Abstract:
The frame of a machine tool forms a closed force loop design that surrounds a workzone containing the spindle head. The X and Y-axis drive motors are mounted on stationary elements of the machine. The Y-axis column is fixed, and supports a Z-axis ram. The mounting and positioning of the Z-axis ram minimizes the change in droop and the effects of acceleration forces on the Z-axis structure. The X, Y, and Z-axis drive motors are all mounted outside of the workzone to shield them from contamination and debris generating during the machining process, and for ease of maintenance.

Description:
FIELD OF THE INVENTION 
     A machine tool has a closed force loop design, a fixed bifurcated Y-axis column, and a Z-slide design that optimizes the consistency of the rigidity of the machine. 
     BACKGROUND OF THE INVENTION 
     A plate mill is a type of machine tool that is used to machine large flat workpieces having a substantial length and width, but relatively little height. Because the workpiece is large, the plate mill itself is relatively large, and in large machines, rigidity and the ability to resist deformation during operation are important design considerations. In machine tools that use a Z-axis ram, it is important to keep the center of gravity of the ram as close as possible to the suspension points for the ram to minimize the effects of acceleration forces that occur during machine operation. It is also important in high performance machines that the rigidity of the machine remain as constant as possible throughout the working envelope of the machine. This allows for optimal process parameters to be utilized throughout the envelope instead of having to vary the process parameters depending on workpiece location in the workzone. In high speed machining there are stability lobes where based on the cutter tooth pass frequency and the rigidity of the system greater metal removal rates can be achieved without chatter. These stability lobes exist in fairly narrow ranges and changes in system stiffness within the work envelope can cause parameters that allow chatter free cutting in one area to cause chatter in another. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a perspective view of a machine tool according to the invention. 
         FIG. 2  is a simplified view of the X, Y, and Z-axis elements of the machine tool of  FIG. 1 . 
         FIG. 3  is a sectional view of the machine tool taken along lines  3 - 3  of  FIG. 1 . 
         FIG. 4  is a side sectional view of the machine tool taken along lines  4 - 4  of  FIG. 1 . 
         FIGS. 5 and 6  are graphical drawings showing a conventional suspension system for a Z-axis ram and the resulting load path. 
         FIGS. 7 and 8  are graphical drawings showing the C-axis droop for the Z-axis ram of  FIGS. 5 and 6 . 
         FIGS. 9 and 10  are graphical drawings showing a suspension system for a Z-axis slide with a front mounted saddle and the resulting load path. 
         FIGS. 11 and 12  are graphical drawings showing a suspension system for a Z axis slide with a middle mounted saddle and the resulting load path. 
         FIGS. 13 and 14  are graphical drawings showing the C-axis droop for the Z-axis slide of  FIGS. 11 and 12 . 
     
    
    
     BRIEF SUMMARY OF THE INVENTION 
     The frame of a machine tool is configured to form a closed force loop design that surrounds a workzone containing the spindle head. The front of the loop comprises a fixed Y-axis bifurcated column and the back of the loop comprises a fixed X-axis frame. The top and bottom of the loop is formed by structural tubes that tie the Y-axis column and the X-axis frame together. A pallet receiver that supports the workpiece is mounted to move on X-axis rails. A vertically movable saddle is mounted near the center of Z-axis stroke on the Y-axis column and carries a Z-axis slide. The support of the saddle on the Y-axis column and of the Z-axis slide on the saddle maintains the load path for the working tool relatively constant throughout its stroke, and adds to the rigidity of the machine. 
     The X, Y, and Z-axis drive motors are all mounted outside of the workzone. The X-axis drive is mounted on a fixed wall that is attached to an X-axis frame member. The Y-axis drive is mounted on the fixed Y-axis column on the side of the Y-axis column that is opposite the workzone. The Z-axis drive is mounted on a saddle on the opposite side of the Y-axis column from the workzone. The positioning of the X and Y-axis drives on a stationary part of the machine adds to the rigidity of the machine, and eliminates the need for flexible cables to power and control these drives. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a machine tool generally designated by the reference numeral  10 . The machine tool is surrounded by standard guarding  12 , and an operator station  14  is positioned outside of the guarding. The machine tool receives a pallet  15  with a workpiece from a pallet manipulator  17  that may be positioned adjacent to a pallet access opening  18  in the guarding. In operation, the pallet  15  is transferred from the pallet manipulator  17  to a pallet receiver  19  that is a part of the machine. The pallet receiver  19  is then driven to the working zone of the machine in front of the spindle and the working tool. 
       FIG. 2  shows the X, Y, and Z-axis elements of the machine tool. The pallet  15  with a workpiece  16  is positioned in front of a spindle or multi-axis head  23  that carries the working tool  24  and this establishes a workzone. The Y-axis column  25  is fixed and is bifurcated. As shown in  FIG. 2  and also in  FIG. 3 , the Y-axis column  25  carries a vertically movable saddle  26  that is mounted in a center opening  21  between the two sides of the bifurcated column  25  on vertical linear guides or ways  27  best seen in  FIG. 3 . Although not separately shown, feedback sensors for the vertical position of the saddle are also located adjacent to the ways  27 . A servomotor  28  is mounted on each side of the Y-axis column  25 , and each servomotor  28  is coupled to a drive screw  29 . The drive screws  29  engage drive nuts  31  on opposite sides of the saddle  26 , and the servomotors  28  are used to raise and lower the saddle to the desired vertical position. 
     The vertically movable saddle  26  carries a Z-axis slide  32 . Bearings mounted on the Z-axis slide  32  support the slide on bearing ways  34  that are mounted on the saddle. A Z-axis drive assembly  36  comprises a servomotor  37  and a drive screw  38  that are mounted on the saddle  26 . The Z-axis drive assembly  36  may be selectively controlled to position the Z-axis slide  32  and the working tool  24  in the desired position along the Z-axis. As used herein, the term Z-axis slide is used to designate a structure in which the bearings are mounted on the slide  32  and the bearing ways  34  are mounted on the saddle  26 . This is to distinguish the structure from a Z-axis ram in which the bearings are mounted on the saddle and the ways are mounted on the ram. 
     X-axis frame members  40  and  44  support an X-axis wall  42 , and a plurality of X-axis rails  41  are mounted on the X-axis wall  42 . The pallet receiver  19  is mounted on the X-axis rails  41  for horizontal movement along the X-axis. One or more X-axis drive motors  43  shown in phantom are mounted on the X-axis wall  42  to drive the pallet receiver back and forth along the X-axis rails. The X-axis frame member  40  is coupled to the Y-axis column  25  by upper and lower tubular frame members  46  and  47 , respectively, to form a rigid closed force loop design. 
       FIG. 3  is a sectional view of the machine tool taken along lines  3 - 3  of  FIG. 1  showing certain elements of the machine  10  in greater detail. Two Y-axis ways  27  that guide the vertical movement of the saddle  26  are positioned on the bifurcated column  25  on either side of the center opening  21 . Two Y-axis flexible cable guides  51  are provided to carry electrical and hydraulic cables and the like from the stationary part of the Y-axis column  25  to the movable saddle  26 . One or more Z-axis flexible cable guides  52  are provided to carry electrical and hydraulic cables from the saddle  26  to the Z-axis slide  32 .  FIG. 3  shows the X-axis rails  41  that extend from one side of the machine to the other to support the pallet receiver  19  and to position the pallet  15  in front of the working tool  24 . In this view, the pallet  15  and the receiver  19  are centered in front of the Y-axis column  25 . 
       FIG. 4  is a side sectional view of the machine tool taken along lines  4 - 4  of  FIG. 1 .  FIG. 4  shows that the drive screws  29  that move and support the saddle  26  are positioned at approximately the midpoint of the saddle measured along the Z-axis. Allowing for the mass and overhang of the spindle head  23 , this positioning of the drive screws  29  relative to the saddle  26  allows the center of gravity  30  of the combination of the saddle  26  and the Z-axis slide  32  to be maintained in relative proximity to the drive screws  29 . On high acceleration and deceleration machines, the acceleration forces become a greater concern than the cutting forces. These forces can be considered to act at the center of gravity of the moving mass. The greater the distance from the center of gravity to the drive system, the greater the moment load that is added to the system and therefore the deflection caused during acceleration. The greater the distance from the center of gravity to the way system, the greater the moment that is exerted on the bearings, again causing greater deflection. The greater the distance from the feedback sensors to the drive system, the greater the Abbe Error (angular error) that will be seen in the linear feedback which will decrease the electronic stiffness of the servo system. The Z-axis slide  32  is fitted with bearing trucks or carriages  33  that ride on the ways  34  that are mounted on the saddle. 
       FIG. 4  shows the two Z-axis cable guides  52  that carry electrical and hydraulic cables from the saddle  26  to the Z-axis slide  32 .  FIG. 4  also shows the X-axis wall  42  that extends along the back of the workzone and is supported by the X-axis frame members  40  and  44 . A plurality of X-axis rails  41  are mounted on the wall  42 . The pallet receiver  19  is mounted on the X-axis rails  41  for horizontal movement along the X-axis. The X-axis drive motors  43  (only one shown) are used to drive the pallet receiver  19  back and forth along the X-axis rails  41 . 
     The Z-axis slide  32  carries the spindle  22 , the head  23 , and the tool  24 , and is movably mounted on the saddle  26  to travel in the Z-axis direction. Since the Z-axis slide  32  is relatively slender (its length is much greater than its width and height) and the working element or tool  24  overhangs from the support point of the bearing trucks or carriages  33  on the Z-axis slide on the ways  34 , placing the Z-axis ways  34  on the saddle  26  and the bearing trucks or carriages on the Z-axis slide  32  causes the droop and stiffness of the constant overhang Z-axis slide to be more constant than with a conventional ram design in which the guide ways are mounted on the ram and the bearing trucks or carriages are mounted at a fixed location on the saddle. Drawing  FIGS. 5-14  illustrate these principles and are explained in greater detail below. 
       FIGS. 5 and 6  show the change in the load path  55  in a conventional ram  56  when the bearings  57  for the ram  56  are mounted on the saddle  58  and the ways  59  are mounted on the ram.  FIG. 5  shows the ram  56  fully extended in the Z-axis direction. The length of the load path  55  measured from the midpoint  61  of the suspension point for the saddle  58  on the Y-axis ways  62  to the tip  63  of the ram  56  is nominally taken to be 1.0.  FIG. 6  shows the ram  56  fully retracted. The length of the load path  55  measured from the midpoint  61  of the suspension point for the saddle  58  on the Y-axis ways to the tip  63  of the ram is 0.5, half the load path length shown in the configuration of  FIG. 5 . 
       FIGS. 7 and 8  show how the angle or droop of the C-axis (the longitudinal axis of the ram  56 ) changes as the load path changes. In  FIG. 7 , with the ram  56  fully extended, the angle α of the C-axis is significant. In  FIG. 8 , with the ram  56  fully retracted, the angle α is zero. Thus, when using a ram with this design, the Z-axis ram stiffness and the C-axis angle vary with the position of the Z-axis ram  56  relative to the saddle  58 . 
     When the bearings trucks move with the Z-axis element and the ways are mounted on the supporting structure, the Z-axis element is called a slide.  FIGS. 9 and 10  show the change in the load path  65  in a slide  66  when the bearings  67  are mounted on the slide  66 , the ways  68  are mounted on the saddle  69 , and the saddle  69  is suspended from one end.  FIG. 9  shows the slide  66  fully extended in the Z-axis direction. The length of the load path  65  measured from the midpoint  61  of the suspension point for the saddle  69  on the Y-axis ways  62  to the tip  71  of the slide  66  is nominally taken to be 1.0. Although not shown, the load path length for the slide  66  at mid-stroke is 1.25.  FIG. 10  shows the slide  66  fully retracted. The length of the load path  65  measured from the midpoint  61  of the suspension point for the saddle on the Y-axis ways  62  to the tip  71  of the slide is 1.5, a change of fifty percent from the length of the load path  65  shown in the configuration of  FIG. 9 . 
       FIGS. 11 and 12  show the change in the load path for a Z-axis slide  66  in another suspension arrangement in which the bearings  72  for mounting the saddle  73  to the Y-axis ways  62  are positioned in the center of the saddle  73  instead of at one end.  FIG. 11  shows the slide  66  fully extended in the Z-axis direction. The length of the load path  75  measured from the midpoint  61  of the suspension point for the saddle on the Y-axis ways  62  to the tip  71  of the slide is nominally taken to be 1.1. Although not shown, the load path length for the slide at mid-stroke is 0.9.  FIG. 12  shows the slide  66  fully retracted. The length of the load path  75  measured from the midpoint  61  of the suspension point for the saddle on the Y-axis ways to the tip  71  of the ram is 1.1, the same as the load path length shown in  FIG. 11 . 
       FIGS. 13 and 14  show how the angle of the C-axis changes as the load path changes with the suspension arrangement shown in  FIGS. 11 and 12 . A comparison of  FIGS. 13 and 14  shows that the overhang of the slide  66  remains constant as the slide moves from a fully extended to a fully retracted position on the saddle  73 , and as a result, angles α 1 , and α 2  are substantially the same with the slide  66  in the two positions. 
     A comparison of the change in the load path length in the examples shown in  FIGS. 5 ,  9 , and  11  shows that the load path changes the least with the suspension arrangement shown in  FIGS. 11-14 . It will be appreciated by those skilled in the art that minimizing the change in length of the load path minimizes the change in the angle of the C-axis of the working tool, thus increasing the consistency of the rigidity of the tool, and improving the mechanical accuracy that can be maintained by the tool over its range of operation. 
     Because of the fixed overhang of the Z-slide design as shown in  FIGS. 13 and 14 , the drives, bearings and external linear feedback for the Y-axis servo system can be positioned behind and near the center of gravity  77  of the saddle  73  and the Z-axis slide structure throughout the stroke of the Z-axis slide  66 . This keeps the center of gravity of the Y-axis moving mass as close as possible to the Y-axis drive screws  29  and Y-axis ways  34  to minimize twisting moments in the saddle structure caused by acceleration forces in the Y-axis direction. The fixed overhang of the Z-axis slide  66  also minimizes variation in length of the load path  75  throughout the Z-axis stroke as shown in  FIGS. 11 and 12 . The load path length variation is roughly proportional to the stiffness variation. This location of the Y-axis supporting elements for the saddle also allows the Y-axis column structure  25  in the preferred configuration to be placed in front of the Y-axis drive components  28 ,  29 , and  31 , the Y-axis ways  49 , and feedback systems. This in turn allows the maintenance removal of the entire saddle, the Y-axis drives, the Y-axis ways and linear feedback from the outside of the workzone, greatly improving maintainability of the systems. Furthermore, the location of these elements on the side of the Y-axis column that is opposite the workzone also makes these sensitive systems less likely to be contaminated by chips and cutting fluids from the workzone. 
     Having thus described the invention, various alterations and modifications may be apparent to those skilled in the art, which modifications and alterations are to be considered to be within the scope of the invention as defined by the appended claims.