Patent Publication Number: US-6666120-B2

Title: Facedriver for miniature workpieces

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of patent application Ser. No. 09/546,882, filed Apr. 10, 2000, titled “Facedriver with Fully-Adaptable Workpiece Engagement and Enhanced Centerpoint Force,” now U.S. Pat. No. 6,374,713. 
    
    
     FIELD OF THE INVENTION 
     The invention described herein relates generally to rotary-drive machining devices and, more particularly, to facedrivers for engaging workpieces along the axes of rotary-drive machining devices. 
     BACKGROUND OF THE INVENTION 
     Many metalworking machines employ rotary-drive devices which support and rotate the workpieces (typically shaft-like pieces) at locations permitting metalworking operations to be performed on the workpieces as they are rotated. Such metalworking machines include lathes, gear-shaping tools, hobbing tools, spline-milling tools and grinding tools. Workpieces are mounted, supported and turned in such machines along a turning axis. 
     One widely-used device for mounting workpieces is known as the facedriver. Facedrivers engage one end of a workpiece while the opposite end is engaged by a tailstock. The term “facedriver” is used because such device applies force to the end of the workpiece, thereby permitting metalworking operations to be performed along the entire axial length of the workpiece in a single operation. The facedriver is a preferred workpiece-mounting device for rotary-drive machining apparatus because it contacts and applies forces to only the end face of a workpiece. 
     Each facedriver typically includes a base member (or “driving head”) with a forward end, an axially-aligned, forwardly-spring-biased centerpoint member slidably disposed in the base member, and drivepins slidably disposed in the base member around the centerpoint member and supported rearwardly by some sort of means to cause some interactive adjustment of the drivepins as they engage the workpiece. The centerpoint member is a male member with a pointed distal end for engaging a female opening at the end of the workpiece, and serves the purpose of providing axial support for the workpiece throughout the machining operations. The drivepins have distal ends engaging the workpiece and imparting to the workpiece the turning motion of the facedriver as it rotates. The drivepins are adjustably held in drivepin holes in the carrier body and are supported rearwardly (at their proximal ends) in various ways. 
     The two most widely used commercially-available facedrivers have two principal types of structures for rearward support of drivepins. One involves the pins bearing on a wobble ring and the other involves the pins bearing on a hydraulic fluid—an oil reservoir. These rearward support means permit the drivepins to adjust in different ways to irregular face variations of the workpiece end—such as end surfaces which are off-normal (i.e., non-perpendicular) with respect to the axis of the workpiece or concave, or which have various other irregular or erroneous configurations. Each of the two principal types of face-driver structures, however, has significant disadvantages and/or shortcomings. 
     In facedrivers with wobble rings, the wobble ring is able to move so that axial movement of one or two of the drivepins in one direction on one side of the centerpoint member causes compensating opposite axial movement of some of the drivepins on the opposite side of the centerpoint member. However, because of the rigid connections, less than all of the drivepins (e.g., only three of the drivepins) will make fully-solid contact with the workpiece at any one time unless there is an absolute true perpendicular mounting. 
     While this type of facedriver has imperfect workpiece engagement by the drivepins, it does provide relatively good workpiece engagement by the centerpoint member. More specifically, an advantage of wobble-ring structures is that rearward movement of the wobble ring caused by drivepins applies rearward force on a cone or a sleeve which in turn causes collapse of a collet, thereby gripping the centerpoint member and enhancing the axial force with which it engages the workpiece. This action gives stronger workpiece support for better on-axis stability when trans-axial tool pressure is applied on the workpiece in various metal-working operations. That is, the workpiece is supported fairly well against side-to-side shifting. As indicated, however, the problem of facedrivers with wobble-ring support for their drivepins is a lack of solid engagement of all drivepins with the workpieces—because of irregular or imperfect workpiece ends as described above. 
     Different advantages and disadvantages exist for facedrivers of the type in which the drivepins are supported on an oil reservoir. Given the non-compressibility of oil, the oil serves to provide compensation for each of the pins and gives you a true equalizing adjustment. The drivepins are supported on a single reservoir the shape of which accommodates the drivepin array. This hydraulic support tends to provide excellent workpiece engagement for all of the drivepins, regardless of the irregularities or imperfections of workpiece ends. All pins engage varying workpieces with equal pressure, regardless of the axial positions of any of the pins, because they are suspended on a common reservoir. 
     However, a significant disadvantage and shortcoming of such oil-reservoir facedrivers is that the drivepins are required to serve too much of a role in holding workpieces in proper axial alignment. While it is supposed to be the role of the centerpoint member to insure proper axial alignment during operations, when the workpiece is loaded against the drivepins (against the resistance of the tail stock engaging the other end of the workpiece), the force with which the drivepins engage the workpiece exceeds the force by which the centerpoint member engages the workpiece. 
     The result is that there is a tendency for the centerpoint member to float because the only thing that helps it apply force to the workpiece is a spring behind it. The problem is that because the spring pressure does not equal the hydraulic pressure, the drivepins end up both driving and more or less suspending the workpiece. This is potentially problematic, particularly when substantial radial loads are applied to the workpiece during machining operations. 
     In summary, there is a need for an improved facedriver which overcomes each of the differing problems in prior art devices. 
     OBJECTS OF THE INVENTION 
     It is an object of this invention to provide a facedriver overcoming the problems and shortcomings of the prior art, including those noted above. 
     Another object of this invention is to provide an improved facedriver which exhibits even and consistent engagement of the drivepins with the workpiece, without any sacrifice in the workpiece-centering and workpiece-holding functions of the centerpoint member. 
     Another object is to provide an improved facedriver which provides enhanced workpiece-engagement under increased loading conditions without any sacrifice in the ability of drivepins to conform to the workpiece-end irregularities. 
     Still another object of the invention is to provide an improved facedriver with balanced drivepin engagement with workpiece ends despite workpiece-end variations and commensurately enhanced centerpoint member loading force as drivepin forces increase. 
     It is a further object of this invention to provide an improved facedriver with excellent miniature workpiece engagement and holding properties and simple construction and operation. 
     Yet another object of this invention is to provide an improved facedriver with the capability of handling miniature workpieces. 
     These and other objects of the invention will be apparent from the following descriptions and from the drawings. 
     SUMMARY OF THE INVENTION 
     This invention is an improved facedriver which overcomes significant problems in the industry, including those described above. The facedriver of this invention is an improvement in facedrivers of the type including a base member with a forward end, an axially-aligned, forwardly-biased centerpoint member slidably disposed in the base member, and drivepins slidably disposed in the base member around the centerpoint member and supported rearwardly by hydraulic fluid in drivepin portions of a fluid-containing chamber within the base member. 
     The facedriver of this invention includes a grip member fixed to the base member and contacting the lateral surface of the centerpoint member to apply radial gripping force thereon to enhance the axial loading force applied by the centerpoint member to the workpiece under certain conditions. The fluid chamber, in addition to its drivepin portions, includes a grip portion adjacent to the grip member for fluid contact therewith. Thus, loading forces applied through the drivepins are transmitted to the grip portion of the fluid chamber, so that such fluid pressure results in application of radial force on the lateral surface of the centerpoint member through the grip member. 
     The grip portion of the fluid chamber and the grip member are configured and arranged such that varying hydraulic fluid pressure in the fluid chamber, caused by forces transmitted to the fluid by the drivepins, results in varying loading forces being applied by the centerpoint member on the workpiece. More specifically, the grip member, the drivepins, and the grip portion and drivepin portions of the fluid chamber are preferably configured and arranged, and the biasing of the centerpoint member selected, such that the force of the drivepins applied to the workpiece upon facedriver loading and the resulting increased fluid pressure in the fluid chamber serve to cause the centerpoint member to apply force to the workpiece upon facedriver loading which approaches the force applied by the drivepins. 
     One particularly novel aspect of the invention is the engagement of the centerpoint member to the base member at a position radially outside of the connection between the drivepins and the base member. 
     In highly preferred embodiments of this invention, the grip member, at the grip portion of the fluid chamber, has a radially-compressible portion bearing on the lateral surface of the centerpoint member. The radially-compressible portion is preferably in the form of a sleeve, and such sleeve is preferably of metal. It is most preferred that the radially-compressible sleeve be integrally formed with the remainder of the grip member. It is highly preferred that the grip member be sleeved over the base member and axially fixed such that the internal surface of the grip member defines a grip member cavity which receives a portion of the base member. 
     In such embodiments, it is highly preferred that the fluid chamber includes at least one connecting portion extending from the grip portion to the drivepin portions of the fluid chamber. Such connecting portion preferably extends rearwardly from the grip portion to the drivepin portions of the fluid chamber. 
     Preferred embodiments are now described in more detail. The base member of the facedriver of this invention preferably forms various voids and spaces in it for the movable components and to accommodate the functions involving hydraulic fluid. More specifically, the base member forms drivepin voids around the axis, a centerpoint-member void around the axis and a fluid chamber around the axis which includes a grip portion and drivepin portions adjacent to the drivepin voids. 
     It is preferred that the grip portion of the fluid chamber be formed by the grip member and an annular void on the surface of the base member adjacent to the radially-compressible portion of the grip member, such that the grip member closes and seals the void. 
     The centerpoint-member void is formed by the connection of a centerpoint housing to the base member. The centerpoint housing has a substantially cylindrical inner wall which has a diameter greater than the diameter of the external surface of the grip member, thereby creating the substantially cylindrical centerpoint-member void. The centerpoint-member void extends from a shoulder adjacent to the connection point to an opening toward the forward end of the base member. At least one coil spring is positioned at the shoulder to provide forward bias to the centerpoint member. 
     The centerpoint member is positioned in the centerpoint-member void such that its proximal end abuts the one or more coil springs. The forwardly-biased centerpoint member has a lateral surface and extends axially out of the centerpoint-member void to a centerpoint-member distal end beyond the forward end of the base member. The lateral surface slidably engages the external surface of the grip member. The centerpoint-member distal end includes drivepin openings which are axially aligned with the drivepin voids. Likewise, the outer end of the grip member includes drivepin orifices which are axially aligned with the drivepin voids. 
     The drivepins are in the drivepin voids and extend forwardly through the drivepin openings and orifices to drivepin distal ends and rearwardly to drivepin proximal ends which are adjacent to the drivepin portions of the fluid chamber for support on the hydraulic fluid. The grip member has an external surface against which the centerpoint member is slidably engaged when positioned in the centerpoint-member void and an internal surface with a mid-portion adjacent to the grip portion of the fluid chamber for fluid contact. As already noted, the grip portion of the fluid chamber and the grip member are configured and arranged such that varying hydraulic pressure in the fluid chamber applies varying radial gripping force on the centerpoint member through the grip member. The preferred arrangement places the grip member between the centerpoint member and the fluid. 
     The centerpoint member preferably includes a sleeve portion with an inner surface defining a centerpoint cavity. This inner surface is the lateral surface which contacts the external surface of the grip member. It is preferred that the base member be slidably disposed within the centerpoint cavity. 
     In preferred embodiments, the coil spring is selected such that its axial force, along with the resistance forces in the centerpoint member generated hydraulically due to the particular configuration and arrangement of parts causes the centerpoint member to apply force to a workpiece which approaches the force applied by the drivepins. 
     In a broad form, this invention is an apparatus for applying force on an object which includes: a base member with a forward end; a forwardly-biased first slidable member slidably disposed with respect to the base member along an axis; one or more additional slidable members each of which is slidably disposed with respect to the base member in a generally axially-parallel position and supported rearwardly by hydraulic fluid in a chamber within the base member; a grip member fixed to the base member and contacting the first slidable member; and a grip portion of the fluid chamber, such grip portion being adjacent to the grip member for fluid contact therewith such that varying fluid pressure in the fluid chamber applies varying gripping force (e.g., radial gripping force) on the first slidable member through the grip member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate a preferred embodiment which includes the above-noted characteristics and features of the invention. The invention will be readily understood from the descriptions and drawings. In the drawings: 
     FIG. 1 is a fragmentary side elevation of a rotary-drive machining device including a facedriver in accordance with an embodiment of this invention in position engaging a workpiece. 
     FIG. 2 is a side elevation with breakaway portions, illustrating the facedriver as it begins to come into engagement with a workpiece during workpiece mounting. 
     FIG. 3 is a side elevation as in FIG. 2, but illustrating the facedriver in complete engagement with a workpiece. 
     FIG. 4 is an exploded side elevation of the facedriver with breakaway portions and portions in section. 
     FIG. 5A is an enlarged side elevation of the facedriver body (or “base member”) with breakaway portions. 
     FIG. 5B is a right side elevation of FIG.  5 A. 
     FIG. 5C is a left side elevation of FIG.  5 A. 
     FIG. 6 is a fragmentary side elevation of a rotary-drive machining device including a facedriver in accordance with a highly preferred embodiment of this invention in position engaging a workpiece. 
     FIG. 7 is a side elevation with breakaway portions, illustrating the facedriver as it begins to come into engagement with a workpiece during workpiece mounting. 
     FIG. 8 is a side elevation as in FIG. 7, but illustrating the facedriver in complete engagement with a workpiece. 
     FIG. 9 is an exploded side elevation of the facedriver with breakaway portions and portions in section. 
     FIG. 10A is an enlarged side elevation of the facedriver body (or “base member”) with breakaway portions. 
     FIG. 10B is a right side elevation of FIG.  10 A. 
     FIG. 10C is a left side elevation of FIG.  10 A. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a facedriver  10  in accordance with this invention on a lathe  12 , which includes a main portion  12   a  and a tailstock portion  12   b . Facedriver  10  may be coupled to lathe  12  in various ways. Most preferably, facedriver  10  is quickly and easily coupled to lathe  12  (or to another rotary-drive machining device) by use of the quick-coupling assembly described and claimed in U.S. Pat. No. 5,771,762 (Bissett). An elongate workpiece  14  is mounted between facedriver  10  and tailstock portion  12   b , along an axis  16  defined by lathe  12 . Facedriver  10 , of course, serves the normal purposes of mounting workpiece  14  and transmitting the turning force of lathe  12  to workpiece  14 . The details of facedriver  10  are illustrated in FIGS. 2-5C. 
     Facedriver  10  includes a main body or base member  20  formed of two principal parts bolted together, a front member  20   a  and a rear member  20   b , and has a forward end  20   c  and a rear end  20   d . The remaining other parts of facedriver  10  are secured to or mounting in base member  20  in various ways. Base member  20  has various voids and spaces formed in it to accommodate the other parts. These voids and spaces include a center void  22  which is centered about axis  16 , eight drivepin voids  24  which are radially spaced from center void  22  and arranged around axis  16 , and a fluid chamber  26  which is around axis  16  and designed to accommodate the functions described herein. 
     Facedriver  10  also includes an axially-aligned, forwardly-biased centerpoint member  28  which extends axially along center void  22 . Centerpoint member  28  has a lateral surface  28   a  and terminates forwardly in a pointed distal end  28   b  which engages workpieces such as workpiece  14 . Centerpoint member  28  is slidably disposed in base member  20  and is forwardly biased by a coil spring  30  which extends in known fashion between a retention knob member  32  and the back end of centerpoint member  28 . 
     Facedriver  10  further includes drivepins  34  slidably disposed in drivepin voids  24  in base member  20 . Each drivepin  34  includes a drivepin main portion  34   a  and a separate drivepin piston portion  34   b . Drivepin main portion  34   a  terminates forwardly in a drivepin distal end  34   c , which serves to engage and turn workpiece  14 , and at its rear end abuts drivepin piston portion  34   b . Each drivepin piston portion  34   b , at its back end, is in contact with hydraulic fluid  36  in fluid chamber  26 . Each drivepin  34  also includes an O-ring  34   d  around piston portion  34   b  for fluid-sealing engagement with the wall of its corresponding drivepin void  24 . 
     Facedriver  10  includes a sleeve-like annular grip member  40  around centerpoint member  28  in center void  22 . Grip member  40  has an outer surface  40   a  and an inner surface  40   b . Inner surface  40   b  is in annular engagement with lateral surface  28   a  of centerpoint member  28 ; thus, the slidable relationship of centerpoint member  28  to base member  20  is by slidable engagement with grip member  40 . Grip member  40 , which is in fixed axial position with respect to base member  20  by its engagement thereto, has a radially-compressible thin-walled mid-portion  40   c  which bears on lateral surface  28   a  of centerpoint member  28 . Mid-portion  40   c , like all of grip member  40 , is in the form of a sleeve and is integrally formed with all of grip member  40 . Grip member  40  is metal. 
     Fluid chamber  26 , in addition to having drivepin portions  26   a  behind drivepins  34 , has an annular portion (“grip portion”)  26   b  which is adjacent to and defined in part by mid-portion  40   c  of grip member  40 . Outer surface  40   a  of grip member  40 , at mid-portion  40   c , is in a necked-in configuration to form annular portion  26   b  of fluid chamber  26 . Thus, fluid chamber  26  is designed to apply the force of its hydraulic fluid pressure on the outer surface of radially-compressible thin-walled mid-portion  40   c ; hydraulic fluid  36  in annular portion  26   b  of fluid chamber  26  engages and applies radially-inward force on mid-portion  40   c.    
     The wall thickness of thin-walled mid-portion  40   c  can vary depending upon the choice of materials and other parameters of the system, including the diameter of grip member  40 , the length of mid-portion  40   c , and the size of the overall equipment, to name just some. In the embodiment illustrated, the outside diameter of mid-portion  40   c  is 1.125 inch, and the length of mid-portion  40   c  is 1.25 inch. Wall thickness of the embodiment illustrated, in which grip member  40  is made of high-carbon steel such as 4140 steel, is 0.040 inch. A suitable thickness range for such material is about 0.030-0.080 inch. Variations in materials will dictate a variation in suitable thickness ranges. 
     With the inventive device, varying loading forces (i.e., forces on facedriver  10 ) applied through drivepins  34  are transmitted to annular portion  26   b  of fluid chamber  26 , and result in application of radial force on lateral surface  28   a  of centerpoint member  28  through grip member  40 . As explained above, this allows centerpoint member  28  to increase its axial force on workpiece  14 , which allows centerpoint member  28  to better serve its holding/centering function even when substantial axial forces are applied to workpiece  14  during machining operations. 
     The dimensioning of mid-portion  40   c  is chosen to meet the particular machining operation. The illustrated embodiment, in which the mid-portion is 1.25 inch in length, is appropriate for grinding and hobbing applications, both of which involve substantial radial metal-working loads on workpiece  14 . Increasing the length of mid-portion  40   c  increases the enhancement of axial loading force of centerpoint member  28  on workpiece  14 ; decreasing the length of mid-portion  40   c  decreases the enhancement of axial loading force of centerpoint member  28  on workpiece  14 . Lathe metal-working applications such as chip removal of workpiece  14  typically involve substantially less radial metal-working loads and typically involves some axial metal-working loads. Under such circumstances, mid-portion  40   c  could be of shorter length, because that is sufficient to provide the less amount of force enhancement needed. 
     Acceptable variations in dimensioning will be apparent to those skilled in the art who are made familiar with this invention. 
     The relationship of grip member  40  to base member  20  is better described with further reference to further details of base member  20 . Center void  22  in base member  20  has: a forward portion  22   e  formed by a forward cylindrical wall  20   e  of first diameter; a middle portion  22   f  formed by a middle cylindrical wall  20   f  of second diameter, which is greater than the first diameter; and a rearward portion  22   e  formed by rearward cylindrical wall  20   g  of third diameter, which is greater than the second diameter. Grip member  40  has a forward portion  40   d  which is forward of mid-portion  40   c  and sealingly engages forward cylindrical wall  20   e . Grip member  40  also has a rearward portion  40   e  which is rearward of mid-portion  40   c  and sealingly engages rearward cylindrical wall  20   g . Such sealing engagement is facilitated by O-rings  42 , so that hydraulic fluid  36  is properly contained. 
     The front and rear ends of grip member  40  are engaged with front member  20   a  and rear member  20   b , respectively, of base member  20 . Front member  20   a  and rear member  20   b  of base member  20  are in sealing engagement with one another by means of large O-ring  44 . 
     Along the length of grip member  40  are end-to-end axial bores of two different diameters, including a forward cylindrical bore  48 , which is defined by inner surface  40   b  and extends along most of the length of grip member  40 , and a rearward cylindrical bore  50 . Rearward cylindrical bore  50  receives coil spring  30 , which bears on centerpoint member  28  to provide the forward bias of centerpoint member  28 . The ledge  52  between forward and rearward cylindrical bores  48  and  50  provides a stop for centerpoint member  28 , defining its forwardmost (unengaged) position. 
     Fluid chamber  26 , in addition to its eight drivepin portions  26   a  and annular portion  26   b , includes various generally radially-oriented connecting portions  26   c  such that, despite its irregular shape, fluid chamber  26  is a single chamber and can perform the plural hydraulic functions of this invention. Connecting portions  26   c  of fluid chamber  26  are along middle cylindrical wall  20   f  at and near the rear portion thereof, and extend rearwardly and radially outwardly from annular portion  26   b  of fluid chamber  26  to join drivepin portions  26   a  of fluid chamber  26 . 
     Facedriver  10  can be made using metals of the type commonly used in facedrivers and other rotary-drive apparatus. A variety of material choices and alternative materials, including non-metals, may be used depending on the particular requirements and needs. A great number of variations are possible in the facedriver shown in the drawings. The shape of the fluid chamber, including the number and location of connecting portions, can vary greatly. Likewise, the specific applications for this invention for holding and turning devices are not limited. 
     FIG. 6 illustrates a facedriver  10  in accordance with the most highly preferred embodiment of the invention on a lathe  12 , which includes a main portion  12   a  and a tailstock portion  12   b . Facedriver  10  may be coupled to lathe  12  in various ways. Most preferably, facedriver  10  is quickly and easily coupled to lathe  12  as described in relation to FIG.  1 . An elongate miniature workpiece  14  is mounted between facedriver  10  and tailstock portion  12   b , along an axis  16  defined by lathe  12 . Facedriver  10 , of course, serves the normal purposes of mounting workpiece  14  and transmitting the turning force of lathe  12  to miniature workpiece  14 . The details of facedriver  10  are illustrated in FIGS. 7-10C. 
     Facedriver  10  includes a main body or base member  20  formed of two principal parts bolted together, a front member  20   a  and a rear member  20   b , and has a forward end  20   c  and a rear end  20   d . The remaining other parts of facedriver  10  are secured to or mounting in base member  20  in various ways. Facedriver  10  has various voids and spaces formed in it to accommodate the other parts. These voids and spaces include eight drivepin voids  24  which are arranged around axis  16 , a centerpoint-member void  22  which is positioned around axis  16  and a fluid chamber  26  which is around axis  16  and designed to accommodate the functions described herein. 
     Facedriver  10  includes a sleeve-like annular grip member  40  positioned around base member  20 . Grip member  40  has an outer end  40   e  and a sleeve portion  40   h  with an external surface  40   a  and an internal surface  40   b . Internal surface  40   b  defines grip-member cavity  40   i  which receives a portion of base member  20 . When centerpoint member  28  is positioned in centerpoint-member void  22 , external surface  40   a  is in annular engagement with lateral surface  28   a  of centerpoint member  28 ; thus, the slidable relationship of centerpoint member  28  to base member  20  is by slidable engagement with grip member  40 . Grip member  40 , which is in fixed axial position with respect to base member  20  by its engagement thereto, has a radially-compressible thin-walled mid-portion  40   c  of sleeve portion  40   h  which bears on lateral surface  28   a  of centerpoint member  28 . Mid-portion  40   c  is integrally formed with all of grip member  40 . Grip member  40  is metal. Outer end  40   e  includes drivepin orifices  40   f  which are axially aligned with drivepin voids  24 . 
     Facedriver  10  includes an annular centerpoint-member housing  20   h  centered around axis  16  and connected to base member  20  to create the cylinder-shaped centerpoint-member void  22 . Centerpoint-member void  22  extends from shoulder  60  to an opening toward the base member forward end  20   c . Spring  30  is positioned at shoulder  60  to provide forward bias to centerpoint member  28  as discussed below. 
     Facedriver  10  further includes an axially-aligned, forwardly-biased centerpoint member  28  which is positioned in centerpoint-member void  22  and extends axially along external surface  40   a . Centerpoint member  28  has a lateral surface  28   a  and terminates forwardly in a pointed distal end  28   b  which engages workpieces such as workpiece  14 . Lateral surface  28   a  is the inner surface of a sleeve portion of centerpoint member  28  and defines centerpoint-member cavity  28   e . Centerpoint member  28  is slidably disposed in centerpoint-member void  22 , such that base member  20  is received in centerpoint cavity  28   e , and is forwardly biased by coil spring  30  which extends in known fashion between shoulder  60  and the proximal end  28   c  of centerpoint member  28 . Centerpoint member includes drivepin openings  28   d  axially aligned with drivepin voids  24 . 
     Facedriver  10  further includes drivepins  34  slidably disposed in drivepin voids  24  in base member  20 . Each drivepin  34  includes a drivepin main portion  34   a  and a separate drivepin piston portion  34   b . Drivepin main portion  34   a  terminates forwardly in a drivepin distal end  34   c , which serves to engage and turn workpiece  14 , and at its rear end abuts drivepin piston portion  34   b . Each drivepin  34  passes through a respective drivepin opening  28   d  and drivepin orifice  40   f . Each drivepin piston portion  34   b , at its back end, is in contact with hydraulic fluid  36  in fluid chamber  26 . Each drivepin  34  also includes an O-ring  34   d  around piston portion for fluid-sealing engagement with the wall of its corresponding drivepin void  24 . 
     Fluid chamber  26 , in addition to having drivepin portions  26   a  behind drivepins  34 , has an annular portion (“grip portion”)  26   b  which is adjacent to and defined in part by mid-portion  40   c  of grip member  40 . Grip portion  26   b  of fluid chamber  26  is partly formed by an annular void  20   i  along the outer surface of base member  20 . Fluid chamber  26  is designed to apply the force of its hydraulic fluid pressure on the internal surface of radially-compressible mid-portion  40   c ; hydraulic fluid  36  in annular portion  26   b  of fluid chamber  26  engages and applies radially-outward force on mid-portion  40   c.    
     The wall thickness of thin-walled mid-portion  40   c  can vary depending upon the choice of materials and other parameters of the system, including the diameter of grip member  40 , the length of mid-portion  40   c , and the size of the overall equipment, to name just some. In the embodiment illustrated, the outside diameter of mid-portion  40   c  is 1.125 inch, and the length of mid-portion  40   c  is 1.25 inch. Wall thickness of the embodiment illustrated, in which grip member  40  is made of high-carbon steel such as 4140 steel, is 0.040 inch. A suitable thickness range for such material is about 0.030-0.080 inch. Variations in materials will dictate a variation in suitable thickness ranges. 
     With the inventive device, varying loading forces (i.e., forces on facedriver  10 ) applied through drivepins  34  are transmitted to grip portion  26   b  of fluid chamber  26 , and result in application of radial force on lateral surface  28   a  of centerpoint member  28  through grip member  40 . As explained above, this allows centerpoint member  28  to increase its axial force on workpiece  14 , which allows centerpoint member  28  to better serve its holding/centering function even when substantial axial forces are applied to workpiece  14  during machining operations. 
     The dimensioning of mid-portion  40   c  is chosen to meet the particular machining operation. The illustrated embodiment, in which the mid-portion is 1.25 inch in length, is appropriate for grinding and hobbing applications, both of which involve substantial radial metal-working loads on workpiece  14 . Increasing the length of mid-portion  40   c  increases the enhancement of axial loading force of centerpoint member  28  on workpiece  14 ; decreasing the length of mid-portion  40   c  decreases the enhancement of axial loading force of centerpoint member  28  on workpiece  14 . Lathe metal-working applications such as chip removal of workpiece  14  typically involve substantially less radial metal-working loads and typically involves some axial metal-working loads. Under such circumstances, mid-portion  40   c  could be of shorter length, because that is sufficient to provide the less amount of force enhancement needed. 
     Acceptable variations in dimensioning will be apparent to those skilled in the art who are made familiar with this invention. 
     The relationship of grip member  40  to base member  20  is better described with further reference to further details of base member  20 . Grip member  40  extends from a rearward end  40   g  which is adjacent to shoulder  60  to outer end  40   e  and sealingly engages the outside cylindrical surface of base member  20 . Outer end  40   e  includes drivepin orifices  40   f  to allow drivepins to slide with respect to grip member  40 . Grip member  40  is preferably connected to base member  20  along axis  16  to prevent axial movement of grip member  40  with respect to base member  20 . 
     Mid-portion  40   c  of grip member  40  seals the annular void of base member  20  to create grip portion  26   b  of fluid chamber  26 . O-rings  42  ensure sealing engagement around grip portion  26   b  to prevent leakage of hydraulic fluid  36 . 
     Grip member  40  has a first internal diameter which corresponds to the outer diameter of the portion of base member  20  which includes grip portion  26   b , and a second internal diameter, smaller than the first internal diameter to correspond to the outer diameter of base member  20  at its forward end  20   c.    
     Fluid chamber  26 , in addition to its eight drivepin portions  26   a  and annular portion  26   b , includes various generally radially-oriented connecting portions  26   c  such that, despite its irregular shape, fluid chamber  26  is a single chamber and can perform the plural hydraulic functions of this invention. Connecting portions  26   c  of fluid chamber  26  are along the rear portion of drivepin portion  26   a  of fluid chamber and extend radially outward and axially outward to join grip portions  26   b  of fluid chamber  26 . 
     Facedriver  10  can be made using metals of the type commonly used in facedrivers and other rotary-drive apparatus. A variety of material choices and alternative materials, including non-metals, may be used depending on the particular requirements and needs. A great number of variations are possible in the facedriver shown in the drawings. The shape of the fluid chamber, including the number and location of connecting portions, can vary greatly. Likewise, the specific applications for this invention for holding and turning devices are not limited. 
     Among other things, the form of the apparatus for applying hydraulic gripping forces onto a centerpoint member can vary greatly. It is not essential that the grip member be an annular sleeve-like device, nor is it necessary that radial forces be applied evenly all around the centerpoint member. A great number of variations are possible. 
     While the principles of this invention have been described in connection with specific embodiments, it should be understood clearly that these descriptions are made only by way of example and are not intended to limit the scope of the invention.