Abstract:
A chuck includes a holder body, a clamp sleeve, and a large number of needle rollers. The holder body has a shank portion, a flange, and a chuck sleeve. The clamp sleeve is fitted onto the outer circumference of the chuck sleeve such that the clamp sleeve is rotatable and axially movable relative to the chuck sleeve. The needle rollers are disposed between the chuck sleeve and the clamp sleeve over the entire circumference. The clamp sleeve is rotated to cause the needle rollers to revolve spirally along the outer circumferential surface of the chuck sleeve while rotating about their own axes in order to decrease and restore the diameter of the chuck sleeve. A thrust member is disposed between the end surface of the flange opposite the shank portion and a corresponding end surface of the clamp sleeve.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a chuck for chucking a cutting tool such as a drill or end mill, and more particularly to a chuck in which a clamp sleeve is fitted onto the outer circumference of a chuck sleeve of a holder body via needle rollers, and in which, through rotation of the clamp sleeve in a clamping direction, the diameter of the chuck sleeve is reduced in order to tightly chuck a desired tool. 
   2. Description of the Related Art 
   A conventional chuck will be described with reference to  FIG. 12 .  FIG. 12  is a cross-sectional view of a conventional tool chuck. 
   As shown in  FIG. 12 , the tool chuck has a holder body  1 . The tool holder body  1  has a tapered shank portion  1 A to be inserted into a spindle of an unillustrated machine tool, a gripping flange  1 B formed at the larger-diameter end of the tapered shank portion  1 A, and a chuck sleeve  1 C that extends from the end surface of the flange  1 B opposite the tapered shank portion  1 A. The chuck sleeve  1 C extends in the direction away from the tapered shank portion  1 A such that the axis of the chuck sleeve  1 C coincides with the axis of the tapered shank portion  1 A. A tapered surface  1 Ca is formed on the outer circumference of the chuck sleeve  1 C such that the diameter of the surface  1 Ca decreases toward the tip end thereof. 
   Reference numeral  2  denotes a roller-retaining sleeve which is fitted onto the outer circumference of the chuck sleeve  1 C with a clearance therebetween. The diameter of the roller-retaining sleeve  2  decreases toward the tip end thereof substantially at the same rate as that of the tapered surface  1 Ca of the chuck sleeve  1 C. The roller-retaining sleeve  2  is held by a retainer ring  3  provided on the outer circumference of the tip end portion of the chuck sleeve  1 C such that the roller-retaining sleeve  2  is prevented from coming off the chuck sleeve  1 C. 
   In the roller-retaining sleeve  2 , numerous needle rollers  4  are disposed to form a plurality of rows. The needle rollers are fitted into the roller-retaining sleeve  2  in such a manner that the needle rollers incline in the circumferential direction at a predetermined angle with respect to the center axis of the roller-retaining sleeve  2 . Each of the needle rollers  4  has a diameter greater than the wall thickness of the roller-retaining sleeve  2 , so that the portion of each needle roller  4  projecting inward from the inner circumferential surface of the roller retaining sleeve  2  is in contact with the tapered surface  1 Ca of the chuck sleeve  1 C, while the portion of each needle roller  4  projecting outward from the outer circumferential surface of the roller retaining-sleeve  2  is in contact with the inner circumferential surface of a clamp sleeve  5 , which will be described below. 
   The clamp sleeve  5  is adapted to reduce the diameter of the chuck sleeve  1 C to thereby tightly chuck a desired tool. The clamp sleeve  5  is fitted onto the outer circumference of the chuck sleeve  1 C via the needle rollers  4  held by the roller-retaining sleeve  2 . The clamp sleeve  5  has a tapered inner circumferential surface whose diameter decreases from the end facing the flange  1 B toward the tip end thereof. Further, a sealing/retaining ring  6  is attached to the inner circumference of the bottom end portion of the clamp sleeve  5  adjacent to the flange  1 B. The sealing/retaining ring  6  is in contact with the outer circumferential surface of the chuck sleeve  1 C so as to provide a sealing function. The sealing/retaining ring  6  also abuts the end surface of the roller retaining sleeve  2  in order to prevent the clamp sleeve  5  from coming off the chuck sleeve  1 C. 
   In the conventional chuck having the above-described structure, when a tool  7  is to be chucked, a spring collet  8  is fitted onto the shank  7 A of the tool  7 , and the shank  7 A of the tool  7 , together with the spring collet  8 , is inserted into the cylindrical bore of the chuck sleeve  1 C. When the clamp sleeve  5  is rotated clockwise, each needle roller  4  in contact with the tapered inner surface of the clamp sleeve  5  revolves spirally along the outer surface of the chuck sleeve  1 C while rotating about its own axis. Thus, the roller-retaining sleeve  2  moves toward the flange  1 B, while rotating, in accordance with the revolution of the needle rollers  4 , and at the same time the clamp sleeve  5  also moves toward the flange  1 B. The movements of the roller-retaining sleeve  2  and the clamp sleeve  5  toward the flange  1 B reduce the clearance between the tapered inner surface of the clamp sleeve  5  and the tapered outer surface of the chuck sleeve  1 C, so that the clamp sleeve  5  strongly presses the chuck sleeve  1 C from the entire outer circumference thereof via the needle rollers  4  in order to reduce the diameter of the chuck sleeve  1 C. As a result, the shank  7 A of the tool  7  inserted into the cylindrical bore of the chuck sleeve  1 C is clamped and held by the chuck sleeve  1 C via the spring collet  8 . When the clamp sleeve  5  in a clamped state is rotated counterclockwise, the roller-retaining sleeve  2  moves toward the tip end of the chuck sleeve  1 C, and the clamp sleeve  5  also moves in the same direction. Therefore, the diameter of the chuck sleeve  1 C is restored to its original size, so that the clamping force applied to the tool  7  is released. This enables the tool  7  to be removed from the chuck sleeve  1 C or to be replaced with a different tool. 
   In such a conventional chuck, the chuck sleeve  1 C extends from the end surface of the flange  1 B opposite the tapered shank portion  1 A. Therefore, within an area of the tapered surface  1 Ca adjacent to the base portion of the chuck sleeve  1 C continuous with the flange  1 B, the rigidity against elastic deformation in the radial direction is considerably higher than that in the remaining area. Therefore, when the clamp sleeve  5  is rotated in the clamping direction so as to reduce the diameter of the chuck sleeve  1 C, within the above-described area of the tapered surface  1 Ca the ratio of reduction in diameter is very small as compared to that in the remaining area. 
   In order to solve the above-described drawback, one of the present inventors has proposed an improved chuck as shown in U.S. Pat. No. 6,105,974. In the improved chuck, a groove having a predetermined depth is formed in the end surface of the flange opposite the shank portion such that the groove extends along the outer circumference of the bottom end of the chuck sleeve through which the chuck sleeve is joined with the flange. Since the groove increases the effective chucking length of the chuck sleeve; i.e., the length of a portion of the chuck sleeve in which the chuck sleeve can elastically deform in a diameter-reducing direction, elastic deformation of the chuck sleeve in the diameter-reducing direction easily occurs over substantially the entire length of the chuck sleeve. Therefore, the effective chucking length of the chuck sleeve can be increased with no accompanying increase in the overall length of the chuck sleeve. 
   In the conventional chuck, the change ratio of the inner diameter of the chuck sleeve can be increased to some extent when the clamp sleeve  5  disposed to surround the outer circumferential surface of the chuck sleeve  1 C is tightened to a degree such that the end surface of the clamp sleeve  5  abuts the end surface of the flange  1 B opposite the tapered shank portion  1 A. However, if the tightening operation is continued even after the end surface of the clamp sleeve  5  has abutted the end surface of the flange  1 B opposite the tapered shank portion  1 A, the tightening force becomes excessive, whereby the chuck sleeve  1 C deforms because of a torsion phenomenon. This causes a problem in that the torsional deformation of the chuck sleeve  1 C causes deflective vibration in the tool holder  1  and a cutting tool fitted into the chuck sleeve when the tool holder  1  is rotated, making accurate cutting operation impossible. 
   SUMMARY OF THE INVENTION 
   The present invention has been accomplished to solve the above-mentioned problems, and an object of the invention is to provide a chuck which can prevent torsional deformation of a chuck sleeve, which would otherwise occur because of excessive tightening of a clamp sleeve against the chuck sleeve, to thereby eliminate adverse effects of the torsional deformation on the chuck sleeve, which in turn prevents deflective vibration in the chuck sleeve, which is a portion of a tool holder, to thereby improve machining accuracy. 
   The present invention provides a chuck comprising: a holder body having a shank portion for insertion into a spindle of a machine tool, a gripping flange formed at one end of the shank portion, and a chuck sleeve that extends from the flange in a direction away from the shank portion in alignment with an axis of the shank portion, the chuck sleeve having a tapered outer circumferential surface whose diameter decreases from a bottom end adjacent to the flange to a tip end of the chuck sleeve; a clamp sleeve fitted onto the outer circumference surface of the chuck sleeve such that the clamp sleeve is rotatable and axially movable relative to the chuck sleeve, the clamp sleeve having a tapered inner circumferential surface whose diameter decreases from a bottom end adjacent to the flange toward a tip end of the clamp sleeve; needle rollers disposed between the outer circumferential surface of the chuck sleeve and the inner circumferential surface of the clamp sleeve over the entire circumference, the needle rollers inclining in the circumferential direction at a predetermined angle with respect to the center axes of the chuck sleeve and the clamp sleeve, and the clamp sleeve being rotated to cause the needle rollers to revolve spirally along the outer circumferential surface of the chuck sleeve while rotating about their own axes in order to decrease and restore the diameter of the chuck sleeve; and a thrust member disposed between the end surface of the flange opposite the shank portion and a corresponding end surface of the clamp sleeve. 
   Preferably, a groove having a predetermined depth and a ring-like shape is formed in the end surface of the flange opposite the shank portion such that the groove extends along the outer circumference of the bottom end of the chuck sleeve through which the chuck sleeve is joined with the flange. 
   Preferably, a radially outer portion of the corresponding end surface of the clamp sleeve is raised so as to abut a radially outer portion of the thrust member. 
   Preferably, the thrust member is held by means of a thrust-member holder of an annular flat shape, the thrust-member holder having at a central portion an opening for allowing passage of the chuck sleeve therethrough, and an end of the thrust-member holder being accommodated in an annular groove formed in the end surface of the flange. 
   Preferably, the thrust-member holder includes an elastic member which is provided on the end of the thrust-member holder accommodated in the groove and engages with a wall surface of the groove so as to restrict movement of the thrust-member holder in the axial direction. 
   Preferably, at least one of opposite surfaces of the thrust member is surface treated so as to reduce friction resistance produced when the end surface of the clamp sleeve comes into contact with the thrust member. 
   In the chuck according to the present invention, the thrust member can prevent deformation, such as torsional deformation, of the chuck sleeve, which would otherwise occur because of excessive tightening of the clamp sleeve against the chuck sleeve, to thereby prevent adverse effects of the torsional deformation on the chuck sleeve, which in turn prevents deflective vibration in the chuck sleeve, which is a portion of a tool holder, to thereby improve machining accuracy. 
   Moreover, in the case where a groove having a predetermined depth is formed in the end surface of the flange opposite the shank portion such that the groove extends along the outer circumference of the bottom end of the chuck sleeve through which the chuck sleeve is joined with the flange, the chuck sleeve can be extended toward the flange side with no accompanying increase in the overall length of the holder body. The radially inward elastic deformation of the chuck sleeve in a bottom end area adjacent to the flange can be made substantially equal to that in the remaining area. In addition, the effective chucking length of the chuck sleeve over which the chuck sleeve can elastically deform in the diameter-reducing direction can be increased without an increase in the overall length of the holder body. As a result, elastic deformation of the chuck sleeve in the diameter-reducing direction occurs more easily, thereby allowing an operator to tighten the clamp sleeve with ease until the end surface of the clamp sleeve abuts the flange of the holder body, without need of large tightening force. 
   Moreover, when the clamp sleeve is tightened excessively, the end surface of the clamp sleeve slides because of presence of the thrust member, whereby torsional deformation of the clamp sleeve is reduced, and thus no torsional force is transmitted to the chuck sleeve. Thus, vibration, such as deflective vibration or chatter vibration, does not occur during machining by use of a tool, whereby cutting operation can be performed under favorable conditions, and machining accuracy can be improved. 
   Moreover, in the case where at least one of the opposite surfaces of the thrust member is surface treated so as to reduce friction resistance, even when the end surface of the clamp sleeve comes into contact with the thrust member because of excessive tightening of the clamp sleeve, only friction resistance of a reduced level is generated between the clamp sleeve and the thrust member, and thus sliding movement of the clamp sleeve is facilitated, whereby the function of the thrust member is improved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is a cross-sectional view of a chuck according to a first embodiment of the present invention in a state in which a clamp sleeve is loosened; 
       FIG. 2  is a cross-sectional view of the chuck according to the first embodiment in a state in which the clamp sleeve is tightened; 
       FIGS. 3A and 3B  show a thrust member used in the first embodiment, wherein  FIG. 3A  is a plan view of the thrust member, and  FIG. 3B  is a cross-sectional view taken along line  3 B— 3 B of  FIG. 3A ; 
       FIG. 4  is a cross-sectional view of a chuck according to a second embodiment of the present invention in a state in which a clamp sleeve is loosened; 
       FIG. 5  is a cross-sectional view of the chuck according to the second embodiment in a state in which the clamp sleeve is tightened; 
       FIG. 6  is a cross-sectional view of a main portion of a chuck according to a third embodiment of the present invention in a state in which a clamp sleeve is loosened; 
       FIG. 7  is a cross-sectional view of the main portion of the chuck according to the third embodiment in a state in which the clamp sleeve is tightened; 
       FIGS. 8A and 8B  show a thrust member used in the third embodiment, wherein  FIG. 8A  is a plan view of the thrust member, and  FIG. 8B  is a cross-sectional view taken along line  8 B— 8 B of  FIG. 8A ; 
       FIG. 9  is a cross-sectional view of a chuck according to a fourth embodiment of the present invention in a state in which a clamp sleeve is tightened; 
       FIG. 10  is a cross-sectional view of a main portion of the chuck according to the fourth embodiment in a state in which the clamp sleeve is tightened; 
       FIG. 11  is a partial, enlarged cross-sectional view of a thrust member and a thrust member holder used in the fourth embodiment; and 
       FIG. 12  is a cross-sectional view of a conventional chuck. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described with reference to the accompanying drawings. 
   First Embodiment: 
     FIG. 1  shows a cross-sectional view of a chuck according to a first embodiment of the present invention. 
   As shown in  FIG. 1 , the tool chuck has a holder body  10 , which has a tapered shank portion  11  to be inserted into a spindle of an unillustrated machine tool, a gripping flange  12  formed at the larger-diameter end of the tapered shank portion  11 , and a chuck sleeve  13  that extends from an end surface  12   a  of the flange  12  opposite the tapered shank portion  11 . The chuck sleeve  13  extends in the direction away from the tapered shank portion  11  such that the axis of the chuck sleeve  13  coincides with the axis of the tapered shank portion  11 . The chuck sleeve  13  has a tapered outer circumferential surface  13   a  whose diameter decreases from the bottom end adjacent to the flange  12  toward the tip end. 
   Reference numeral  14  denotes a roller-retaining sleeve fitted onto the outer circumference of the chuck sleeve  13  with a clearance therebetween. The diameter of the roller retaining sleeve  14  decreases toward the tip end thereof at the same rate as that of the tapered surface  13   a  of the chuck sleeve  13 . The roller-retaining sleeve  14  is held by a retainer ring  15  provided on the outer circumference of the tip end portion of the chuck sleeve  13  such that the roller-retaining sleeve  14  is prevented from coming off the chuck sleeve  13 . 
   In the roller-retaining sleeve  14 , numerous needle rollers  16  are arranged in the circumferential direction to form a plurality of rows in the longitudinal direction. The needle rollers  16  are fitted into the roller-retaining sleeve  14  in such a manner that the needle rollers  16  incline in the circumferential direction at a predetermined angle with respect to the center axis of the roller retaining sleeve  14 . Each of the needle rollers  16  has a diameter greater than the wall thickness of the roller-retaining sleeve  14 , so that the portion of each needle roller  16  projecting inward from the inner circumferential surface of the roller-retaining sleeve  14  is in contact with the tapered surface  13   a  of the chuck sleeve  13 , while the portion of each needle roller  16  projecting outward from the outer circumferential surface of the roller retaining sleeve  14  is in contact with the inner circumferential surface of a clamp sleeve  17 , which will be described below. 
   The clamp sleeve  17  is adapted to reduce the diameter of the chuck sleeve  13  to thereby tightly chuck a desired tool. The clamp sleeve  17  is fitted onto the outer circumference of the chuck sleeve  13  via the needle rollers  16  held by the roller-retaining sleeve  14 . The clamp sleeve  17  has a tapered inner circumferential surface whose diameter decreases from the end facing the flange  12  toward the tip end thereof. Further, a sealing/retaining ring  18  is attached to the inner circumference of the bottom end portion of the clamp sleeve  17  adjacent to the flange  12 . The sealing/retaining ring  18  is in contact with the outer circumferential surface of the chuck sleeve  13  so as to provide a sealing function. The sealing/retaining ring  18  also abuts the end surface of the roller-retaining sleeve  14  in order to prevent the clamp sleeve  17  from coming off the chuck sleeve  13 . 
   Reference numeral  19  denotes a thrust member, which assumes a flat annular shape and has a circular central opening, as shown in  FIGS. 3A and 3B . The thrust member  19  is disposed between the end surface  12   a  of the gripping flange  12  opposite the tapered shank portion and a corresponding end surface  17   a  of the clamp sleeve  17 , which is attached to the chuck sleeve  13 . At least one of the opposite surfaces of the thrust member  19  (preferably, both the opposite surfaces of the thrust member  19 ) is surface treated by means of, for example, TiN coating, in order to reduce the coefficient of friction of the surface. 
   In the chuck of the first embodiment having the above-described structure, when a tool  20  is to be chucked, a spring collet  21  is fitted onto a shank  20 A of the tool  20 , and the shank  20 A of the tool  20 , together with the spring collet  21 , is inserted into the cylindrical bore of the chuck sleeve  13 . When the clamp sleeve  17  is rotated clockwise, each needle roller  16  in contact with the tapered inner surface of the clamp sleeve  17  revolves spirally along the outer surface of the chuck sleeve  13  while rotating about its own axis. Thus, the roller-retaining sleeve  14  moves toward the flange  12 , while rotating, in accordance with the revolution of the needle rollers  16 , and at the same time the clamp sleeve  17  also moves toward the flange  12 . The movements of the roller-retaining sleeve  14  and the clamp sleeve  17  toward the flange  12  reduce the clearance between the tapered inner surface of the clamp sleeve  17  and the tapered outer surface of the chuck sleeve  13 , so that the clamp sleeve  17  strongly presses the chuck sleeve  13  from the entire outer circumference thereof via the needle rollers  16  in order to reduce the diameter of the chuck sleeve  13 . As a result, the shank  20 A of the tool  20  inserted into the cylindrical bore of the chuck sleeve  13  is clamped and held by the chuck sleeve  13  via the spring collet  21  (see  FIG. 2 ). 
   Upon tightening the clamp sleeve  17 , one end surface of the thrust member  19  abuts the end surface  17   a  of the clamp sleeve  17 , and the other end surface of the thrust member  19  abuts the end surface  12   a  of the flange  12 , whereby the end surface  17   a  of the clamp sleeve  17  abuts the end surface  12   a  of the flange  12  via the thrust member  19 . When the tightening operation is continued after such abutment, an excessive clamp force may be applied. However, the end surface  17   a  of the clamp sleeve  17  slides on the surface of the thrust member  19 , to thereby prevent deformation, such as torsional deformation, of the clamp sleeve  17 . 
   When the clamp sleeve  17  in a clamped state is rotated counterclockwise, the roller-retaining sleeve  14  moves toward the tip end of the chuck sleeve  13 , and the clamp sleeve,  17  also moves in the same direction. Therefore, the diameter of the chuck sleeve  13  is restored to its original size, so that the clamping force applied to the tool  20  is released. This enables the tool  20  to be removed from the chuck sleeve  13  or to be replaced with a different tool. 
   Second Embodiment: 
   Next, a chuck according to a second embodiment of the present invention will be described. 
   The chuck of the second embodiment shown in  FIG. 4  is identical with that of the first embodiment, except that in the end surface  12   a  of the flange  12  opposite the tapered shank portion, a groove  22  having a predetermined depth (approximately 2 to 5 mm) is formed, at the bottom portion of the chuck sleeve  13  through which the chuck sleeve  13  is joined with the flange  12 , such that the groove  22  extends along the outer circumference of the chuck sleeve  13  to form a ring-like shape. The ring-shaped, groove  22  allows the chuck sleeve  13  to be extended toward the flange  12  side without changing the tool length L 1  of the holder body  10 . 
   By virtue of the above-described structure, the radially inward elastic deformation of the chuck sleeve  13  in a bottom end area adjacent to the flange  12  is made substantially equal to that in the remaining area of the chuck sleeve  13 . 
   Since the remaining portions are identical with those of the first embodiment shown in  FIG. 1 , those portions are denoted by the same reference numerals, and their repeated descriptions are omitted. 
   The chuck of the second embodiment has the same deformation prevention function as that of the first embodiment. That is, even when an excessive clamp force is applied because of a tightening operation continued after abutment, deformation, such as torsional deformation, of the clamp sleeve  17  can be prevented, because the end surface  17   a  of the clamp sleeve  17  slides on the surface of the thrust member  19 . In addition, the clamp sleeve  17  can be tightened with ease until the end surface  17   a  of the clamp sleeve  17  abuts the end surface  12   a  of the flange, without need of large tightening force, whereby a sufficiently large chucking force can be applied to the tool shank  20 A in an area corresponding to the bottom end of the chuck sleeve  13  through which the chuck sleeve  13  is joined with the flange  12 . Moreover, the radially inward elastic deformation of the chuck sleeve  13  in a bottom end area adjacent to the flange  12  can be made substantially equal to that in the remaining area. 
   Furthermore, since a larger chucking force can be applied to the tool shank  20 A in the bottom end of the chuck sleeve  13  through which the chuck sleeve  13  is joined with the flange  12 , even when heavy cutting is performed, the tool  20  is prevented from causing defecting movement about the tip end of the chuck sleeve  13 . Further, this structure can prevent rubbing between the outer circumferential surface of the tool shank  20 A and the inner wall surface of the chuck sleeve  13 , to thereby prevent damage to the outer circumferential surface of the tool shank  20 A and the inner wall surface of the chuck sleeve  13 . 
   Third Embodiment: 
   Next, a chuck according to a third embodiment of the present invention will be described with reference to  FIGS. 6 to 8 . 
   In  FIG. 6 , structural elements identical with those shown in  FIGS. 1 to 5  are denoted by the same reference numerals, and their repeated descriptions are omitted. Only portions different from those shown in  FIGS. 1 to 5  will be mainly described. 
   The feature of the chuck according to the third embodiment resides in that the annular groove  22  is formed in the end surface of the flange  12  in order to increase the effective length of the chuck sleeve  13 , and that a thrust-member holder  23  is provided in order to prevent play or movement of the thrust member  19  along the axial direction. 
   In the chuck of the third embodiment, the thrust-member holder  23  is fitted into the central opening of the thrust member  19 . The thrust-member holder  23  has an opening for allowing passage of the chuck sleeve  13  therethrough, and a stepped portion  23   a  which projects outward from a first end of the thrust member holder  23  for engagement with the edge portion of the central opening of the thrust member  19 . An elastic member  24 , such as an O-ring, is fitted into a circumferential groove formed at a second end of the thrust-member holder  23 . When the second end of the thrust-member holder  23  is fitted into the groove  22 , the elastic member  24  engages with the wall surface of the groove  22  to thereby prevent axial movement of the thrust-member holder  23 . 
   Accordingly, the thrust member  19  in engagement with the stepped portion  23   a  of the thrust member holder  23  is prevented from playing or moving in the axial direction after tightening of the clamp sleeve  17 . 
   In the third embodiment, a radially outer portion of the end surface  17   a  of the clamp sleeve  17 , which portion comes into contact with the thrust member  19 , is raised in the axial direction so as to form a raised portion  17   b.  Thus, the raised portion  17   b  abuts a radially outer portion of the thrust member  19 . 
   Fourth Embodiment: 
   Next, a chuck according to a fourth embodiment of the present invention will be described with reference to  FIGS. 9 and 10 . 
   In  FIG. 9 , structural elements identical with those shown in  FIGS. 1 to 8  are denoted by the same reference numerals, and their repeated descriptions are omitted. Only portions different from those shown in  FIGS. 1 to 8  will be mainly described. 
   The feature of the chuck according to the fourth embodiment resides in the shape of the chuck sleeve  13 . The chuck sleeve  13  has a larger diameter portion  13   b  extending from the flange  12  in the direction opposite the shank portion, and a smaller diameter portion extending from the large diameter portion  13   b  via a stepped portion  13   c.  The clamp sleeve  17  is fitted onto the outer circumference of the smaller diameter portion of the chuck sleeve  13  via needle rollers  16 . 
   An annular groove  22  is formed in the stepped portion  13   c  in such a manner that an inner wall surface is substantially flush with the outer circumference of the smaller diameter portion of the chuck sleeve  13 . The second end of the thrust-member holder  23  carrying the thrust member  19  is fitted into the groove  22 . Thus, by the action of the elastic member  24 , axial movement of the thrust-member holder  23  is prevented. Moreover, in the present embodiment, the outer circumferential surface of the larger diameter portion  13   b  of the chuck sleeve  13  is flush with the outer circumferential surface of the clamp sleeve  17 . 
   Since the remaining structure is identical with those of the above-described embodiments, description therefor is omitted. Needless to say, the chuck of the fourth embodiment are expected to provide the same action and effects as those of the above-described embodiments. 
   Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.