Patent Publication Number: US-6217496-B1

Title: Symmetrical multi-axis linear motor machine tool

Description:
This is a continuation of prior application Ser. No. 08/920,823, filed Aug. 29, 1997 , U.S. Pat. No. 5,938,577 which in turn is a continuation of application Ser. No. 08/440,416, filed May 12, 1995, and issued as U.S. Pat. No. 5,668 568. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to machine tools driven by linear motors along a plurality of axes and, more particularly, to a machine tool supported in a frame and driven by linear motors for high speed movement of the tool along at least three (3) mutually transverse axes. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to a machine tool having three (3) axes of movement and a working element moved along these axes by linear motors. The working element is capable of moving in a work zone. The present invention is thus applicable to various machines of this type; but for sake of brevity, the invention will be described in connection to a cutting tool machine having a cutting tool. However, the invention is not to be limited as to only covering cutting tool machines. U.S. Pat. No. 5,368,425 discloses a linear motor-driven, cutting tool machine, and as present in numerous conventional screw-driven cutting tool machines, a vertical column is mounted on a slide to travel in a horizontal X-axis direction and a vertically-movable slide is slidable vertically along the column in the Y-axis direction. A spindle carrying a tool is mounted on the slide for sliding horizontally in a Z-axis direction at a location spaced above the X-axis and normal to both the X and Y axes, respectively. Each of these three axes is typically stacked one on another, resulting in the axis driving forces being offset from the spindle and cutting tool, e.g., a milling cutter, for cutting a piece of steel. This asymmetrical arrangement of the driving forces relative to the axis of the cutting tool results in deflections in the machine structure and inaccuracies in the cutting process. This is true whether the driving force is supplied by a servomotor and ball screw/nut combination or by way of linear motor drives, as in the &#39;425 Patent. This asymmetrical arrangement results in: 1) structural deflections due to working forces or acceleration/deceleration forces, 2) in bending as experienced in the cantilevered load of the spindle slide and spindle, and 3) in less stiffness per unit of mass due to the large mass needed to achieve the necessary stiffness. 
     U.S. Pat. No. 5,368,425 discloses a cutting tool machine having its large upright column mounted for traversing in the X-direction along a large stationary, heavy base that supports the column. The driving force from the linear motors is applied only to the lower end of the column to propel it in the X-axis direction. The large stationary base provides the horizontal alignment of the respective X and Z-axis movements of the column and of the spindle slide and its tool spindle that travel vertically in the Y-axis direction along a vertical wall of the massive column. The spindle travels in a direction of the Z-axis and is carried on a cantilevered or overhung slide that travels along the vertical column wall. Thus, the machine alignment is predicated on the foundation remaining stable. If a corner of the foundation sinks or becomes misaligned, then the horizontal axes will be misaligned. Because of the space needed to support the column at its bottom end, and the size of the tool-carrying slide, the tool spindle and tool could not reach the bottom of a workpiece easily. To overcome this, the foundation required a pit to be dug to lower the spindle to reach a bottom portion of the workpiece to machine the same. Such foundation pits are expensive, as are the large and often custom-sized, foundations for the kind of machine tools illustrated in this patent. Further, adding to the size of the foundation and to the size of the machine, per se, are the covers for the ways which are bellow-shaped and which are located at the ends of the ways. These bellow-shaped covers are located at positions on the machine base at the ends of the column travel in the X-axis; and hence, add to the overall length footprint of the machine. The covers keep chips and metallic dust from the ways and from being magnetically attracted to the linear motor parts. 
     To move the massive column in the X-axis in the machine disclosed in the aforesaid patent, there is a pair of linear motors with one-half of each linear motor being mounted on the stationary base and the other half being mounted on the X-axis carrier supporting the column. The one-half of each of these two motors on the column adds to its overall weight, and thereby requires more linear motor thrust for the column to be moved along the X-axis. In addition to this linear motor weight on the column, there is an attractive, magnetic force, i.e., a normal magnetic force component which, in this instance, is a downward magnetic force between the coils and magnets of these linear motors, e.g., 24,000 lbs. of downward force that causes an increase in friction that must be overcome to traverse the column-supporting carrier along the X-axis. If the column weighs 8,000 pounds, and a normal 24,000 lb. attractive, magnetic force is present, the latter adds significantly to the mass that needs to be overcome. The linear motors apply the thrust only to the bottom of the column. To resist deflection of the unsupported upper end of the column as the lower end accelerates, the column is formed with heavy structural members that add to the weight of the column. Hence, the linear motor force required to move the column at accelerations of one G or more is increased significantly from the force needed if the column were not driven only at its lower end, and if large no such attractive, magnetic, normal force was present. 
     Generally speaking, at the present time, it takes about one pound of thrust from the linear motor to move about one pound of mass. The larger the thrust needed, the larger linear motor weight that must be added to the column. Stated differently, the effect is cumulative because the more force needed to be obtained from the linear motors, the heavier the linear motor is in weight, e.g., the coils on the column carrier and this weight increase adds more weight requiring more thrust. 
     In the linear motor cutting machine, disclosed in the aforesaid patent, the massive column is constructed of lightweight materials and has a skeletized structure in order to reduce its weight; and thereby, reduces the linear thrust force needed to accelerate it rapidly and to reduce deflection of the upper end of the column relative to the lower end of the column, which is being propelled by the linear motors. The column is formed with an aluminum braced, trapezoidal-shaped frame and with an aluminum skin covering the skeleton frame and with a reinforcing ladder of frame elements thereon to add rigidity and stiffness to the column. Because one is machining metals, a high degree of stiffness is needed for the column which carries the cutting tool slide and spindle in order to obtain the precision needed for the cut workpiece surfaces. 
     In the aforesaid patent, the spindle-carrying slide was overhung or cantilevered on the column to move vertically along a Y vertical axis. The column carried on three sides thereof, permanent magnets for three (3) linear motors used to accelerate and decelerate the spindle slide. The stator coils of the motors were on three sides of this slide and located closely adjacent to and traveling along the three, associated vertical rows of permanent magnets on the column. Three linear motors were needed to accelerate and decelerate the spindle slide vertically, and these three linear motors generated normal attractive, magnetic forces directed inwardly along three sides of the column. The spindle-carrying slide was cantilevered on the column and the weight of the ram and spindle as well as the applying of the thrust only to the side of the slide adjacent the column could cause bending and deflection and sufficient structural members had to be used in the slide to offset any such bending or deflection. The three linear motors not only applied their thrust to only one side of the slide adjacent the column wall, but these linear motors also increased the friction between the spindle slide and the column that had to be overcome. Thus, if 18,000 lbs. of normal attractive, magnetic force existed between the column and the ram, this increased frictional force generated thereby had to be overcome to accelerate the spindle slide in the Y-axis direction and the slide structure had to be reinforced with structural members to resist bending and deflection of the remote end of the cantilevered slide as the motor thrust was applied to the near side of the slide at the column wall. 
     The size and weight of this spindle slide were quite large in order to support the three rows of coils for the three Y-axis motors and to provide a stiff, cantilever support for the slide and spindle thereon to provide the cutting tool with the necessary stiffness against the workpiece. The spindle slide, illustrated in this patent, had a ribbed structure that was quite large in cross-section and carried an outer aluminum skin about the ribs. The large size of the spindle slide meant that the tool could not be brought down as low as desired relative to the workpiece because the spindle slide&#39;s height below the spindle limited the amount of downward travel of the spindle. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a machine tool for advancing a rotary cutting tool along at least three (3) mutually transverse axes and driven by linear motors is provided, which overcomes the aforementioned problems of the prior art. 
     In accordance with the present invention, the machine tool has linear motor drives distributed symmetrically relative to at least two axes to minimize deflections that cause cutting process deviations and inaccuracies. By having the machine elements driven symmetrically on opposite sides of the cutting spindle axis, there has been achieved a reduction in structural deflection, a lighter weight machine having a high stiffness per mass unit, a balancing or cancelling of normal magnetic forces that would increase friction, and an elimination of bending from a cantilevered load. In accordance with the present invention, the structural member or gantry driven in the X-axis direction is driven by linear motors at both the upper and lower ends thereof, rather than only at the bottom thereof; and the spindle-carrying slide is driven in the Y-axis direction by linear motors on opposite sides of the slide rather than only at one side of the slide. By balancing these driving forces on opposite sides of the movable X and Y-axis structures, rather than applying these driving forces at one side of the movable X and Y-axis structures, as in the machine of the aforesaid patent, the structures may be of lighter weight construction by eliminating structural members added to resist deflection or bending of the side remote from the force application side. Hence, the present invention provides lighter weight X and Y-axis structures relative to the massive weights of the vertical column and of the cantilevered spindle slide, and reduces the friction by cancelling the effect of attractive forces. The X and Y linear motors are symmetrically arranged on opposite sides of the tool-carrying spindle to drive the X and Y moving machine elements with forces applied symmetrically on opposite sides of the tool-carrying spindle; and this results in a minimum of structural deflection from the motor attractive forces, acceleration forces and the cutting forces. This symmetrical machine structure eliminates much of the bending experienced in the asymmetrical machine structure from the cantilevered loads. Preferably, the linear motors are distributed optimally throughout the machine structure to balance loads and forces about the tool-carrying spindle, thereby minimizing the cutting deviations and inaccuracies. By eliminating the need for many structural members used in the asymmetrical machines to resist bending and/or deflection, the symmetrical machine can be lighter in weight with a higher stiffness per unit of mass. Thus, it is possible to achieve higher accelerations and decelerations with less linear motors. 
     In accordance with the present invention, the linear motor machine includes a slide-carrying gantry in which the linear motors for moving the gantry are mounted on opposite sides thereof to provide equal propelling forces on two sides of the gantry and to provide an attractive magnetic force balancing principle in which a reduced net downward force exists between the gantry and the underlying frame support member. Preferably, this is achieved by having the linear motor drive on one side of the gantry, e.g., the bottom side, pushing on the bottom side of the gantry in the X-direction and attracting downwardly and by having the linear motor drive on the other top side, pushing on the top side of the gantry in the X-direction and attracting oppositely, e.g., upwardly. This is in contrast to the aforementioned machine in which the linear motors for shifting column all push on the lower end of the column and all attract in the same downward direction. Likewise, it is preferred that the tool-carrying slide or saddle (hereinafter called a “saddle”) be driven by linear motors mounted on opposite sides thereof, e.g., left and right vertical sides, rather than applying the motor driving force to only one side of the slide. Placing the linear motors on opposite sides of the saddle provides propelling forces in the Y-direction on opposite sides of the saddle and provides oppositely-directed, attractive motor forces in opposition with one another. This achieves a force-balancing principle that results in a reduced net magnetic, normal force, preferably about a zero (0) net magnetic, normal force in either the left or right direction. This is in contrast to the aforesaid patent wherein the illustrated machine has three (3) linear motors for moving the slide, all providing propelling forces on one side of the slide and all exerting inwardly-directed, magnetic normal forces against three sides of a vertical column. 
     In the gantry, linear motor machine of the present invention, the tool-carrying spindle is supported on an underneath saddle that spans a pair of adjacent, vertical gantry frame members, rather than being cantilevered on one vertical wall of the column, as disclosed in the aforesaid patent. The underneath support of the saddle allows a large reduction in the size thereof relative to the size of the cantilevered ram. The reduction in size, and also of weight, results in less weight that needs to be moved in the Y-axis direction and less weight to be moved in the X-axis direction. The reduction in weight and size results in lower cost and less motor thrust needed. Indeed, the number of linear motors can be reduced from six or eight, in the aforesaid &#39;425 Patent, to five, illustrated herein. 
     In accordance with another aspect of the invention, the machine frame is box-like in configuration with upper and lower parallel, horizontal frame members and a pair of spaced vertical frame members joined to ends of the upper and lower frame members. There is no large foundation necessary for alignment of the machine; rather the frame members themselves provide the alignment. In theory, the machine tool can be rotated through 90°, 180° or 270° and still perform equally as well, which is not true of the column stack machine disclosed in the aforesaid patent. 
     In the illustrated and preferred embodiment, the machine frame is a stationary box-type structure which encompasses the sides, rears, top and bottom of the movable, linear motor-driven, gantry traveling in the X-direction, the linear motor-driven saddle movable along the gantry in the Y-direction, and the linear motordriven spindle movable along the saddle in the Z-direction. In this preferred embodiment, the gantry is supported on and is driven by top and bottom linear motor drives spaced on opposite sides of the spindle. Preferably, the upper and lower linear motors are substantially identical. Thus, the forces applied to the top and bottom of the gantry are substantially equal; and the opposed, normal magnetic forces between stator coils and magnets are self-cancelling. Thus, there is symmetry about the X-axis. 
     In this illustrated and preferred embodiment of the invention, the spindle-carrying saddle is mounted to travel vertically in a slot at the vertical centerline of the gantry. The saddle is driven by left and right-hand, linear motors mounted on the gantry at opposite vertical sides of the slot. The linear motors are identical and are positioned so that the forces applied to the opposite sides of the saddle are substantially equal and so that the opposed right and left-hand, normal magnetic forces are self-cancelling. Thus, the Y-axis has symmetry as well as along the X-axis. 
     The spindle and ram are mounted on the saddle to travel in the Z-direction. If desired, a linear motor may be mounted both above and below the same to drive the ram and spindle in the Z-direction and to provide symmetry. It is preferred, however, that a single linear motor be used either above or below the ram and extending in the Z-direction to reduce the weight of the ram and to provide an attractive “preload” on the ways of the ram and saddle for greater stability of the spindle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a machine tool movable in three axes and illustrating a box frame for supporting the three axis machine tool according to the invention; 
     FIG. 2 is a front elevational view of the machine tool of FIG. 1 having a portion of the front face of the box frame removed to illustrate the vertical gantry, the saddle, and the ram and spindle arrangement; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG. 2 illustrating the vertical gantry, the saddle, and the ram and spindle and showing movement in phantom of the ram along the Y-axis; 
     FIG. 4 is an enlarged, exploded, perspective view of the ram and spindle assembly including permanent magnets of a linear motor associated with the ram and a wiper element; 
     FIG. 5 is an enlarged, exploded perspective view of an alternatively shaped ram; 
     FIG. 6 is an enlarged perspective view of a “doghouse” and bellow assembly for the ram of FIG. 5; 
     FIG. 7 is a front elevational view of the ram of FIG. 5; 
     FIG. 8 is a front elevational view of an alternative saddle and ram, similar to the ram of FIG. 4, with the ram being driven by linear motors at the top and bottom of the ram; 
     FIG. 9 is a sectional end view of a prior art cantilevered ram and linear motors associated therewith; 
     FIG. 10 is an enlarged, exploded perspective view of the vertical gantry; 
     FIG. 11 is an enlarged, exploded perspective view of the saddle and coil stator sections of linear motors associated therewith and illustrates the detachable bottom plate and stator coils subassembly; 
     FIG. 12 is a partially exploded perspective view of the box frame for housing the vertical gantry, the saddle, and the ram and spindle assembly; 
     FIG. 13 is a sectional, side elevation view similar to FIG. 3, illustrating an alternative embodiment location for the gantry top guides and linear motor; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawings for purposes of illustration, the machine  10  has a frame  12  that supports a gantry  38  movable in a horizontal X-axis direction and slidable along an upper supporting, frame member  20  and a lower supporting, frame member  22 . The gantry  38  carries a tool-carrying slide or saddle  56  that travels vertically in the Y-axis direction. The saddle  56  carries a reciprocating ram  68  and tool spindle  78  which has a tool  11  thereon for cutting a workpiece  30  mounted on a workpiece support  34 . The gantry  38 , saddle  56  and ram  68  are driven by linear motors, generally designated  108 , and each having permanent magnets  110  and coil stator sections  112 . 
     In accordance with the present invention, upper and lower driving forces are applied to the upper and lower ends  38   a  and  38   b  of the gantry  38  by an upper linear motor  108   b  secured to the upper, horizontal frame member  20  and a lower linear motor  108   a  secured to the lower, horizontal frame member  22 . As best seen in FIG. 2, the saddle  56  and the tool-carrying spindle  78  are both mounted between the upper and lower linear motors,  108   b  and  108   a,  respectively, so that there is symmetry about the spindle  78 . This is in contrast to the asymmetrical machine disclosed in the aforesaid &#39;425 Patent where the linear motors are all mounted at the bottom of the column and all push on the lower end of the column. In the machine  10 , shown in FIGS. 2 and 3, substantially equal forces from linear motors exert propelling forces at the top of the gantry  38  adjacent the upper frame member  20  and at the bottom of the gantry  38  adjacent the lower frame member  22 . Because of the force application at the top and bottom of the gantry  38  on opposite sides of the spindle-carrying ram  68 , the gantry  38  need not have the structural members therein to resist bending and deflecting as did the massive column of the &#39;425 Patent, which had the tool spindle located above all of the linear motors in the machine base for driving the column in X-axis direction. 
     Also, in accordance with an important aspect of the invention, the attractive magnetic normal force, 6,000 pounds per linear motor, are offsetting in that the upper linear motor  108   b  pulls upwardly with a 6,000 pound force, indicated by the arrow A in FIGS. 2 and 3, and the lower linear motor  108   a  pulls downwardly with a 6,000 pound force as indicated by the arrow B in FIGS. 2 and 3. The net magnetic, normal force is preferably about zero because the upper and lower linear motors are substantially identical in size and field strength, and are controlled simultaneously in the same manner. In the machine tool of the &#39;425 Patent, all of the X-direction linear motors exerted downward forces, e.g., 24,000 pounds of downward force. This created additional frictional resistance and the need for heavier structural members in the column to resist deflection. 
     In accordance with the invention, the saddle  56  is supported on its left and right sides by vertical gantry frame members  40  and  42 , each of which carries vertically-disposed portions of linear motors  108   c  and  108   d  (FIG. 10) on opposite sides of the tool-carrying spindle  78 . That is, the linear motor drives are mounted on opposite sides of the spindle  78  and are symmetrical to the spindle  78  to exert equal, vertically-directed forces on opposite sides of the saddle  56 . Thus, the saddle  56  is supported on opposite sides thereof and the propelling force in the Y-direction is applied on opposite sides of the saddle  56  by the left and right-hand linear motors  108   c  and  108   d.  This is in contrast to the spindle-carrying slide in the &#39;425 Patent which had one side thereof mounted to slide along the column and in which the spindle-carrying slide was cantilevered supported. The linear motors for the Y-axis movement of the tool-carrying slide, as in the &#39;425 Patent, were all mounted on the column wall so that the propelling force was along the one side of the slide, i.e., the side of the slide adjacent column. Because each of the three linear motors in the &#39;425 Patent machine pulled inwardly with 6,000 lbs. of normal, attractive magnetic force, there was increased friction. More importantly, the structure of the cantilevered spindle slide had to be strengthened by a heavy framework of ribs in order to resist bending and deflection of the spindle slide in order to provide the stiffness needed for the cutting tool on the spindle. The saddle  56  can be much lighter and provide a higher stiffness to mass ratio. The lighter weight saddle  56  allows faster acceleration and the use of less linear motor thrust forces. 
     Thus, it will be seen that the spindle  78  may be mounted symmetrically with respect to the X-axis and Y-axis motors resulting in a minimum of structural deflection, an elimination of bending as experienced in cantilevered loads, and a cancellation of normal attractive, magnetic forces. While it is possible to mount linear motors both above and below the spindle  78  and on the saddle  56 , it is preferred to use only a single linear motor drive located beneath the spindle  78  and to use the 6,000 lb. normal downward, attractive force to preload the ram  68  into tight engagement with the ways on the spindle. 
     In the &#39;425 patented machine, a very large foundation with a pit was needed to support the base and to provide alignment in the X-axis direction for the stacked movable X, Y and Z movable elements. If a corner of the foundation dropped, this alignment was lost. Herein, the square or rectangular frame receives its alignment from the steel frame members  16 ,  18 ,  20  and  22  that encompass the gantry, the saddle and the spindle-carrying ram, hereafter referred to in a group as movable members  14 . Preferably, the frame  12  is formed from steel with the height of the machine in the Y-direction as measured between the members  20  and  22  being equal to or less than the width of the machine in the X-direction, measured between members  16  and  18  to provide a very stable box-frame support for the movable members  14  as they slide along their respective axes of movement. The machine can be shipped, ready to install and it need not be set up and leveled on a massive foundation as was the &#39;425 patented machine. As seen in FIG. 13, vibration isolators  150  may be positioned between the bottom frame member  22  and the floor, and the machine should be ready to use without leveling or a specialized foundation therefor. Theoretically, the bottom frame member  22 , as well as the rest of the frame  12 , could be rotated through 90°, 180° or 270° and still perform the same because of the symmetry of the machine and because of the box-like nature of the frame. 
     The box-shaped frame  12  and the symmetrical location of the X-axis motors  108   b  and  108   a  above and below the X-axis movable member (gantry), allows a much smaller geometric size or footprint for the illustrated machine than the machine shown in the &#39;425 Patent. In the vertical column machine of the prior art, the ends of the ways had large bellow-shaped covers that extended beyond the ways and added significantly to the length of the machine. In this invention, the box-shaped frame  12  is enclosed in a containment housing that is pressurized by air from a fan to keep metal dust particles and chips from the ways and from the linear magnetic motors. The shorter length is of particular importance where a plurality of machine tools  10  are disposed side-by-side in a transfer line. 
     Referring now in greater detail to the illustrated embodiment of the invention, and specifically to FIG. 10, a vertical gantry  38  as used as the movable member  14  movable along the X-axis is illustrated. The vertical gantry  38  is driven for movement along the X-axis and is mounted in the frame  12  to extend between the top and bottom members,  20  and  22 , thereof. The vertical gantry  38  includes parallel elongate side trusses  40  and  42  joined to top and bottom walls  44  and  46 , respectively. Front and rear walls  48  and  50 , respectively, are provided between the trusses  40  and  42  and top and bottom walls  44  and  46 , each having elongate slots  52  and  54  therein which define the Y-axis of travel for the cutting tool  11  and allow the cutting tool  11  to be advanced therethrough along the Z-axis. 
     Turning to FIG. 11, a saddle  56  as used as the member  14  movable along the Y-axis is illustrated. The saddle  56  has a pair of laterally spaced, parallel sidewalls  58  and  60  each having an L-shaped profile. The saddle  56  is mounted for vertical sliding movement in the gantry  38  between the trusses  40  and  42 . The gantry side-trusses  40  and  42  are spaced at a distance slightly greater than the distance between the saddle sidewalls  58  and  60  to provide a close fit between the gantry trusses  40 ,  42  and saddle sidewalls  58 ,  60  as the saddle  56  is carried by the gantry  38  and is driven for sliding movement along the Y-axis in the gantry  38 . The saddle  56  further includes a bottom, detachable web wall  62  in the form of a flat plate which extends between the sidewalls  58  and  60 . The top wall  64  and a forward wall  66  complete the framing for the saddle  56  and form a cradle for carrying a ram  68  as will be more fully described herein. The front wall  66  of the saddle  56  is cut out as at  70  to provide an arched opening  72  through which the ram  68  can extend when driven along the Z-axis. Thus, the saddle  56  supports the ram  68  from beneath the ram  68  eliminating the cantilever support as shown in the &#39;425 Patent and the undue deflection stresses created thereby. 
     The structure of the ram  68  will now be described with reference to FIGS. 4-8. The ram  68  includes a hollow body  76  for supporting a spindle  78  therein. The body  76  has an annular bore  77  extending therethrough. The outer surface  79  of the body  76  can have a number of different shapes when viewed in cross-section. The preferred polygonal shape for the ram  68  is shown in FIG. 4 as a pentagon. The exact shape of the pentagon itself can vary (FIG.  8 ), as will be more fully described herein. As seen in FIG. 4, with the preferred pentagon-shaped ram  68 , a resilient plastic wiper element  81  can be utilized with the wiper  81  having an similar cross-sectional shape as the ram  68  to closely fit around the ram outer surface  79  and limit the travel of metallic chips and dust from being carried into the internal space  13  as the ram  68  slides along the Z-axis. The ram  68  further includes a series of holes  86  in the front face  87  thereof around the bore  77 . The spindle  78  has an enlarged, annular forward flange  82  formed integrally with a cylindrical body  84  extending rearwardly therefrom with the flange  82  including a series of circumferentially spaced holes  88  therethrough. The cylindrical body  84  has a diameter sized so that it tightly fits within the bore  77  and is fixed therein by aligning holes  86  and  88  on the ram front face  87  and spindle forward flange  82  and inserting attachment members, such as threaded bolts (not shown), through the holes. 
     In another form, a ram  68   a  includes a hollow body  76   a  having a cylindrical shape surface  79   a  is provided as seen in FIG.  5 . The ram  68   a  has an enlarged, annular front flange  80 , with the annular flange  80  having an inner diameter slightly less than the diameter of the bore  77  and a series of holes  86   a  therearound, as best seen in FIG.  7 . The annular flanges  80  and  82  cooperate to secure the spindle  78  in the ram  68 . The cylindrical body  84  has a diameter slightly less than the inner diameter of the annular front flange  80  such that the spindle  78  can be fit into the ram bore  77  with the cooperating front and forward flanges  80  and  82  being secured to one another as by aligning holes  86   a  and  88  therein, respectively, and attaching, as previously described. The ram body  76   a  is attached to a bottom support plate  81  to mount the linear motor portion associated with the ram  68   a  for driving the ram  68   a  along the Z-axis. 
     The ram  68   a  can be further provided with a protective “doghouse” housing  90  for guarding portions of the ram  68   a  which extend outwardly from the frame  12  through the opening  28  as the ram  68   a  is driven along the Z-axis to protect the portions of ram  68   a  which extend outwardly from the frame internal space  13  during sliding movement along the Z-axis. Accordingly, the wiper element  81  is not necessary when the housing  90  is employed. The housing  90  can have a steel roof  92  having a forward end  94  and a rearward end  96 . At the forward end  94 , a vertical front plate  98  is provided having an entry hole  100  centrally cut therein and of sufficient diameter such that the ram  68  can slide therethrough. Side bellows  102  and bottom bellows  104  are attached to the roof  92  and the front face  66  of the saddle  56  with the bellows  102  and  104  and the roof  94  cooperatively defining an expansible chamber  106  in which the ram  68  can be advanced along the outside of the saddle  56 . The housing  90  can be attached to the cylindrical body  76  by connecting the bottom of the roof  92  at the forward end  94  thereof to the flange  80  as by bolting or the like. As the roof  92  extends longitudinally further along the ram body  76  than the bellows, a peaked cut-out section as at  74  is provided above the arched opening  72  and corresponds to the shape of the roof  92  to allow it to extend therethrough as the ram  68  slides along the Z-axis. Similarly, the slot  52  in the front wall  48  of the gantry  38  is cut so that the front plate  98  along with the bellows  102 ,  104  and steel roof  92  can slide therethrough. On the other hand, the front wall  66  of the saddle  56  limits the distance to which the housing  90  can extend into the saddle  56  as movement to the right in FIG. 10 is permitted until the front plate  98  of the housing  90  reaches the front wall  66  of the saddle  56 . In this manner, the expansible chamber  106  alternatively collapses and expands as the ram moves in and out of the saddle  56  along the Z-axis. 
     As described above, it can be seen that the movable members  14  including the horizontal ram  68 , the saddle  56  and the gantry  38  are all nested one within the other for three-axis movement of the tool  10 . More specifically, the gantry  38  carries the saddle  56  and the ram  68  and spindle  78  along the X-axis, the saddle  56  carries the ram  68  and spindle  78  along the Y-axis, and ram  68  only carries the spindle  78  along the Z-axis. 
     To drive the movable members  14  for three-axis movement of the tool  10 , linear motors  108  are employed. The linear motors  108  each have a stator coil section  110  and a row of permanent magnets  112 . The permanent magnets  112  of the linear motor  108  are arranged in a row and are bonded to the movable members  14  and the frame  12  with spaces between magnets in a row being filled with epoxy. A thin strip of a protective non-magnetic, metallic material covers each row of magnets  112 . 
     The linear motors  108  can be any motor which operates based on attractive magnetic forces producing a thrust driving force such that there is no contact between the driving (stationary) and driven (moving) members. For example, the linear motors  108  can be brushless DC motors and each may have maximum thrust force of 2,000 pounds. The linear motors  108  are generally positioned between the movable members  14  and the movable member in the X-axis (the gantry  38 ) and the frame  12 . The control for the linear motors  108  is integrated into the machine frame  12  and is operable as by control panel  116 . A position feedback (not shown) is utilized and can include optical, mechanical or laser sensors used to detect the position of the movable members and send signals to allow the control to energize only those coils needed to drive the member along its axis of movement, as is known in the art. 
     Similar to the linear motors  108 , bearing structure  118  in the form of cooperating anti-friction, elongate linear rails  120  and rail receiving trucks  122  are positioned between the movable members  14  and the movable member in the X-axis (the gantry  38 ) and the frame  12  to support and guide sliding movement thereof in the X, Y and Z-axes as they are driven by the linear motors  108 . The rails  120  and trucks  122  provide their guiding and supporting functions through roller or ball bearings, or through surfaces coated with low coefficient of friction materials, such as Teflon. 
     Returning to the box frame  12 , the configuration thereof will be more fully described with reference to FIGS. 1,  12  and  13  although, as will be apparent to one skilled in the art, the exact configuration of the box frame  12  can be modified while still housing and supporting the movable members  14  for sliding movement along the X, Y and Z-axes. The box frame internal space  13  can be divided into front and rear regions  126  and  136 , respectively, by an intermediate partition wall  124  which extends between the side members  16  and  18 . The front frame panel  24  can be attached to a T-shaped base  15  for the box frame  12  at the junction of the arm and leg of the “T” and extends upwardly from the base and is attached to the machining housing  32  for the support  34  about the rear thereof. The panel  24  extends parallel to the intermediate wall  124  and has a central, rectangular-shape cut out  26  to fit around the housing  32  and to allow the ram  68  to extend into the housing  32  along the Z-axis to engage a workpiece  30  therein. The intermediate partition wall  124  along with the side members  16  and  18 , front panel  24  and the top and bottom members  20  and  22  define a front gantry region  126  in the box frame internal space  13 . Similar to the front panel  24 , the intermediate partition wall  124  has a rectangular shaped cut-out section therein which allows for the bottom leg  130  of the L-shaped side walls  58  and  60  of the saddle  56  to extend therethrough during movement of the saddle along the Y-axis, and allows the ram  68  to extend therethrough as the ram  68  moves along the Z-axis. 
     Two parallel side support panels  132  and  134  can be spaced inwardly from the box frame side members  16  and  18 , respectively, and extend from the intermediate partition wall  124  towards the rear of the box frame  12 . The intermediate partition wall  124 , the side panels  132  and  134  and the top and bottom members  20  and  22  cooperatively define the rear region  136  in the box frame internal space  13  into which the saddle bottom legs  130  and ram  68  can project, as previously described. Further, the rear region  136  provides an enclosure for housing the various services and support equipment, such as a tank, pump and chiller, for supplying the linear motors with cooling water. 
     The box frame  12  can be sealed at the forward central opening  28  thereof as by structure for blocking the opening while allowing the ram  68  to extend therethrough during its sliding movement along the Z-axis. The sealing structure can take the form of X and Y-curtains  142  or baffles which allow the saddle  38  and ram  68  to move in the X and Y-axes while still blocking the opening  28  around the ram  68 . The curtain  142  includes an upper curtain roll  142   a  above the saddle and a lower curtain roll  142   b  at the bottom of the frame and below the saddle. The upper and lower curtains roll and unroll respectively as the saddle moves vertically. The rear of the machine can be sealed by sheet metal to complete the rear region enclosure. The box frame internal space  13  can be pressurized by a positive pressure means in the form of a fan  146  to prevent dust and workpiece chips generated during a machining operation from entering the internal space. The fan  146  can be placed in the box frame rear region  136  and supplied with ambient filtered air. The pressurized air from the fan  146  can effectively isolate the internal space from any cast-iron or steel dust or chips generated from the workpiece which might be attracted to the magnetics of the linear motors  108  which could adversely affect operation of the same. 
     Referring to FIG. 13, it is also possible for the base  15  as described earlier to be removed while providing the frame  12  with a thicker bottom member  22   a  which is directly mounted to a support surface. Vibration-absorbing resilient pads  150  can be attached to the bottom frame member  22   a  to isolate the machine and the support surface one from the other so that vibrations in either are not transmitted from one to the other. Because of the symmetrical box-frame construction of the machine, the alignment of the machine does not depend upon the foundation such that only three support pads  150  need be utilized to provide the machine with an effective three-point mounting to the support surface. 
     To drive the vertical gantry  38  along the X-axis in the box frame  12 , a pair of opposed linear motors  108   a  and  108   b  are provided, as seen in FIG.  3 . By providing for top and bottom driving of the gantry  38 , the deflection loads caused by driving the gantry solely from the bottom is substantially obviated. As seen in the &#39;425 Patent, the bottom driving of the long freestanding column causes a deflection near the free top end of the column due to the concentration of drive forces at the bottom of the column thereby causing a deflection lag at the top as the column is driven. In the box-frame machine, the coil stator section  112   a  of the bottom gantry linear motor  108   a  is mounted on the gantry bottom member  46  while the row of permanent magnets  110   a  is mounted along the X-axis on the bottom frame member  22  in the front gantry region  126  (FIG.  12 ). Adjacent the coil stator section  112   a  mounted on the frame bottom member  22  are a pair of linearly aligned rail-receiving trucks  122   a  (FIG.  10 ). Aligned therewith is a slide rail  120   a  positioned adjacent the magnets  110   a  so that the gantry  38  is supported and guided for sliding movement along the X-axis by the cooperating rail and trucks,  120   a  and  122   a , respectively. 
     The top linear motor  108   b  is positioned at or near the top of the gantry  38  and frame  12 . As seen in FIGS. 2 and 3, in the preferred embodiment, a horizontally extending frame member  154  is provided in the front gantry region  126  parallel and spaced from the top frame member  20 . The frame member  154  extends from the intermediate wall  124  towards the front of the frame  12  and is attached to a vertical wall portion  158  between the front panel  24  and intermediate wall  124  extending between the wall portion  154  and the top frame member  20 . To drive the vertical gantry  38  from the top as well as the bottom of the gantry, the coil stator section  112   b  is mounted to the gantry top wall  44  and the row of permanent magnets  110   b  is aligned therewith along the X-axis and is mounted to the bottom of the horizontal wall portion  154 . Proximate the linear motor  108   b,  a rail  120   b  and trucks  122   b  are provided to guide and support the gantry  38  near the top thereof as it is advanced along the X-axis. The gantry  38  includes a vertical flange  162  mounting the linearly arranged pair of trucks  122   b  (FIG.  10 ). The vertical wall portion  158  of the frame  12  mounts the slide rail  120   b  such that it is in alignment with the trucks  122   b  for sliding movement therein. In an alternative arrangement, both the trucks  122   b  and the coil section  112   b  are mounted in side-by-side relation on the gantry top wall  44  with the rail  120   b  and row of permanent magnets  110   b  being aligned therewith mounted on the top frame member  20 , as illustrated in FIG.  13 . As will be apparent to one skilled in the art, the arrangement of the cooperating rail  120   b  and trucks  122   b  and the linear motor  108   b  can be altered, such as by placement thereof on the intermediate wall  124 , as long as they remain near the top of the gantry to provide for top driving of the same. In this manner, the attractive magnet force in the top and bottom linear motors  108   a  and  108   b  are self-canceling so as to limit structural deflections as caused by the unbalanced magnetic forces as applied in the machine of the &#39;425 Patent. 
     Similarly, with the saddle  56  mounted as a balanced load in the gantry  38  between the gantry side trusses  40  and  42  for sliding travel therealong in the Y-axis, linear motors  108   c  and  108   d  can be positioned between the saddle sidewalls  58  and  60  and the gantry side trusses  40  and  42 , respectively, to balance the magnetic forces applied between the gantry  38  and saddle  56  about the Y-axis. More specifically, each gantry side truss  40  and  42  mounts one-half of the linear motors  108   c  and  108   d,  respectively, which in the preferred form is a row of permanent magnets  112   c  and  112   d  extending along the Y-axis (FIG.  10 ). The respective coil stator sections  112   c  and  112   d  are mounted on the saddle sidewalls  58  and  60 , respectively, to drive the saddle  56  and the ram  68  carried thereby along the Y-axis. Thus, the saddle  56  is symmetrically driven from either side thereof about the Y-axis while simultaneously providing a support beneath the ram  68  in the form of the detachable web wall  62 , thereby eliminating any overhung or cantilevered loads as seen with the ram, slide and column configuration in the &#39;425 Patent. 
     To counterbalance the saddle  56  as it moves in the Y-axis, a pair of counterbalance cylinders  166  and  168  are attached between the gantry  38  and the saddle  56  such that when the linear motors  108   c  and  108   d  are not activated, the saddle  56  remains stationary in the gantry  38  along the Y-axis. The cylinders  166  and  168  can be, for example, either air or hydraulic cylinders and each include a spring brake  170  and  172 , respectively, as is known in the art. Thus, when there is a loss of power and pressure is removed from the counterbalance cylinders  166  and  168 , the spring brakes  170  and  172  are activated to maintain the saddle  56  in position along the Y-axis. 
     As previously mentioned, the saddle  56  includes a detachable bottom web wall  62  for providing the ram  68  with a cradle or support thereunder as it slides along the Z-axis. The web wall  62  can be attached to the main portion of the saddle  56 , as seen in FIG. 11, by any variety of well-known attaching means such as by threaded holes and bolts which also allows the web wall  62  to be adjustable so as to correct for any variations in tolerances between the ram  68  and saddle  56 . 
     To drive the ram  68  along the Z-axis, it is preferred to utilize a single linear motor  108   e  with one-half of the motor  108   e  on the web wall  62  and the other half below the ram  68 . Preferably, the coil stator section  112   e  is mounted on the bottom web wall  62  with a pair of linearly arranged trucks  122   e  positioned on either side thereof. A row of permanent magnets  110   e  is mounted beneath the ram  68  and is flanked by two parallel rails  120   e  with the magnets  110   e  and rails  120   e  being aligned with the coils  112   e  and the trucks  122   e , respectively, to allow the ram  68  to be driven for sliding movement along the Z-axis and supported by the web wall  62 . With this arrangement, the strong attractive force provided by the linear motor  108   e  produces a preload on the rails  120   e  and trucks  122   e  to provide the spindle  78  with greater stability as the ram  68  is reciprocated along the Z-axis. 
     Due to the symmetrical box frame configuration with the bulk of the mass of the machine in the stationary steel frame  12  and the application of balanced driving forces about the gantry  38  and saddle  56 , the movable elements  14 , i.e., the gantry  38 , the saddle  56  and the ram  68  can all be relatively light in weight and still be formed from a material having a high degree of material stiffness, such as steel, to be accurately driven for high-speed movement by their respective linear motors  108  so that the tool  10  can be advanced at speeds of at least 1500 inches per minute and at an acceleration or deceleration of at least 1 G. 
     In another embodiment, balanced magnetic drive forces are applied to the ram  68  by utilizing a slightly modified pentagonal shape, as illustrated in FIG. 8, wherein the flat top surface thereof is extended in the X-direction so that it is adapted to mount a row of permanent magnets  110   f  thereon. The modified ram  68   b  utilizing a top mounted linear motor  108   f  also employs a slightly modified saddle construction wherein the saddle  56   a  forms a square-shaped frame about the circumference of the ram  68   b  so that the top horizontal member  174  of the saddle  56   a  can mount a coil pack  112   f  thereon in alignment with the magnets  10   f  to drive the ram  68   b  from both the top and bottom thereof. 
     As is apparent, the use of linear motors  108  in the box-frame machine tool described herein provides for flexibility in the design of its movable members  14  as the linear motors  108 , unlike conventional ball-screw motors or the linear motors in the machine of the &#39;425 Patent, can be distributed throughout the machine tool box-frame  12  to balance forces about the various axes of movement. As such, the exact placement of the motors  108  and their components, i.e., the magnets  110  and coil section  112 , can be varied in accordance with the force and load balancing principles described herein. Thus, with symmetrically arranged movable members and drive forces, improved accuracy, stiffness, speed and acceleration can be obtained over conventional machine tools.