BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a tunable toolholder for suppressing vibrations caused in the machining processes and, more particularly, to a tunable toolholder which utilizes a dynamic vibration absorber to suppress vibrations.  
           [0003]    2. Description of Related Art  
           [0004]    During a metal cutting operation, any vibration between a cutting tool and a workpiece may lead to undesirable cutting performances, such as poor workpiece surface finish and out-of-tolerance finished workpieces. Furthermore, such vibration may cause the cutting tool, or the associated machine tool, to become damaged.  
           [0005]    To reduce this vibration, the metal removal rate can be decreased. However, this approach interferes with production and only minimally reduces the amount of vibration.  
           [0006]    The cutting tool is typically secured within a toolholder. Attempts to eliminate the vibration in the toolholder, such as a boring bar, may include using a boring bar fabricated from solid carbide. Solid carbide, because of its inherently high density, reduces the amount of chatter and vibration transferred to the boring bar. However, solid carbide is extremely expensive. Furthermore, although chatter and vibration are reduced by the inherently high density of the solid carbide bar, vibration nonetheless may build up to an unacceptable level. Still furthermore, solid carbide is fairly brittle and a minor impact upon the boring bar during use or setup may inadvertently damage the bar.  
           [0007]    Other attempts to reduce vibration in boring bars include mounting upon or within the bar a dynamic vibration absorber, such as that absorber disclosed in U.S. Pat. No. 3,774,730, which is comprised of a cylindrical mass of a high density material supported on rubber bushings. When optimally tuned, the mass oscillates in response to the vibration produced in the boring bar to cancel out vibration. The absorber may be tuned to accommodate the boring bar for the speed at which the workpiece or boring bar is rotating, the length of the boring bar, and the type of cutting tool connected at the end of the bar. Such an adjustment is made by longitudinally urging pressure plates at opposing ends of the cylindrical mass, thereby compressing the rubber bushings against the mass. This simultaneously shifts the position of the mass and alters the stiffness of the rubber bushings to change the dynamics of the bar.  
           [0008]    Typically, the adjustment of the pressure plates is made from the rear of a boring bar as illustrated in U.S. Pat. No. 3,838,936 or from the side of the toolholder, as illustrated in FIG. 1 of U.S. Pat. No. 5,518,347.  
           [0009]    While these mechanisms may be suitable for toolholders and boring bars having relatively large diameters, when a toolholder has a small diameter, such as, for example, less than one inch, then because of space limitations, access from the rear of the toolholder, or from the side of the toolholder, may become difficult and passageways providing such access may substantially weaken the boring bar structure.  
           [0010]    U.S. Pat. No.  5 , 518 , 347  (the &#39;347 patent) is also directed to a tuned damping system for an end mill  16 , (FIG. 2), wherein a tuning fixture  27  has a longitudinally oriented adjusting screw to apply force against elastomeric support  21   a,    21   b,  which acts against a damper mass  19 . However, after adjusting the supports against the mass, the tuning fixture  27  must be removed and replaced with a cutting head to perform various cutting tasks. The damper mass  19  is kept in compression after removal of the tuning fixture  27  by a clamping screw  23 , which locks the system adjustment in place. While the cutting head is in place, the tuning of the system is fixed and may not be adjusted further.  
           [0011]    The system disclosed in the &#39;347 patent permits front adjustment of the supports  21   a ,  21   b , but does so in a fashion that creates yet another disadvantage. The tuning fixture  27  is placed on the end mill  16  during a tuning operation and, once the system is tuned utilizing the tuning fixture  27 , the clamp screw  23  must be secured and the tuning fixture  27  removed and replaced with an appropriate cutting head. The tuning fixture  27  is sized such that the mass is very near to the mass of the cutting head to simulate the presence of the cutting head during a machining operation. As a result, if a different cutting head were to be used, then a different tuning fixture  27  having a different mass would be required.  
           [0012]    A design is needed to partially or totally overcome each of these disadvantages to provide a tunable toolholder that may be used for small diameter tools.  
         SUMMARY OF THE INVENTION  
         [0013]    A toolholder used for metalworking operations has a shank having a front end, back end and a longitudinal axis, wherein a cavity extends within the shank along the axis and wherein the cavity defines a cavity wall. An absorber mass is positioned within the cavity, wherein the mass has a first end and a second end. A resilient support circumscribes each end of the mass and is positioned to suspend the mass within the cavity. There is a pressure plate at each end of the absorber mass adjacent to each resilient support, wherein at least one pressure plate is movable along the longitudinal axis to compress each resilient support against the absorber mass. A toolholder head has a pocket adapted to receive a cutting tool and furthermore has a front face and a rear face, wherein the head is secured at the rear face to the front end of the shank. An adjustment screw extends through the front face of the toolholder head and is oriented to displace the movable pressure plate along the longitudinal axis within a desired range against the resilient support at the first end of the shank. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Further features of the present invention, as well as the advantages derived therefrom, will become clear from the following detailed description made with reference to the drawings in which:  
         [0015]    [0015]FIG. 1 illustrates a top view of a toolholder, which, in this instance, is a boring bar, in accordance with the subject invention;  
         [0016]    [0016]FIG. 2 is a cross sectional view of the toolholder illustrated in FIG. 1;  
         [0017]    [0017]FIG. 3 is a perspective view of a toolholder head in accordance with the subject invention;  
         [0018]    [0018]FIG. 4 is a top plan view of a boring bar wherein the head is offset relative to the shank;and  
         [0019]    [0019]FIG. 5 is a side view from arrows “V-V” in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Directing attention to FIGS.  1 - 3 , a toolholder  10  used for metal operations is comprised of a shank  15  having a front end  17  and a back end  19 . A longitudinal axis L extends along the length of the toolholder  10 . A cavity  25  extends within the shank  15  along the longitudinal axis L. The cavity  25  defines a cavity wall  27 .  
         [0021]    An absorber mass  30  is positioned within the cavity  25 . The absorber mass  30  has a first end  32  and a second end  34 . A first resilient support  40  circumscribes the first end  32  of the absorber mass  30  and is positioned against the cavity wall  27  to suspend the absorber mass  30  within the cavity  25 . A second resilient support  45  circumscribes the second end  34  of the absorber mass  30 , and is positioned against the cavity wall to suspend the mass  30  within the cavity  25 . A first pressure plate  50  is positioned at the first end  32  of the absorber mass  30  adjacent the first resilient support  40 . A second pressure plate  55  is positioned at the second end  34  of the absorber mass  30  adjacent to the second resilient support  45 . At least one pressure plate, in this instance pressure plate  50 , is movable along the longitudinal axis L to compress each resilient support  40 ,  45  against the absorber mass  30 .  
         [0022]    A toolholder head  60  has a pocket  65  adapted to receive a cutting tool  70 . The toolholder head  60  additionally has a front face  62  and a rear face  64 . The toolholder head  60  is secured at the rear face  64  to the front end  17  of the shank  15 .  
         [0023]    Directing attention to FIG. 2, an adjustment screw  75  extends through the toolholder head  60  and is oriented to displace the pressure plate  50  along the longitudinal axis L within a desired range against the first resilient support  40  at the front end  17  of the shank  15 . In turn, the absorber mass  30  will be urged against the second resilient support  45 , which is then urged against the second pressure plate  55  to provide compression of both the first resilient support  40  and the second resilient support  45  against the absorber mass  30 .  
         [0024]    The adjustment screw  75  extending through the toolholder head  60  to engage the first pressure plate  50  provides a simple adjustment mechanism, while, at the same time, the integrity of the shank  15  is maintained since no material is removed from the shank  15  to provide a bore therein for an adjustment screw. By incorporating the adjustment screw  75  in the toolholder head  60 , material is taken from the toolholder head  60  to provide the bore for the adjustment screw  75 . Additionally, by incorporating this feature into the toolholder head  60 , the fabrication and assembly of the remaining portions of the toolholder  10  are simplified.  
         [0025]    The toolholder head  60  is removably secured to the shank  15 . In particular, toward the rear face  64  of the toolholder head  60  is a threaded portion  80  that is secured within a threaded portion  85  of the cavity wall  27 . Additionally, the toolholder head  60  may further include a shoulder  82  circumferentially extending from the threaded portion  80  to abut with the front end  17  of the shank  15 .  
         [0026]    The adjustment screw  75  may be threadably engaged within a mating adjustment screw bore  90  having a bore centerline CL within the head  60 , wherein the centerline CL forms an angle A of between approximately 10 to 80° with the longitudinal axis L.  
         [0027]    As illustrated in FIG. 2, the adjustment screw  75  may enter the front face  62  of the toolholder head  60  at a location below the pocket  65 . It is also possible for the adjustment screw  75  to enter the front face  62  of the toolholder head  60  above the pocket  65 . While such a design is not illustrated in FIG. 2, it should be obvious from the details so far disclosed and visualization of such an embodiment is apparent therefrom. Furthermore, it is also possible for the adjustment screw  75  to enter the toolholder head  60  at an angle from the side of the toolholder head  60  and not directly through the front face  62 .  
         [0028]    As illustrated in FIG. 2, the adjustment screw  75  may have a flat end  77  which contacts a front surface  52 . The adjustment screw  75  may be angled to provide a flush contact between the flat end  77  of the adjustment screw  75  and the front face  52  of the first pressure plate  50 .  
         [0029]    The toolholder head  60  may have a bore open toward the back end  19 , while the first pressure plate  50 , which is adjacent to the toolholder head  60 , may have a projecting portion  100  which is matingly received and slidable within a bore  95 . The longitudinal clearances between the projecting portion  100  and the bore  95  in the toolholder head  60  should be sufficient for the desired range of motion of the first pressure plate  50  within the cavity  25  of the shank  15 .  
         [0030]    The absorber mass  30  may have at the first end  32  and the second end  34  conical surfaces  33 ,  35 , such that each resilient support  40 ,  45  circumscribes the first end  32  and the second end  34  of the absorber mass  30 . Additionally, each pressure plate  50 ,  55  may have an inwardly facing conical surface  53 ,  58 , which surrounds the first resilient support  40  and the second resilient support  45 .  
         [0031]    Each end  32 ,  34  of the absorber mass  30  may furthermore have a cylindrical protrusion  105 ,  107  extending past the conical surfaces  33 ,  35  to position and to provide additional support to the first resilient support  40  and the second resilient support  45 .  
         [0032]    To positively position the adjustment screw  75  within the adjustment screw bore  90 , the adjustment screw  75  has at least one O-ring  110  about a groove  78  in a head  79  of the adjustment screw  75 . The head  79  may be rotated using any number of conventional designs such as a hexagonal cavity  81  extending within the head  79  into which a mating wrench may be inserted. The O-ring  110  is sized such that it contacts a wall  112  of the adjustment screw bore  90  which accommodates the head  79  of the adjustment screw  75 .  
         [0033]    The resilient supports  40 ,  45  may be made of a number of resilient materials including rubber or an elastomer material. The material may be made of a Durometer A Scale  50  material.  
         [0034]    Directing attention to FIG. 3, the pocket  65  may have a threaded bore  67  extending therein to accommodate a matingly threaded screw  115  intended to fit through a central bore  72  of the cutting tool  70  mounted within the pocket  65 .  
         [0035]    While the shank  15  illustrated in FIG. 1 is cylindrical and the toolholder head  60  generally lies along the centerline L, as illustrated in FIGS. 4 and 5, it is entirely possible for the shank  215  to be non-cylindrical, and for the toolholder head  260  to be offset relative to the shank  215 . With such an arrangement, the absorber mass (not shown) would preferably remain mounted within a cavity which extends within the shank  215 . The remaining features of this embodiment are similar to those features already discussed in association with FIGS.  1 - 3 .  
         [0036]    Although this invention has been described with respect to certain embodiments, various modifications, revisions and additions will become evident to persons of ordinary skill in the art. All such modifications, revisions and additions are intended to be encompassed in the scope of this invention, which is limited only by the claims appended hereto.
A tunable toolholder with a dynamic vibration absorber wherein an absorber mass is compressed between two resilient supports utilizing at least one longitudinally movable pressure plate to dynamically tune the toolholder. The pressure plate is displaced by an adjustment screw extending through the front face of a toolholder head removably mounted at one end of the toolholder.