Abstract:
A tuned mass damper is designed for a machine tool head that moves in the X, Y, and Z directions and rotates around a C-axis that is coincident with the axis of an arm that supports the head. The damper comprises a pair of damper masses symmetrically arranged around the head. Each damper mass has a center of gravity which is approximately in the geometric center of the mass. A resilient mounting is provided for each of the damper masses allowing the damper masses to damp out vibration in the X and Y directions regardless of the orientation of the head.

Description:
FIELD 
     The device relates a pair of tuned mass dampers that are symmetrically mounted on the multi-axis head of a machine tool. 
     BACKGROUND 
     Precision machine tools often have vibration which has to be damped out in order to obtain the best performance out of the tool. In a multi-axis machine tool, the vibration of the tool is the greatest at the far end of the unsupported mass, i.e., at the head of the tool, since this is where the greatest absolute motion occurs. This is also the location where the mass of a mass damper could be minimized. However, the head of the tool is a difficult area to mount a tuned mass damper, since the space is limited and the head must move in 2 or 3 linear axes, and in 1 or 2 rotary axes. 
     SUMMARY 
     A tuned mass damper is provided for a machine tool head that has a rotary tool and moves in the X, Y, and Z directions. The head pivots around the A-axis of the head, and rotates around a C-axis that is coincident with the long axis of the arm. The damper comprises a pair of damper masses symmetrically arranged around the head. Each damper mass has a center of gravity which is approximately in the geometric center of the mass. A resilient mounting is provided for each of the damper masses allowing the damper masses to damp out vibration in the X and Y directions regardless of the orientation of the C-axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a machine tool with a multi-axis head mounted on the end of a cantilever arm. 
         FIG. 2  is a side view of a multi-axis head showing pivoting motion of the head about the A-axis  25  of the head. 
         FIG. 3  is a front view of a multi-axis head. 
         FIG. 4  is an exploded perspective view of the underside of a cover for one of the dampers for the multi-axis head. 
         FIG. 5  is a detail view showing the step of tightening the donut mounting blocks to the damper mass. 
         FIG. 6  is a detail view showing the step of loosening the donut mounting blocks on the damper mass. 
         FIG. 7  is a plan view showing the underside of the cover with the damper mass, the support rods, and the biasing bolts in place. 
         FIG. 8  is a sectional view taken along line  8 - 8  of  FIG. 7  showing the support rods in place in the damper mass. 
         FIG. 9  is a detail view of section  9  of  FIG. 7  showing the donut support block bolted in place in the cover. 
         FIG. 10  is a detail view of an alternate form of the device showing the donut support bracket, a biasing bolt and a rubber donut. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a plan view of a machine tool  10  with a multi-axis head  12  mounted on the end of a cantilever arm  14 . The multi-axis head  12  supports a tool  15  which is used to machine a workpiece (not shown) and is able to change its position and orientation relative to the arm  14  and the workpiece. The head  12  has a C-axis of rotation  16  that is in alignment with the long axis of the arm  14 . The head  12  includes a spindle  17  that is moveably mounted between two arms  18  of a fork  19  for pivoting motion about the A-axis of the head so that the spindle  17  may change the angle of the tool relative to the cantilever arm  14  as best seen in  FIG. 2 . Dampers  20  are symmetrically mounted on both sides of the fork  19  to dampen out unwanted vibration. The dampers  20  include removable covers  22  that protect the dampers from debris created during a machining operation. Safety hoist rings  23  may be provided on the covers  22  to aid in removal of the dampers  20  from the head  12 . 
       FIG. 2  is a side view of a multi-axis head  12  showing pivoting motion of the spindle  17  about the A-axis  25  of the head. The pivoting motion allows the tool  15  to change its angular orientation relative to the C-axis  16  of the arm  14  that supports the head and relative to a workpiece. 
       FIG. 3  is a front view of the multi-axis head  12  of  FIGS. 1 and 2  showing a spindle  17  mounted between the two arms  18  of a fork  19 . Dampers  20  are symmetrically mounted on both sides of the head  12 . A damper mass  26  is shown in phantom within each of the damper covers  22 . 
       FIG. 4  is an exploded perspective view of the damper  20 . The damper  20  comprises a damper cover  22  and a damper mass  26  of material such as lead. The damper mass  26  is supported in the cover by two support rods  28  that pass through elongated passages  29  formed in the damper mass as best seen in  FIGS. 7 and 8 . Each damper mass  26  has a center of gravity  30  which is located approximately midway between the support rods  28  and midway between the two ends  32  of the damper mass. Support rod mounting brackets  33  and  34  are attached to the inside of the cover  22  and the support rods  28  pass through the support rod mounting brackets. The two support rods  28  are symmetrically located around the center of gravity  30  of the damper mass and are attached to the damper mass  26  near the center of their lengths as best seen in  FIGS. 7 and 8 . One or more nuts  35  may be threaded on the ends of each of the support rods  28  to secure the rods to the support rod mounting brackets  33  and  34 . A donut support block  36  is mounted on each end of the damper mass  26  by biasing bolts  38 . Resilient mounts such as rubber donuts  39  are mounted on the biasing bolts  38  and are captured between the donut support block  36  and the end  32  of the damper mass  26 . The donut support blocks  26  are mounted to the inside of the cover  22  by support block mounting screws  53 . First sets of access holes  40  are provided in both ends of the cover  22  to allow insertion and tightening of the support rods  28  that support the damper mass  26  as best seen in  FIGS. 4 ,  7 , and  8 . Second sets of access holes  41  are provided to allow access to the biasing bolts  38  that are used to preload rubber donuts  39  against the end of the damping mass  26  best seen in  FIGS. 4-6 . Plugs  42  may be used to seal the first and second sets of access holes  40  and  41 , respectively, from dirt and debris once access to the support rods  28  and the biasing bolts is no longer needed. Biasing springs  43  and washers  44  are mounted on the ends of the biasing bolts  38  as best seen in  FIGS. 5 and 6 . 
       FIG. 5  shows a biasing bolt  38  tightened in the donut support block  36  to draw the donut support block toward the damping mass  26  to compress a compression member such as a biasing spring  43  to a solid condition. This positions the donut support block  36  a distance S 1  from the end  32  of the damper mass. 
       FIG. 6  shows a biasing bolt  38  loosened in the donut support block  36  to allow the donut support block to move away from the end  32  of the damping mass in order to release some of the compressive force exerted by the biasing spring  43  on the rubber donut  39 . This positions the donut support block  36  a distance S 2  from the end  32  of the damper mass. The distance S 2  is greater than the distance S 1 . 
       FIG. 7  is a plan view showing the damper mass  26  in the cover  22  with the support rods  28  and the biasing bolts  38  in place. The donut support blocks  36  are not yet fastened to the cover  22  and are held in place relative to the damper mass by the biasing bolts  38 . The support rods  28  are threaded through the first set of access holes  40  in one end of the cover, through the suspension rod mounting bracket  33  in the near end of the cover, and through the elongated passageways  29  in the damper mass until they reach the suspension rod mounting bracket  34  in the far end of the cover. Each support rod  28  has a threaded middle portion  48  to engage a threaded middle portion  49  of the passageway. Except for the reduced diameter threaded middle portion  49  of the passageway, the diameter of the passageway  29  is larger than the diameter of the support rods  28  to allow the support rods  28  to flex in response to motion of the machine tool head  12 . The support rods  28  are coupled to their respective damper mass  26  only at their midpoint defined by the threaded portion  48 . 
       FIG. 8  is a sectional view taken along line  8 - 8  of  FIG. 7  showing a support rod  28  supporting the damper mass  26  in the cover. A set screw  51  engages the threaded middle portion  48  of the support rod  28  to prevent the support rod  28  from rotating relative to the damper mass  26  once the final position of the damper mass in the cover  22  has been established. 
       FIG. 9  is a detail view of section  8  of  FIG. 7  showing the biasing bolts  38  holding the rubber donuts  39  and the donut support block  36  in place against the end  32  of the damping mass  26  when the damping mass is positioned in the cover  22 . Mounting screws  53  are used to fasten the donut support block  36  in place in the cover  22 . 
     To assemble the damper mass  26  into the cover  22 , the biasing bolts with the washers  44  and springs  43  are first threaded into the support blocks  36 , through the rubber donuts  39  and into the end  32  of the damper mass. The biasing screws  38  are tightened to compress the springs  43  to a solid condition as shown in  FIG. 5 , and are then loosened a desired amount as shown in  FIG. 6  to apply the required compressive force to the rubber donuts  39 . The damper mass  26  with the donut support blocks  36  attached to each end  32  is then placed into the cover  22 . The support rods  28  are threaded through the first set  41  of access holes in one end of the cover  22 , through the elongated passageways  29  in the damper mass, and are tightened through the reduced diameter threaded middle portion  49  of the passageway  29  until the damper mass is pulled tight against near side mounting bracket  33 . The support rods  28  are then loosened to center the damper mass  26  in the cover  22 . Set screws  51 , best seen in  FIG. 8 , are then threaded into the damper mass until they are in contact with the threaded portion  48  of the support rods  28  to lock the damper mass  26  in place on the support rods. Nuts  35  are locked onto the ends of the support rods  28 , and the support block mounting screws  53  are used to fasten the donut support blocks  36  in place in the cover. Plugs  42  may be placed over the first and second sets of access holes  40  and  41 , respectively, to seal the interior of the cover from dirt and debris. 
     The diameter of the support rods  28  is calculated to bring the frequency of vibration of the damper mass  26  on the support rods in line with the required damper frequency for the machine tool head  12 . In use, the support rods  28  act as springs for the damper mass  26 . Although the ends of each support rods  28  is fixed in the mounting brackets  33  and  34  in the cover, the remainder of the support rod is free to vibrate in response to the suspended weight of the damper mass  26 . The two dampers  20  are paired to create symmetry around the center of the C-axis of rotation of the head  12 , and are symmetrically located around the center of gravity of the head. The center of gravity of each damper mass  26  is centered in the Z-direction between the two ends  32  of the damper mass to prevent inducing torsional forces about the X-axis, and between the support rods  28  and the resilient mounts  39  in the Y-direction to avoid torsional forces about the Z-axis. The center of gravity of the damper mass  26  is located on the plane of the support rods  28  and the resilient mounts  39  in the X-direction to avoid torsional forces about the Z-axis. 
     Because the dampers  20  are mounted on the head  12  and move with the head, the dampers are able to damp vibrations in the X and Y directions. Because the dampers  20  are mounted in the removable covers  22 , the covers may be removed for easy access to the head without disturbing the damper set-up. The durometer of the rubber donuts  39  is selected to achieve the desired damper frequency. 
       FIG. 10  shows an alternate embodiment in which springs are not used to set a preload on the rubber donuts  39  that suspend the damper mass  26 . The biasing bolts  38  are in direct contact with the donut support block  36 , and the rubber donut  39  is positioned between the donut support block and the end  32  of the damper mass. The biasing force on the rubber donut  39  is determined by the spacing S 3  between the donut support block  36  and the end  32  of the damper mass  26 . The biasing bolt  38  holds the rubber donut  39  between the donut support block  36  and the end of the damper mass  26 . The biasing bolt  38  is tightened until the gap S 3  between the mounting bracket  36  and the damper mass  26  is a preselected amount, corresponding to the proper amount of preload being applied to the rubber donut  39 . After all four of the biasing bolts have been threaded into the damper mass  26  and the biasing bolts  38  have been tightened the required amount, the damper mass and the donut support blocks  36  are mounted in the cover  22  in the same way as described above in connection with  FIGS. 8 and 9 . After the mounting screws  53  are used to fasten the donut support blocks in place in the cover, the biasing bolts  38  are then removed from the ends of the damper mass  26  and the donut support block  36  by means of the second set access holes  41  in the ends of the cover. Although the biasing bolts  38  are removed, the rubber donuts will be held in place by the pressure between the donut support blocks  36  that are attached to the cover by the mounting screws  53  and the ends  32  of the damping mass. Plugs  42  may be used to cover the access holes  40  and  41  to prevent dirt and debris from entering the cover  22 . 
     Having thus described the device, various alterations and modifications will occur to those skilled in the art, which alterations and modifications will be within the scope of the attended claims.