illustrated in more detail). In the region of its opposite axial end, the tool holder  10  has a sleeve-shaped tool holding section  16  with a substantially cylindrical holding opening  18 , centered in relation to the axis of rotation  12 , into which a shank  20  of a rotary tool  22  (otherwise not illustrated in more detail in  FIG. 1 ) can be inserted. The tool  22  can be, for example, a drill, a milling cutter or a reaming tool. The outside diameter of the shank  20  is somewhat greater than the nominal diameter of the holding opening  18 , and so when it is inserted into the sleeve section  16  the shank  20  is held therein in a press fit guaranteeing the transfer of the desired operating torque. 
     In order to be able to insert the tool shank  20  into the tool holder  10  and remove it from the latter, the sleeve section  16  is widened by heating. The heating is performed by means of an inductive heating unit, denoted in general by  23 , that comprises at least one induction coil  24  that is held (in a way not shown in more detail), on a holder of an induction-shrinking device equipped with the heating unit  23  and is fed from a current generator  26  with alternating current or pulsed direct current with a frequency in the kHz range, for example a few 10 kHz. The induction coil  24  is not mounted on the sleeve section  16  during heating operation of the heating unit  23 . Rather, with reference to the axis  12  of the tool holder  10 , it is offset eccentrically from the latter, that is to say the induction coil  24  does not wrap around the tool holder  10 . In the example of  FIG. 1 , the coil  24  for heating the sleeve section  16  is arranged approximately at the same axial level to the side at a radial spacing next to the sleeve section  16 . The magnetic flux that is generated by the induction coil  24 , which is wound here in an approximately cylindrical fashion with a straight coil axis  28  and is arranged in an axially parallel fashion with the tool holder axis  12 , is guided in a magnetic circuit through the sleeve section  16  and induces eddy currents there that heat the sleeve section  16  comparatively quickly and thus bring about an enlargement in the diameter of the holding opening  18  that permits the tool shank  20  to be pushed in and withdrawn. 
     In order to focus the magnetic flux of the induction coil  24  and to direct it onto the sleeve section  16 , the interior of the induction coil  24  includes a coil core  30  that penetrates the coil over its entire axial length and preferably completely fills up the coil interior while avoiding empty air spaces, and that is connected in a magnetically conducting fashion to plate-shaped yoke elements  32 ,  34  that are arranged on the axial end faces of the induction coil  24  and cover the latter, if appropriate, and that for their part bridge the radial spacing between the induction coil  24  and the sleeve section  16  and, in a fashion closely adjoining magnetic pole regions  36 , reach as far as the sleeve section  16 . Closely adjoining here covers both a direct bearing contact and a slight air gap between the yoke plates  32 ,  34  and the sleeve section  16 . The coil core  30  and the yoke plates  32 ,  34  can consist of ferromagnetic metal or of a magnetic composite material, such as ferrite, for example. The induction coil  24  can be wound onto a coil former of plastic or ceramic into which, for its part, the coil core  30  is inserted. However, it is also possible for the induction coil  24  to be wound directly onto the coil core  30 . It is an advantageous feature of the eccentrically offset arrangement of the induction coil  24  that the risk of heating the induction coil  24  and a possible coil former is substantially reduced because of the large spacing, by comparison with conventional solutions, between induction coil  24  and sleeve section  16  such that—if it is at all necessary—the outlay on cooling need not be as high as previously for the induction coil  24 . 
     In the exemplary embodiment of  FIG. 1 , the magnetic flux permeates the sleeve section  16  in the axial direction and in a fashion distributed substantially uniformly about the tool holder axis  12 . For this purpose, the yoke plates  32 ,  34  have in the region of the sleeve section  16  annular regions  38 ,  40  that form the magnetic pole regions  36  on their inner circumference. One of the annular regions  38 ,  40 , here the annular region  38 , surrounds the sleeve section  16  close to the end of the sleeve section  16  remote from the tool. The other annular region  40  is situated adjoining the end of the sleeve section  16  close to the tool; it can likewise at least partially surround the sleeve section  16 . Likewise, it can extend at least partially axially on the other side of the end face of the sleeve section  16  close to the tool in a radial fashion beyond the outer circumference of the sleeve section  16  in the direction of the inner circumference thereof, as is shown in FIG.  1 . In this case, it can rest with its area axially facing the sleeve section  16  on the end face of the sleeve section  16  close to the tool. The annular regions  38 ,  40  are illustrated here as flat disks extending radially relative to the tool holder axis  12 . Of course, they can also be of another shape, for example that of a conical shell. The concrete shape of the annular regions  38 ,  40  is selected, in particular, as a function of how the best possible magnetic shielding of the tool shank  20  can be achieved. 
     It is generally desired to be able to use induction-shrinking devices for tool holders with sleeve sections of different diameters. In the case of a unipartite design of the yoke plates  32 ,  34 , it would be necessary to exchange the yoke plates for the purpose of adapting diameters. In order to avoid such an exchange, it is conceivable to make use of a plurality of yoke pieces that are arranged distributed uniformly around the tool holder axis  12  and are guided moveably relative to one another, for example by means of a cam/cam follower arrangement, in order to form the annular regions  38 ,  40 . Reference may be made in this regard to WO 01/89758 A1, where various examples of such yoke pieces guided moveably relative to one another are shown in  FIGS. 12-20 . Reference is expressly made to the contents of said document. 
     To the extent that identical or identically acting components are concerned, the following description of the further figures has recourse to the same reference numerals as in  FIG. 1 , but with the addition of a lower case letter for the purpose of distinction. If nothing different results from the following, reference is made to the discussion above in relation to  FIG. 1  for the purpose of explaining these components. 
     The exemplary embodiment of  FIGS. 2   a  and  2   b  shows a heating unit  23   a  that corresponds essentially to the exemplary embodiment of FIG.  1 . The, once again, plate-shaped yoke elements  32   a ,  34   a  of the heating unit  23   a  are divided, however, in each case into two partial yoke elements  42   a ,  44   a  (see  FIG. 2   b ), of which each forms approximately a half of the annular region  38   a ,  40   a  of the relevant yoke element  32   a ,  34   a . By means of a swivel joint arrangement  46   a , the partial yoke elements  42   a ,  44   a  of the two yoke elements  32   a ,  34   a  are held such that they can be swiveled relative to one another about a swiveling axis  48   a  offset eccentrically from the tool holder axis  12   a  and from the coil axis  28   a  and parallel to these axes. Formed in this way is a yoke clamp  50   a  with two clamp halves  52   a ,  54   a  that are articulated relative to one another about the swiveling axis  48   a  and of which one clamp half  52   a  comprises the partial yoke element  42   a  of each of the yoke elements  32   a ,  34   a , and whose other clamp half  54   a  comprises the partial yoke element  44   a  of each of the yoke elements  32   a ,  34   a.    
     If the tool  20   a  is to be shrunk into the tool holder  10   a  or outshrunk therefrom, the yoke clamp  50   a  can be opened so far by folding out the clamp halves  52   a ,  54   a  relative to one another such that the sleeve section  16   a  can be inserted radially between the clamp halves  52   a ,  54   a . Subsequently, the yoke clamp  50   a  can be closed again by folding the clamp halves  52   a ,  54   a  together relative to one another such that the magnetic pole regions  36   a  situated on the mutually facing insides of the clamp halves  52   a ,  54   a  are brought into close proximity with the sleeve section  16   a , and current can be applied from the generator (not illustrated in more detail here) to the induction coil  24   a  for the purpose of thermal expansion of the sleeve section  16   a.    
     The clamp solution is particularly advantageous when the aim is to shrink or outshrink a tool whose greatest outside diameter is substantially greater than the outside diameter of the sleeve section  16   a , for example when the tool  22   a  bears a grinding plate  56   a , as shown in  FIG. 2   a . Specifically, with the yoke clamp  50   a  closed, a tool holder in which such a tool is clamped cannot be moved axially into the heating unit  23   a  or moved out of the latter, otherwise than in the case of a tool whose operating range can fundamentally be conceived as not being greater than the shank diameter, or only insubstantially so. However, by opening the yoke clamp  50   a , it is possible to create a radial passage through which it is possible to bring between the clamp halves  52   a ,  54   a  even a tool holder that bears a tool whose operating range is substantially greater than the sleeve section  16   a  of the tool holder. 
       FIGS. 3   a  and  3   b  show a heating unit  23   b  that differs from the exemplary embodiment of  FIGS. 2   a  and  2   b  essentially in that it has not only a single induction coil  24   b  in the form of a right cylinder, but a plurality of such induction coils  24   b  that each include a coil core  30   b  and for the purpose of heating the sleeve section  16   b  of the tool holder  10   b  are arranged distributed uniformly in the circumferential direction about the sleeve section  16   b  and at a radial spacing close to the latter, in which case they are situated with their coil axes  28   b  parallel to the tool holder axis  12   b . Here, the yoke elements  32   b ,  34   b  arranged on the axial end faces of the induction coils  24   b  and connected in a magnetically conducting fashion to the coil cores  30   b  form approximately annular disks. The yoke elements  32   b ,  34   b  form a yoke clamp  50   b  in a fashion similar to the exemplary embodiment of  FIGS. 2   a  and  2   b . For this purpose, the yoke elements  32   b ,  34   b  are each divided into two partial yoke elements  42   b ,  44   b  (see  FIG. 3   b ) in the shape of semiannular disks that are held such that they can be swiveled relative to one another by means of the swivel joint arrangement  46   b .  FIG. 3   b  shows the yoke clamp  50   b  thus formed in the open state. Otherwise, the yoke clamp  50   b  corresponds in terms of its function to the yoke clamp  50   a  of  FIGS. 2   a  and  2   b.    
     The variant of  FIGS. 4   a  and  4   b  corresponds to the exemplary embodiment of  FIGS. 3   a  and  3   b  to the extent that the heating unit  23   c  there likewise comprises a plurality of induction coils  23   c , wound in the form of right cylinders, that, for the purpose of heating the sleeve section  16   c  of the tool holder  10   c , are distributed at regular angular spacings around the sleeve section  16   c  and are arranged in this case with coil axis  28   c  parallel to the tool holder axis  12   c  at a radial spacing from the sleeve section  16   c  approximately at the same axial level as the latter. However, the heating unit  23   c  has no yoke clamp. Rather, each induction coil  24   c  is provided separately from the remaining coils at its axial end faces with yoke pieces  32   c ,  34   c  that are connected in a magnetically conducting fashion to the coil core  30   c  of the relevant coil  24   c . A plurality of magnetically unconnected, separate heating elements  56   c  are formed, so to say, in this way and together form the heating unit  23   c , being constructed in each case from an induction coil  24   c , the coil core  30   c  thereof and the associated yoke pieces  32   c ,  34   c . In order to permit adaptation to tool holders of different diameter, the radial position, referred to the tool holder axis  12   c , of the heating elements  56   c  can be varied in operational terms. For this purpose, the heating elements can be guided moveably relative to one another on guide means (not illustrated in more detail), for example in the radial direction referred to the tool holder axis  12   c . It goes without saying that instead of this it is also possible for only the yoke pieces  32   c ,  34   c  to be adjustable in their radial position relative to the sleeve section  16   c , if it is ensured that the magnetically conducting contact with the coil cores  30   c  is retained upon adjustment of the yoke pieces  32   c ,  34   c.    
     In the case of the variant of  FIGS. 5   a  and  5   b , as well, the heating unit  23   d  there has a plurality of heating elements  56   d  that, for the purpose of heating the sleeve section  16   d  of the tool holder  10   d , are arranged distributed at regular spacings around the sleeve section  16   d . In a departure from the exemplary embodiment of  FIGS. 4   a  and  4   b , each heating element  56   d  has, by contrast, a toroidally wound cylindrical coil  24   d  with a coil axis  28   d  running approximately in the shape of a circular arc, the coil core  30   d  of each induction coil  24   d  being designed appropriately in the shape of a torus. Formed at the ends of each coil core  30   d  are yoke regions  32   d ,  34   d  with the aid of which the relevant heating element  56   d  is closely adjacent to the sleeve section  16   d  for the purpose of heating it. The yoke regions  32   d ,  34   d  can be formed by unipartite projections of the coil cores  30   d . However, it is not excluded to form the yoke regions  32   d ,  34   d  by using separate material pieces of magnetizable material that are connected in a magnetically conducting fashion to the coil cores  30   d.    
     In order to heat the sleeve section  16   d , the heating elements  56   d  are respectively situated in a three-dimensional disk running parallel to the tool holder axis  12   d  and including the latter. Such a three-dimensional disk is indicated by dashes and denoted by RS in  FIG. 5   b  for one of the heating elements  56   d . It goes without saying that the heating elements  56   d  can be adjusted in their radial position relative to the tool holder  10   d , for example by guiding them moveably in a radial fashion referred to the tool holder axis  12   d  in order, on the one hand, to permit adaptation to tool holders  10   d  of different diameter of the sleeve section and, on the other hand, to facilitate the insertion of the tool holder  10   d  between the heating elements  56   d.    
     In the exemplary embodiments explained so far, the magnetic flux permeates the sleeve section of the tool holder in the axial direction. The magnetic flux can, however, also permeate the sleeve section transversely, in particular orthogonal to the axial direction.  FIG. 6  shows an example of this. The heating unit  23   e  in accordance with this variant in turn comprises only a single induction coil  24   e  that is designed as a toroidal coil with a coil axis  28   e , curved in the shape of a circular arc, and in accordance with a toroidal coil core  30   e . For the purpose of heating the sleeve section  16   e  of the tool holder  10   e , the induction coil  24   e  is arranged in a three-dimensional disk normal to the tool holder axis  12   e . Provided at the ends of the coil core  30   e  are yoke elements  32   e ,  34   e  that have an approximately cylindrical shape of shell and face the sleeve section  16   e  with the inside of their shells and closely adjoin said section for the purpose of heating it. The yoke shells  32   e ,  34   e  extend in the axial direction substantially over the entire length of that region of the tool holder  10   e  which is to be heated. In the case of the example of  FIG. 6 , they surround the sleeve section  16   e  in the circumferential direction on a substantial portion of its outer circumference. A sufficiently large air gap should be present between the shell edges, adjacent in the circumferential direction, of the yoke shells  32   e ,  34   e , in order to prevent a magnetic short circuit passing the sleeve section  16   e . This holds, in particular, whenever, for the purpose of heating the sleeve section  16   e , the yoke shells do not bear against said section but there is a radial air gap between the outer circumference of the sleeve section  16   e  and the shell inside of the yoke shells  32   e ,  34   e.    
     The diametrically opposite arrangement of the yoke shells  32   e ,  34   e  on the outer circumference of the sleeve section  16   e  effects a magnetic flux within the sleeve section  16   e , substantially along a plane orthogonal to the tool holders axis  12   a . The points of entry and exit of the magnetic flux into and out of, respectively, the sleeve section  16   e  are therefore situated at points offset in the circumferential direction of the sleeve section  16   e , and so it is possible, as it were, to talk of a transverse flux of the sleeve section  16   e . It has emerged that the sleeve section  16   e  can be heated sufficiently rapidly even in the case of transverse flux in order to achieve the desired expansion of the holding opening  18   e.    
     The yoke shells  32   e ,  34   e  can be designed in one piece with the coil core  30   e , but they can also be formed by separate material pieces of magnetizable material. They are part of a yoke clamp  50   e  that is formed by virtue of the fact that the coil core  30   e  is divided into two, and its two parts are held such that they can swivel relative to one another by means of a swivel joint arrangement  46   e.    
     Finally,  FIGS. 7   a  and  7   b  show a variant with a heating unit  23   f  that has a plurality of, here four flat coils  24   f  that, for the purpose of heating the sleeve section  16   f  of the tool holder  10   f , are arranged at equal angular spacings around said section. The induction coils  24   f  are situated in this case in such a way that they cut the tool holder axis  12   f  approximately orthogonally with their coil axes  28   f . They are held on a yoke clamp  50   f  whose magnetizable clamp material simultaneously forms the coil cores  30   f  of the induction coils  24   f.    
     Magnetic pole regions  36   f  are formed on the sides, facing the outer circumference of the sleeve section  16   f , of the coil cores  30   f  and, if appropriate, on the inner circumference of an annular yoke region  40   f , formed by the yoke clamp  50   f  in its closed state, at the end of the sleeve section  16   f  near the tool. A more or less strong transverse flux component of the sleeve section  16   f  can be achieved depending on the direction in which the coils  24   f  are flowed through.