Abstract:
A tool holder includes a shank including a tool holder portion and a central bore for accommodating a cutting tool, and a first shrink fit sleeve disposed about the tool holder portion of the shank. An application of heat to the tool holder causes the tool holder portion and the first shrink fit sleeve to expand outwardly, and the removal of heat will cause the tool holder portion and the first shrink fit sleeve to contract inwardly to rigidly maintain the cutting tool within the tool holder by shrink fit. The tool holder may also include a second shrink fit sleeve disposed about the first shrink fit sleeve to provide increased torque capability.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    It is known in the art to utilize heat shrink or shrink fit tool assemblies in industrial applications. The prior designs have utilized unitary chucks having an inner diameter slightly smaller than the exterior diameter of the shank of the cutting tool or other work piece. During use, the tool chuck is heated, typically by means of induction heating, causing the chuck to expand a sufficient amount to allow the tool shank to be inserted within the chuck. The tool is then allowed to cool during which the thermal contraction of the chuck exerts a uniform pressure on the tool shank, allowing the chuck to shrink down around the tool shank to securely lock or hold the shank in place. It has long been recognized that these types of heat shrink tool assemblies provide an improvement in accuracy and rigidity over standard milling chucks or collet chucks. 
         [0002]    Referring now to  FIG. 6 , a conventional shrink fit tool holder is shown generally at  10  for detachably retaining a rotary cutting tool  14 . In general, the shrink fit tool holder  10  comprises a shank or body  12  that includes a tool holder portion  12   a , a flange member  12   b , a tapered outer surface  12   c  that generally corresponds to a tapered bore of a spindle (not shown), and a central bore or aperture  12   d  for accommodating the cutting tool  14 . 
         [0003]    The central aperture  12   d  is formed to be slightly less in diameter than a shank portion  14   a  of the cutting tool  14 . This amount depends on the nominal size and the required torque transmission capacity. In order to insert the cutting tool  14  within the tool holder  10 , the tool holder portion  12   a  of the tool holder  10  is externally heated, for example, to a temperature of approximately 650° F. Due to the thermal expansion characteristics of the tool holder  10 , the application of heat to the tool holder  10  causes the tool holder  10 , and in particular, the tool holder portion  12   a  of the body  12  to expand resulting in the central aperture  12   d  enlarging or increasing in diameter a sufficient distance to allow the tool shank  14   a  to be inserted within the central aperture  12   d . When the tool shank  14   a  is inserted a sufficient distance within the central aperture  12   d , the external application of heat is discontinued and the tool holder  10  is allowed to cool back to ambient temperature, wherein thermal contraction causes the aperture  12   d  to contract and form a rigid bond between the tool holder  10  and the shank portion  14   a  of the cutting tool  14 . As such, the cutting tool  14  is rigidly maintained within the tool holder  10  in a concentric fashion for high tolerance machining applications. To remove the cutting tool  14 , heat is again applied to the tool holder  10  causing the tool holder  10 , and in particular, the tool holder portion  12   a  to expand outwardly. As the tool holder  10  expands, the tool shank  14   a  can be withdrawn from within the central aperture  12   d  of the tool body  12 . 
         [0004]    However, the use of shrink fit tool holders for machining of high strength materials, such as in the aerospace industry, has some limitations. The maximum grip is proportional to the interference fit levels, but a maximum designed interference is limited by the temperature required to remove the tool from the tool holder. Therefore, the maximum grip is limited. 
         [0005]    Some end mills with high helix angles may be pulled out from the tool holder during severe operating conditions, thus scrapping costly components. One solution is to use Weldon shank holders that provide high torque, but are not suitable for high speed applications. 
         [0006]    Accordingly, there is a need for a heat shrink tool assembly that provides an increased torque transmission capability of the assembly. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    To solve these and other problems associated with conventional shrink fit or heat shrink tool holders, a tool holder comprises a shank including a tool holder portion and a central bore for accommodating a cutting tool, and a first shrink fit sleeve disposed about the tool holder portion of the shank. In one embedment, the first shrink fit sleeve has an internal diameter less than the external diameter of the tool holder portion of the shank. In an alternate embodiment, the first shrink first sleeve has a tapered internal surface and the tool holder portion of the shank has a tapered external surface. 
         [0008]    Upon heating of the first shrink fit sleeve the first shrink fit sleeve expands outwardly such that the shrink fit sleeve can be disposed about the tool holder portion of the shank. Upon cooling of the shrink fit sleeve the first shrink fit sleeve contracts inwardly around the tool holder portion to rigidly maintain the cutting tool within the tool holder by shrink fit. The tool holder may be a shrink fit type tool holder. 
         [0009]    In another aspect of the invention, a tool holder comprises a shank including a tool holder portion and a central bore for accommodating a cutting tool; a first shrink fit sleeve disposed about the tool holder portion of the shank; and a second shrink fit sleeve disposed about the first shrink fit sleeve. Heat is applied to the first shrink fit sleeve and then disposed about the tool holder portion and then cooled. Next, heat is applied to the second shrink fit sleeve and then disposed about the first shrink fit sleeve and then cooled. The first shrink fit sleeve and the second shrink fit sleeve upon cooling contract inwardly to rigidly maintain the cutting tool within the tool holder by shrink fit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Further features of the present invention, as well as the advantages derived therefrom, will become clear from the following detailed description made with reference to the drawings in which: 
           [0011]      FIG. 1  is a cross-sectional view of a shrink fit tool holder with a shrink fit sleeve according to an embodiment of the invention. 
           [0012]      FIG. 2  is a cross-sectional view of a shrink fit tool holder with a shrink fit sleeve according to another embodiment of the invention. 
           [0013]      FIG. 3  is a cross-sectional view of a shrink fit tool holder with a shrink fit sleeve according to yet another embodiment of the invention. 
           [0014]      FIG. 4  is a cross-sectional view of a shrink fit tool holder with a pair of shrink fit sleeves according to an embodiment of the invention. 
           [0015]      FIG. 5  is a cross-sectional view of a shrink fit tool holder with a pair of shrink fit sleeves according to another embodiment of the invention. 
           [0016]      FIG. 6  is a partial cut-away perspective view of a conventional shrink fit tool holder. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    As discussed above, the conventional design of  FIG. 6  suffers from the lack of sufficient gripping force under severe operating conditions. One aspect of the invention is to increase the torque transmission capability of the tool holder assembly. This aspect is accomplished by providing a sleeve made of steel or other suitable material of high modulus of elasticity that can be shrink fit externally to the conventional tool holder of  FIG. 6 , thereby increasing the torque transmission capability of the tool holder assembly. It will be appreciated that the tool holder may be a conventional tool holder  10  or a shrink fit type tool holder  100 . 
         [0018]    As used herein, “shrink fit” means a firm fit which is effected by heating and expanding a sleeve, fitting the expanded sleeve on another member and cooling and shrinking the sleeve to fit the member firmly. The member may be the shank of tool, a tool holder or another sleeve. 
         [0019]    Referring now to  FIG. 1 , a tool holder  100  is shown according to an embodiment of the invention, wherein like reference numerals are given the same designations as in  FIG. 1 , and thus not be described in further detail. In a preferred embodiment the tool holder  100  is a shrink fit type tool holder. The tool holder  100  differs from the conventional tool holder  10  of  FIG. 6  in that the tool holder  100  includes a generally tubular shrink fit sleeve, shown generally at  102 , that provides increased torque transmission capability to the tool holder  100 . 
         [0020]    The shrink fit sleeve  102  includes an inner surface  104  and an outer surface  106 . The shrink fit sleeve  102  can be made of the same material as the tool body  12 , or alternatively, can be made of a different material having the desired shrink fit properties. The inner surface  104  is tapered at approximately the same angle as an outer surface  12   e  of the tool body  12 . The sleeve  102  has a varying thickness  108  such that the thickness  108  is at a maximum at a distal end with respect to the flange member  12   b  and is at a minimum at a proximate end with respect to the flange member  12   b  of the tool body  12 . As a result, the outer surface  106  of the sleeve  100  is generally parallel with respect to the longitudinal axis of the tool body  12 . 
         [0021]    Referring now to  FIG. 2 , a shrink fit tool holder  200  is shown according to another embodiment of the invention. In this embodiment, the tool holder  200  includes a shrink fit sleeve  202  that includes an inner surface  204 , an outer surface  206 , a bottom surface  210  and a top surface  212 . When positioned about the tool body  12 , the outer surface  206  of the sleeve  200  provides a substantially continuous surface with an outer surface  12   g  of the tool body  12 , and the top surface  212  is substantially continuous with a top surface  12   h  of the tool body  12 . When the sleeve  202  is shrink fit in place with a pre-determined amount of interference, the bottom surface  210  of the sleeve  202  may or may not abut a ledge  12   f  of the tool body  12 , thereby acting as a stop to correctly position the sleeve  202  with respect to the tool body  12 . This pre-determined amount of interference depends on the amount of heat applied to the sleeve  202 , the material and the dimensions of the sleeve. The abutment between the bottom surface  210  and the ledge  12   f  may be used as a limit to the interference fit and as to the amount of heat that should be applied to remove the sleeve  202 . In this embodiment, the sleeve  200  is not tapered like the sleeve  100 , but has a substantially uniform thickness  208 . 
         [0022]    Referring now to  FIG. 3 , a shrink fit tool holder  300  is shown according to yet another embodiment of the invention. In this embodiment, the tool holder  300  includes a shrink fit sleeve  302  that is substantially identical to the sleeve  202 , except that the sleeve  302  has a varying thickness  308  such that the thickness  308  is at a minimum a distal end with respect to the flange member  12   b  of the tool body  12 . This variation is opposite to the variation in thickness of the sleeve  100  of  FIG. 1 . When the sleeve  302  is shrink fit in place with a pre-determined amount of interference, it is desirable that the bottom surface  310  of the sleeve  302  abuts the ledge  12   f  of the tool body  12 , thereby acting as a stop to correctly position the sleeve  302  with respect to the tool body  12 . Because the tool holder portion  12   a  and the inner surface  304  of the sleeve  302  are substantially cylindrical in shape, this pre-determined amount of interference can not be controlled in the same manner as the sleeve  202  in  FIG. 2 , which has a tapered inner surface  204 . 
         [0023]    Referring now to  FIG. 4 , a shrink fit tool holder  400  is shown according to another embodiment of the invention. In this embodiment, the tool holder  400  includes a first shrink fit sleeve  402  and a second shrink fit sleeve  404  disposed about the first shrink fit sleeve  402 . The second shrink fit sleeve  404  provides increased torque transmission capability to the shrink fit tool holder  400  as compared to the earlier embodiments. As shown in  FIG. 4 , the first shrink fit sleeve  402  is substantially identical to the shrink fit sleeve  102  and is assembled to the tool holder  400  in the direction of the arrow  406 . That is, the sleeve  402  has a greater thickness distal to the flange member  12   b  of the tool body  12 . By contrast, the second shrink fit sleeve  404  has a greater thickness proximate to the flange member  12   b  of the tool body. Thus, the second shrink fit sleeve  404  is assembled in a direction of the arrow  408 . 
         [0024]    To install the shrink fit sleeves  402 ,  404 , the inner shrink fit sleeve  402  is first shrink fit in place and then is allowed to cool to room temperature. Then, a second shrink fit operation is performed on the outermost shrink fit sleeve  404 . To remove the shrink fit sleeves  402 ,  404 , heat is applied to the outermost shrink fit sleeve  404  to allow the sleeve  404  to expand a sufficient amount to enable the sleeve  404  to be removed from the tool holder  400 . Next, heat is applied to the inner shrink fit sleeve  402  to allow the sleeve  402  to expand a sufficient amount to enable the sleeve  402  to be removed from the tool holder  400 . Lastly, heat is applied to the body  12  to allow the bore  12   d  to expand, thereby enabling the cutting tool  14  to be removed from the tool holder  400 . 
         [0025]    Referring now to  FIG. 5 , a shrink fit tool holder  500  is shown according to yet another embodiment of the invention. In this embodiment, the tool holder  500  includes a first shrink fit sleeve  502  and a second shrink fit sleeve  504  disposed about the first shrink fit sleeve  502 . As shown in  FIG. 5 , the first shrink fit sleeve  502  is substantially identical to the shrink fit sleeve  202  having a substantially uniform thickness and is assembled to the tool holder  500  in the direction of the arrow  506 . The second shrink fit sleeve  504  has a greater thickness distal to the flange member  12   b  of the tool body, and is assembled in a direction of the arrow  508 , which is in the same to the direction as the arrow  506 . The installation and removal of the shrink fit sleeves  502 ,  504  is substantially similar to the installation and removal of the shrink fit sleeves  402 ,  404  described above. 
         [0026]    Many variations of the illustrated embodiments set forth above are within the spirit and scope of the invention. For example, the tool body  12  of the tool holders  400 ,  500  may include a ledge similar to the ledge  12   f  such that the outer surface of the second shrink fit sleeve  404 ,  504  may be continuous with the outer surface  12   g  of the tool body  12 , as shown in  FIGS. 2 and 3 . In addition, a set screw of a type well known in the art may be used to firmly secure the sleeve to the tool holder. 
         [0027]    To assemble the tool holders of the invention, the shrink fit sleeve or sleeves are shrink fit in separate steps. First, the tool is assembled on the toolholder by applying the heat to the toolholder assembly, inserting the tool in the sleeve and then allowing the assembly to cool down. Then, heat is applied to a sleeve and then the sleeve is disposed about the tool holder portion and is then allowed to cool to collapse the sleeve on the tool holder. Additional shrink fit sleeves may be added using essentially the same sequence of steps. 
         [0028]    In order to remove the tool, the outermost shrink fit sleeve or sleeves are removed first (heat applied to the outermost sleeve of the tool holder assembly, allowing the outermost sleeve to expand first and be removed, additional sleeves may be removed applying the same sequence of steps). Subsequently, heat is applied again on the tool holder, and then the tool is removed. This procedure provides a better distribution of stresses across the whole wall thickness and allows the tool holder to apply more pressure on the shank of the tool, as compared to conventional shrink fit tool holders. 
         [0029]    It is noted that when using the sleeves with taper bores, the amount of interference fit that can be achieved depends on the temperature applied to the sleeve(s). The more the sleeve expands, the more it drops on the external taper of the tool holder (or the sleeve that was previously shrunk). This only works for shallow taper angles, which are self locking. As a result, there is no need to be concerned about the exact size of the taper as long as the angles match the mating surface of the inner component. However, the maximum temperature applied on the sleeve needs to be limited so that the sleeve can be properly removed. 
         [0030]    Due to the heat that goes to the internal component, the temperature necessary to remove the sleeve (or the tool) is normally higher than the temperature used when shrink fitting the sleeve. On the cylindrical sleeve, the interference is defined by design and manufacturing tolerances so the interference does not change with temperature, but tighter tolerances may be required. 
         [0031]    The shrink fit sleeve of the invention can be applied to a wide variety of different tool holder designs. For example, the shrink fit sleeve can be applied to a Weldon style shank design that is popularly used in end mills for severe operating conditions. However, in a Weldon style shank design, the inside corner of the flat in the Weldon design can act as a stress riser that might result in possible cracking or failure of the end mill shank. In addition, the sharp points on the side of the Weldon flat and the shank outer diameter can also cause scratching and marring of the central bore or aperture  12   d.    
         [0032]    To alleviate these and other problems associated with using a Weldon style shank design, a shallow relief groove can be provided on the inner surface  104  of the shrink fit sleeve  102  (as well as the other embodiments of the shrink fit sleeve). The shallow relief groove prevents the otherwise desirable extra holding force created by the shrink fit sleeve from acting on the Weldon flat, thereby relieving the stress in the Weldon flat. 
         [0033]    The documents, patents and patent applications referred to herein are hereby incorporated by reference. 
         [0034]    While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.