Abstract:
A collet chuck assembly includes an annular mount adapted to be attached to a spindle of a turning machine, a tubular collet sleeve disposed internally to the mount including an inner tapered cam surface and adapted to be coupled to an axial moving mechanism of the turning machine, and a tubular collet structure dispose interiorly of the collet sleeve and having a cooperating tapered, outer cam surface. An annular retaining cap has retaining lugs adapted to be engaged with locking flanges of the mount upon engagement of the cap with the mount and rotation of the cap with respect to the mount. The installed cap engages a front end of the collet structure to retain the collet structure within the collet sleeve. A cap anti-rotation mechanism includes an anti-rotation pin carried within the mount for reciprocating axial movement with respect to the mount between extended and retracted positions and a pin actuating mechanism for selectively effecting the reciprocating axial movement. When in the extended position, the anti-rotation pin engages the cap to prevent rotation of the installed cap with respect to the mount. The cap can be rotated with respect to the mount and removed therefrom be moving the anti-rotation pin to the retracted position.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/099,317, filed Sep. 4, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a collet chuck assembly for holding a tool or workpiece on a spindle of a turning machine and particularly to a collet chuck that can be changed quickly and easily. 
     Collets are sleeves or collars used in various machine tools such as lathes for clamping or gripping workpieces or tools. Collets include a plurality of circumferentially-spaced, workpiece-gripping segments and conical surfaces or cams, which, when the collet is moved axially, interact with corresponding, opposing surfaces or cams on a mounting fixture or on the spindle. This interaction causes the workpiece-gripping segments of the collet to contract or expand to grasp or release a workpiece or tool, depending on the direction of movement. Collet chucks have commonly been used in machine tools in place of other types of chucks. Collet chucks are typically more accurate and have a greater gripping characteristic than a typical jaw chuck, for example. An advantage of collets is that they continue to grasp the workpiece or tool even at high rotational speeds when jaw chucks would have a tendency to loosen their grip due to centrifugal force. 
     One problem encountered with collet chucks is that slight variations in the diameter of the workpiece or stock could cause the collet to position the workpiece differently. When and where a collet will grasp a work piece depends on the difference in diameter between the open collet and the diameter of the workpiece. Precise workpiece diameter is therefore required if the workpiece is to be positioned precisely and consistently in machining operations such as facing, side finishing or cutting to precise lengths. 
     Another problem encountered with conventional collet assemblies is that collet cannot be easily and quickly removed from the spindle. Accordingly, changing collets can be time consuming. 
     For example U.S. Pat. Nos. 5,096,213 and 5,330,224, the respective disclosures of which are hereby incorporated by reference, disclose collet chucks in which a collet body includes an annular groove and hook portion on a rear end of the body which engages an annular flange portion of a spindle of the turning machine or a drawbar adapter of the collet assembly. A collet is installed by engaging the hook portion of the collet to the flange portion of the spindle or adapter, a special tool, such as those disclosed in U.S. Pat. Nos. 4,589,938 or 5,087,059, the respective disclosures of which are hereby incorporated by reference, is needed to compress the rear portions of the collet segments a sufficient distance radially inwardly so that the hook portion of the collet clears the annular flange portion. Similarly, to remove the collet from the turning machine, the tool must again be used to compress the rear portions of the segments so that the hook portions clear the flange portions so that the collet can be disengaged. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the forgoing disadvantages which accompany prior art collets. This object is achieved by a collet chuck assembly for holding a tool or workpiece on a spindle of a turning machine constructed in accordance with principles of the present invention. The collet chuck assembly comprises a mount, a collet assembly, and a cap. 
     The mount has a rear end adapted to be connected to the spindle and cap engaging structure. The collet assembly includes a collet structure for holding a tool or workpiece and operatively interacting cam surfaces. The collet assembly permits relative movement between the cam surfaces, whereby relative movement between the cam surfaces in one direction closes the collet structure to hold the tool or workpiece and relative movement between the cam surfaces in another direction opens the collet structure to release the tool or workpiece. The cap is secured to the mount and engages a front end of the collet structure to maintain the front end at a fixed axial position with respect to the cap and the mount during the relative movement between the cam surfaces of the collet assembly. The cap includes mount engaging structure adapted to coact with the cap engaging structure of the mount to prevent relative axial displacement between the cap and the mount when the cap is in a cap-locked position with respect to the mount. The cap is placed in the cap-locked position by coupling the cap to the mount with the cap-engaging structure of the mount and the mount-engaging structure of the cap disengaged from one another and then rotating the cap less than one revolution with respect to the mount to interengage the cap-engaging structure of the mount with the mount-engaging structure of the cap. 
     Accordingly, the collet structure does not move axially with respect to the assembly so that tools and workpieces can be gripped and positioned consistently. Furthermore, installing a collet is easily accomplished by merely inserting a collet structure into the collet assembly, engaging a cap over the collet structure onto the mount, and rotating the cap into the cap-locked position. 
     Other objects, features, and characteristics of the present invention, as well as the methods of operation of the invention and the function and interrelation of the elements of structure, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this disclosure, wherein like reference numerals designate corresponding parts in the various figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the collet chuck assembly according to the present invention; 
     FIG. 2 is an exploded cross-sectional view of the collet chuck assembly; 
     FIGS. 3A,  3 B, and  3 C are a side elevation, bottom plan view, and top plan view, respectively, of a mount of the collet chuck a assembly; 
     FIG. 3D is a side elevation of the mount rotated 90 degrees with respect to the side elevation of FIG. 3A; 
     FIG. 4 is a side elevation of a rotation tool used in conjunction with the collet chuck assembly; 
     FIGS. 5A,  5 B, and  5 C are a side cross-sectional view, a bottom plan view, and a top plan view, respectively, of a quick-change cap of the collet chuck assembly; 
     FIG. 5D is an enlarged view of the quick-change cap within the circle “D” in FIG. 5A; 
     FIG. 5E is an enlarged view of the portion of the quick-change cap within the circle “E” in FIG. 5B; 
     FIGS. 6A,  6 B, and  6 C are a top plan view, right side elevation, and left side elevation, respectively, of a pin-actuating cam of the collet chuck assembly; 
     FIG. 7 is a cross-sectional view of an alternative collet used in conjunction with the collet chuck assembly; 
     FIG. 8 is a side elevation of a solid stop assembly optionally used in conjunction with the collet chuck assembly; and 
     FIG. 9 is a cross-sectional view of a rear guide bushing optionally used in conjunction with the collet chuck assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For convenience in the following description, various directional or other spatial references are made with regard to references to the drawings. It is understood, however, that such references, including without limitation, front, back, forward, rearward, upper, lower, top, bottom, left, right, lateral, or longitudinal, are made for convenience only and should not be construed to be limiting on the invention described herein. 
     A collet chuck assembly according to the present invention for holding a tool or workpiece is indicated generally by reference number  30  in FIGS. 1 and 2. The collet chuck assembly  30  is attached to a spindle  10  and draw bar  12  of a machine, for example a lathe. The collet chuck assembly  30  generally comprises a mount  60 , a collet assembly which includes a collet sleeve  100 , a collet  80 , and a collet spring  36 , and a quick-change cap  40 . 
     An O-ring  16  may be disposed between the draw bar  12  and spindle  10  to prevent lubricants from escaping between the draw bar  12  and spindle  10  and to prevent debris from entering the draw bar  12  and spindle  10 . 
     As shown in FIGS. 1 and 2 and FIGS. 3A-3D, the mount  60  includes equally spaced, counterboard axial through-holes  62  formed about the outer periphery of the mount  60 . Mount  60  also includes an inner tapered surface  64  shown at the left side thereof in the figures, which, for convenience, will be referred to as the rear side. Surface  64  cooperates with outer tapered surface  20  of the machine spindle  10  to appropriately position the mount  60  with respect to the spindle  10 , as described, for example, in U.S. Pat. No. 5,096,213, the disclosure of which is hereby incorporated by reference. Mount  60  also includes a pair of diametrically opposed blind holes  76  formed in the rear end face  77  which cooperate with a pair of mating projections (not shown) extending from the end of the spindle  10  as disclosed in the previously mentioned U.S. Pat. 5,096,213. Fasteners  63  (e.g., socket head cap screws) extend through the counterboard through-holes  62  into threaded blind apertures  18  formed in the spindle  10  to secure the mount  60  to the spindle  10 . 
     As best shown in FIG. 2, mount  60  further includes a cam aperture  65  extending radially to an outer surface of the mount  60 . An axial aperture  66  extends from rear end-face  77  of the mount  60  to the cam aperture  65 . A second axial aperture  67  extends from a front annular shoulder  73  of the mount  60  opposite rear end-face  77  to a position beyond the cam aperture  65 . Axial apertures  66  and  67  are radially offset and radially aligned with one another in a parallel relation. A tangential aperture  68  (see FIG. 3C) extends from a side surface of the mount  60  to the axial aperture  67 . As shown in FIG. 3D, a slot  71  extends radially from the cam aperture  65  and communicates with an arcuate slot  69  formed below the outer surface of the mount  60 . 
     To compensate for the radial imbalance caused by the removal of material for apertures  65 ,  66 ,  67 , and  68  on one side of the mount  60 , balance holes  61  are formed on a diametrically opposite side of the mount  60 . In the illustrated embodiment, two balance holes  61  are formed, one on either side of one of the through-holes  62 . 
     A cylindrical extension  70  extends from one end of the mount  60 . Extension  70  has a diameter generally less than the remainder of the mount  60  and therefore defines the annular shoulder  73  at the base of the cylindrical extension  70 . Cap-engaging structure, such as radial flanges  72 , extend from the axial end of the cylindrical extension  70 . In the illustrated embodiment, three such flanges  72  extend from the extension  70 . In the preferred embodiment, flanges  72  are not equally spaced about the perimeter of the cylindrical extension  70 . The radial flanges  72  define a peripheral channel  81  (see FIG. 3A) extending about the base of the cylindrical extension  70 . A dowel pin  74  extends radially from the outer surface of the cylindrical extension  70  at a generally central portion of the peripheral channel  81  adjacent to an edge of one of the radial flanges  72 . 
     Mount  60  is preferably machined from 8620-C steel and is hardened to Rockwell C hardness of about  61 . 
     As shown in FIGS. 1 and 2, collet sleeve  100  is disposed radially inwardly of the mount  60  within the assembly  30 . Collet sleeve  100  includes an enlarged portion  110  (at the right hand side thereof in the figure), intermediate portion  112 , and narrow end portion  114 . An interior annular shoulder  116  is defined between the intermediate portion  112  and narrow end portion  114 . External threads  106  are formed on the narrow end portion  114 , and threads  106  engage with internal threads  14  formed in the draw bar  12  (see FIG. 1) to secure the sleeve  100  to the draw bar  12 , thereby coupling the assembly  30  to the draw bar  12 . 
     Collet sleeve  100  includes an inner tapered, frusto-conical cam surface  102  formed on the interior of the enlarged head portion  110 . Axially extending slots  108  are formed about the outer peripheral surface of the enlarged head portion  110 . Although only one such slot  108  is shown in FIGS. 1 and 2, in the preferred embodiment, three equally spaced slots are provided. One of the slots  108  is engaged by a key screw  79  extending through a radial key hole  75  formed in the mount  60  (see FIG.  3 C). The key screw  79  engaging one of the slots  108  prevents relative rotation between the collet sleeve  100  and the mount  60 . 
     Sleeve  100  is preferably machined from 8620-C steel, and the entire sleeve is initially hardened to a Rockwell “a” hardness of about 81.8-82.8 (approximately 61-63 on the Rockwell C scale). Subsequently, a rear portion of the sleeve  100  extending from end portion  114  to a location just forward (i.e., to the right in the drawings) of annular shoulder  116  is drawn down to a Rockwell “a” hardness of about 70.4-73.1 (approximately 42-45 on the Rockwell C scale). 
     As shown in FIGS. 1 and 2, the collet  80  is disposed inside the collet sleeve  100 . A collet spring  36  is disposed between an annular end-face  96  of the collet  80  and the interior annular shoulder  116  of the collet sleeve  100 . Collet spring  36  is preferably a rectangular wire spring. Collet  80  includes an outer tapered cam surface  82  and a plurality of collet segments  89  separated by slots  90 . In the preferred embodiment, three equally spaced collet segments  89  are provided, although one skilled in the art will readily appreciate that more than three segments can be employed. Holes  84  extending radially inwardly from the outwardly tapered surface  82  are drilled into each of the slots  90 . The holes  84  receive an elastomeric sealing member (not shown) inserted therein to prevent debris from entering into the machine spindle  10  through the collet slots  90 . 
     The segments  89  contract radially when the collet  80  is closed and expand radially when the collet  80  is opened in response to the camming interaction of the tapered cam surface  82  of the collet  80  and the tapered cam surface  102  of the collet sleeve  100  during relative axial movement of the cam surfaces  82 ,  102  with respect to each other. A slot relief opening  92  is preferably provided at the axial end of each of the slots  90  to facilitate the radial expansion and contraction of the segments  89 . 
     Collet  80  is preferably machined from 2317 steel, and the entire collet is hardened to a Rockwell C hardness of 61-63. Subsequently, a portion of the collet  80  generally rearwardly (i.e. to the left in the drawings) of the cam surface  82  is drawn down to a Rockwell C hardness of 42-45. The generally softer rear end of the collet  80  facilitates radial contraction of the collet segments  89  during gripping. 
     An axially extending key-way slot  85  is formed in the outer surface of the collet  80  adjacent the annular rear end-face  96 . Key way slot  85  receives a dowel pin  105  extending through a radial opening  104  formed in the collet sleeve  100  to prevent relative rotation between the collet  80  and the collet sleeve  100 . Pin  105  is preferably formed from 8620-C steel hardened to a Rockwell C hardness of about 35. The collet  80  may include drilled holes  86  formed in the annular front end-face  94 . Although only a single hole  86  is shown in FIGS. 1 and 2, the preferred embodiment includes three equally-spaced holes formed in the annular front end-face  94 . Holes  86  accommodate a collet pad clamp (not shown) for a master collet, such as collet  80  shown in FIGS. 1 and 2. 
     An annular shoulder  98  extends about the periphery of the collet  80 . Shoulder  98  is axially displaced from the annular front end face  94  and defines an axial boundary of the outer tapered cam surface  82 . Shoulder  98  is preferably beveled at a slight angle of approximately 10 degrees. 
     As shown in FIGS. 1,  2 , and  5 A- 5 D, the quick-change cap  40  includes a radially extending portion  42  and an axially extending portion  44 . The outer surface of the axially extending portion  44  may be knurled so as to facilitate the gripping thereof. A circular opening S 0  is centrally formed in the radial portion  42  of the quick-change cap  40 . 
     As shown in FIG. 5B, the quick-change cap  40  includes mount-engaging structure, such as lugs  46 , extending radially inwardly from the axial end of the axially extending portion  44  of the quick-change cap  40 . In the preferred embodiment, three unequally-spaced lugs  46  are provided. The lugs  46  define cut-out areas  47  between adjacent lugs  46 . As shown in FIGS. 5A and 5D, the radially extending portion  42  of the cap  40  defines an annular inner face  54  extending about the periphery of the opening  50 . Annular inner face  54  includes a beveled surface  52  extending about the edge of the opening  50 . Beveled surface  52  is preferably formed at an angle δ, which is approximately 10 degrees. 
     Cap  40  is preferably machined from 8620-C steel and then hardened to a Rockwell C hardness of about 61. 
     The assembly  30  is assembled by first threading the collet sleeve  100  to the draw bar  12  and then securing the mount  60  to the spindle  10  with screws  63 . The collet sleeve  100  is rotated until one of the slots  108  is aligned with the radial keyhole  75  formed in the mount  60 , and key screw  79  is then turned into the keyhole  75  to engage the aligned slot  108 . The collet  80  includes a cylindrical extension  99  having an outside diameter slightly smaller than the diameter of the opening  50  of the quick-change cap  40 . The collet  80  is coupled to the cap by inserting the cylindrical extension  99  into the opening  50 . The quick-change cap  40  is secured to the mount  60  by engagement of the cap-engaging structure of the mount  60  with the mount-engaging structure of the cap  40  when the cap  40  is in a cap-locked position with respect to the mount  60 . More particularly, the collet  80  and quick-change cap  40  are coupled to the mount  60  by securing the lugs  46  of the quick-change cap  40  behind the radial flanges  72  of the mount  60  (as will be described in more detail below) while compressing the collet spring  36 . The beveled annular shoulder  98  of the collet  80  bears against the beveled surface  52  of the quick-change cap  40 , both surfaces having approximately the same beveled angle. 
     Collet spring  36  urges the collet  80  against the quick-change cap  40  which is held axially immovable by the engagement thereof with the mount  60 , thus maintaining the annular front end face  94  in a fixed position with respect to the mount  60 . Expansion and contraction of the segments  89  of the collet  80  during opening and closing thereof is effected by relative axial movement of the collet sleeve  100 , as actuated by the draw bar  12 , with respect to the collet  80 , thereby causing relative movement between the tapered cam surfaces  82 ,  102 . Because the collet  80  is held axially fixed while the collet sleeve  100  moves axially with respect to the collet  80 , the axial position of the collet  80  does not change regardless of the diameter of the workpiece secured within the collet  80 . 
     The manner in which the quick-change cap  40  is operatively secured to the mount  60  will now be described in detail. 
     An anti-rotation pin  130  is disposed within the axial aperture  67  formed in the mount  60 . Anti-rotation pin  130  generally includes an enlarged portion  132  with a slot  134  formed therein, and an extension portion  136  extending from an end of the enlarged portion  132 . Anti-rotation pin  130  is preferably formed from 8620-C steel and is hardened to a Rockwell C hardness of about 61. The anti-rotation pin  130  is inserted into the axial opening  67  with a coil spring  128  disposed at the blind end of the opening  67  and with the extension portion  136  of the anti-rotation pin  130  extended into the spring  128 . 
     A pin-actuating cam  140  is disposed within the cam aperture  65  formed in the mount  60 . As shown in FIGS. 6A-6C, the pin-actuating cam  140  includes a cylindrical main body  142  having a central blind aperture  144  formed therein and a radial slot  146  extending from the aperture  144 . An arcuate peripheral slot  148  is formed in an outer surface of the main body  142 , and an eccentric protrusion  150  extends from a bottom surface of the cylindrical main body  142 . 
     Pin-actuating cam  140  is preferably formed from 8620-C steel and is hardened to a Rockwell C hardness of about 61. 
     The pin-actuating cam  140  is placed in the cam aperture  65  of the mount  60  with the eccentric protrusion  150  engaging the slot  134  of the anti-rotation pin  130 . A retaining screw  37 , having a threaded portion  38  and a non-threaded lead portion  39 , is turned into the axial aperture  66  having like threads until the lead portion  39  of retaining screw  37  extends into the arcuate slot  148  formed in the cam  140 . Cam  140  is thereby held in the cam aperture  65  by the retaining screw  37  and is permitted to rotate within the cam aperture  65  over the angular extent of the slot  148 , which is preferably 90 degrees. 
     A rotation tool  120 , shown in FIG. 4, is provided for use with the collet chuck assembly  30 . Rotation tool  120  includes a T-handle  122 , a shaft  124 , and a radial dowel  126  extending from the end of the shaft  124 . Tool  120  is preferably formed from 8620-C steel and is hardened to a Rockwell C hardness of about 35. 
     The end of the tool  120  is inserted into the cam  140  disposed in the cam opening  65 . The diameter of the shaft  124  fits inside the diameter of the central blind aperture  144  formed in the cam  140 , and the radial extent of the dowel  126  conforms to the radial extent of the slot  71 . Spring  128  urges the anti-rotation pin  130  forwardly in an extended position so as to project past the annular shoulder  73  of the mount  60 . The engagement of the slot  134  with the eccentric protrusion  150  of the cam  140  rotates the cam so that, in this biased position, the slot  146  of the cam  140  is aligned with the radial slot  71  of the mount  60 . Accordingly, when the tool  120  is inserted into the cam  140 , dowel  126  engages the slot  146  of the cam. Tool  120  can then be turned (counter-clockwise in the illustrated embodiment) to rotate the cam  140 , thereby retracting the anti-rotation pin  130  into the aperture  68  by the camming action of the eccentric protrusion  150  in the slot  134 . 
     To install the quick-change cap  40  onto the mount  60 , the anti-rotation pin  130  is first retracted using the rotation tool  120 . Quick-change cap  40  is oriented with respect to mount  60  so that the cutouts  47  between the lugs  46  of the quick-change cap  40  are aligned with the radial flanges  72  of the mount  60 . Because the flanges  72  and lugs  46  are asymmetrically arranged about the mount  60  and quick-change cap  40 , respectively, the cutouts  47  are aligned with the flanges  72  in only one orientation of the quick-change cap  40  with respect to the mount  60 . Quick-change cap  40  is then pressed onto the mount  60  so that the annular end face  43  contacts the annular shoulder  73  of the mount  70 , thereby coupling the cap  40  to the mount  60 . Dowel  74  protruding into the channel  81  of the mount  60  contacts one of the lugs  46  of the quick-change cap  40 , thereby providing a hard stop which permits rotation of the quick-change cap  40  with respect to the mount  60  in only one direction, thereby ensuring that pin  130  and hole  48  are oriented in mating positions. 
     Cap  40  is rotated a portion of a single revolution with respect to the mount  60  until an anti-rotation hole  48  is aligned with the anti-rotation pin  130 . In this cap-locked position, the lugs  46  of the cap  40  are behind the flanges  72  of the mount  60 , thereby preventing axial displacement of the cap  40  with respect to the mount  60 . As shown in FIG. 5E, anti-rotation hole  48  is preferably slightly elongated in the radial direction so as to compensate for slight misalignments between the hole  48  and the anti-rotation pin  130 . As shown in FIG. 3C, spring biased plungers  83  are preferably installed in drilled and tapped holes formed in the annular shoulder  73  of the mount  60 . In the preferred embodiment, three equally spaced plungers  83  are provided. Plungers  83  engage mating detents  56  formed in the annular end face  43  of the quick-change cap  40  (see FIG. 5B) to assist in aligning the anti-rotation hole  48  with the anti-rotation pin  130 . 
     The tool  120  is then rotated (clockwise in the illustrated embodiment) to permit the spring  128  to urge the anti-rotation pin  130  into engagement with the anti-rotation hole  148  to link the cap  40  to the mount  60 , thereby preventing rotation of the cap  40  with respect to the mount  60 . A spring plunger  78  inserted into the tangential aperture  68  of the mount  60  engages a detent  138  formed in the side of the anti-rotation pin  130  to assist in holding the anti-rotation pin  130  in the extended position. With the tool  120  thus rotated, dowel  126  is again aligned with slot  71  formed in the mount  60  so that the tool may be removed from the cam  140 . Note that because of the arrangement of the radial slot  71  and the arcuate slot  69  of the mount  60 , the tool  120  cannot be retracted from the pin-actuating cam  140  until the cam rotates to a position in which the anti-rotation pin  130  is extended to engage the anti-rotation hole  48  of the quick-change cap  40 . 
     While the anti-rotation cam  140  operated by the tool  120  constitutes a preferred pin-actuating mechanism for moving the anti-rotation pin  130  between extended and retracted positions, other mechanisms for effecting movement of the pin  130  may be used as well. For example a linearly sliding pin-actuating mechanism can be installed in an outer wall of the mount and coupled to the anti-rotation pin  130  so that sliding movement of the mechanism will cause corresponding movement of the pin  130 . It is especially preferred, however, that the pin actuating mechanism be constructed and arranged so that a tool for moving the mechanism and causing corresponding movement of the pin  130  can only be disengaged from the mechanism when the pin is in the extended position. This provides a safety check to the user so that the tool is not disengaged from the pin-actuating mechanism before the pin  130  has extended into the aperture  48  of the cap  40  and thereby locked the cap  40  with respect to the mount  60 . 
     Thus, it can be appreciated that the collet  80  can be installed by simply inserting it into the sleeve  100  and installing the cap  40  in the simple manner described above. Removing the collet  80  is equally simplified. The sleeve  100  is engaged with the draw bar  12  and need not be disengaged every time the collet  80  is removed. Accordingly, the collet  80  need not be disengaged from the draw bar when removed or engaged with the draw bar when installed. 
     An alternate collet  180  for use with the assembly of the present invention is shown in FIG. 7 in which features that are common to the collet  80  shown in FIGS. 1 and 2 have corresponding reference numbers. Collet  180  is a solid collet having segments  189  with solid portions  182  defining an axial opening  185  therethrough. Solid portions  182  define aback annular edge  184 . Solid collet  180  can be custom bored by the end user to accommodate a particular size of tool or workpiece. Accordingly, collet  180  does not require collet pads and therefore a collet pad clamp and the corresponding openings  86  shown on the master collet  80  are not necessary for the solid collet  180 . 
     Solid collet  180  is preferably machined from the same material and given the same heat treatment as collet  80  described above. 
     An optional feature which may be advantageously employed with the collet assembly  30  of the present invention is a rear guide bushing  160  shown in FIG.  8 . Bushing  160  includes a body  162  with a first end face  164  and a second end face  166 . A first conical section  168  and a second conical section  172 , separated by a cylindrical section  170 , are formed so as to extend from the second end face  166 . First and second conical sections  168 ,  172  are preferably formed at an angle γ of approximately 132 degrees. A through hole  173  is formed centrally through the second conical section  172  to the first end face  164 . The rear guide bushing  160  can be installed into a rear end of a collet  80  ( 180 ) by turning external threads  174  formed in the outer surface of the body  162  into internal threads  88  formed on the interior of the collet  80  ( 180 ). A custom-sized through hole, indicated by phantom lines  176 , can be formed centrally through the rear guide bushing  160  by an end user. The rear guide bushing  160  facilitates alignment of elongated bar stock inserted from the rear of a machine, such as a lathe, with the collet  80  ( 180 ). With a solid collet  180  or with collet pads installed in a master collet  80 , alignment of the bar stock with the collet opening, such as collet opening  185  formed in solid collet  180 , can be difficult without the benefit of the rear guide bushing  160 . 
     Bushing  160  is preferably formed from 8620-C steel and is hardened to a Rockwell C hardness of about 35. 
     Another optional feature that can be advantageously used in conjunction with the collet assembly of the present invention is a solid stop assembly  190 , as shown in FIG.  9 . The solid stop assembly  190  includes a solid stop body  196  having external threads  198  formed on the outer periphery thereof and a centrally formed threaded aperture that is engaged by a threaded stop rod  194 . Solid stop body  196  is preferably formed from steel and is hardened to a Rockwell C hardness of about 35. A nut  192  secures the stop rod  194  with respect to the body  196 . A slot  200  may be formed in one end of the stop rod  194  to facilitate adjustment of the rod  194  with a tool such as a screwdriver. Solid stop assembly  190  can be secured to a collet  80  ( 180 ) by turning the external threads  198  of the body  196  into the internal threads  88  of the collet  80  ( 180 ). With the solid stop assembly installed in a collet  80  ( 180 ), the stop rod  194  limits the extent to which a workpiece can be inserted into the collet  80  ( 180 ), thereby permitting repeatable positioning of the workpiece in the collet. The construction, function, and operation of the solid stop assembly is similar to a solid stop assembly disclosed in U.S. Pat. No. 5,330,224, the disclosure of which is hereby incorporated by reference. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
     Furthermore, it should be noted that where the appended claims do not include language in the ‘means for performing a specified function’ format permitted under 35 U.S.C. §112(¶6), it is intended that the appended claims not be interpreted under 35 U.S.C. §112(¶6) as being limited to the structure, material, or acts described in the present specification and their equivalents.