Patent Publication Number: US-2023150043-A1

Title: Actively preloaded drive-guideway system

Description:
SUMMARY 
     The present invention is directed to an apparatus. The apparatus comprises a first frame, a second frame and a plurality of drive assemblies. The first frame comprises a first cylindrical rail and a second cylindrical rail. The second cylindrical rail is parallel to and spaced apart from the first cylindrical rail. 
     The second frame comprises a plurality of drive assemblies, each comprising a motor and a roller rotatably attached to the motor. Each roller in the plurality of drive assemblies has an external concave profile complementary to a portion of the first cylindrical rail. The second frame is supported on the first frame at each roller of the plurality of drive assemblies. 
     In another aspect, the invention is directed to an assembly for driving a second frame along a first frame. The first frame comprises vertically offset first and second cylindrical rails. The assembly comprises a first drive member and a first preloading member. 
     The first drive member comprises a motor, a roller, a bearing assembly, and a position sensor. The roller is coupled to the motor and has a concave profile complementary to a surface of the first cylindrical rail. The bearing assembly comprises inner and outer portions. The inner portion rotates relative to the outer portion and the outer portion does not rotate relative to the second frame. The position sensor is configured to determine a position of the second frame along the first frame. 
     The first preloading member comprises a roller and a biasing member. The roller has a concave profile complementary to the surface of the second cylindrical rail. The biasing member is configured to force the first preloading member in a direction toward the second cylindrical rail and away from the first cylindrical rail. 
     In another aspect, the invention is directed to an apparatus having three degrees of linear freedom. The apparatus comprises a first pair of drive-guide assemblies, a second pair of drive-guide assemblies, and a third pair of drive-guide assemblies. The first pair of drive-guide assemblies is adapted to move a work tool in a first direction along a corresponding first set of linear rails. The second pair of drive-guide assemblies is adapted to move a work tool in a second direction along a corresponding second set of linear rails. The third pair of drive-guide assemblies is adapted to move a work tool in a third direction along a corresponding third set of linear rails. Each set of linear rails comprises opposed pairs of first and second rails. The first direction, second direction, and third direction are each perpendicular to each of the other directions. 
     Each of the drive-guide assemblies comprises a drive roller assembly, a support guide assembly, and a loaded guide assembly. The drive roller assembly comprises a motor and a roller. The roller is coupled to the motor and has a concave radial surface conforming to a first member of a selected one of the first, second, and third linear rails. 
     The support guide assembly comprises a support roller having a concave radial surface conforming to the first member of a selected one of the first second and third set of linear rails. The loaded guide assembly comprises a loaded roller and a biasing member. The loaded roller has a concave surface conforming to a second member of the selected one of the first, second, and third linear rails. The biasing member is configured to bias the loaded roller towards the second member of the selected one of the first, second, and third set of linear rails and away from the first member of the selected one of the first, second, and third set of linear rails. 
     BACKGROUND 
     Traditional gantry systems are typically large and heavy metal assemblies. The primary purpose of these gantry systems is to control the movement of tools and other attachment pieces throughout machining and/or assembly processes. These gantry systems can be situated overhead or on the ground. Notably, these gantry systems consist of separate drive and guide systems. The separate drive and guide systems result in axes of contact at each location of a drive member. Such separate systems have known limitations in accurate placement of a tool, due to the inherent tendency towards backlash and runout in these systems. 
     Drive systems are configured to power movement and “drive” whatever is attached to the gantry-often cutting/machining tools or assembly tools. The most common drive systems for large motion platforms are gear systems. These gear systems usually comprise a toothed gear rack and at least one pinion gear configured to fit inside the gear rack. The pinion may be driven by a gearbox, in which many gears may allow the alteration of a gear ratio in order to meet mechanical need. These gear systems experience backlash, however, which results in increased margins of error and higher machining tolerances. As more gears are added, the overall amount of backlash increases. Backlash can cause issues in later phases of production, such as misaligned holes or unanticipated gaps between machined pieces. It is therefore ideal to reduce backlash as much as possible in order to increase accuracy and efficiency. 
     Some artisans have attempted to eliminate backlash by creating pre-loaded pinion gears that manipulate against each other. However, even preloaded pinions wear over time, resulting in increased gap tolerances and eventually backlash. Therefore, even contemporary pre-loaded pinion gears do not completely eliminate backlash. 
     Other forms of drive systems have provided half-measures against backlash and often possess other limitations. For example, ball-nut drive systems are common in industrial settings. These ball-nut drive systems typically comprise a threaded rod with a spiral trapezoidal groove, and a plurality of balls that are preloaded inside of the nut to fit inside the grooves and allow the nut to slide up and down the rod. However, these ball-nut drive systems often wear down over time, resulting in increased gaps and, again, backlash. Moreover, these ball-nut systems are limited in size because the necessary rods are only manufactured up to a certain length. Thus, all current drive systems possess notable flaws. 
     While guide systems are separate from drive systems, they often experience similar backlash and accuracy issues. Guide systems are designed to support the weight of machining or tooling assemblies and provide axial limitations in the movement of the assemblies. These guide systems sit on top of drive systems, providing a second point of contact between the gantry and the machining or assembly tool. 
     Both box linear guideways and profile linear guideways contain recirculating rollers or balls that allow movement of the gantry system. These recirculating elements, however, often wear down, which leads to backlash and other inaccuracies during operation. Additionally, the surface that these guideways sit on are often unforgiving, resulting in extra forces exerted on the system. 
     Thus, there is a current industry need for both drive and guide systems that do not experience backlash or other inefficiencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a top front right side perspective view of a gantry. The gantry has a plurality of drive-guide systems which enable the movement of a work tool carried by the gantry along three perpendicular axes. A box is shown from which  FIG.  1 B  is taken. 
         FIG.  1 B  is a top front right side perspective view of the highlighted box from  FIG.  1 A . A portion of the stationary frame is cut away such that the drive-guide system of the present invention may be highlighted. 
         FIG.  2    is a front right top perspective view of a movable frame having a drive-guide system on each side, with a tool carrying frame carried thereon along a second set of rails. The stationary frame is not shown in  FIG.  2   . 
         FIG.  3    is a top right perspective view of the movable frame with the drive-guide system shown. The first set of rails of the stationary frame are truncated, shown only in a short segment below one of the rollers. 
         FIG.  4    is an exploded side view of a drive roller assembly. Frame elements unrelated to the assembly are represented by the truncated rail and frame beneath the roller and a frame element disposed about the motor. 
         FIG.  5    is a sectioned view of the drive roller assembly of  FIG.  4   , with parts re-assembled. 
         FIG.  6 A  is a front left perspective view thereof. 
         FIG.  6 B  is a back right perspective view thereof. 
         FIG.  7 A  is a front left perspective view of a support guide assembly for use with the drive-guide system of the present invention. 
         FIG.  7 B  is a back right perspective view thereof. 
         FIG.  8 A  is a front left perspective view of a loaded guide assembly. 
         FIG.  8 B  is a back right perspective sectioned view thereof, with a sectional view of the biasing assembly in the foreground. 
         FIG.  9 A  is a diagrammatic end view of a roller engaging with a rail. An arrow indicates a small amount of force is provided to the roller in the radial direction. The diagram includes an enlarged view of the interface between the roller and rail. 
         FIG.  9 B  is a diagrammatic end view of a roller engaging with a rail. An arrow indicates a small amount of force is provided to the roller in the radial direction and a second arrow indicates a small amount of force is provided to the roller in the axial direction. The diagram includes an enlarged view of the interface between the roller and rail, with centers of the roller and rail offset from one another. 
         FIG.  9 C  is a diagrammatic end view of a roller engaging with a rail. Arrows indicate a moderate amount of force is provided to the roller in the radial direction and a separate arrow indicates a small amount of force is provided to the roller in the axial direction. The diagram includes an enlarged view of the interface between the roller and rail, with centers of the roller and rail offset from one another. 
         FIG.  9 D  is a diagrammatic end view of a roller engaging with a rail. Arrows indicate a large amount of force is provided to the roller in the radial direction and a second set of arrows indicate a moderate amount of force is provided to the roller in the axial direction. The diagram includes an enlarged view of the interface between the roller and rail, with centers of the roller and rail offset from one another. 
         FIG.  9 E  is a diagrammatic end view of a different roller engaging with a rail. In this figure, the radius of the concave portion of the roller is less than the radius of the rail. The diagram includes an enlarged view of the interface between the roller and rail. 
         FIG.  9 F  is a diagrammatic end view of a different roller engaging with a rail. In this figure, the concave portion of the roller has a profile similar to a gothic arch. The diagram includes an enlarged view of the interface between the roller and rail. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the figures,  FIG.  1 A  shows a dynamically preloaded drive-guideway system. The system is placed on a machine  10 , which, as shown in the figures, is an overhead gantry. While the gantry  10  will be described herein, it should be understood that the present invention is equally applicable on other axis layouts for machine tools. The drive-guideway system disclosed herein will allow for the precise and repeatable movement of a motion platform. Gantry  10  is shown herein as one applicable axis system for using the drive-guideway system  40  described. 
     The gantry  10  comprises a work tool assembly  12  which is supported in three dimensions, and rotational about three axes, due to components of the gantry  10 . This tool  12  may be a probe, a milling tool, a waterjet cutting head, or any other tool for use in machining, inspection or assembly of large parts. The identity of the tool  12  is not limiting on the present invention. 
     The gantry  10  comprises a plurality of rails, each of which extends about a longitudinal axis. A first set of rails  14  is supported by one or more stationary frames  16 . As shown, the stationary frames  16  are spaced apart, but it should be understood that these frames may be joined at one or more points for added rigidity. 
     The first set of rails  14  define a first longitudinal direction that is parallel to the longitudinal axis of each of the first set of rails. For the purposes of this disclosure, the first longitudinal direction is defined as “x”, though naming conventions are not limiting on this invention. 
     As shown, the first set of rails  14  is suspended some distance above the ground by the stationary frame  16 , and supported by one or more beams extending in the first longitudinal direction. The first set of rails  14 , alternatively, may be placed near to the ground level with a vertical offset allowing the operation of the gantry  10 . It should be understood that the first set of rails  14  includes all rails on the gantry  10  which extend in the “x” direction. 
     A movable frame  20  is supported on the first set of rails  14 . The movable frame  20  is movable along the first set of rails  14  in the first longitudinal direction. The movable frame  20  comprises a second set of rails  22 . The second set of rails  22  define a second longitudinal direction, wherein the second longitudinal direction is perpendicular to the first longitudinal direction. For the purposes of this disclosure, the second longitudinal direction is defined as “y”. 
     As shown, the first longitudinal direction and second longitudinal direction define a plane which is parallel to a surface of the ground. A third longitudinal direction, therefore, is defined as “z” and is vertical - that is - perpendicular to both the first and second longitudinal directions. 
     A tool holding frame  30  is supported on the movable frame  20 . The tool holding frame  30  comprises a third set of rails  32  ( FIG.  2   ). The tool holding frame is actuated by one or more counterbalancing cylinders  34  which drive the work tool  12  in the z direction. Elements of the tool holding frame  30  as it moves relative to the movable frame  20  are better shown on  FIG.  2   . 
       FIG.  1 B  shows portions of the stationary frame  16  cut away so that the first set of rails  14  may be shown. The first set of rails  14  on each side of the stationary frame  16  comprises a bottom rail  14   a  and a top rail  14   b . A matching top and bottom rail are disposed on the stationary frame at an opposite end of the movable frame  20 . Preferably, the top rail  14   b  and bottom rail  14   a  are parallel and both situated with their longitudinal axes within a vertical plane. The first set of rails  14  are supported on the stationary frame  16  at structural beams  18 . 
     The movable frame  20  comprises a drive-guide system  40  disposed at each of its ends. The drive-guide system  40  interacts with the first set of rails  14  to support and drive the movement of the movable frame  20  (and thus the tool  12 ) in the x direction. Likewise, a drive-guide system  40  is disposed on each side of the tool holding frame  30 , and moves the tool holding frame in the y direction along the second set of rails  22 . And further, a drive-guide system  40  is fixed in position on a tool frame  38  relative to the work tool  12  and interacts with the third set of rails  32  on the tool-holding frame  30 , allowing movement in the z direction. 
     Each drive-guide system  40  is substantially identical, with a set of rollers, as described below, which interact with a selected one of the first, second and third set of rails  14 ,  22 ,  32  to move the work tool  12  in a corresponding x, y, or z direction. 
     As best shown in  FIGS.  1 B and  2   , a pair of drive-guide systems  40  are on opposed sides of the movable frame  20 , the tool holding frame  30 , and on the tool frame  38 . One or both of the pair of drive-guide systems  40  may include one or more magnets  36 . The magnets  36  act to provide a force to the frame  20 ,  30 ,  38  which is transverse to the length of the set of rails  14 ,  22 ,  32  on which the respective frame is carried. 
     While the term “frame” is used to describe the elements on a gantry  10 , it should be appreciated that each frame which provides a separate degree of freedom to the work tool  12  is accurately described as an “axis stage”. In a gantry  10 , each axis stage may indeed be a separate frame. However, other, non-frame structures may be used with other machine tools to provide degrees of linear freedom to a work tool  12 . These alternative structures may also utilize the drive-guide system  40  of the present invention without departing from the spirit thereof. 
     The drive-guide system  40  comprises a support guide assembly  42 , a drive roller assembly  44 , and a loaded guide assembly  46 . Each of the support guide assembly  42 , drive roller assembly  44 , and loaded guide assembly  46  are shown in the figures with reference to a single side of the movable frame  20 , but substantially identical drive-guide systems  40  are provided with the tool frame  38  and the tool holding frame  30 . Thus, an identical set of assemblies  42 ,  44 ,  46  is provided at a second end of the movable frame  20  and engage with the first set of rails  14  disposed on that portion of the secondary frame  16 . Preferably, the movable frame  20  is only supported at the drive-guide system shown, and does not have additional guide members disposed elsewhere on the gantry  10  for supporting the movable frame  20 . 
     As shown in  FIG.  3   , a pair of magnets  36  is carried on a single drive-guide system  40  of the pair carried by the movable frame  20 . However, it should be understood that a magnet  36  could be carried on each of the pair of drive-guide systems  40 , so long as the resultant magnetic forces are additive. For example, the magnets  36  shown in  FIG.  3    may be an attractive force, pulling the movable frame  20  towards the portion of the stationary frame  16  on which the rails are carried. Any magnet  36  on the opposite one of the pair of drive-guide systems  40  should provide a repulsive force relative to its portion of the stationary frame  16 . 
     With reference to  FIGS.  4 ,  5 , and  6 A and  6 B , an example drive roller assembly  44  is shown. The drive roller assembly  44  comprises a motor  102 , an adaptor  104 , a bearing assembly  106 , and a roller  108 . 
     The motor  102  may be a rotational drive motor having one or more intermediate rotor pieces  110  between the motor  102  and the roller  108 . The motor  102  is mounted on the frame upon which the drive-guide system is supported, here, the movable frame  20 . The adaptor  104  is connected to non-rotating components of the motor  102  and is sized to connect via pins  112  to the bearing assembly  106 . 
     The bearing assembly  106  has a rotating race or portion  114  and a non-rotating race or portion  116 . Preferably, the bearing assembly  106  is a cross-roller bearing. In a cross-roller bearing, rollers between the rotating  114  and non-rotating  116  portions of the bearing assembly  106  alternate at a ninety degree angle to adjacent rollers within a substantially “v” shaped groove. This allows the bearing assembly  106  to have radial and axial loading. 
     The non-rotating portion  116  is connected to the adaptor  104 . The rotating portion  114  is connected to the motor  102  at its intermediate rotor pieces  110 . The rotating  114  and non-rotating  116  portions abut at a sliding surface, where ball bearings, cylinder bearings or other bearing devices (not shown) disposed between the portions  114 ,  116  allow for relative rotation between them. Preferably, the bearing assembly  106  is preloaded and constrained in the axial direction to provide accurate and repeatable linear motion. 
     The drive roller assembly  44  is preferably connected to the movable frame  20  at a position close to its end, with as little of a drive roller assembly  44  extending from the frame  20  as possible. As shown in  FIGS.  1 B,  2  and  3   , the roller  108  extends just beyond the end of the movable frame  20  to engage with the first set of rails  14 . In this way, the length of any moment arm is limited. 
     The roller  108  is attached to the rotating section  114  and other rotating components  110  by large pins  118 . These pins  118  should be of sufficient strength and length to carry forces exerted on the roller  108  by the weight of the frame it carries. For example, a plurality of drive roller assemblies  44  adapted to move the movable frame  20   along the first set of rails should be able to carry its weight, even as the tool holding frame  30  is moved along the second set of rails  22  to different locations. 
     A ring  119  is provided at which a micrometer (not shown) or other instrument may be attached to measure the runout of any individual drive roller assembly  44 . Further, an encoder  130  may be carried by a flange  109  on the stationary bearing of the bearing assembly  106  to measure the position of the drive roller assembly  44  along a scale  132  which is attached to an element of the frame supporting the rail  14 ,  22 ,  32  along which the drive roller assembly  44  is moving. 
     The roller  108  has a concave surface  120  which largely conforms to the cylindrical rail  122  shown. The cylindrical rail  122  shown here may represent the first  14 , second  22  or third  32  set of rails, and is sectioned for clarity. If magnets  36  are in use, the roller  108  will not center on the cylindrical rail  122 , but rather will be offset due to the magnetic force associated with the field of the magnets  36 . ( FIGS.  9 B- 9 D ) Such force reduces wear on the roller  108  due to oscillation or vibration and allows the concave surface  120  to center on the cylindrical rail  122  at a location offset from the top of the rail. 
     The amount of axial force  105  ( FIGS.  9 B- 9 D ) provided by the magnets  36  may be precisely adjusted by adjusting a distance between the magnet itself and attractive frame elements. For example, bolts may be provided with each magnet  36 , adjusting a position of the magnet relative to the nearby frame (such as stationary frame  16 ). 
     An example of how pre-loaded forces affect the position of the roller  108  is shown in  FIGS.  9 A- 9 D . Forces  103  are perpendicular to both the axis of the roller  108 ,  108 A and the length of the rail  122 . These will be referred to as “radial forces”, and may be added by manipulation of the loaded guide assembly  46  ( FIGS.  8 A- 8 B ). Forces  105  are perpendicular to the length of the rail  122  but parallel to the axis of the roller  108 . Forces  105  are provided by the magnets  36 , and will be referred to as “axial forces”. 
     As shown in  FIG.  9 A , when only radial force is provided, the roller  108 A is centered on the rail  122 . When axial force is added as shown in  FIG.  9 B , the roller  108 A is offset from the center of the rail  122  by a distance  107 . Because these forces  103 ,  105  are equal, the offset distance  107  will tend to be large as rail  122  moves up the “ramp” of the roller  108 A. As shown in  FIGS.  9 C- 9 D , axial forces that are less than the radial forces applied result in a smaller offset distance  107 . Consistent positioning of the contact points between the rail  122  and roller  108  will reduce rubbing between the two, cutting down on wear and improving positional accuracy. 
     In  FIGS.  9 A- 9 C , roller  108 A is shown, having a profile which less particularly matches the rail  122 . This roller  108 A may be suitable for the loaded roller assembly  46  or support guide assembly  42 . In these figures, this roller  108 A is shown for illustrative purposes, the tight tolerances of roller  108  being more difficult to visualize. In  FIG.  9 D , the profile of roller  108  more closely matches the rail  122 , resulting in a steeper ramp and more precise positioning 
     The relative profiles of  FIGS.  9 A- 9 D  are preferable to both the spherical roller  108 B of  FIG.  9 E  and the gothic arch style roller  108 C of  FIG.  9 F . These profiles provide for precise axial positioning in an initial position because of the two points of contact. However, as wear occurs, it becomes less reliable. Therefore, systems employing two points of contact between a rail  122  and roller  108  will require lubrication, which is practically disadvantageous and unsuitable for a roller which operates to drive the motion of various frames within the gantry  10 . 
     With reference to  FIGS.  7 A and  7 B , the support guide assembly  42  is shown, without the frame elements around it. As shown in  FIG.  1 B , the support guide assembly  42  is secured to a frame element (such as the movable frame  20 ). The support guide assembly  42  comprises a roller  150  and a bearing assembly  152 . The bearing assembly  152  comprises a movable and non-movable portion, with ball or roller bearings disposed therebetween suitable for repeated rotation. The bearing assembly  152  is secured to the movable frame by a plurality of pins  154 . Likewise, the roller  150  is secured to the movable portion of the bearing assembly  152  by a plurality of pins  156 . A ring  158  may be placed proximate the roller  150  such that a micrometer or other measurement device may detect runout in the roller  150  and adjust accordingly. 
     The support guide assembly  42  has a flat portion  160  at which the assembly may be attached to frame elements. As with the roller  108  of drive roller assembly  44 , roller  150  has a concave surface  162  which conforms to an outer surface of a cylindrical rail. 
     While one support guide assembly  42  is shown as a part of the drive-guide system  40 , multiple support guide assemblies  42  may be utilized to provide additional support to the particular frame being guided. It should be appreciated that if more than one is used, the additional support guide assemblies should be carefully aligned such that the drive-guide system will interact with the applicable set of rails  14 ,  22 ,  32  at each point. Alternatively, as in the Figures, one and only one support guide assembly  42  is used with each drive roller assembly  44  to provide two and only two points of contact on the lower rail  14   a . 
     It may be preferable to allow the bearing assembly  152  of the support guide assembly  42  to float axially - that is, in a direction along the longitudinal axis of the assembly  42  and perpendicular to each of the first set of rails  14 . The drive roller assembly  44  will be fixed in axial position and not allowed to float, which will maintain a preferred position of the assemblies  42 ,  44  relative to the rail. Allowing axial float in the roller  150  through the bearings  152  allows for mounting on imperfect surfaces without the drive-guide system  40  getting in a bind. 
     With reference to  FIGS.  1 B,  2 ,  8 A and  8 B , the loaded guide assembly  46  engages a top rail  14   b . The top rail  14   b  is secured to the stationary frame  16 . Thus, any force applied by the drive-guide assembly  40  to the top rail  14   b  will tend to press the support guide assembly  42  and the drive roller assembly  44  into the lower rail  14   a . This “preload” on the set of rails  14  provides for consistent positioning of roller  150  and roller  108 . This preload force is shown as force  103  in  FIGS.  9 A- 9 D . 
     The loaded guide assembly  46  comprises a roller  180 , a bearing assembly  182 , and a biasing assembly  184 . The bearing assembly  182  is allowed to float axially for the same reasons as bearing assembly  152 . The non-moving portion of the bearing assembly  182  is fixed to the biasing assembly  184 , which allows the selective positioning of the roller  180  relative to the movable frame  20  by adjusting the distance between the engagement point of roller  180  and that of rollers  108 ,  150 . 
     The biasing assembly  184  comprises a plate  186 , a frame attachment point  188 , and one or more pre-load nuts  190 . The loaded guide assembly  46  is attached to the movable frame  20  (or other frame  30 ,  38  as the case may be) at the attachment point  188 . Adjustment of the one or more nuts  190  adjusts the distance between the plate  186  and the attachment point  188 . Preferably, a plurality of springs  191 , such as disc springs, are utilized to provide the biasing force. Linear guideways  194  allow the plate  186  to slide, which carries the bearing assembly  182  and roller  180  to provide radial force. 
     As the roller  180  and bearing assembly  182  are movable with and supported on the plate  186 , adjustment of the nuts  190  also adjusts the resultant force between the roller  180  and rollers  108 ,  150 . Thus, adjusting the nuts  190  to force the roller  180  away from the lower rail  14   a  increases the load applied to the first set of rails  14 , or radial force  103  ( FIG.  9 D ). 
     As with the roller  108  of drive roller assembly  44  and roller  150  of the support assembly  42 , roller  180  has a concave surface  192  which conforms to an outer surface of a cylindrical rail. In this instance, the cylindrical rail is upper rail  14   b . 
     For drive-guide systems  40  engaging the first set  14  and second set  22  of rails, the biasing assembly adjusts a “z” distance between the roller  180  and rollers  108 ,  150 . For drive-guide systems  40  engaging the third set of rails, the biasing assembly  184  adjusts either an “x” or “y” distance between these elements, depending upon the orientation of the drive-guide systems on the tool frame  38 . 
     Components, such as the rollers  108 ,  150 ,  180 , rings  119 ,  158 , bearing assemblies  106 ,  152 ,  182  and the like may be identical between the described assembly, or may include differences between them without departing from the spirit of the invention. For example, the roller  180  of the loaded guide assembly  46  may be shaped similarly to roller  108 A as shown in  FIG.  9 A , or it may be identical to roller  108 . Roller  180  may conform less to the rail  122  (or  14   b ) at the loaded guide assembly as axial movement is controlled by other elements and some axial float may be desired to avoid binding. 
     In addition to the linear systems above, a radial adjustment mechanism may use similar rollers and rails. In some applications, such a radial adjustment mechanism may be disposed on the tool frame  38 . However, depending upon the type of apparatus, linear guides may be provided between rotary systems. 
     The radial adjustment mechanism comprises a radial drive motor and one or more bearing assemblies. Each of the radial drive motor and the bearing assemblies are disposed about a disc having a circular outer profile. Rollers are provided with each of the radial drive motor and bearing assemblies which engage the disc. It is preferable to have three points of contact on the disc - the radial drive motor and bearing assemblies are approximately 120 degrees apart. 
     The drive motor may be mounted on a radial pre-load system  210  which provides force to the disc to ensure a proper frictional connection between the rollers and the disc. 
     The tool frame may have three or more radial adjustment mechanisms. For example, a first mechanism a rotates a bottom portion of the tool frame  38  about a substantially vertical axis. The second mechanism rotates the tool  12  about a substantially horizontal axis. 
     The third adjustment mechanism rotates the tool  12  relative to the tool frame  38  itself. The third adjustment mechanism is rotatable about an axis which is perpendicular to the substantially horizontal axis of the second mechanism, but its orientation relative to the directions “x”, “y”, and “z” is adjusted due to the operation of the second adjustment mechanism. Additional degrees of rotational freedom may be added, and various arrangements may be utilized depending upon the type of machine. 
     The gantry  10  thus can manipulate the tool  12  accurately along three perpendicular axes, and can rotate the tool  12  about three additional axes. 
     Given that the above system does not utilize any gear boxes nor does it provide a guide system separated from its drive system, backlash and runout are both significantly reduced. Total volumetric accuracies in large machine tools utilizing existing guide/drive systems may be as large as 0.03″ or 750 microns. The present gantry  10 , due to the near-elimination of backlash and runout, may be able to achieve accuracy that is one hundred times greater, with total volumetric accuracy in the five to fifteen micron range. 
     Used in this specification and the claims, the phrase “complementary to the profile” or “conforms to” when discussing the interface of the cylindrical rail and roller  108 , 150 , 180 , means that the roller is optimized to fit the rail. For example, a rail having a diameter of approximately five centimeters has an ideal complementary concavity on an associated roller. If the diameters too closely match, frictional resistance between the rollers and rail will cause problems. Conversely, if the diameters are too disparate, axial position of the roller relative to the rail will not be maintained, which can cause inaccuracies and unfavorable stress profiles. 
     If the diameter of the concavity of the roller and rail substantially match, more resistance to lateral (axial) movement will exist, and load capacity will increase. At a 14 degree roller pressure angle on a 49.983 millimeter rail, a 2.91 millimeter gap would exist between the roller and the rail, as the roller would have a concavity with a diameter of 75 mm. 
     Analytical tools reveal a best mode for a 49.983 millimeter rail being a roller with a 50.1 millimeter edge diameter. The roller pressure angle in this instance is 22.5 degrees with a gap of one one-thousandth of an inch between the rail and roller. This tight tolerance effectively reduces lateral movement and wear. While this is the best relationship between rail and roller diameter, it should not be construed as limiting. The example shown with a 75 millimeter diameter concavity may work in many applications, without departing from the scope and spirit of these claims. 
     The various features and alternative details of construction of the apparatuses described herein for the practice of the present technology will readily occur to the skilled artisan in view of the foregoing discussion, and it is to be understood that even though numerous characteristics and advantages of various embodiments of the present technology have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the technology, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts within the principles of the present technology to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.