Patent Publication Number: US-6991411-B2

Title: Device for clamping tools

Description:
RELATED APPLICATIONS 
   The present application claims the benefit of U.S. patent application Ser. No. 09/958,704, filed on Oct. 9, 2001, which issued as U.S. Pat. No. 6,712,367 on Mar. 30, 2004 entitled “DEVICE FOR CLAMPING TOOLS”, which was a national stage of PCT/EP00/02123 filed Mar. 10, 2000 and based upon DE 199 15 412 filed Apr. 6, 1999 under the International Convention, the specification of which is hereby incorporated in its entirety by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention concerns a device for clamping and releasing a tool having a tool shaft, the device including a tool receptacle having on its free end an open socket part of an electrically conductive material for receiving the tool shaft with frictional engagement, and with a ring or cylindrical coil shaped induction coil surrounding the socket part of the tool receptacle, which coil can be energized with a preferably high frequency alternating current. 
   2. Description of the Related Art 
   Devices of this type are intended to be used for example for clamping or releasing a tool in the form of a milling cutter or drill in the tool receptacle. For this, the tool receptacle is heated in the area of the socket part with the aid of the induction coil, so that the borehole of the socket part expands. The tool is then introduced via its shaft into the heating-enlarged receptacle. During the subsequent cooling of the socket part, the tool shaft is frictionally held in the borehole of the socket part which shrinks as it cools. The diameter of the tool shaft and the socket part are therein so selected, that upon cooling a form-fitting connection results which is fixed against rotation, and which is not released even due to centrifugal forces during rapid rotation. For de-tensioning, the socket part is warmed again via the induction coil, until the tool can be pulled out of the receptacle. The de-tensioning is only possible because the heating spreads from outside towards inwards, so that first the socket part is warmed, prior to the warmth reaching the tensioned tool part. Thereby it is achieved, that first the socket segment expands, so that the still cooler tool can be released out of the receptacle during de-tensioning. This however functions only when the employed tool has low thermal coefficient of expansion and/or low electrical conductivity, for example hardened metal or ceramic. In the case of employment of insertion tools made of tool steel, there are however always problems during de-tensioning. 
   SUMMARY OF THE INVENTION 
   The invention is thus concerned with the task of further developing the known device of the above described type in such a manner that even tools with a large thermal coefficient of expansion and/or comprised of electrically conductive material can reliably be clamped and released with the aid of induction heating. 
   The inventive solution is based upon the recognition, that tools of electrically conductive material are heated so rapidly by the not insignificant field of the conventional induction coil in the vicinity of the clamping area, so that the de-tensioning is made difficult or impossible. In order to prevent this, in accordance with the invention it is proposed to reduce the electromagnetic fields in the area of the free end of the socket part to such an extent that a heating of the tool situated in the tool receptacle is prevented. In accordance with the invention this is achieved thereby, that the induction coil is covered over by a pole shoe on its end surface adjacent the free end of the socket part, the pole shoe exhibiting a central opening for the tool and comprised of a magnetically conductive and electrically non-conductive material. By this means it is achieved, that the magnetic field lines are concentrated in the pole shoe on the concerned surface of the induction coil, so that also the part of the tool extending beyond the tool receptacle is effectively shielded against electromagnetic flux or leakage fields. 
   A particularly effective shielding is achieved thereby, that the pole shoe partially overlaps the free end of the socket part and lies axially and/or radially against the free end of the socket part. The through-hole opening in the pole shoe exhibits a greater dimension than the tool diameter, so that an air gap with high magnetic resistance exists in the direction of the tool. A further improvement in this respect is achieved thereby, that the pole shoe exhibits a cross-section which narrows partially in the direction towards the opening. 
   The pole shoe can be formed as a ring disk, which exhibits a concentrically conical fluting or groove towards the opening. Basically it is also possible to form the pole shoe of multiple, spoke-like arranged radial segments. 
   In order to achieve a rapid as possible warming of the socket part with as low as possible electrical consumption, it is proposed in accordance with a further preferred embodiment of the invention, that the induction coil further includes on its surface opposite to the pole shoe and/or on its outer circumference surface supplementally a magnetic shield of magnetic conductive and electrically non-conductive material. The face of the magnetic shield can be formed as a ring disk with a through hole for the tool receptacle of the socket part. Alternatively to this, the face of the magnetic shield can also be formed of multiple spoke-arranged radial segments. The outer circumference or casing outer-side shield is preferably formed as a cylindrical cage, which is either closed in the circumference direction or can be comprised of multiple axially parallel segments arranged spaced apart from each other in the circumference direction. 
   For better centering of the induction coil with respect to the socket part it is of advantage, when the pole shoe includes a concentric, ring shaped sintering shoulder for receiving the free end of the socket part and/or for supporting the ring opening of the induction coil. 
   In order to keep the current strength as low as possible for a pre-determined amount of heat production, it is proposed in accordance with a further preferred embodiment of the invention, that the induction coil includes a preferably multi-layer wound coil winding of a high frequency stranded conductor. It has found that, in this case, the otherwise conventional water cooling of the coil winding can be dispensed with. Since the heating process only requires a few seconds, generally convection cooling via the ambient air suffices. At higher power settings the coil winding can be cooled as necessary by forced air. 
   The induction coil preferably includes a coil body or bobbin of a ceramic material, which can in addition be used as carrier for the pole shoe and/or the magnetic shield. 
   Tests have shown that the outer circumference part is heated less rapidly in the vicinity of its free end than in the central area. In order to avoid gap problems, it is thus of advantage, when the socket part includes a registry or fitting borehole for receiving the tool shaft, which is in communication towards its free with a segment of larger diameter. The socket part can exhibit a cylindrical or barrel shaped outer surface. 
   The tool receptacle could on the one hand be provided directly on a rapidly rotating machine spindle. Alternatively thereto, the tool receptacle can carry a coupling element which carries the socket part and is connectable with a rapidly rotating machine spindle. 
   The tool receptacle and the induction coil are associated in the above-described orientation for the purpose of coupling and de-tensioning procedures. In order to make possible a simple operation, it is of advantage when the tool receptacle and the induction coil are moveable relative to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In the following the invention will be described on the basis of a illustrative embodiment represented schematically in the drawing. The single FIGURE shows a section through a device for coupling and releasing a tool in a tool receptacle with a heating device formed as an induction coil. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The tool receptacle  10  shown in the drawing includes a socket part  12  open towards its free end for the frictional reception of the tool shaft  14  of a tool  16  formed as a spiral milling cutter or drill. The tool receptacle  10  in the illustrated embodiment is formed as an adapter, which on its rear end includes a coupling element  18  in the form of a hollow shaft with conical or tapered outer surface for the connection to a—not shown—rapidly rotating machine spindle. The socket part  12  includes a fitting borehole  20  for the cylindrical work tool shaft  14  as well as a truncated conical shaped outer surface  22 . On its end, the socket part  12  is bordered by circumscribing ring surface. The diameter of the fitting borehole has a smaller dimension than the tool shaft  14  at ambient temperature. The clamping and release of the work tool  16  is thus possible at elevated temperature of the socket part  12  up to about 450° C. 
   The warming of the socket part  12  occurs with the aid of an induction coil  26 , which is acted upon by a high frequency alternating current. The socket part  12  is comprised for this purpose of an electric conductive material, in which the field lines  28  of the induction coil  26  penetrate into the outer circumference area and cause a temperature elevation by production of an eddy current. The tool receptacle is for this purpose introduced with its outer surface or outer circumference part  12  in the central opening  30  of the induction coil  26  which is in the form of a cylindrical coil. The induction coil includes a bobbin or coil body  32  comprised of a ceramic material. Upon the coil body there is situated a multi-layer winding  33  of high frequency conductive stranded conductor, which is acted upon by the high frequency alternating current. The induction coil  26  is closed off on the end adjacent the free end of the socket part  12  by a pole shoe  34  laid upon the coil body  32 , which pole shoe is comprised of a magnetically conductive and electrically non-conductive material which exhibits a central opening  36  for the insertion of the tool  16 . In the illustrated embodiment the pole shoe  34  is formed as a ring disk, which partially overlaps the ring surface of the socket part  12  from outside and which exhibits a conical recess  38  extending towards the central opening  36 . The pole shoe  34  lies upon the ring surface and borders towards the work tool shaft . 14  with a magnetic resistance increasing ring shaped air gap  40 . By this means it is achieved that the field lines  28  coming from the coil winding  33  are concentrated in the pole shoe  34  and are conducted directly via the ring surface into the socket part  12 . The part of the work tool  16  projecting beyond the ring surface of the socket part  12  is in this manner effectively shielded from the electro-magnetic flux. Thereby it is achieved, that also tools made of electrically conductive material, for example tool steel, can be employed, without being heated by the electro-magnetic flux. Finally, this is also necessary in order to make possible a reliable de-tensioning. 
   For further improvement of effectiveness, the induction coil  26  is provided on its end surface lying opposite the pole shoe  34  and also on its cylindrical outer surface respectively with a magnetic shield  42 ,  44  of magnetically conductive and electrically non-conductive material. The magnetic shield  42  on the end surface opposite the shoe pole side is thereby formed as a ring disk with a through hole  46  for the inner wall  48  of the coil body  32  provided, while the outer circumference side shield  44  is formed as a cylindrical cage. The field lines  28  are concentrated in the shields  42 ,  44 , so that the flux loss is minimized. The pole shoe  34  and the shield  42 ,  44  are comprised of a soft magnetic, in particular a soft ferritic ceramic oxide material, in which little or no flux loss occurs due to the electrically insulating characteristic. 
   The tool receptacle  10  and the induction coil  26  are moveable relative to each other for the clamping and releasing of the work tool. Besides the heating station, which includes the induction coil  26 , there is preferably provided a not shown cooling station, in which the tensioning process can be accelerated. 
   In summary the following can be concluded: The invention is concerned with a device for clamping and releasing work tools  16  in a tool receptacle  10 . The tool receptacle  10  includes for this purpose on its free end an open socket part  12 , which is adapted for receiving the work tool shaft  14  with frictional engagement. The work tool shaft is thereby shrink-fitted into a borehole  20  in the socket part  12 . For this purpose the socket part is provided with a tool receptacle  10  surrounding induction coil  26  which can be acted upon by high frequency alternating current for heating the socket part  12 . For avoidance of leakage or flux fields, which could result in a undesired heating of the work tool  16 , it is proposed in accordance with the invention, that the surface of the induction coil  26  adjacent the free end of the socket part  12  is covered over by a pole shoe  34  having a central through-hole  36  for the work tool  16  and comprised of a magnetically conductive and electrically non-conductive material.