Abstract:
It is an object of the invention to provide a constituent member of a collet chuck, a collet and a fastening nut as well as a collet chuck capable of reinforcing a holding force of a tool. As means therefor, in a collet chuck including a chuck body having a taper hole in an inwardly converging shape, a diameter contractable collet having a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, there is provided a force increasing collet with a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein in a state of fastening the fastening nut to the chuck body, a front side of the first contact face of the force increasing collet is not brought into contact with an outer peripheral face of the collet in a state of pressing the collet frontwardly.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a collet chuck for attaching a tool of a drill, an end mill or the like to a machine tool spindle. 
         [0002]    In a background art, there is known a collet chuck for holding a tool of a drill, an end mill or the like to attach to a machine tool spindle, and in such a collet chuck, various proposals have been made to enable to carry out highly accurate machining by firmly holding the tool. 
         [0003]    For example, a collet chuck  100  shown in  FIG. 7  comprises a chuck body  104  having an inwardly converging taper hole  102  at a front end portion thereof, a diameter contractable collet  108  having a first taper face  106  to be fitted to the taper hole  102 , and a fastening nut  110  removably connected to the collet  108  and screwed to the chuck body  104 . 
         [0004]    A circular groove  112  is formed at an outer peripheral face of a front end portion of the collet  108 , a second taper face  114  inclined inverse to the first taper face  106  is formed at a peripheral side wall of the circular groove  112  contiguous to the first taper face  106 , further, an inner peripheral face of the fastening nut  110  is formed with a taper face  116  (refer to, for example, Patent Reference 1). 
         [0005]    Although in such a collet chuck  100 , a diameter of the collet  108  is effectively contracted by operating a press force F 2  inwardly in a diameter direction in addition to operating a press force F 1  in an axial direction L from the fastening nut  110  to the collet  108  by bringing the taper face  116  of the fastening nut  110  into press contact with the second taper face  114  of the collet  108  by fastening the fastening nut  110 , it is desired to further reinforce a tool holding force in accordance with further severe formation of a cutting condition. 
         [0006]    Meanwhile, as shown in  FIG. 8 , as for a fastening piece  120  for pressing to fasten a fastened object  130  of a tool or the like to a flange portion  129  of a fastened member  122  in an axial direction L by way of a fastening flange  126  by fastening a fastening nut  124  screwed with a male screw  122   a  at a shaft end of the fastened member  122 , there is known to fasten strongly pressing the fastened object  130  in an axial direction L by a wedge action (refer to, for example, Patent Reference 2). 
         [0007]    The fastening piece  120  includes a collet  128  elongatably and contractably supported in a diameter direction by guide faces  124   a  and  126   a  of the fastening nut  124  and the fastening flange  126  having a section in a V-like shape opposed to each other, and by moving the fastening flange  126  to a side of the fastened object  130  by compressing an outer diameter of the collet  128  by a force from a taper ring  132  moved to the axial direction L, the collet  128  is made to function as a wedge to thereby enhancedly fasten the fastened object  130  in the axial direction L. 
         [0008]    When such a collet  128  is applied to a front end portion of the collet  106  of the collet chuck  100  described in Patent Reference 1, since the collet  128  functions as the wedge for widening an interval of the guide faces having a section in the V-like shape, and therefore, there poses a problem that the tool holding force cannot efficiently be reinforced. 
         [0009]    That is, the collet  128  exerted with the force in the axial direction L presses the collet  108  holding the tool from side faces  128   a  and  128   b  thereof. Then, the collet  108  is operated with a force for pressing rearwardly in the axial direction from the side face  128   a  and also operated with a force of pressing forwardly in the axial direction from the side face  128   b , the forces in the two directions are cancelled by each other and a large press force is not exerted rearwardly in the axial direction and it is difficult to efficiently reinforce the tool holding force. 
         [0010]    Patent Reference 1: JP-A-2001-219310 
         [0011]    Patent Reference 2: JP-A-10-309612 
       SUMMARY OF THE INVENTION 
       [0012]    The invention has been carried out in view of the above-described problem and it is an object thereof to provide a constituent member of a collet chuck, a collet and a fastening nut as well as a collet chuck capable of reinforcing a force of holding a tool. 
         [0013]    A first aspect of the invention is a constituent member of a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the constituent member comprising: a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein in a state of fastening the fastening nut to the chuck body, a front side of the first contact face is not brought into contact with an outer peripheral face of the collet in a state of pressing the collet forwardly. 
         [0014]    A second aspect of the invention is a collet of a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the collet comprising: a force increasing collet with a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein in a state of fastening the fastening nut to the chuck body, a front side of the first contact face of the force increasing collet is not brought into contact with an outer peripheral face of the collet in a state of pressing the collet frontwardly. 
         [0015]    A third aspect of the invention is a fastening nut of a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the fastening nut comprising: a force increasing collet having a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein in a state of fastening the fastening nut to the chuck body, a front side of the first contact face of the force increasing collet is not brought into contact with an outer peripheral face of the collet in a state of pressing the collet frontwardly. 
         [0016]    A fourth aspect of the invention is a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the collet comprising: a force increasing collet having a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein in a state of fastening the fastening nut to the chuck body, a front side of the first contact face of the force increasing collet is not brought into contact with an outer peripheral face of the collet in a state of pressing the collet frontwardly. 
         [0017]    According to the first through fourth aspects of the invention, the constituent member (force increasing collet) of a collet chuck including the first contact face brought into contact with the second taper face in the frontwardly converging shape formed at the outer peripheral face of the front end portion of the collet, and the second contact face brought into contact with the third taper face in the frontwardly converging shape formed at the inner peripheral face of the front end portion of the fastening nut is arranged at the collet chuck, and therefore, a press force in an axial direction generated by fastening the fastening nut  60  can be amplified. 
         [0018]    Further, in the state of fastening the fastening nut to the chuck body, the front side of the first contact face of the constituent member (force increasing collet) is not brought into contact with the outer peripheral face of the collet in the state of pressing the collet forwardly, and therefore, the amplified force can be exerted to the collet without being reduced by being cancelled by each other in the axial direction and the tool holding force can efficiently be reinforced. 
         [0019]    A fifth aspect of the invention is a constituent member of a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the constituent member comprising: a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein the constituent member further includes a third contact face brought into contact with a fourth taper face formed on a front side of the second taper face of the collet and inclined to increase an outer diameter as proceeding to a front end thereof in a state of fastening the fastening nut to the chuck body, wherein an angle of the third contact face relative to a diameter direction of the collet chuck is larger than an angle of the first contact face relative to the diameter direction of the collet chuck. 
         [0020]    A sixth aspect of the invention is a collet of a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the collet comprising: a force increasing collet having a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein the force increasing collet further includes a third contact face brought into contact with a fourth taper face formed on a front side of the second taper face of the collet and inclined to increase an outer diameter as proceeding to a front end thereof in a state of fastening the fastening nut to the chuck body, and wherein an angle of the third contact face relative to a diameter direction of the collet chuck is larger than an angle of the first contact face relative to the diameter direction of the collet chuck. 
         [0021]    A seventh aspect of the invention is a fastening nut of a collet chuck including a chuck body with a taper hole in an inwardly converging shape, a diameter contractable collet with a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the fastening nut comprising: a force increasing collet with a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein the force increasing collet further includes a third contact face brought into contact with a fourth taper face formed on a front side of the second taper face of the collet and inclined to increase an outer diameter as proceeding to a front end thereof in a state of fastening the fastening nut to the chuck body, and wherein an angle of the third contact face relative to a diameter direction of the collet chuck is larger than an angle of the first contact face relative to the diameter direction of the collet chuck. 
         [0022]    An eighth aspect of the invention is a collet chuck including a chuck body having a taper hole in an inwardly converging shape, a diameter contractable collet having a first taper face fitted to the taper hole, and a fastening nut screwed to the chuck body, the collet chuck comprising a force increasing collet having a first contact face and a second contact face respectively brought into contact with a second taper face formed at an outer peripheral face of a front end portion of the collet and inclined to reduce an outer diameter as proceeding to a front end thereof, and a third taper face formed at an inner peripheral face of a front end portion of the fastening nut and inclined to reduce an inner diameter as proceeding to a front end thereof, wherein the force increasing collet further includes a third contact face brought into contact with a fourth taper face formed on a front side of the second taper face of the collet and inclined to increase an outer diameter as proceeding to a front end thereof in a state of fastening the fastening nut to the chuck body, and wherein an angle of the third contact face relative to a diameter direction of the collet chuck is larger than an angle of the first contact face relative to the diameter direction of the collet chuck. 
         [0023]    In the fifth through eighth aspects of the invention, the collet chuck is provided with the constituent member (force increasing collet) of the collet chuck including the first contact face brought into contact with the second taper face of the frontwardly converging shape formed at the outer peripheral face of the front end portion of the collet, and the second contact face brought into contact with the third taper face in the frontwardly converging shape formed at the inner peripheral face of the front end portion of the fastening nut, and therefore, the press force in the axial direction generated by fastening the fastening nut can be amplified. 
         [0024]    Further, although the above-described constituent member (force increasing collet) includes the third contact face brought into contact with the fourth taper face in the inwardly converging shape formed on the front side of the second taper shape of the collet in the state of fastening the fastening nut to the chuck body, and the collet is pressed to the front side of the third contact face, since the angle of the third contact face relative to the diameter direction of the collet chuck is set to be larger than the angle of the first contact face relative to the diameter direction of the collet chuck, while restraining a press force to a rear side in the axial direction exerted to the second taper face from being reduced by setting a component of a force to the front side in the axial direction to be small, by setting a component force inwardly in the diameter direction of the collet chuck to be large, the press force in a direction of holding the tool can be exerted efficiently to the front end portion of the collet, and the tool holding force can be reinforced. 
         [0025]    Further, in the first through eighth aspects of the invention, the angle of the second contact face relative to the diameter direction of the collet chuck may be provided to be larger than the angle of the first contact face relative to the diameter direction of the collet chuck, thereby, the holding force of the tool can further effectively be reinforced. 
         [0026]    According to the invention, the force increasing collet for amplifying the press force from the fastening nut is arranged between the collet and the fastening nut, and therefore, the tool holding force of the collet chuck can further be reinforced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a sectional view of a chuck collet according to a first embodiment of the invention. 
           [0028]      FIG. 2  is a sectional view enlarging an essential portion of the chuck collet according to the first embodiment of the invention. 
           [0029]      FIG. 3  is a perspective view showing a force increasing collet. 
           [0030]      FIG. 4  is a sectional view enlarging an essential portion of the chuck collet according to the first embodiment of the invention. 
           [0031]      FIG. 5  is a sectional view of a chuck collet according to a second embodiment of the invention. 
           [0032]      FIG. 6  is a sectional view enlarging an essential portion of the chuck collet according to the second embodiment of the invention. 
           [0033]      FIG. 7  is a sectional view showing a chuck collet of a background art. 
           [0034]      FIG. 8  is a sectional view showing a fastening piece of a background art. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
       [0035]    A first embodiment of the invention will be explained in reference to  FIG. 1  through  FIG. 4  as follows. 
         [0036]      FIG. 1  is a sectional view of a collet chuck  10  according to the embodiment,  FIG. 2  and  FIG. 3  are sectional views enlarging an essential portion of the collet chuck  10 , and  FIG. 4  is a perspective view of a force increasing collet  80 . 
         [0037]    As shown by  FIG. 1  and  FIG. 2 , the collet chuck  10  according to the embodiment includes a chuck body  20  having a taper hole  22  at a front end portion thereof, a collet  40  fitted to the taper hole  22 , a fastening nut  60  mounted to the chuck body  20 , and the force increasing collet  80  arranged between the collet  40  and the fastening nut  60 . 
         [0038]    The chuck body  20  includes the taper hole  22  in an inwardly converging shape converging to a rear end side thereof at a front end side thereof inserted with a tool  1 , and formed with a shunk portion  24  attachably and detachably mounted to a main spindle (not illustrated) of a machine tool on a rear end side thereof. Further, an outer peripheral face of the front end portion of the chuck body  20  is formed with a male screw portion  26  to be screwed with a female screw portion  62  of the fastening nut  60 . 
         [0039]    The collet  40  constitutes substantially a shape of a cylinder having a tool inserting hole  42  to be inserted with the tool  1 , and a diameter thereof is made to be able to be contracted by split grooves, not illustrated, cut to be formed alternately from a front end side and a rear end side. An outer peripheral face of a rear end portion of the collet  40  is formed with a first taper face  44  fitted to the taper hole  22  of the chuck body  20 . Further, at an outer peripheral face of the front end portion, a second taper face  46  inclined such that the more proceeding to the front end, the more reduced the outer diameter of the collet  40  is formed contiguous to the first taper face  44 , and a front end side of the second taper face  46  is formed with a circular projected portion  48 . According to the circular projected portion  48 , as shown by  FIG. 3 , a front end side peripheral side wall  48   a  thereof constitutes a frontwardly converging taper shape reducing an outer diameter thereof as proceeding to a front end thereof, and a rear end side peripheral side wall  48   b  constitutes a rearwardly converging taper shape of reducing an outer diameter as proceeding to a rear end thereof. 
         [0040]    The fastening nut  60  includes a nut body  64  formed with the female screw portion  62  to be screwed with the male screw portion  26  of the chuck body  20  at an inner peripheral face of a rear end portion thereof, and a ring  66  rotatably mounted to an inner peripheral face of a front end portion of the nut body  64 , circular grooves having a section in a semicircular shape are formed at respectively corresponding positions of the inner peripheral face of the nut body  64  and the outer peripheral face of the ring  66 , and by closely integrating a ball  68  between the two circular grooves, the nut body  64  and the ring  66  are connected pivotably with each other and integrally in an axial direction L. An inner peripheral face of a front end portion of the fastening nut  60 , that is, the inner peripheral face of the ring  66  is formed with a third taper face  70  inclined such that the more proceeding to the front end, the more reduced the inner diameter. 
         [0041]    The third taper face  70  is provided such that an angle θ 2  relative to a diameter direction of the collet chuck  10  (incidentally, 0°&lt;θ2&lt;90°) is larger than an angle θ 1  of the second taper face  46  relative to the diameter direction of the collet chuck  10  (incidentally, θ 0 &lt;θ 1 &lt;90°), and according to the embodiment, for example, the angle θ 1  is provided in a range of 30° through 40°, and the angle θ 2  is provided in a range of 60° through 80°. 
         [0042]    As shown by  FIG. 4 , the force increasing collet  80  is a member in a circular shape an outer shape of which constitutes a shape of a cone a front end of which is cut off, and a diameter thereof is made to be able to be contracted by split grooves  88  cut to be formed alternately from a front end side and a rear end side thereof. The force increasing collet  80  is removably connected to the collet  40  and is arranged between the collet  40  and the fastening nut  60  at the front end portion of the collet chuck  10 . 
         [0043]    In details, as shown by  FIG. 2  and  FIG. 3 , an inner peripheral face of the force increasing collet  80  is formed with a first contact face  82  in which an angle thereof relative to the diameter direction of the collet chuck  10  is equal to that of the second taper face  46  on a rear end side and which can be brought into face contact with the second taper face  46 , and formed with a circular recessed portion  84  with which the circular projected portion  48  of the collet  40  is engaged to be spaced apart therefrom by a clearance on a front end side of the first contact face  82 . 
         [0044]    The circular recessed portion  84  constitutes a frontwardly converging taper shape at a front end side peripheral side wall  84   a  and constitutes a rearwardly converging taper shape at a rear end side peripheral side wall  84   b  in correspondence with the circular projected portion  48  of the collet  40 . 
         [0045]    On the other hand, an outer peripheral face of the force increasing collet  80  is formed with a second contact face  86  in which an angle relative to the diameter direction of the collet chuck  10  is equal to that of the third taper face  70  of the ring  66  and which can be brought into face contact with the third taper face  70 . 
         [0046]    According to the collet chuck  10  of the embodiment, by fastening the fastening nut  60  to the chuck body  20 , the third taper face  70  is brought into press contact with the second contact face  86  of the force increasing collet  80 , and the force increasing collet  80  receiving a press force from the fastening nut  60  is brought into press contact with the second taper face  46  of the collet  40  at the first contact face  82 . 
         [0047]    There is provided a rigidity capable of maintaining the clearance formed between the circular recessed portion  84  provided on a front side of the first contact face  82  and the circular projected portion  48  provided at the collet  40  even in a state of elastically deforming the force increasing collet by an operation of the split grooves  88  by fastening the fastening nut  60  in this way. 
         [0048]    That is, the rear end side peripheral side wall  48   b  constituting the rearwardly converging taper shape of the circular projected portion  48  and the force increasing collet are not brought into contact with each other, and a front side of the first contact face  82  of the force increasing collet is not brought into contact with the outer peripheral face of the collet  40  in a state of pressing the collet  40  frontwardly. 
         [0049]    Further, the circular recessed portion  84  of the force increasing collet  80  constitutes the frontwardly converging taper shape at the front end side peripheral side wall  84   a  and constitutes the rearwardly converging taper shape at the rear end side peripheral side wall  84   b  in correspondence with the circular projected portion  48  of the collet  40 , and therefore, attachment and detachment of the force increasing collet  80  to and from the collet  40  are facilitated. 
         [0050]    In order to fix the tool  1  to the collet chuck  10  of the embodiment, first, the force increasing collet  80  is fitted to the fastening nut  60  by bringing the second contact face  86  of the force increasing collet  80  into contact with the third taper face  70  of the fastening nut  60 . Successively, the front end portion of the collet  40  is fitted to the force increasing collet  80  attached to the fastening nut  60 , and the tool  1  is inserted into the tool inserting hole  42  of the collet  40 . Successively, the first taper face  44  of the collet  40  is fitted to the taper hole  24  of the chuck body  20 , the female screw portion  62  of the fastening nut  60  is screwed to the male screw portion  26  of the chuck body  20  from the state, and the tool  1  is fixed to the collet chuck  10  by pivoting the nut body  64  of the fastening nut  60  in a fastening direction. 
         [0051]    By the fastening, the third taper face  70  formed at the ring  66  of the fastening nut  60  is brought into press contact with the second contact face  86  of the force increasing collet  80 , as shown in  FIG. 2 , the force increasing collet  80  is operated with a press force α 1  in the axial direction L from the fastening nut  60  and is operated with a press force α 2  inwardly in the diameter direction. Here, a relationship of equation (1) described below is established among the angle θ 2  of the second contact face  86  relative to the diameter direction of the collet chuck  10  and the press forces α 1  and α 2 . 
         [0000]      α2=α1×tan θ2  (1) 
         [0052]    Further, the first contact face  82  of the force increasing collet  80  receiving the press force from the fastening nut  60  is brought into press contact with the second taper face  46  of the collet  40 , and the collet  40  is operated with a press force β 1  in the axial direction L and operated with a press force β 2  inwardly in the diameter direction from the force increasing collet  80 . Here, a relationship of equation (2) described below is established among the angle θ 1  of the first contact face  82  relative to the diameter direction of the collet chuck  10  and the press forces β 1  and β 2 . Further, the press force β 2  inwardly in the diameter direction operated to the collet  40  is equal to the press force α 2  inwardly in the diameter direction operated to the force increasing collet  80 , and therefore, equation (2) described below becomes equation (3) described below. 
         [0000]      β1=β2/tan θ1  (2) 
         [0000]      β1=α1×tan θ2/tan θ1  (3) 
         [0053]    According to the collet chuck  10  of the embodiment, in the state of fastening the fastening nut  60 , the front side of the first contact face  82  of the force increasing collet  80  is not brought into contact with the outer peripheral face of the collet  40 , the force increasing collet  80  does not press the collet  40  to the front side, and therefore, a synthesized force F in the axial direction L operated to the collet  40  becomes equation (4) described below. 
         [0000]        F=α 1+α1×tan θ2/tan θ1  (4) 
         [0054]    As described above, according to the collet chuck  10  of the embodiment, the press force F larger than the press force α 1  in the axial direction L generated by fastening the fastening nut  60  is operated to the collet  40  without being reduced by being cancelled by each other in the axial direction, and the force of holding the tool  1  can be reinforced. Particularly, as in the embodiment, tan θ 2 /tan θ 1 &gt;1 by providing the angle θ 2  of the second contact face  86  relative to the diameter direction of the collet chuck  10  larger than the angle θ 1  of the first contact face  82 , and the press force F in the axial direction L can further effectively be reinforced. 
         [0055]    Further, although according to the embodiment, the second taper face  46  is formed at the outer peripheral face of the front end portion of the collet  40  contiguous to the first taper face  44 , the invention is not limited thereto but, for example, there may be constructed a constitution in which the peripheral side wall  48   a  on the front end side of the circular projected portion  48  is brought into face contact with the first contact face of the force increasing collet  80  as the second taper face. 
       Example 
       [0056]    Although a specific example and effect of the invention will be shown by an example and a comparative example with regard to the above-described first embodiment, the invention according to the embodiment is not limited to the example. 
         [0057]    Collet chucks of examples 1 and 2 are the collet chucks  10  each arranged with the force increasing collet  80  between the collet  40  and the fastening nut  60  as shown by  FIG. 1  and  FIG. 2 , and the angles θ 1  and θ 2  of the first contact face  82  and the second contact face  86  relative to the diameter direction are set as θ 1 =30°, θ2=60° in example 1 and θ 1 =40 °, θ2=80° in example 2. 
         [0058]    As shown by  FIG. 7 , a collet chuck of comparative example includes the chuck body  104  having the taper hole  102  in the inwardly converging shape at the front end portion, the diameter contractable collet  108  having the first taper face  106  fitted to the taper hole  102 , and the fastening nut  110  removably connected to the collet  108  and screwed to the chuck body  104 , the outer peripheral face of the front end portion of the collet  108  is formed with the circular groove  112 , the peripheral side wall of the circular groove  112  contiguous to the first taper face  106  is formed with the second taper face  114  inclined inverse to the first taper face, the inner peripheral face of the fastening nut  110  is formed with the taper face  116 , and the angle θ 1  of the second taper face  114  relative to the diameter direction of the collet chuck  100  is set to 30°. 
         [0059]    A holding force test of the tool is carried out for the collet chucks of examples 1 and 2 and comparative example. In the holding force test, a tool holding force generated when the fastening nut is fastened by respective nut fastening torques for respective 1.0 Kgf/m from 3.0 to 6.0 Kgf/m is measured and an average value of a result of 4 times measurement is shown in Table 1. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Angle θ1 
                 Nut fastening torque (Kgf/m) 
                   
               
             
          
           
               
                   
                 Angle θ2 
                 3.0 
                 4.0 
                 5.0 
                 6.0 
               
               
                   
                   
               
             
          
           
               
                 Example 1 
                 θ1 = 30° 
                 5.8 
                 6.6 
                 9.4 
                 11.9 
               
               
                   
                 θ2 = 60° 
               
               
                 Example 2 
                 θ1 = 40° 
                 6.5 
                 8.1 
                 10.4 
                 12.3 
               
               
                   
                 θ2 = 80° 
               
               
                 Comparative 
                 θ1 = 30° 
                 3.7 
                 5.7 
                 6.2 
                 8.0 
               
               
                 example 
               
               
                   
               
             
          
         
       
     
         [0060]    As is apparent from Table 1, according to the collet chucks of examples 1 and 2, the holding force is 1.2 times through 1.8 times as much as that of the collet chuck of comparative example and the tool holding force can be reinforced. 
       Second Embodiment 
       [0061]    Next, a second embodiment of the invention will be explained in reference to  FIG. 5  and  FIG. 6 . Elements the same as or corresponding to those of the above-described first embodiment are attached with the same notations and a duplicated explanation thereof will be omitted. 
         [0062]    A point of difference between the embodiment and the above-described first embodiment resides in that in a state of fastening the fastening nut  60  to the chuck body  20 , the front side of the first contact face  82  of the force increasing collet  80  is brought into contact with the collet  40 . 
         [0063]    In details, as shown by  FIG. 5  and  FIG. 6 , the inner peripheral face of the force increasing collet  80  is formed with the first contact face  82  in which the angle relative to the diameter direction of the collet chuck  10  is equal to that of the second taper face  46  on the rear end side and which can be brought into face contact with the second taper face  46 , and formed with the circular recessed portion  84  to be engaged with the circular projected portion  48  of the collet  40  on the front end side of the first contact face  82 . 
         [0064]    The circular recessed portion  84  is provided such that the front end side peripheral side wall  84   a  constitutes the frontwardly converging taper shape, the rear end side peripheral side wall  84   b  constitutes the rearwardly converging taper shape in correspondence with the circular projected portion  48  of the collet  40 , and the rear end side peripheral side wall  84   b  of the circular recessed portion  84  is made to be able to be brought into face contact with the rear end side peripheral side wall  48   b  of the circular projected portion  48 . That is, according to the embodiment, the rear end side peripheral side wall  84   b  of the circular recessed portion  84  constitutes the third contact face  84   b , and the rear end side peripheral side wall  48   b  of the circular projected portion  48  constitutes the fourth taper face  48   b . According to the rear end side peripheral side wall (third contact face)  84   b  of the circular recessed portion  84 , an angle θ 3  relative to the diameter direction of the collet chuck  10  (incidentally, 0°θ3&lt;90°) is provided to be larger than the angle θ 1  of the first contact face  82  relative to the diameter direction of the collet chuck  10 , and according to the embodiment, for example, the angle θ 3  is provided in a range of 60° through 80°. 
         [0065]    According to the collet chuck  10  of the embodiment, by fastening the fastening nut  60  to the chuck body  20 , the third taper face  70  is brought into press contact with the second contact face  86  of the force increasing collet  80 . By contracting the diameter by elastically deforming the force increasing collet  80  receiving the press force from the fastening nut  60 , the first contact face  82  is brought into press contact with the second taper face  46  of the collet  40 , and the third contact face  84   b  is brought into press contact with the fourth taper face  48   b  of the collet  40 . Further, before fastening the fastening nut  60 , the third contact face  84   b  may not be brought into contact with the fourth taper face  48   b  of the collet  40 . 
         [0066]    Thereby, the collet  40  is operated with the press force β 1  frontwardly in the axial direction L and the press force θ 2  inwardly in the diameter direction from the first contact face  82  of the force increasing collet  80 , and is operated with a press force γ 1  rearwardly in the axial direction L and a press force γ 2  inwardly in the diameter direction from the third contact face  84   b  of the force increasing collet  80 . 
         [0067]    Here, a relationship of equation (5) described below is established among the angle θ 3  of the third contact face  84   b  relative to the diameter direction of the collet chuck  10  and the press forces γ 1  and γ 2 . 
         [0000]      γ 1 =γ 2 /tan θ3  (5) 
         [0068]    Although according to the collet chuck  10  of the embodiment, in the state of fastening the fastening nut  60 , the third contact face  84   b  of the force increasing collet  80  is brought into contact with the fourth taper face  48   b  of the collet  40 , and the force increasing collet  80  presses the collet  40  forwardly, since the angle θ 3  of the third contact face  84   b  relative to the diameter direction of the collet chuck  10  is set to be larger than the angle θ 1  of the first contact face  82  relative to the diameter direction of the collet chuck  10 , a component of force γ 2  inwardly in the diameter direction of the collet chuck  10  can be set to be large while restraining the press force β 1  rearwardly in the axial direction L exerted to the second taper face  46  from being reduced by setting a component of force γ 1  frontwardly in the axial direction L to be small. 
         [0069]    The large force γ 2  can be exerted inwardly in the diameter direction of the collet chuck  10  to the fourth taper face  48  provided at the front end portion of the collet  40  in this way, and therefore, the diameter of the collet  40  is easy to be contracted, and the tool holding force can efficiently be reinforced.