Abstract:
A tool holder includes a plurality of elastic engagement pieces arrayed along an outer perimeter of a shank. During and after assembly, the elastic engagement pieces elastically abut at least an inner surface of a tapered hole in a main shaft joined to the tool holder and deform slightly in a radial direction. The elastic engagement pieces provide uniform engagement force with the main shaft over a broad surface area. An elastic flange provides additional elastic engagement and easy separation from the main shaft. Together, the elastic engagement pieces and elastic flange reliably engage and securely attach the main shaft to the tool holder while aiding disengagement.

Description:
This is a U.S. national phase application under 35 U.S.C. ¶371 of International Patent Application No. PCT/JP00/02338 filed Apr. 10, 2000. The International Application was published in Japanese on Oct. 18, 2001 as WO 01/76814 A1 under PCT Article  21(2).   
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tool holder that reliably secures a tool along a main shaft of a machine tool. 
     2. Description of the Related Art 
     Conventionally a tool holder is attached to the end of a main shaft to support a tool. This type of attachment is used in various machine tools including drill presses, milling machines, and machining centers. 
     With conventional tool holders, a main shaft rotates the tool to perform machining on a workpiece. Since the tool holder is removably attached to the main shaft, the main shaft is equipped with a retraction mechanism to draw in and secure a shank of the tool holder. 
     Conventional tool holders include a tool support section for attaching a tool, a shank including a tapered outer perimeter surface, a pull stud secured to the shank, and a flange having a larger diameter than the tapered hole. 
     Conventionally, the retraction mechanism pulls in the tool holder and fits and secures the shank to the tapered hole of the main shaft. 
     Unfortunately, high machining precision, of 1 micron error or less, is demanded from recent machining tools. During use, machining tools must operate while rotating the main shaft at high speeds of 30,000-40,000 rpm. 
     Consequently, the tool holder and the main shaft (rotating at high speeds) undergo undesirable rotary vibrations, which are a factor in degrading machining precision. 
     To minimize precision loss, it is desirable to increase securing strength by tightly securing the entire shank of the tool holder against the tapered hole of the main shaft. Unfortunately, due to machining tolerances in the tapered hole of the main shaft and the tool holder, the tool can heat up during use, resulting in detrimental thermal expansion of both the main shaft and the tool holder. Detrimental thermal expansion makes it impossible to secure the entire shank of the tool holder firmly and tightly against the tapered hole of the main shaft. 
     It is also impossible to abut the flange of the tool holder tightly to the end surface of the main shaft while having the shank of the tool holder tightly secured in the tapered hole of the main shaft. Consequently, conventional tool holders are not designed to abut the end surface of the main shaft. 
     Japanese laid-open patent publication number 8-108302 discloses a tool holder which includes a main holder unit having a shank and a flange. A sleeve is outwardly fitted to the shank so that it can move along an axial direction relative to the shank. The sleeve also has an outer perimeter surface with the same tapered shape as the tapered hole of the shank. An elastic member is interposed between the flange and the sleeve. 
     The sleeve is divided at one section along the perimeter, with the elastic body mounted in the resulting gap. When the retraction mechanism draws in the tool holder, the flange abuts the end surface of the main shaft. The sleeve is elastically pressed by the elastic member toward the base end of the main shaft. The sleeve is narrowed and engages the tapered hole and couples with the shank. 
     Japanese laid-open patent publication number 9-248727 discloses a tool holder, formed as described above, but instead of the sleeve with a division at one section, a sleeve with an inner groove at one section along the perimeter is used. With both above-described tool holders, the overall sleeve diameter is narrowed when the retraction mechanism pulls in the tool holder. As a result of this narrowing, the tapered outer perimeter surface of the sleeve cannot be tightly fitted and secured against the tapered hole. This makes it difficult to have a uniform contact force over the entire perimeter and provide secure coupling with the inner surface of the tapered hole. As a further detriment to this design, an increased number of parts is required and the structure is correspondingly complex, thus increasing manufacturing costs. 
     In a tool holder presented in Japanese laid-open patent publication number 7-96437, a ring-shaped groove is formed at the boundary between the shank and the flange. The ring-shaped groove allows a section of the flange to have a reduced thickness. When the tool holder is drawn in by the retraction mechanism, the flange abuts the end surface of the main shaft and is elastically deformed slightly, and acts as a disc spring. This mechanism engages the shank and the tapered hole. Unfortunately, the structure of the shank is the same as previous conventional shank structures, and roughly the same problems therefore exist as in conventional tool holders. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a tool holder which overcomes the drawbacks of the related art noted above. 
     It is another object of the present invention to provide a tool holder which provides both a firm and elastic fit of an entire shank in a tapered hole of a main shaft. 
     It is another object of the present invention to provide a tool holder which allows a flange to elastically abut an outer end surface of a main shaft. 
     It is another object of the present invention to provide a tool holder which prevents vibrations, greatly increases stability, and prevents loosening during use. 
     The present invention relates to a tool holder including a plurality of elastic engagement pieces arrayed along an outer perimeter of a shank. During and after assembly, the elastic engagement pieces elastically abut at least an inner surface of a tapered hole in a main shaft joined to the tool holder and deform slightly in a radial direction. The elastic engagement pieces provide uniform engagement force with the main shaft over a broad surface area. An elastic flange provides additional elastic engagement and easy separation from the main shaft. Together, the elastic engagement pieces and elastic flange reliably engage and securely attach the main shaft to the tool holder while aiding disengagement. 
     According to an embodiment of the present invention there is provided a tool holder apparatus, for securing a tool support section on a tool to a main shaft of a machining tool, comprising: at least a shank on the tool support section, the main shaft including a tapered hole, means for elastically securing the shank in the tapered hole of the main shaft and eliminating vibration and attachment failure during a use of the machining tool, a plurality of elastic engagement pieces in the means for removably securing, the elastic engagement pieces arrayed concentrically around an outer perimeter section of the shank, the elastic engagement pieces extending away from the shank and elastically abutting an inner surface of the tapered hole during an insertion of the shank into the tapered hole, and the elastic engagement pieces elastically deforming in a radial direction during the insertion, whereby the means for removably securing enables the elastic engagement pieces to absorb a plurality of use vibrations while maintaining a secure attachment to the main shaft. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, further comprising: a plurality of ring-shaped grooves on an outer perimeter section of the shank, the plurality of ring-shaped grooves arrayed alternatingly with the plurality of elastic engagement pieces in an axial direction along the outer perimeter of the shank, the plurality of ring-shaped grooves being smooth radius contours, thereby minimizing elastic stress concentration, and the plurality of elastic engagement pieces extending integrally from shank, thereby simplifying manufacturing and assembly of the tool holder. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: the plurality of ring-shaped grooves being positioned along the axial direction at a first diameter parallel to a central axis of the shank, the plurality of elastic engagement pieces each being a first length extending from each the respective ring-shaped groove to a sloped inner surface of the tapered hole, ends of each the elastic engagement piece arrayed in a plane parallel to the sloped inner surface, whereby each the elastic engagement piece contacts the sloped inner surface simultaneously during the insertion to provide easy alignment and tight attachment, and the first length of each the elastic engagement piece being dependant upon a position on the shank relative to the sloped inner surface, whereby the first length of the elastic engagement piece at a narrow side of the shank is smaller than the first length of the elastic engagement piece at a wide side of the shank, thereby increasing an elastic modulus of each the elastic engagement piece along the axial direction to beneficially resist vibrational loosening. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: each the elastic engagement piece is sloped toward the narrow side of the shank relative to a plane perpendicular to the central axis, and each the ring-shaped groove is oriented toward the narrow side of the shank relative to a plane perpendicular to the central axis, whereby the plurality of elastic engagement pieces resist removal of the shank from the tapered hole. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: each the elastic engagement piece extends from the shank in a plane perpendicular to the central axis. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: each elastic engagement piece is sloped toward the wide side of the shank relative to a plane perpendicular to the central axis, and each the ring-shaped groove is oriented toward the wide side of the shank relative to a plane perpendicular to the central axis, whereby the plurality of elastic engagement pieces enables easy removal after use while ensuring tight attachment to the main shaft. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, further comprising: at least one elastic flange on the shank, the elastic flange extending from the shank and abutting an outer end surface of the main shaft during the insertion, the elastic flange abutting the outer end surface along a continuous radius and providing an increased stability between the shank and the main shaft during the insertion and the use, and the elastic flange elastically deforming away from the outer end surface shank during the insertion, thereby urging the shank out of the tapered hole during a removal. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: the at least one elastic flange having a ring shape with a circular abutting piece at an outer diameter, a ring-shaped groove on the elastic flange at a radially inward position opposite the circular abutting piece, and a ring-shaped sloped groove on the elastic flange at a radially outward position opposite the ring-shaped groove, whereby the ring-shaped groove and the ring-shaped sloped groove assist the elastic flange in elastically urging the tool holder from the tapered hole during the removal. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: the plurality of elastic engagement pieces are a plurality of collar-shaped members individually secured to an outer perimeter section of the shank. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: the plurality of elastic engagement pieces are a plurality of flat springs mounted on an outer perimeter section of the shank, and the plurality of flat springs being between the shank and the tapered hole. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, further comprising: at least a first groove on the shank, the at least first groove on an outer diameter of the shank, and at least a first key extending from the tapered hole and engaging the first groove on the shank and allowing easy torque transfer from the main shaft to the tool support section during the use while enabling the insertion. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein: a cross-section of the tapered hole and the shank along a plane perpendicular to the center axis being a non-circular shape, the tapered hole having a non-circular inner surface parallel to a non-circular outer surface of the shank, at least a first groove in the plurality of elastic engagement pieces, and at least a first projection extending from the shank and engaging the non-circular inner surface during the use, whereby the at least first projection engages the non-circular inner surface and prevents rotation of the tool holder relative to the main shaft during the use. 
     According to another embodiment of the present invention there is provided a tool holder apparatus, wherein the plurality of ring-shaped grooves have smooth radius contours, thereby minimizing elastic stress concentration. 
     The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-section view of a tool holder including a section of a main shaft. 
     FIG. 2 is a side-view of the tool holder from FIG.  1 . 
     FIG. 3 is a drawing of FIG. 2 as seen from the direction indicated by arrow A in FIG.  2 . 
     FIG. 4 is a cross-section taken along the IV—IV line in FIG.  2 . 
     FIG. 5 is a cross-section taken along the V—V line in FIG.  3 . 
     FIG. 6 is a detail drawing of an elastic engagement piece. 
     FIG. 7 is a vertical cross-section of a second embodiment of the present invention. 
     FIG. 8 is a vertical cross-section of a third embodiment of the present invention. 
     FIG. 9 is a vertical cross-section of a fourth embodiment of the present invention. 
     FIG. 10 is a close-up schematic vertical cross-section of an alternative embodiment of the present invention. 
     FIG. 11 is a close-up schematic vertical cross-section of an alternative embodiment of the present invention. 
     FIG. 12 is a vertical cross-section of a tool holder and a main shaft along a plane perpendicular to an axial center according to another embodiment of the present invention. 
     FIG. 13 is a vertical cross-section of a tool holder and a main shaft along a plane perpendicular to an axial center according to another embodiment of the present invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a tool holder  1 , includes a main holder unit  10 , mounted to a main shaft  2  of a machine tool (not shown). A tapered hole  3  extends inward from an end of main shaft  2 . Tapered hole  3  is larger at a first end of main shaft  2  and narrows towards a center axis, as shown. The axial center of tapered hole  3  is aligned with an axis a of main shaft  2 . 
     The end surface of main shaft  2  is a flat surface extending perpendicular to axis a. A holding hole  4 , inside main shaft  2 , extends continuously with tapered hole  3 . A collet  5  is disposed at the end of a draw bar extending from a retraction mechanism (not shown). Collet  5  retracts and secures a shank  12  of tool holder  1  in tapered hole  3  of main shaft  2 . 
     Tool holder  1  includes main holder unit  10 , shank  12 , a locking bolt  20 , and a pull stud  25 . A tool support  11  attaches a tool T to shank  12  to allow tool T to be removably fitted and secured in tapered hole  3 . A plurality of elastic engagement pieces  13  are disposed along an outer perimeter section of shank  12 . A flange  14 , extending around main holder unit  10 , has a diameter larger than that of tapered hole  3 . Tool support  11 , shank  12 , elastic engagement pieces  13  and flange  14  are integrally formed during manufacture. Elastic engagement pieces  13  may be separately formed, as will be explained. 
     A tapered hole  10   a , an insertion hole  10   b , and a threaded hole  10   c  extend in series inside main holder unit  10 . During assembly, a locking bolt  20  is inserted through threaded hole  10   c  into insertion hole  10   b . Locking bolt  20  includes a threaded section which projects through insertion hole  10   b  and extends into tapered hole  10   a.    
     A connecting projection at the base of tool T tightly engages tapered hole  10   a  and threadably meshes with threaded locking bolt  20  to lock tool holder  1  to tool T. Tool support  11  includes tapered hole  10   a , insertion hole  10   b , and locking bolt  20 . 
     A pull stud  25  meshes with threaded hole  10   c  at a base end of main holder unit  10 . A collar  25   a  extends radially from pull stud  25  abutting the end of shank  12 . Since collar  25   a  has a radial surface in contact with shank  12  and threadably engages threaded hole  10   c , collar  25   a  provides strong lateral support to secure stud  25  and assists vibration compensation, as will be explained. 
     An engagement section  25   b  extends at the base end of pull stud  25  and positively engages collet  5  and draws inward shank  12  by pulling the draw bar (not shown) to the left in FIG.  1 . 
     A hexagonal hole  20   a  extends axially in locking bolt  20  to allow easy access for a hexagonal wrench (not shown) to engage and rotate locking bolt  20 . 
     An insertion hole  25   c  extends axially in pull stud  25  to provide access to locking hexagonal hole  20   a.    
     Elastic engagement pieces  13  extend from shank  12  along an outer perimeter away from the axial center of shank  12 . When assembled, elastic engagement pieces  13  abut the inner surface of tapered hole  3  and are elastically deformed in the radial direction. 
     A plurality of ring-shaped grooves  12   a  are arrayed at the outer perimeter section of shank  12 , positioned at predetermined axial intervals alternating with collar-shaped elastic engagement pieces  13 . Ring-shaped grooves  12   a  are formed at the outer perimeter section of shank  12  to allow elastic engagement pieces  13  to extend integrally from shank  12 . 
     An outer perimeter end of each elastic engagement piece  13  is sloped relative to a plane perpendicular with the axis of the shank  12  extending along the tapered shape of shank  12  (and tapered hole  3 ). 
     Each elastic engagement piece  13  has a high elasticity modulus, which makes them more easily elastically deformed during assembly due to the sloped outer perimeter end, described above. 
     An axial thickness of each elastic engagement piece  13  is pre-selected to provide an appropriate elastic modulus. The plurality of elastic grooves  12   a  are sloped grooves oriented toward the narrower end of shank  12 . 
     During engagement of elastic engagement pieces  13  into tapered hole  3 , the distance of respective elastic engagement pieces  13  from axial center a is greater at the bottom side of shank  12  (toward the right side in FIG.  1 ). The bottoms of ring-shaped grooves  12   a  are positioned at roughly a fixed radius and distance from axial center a. Consequently, a radial width of the plurality of elastic engagement pieces  13  is longer toward the wider end (bottom side) of the taper of shank  12 . The last elastic engagement piece  13  at of tapered hole  3 , is formed between ring-shaped groove  12   a  and a securing groove  15   a , as will be described. 
     Referring now to FIGS. 2 through 4, a pair of grooves  12   b , at the outer perimeter section of shank  12 , extend longitudinally along shank  12  at symmetrical positions around axial center a. Consequently, elastic engagement pieces  13  are divided in two sections extending along the outer perimeter of shank  12 . Each section of elastic engagement pieces  13  extends along an arc of approximately 170 degrees along the outer perimeter of the shank  12 . 
     Combining grooves  12   b  and the division in elastic engagement pieces  13  by ring-shaped grooves  12   a  allows elastic engagement pieces  13  to be easily elastically deformed during use. In other words, these structures reduce the effective elastic modulus of elastic engagement pieces  13 . 
     Referring now to FIG. 5, two keys  6  fixably extend from the inner surface of tapered hole  3  of main shaft  2  and engage grooves  12   b  when tool holder  1  is mounted on main shaft  2 . Keys  6  and grooves  12   b  allow a rotational torque to be transferred from main shaft  2 , through keys  6  to tool holder  1 . 
     An outer perimeter section of the flange  14  includes an integrally formed engagement groove  14   a  that engages an arm of an automatic tool changing device (not shown) and allows easy and rapid replacement. 
     A ring-shaped elastic flange  15  is formed at a base end of flange  14  and has a high elastic modulus. When mounting main holder unit  10  on main shaft  2 , elastic flange  15  is elastically deformed toward the axial center when it abuts the outer end surface of main shaft  2 . Elastic flange  15  has a ring that slopes toward the outer end surface of main shaft  2 . Additionally, when elastic flange  15  is elastically deformed, elastic flange  15  exerts a force against the outer end surface of main shaft  2  in the axial direction of shank  12 . 
     A ring-shaped groove  15   a  is formed at a radially inward position on one side of elastic flange  15 . A ring-shaped sloped groove  15   b  is formed at a radially outward position of elastic flange  15  opposite groove  15   a . Groove  15   a  and sloped groove  15   b  reduce the high elastic modulus of elastic flange  15  to facilitates elastic deformation during assembly and use. 
     During assembly, shank  12  of tool holder  1  is inserted into tapered hole  3  of main shaft  2 , and collet  5  of the retraction mechanism forcibly draws shank  12  into tapered hole  3 . 
     Referring now to FIG. 6, during assembly and drawing, the plurality of elastic engagement pieces  13  abut the inner surface of tapered hole  3  (solid lines) and are elastically deformed slightly in the radial direction, as indicated by the dotted lines. By having elastic engagement pieces  13  elastically deform radially and abut tightly against tapered hole  3 , the entire shank  12  is firmly and removably fitted against the inner surface of tapered hole  3  with a uniform force along the entire length of shank  12 . 
     Additionally, during assembly when tool holder  1  mounts on main shaft  2 , elastic flange  15  abuts the end surface of main shaft  2  prior to elastic engagement pieces  13  abutting the inner surface of tapered hole  3 . Consequently, when pull stud  25  is drawn in by the retraction mechanism (not shown), elastic flange  15  elastically deforms radially. 
     Since elastic flange  15  elastically contacts the end surface of main shaft  2 , small gaps between main shaft  2  and tool holder  1  are easily eliminated while firmly fitting shank  12  against tapered hole  3 . These two elastic engagement mechanism prevent undesirable vibrations in main shaft  2  and tool holder  1  even when main shaft  2  is operated at high speeds. As discussed above, eliminating undesirable vibrations significantly improves machining precision. 
     Since elastic flange  15  engages the end surface of main shaft  2  at a position where its radius from axial center a is maximized (large), vibrations and flexure in tool holder  1  is greatly reduced, thereby improving the stability of a tool in tool holder  1 . 
     Since elastic engagement pieces  13  extend along the outer perimeter and axis of shank  12 , the entire shank  12  is easily and reliably secured against the inner surface of tapered hole  3  with a uniform force. Additionally, the plurality of ring-shaped grooves  12   a  are integrally formed at uniform intervals along shank  12  to allow elastic engagement pieces  13  to be easily and integrally formed with shank  12 . This integral formation simplifies the structure of main holder unit  10  and reduces manufacturing costs. 
     The elasticity of elastic engagement pieces  13  is regulated and controlled by multiple mechanisms. First, the elasticity of elastic engagement pieces  13  is regulated by forming elastic engagement pieces  13  in a slope along a plane generally perpendicular to axial center a. Second, the ends of elastic engagement pieces  13  are sloped relative to tapered hole  3  and allow uniform contact during insertion. Third, elastic engagement pieces  13  are split by grooves  12   b  to engage keys  6  extending from tapered hole  3  and provide a torque transfer link with main shaft  2  while increasing elasticity. Together, these mechanisms allow uniform tight fitting of shank  12  into tapered hole  3  while permitting shank  12  to be formed of a strong material with a high elastic modulus. 
     An additional benefit of sloping the outer ends of elastic engagement pieces  13 , allows elastic engagement pieces  13  to be elastically deformed toward the narrower end of the tapered shape when tool holder  1  is mounted into tapered hole  3 . Since the outer ends elastic engagement pieces  13  tightly fit against the inner surface of tapered hole  3  by friction, the elastic restorative force of elastic engagement pieces  13  moves tool holder  1  toward the base end of main shaft  2 . Consequently, the retraction force drawing in tool holder  1  against collet  5  is increased and the drive force of the retraction device may be reduced. 
     As an additional benefit of the present design, when tool holder  1  is removed from main shaft  2 , the drive force of the retraction mechanism is stopped, and the elastic restorative force of elastic flange  15  urges tool holder  1  in a release direction. This elastic restorative forces releases the elastic deformation of elastic engagement pieces  13  and allows easy removal of tool holder  1  with the arm of the automatic tool changer device (not shown). When necessary, an axial thickness of elastic flange  15  may be increased to accommodate different needs and groove  15   a  may be eliminated where necessary or desirable. 
     In additional embodiments, where desirable elastic engagement pieces  13  and elastic flange  15  may be formed to abut main shaft  2  simultaneously or engagement pieces  13  may engage main shaft  2  prior to elastic flange  15 . When elastic flange  15  contacts main shaft  2  prior to elastic engagement pieces  13 , as in the present design, there is an additional stability benefit, but alternative timing choices may be selected according to manufacturer or customer needs. 
     Referring now to FIG. 7, an alternative embodiment of a tool holder  1 A includes a plurality of elastic engagement pieces  13 A extending parallel to a plane perpendicular with axial center a of a shank  12 A. 
     A plurality of ring shaped grooves  12   c  are on an outer perimeter section of shank  12 A at predetermined axial intervals relative to elastic engagement pieces  13 A. Ring shaped grooves  12   c  are parallel to a plane perpendicular to axial center a of shank  12 A. 
     Elastic engagement pieces  13 A are formed integrally with shank  12 A and elastically engage the inner surface of tapered hole  3 , during assembly, as noted above. 
     An elastic flange  15 A extends from shank  12 A perpendicular to axial center a, and an abutting piece  15   c  from an outer perimeter end. Abutting piece  15   c  extends from elastic flange  15 A toward the end surface of main shaft  2 . Abutting piece  15   c  abuts the end surface of main shaft  2  during assembly and aids in securing tool holder  1 A to main shaft  2 . 
     A ring-shaped groove  15   d  extends at a base end of elastic flange  15 A between shank  12 A and abutting piece  15   c . Ring-shaped groove  15   e  is parallel with a plane perpendicular to axial center a of shank  12 A and extends along elastic flange  15 A, thus allowing elastic flange  15 A to be easily and integrally formed with flange  14 . 
     During assembly with this embodiment, when tool holder  1 A is drawn in to main shaft  2 , a force perpendicular to tapered hole  3  is applied to the outer perimeter surface of elastic flange  15 A. This force elastically deforms elastic flange  15 A slightly in the radial direction and provides a tight fit between tool holder  1 A and main shaft  2 . 
     Referring now to FIG. 8, another alternative embodiment of a tool holder  1 B includes a plurality of elastic engagement pieces  13 B extending in a sloped direction toward the wider end of shank  12 B relative to a plane perpendicular with axial center a of a shank  12 B. 
     Elastic engagement pieces  13 B are integrally formed with shank  12 B. A plurality of ring-shaped grooves  12   d  are formed at the outer perimeter section of shank  12 B at a predetermined intervals with elastic engagement pieces  13 B. Ring-shaped grooves  12   d  sloped toward the wider end of shank  12 B. 
     During assembly, when tool holder  1 B is secured in tapered hole  3 , elastic engagement pieces  13 B elastically deformed slightly in the radial direction. In this state, due to the formation of elastic engagement pieces  13 B and ring-shaped grooves  12   d , elastic engagement pieces  13 B apply an outward force to tool holder  1 B, away from main shaft  2 . This outward force allows easy removal of tool holder  1 B from main shaft  2 . 
     Referring now to FIG. 9, another alternative embodiment of a tool holder  1 C exists without elastic flange  15 , groove  15   a , and sloped groove  15   b . This embodiment is useful where there is minimal need for the benefits provided by these additional elements and may be selected by customers operating at lower speeds. 
     Referring now to FIG. 10 another alternative embodiment of a tool holder  1 D includes a plurality of collar-shaped members  13 D separately secured to respective ring shaped grooves along an outer perimeter of a shank  12 D. Collar-shaped members  13 D slope toward the narrow end of shank  12 B and extend relative to a plane perpendicular with axial center (not shown) of shank  12 D. 
     In the present embodiment, collar-shaped members  13 D may alternatively be formed as rings, and a plurality of dividing bodies symmetrical relative to the axial center may be additionally placed between collar-shaped members  13 D. 
     Collar-shaped members  13 D may be separately secured to shank  12 D in similar directions as elastic engagement pieces  13 A or  13 B. 
     The present embodiment allows collar-shaped members  13 D to be selected according to a desired elastic modulus or material different from shank  12 D, thereby allowing easy adaptability to customer and manufacturer demand. 
     Referring now to FIG. 11, another alternative embodiment of a tool holder  1 E includes a plurality of elastic engagement pieces formed as a plurality of ring-shaped disk springs  13 E. Ring-shaped disk springs  13 E have a high elastic modulus and are mounted on the outer perimeter of a shank  12 E. Ring-shaped disk spring  13 E are arranged between shank  12 E and tapered hole  3  (not shown in this embodiment) to form a zig-zag cross-section pattern with the ends of adjacent disk springs  13 E contacting each other. 
     During manufacture, a knurled section  12   e  is formed on an outer perimeter of shank  12 E and provides a frictional link between the inner ends of disc springs  13 E and shank  12 E. This embodiment allows easy use of disk springs  13 E and easy adaptability to differing elastic requirements. 
     Referring now to FIG. 12, an alternative embodiment of a tool holder  1 F includes a plurality of elastic engagement pieces  13 F extending from a shank  12 F to engage a tapered hole  3 F of tool holder  2 . 
     In the present embodiment, a cross-sectional shape of tapered hole  3 F of the main shaft and shank  12 F, along a plane perpendicular to the axial center of shank  12 F, forms a non-circular triangular shape, as shown. 
     The triangular shape is formed by three projections  31  formed at 120 degree intervals along an outside of an inscribed reference circle  30 . Projections  31  allow rotational torque to transfer from tapered hole  3 F of the main shaft to shank  12 F without the use of additional keys. Three grooves  12   f  are formed at symmetrical positions at the corners on the outer perimeter section of shank  12 F, relative to the axial center of shank  12 F. During use, since projections  31  cannot rotate beyond the apex of their respective non-circular triangular position without encountering increased resistance, they easily operate to transfer torque, 
     Referring now to FIG. 13, an alternative embodiment of a tool holder  1 G includes a plurality of elastic engagement pieces  13 G extending from a shank  12 G to engage a tapered hole  3 G of tool holder  2 . 
     In the present embodiment, a cross-sectional shape of tapered hole  3 G of the main shaft and shank  12 G, along a plane perpendicular to the axial center of shank  12 F, forms a non-circular four-sided shape, as shown. 
     This four-sided shape is formed by four projections  33  extending at from shank  12 G at 90 degree intervals along an outside of a reference circle  32 . As with tool holder  1 F in FIG. 12, projections  33  allow rotational torque to be transferred from tapered hole  3 G of the main shaft to shank  12 G without the use of keys, thereby simplifying assembly and use. 
     Four grooves  12   g  are in shank  12 G at symmetrical positions relative to the axial center and at the corners of the outer perimeter. 
     In addition to the non-circular shapes indicated in the embodiments shown in FIGS. 12 and 13, other non-circular shapes may be used to provide the same benefit. 
     In other alternative embodiments, the grooves formed on the elastic engagement pieces  13 - 13 B may be optionally omitted. One skilled in the instant art will also recognize that the number, thickness, and region of the elastic engagement pieces along the outer perimeter of the shank is readily adaptable to a desired retaining and elastic force. 
     Although only a single or few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment(s) without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the spirit and scope of this invention as defined in the following claims. 
     In the claims, means- or step-plus-function clauses are intended to cover the structures described or suggested herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, for example, although a nail, a screw, and a bolt may not be structural equivalents in that a nail relies entirely on friction between a wooden part and a cylindrical surface, a screw&#39;s helical surface positively engages the wooden part, and a bolt&#39;s head and nut compress opposite sides of at least one wooden part, in the environment of fastening wooden parts, a nail, a screw, and a bolt may be readily understood by those skilled in the art as equivalent structures. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.