Abstract:
A tunable toolholder with a dynamic vibration absorber is disclosed wherein an absorber mass is compressed between two elastomer supports utilizing at least one longitudinally movable pressure plate to dynamically tune the toolholder.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a tunable toolholder for suppressing vibrations caused in machining processes and, more particularly, to a tunable toolholder which utilizes a dynamic vibration absorber to suppress vibrations. 
     2. Description of the Prior Art 
     During a metal cutting operation, any vibratory motion between a cutting tool and 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 machine tool to become damaged. 
     To reduce these vibrations, the metal removal rate can be decreased. However, this approach interferes with production and only minimally reduces the amount of vibration. 
     Attempts to eliminate the vibration in the boring bar may also 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 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. 
     A further attempt to reduce vibration in boring bars includes 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 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 which simultaneously shifts the position of the mass and alters the stiffness of the rubber bushings to change the dynamics of the cylindrical mass. 
     However, even with such a design available, each time the boring bar is to be used under different conditions, it must be tuned using sophisticated equipment that may or may not be available on the shop floor. 
     U.S. Pat. No. 3,774,730 generally identifies the design of a tunable toolholder with a dynamic vibration absorber, however, this toolholder also must be tuned each time it is used under different conditions using equipment that may or may not be available on the shop floor. 
     Therefore, an object of the subject invention is to provide a tunable boring bar with a dynamic absorber capable of reliably suppressing vibration, and capable of being tuned without the need to employ sophisticated equipment each time the cutting conditions change. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method for tuning a toolholder having a diameter D, wherein the toolholder may be supported on a metalworking machine at different lengths along the tool to define different length to diameter ratios. The toolholder has a shank with a longitudinal axis and a central cavity extending within the shank along the axis. The central cavity defines a cavity wall. The toolholder also has a toolholder head adapted to receive a cutting tool. The head is attached, either as a separate piece to or integral with, the shank at a tool end of the shank. Additionally, the toolholder has a tunable absorber with an absorber mass inserted within the central cavity. The mass has a first end, a second end and an elastomer support circumscribing each end. Finally the toolholder has a pressure plate at each end of the absorber mass adjacent each elastomer support, wherein at least one pressure plate is movable along the longitudinal axis to compress the elastomer supports against the absorber mass. The method is comprised of the steps of: 
     a) positioning the at least one movable pressure plate to a reference location in which the compression of each elastomer support is known, 
     b) supporting the shank on the metalworking machine at a first length L 1  from the end of the shank to define a first length to diameter ratio, and 
     c) moving the pressure plate from the reference location to a predefined first tuned location to adjust the compression upon each elastomer support thereby minimizing vibration for the tool supported at the first length to diameter ratio. 
     The invention is further directed to a tunable toolholder having an outside diameter D and which may be supported on a metalworking machine at different lengths L along the tool length to define different length to diameter ratios. The toolholder has a shank with a longitudinal axis, wherein a central cavity extends within the shank along the axis and wherein the central cavity defines a cavity wall. The toolholder also has a toolholder head adapted to receive a cutting tool and attached, as a separate piece or integral, to the toolholder at a tool end. The toolholder also has a tunable absorber having an absorber mass inserted within the central cavity, wherein the mass has a first end, a second end and an elastomer support circumscribing each end of the shank. Additionally the toolholder has a pressure plate at each end of the absorber mass adjacent each elastomer support, wherein at least one pressure plate is movable along the longitudinal axis to compress the elastomer supports against the absorber mass. Additionally, the toolholder has a positioning element for displacing the movable plate from one location to another along the longitudinal axis. Finally, the toolholder has tuning indicia for indicating the position along the longitudinal axis of the at least one movable pressure plate. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention, along with the object and advantages thereof, will be more apparent with the consideration of the detailed description read in conjunction with the accompanying drawings in which: 
     FIG. 1 is prior art and illustrates a cross-sectional view of a toolholder; 
     FIG. 2 illustrates an alternate embodiment of the enlarged sectional portion labeled as II in FIG. 1 in accordance with one embodiment of the subject invention; 
     FIG. 3 illustrates a top view of the portion highlighted by arrows  3 — 3  in FIG. 1 in accordance with a second embodiment of the subject invention; 
     FIG. 4 illustrates a sectional view of an alternate embodiment of detail “AA” in FIG. 1; 
     FIG. 5 illustrates a sectional view of another alternate embodiment of detail “AA” in FIG. 1; and 
     FIG. 6 illustrates a sectional view of yet another alternate embodiment of detail “AA” in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a toolholder  10  which for purposes of this discussion will be a boring bar used for boring deep holes in work pieces. 
     FIG. 1 illustrates a prior art boring bar  10  which is one type of toolholder addressed by the method of the subject invention. Modifications of this design will be discussed and will provide embodiments of the subject invention directed to an apparatus. 
     A cutting tool, such as a cutting insert  15 , may be mounted in a conventional manner to a boring bar head  20  attached to the boring bar shank  25  at one end  27  of the shank. Use of the boring bar  10  in a metalworking operation will produce vibrations which travel through the boring bar  10  affecting the stability of the cutting process. For this reason, the boring bar  10  is provided with a tunable absorber  30  which will dampen the frequency vibration generated in the boring bar  10 . 
     The boring bar  10  has a central cavity  35  extending inwardly from the boring bar end  27  to a cavity base  36 . The boring bar  10  is supported at end  45  of the shank  25 . 
     The boring bar  10 , in accordance with the subject invention, may have an outside diameter from three-eighths to nine inches. 
     The tunable absorber  30  is comprised of a generally cylindrical absorber mass  50  having a first end  55  with an end portion  57  and a second end  60  with an end portion  62 . The absorber mass  50  is inserted within the central cavity  35  of the boring bar shank  25 . Each end portion has an outwardly facing conical surface  58 , 61  respectively. The conical surfaces  58 , 61  form with a line parallel to the longitudinal axis L an angle A of between 40-90 degrees. First elastomer support  65  and second elastomer support  70  circumscribe the conical surface  58  on the first end  55  and the conical surface  61  on the second end  60 , respectively, of the absorber mass  50 . A first pressure plate  75  and a second pressure plate  80  are positioned within the central cavity  35  proximate to the ends  55 , 60  of the absorber mass  50 . The first pressure plate  75  has an inwardly facing conical surface  77  while the second pressure plate  80  also has an inwardly facing conical surface  82 . The conical surface  77 , 82  form with a parallel to the longitudinal axis L an angle B of between 40-90 degrees. 
     Each pressure plate  75 , 80  surrounds an elastomer support  65 , 70  such that the inwardly facing conical surfaces  77 , 82  of the pressure plates  75 , 80  urge each elastomer support  65 , 70  against the respective conical surface  58 , 61  of the first end  55  and the second end  60  of the absorber mass  50 . 
     The first pressure plate  75  is movable within the central cavity  35  along the longitudinal axis L. A positioning member  85 , such as an adjusting screw, may be used to adjust the compression of the elastomer supports  65 , 70  against the absorber mass  50 . As a positioning member, the adjusting screw  85  extends through a bore  90  from the outer surface of boring bar  10  to contact the first pressure plate  75 . The adjusting screw  85  is threadably mated with the bore  90  such that the rotation of the adjusting screw  85  at the screw head  87  urges the contact end  89  of the adjusting screw  85  against or away from the first pressure plate  75  thereby displacing the first pressure plate  75  along the longitudinal axis L to increase or decrease the compression of the elastomer supports  65 , 70 . 
     To tune the subject boring bar  10  it has, in the past, been necessary to monitor the vibration of the boring bar  10  and tighten or loosen the adjusting screw  85  thereby adjusting the pressure of the elastomer supports  65 , 70  against the absorber mass  50 . However, this approach becomes cumbersome and the Applicant realized it is possible to predefine the amount of compression necessary on the elastomer supports against the absorber mass to minimize vibration under different tool conditions. In this manner, a machine operator may simply adjust the compression of the elastomer supports  65 , 70  to predetermine levels for tuning. 
     Specifically, the Applicant has discovered a method for tuning a toolholder comprised of the following steps. The at least one movable pressure plate  75  is positioned to a reference location in which the compression of each elastomer support  65 , 70  is known. The shank  25 , which has a diameter D, is supported on a metalworking machine at a first length L 1  from the tip of the cutting insert  15  to define a first length to diameter (L/D) ratio. The pressure plate  75  is then moved from the reference location to a predefined first tuned location to adjust the compression upon each elastomer support  65 , 70  thereby minimizing vibration from the toolholder supported at the first length L 1 . 
     The method may be further comprised of the step of supporting the shank  25  on the metalworking machine at a second length L 2  from the end  27  of the shank  25  to define a second L/D ratio. The pressure plate  75  is then moved to a predefined second tuned location to adjust the compression upon each elastomer support  65 , 70  to a second tuned location thereby minimizing vibration for the toolholder  10  supported at the second length. 
     The reference location may be any position of the moveable pressure plate  75  in which the compression of the elastomer supports is known. As an example, the reference location may be defined by compressing each elastomer support  65 , 70  an amount between 5% and 30% of the elastomer uncompressed width between the pressure plates  75 , 80  and the ends  55 , 60  of the tunable absorber mass  50 . Under these circumstances, moving the pressure plate  75  from the reference position to the first tuned location comprises increasing the compression of the elastomer supports  65 , 70 . In an alternative embodiment the reference location is defined by compressing each elastomer support  65 , 70  an amount greater than 70% of the elastomer uncompressed with between the pressure plate  75  and the ends  55 , 60  of the absorber mass  50 . Under these circumstances, the pressure plate  75  may be moved from the reference location to the first tuned location by reducing the compression on the elastomer supports  65 , 70 . 
     The amount of compression of the pressure plate  75  upon the elastomer supports  65 , 70  is determined by the location of the pressure plate  75  along the longitudinal axis L. 
     There are different methods in which to identify the location of the pressure plate  75  along the longitudinal axis L and one method involves the use of a hole which extends through the cavity wall  35 . 
     One such hole may be the locking screw hole  97  into which the locking screw  95  is positioned. The locking screw  95  is generally used to secure the movable pressure plate  75  in any of a number of different positions. However, in order to have a clear view through this hole  97  it is necessary to remove the locking screw  95 . This provides a view of the movable pressure plate  75  so that the longitudinal position of the pressure plate  75  may be visually determined, which will reveal the amount the elastomer supports  65 , 70  are compressed. To prevent the movable pressure plate  75  from displacing the adjusting screw  85 , the locking screw  95  is radially urged against the movable pressure plate  75  thereby securing it in one location. 
     In the alternative, illustrated in FIG. 2, a pin  105  may extend radially outwardly from the pressure plate  75  through a slot  110  in the cavity wall  39  of the shank  25 . By the longitudinal position of the pin  105 , it is possible to visually determine the longitudinal location of the pressure plate  75 . 
     In the alternative the adjusting screw  85  may be turned a predetermined amount to move the pressure plate  75  to a desired location. More specifically and with reference to FIG. 3, the screw head  87  of the adjusting screw  85  may have radial markings  88  associated with matching radial markings indicated by letters A, B, C, D in FIG.  3 . The adjusting screw  85  is turned a predetermined amount based upon these marks to move the pressure plate  75  to a desired location thereby achieving a desired compression of the elastomer support  65 , 70 . 
     With the understanding that by displacing the movable pressure plate  75  a predetermined amount, it is possible to tune the toolholder  10  under different L/D ratios, then different mechanisms may be used to impart such displacement. Such mechanisms may include a hydraulic piston or a rack and pinion gear arrangement which design is known to those skilled in the art. 
     Furthermore, while adjusting screw  85  has been discussed as the primary mechanism for displacing the movable pressure plate  75 , other devices are also possible. Directing attention to FIG. 4, a wedge  120  is radially positioned within a hole  125  extending through the cavity wall  39  of the shank  25 . A ramp  122  on the wedge  120  longitudinally displaces an adjusting ball  130  against the surface of the movable pressure plate  75 , thereby again urging the movable pressure plate  75  against the elastomer supports  65 , 70 . 
     FIG. 5 illustrates an alternate embodiment for displacing the movable pressure plate  75 . In particular, concentric threads  133  on shaft  135  are engaged by threads  140  of a bolt  145  to provide rack and pinion arrangement. 
     FIG. 6 illustrates yet another alternate embodiment for displacing the movable pressure plate  75 . In particular, the shaft  155  is displaced by a piston  160  which has an adjacent hydraulic chamber  165  that may be pressurized with hydraulic fluid to displace the piston  160  and consequently displace the shaft  155  and the pressure plate  75 . 
     Since the purpose of displacing the movable pressure plate  75  has been to adjust the compression of the elastomer support  65 , 70  upon the absorber mass  50 , it is also possible to monitor the longitudinal force exerted by the movable pressure plate  75  against the elastomer support  65 , 70  and to laterally displace the pressure plate  75  in accordance with the desired force. This may be accomplished by attaching to the pressure plate  75  a force-measuring device such as a transducer and to tighten or loosen the adjusting screw  85  in accordance with the force revealed by the transducer. Such a transducer may, for example, be mounted between the adjusting ball  130  and the plate  75  in FIG.  4 . Additionally, the force may be measured by monitoring the torque applied to the adjusting screw  85  using, for example, a torque wrench. 
     A guide pin  98  is threadably secured within the shank  25  and engages a surface of the pressure plate  75  to limit rotations of the pressure plate  75  about the longitudinal axis L. FIG. 2 indicates a pin  105  which may be visually monitored to determine the position of the pressure plate  75  and FIG. 3 illustrates calibrated radial markings  115  used to determine the position of the pressure plate  75 . Each of these may be generally referred to as tuning indicia for indicating the position along the longitudinal axis L of the at least one movable pressure plate  75 . 
     Under certain circumstances, the boring bar  10  may be oriented such that the longitudinal axis L is in a vertical direction. In order to compensate for the weight of the absorber mass  50 , a resilient support, illustrated by spring  127 , may be inserted between the absorber mass  50  and the pressure plate  80  such that the entire weight of the absorber mass  50  is not directly against the lowermost elastomer support. By providing such a spring, the force upon both elastomer supports  65 ,  70  may be equal when the movable pressure plate  75  is used to compress these supports. 
     The absorber mass  50  may be made of carbide or any other material which preferably has a density greater than that of steel. Additionally, the elastomer supports  65 , 70  may be made of an elastomer material having a Durometer A Scale  50  material. 
     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.