Abstract:
Precision movement of a table part by movement of a nut over a lead screw is achieved by a less rigorously precise structure than has heretofore been thought necessary. To that end the screw is fixedly supported only at one end and is rotatable in bearings mounted within a rigidly mounted tube which at least partially surrounds the screw, play in those bearings is minimized by exerting axial pressure thereon, the nut is especially designed to engage the screw only at separated points, thereby to reduce friction and play in the nut, and the nut is connected to the moveable table part by structure passing through an opening in the tube and resiliently engaging the moveable table part to compensate for displacement and play.

Description:
FIELD OF THE INVENTION 
   The invention relates to a mechanical precision device such as a precision table that can be useful in a variety of linear movement applications. 
   BACKGROUND OF THE INVENTION 
   A wide variety of linear translation devices are known, most of them involving a table slideable over a supporting part and moved by rotation of an externally threaded shaft on which an internally threaded nut is mounted for movement along the axis of the shaft. The shaft is driven either directly or indirectly by a motor or other appropriate drive mechanism. A typical mechanical arrangement involves fixedly supporting the shaft at two spaced points precisely rigidly mounted independently of the moveable part, and also providing very precise machined surfaces on the table and on the supporting part over which the table is slideable, precise alignment of the shaft with the direction of movement of the table being necessary to avoid jamming or stiff resistance to shaft rotation. The relationship between nut and screw also presents problems. If the nut fits the screw too tightly rotation is impeded and vibration tends to occur. If the nut fits too loosely the resultant play reduces precision. All of this in turn requires the use of costly manufacturing equipment and adds complication and expense to the manufacture of the tables. A need exists, therefore, for a simple yet rigid linear translating device that is relatively easy and inexpensive to manufacture, and which is both accurate and useable for a variety of needs for linear precise movement. This is today particularly important in various industries, in carrying out research, and in laser positioning. 
   SUMMARY OF THE INVENTION 
   The prime object of the present invention is therefore to provide a linear translation device which is highly accurate but which is composed of easily manufactured parts, reliable and simple to maintain, and insensitive to shock or bad handling. This object is achieved, in accordance with the present invention, by a novel mounting of the lead screw and a novel connection of the lead screw to the moveable table parts. 
   The moveable table part slides relative to a stationary table part. The lead screw is mounted at one end so as to be supported fixedly with respect to the stationary table part. The other end of the screw need not be so mounted. Rigid parallelism between the screw and the direction of movement of the table is not required. 
   The nut on the screw is operatively connected to the moveable table part by structure which compensates for any departure of the screw action from such exact parallelism. A structural element such as a tube at least partially surrounds the screw, operatively engages the far end of the screw and helps to support it. The screw rotates in bearings, some of which may be supported by the structural element, and axial pressure may be exerted on those bearings, from that structural support or otherwise, in order to minimize bearings play. The mechanical connection between the nut and the moveable table part resiliently engages the latter by means of structure which will act without substantial loss of precision, despite minor displacements of the part relative to its ideal location. When, as mentioned, the screw is at least partially surrounded by a tube, that mechanical connection between nut and moveable table part passes through a slot in that tube. In addition, to reduce friction and play in the nut on the screw, the nut may be partly split intermediate its length with means provided for adjusting the relative position of the two parts produced by the split, thereby to cause the nut to engage the screw only at a limited number of spaced areas, reducing friction and play. 
   Each of these structural arrangements contributes to the achievement of very high precision in moving the moveable table part while utilizing relatively inexpensive mechanical parts which can be readily assembled and disassembled without compromising the overall precision of the assembly. 

   
     DESCRIPTION OF THE DRAWINGS 
     To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to the construction of a precision table or the like as described in this specification and as disclosed in the following drawings, in which: 
       FIG. 1  is a three-quarter perspective view of a preferred embodiment of the present invention with the parts directly connecting the nut with the moveable table separated from their normal operating position for purpose of illustration; 
       FIG. 2  is an exploded view of the moveable table of  FIG. 1 ; 
       FIG. 3  is an exploded perspective view on an enlarged scale of the parts which support one end of the lead screw; 
       FIG. 4  is a view similar to  FIG. 3  but showing the parts which support the other end of the lead screw; 
       FIG. 5  is an enlarged perspective view of the lead screw, nut and connection of the nut to the moveable table with the latter in its normal operating position; and 
       FIG. 6  is a three-quarter perspective view on an enlarged scale of a slotted nut which rides over the lead screw. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A typical table comprises a fixed part generally designated  2  and a moveable part generally designated  4  slideable thereover, the engaging surfaces of those tables being cooperatingly shaped and machined so that the table  4 , as it is moved, slides smoothly and accurately over the fixed part or base  2 . The means for accurately moving the table  4  over the fixed part  2 , a preferred embodiment of which is here specifically disclosed, comprises a support plate  6  secured to the base  2  by screws  8  passing through holes  10  in the support plate  6  and being received in holes  12  in the table base  2 . All of the operative parts of the table moving means are mounted on that support plate  6 . 
   In the embodiment here specifically disclosed the table part  4  is designed to be moved by means of a motor  14  secured to the support plate  6  by screws  16  passing through holes  18  in the support plate  6  and engaging the motor  14 . The motor has an output shaft  20  with a driving part  22 , such as a gear or the like. A preferably internally toothed drive belt  24  drivingly connects that driving part  22  to a driven part  26  on the end of an elongated externally threaded lead screw  28 . Mounted on the lead screw  28  is an internally threaded nut  30  which, when the screw  28  is rotated, will move along the screw. The nut  30  is connected by structure generally designated  32  to the moveable table part  4  so that the two will move together. 
   The lead screw  28  is rotatably mounted on the support plate  6  with one end thereof, to which the driven part  26  is connected, passing through a hole  34  in the support plate  6 . The lead screw  28 , as here specifically disclosed, is mounted in cantilever fashion on the support plate  6  by the front end retainer  36  secured to the support plate  6  by screws  38  passing through holes  40  in the support plate  6  and threaded in holes  42  in the front end tube support  36  which includes forwardly extending cylindrical portion  36 a. Received within the front end tube support portion  36   a  are a pair of ball bearings  44  and  46  which support the right-hand end of the lead screw  28 . 
   The lead screw  28  extends through an elongated tube  48  the right-hand end of which extends over the cylindrical portion  36   a  of the front end tube support  36 , and is there held in place by the locking bracket  50  which is split at  52  and which is telescoped over the portion  36   a  and clamped about the right-hand end of the tube  48  by means of a screw  54  passing through the hole  56  in one of the split parts and being threadedly engaged in the opposing split part. The bearing  44  is retained within the front end tube support  36  by means of spring retainer  58  received within slot  60  on the inner surface of the tube support  36  (see  FIG. 3 ). 
   As is particularly shown in  FIG. 4 , the lefthand end  62  of the lead screw  28  is mounted in ball bearing  64  received within a countersunk portion  66  of a tube end bearing housing  68  which is substantially closed at its lefthand end and which is received within the lefthand portion of the tube  48 . Also received within the tube  48  to the left of the bearing housing  68  is a support  70  having an internally threaded central aperture  72  for receiving an adjustment screw  76 . The aperture  70  abuts against the lefthand end of the bearing housing  68  and is retained within the tube  48  by means of expansible locking ring  74 . The adjusting screw  76  engages the lefthand end of the bearing housing  68  so that when it is screwed through the support  70  it urges to the right the bearing housing  68  and the bearing  64  carried thereby, that exerts axial pressure on the lead screw  28 , and that in turn exerts axial pressure on the bearings  46  and  48 . The portions of the screw  28  which are received within the bearings  44 ,  46  and  64  are of lesser diameter than the externally threaded portion. Thus pressure exerted on the lefthand end of the screw  28  by means of the bearing housing  68  as produced by the adjustment of the screw  76  urges the screw  28  to the right and the externally threaded portion of the screw  28  in turn urges the bearings  44  and  46  to the right, thus in effect firmly and precisely locating and loading the bearings to produce minimal play in the bearings while facilitating the rotation of the screw  28 . 
   As is best shown in  FIG. 6 , the nut  30  threadedly received on the screw  28  is of appreciable length and provided with an internally threaded axial opening  78  and a transverse slot  80  approximately mid-way of the length of the screw  30  and extending approximately one-half of the width of the screw  30 , that slot  80  intersecting the axial opening  78  to produce split portions  30   a  and  30   b . A screw  82  passes through split portion  30   a  and engages split portion  30   b , the screw  82  having threaded engagement with at least one of those split portions so that rotation of the screw  82  will cause those split portions  30   a  and  30   b  to move toward and away from one another. If the screw  82  is rotated to cause the split portions  30   a  and  30   b  to separate the actual thread engagement between the nut  30  and the screw  28  will only occur at a limited number of areas. Theoretically the nut will engage the screw at two points at the bottom of the nut at its axial ends and at a third point at the top of the nut approximately mid-way of its length. This minimizes friction, making it easier for the screw to rotate, minimizes play between the screw and the nut, and permits modification of the nut as it wears down in use. 
   The outer surface of the nut  30  is longitudinally slotted, as at  84 . Fixedly mounted within that slot and extending upwardly therefrom is a stud  86  which extends through a longitudinally extending slot  88  in the tube  48  so that it extends alongside the moveable table part  4 . That table part  4  is provided with a laterally extending L-shaped bracket  90  provided with a pair of holes  92  through which the legs  94  of a U-shaped retainer  96  freely pass. Those legs  94  extend to the left of the bracket  90  where they are surrounded by compression springs  98 . The ends of the legs  94  are externally threaded to adjustably receive nuts  100  effective to compress the springs  98  against the bracket  90  and thus resiliently urge U-shaped retainer  96  to the left. The upper end  86   a  of the lug  86  extends up inside that retainer  96 , so that the compression springs  98  are effective to urge the upper end  86   a  of the lug  86  against the right-hand surface of the bracket  90 . That surface is provided, as may best be seen in  FIGS. 1 and 5 , with a pair of protuberances  102 , preferably in the form of balls or portions of balls against and between which the upper portion  86   a  of the lug  86  is received. The normal operating condition of the parts with the lug portion  86   a  thus received is shown in  FIG. 5 . In the other figures, for purposes of illustration, the lug portion  86   a  is artificially shown spaced from the bracket  90  and balls  102  in order to better illustrate the operative parts themselves. The ball-like protrusions  102  are preferably constituted by actual ball bearings housed within and extending from the bracket  90 . The arrangement permits movement of the stud  86  and its extension  86   a  up and down and sideways caused perhaps by slight departure of the screw  28  from precise parallelism with the linear movement of the table part  4 , or relative movement or vibration of the parts, and also effectively eliminates any longitudinal play between the nut  30  and the moveable table part  4 . 
   There are several aspects to the structure of the present invention which, individually and in combination, produce the improvements described above. 
   Mounting the lead screw  28  only at its right-hand end end obviously simplifies the overall structure but gives rise to the problem of a maintaining the screw in close parallelism with the direction of movement of the table. The structure of the present invention solves that problem. The screw  28  is mounted within the tube  48  the right-hand end of which is rigidly held with respect to the table and the lefthand end of which supports the lefthand end of the lead screw  28 . Residual problems of alignment and vibration are solved by the disclosed resilient connection of the screw nut  30  to the moveable table part  4  by pressing the stud part  86   a  against the table-carried bracket  90 , the stud part  86   a  being received between the ball-like protuberances  102 . Play is further minimized by utilizing the tube  48  to carry at its end means for axially loading the bearings  44 ,  46  and  64  which support the screw  28 . Play is further minimized by the use of the disclosed slotted screw  30 , the construction of which also minimizes friction and reduces wear. Together these elements permit one to achieve a high degree of precision with components which individually need not be held to a high degree of precision, and which may be readily assembled, and disassembled and replaced when necessary. The end result is a table which may use less expensive and more readily available parts effective when assembled to produce precision movement of the table part  2  in a reliable manner. 
   While but a single embodiment of the present invention has been here specifically disclosed, it will be apparent that many variations may be made therein, all without departing from the spirit of the invention as defined in the following claims: