Abstract:
A pull-to-close collet chuck for use with lathes equipped with servo-type bar loaders. The chuck is configured to be coupled to a lathe drawtube and translates linear movement of the drawtube to a work-gripping action to a collet. The chuck generally includes a ramp body surrounding a first portion of the collet and integral means for fixing the ramp body within a central bore of the chuck. A piston surrounds the collet and has a bearing surface adjacent a ramp surface of the ramp body, and a wedge is located between the ramp surface of the ramp body and the bearing surface of the piston. The wedge has multiple bearing surfaces through which the work gripping action is transferred from the piston to the collet as a result of radial inward movement of the wedge.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/191,024, filed Mar. 20, 2000. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to chucks for holding workpieces. More particularly, this invention relates to a pull-to-close collet chuck for use with lathes equipped with servo-type bar loaders, in which the chuck provides for dead length positioning by preventing movement of bar stock during closing of the collet. 
     Chucks are often used on lathes, screw machines and other machines for holding workpieces during machining operations. In lathes used to machine bar stock, chucks serve to mount workpieces to the rotating spindle of the lathe and accurately align the workpiece with the cutting tool. Chucks may be equipped with collets which, as is well known in the art, are roughly tubular-shaped with equiangularly-spaced slots in one end to delineate multiple resilient fingers capable of gripping a workpiece disposed within the bore of the collet. Servo-type bar loaders enable bar stock to be automatically fed through a collet chuck, with the axial position of the bar stock often being established by a stop provided by the lathe. Dead length positioning (also known as true position, true length positioning, and fixed length positioning) chucks eliminate the requirement for a stop on the lathe, instead relying on a single stop associated with the bar loader. However, for accurate axial positioning, a dead length positioning chuck must be capable of preventing movement of the bar stock during closing of the collet. In U.S. Pat. No. 3,434,730 to Smrekar, a dead length positioning collet chuck is disclosed in which a sleeve surrounds the end of a collet in which the resilient fingers are formed. The collet is fixed within the chuck, while the sleeve is adapted to be axially actuated relative to the sleeve by a cam and roller assembly. The sleeve and the collet end have cooperating camming surfaces so that upon axial movement of the sleeve into engagement with the collet end, the resilient fingers of the collet are compressed inward to chuck or hold bar stock within the collet. 
     While collet chucks of the type taught by Smrekar have been successfully used for many years, further improvements in collet chucks are desired, particularly where greater gripping forces are necessary to support larger bar stock. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a pull-to-close collet chuck for use with lathes equipped with servo-type bar loaders. Similar to prior art chucks used with servo-type bar loaders, the chuck is configured to be coupled to a lathe drawtube and translates linear movement of the drawtube to a work-gripping action at the collet. The chuck provides for dead length positioning by preventing movement of bar stock as the collet is closed, eliminating the need for a stock stop on the lathe. Instead, bar stock is held securely against a stop on the bar loader during the closing phase of the chuck. The chuck is constructed to promote the strength, rigidity and long wear life of its internal components. 
     The chuck of this invention generally includes a housing with a bore therein that defines a longitudinal axis. A collet is disposed within the bore of the housing, and is configured to include a first portion, a collapsible second portion, and a bore that is substantially coaxial with the bore of the housing. A ramp body is also disposed within the bore and surrounds the first portion of the collet, with the collet being removably secured within the ramp body by any appropriate means. The ramp body is formed to include integral means for securing the ramp body to the housing. The ramp body defines has a ramp surface that is substantially transverse to the longitudinal axis of the housing. A piston surrounds the collet and has a bearing surface adjacent the ramp surface of the ramp body. The piston is longitudinally reciprocable within the bore by a suitable actuating means. Also within the housing is a wedge located between the ramp surface of the ramp body and the bearing surface of the piston. According to the invention, the wedge differs from ball and roller bearings conventionally employed in collet chucks by having multiple bearing surfaces that provide surface-to-surface contact (as opposed to point or line contact provided by ball and roller bearings, respectively) through which the work gripping action is transferred to the collet. Two of the bearing surfaces cam against the ramp surface of the ramp body and the bearing surface of the piston, so that the wedge moves radially inward toward the collet when the piston moves toward the ramp body. Finally, the chuck includes means engaged with a third bearing surface of the wedge for causing the second portion of the collet to collapse radially inward to grip the workpiece when the wedge moves radially inward toward the collet. 
     The chuck of this invention is extremely rugged in terms of the strength and rigidity of its internal structure, including the manner in which the ramp body is held fast within the housing and the configuration of the wedge, whose bearing surfaces allow for better force distribution with the mating surfaces of the piston and ramp body. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exterior perspective view of a collet chuck in accordance with the present invention. 
     FIG. 2 is an exploded view of the collet chuck of FIG.  1 . 
     FIG. 3 is a perspective sectional view of the collet chuck of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the collet chuck of FIG. 1, in which the chuck is shown mounted to a drawtube of a lathe. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 4 show a collet chuck  10  of a type used on computer numerically-controlled (CNC) lathes used for machining bar stock, including steel, brass, aluminum and plastic. As with conventional collet chucks, the chuck  10  closes a collet  12 , causing the collet  12  to close around bar stock  14  (represented in FIG.  4 ). The collet  12  is represented in FIGS. 1 through 3 as being a solid, roughly tubular-shaped body, though as shown in FIG. 4 the collet  12  will typically have equiangularly-spaced slots present in one end to delineate multiple resilient fingers capable of gripping the bar stock  14  disposed within the bore  78  of the collet  12 . The collet  12  is typically the only component of the chuck  10  that comes into contact with the bar stock  14 . The collet  12  can be solid, dedicated to one size stock, or it can be a master collet that accepts segments that can be changed to alternate the gripping diameter. As known in the art, CNC lathe machines may be equipped with servo-positioning bar loaders  16  that feed the bar stock  14  into the lathe machine (now shown) and position the stock  14  with the use of a stock stop located in the turret of the lathe. When coupled with the servo-positioning bar loader  16 , the chuck  10  of this invention does not allow the bar stock  14  to move longitudinally as the chuck  10  closes the collet  12 . Instead, the chuck  10  causes the bar stock  14  to be held securely against a stop  18  provided by the bar loader  16  during the closing phase of the chuck  10 . As a result, the chuck  10  eliminates the need for a stop provided by the lathe machine. 
     The chuck  10  includes a housing  22  shown as comprising two housing members  23  and  24  held together with bolts (not shown). The housing  22  is adapted for mounting to the spindle (not shown) of a lathe via a mounting plate  26 , which can be readily configured for use with different spindle nose configurations including tapered and cylindrical. The rearmost housing member  24  is preferably equipped with four setscrews  27  (FIG. 4) to adjust the concentricity or run out of the bar stock  14 , allowing the lathe operator to virtually eliminate any run out that can occur from bar stock tolerance or manufacturing tolerances within the collet chuck  10 . The internal mechanism of the chuck  10  is contained within a central bore  28  within the housing  22 , and is connected to an operating cylinder (not shown) on the back of the spindle through a drawtube  20  that is threaded into a drawtube connector  30 . The cylinder actuates the drawtube  20  forward to open the chuck  10 , and pulls back to close the chuck  10 . 
     The primary components of the collet chuck  10  are illustrated in FIGS. 1 through 4. Omitted are such hardware items as o-rings, screws and grease fittings, which are known by those skilled in the art as being required in the assembling and mounting of the chuck  10  to a lathe spindle. As seen in FIGS. 3 and 4, the drawtube connector  30  is retained in the chuck  10  by a drawtube cap  32 . The cap  32  is bolted to a two-piece split ring assembly  34 , which in turn is clamp shelled around one end of a piston  36 . For this purpose, the ring assembly  34  has an annular flange or lip  35  that is inserted into a groove  37  formed in the outer surface of the piston  36 . The connector  30 , cap  32 , split ring assembly  34  and piston  36  move in unison when the drawtube  20  is actuated. In contrast, a ramp body  38  is held in a fixed position, both rotationally and axially, within the housing bore  28 . As shown in FIGS. 2,  3  and  4 , the housing members  23  and  24  are formed to have complementary milled pockets that when the members  23  and  24  are assembled define recesses  40  that receive four outward projections  42  of the ramp body  38 . The projections  42  key the ramp body  38  to the housing  22  and prevent rotation of the ramp body  38  within the housing bore  28 . As more readily seen from FIG. 2, the piston  36  has four longitudinal slots  44  in which the projections  42  are received to allow the piston  36  to be assembled with the ramp body  38  in a concentric arrangement, while also allowing the piston  36  to be actuated longitudinally relative to the ramp body  38 . 
     The piston  36  and the ramp body  38  have bearing surfaces  46  and  48 , respectively, contacting a wedge  50  made up of four wedge segments  52 , though fewer or more segments  52  could be used. The bearing surface  46  of the piston  36  faces radially inward toward the wedge  50  and has a generally frustroconical shape tapered to have a decreasing diameter away from the ramp body  38 . The bearing surface  48  of the ramp body  38  is substantially transverse to the axis of the housing bore  28 , though a slight angle to this axis could be used. The wedge segments  52  are shown has having multiple linear surfaces (when viewed in cross-section), three of which are bearing surfaces  72 ,  74  and  76 . A first  72  of the bearing surfaces of each segment  52  contacts the bearing surface  46  of the piston  36 , while a second  74  contacts the bearing surface  48  of the ramp body  38 . The first bearing surfaces  72  of the wedge segments  52  together yield a composite frustroconical shape that is substantially coaxial with the housing bore  28  and tapered to have a decreasing diameter toward the ramp body  38 , thereby complementary to the bearing surface  46  of the piston  36 . The second bearing surface  74  of each segment  52  is planar and transverse to the axis of the housing bore  28 . As a result of the orientations of the respective bearing surfaces  46 ,  48 ,  72  and  74 , actuation of the piston  36  toward the stationary ramp body  38  forces the wedge segments  52  radially inward along the bearing surface  48  of the ramp body  38  and toward the collet  12 . The third bearing surface  76  of each segment  52  contacts a complementary bearing surface  54  of a pusher body  56 . The bearing surfaces  54  and  76  face radially outward and inward, respectively, toward each other, and each has a complementary frustroconical shape tapered to increase in diameter away from the ramp body  38 . 
     The pusher body  56  is accurately guided by the bore  28  of the collect  10  so as to be limited to fore and aft movement. As a result, when the wedge segments  52  are forced inward along the ramp body  38 , the pusher body  56  is forced forward away from the ramp body  38 . The pusher body  56  is shown has having a second bearing surface  58  at an end opposite the bearing surface  54 . The bearing surface  58  also has a frustroconical shape increasing in diameter away from the ramp body  38 , and contacts yet another frustroconical bearing surface  60  on a tapered portion  62  of the collet  12 . The collet  12  is held fixed within the housing  22  by threads  64  formed on a cylindrical portion  66  of the collet  12  and engaged with the ramp body  38 . As a result, the forward motion of the pusher body  56  closes the collet  12 , i.e., deflects the tapered portion  62  of the collect radially inward to reduce the internal diameter of the collet bore  78 . 
     To release the bar stock  14 , the drawtube  20  is actuated forward toward the chuck  10 , which causes the drawtube connector  30 , cap  32 , split ring assembly  34  and piston  36  to move forward, thereby releasing the radially inward force on the wedge segments  52 . Springs  68  housed in complementary cavities  70  in the housing member  23  and the pusher body  56  (FIG. 4) force the pusher body  56  back toward the ramp body  38 , allowing the collet  12  to spring open and release the bar stock  14 . 
     Those skilled in the art will appreciate that various materials and surface treatments may be employed to form the components of the chuck  10 . Suitable materials include 4140 and 8620 steels, with case hardening of the bearing surfaces to increase wear resistance. In a preferred embodiment, the housing members  23  and  24  are formed of 4140 steel and through-hardened, though it is foreseeable that other materials and a different heat treatment could be used. 
     In view of the above, the collet chuck  10  of this invention can be understood to incorporate several significant features. The chuck  10  is designed to work specifically with servo-type bar loaders and eliminates the need for a stop in a lathe. The chuck  10  incorporates dead length positioning, meaning that the bar stock  14  does not move when the chuck  10  (collet  12 ) closes. Importantly, the segmented wedge  50  provides for surface-to-surface contact with the mating bearing surfaces  46 ,  48  and  54  of the piston  36 , ramp body  38  and pusher body  56 , instead of the point or line contact provided with the use of ball and roller bearings. Though this surface-to-surface contact increases internal friction, the chuck  10  is surprisingly efficient with respect to generating desirable gripping loads at the tapered end  62  of the collet  12 . In addition, the manner in which the ramp body  38  is secured within the housing  22  with integral projections greatly increases the gripping force that can be generated at the collet  12  while achieving the true positioning capability desired for the chuck  10 . The chuck  10  also has adjustable concentricity to eliminate run out in the bar stock  14 , and has o-ring protection on the collet  12  and other internal components to reduce chip build-up from the machining operation. Adjustable grip force is possible by varying the pressure from the cylinder. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.