Abstract:
A drive center constituting a machine tool accessory being used on a lathe for holding workpieces that are mounted between the headstock and the tailstock (between centers). The drive center comprises a head having an adjustable spring-loaded center point and a number of removable, adjustable spurs that are capable of penetrating a workpiece sufficiently to provide torque for the turning process. The head is attached to a shank with a standard taper to fit the spindle of a lathe. A cylindrical guard is supported coaxially by the external surface of the head so that the guard radially encircles the head and the spurs but is able to slide axially and rotationally with respect to the head. A spring causes the guard to extend axially beyond the spurs except when pushed back by the workpiece.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus for holding a workpiece as the workpiece rotates in a lathe and, more specifically, to a highly adjustable drive center equipped with a safety guard. 
       BACKGROUND OF THE INVENTION 
       [0002]    Holding devices used in conjunction with lathes for mounting a workpiece between the headstock and tailstock of the lathe, are typically called drive centers, spur centers, or prong centers. These devices are intended to serve both the function of accurately locating the workpiece and rotating the workpiece (i.e., driving it), such that the device maintains correct centering and continued rotation as the operator applies torque on the workpiece by means of cutting tools. 
         [0003]    Typical drive centers have a tapered shank that is received by an identically tapered bore in the spindle of the headstock of the lathe. Drive centers have a center point and a number of spurs or prongs. The spurs embed and engage in the workpiece and are retained there by the force imparted to the workpiece from the tailstock at the opposite end of the workpiece. 
         [0004]    The workpiece is brought into its correct location against the drive center by placing the center point into a previously made indentation in the workpiece. The tailstock center is similarly engaged in the opposite end of the workpiece. After placing the workpiece between the lathe centers, the tailstock center is advanced toward the headstock so that the workpiece is sufficiently forced against the spurs of the drive center for holding and driving the workpiece. This process is referred to as “mounting” the workpiece on the lathe. 
         [0005]    There are a number of drawbacks associated with known drive centers. One of the encountered problems stems from the fact that the density of the material of different workpieces can vary greatly. Due to this, different lengths of center points are required to effectively position the different workpieces. It is generally desirable to use a long center point for working with materials of low density, e.g., soft wood, and a short center point for working with materials of high density, e.g., hardwood. If the material is very dense, it is difficult to force the center point far enough into the workpiece so that the spurs will sufficiently engage with the workpiece. In this case, spurs are unable to impart the required torque and tend to slip against the workpiece. Many turners mistakenly attempt to compensate for this drawback by over-tightening the tailstock so as to increase the amount of force between the headstock and the tailstock. However, it is to be appreciated that a spindle should be turned with a minimum amount of force from the tailstock. The minimum force is defined herein as that force which is just enough to keep the work from slipping. It is known that the increase of force, between the centers, creates an excessive load on the headstock bearings and revolving center which results in premature wear and possible failure of the lathe. Further, the workpiece tends to deflect or flex from the centerline when it is “squeezed” too tightly by the centers. This flexing, sometimes called “whip,” leads to vibration of the workpiece and causes the chisel to “chatter” which creates marks on the workpiece which are undesirable and difficult to repair. 
         [0006]    When turning workpieces between centers, it is essential that the center point in the headstock spindle run true. If any of these components are eccentric, the workpiece will be eccentric about the rotational axis any time the workpiece is removed and replaced, or reversed end-for-end. Eccentricity may result from manufacturing tolerances of the center point, the shank of the drive center or the tapered bore of the spindle. The workpiece often becomes eccentric when it is removed from the lathe and then not accurately re-centered when it is again remounted on the lathe. To overcome eccentricity caused by manufacturing errors of the drive center and center point and the spindle, if the center point is rigidly coupled with the drive center, the drive center can be set into the spindle and the lathe turned on and with the spindle and drive center spinning, the center point can be trued-up so as to create a new axis. After this, the drive center must always be aligned in the spindle as it was when the new axis was created in the center point. In this way, the turner can achieve accuracy greater than that which was originally built into the lathe, but this remedy is only available if the center point is rotationally fixed in the body. 
         [0007]    Truing-up the center point requires the center point to be rotationally fixed to the drive center, and other known drive centers in which the center point is not keyed, cannot be trued-up. 
         [0008]    Although some of the known drive centers may have spring loaded center points and adjustable spurs, a major drawback of the known drive centers lies in the fact that the spurs are exposed, thus creating a safety hazard. It is possible for loose clothing, sleeves, neckties, jewelry or the hair of the turner, for example, to get caught or to become entangled in the spurs as the drive center rotates. Due to the high rate of rotation of the drive centers and the power imparted by the machine, when something becomes entangled with the spurs, this often results in serious injury to the turner. The turner could also inadvertently brush part of his/her body against the sharp spurs, whether the spindle is in motion or not in motion. Further, a collision between the tool rest or a chisel and the drive center can cause injury to the turner and damage to the spur center. 
       SUMMARY OF THE INVENTION 
       [0009]    Wherefore, it is an object of the present invention to overcome the above-mentioned shortcomings and drawbacks associated with the prior art drive centers. 
         [0010]    It is yet another objective of the present invention to provide a drive center that enables the workpiece to be removed or replaced or reversed end-for-end while the spindle and drive center are in motion. This allows for faster, safer, and convenient loading, unloading and/or reversing end-for-end of workpieces on the lathe. This feature is made possible because the center point is positioned by a spring that causes the point to maintain sufficient contact with the workpiece even as the force from the tailstock is removed, thus moving the workpiece away from the spurs, allowing the workpiece to stop rotating while maintaining its positional location between the headstock and tailstock centers. The functionality of the spring loaded center point is enhanced by two separate features of its design: first the length of travel (stroke) of the center point is nearly 10 mm, long enough to allow easy application of the left side of the workpiece to be mounted on the center point without any interference by the guard or the spurs; and second, the force on the spring is adjustable to conform and adapt to the weight, diameter, and rigidity of the workpiece. 
         [0011]    It is still another objective of the present invention to ensure that when the workpiece is removed from the lathe and replaced or reversed end-for-end, it returns exactly to its original concentric position, and thus runs true on its original axis as previously. 
         [0012]    A further objective of the present invention is to provide a spring loaded drive center that functions like a clutch so as to permit the workpiece to be easily stopped for measurement or inspection of the surface quality, such as during a sanding operation, without having to stop the spindle or motor. 
         [0013]    Yet another objective of the present invention is to provide a drive center that is designed to allow the workpiece to be turned with the minimum of force from the tailstock. To accomplish this, the drive center, according to the invention, includes two pertinent features: First, the center point is provided with an adjustable spring that positions the point in a forward most orientation for initial location of the workpiece, but recedes out of the way as the workpiece is gradually advanced toward the spurs, thus causing minimal interference by the center point with the engagement of the spurs into the workpiece; and Second, the spurs are extremely sharp allowing them to embed and engage adequately with the workpiece and impart the necessary torque for the turning process. 
         [0014]    Another objective of the present invention is to provide a drive center with spurs that can be quickly removed for resharpening, and following replacement, each of the spurs can be “micro adjusted” in the drive center so each of the spurs are the same length. Further, if a “two spur” center is desired, for roughing of cross grain bowls for example, two of the four spurs can be removed. The spurs of this drive center are also easily replaceable in the event that they become too short, from repeated sharpening, or possibly damaged by dropping or collision with another object. 
         [0015]    A still further objective of the present invention is to provide a drive center having a guard which functions to enshroud or enclose the spurs thereby preventing objects, such as loose clothing, jewelry, or hair from inadvertently becoming entangled with the spurs, and preventing any part of the operator&#39;s body from coming into contact with the spurs. Because the guard itself is not rotationally fixed, it will stop rotating if the guard becomes entangled with clothing, jewelry, or hair. 
         [0016]    The present invention also relates to a drive center for use with a lathe comprising a headstock and a tailstock. The drive center having a head that has a leading face and an exterior surface. The leading face of the head supports a plurality of spurs and a center point. A shank is integrally connected with a trailing end of the head and has a remote end that is spaced from the head. The center point, the shank, and the head define a rotational axis of the drive center. A cylindrical guard is coaxially supported by the exterior surface of the head such that the guard encases the head, and the guard is axially slidable along the exterior surface of the head between an extended position and a retracted position. A biasing element is supported by the head and biases the guard into the extended position in which a leading portion of the guard projects past the leading face of the head and the spurs but not the center point. The guard, when in the retracted position, encloses the spurs while still allowing engagement between the spurs and a workpiece. 
         [0017]    The present invention also relates to a drive center for use with a lathe having a headstock and a tailstock. The drive center comprises a head that has a leading face and an external surface that has a flange located at an axial end of the head opposite from the leading face. The head supports a center point and at least two spurs which extend axially beyond the leading face of the head. A shank is fixed to a trailing end of the head that is axially opposite the leading face of the head. The shank and the head define a rotational axis of the drive center. A cylindrical guard is coaxially supported by the external surface of the head such that the guard radially encircles the head. The guard has a cylindrical inner surface that is directly supported on the external surface of the head and the flange such that the guard is rotationally and axially movable with respect to the head. The flange axially engages a retainer that is fixed to the guard so as to limit axial movement of the guard with respect to the head. The guard is in a fully extended position when the flange engages the retainer. A spring is radially located between the inner surface of the guard and the external surface of the head. The spring contacts the flange and the retainer so as to continually bias the guard axially toward the fully extended position in which the guard extends axially beyond the leading face of the head and the at least two spurs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate various embodiments of the invention and together with the general description of the invention given above and the detailed description of the drawings given below, serve to explain the principles of the invention. It is to be appreciated that the accompanying drawings are not necessarily to scale since the emphasis is instead placed on illustrating the principles of the invention. The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
           [0019]      FIG. 1  is an isometric view of a drive center prior to the installation of a guard according to the invention; 
           [0020]      FIG. 2  is a side view of the drive center of  FIG. 1  prior to the installation of the guard with holes therein shown in dashed lines; 
           [0021]      FIG. 3  is a front view of the drive center of  FIG. 2  prior to the installation of the guard from a point of view on a rotational axis of the drive center according to the invention; 
           [0022]      FIG. 4  is a side view of the spring pusher according to the invention; 
           [0023]      FIG. 5  is an isometric view of the head of the drive center prior to the installation of the guard according to the invention showing a spur arranged within a bore in the head; 
           [0024]      FIG. 6  is an isometric view of a center point; 
           [0025]      FIG. 7  is a side view of the center point of  FIG. 6 ; 
           [0026]      FIG. 8  is a front view of the guard, according to the present invention, prior to assembly on the head of the drive center; 
           [0027]      FIG. 9  is a cross sectional side view of the guard along section line  9 - 9  shown in  FIG. 8 ; 
           [0028]      FIG. 10  is an isometric view of the drive center according to the invention following installation of the guard on the head; 
           [0029]      FIG. 11  is a diagrammatic cross sectional side view of  FIG. 10 ; 
           [0030]      FIG. 12  is a partial diagrammatic cross sectional view of the drive center of  FIG. 10  with a workpiece being located on the center point; 
           [0031]      FIG. 13  is a partial diagrammatic cross sectional view of the drive center of  FIG. 10  showing initial engagement of the guard with the workpiece; 
           [0032]      FIG. 14  is a partial diagrammatic cross sectional view of the drive center of  FIG. 10  showing initial engagement of the spurs with the workpiece; 
           [0033]      FIG. 15  is a partial diagrammatic cross sectional view of the drive center of  FIG. 10  showing initial engagement of the spurs with a workpiece having a diameter smaller than the workpiece illustrated in  FIGS. 12-14 ; 
           [0034]      FIG. 16  is a diagrammatic side view of the drive center installed on a lathe with the drive center being used to hold the workpiece 
           [0035]      FIG. 17  is a side view of a further embodiment of the drive center according to the invention; 
           [0036]      FIG. 18  is a front view of the embodiment of the drive center illustrated in  FIG. 17 ; and 
           [0037]      FIG. 19  is a pictorial view of the embodiment of the drive center illustrated in  FIG. 17 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The present invention will be understood by reference to the following detailed description, which should be read in conjunction with the appended drawings. It is to be appreciated that the following detailed description of various embodiments is by way of example only and is not meant to limit, in any way, the scope of the present invention. 
         [0039]    Turning now to  FIGS. 1-9 , the various components of the drive center  2 , according to present invention, will now be briefly discussed. As can be seen, the drive center  2  comprises a body  4  that includes a shank  6  and head  8 . The shank  6  is an elongate member that is tapered to fit into the tapered bore of the headstock spindle  10  of a lathe ( FIG. 16 ). The head  8  of the body  4  comprises a plurality of apertures  56 ,  58 ,  67  and  20 . Apertures  58  receive and support a number of spurs  12 , e.g., typically four, a center point  14 , a retaining pin  16 , set screws  18  and adjusting screws  54  which are utilized to adjust and lock the position of these elements in the manner described below. The body  4  has a central bore  20  that extends along the length thereof and supports and aligns the center point  14 , a spring  22 , a spring pusher  24  and an adjustment screw  26  in the manner described below. 
         [0040]    As best seen in  FIGS. 2 and 11 , the central bore  20  of the drive center  2  is oriented so as to be centrally located along the length of the body  4 . The central bore  20  generally defines the rotational axis  28  of the drive center  2  due to the fact that the center point  14  is received within the central bore  20  at the head end of the drive center  2 . It is to be appreciated that the center point  14 , according to the invention, can be any desired axial length. The central bore  20  is cylindrically shaped over the entire length of the body  4  and has a diameter, at least in the head  8  of the drive center  2 , which closely matches, but is slightly larger than, the outer diameter of the center point  14 . Due to this arrangement, the center point  14  is tightly and snugly received by the head  8  within the central bore  20 . It is to be appreciated that the fit between the head  8 , of the drive center  2 , and the center point  14  must be loose enough so as to allow relatively free axial movement of the center point  14  along the rotational axis  28  but, at the same time, be sufficiently tight so as to prevent lateral movement of the center point  14  in relation to the rotational axis  28 . In other words, the center point  14  must be capable of moving axially in the central bore  20 , but is retained therein such that the point  30  of the center point  14  is always located along and coincident with the rotational axis  28 . If the need arises for the workpiece  32 ,  FIG. 16 , to be removed from the lathe, this tight fit enables the point  30  of the center point  14  to be repeatedly located on the rotational axis  28  of the drive center  2 . This is vital for proper re-centering of the workpiece  32 . 
         [0041]    The diameter of the central bore  20  is substantially the same over a majority of the length of the drive center  2 . However, at the remote end  34  of the shank  6 , the central bore  20  has a narrow portion  36  with a reduced diameter. The narrow portion  36  of the central bore  20  is also threaded and receives and matingly engages with a threaded adjustment screw  26 , the purpose of which will be discussed below. In addition to receiving the threaded adjustment screw  26 , the narrow portion  36  of the central bore  20  also receives a trailing end of a spring pusher  24  which functions, in combination with the threaded adjustment screw  26  and the spring  22 , to adjust the tension applied to the center point  14  by the spring  22 . 
         [0042]    The spring  22  is arranged within the central bore  20  between a trailing end of the center point  14  and a leading end of the spring pusher  24 . The spring  22  communicates with a trailing end  38  of the center point  14  and applies a spring force which pushes the center point  14  in a direction projecting out of the head  8  and away from the spurs  12 . The adjustment screw  26 /spring pusher  24 /spring  22  arrangement enable adjustment of the force imparted by the center point  14  against the workpiece. Further, due to the ability of the center point  14  to move along the rotational axis  28  in combination with the ability of the spring  22  to compress, it is possible to remove the workpiece  32  from the drive center  2  without being required to stop the rotation of the lathe spindle and the drive center  2 . 
         [0043]    According to the prior art, when the turner wants to remove the workpiece  32  from the lathe, he/she typically turns the lathe off and waits for the workpiece  32 , which is still engaged with the spurs  12 , to stop rotating. Once the workpiece  32  stops rotating, the tailstock  40  is then retracted or backed off so as to axially separate the drive center  2  from the workpiece  32 . However, with the drive center  2  according to the invention, once the tailstock  40  is sufficiently retracted, the spring force placed on the center point  14  by the spring  22  axially pushes the workpiece  32  away from the drive center  2  such that the spurs  12  disengage from the workpiece  32 . At the same time, however, the workpiece  32  is still supported and centered by the center point  14 . Since the center point  14  only transmits minimal rotational force, if any, to the workpiece  32 , the workpiece  32  can be readily stopped and removed from the lathe by further withdrawal or retraction of the tailstock  40  without stopping the lathe. 
         [0044]    The adjustment screw  26 /spring pusher  24 /spring  22  arrangement of the drive center  2 , according to the invention, enables the force exerted by the center point  14  on the face of the workpiece  32  to be adjusted. The spring force applied on the center point  14  by the spring  22  is adjustable via adjustment of the spring pusher  24  which is arranged within the central bore  20 . The spring pusher  24 , as shown in  FIGS. 4 and 11 , comprises a stem  42  and a disk  44  and is aligned within the central bore  20  with the stem  42  extending toward the trailing remote end  34  of the shank  6  and the disk  44  facing toward the head  8  of the drive center  2 . The stem  42  of the spring pusher  24  is received within and slidably mates with the narrow portion  36  of the central bore  20 , while the disk  44 , on the other hand, has a diameter that is at least slightly smaller than the diameter of the wider portion  46 ,  FIG. 2 , of the central bore  20 . Communication between the stem  42  of the spring pusher  24  and the narrow portion  36  of the central bore  20  facilitates axial alignment of the spring pusher  24  within the central bore  20  while, at the same time, permitting the spring pusher  24  to slide along the central bore  20  and coincident with the rotational axis  28  of the drive center  2 . 
         [0045]    The spring pusher  24  is arranged within the central bore  20  such that the disk  44  communicates with the trailing end of spring  22  opposite from the center point  14 . As briefly discussed above, the leading end of the spring  22  contacts the center point  14  such that the spring  22 , located between the spring pusher  24  and the center point  14 , exerts a constant and continuous spring force on the center point  14  so as to axially bias the center point  14  out of the head  8 . The spring force exerted by the spring  22  on the center point  14  is adjustable by rotating the threaded adjustment screw  26 . That is, rotation of the adjustment screw  26  further into the narrow portion  36  of the central bore  20  causes the spring pusher  24  to gradually move toward the head  8  which compresses the spring  22  so that the spring  22  exerts a greater spring force on the center point  14 . In contrast, rotation of the adjustment screw  26  out of the narrow portion  36  of the central bore  20  causes the spring pusher  24  to gradually move away from the head  8  which permits expansion of the spring  22  so that the spring  22  exerts a smaller spring force on the center point  14 . This induces a reduction of the spring force applied by the spring  22  on the center point  14 . 
         [0046]    As is known, the type of material of the workpiece often correlates to the amount of force of the center point required to effectively position the workpiece. Typically, a greater force is preferred when turning a heavy workpiece. For light or thin workpieces, or other material with similar characteristics, a light force is generally preferred. In view of the above, it should be apparent that due to the ability to adjust, via the adjustment screw  26 , the amount of spring force applied to the center point by the spring  22 , the drive center  2  according to the invention can be utilized with workpieces having a wide range of weight. That is to say, the pressure applied by the center point  14  on the workpiece  32  can be adjusted according to the individual characteristics of the workpiece  32 . 
         [0047]    The spring-loaded center point  14  comprises a milled slot  48  which passes laterally through the center point  14  and the milled slot  48  extends along the rotational axis  28  of the center point  14  so as to retain as well as limit the range of axial movement of the center point  14 . A retaining pin  16 , as seen in  FIGS. 1 and 11 , is frictionally accommodated and retained within a pin passage  67  which is located within the head  8 . The pin passage  67  extends through the head  8  normal to and coincident with the central bore  20 . The retaining pin  16 , when located within the pin passage  67 , passes through the milled slot  48  so as to thereby captively retain and limit to and fro axial movement of the center point  14 , and also prevent relative rotation of the center point  14  as described previously. Axial movement of the center point  14  is thereby limited to the axial length of the milled slot  48  minus the diameter of the retaining pin  16 . The point  30  of the center point  14  has an included angle of precisely 60 degrees. 
         [0048]    As best seen in  FIGS. 5 and 10 , the head  8  of the body  4  supports a plurality of spurs  12 , e.g., typically four, also commonly referred to as driving dogs, prongs, or driving members. These spurs  12  radially surround the center point  14 . Each of these spurs  12  has a flat  50  that is machined longitudinally along the spur  12  and the respective flat  50  matingly engages with a tangentially arranged set screw  18  so as to lock the spur  12  in its adjusted position once the height of the spur  12 , projection above the face  52  of the head  8 , has been adjusted via suitable adjustment of the push screw  54 . As the features of the spurs  12  are generally known in the art, a further detailed discussion concerning the spurs  12  will not be provided herein. 
         [0049]    As noted, the amount that each spur  12  projects beyond the face  52  of the head  8  is individually adjustable by the respective push screw  54 , once the associated set screw  18  is sufficiently loosened. This allows precise setting of the height of each one of the spurs  12  so that all of the spurs  12  can typically project the same distance from the face  52  of the head  8 . In a like manner, the spurs  12  can be individually adjusted if the face of the workpiece  32  is at an angle with respect to the face  52  of the head  8 . In order to adjust the distance by which the spurs  12  project out from the head  8 , the set screw  18  is first sufficiently rotated out of the set screw bore  56  so as to break contact with the flat  50  of the spur  12  and thereby permit relative movement. Such disengagement from the set screw  18  frees the spur  12  such that the spur  12  can slide along its axis within the spur bore  58 . One end of the spur bore  58 , opposite from the face  52  of the head  8 , has a narrow threaded portion  53  which receives the push screw  54 . As the push screw  54  is rotated either into or out of the narrow threaded portion  53  of the spur bore  58 , such movement induces the respective spur  12  to move correspondingly axially along the spur bore  58 . The distance by which the spur  12  projects out of the head  8  is thereby adjusted by a corresponding adjustment of the push screw  54 . Once the spur  12  is adjusted to its desired height, the set screw  18  is then retightened so as to engage the flat  50  and lock the spur  12  in the adjusted position. Although the figures show the drive center  2  as having four spurs  12 , it is to be understood that the drive center  2  may also be utilized with only two spurs  12 , for example, by removing the two excess spurs  12 . It is to be appreciated that a drive center comprising a different number of spurs, e.g., three, six, etc., are also foreseeable. The spurs  12  of the drive center  2  according to the invention are thus able to be readily removed for resharpening or replacement if desired or necessary. 
         [0050]    As shown in  FIGS. 8-11 , the drive center  2  has a guard  60  which intimately and closely surrounds the circumference of the head  8  and is normally spring loaded into an extended position which encircles and encloses the face  52  of the head  8  and the spurs  12  protruding therefrom. The guard  60  is secured to the head  8  in the manner illustrated in  FIG. 11  such that the head  8  is substantially enclosed by the spring loaded guard  60 . The guard  60  is generally cylindrical in shape and has an inwardly facing surface that includes a cylindrical slide portion  64  that mates with and slides along a cylindrical exterior surface  66 ,  FIG. 2 , of the head  8 . A trailing end inner surface of the guard  60  comprises a wider portion  68  which has a larger diameter than a diameter of the slide portion  64 . The slide portion  64  of the guard  60  has a diameter that is substantially equivalent or only slightly larger than a diameter of the exterior surface of the head  8 , e.g., by a few thousands of an inch or so, so as to permit relative sliding movement therebetween. The sliding surface  64  has a sufficiently axial length so as to maintain a concentric arrangement of the guard  60  with respect to the head  8  as the guard  60  slides axially along the exterior surface  66  of the head  8 . As shown in  FIG. 9  for example, since the diameter of the wider portion  68  of the inner surface of the guard  60  is greater than the diameter of the sliding surface  64 , this recessed region thus forms an internal cavity  70 ,  FIG. 11 , between the wider portion  68  and the exterior surface  66  of the head  8 . The transition of the diameters, from the slide portion  64  to the wider portion  68  of the inner surface  62 , form an abutment shoulder  72 , the purpose of which is described below. An annular groove  74  is formed in the trailing end of the guard  60  adjacent the trailing end of the wide portion  68  which is located closest to the shank  6 , and the purpose of the annular groove  74  will be described below. 
         [0051]    As shown in  FIG. 1 , the head  8  has a flange  76  that is located at a trailing end of the head  8 , adjacent the shank  6 . The flange  76  extends radially outwardly from the exterior surface  66  of the head  8  by a distance that is slightly less than the diameter of the wide portion  68  of the inner surface  62  of the guard  60 . An outwardly facing cylindrical surface of the flange  76  mates with and is slidable relative to the wide portion  68  of the inner surface  62  of the guard  60  so as to help maintain the concentric arrangement of the guard  60  with respect to the head  8  as the guard  60  slides to and fro axially along the exterior surface  66  of the head  8 , as described below in further detail. 
         [0052]    A spring  78  is accommodated within the internal cavity  70  and a first end thereof abuts against the flange  76  while an opposite end thereof abuts against the abutment  72  of the guard  60  so as to normally spring load and bias the guard  60  into its fully extended position, as generally shown in  FIGS. 11 ,  12  and  15 , for example. That is, the spring  78  biases the guard  60  in an axial direction away from the flange  76  so that a leading end of the guard  60  extends beyond the leading end of the spurs  82 . A retainer  80 , such as retainer ring or a circlip, is located within the annular groove  74  of the guard  60  and the retainer  80 , in the extended position of the guard  60 , abuts against the flange  76  so as to prevent further axial movement of the guard  60  toward the leading end of the head  8 . It is to be appreciated any type of retainer can be utilized to limit excess movement of the guard  60  so long as the retainer can be secured to and removed from the guard  60 . This enables easy installation or removal of the guard  60  from the head  8 . 
         [0053]    In order to install the guard  60  on the head  8 , the spring  78  is first slid over the head  8  so that one end thereof engages with the flange  76 . Next, the guard  60  is slid over the head  8 , trailing end first, so that the abutment  72  engages with the opposite end of the spring  78 . The guard  60  is further moved toward the shank  6  against the force of the spring  78  until the annular groove  74 , located in the trailing end, is accessible. Thereafter, the retainer  80  is secured within the annular groove  74  of the guard  60  and the guard  60  can then be released so that the retainer  80  then abuts against the flange  76  and captively retains the guard  60  on the head  8 . 
         [0054]    The guard  60  has an axial length such that when the retainer  80  abuts against the flange  76  the guard  60  is fully extended, as seen in  FIG. 11 , and extends axially beyond the face  52  of the head  8  and radially encloses and encases the engaging end  82  of the spurs  12 . However, in the fully extended position, i.e, when no axial forces are exerted on the guard  60  to force the guard  60  toward the shank  6 , the guard  60  still remains axially spaced from the point  30  of the center point  14  so that the point  30  is readily accessible. That is, the guard  60  does not surround or encase the center point  14  which always protrudes and projects from the space  84  enclosed and encased by the guard  60 . 
         [0055]    To emphasize the benefits of the inventive drive center  2 , the process of mounting or remounting the workpiece  32  while the lathe is operating and the drive center  2  rotating will be now described with reference to  FIGS. 11-15 . For the sake of clarity, the process will only be described with regard to mounting the end of the workpiece  32  that is to be supported by the drive center  2 . As generally shown in these figures, the workpiece  32  is initially mounted, in a conventional manner, so that a first end of the workpiece  32  is positioned against the point  30  of the center point  14  while a second end of the workpiece  32  is supported by the tailstock  40 , in the absence of external forces, e.g., see  FIG. 11  where the center point  14  and the guard  60  are both generally shown in an extended position relative to the head  8 . 
         [0056]    With regard to the center point  14 , the center point  14  is fully extended when the retaining pin  16  abuts the axial end  86  of the milled slot  48  opposite the point  30 . As indicated above, the center point  14  is spring loaded into this position by force applied by the spring  22 . The guard  60  is considered to be in its fully extended position ( FIGS. 11 ,  12 ) when the retainer  80  abuts against the flange  76 . In this position, the guard  60  radially encloses the spurs  12  and generally prevents, or at least significantly reduces, the possibility of an object(s), e.g., hair, loose clothing and cutting tools contacting or becoming entangled with the spurs  12 , while the drive center  2  is rotating. To facilitate centering of the workpiece  32 , the center point  14  extends beyond the leading surface  88  of the guard  60  to facilitate centering engagement with the workpiece  32 . This enables the turner to see the center point  14  and properly align the center point  14  with the rotational center of the workpiece  32 , e.g., an indentation previously formed into the end face of the workpiece  32 . 
         [0057]    A workpiece  32  is considered to be “located” when it is supported by both the head and tail centers of the lathe. A “located” workpiece  32  is illustrated in  FIG. 12  in which the center point  14  is sufficiently introduced into the workpiece  32  to center the workpiece along the rotational axis of the lathe, but the workpiece  32  has not yet engaged with either the guard  60  or the spurs  12 . It is noted that in this position, although the center point  14  supports the workpiece  32  and the center point  14  is rotating, the rotation of the center point  14  is insufficient to induce rotation to the workpiece  32 . 
         [0058]    While maintaining proper alignment of the center point  14  and the workpiece  32 , the tailstock  40  is then gradually adjusted, in a conventional manner, so as to move the workpiece  32  gradually axially along the rotational axis of the lathe toward the drive center  2 . As soon as the center point  14  is fully located within the indentation provided within the workpiece  32 , further axial adjustment of the tailstock  40 , toward the drive center  2 , thereby causes the center point  14  to be axially retracted within the head  8 , against the force of the spring  22 . 
         [0059]    After centering the workpiece  32 , the tailstock  40  continues to be further gradually advanced toward the headstock supporting the drive center  2  which, in turn, forces the center point  14  to be retracted further into the head  8  of the drive center  2 . The degree to which the center point  14  is inserted into the workpiece  32  and the amount the tailstock  40  needs to be further advanced and adjusted so as to secure the workpiece  32  depends at least on the type of material comprising the workpiece  32  and the spring force applied to the center point  14  by the spring  22 . 
         [0060]    Once the center point  14  is sufficiently retracted into the head  8  by the distance which it initially extended beyond the guard  60 , the leading circumferential surface  88  of the guard  60  commences engagement with the surface of workpiece  32 , as generally illustrated in  FIG. 13 . Once the guard  60  abuts against the surface of the workpiece  32 , the spurs  12  are completely enclosed by and within the guard  60  thereby preventing an object(s), e.g., hair, loose clothing, and cutting tools, from inadvertently contacting or becoming entangled with the spurs  12 . 
         [0061]    In addition to the enhanced safety aspects, the spring loaded guard  60  also functions as a synchronizer for assisting with engagement between the drive center  2  and the workpiece  32 . In the process of mounting the workpiece  32 , while the lathe spindle  10  and thus the drive center  2  are rotating, the workpiece  32  is, at first, located between the point  30  of the center point  14  and the tailstock center  40 , and initially the workpiece  32  is at least substantially stationary as shown in  FIG. 12 . With drive centers known in the prior art, the tailstock center  40  is next advanced so as to engage the workpiece with the spurs, however, if the tailstock center  40  is advanced too rapidly toward the drive center, such sudden engagement of the spurs with the workpiece  32  can possibly cause the center point to move away from its proper centered location with the workpiece  32 . This happens especially if the end surface of the workpiece  32  is not “square” to the rotational axis, or the spring force on the center point is not correctly adjusted, via adjustment screw  26 , so as to be commensurate with the weight of the workpiece  32 . This problem is overcome with the inventive drive center  2  by the fact that a limited amount of rotation can be imparted to the workpiece  32  as soon as the guard  60  engages with the workpiece  32 , as generally shown in  FIG. 13 , prior to the engagement of the workpiece  32  with the spurs  12  as generally shown in  FIG. 14 . 
         [0062]    Due to frictional connection of the guard  60  with both the spring  78  and the exterior surface of the head  8 , the guard  60  normally rotates along with the head  8 . Although the guard  60  is not strictly fixed rotationally connected to the head  8 , the guard  60  can transmit limited rotational forces to the workpiece  32 . The amount of rotation imparted by the guard  60  to the workpiece  32  depends on the amount of frictional forces between the engaged surfaces of the guard  60  and the workpiece  32  as well as the radius of the guard. For example, a very small workpiece  32 ′, as shown in  FIG. 15  for example, that fits completely inside the guard  60 , would not benefit from the synchronization feature described above. Since the guard  60  is normally able to initially impart a limited amount of rotation to the workpiece  32 , before the spurs  12  eventually engage with the workpiece  32 , the rotational speed difference, between the workpiece  32  and the spurs  12  when those components engage with one another, can be greatly reduced or possibly eliminated. 
         [0063]    From the point at which the workpiece  32  contacts the leading surface  88  of the guard  60 , continued adjustment of the tailstock  40  toward the drive center  2 , gradually drives the workpiece  32  into engagement with the spurs  12  of the drive center  2 . As soon as the surface of the workpiece  32  engages with the leading surface of the guard  60  (see  FIG. 13 ), both the center point  14  and the guard  60  are sufficiently retracted axially backward in the direction of the shank  6  to an at least partially retracted position. The distance over which the center point  14  and the guard  60  travel, before the spurs  12  eventually engage with the surface of the workpiece  32 , is the axial distance of which the guard  60  extends beyond the spurs  12  when the guard  60  is in its fully extended position.  FIG. 14  illustrates a further retracted position at which the spurs  12  engage with the surface of the workpiece  32  and commence driving rotation of the workpiece  32 . At this point, the workpiece  32  is rotationally driven by the drive center  2  at the rotational speed set by the turner, without any slip or speed difference, and the workpiece  32  is now ready to be turned, e.g., worked, turned, cut, sanded, etc., by the turner. 
         [0064]    A further embodiment of the drive center  89  is illustrated in  FIGS. 17-19 . As this embodiment of the drive center  89  is substantially the same as the drive center  2  discussed above, only the differences between the two embodiments will be described below. As many of the features and elements of this further embodiment are identical to features and elements of the drive center discussed above, the elements that are common to both embodiments will be identified with the same reference numbers. 
         [0065]    In this embodiment of the drive center  89  illustrated in  FIGS. 17-19 , the spurs  92  are integral with the head  91 , for example the spurs  92  and the head  91  can be milled from one piece of material or it is conceivable that the spurs and the head milled independently and then permanently secured to each other. The drive center  89  comprises four spurs  92  that are integral with the head  91  and each spur  92  has a leading edge  94  that engages the workpiece  32  being turned. It should be appreciated that because the head  89  of this embodiment lacks the means for enabling adjustment and removal of the spurs  92 , the manufacturing of such a drive center  89  having a head  91  with integral spurs  92  would simplified and therefore less expensive to manufacture. Specifically, in comparison to the prior discussed embodiment, this further embodiment does not include set screws  18  and push screws  54  and the associated bores  56 ,  58 . 
         [0066]    To further simplify the manufacturing of the drive center  89  and reduce the cost associated therewith, the drive center  89  can comprise a blind center bore  93 , i.e., a blind bore that extends along the rotational axis  28  through the head  91  and through only a portion of the shank  6 , as illustrated in  FIGS. 17 and 19 . Similar to the previously described embodiment, the center point  14  is spring loaded by a force applied by the spring  22 . However in this embodiment, the trailing end of the spring  22  abuts the bottom  95  of the blind center bore  93 . This embodiment eliminates the use of the spring pusher and the adjustment screw. 
         [0067]    Although  FIGS. 17-19  illustrate an embodiment comprising both the features of integral spurs  92  and a blind center bore  93 , it is conceivable a drive center to comprise only one of these features, i.e., integral spurs or a blind center bore. 
         [0068]    In the above description and appended drawings, it is to be appreciated that only the terms “consisting of” and “consisting only of” are to be construed in the limitative sense while of all other terms are to be construed as being open-ended and given the broadest possible meaning. 
         [0069]    While various embodiments of the present invention have been described in detail, it is apparent that various modifications and alterations of those embodiments will occur to and be readily apparent to those skilled in the art. However, it is to be expressly understood that such modifications and alterations are within the scope and spirit of the present invention, as set forth in the appended claims. Further, the invention described herein is capable of other embodiments and of being practiced or of being carried out in various other related ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.