Abstract:
A hydrostatic guidance system for a moving carriage upon a lathe bed or other such machining has a plurality of fluid, preferably air bearings mounted on the carriage and a pressurized fluid manifold device for routing the pressurized fluid to the air bearings. The several air bearings are located and oriented on the carriage adjacent the rails or ways of the machine so that the carriage is made to float, contact free, over the ways for smooth, jerk free movement. At least one of the air bearings is mounted on the distal end of a bendable beam which, under pressure of the air, maintains the gap between the bearing and the way despite variations in the straightness or linearity of the way so as to maintain a uniform velocity profile.

Description:
RELATED APPLICATIONS 
     This application concerns subject matter related to that shown in U.S. patent application Ser. No. 09/353,943 of Mueller et al., filed Jul. 15, 1999, the disclosure of which is incorporated herein by reference, and to U.S. patent application Ser. No. 09/500154 filed Feb. 8, 2000, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an apparatus for supporting and compliantly guiding a movable lathe carriage and, more particularly, to such an apparatus for use in the MCVD process for producing optical fiber. 
     BACKGROUND OF THE INVENTION 
     Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027, issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Telephone Laboratories, Inc., has been found to be one of the most useful because the process enables large scale production of preforms which yield very low loss optical fiber. 
     During the fabrication of preforms by the MCVD process, reactant-containing gases, such as SiCl 4  and GeCl 4  are passed through a rotating substrate tube suspended between the headstock and tailstock of a lathe. A torch assembly, which heats the tube from the outside as the gases are passed therethrough, traverses the length of the tube in a number of passes, and provides the heat for the chemical reactions and deposition upon the inner wall of the tube. The torch assembly also supplies the heat for collapsing the tube to form a rod, and, in subsequent operations, for collapsing an overclad tube onto the rod, as explained in the aforementioned Mueller et al.—943 application. In the current manufacture of preforms, the torch is mounted on a carriage which is a solid structure supported and guided on the lathe or machine bed. The guidance of the carriage along a specific path is usually accomplished through the use of a typical three sided gib and way system, with the carriage having rolling or sliding elements attached and in contact with the tops, sides, and bottoms of a dual way system. Linear guide rails having various cross-sections for rolling and sliding elements and mounted to the bed may be used as an alternative. In the systems as currently used, the sliding or rolling elements on the carriage are in direct contact with the bed of the lathe or machine or with the ways. In all such systems, the movement of the carriage and the physical contact between it and the bed requires lubrication to eliminate wear and friction. An initial “stick-skip” condition must be overcome during the start of carriage motion which is a result of the friction, and the friction can also induce “jerk” in the movement of the carriage along the bed. In addition, the friction can cause or induce, over a period of time, freeplay in the system as a result of wear. Thus, where a smooth uniform velocity of the torch down the length of the tube is a necessity for uniformity of heating and deposition and, ultimately, a uniformity of product, the friction effects can, and most often do, cause a non-uniform velocity profile, and, as a consequence, non-uniformity of heating and deposition, which result in non-uniformity of product. In present day practice, friction is overcome, at least in part, through the use of lubricants which, during a production run, become a contaminant to the process and spread throughout the machine. This, in turn, necessitates frequent cleaning of the apparatus which is detrimental to the goal of substantially continuous production. Further, the lubricant does not completely eliminate the stick-slip and jerk problems which, as pointed out in the foregoing, most often lead to a nonuniform velocity profile. 
     The related U.S. patent application Ser. No. 09/500,154 is directed to a carriage guidance system that substantially eliminates physical contact between the carriage and lathe bed and, hence, overcomes most if not all of the aforementioned problems. The arrangement shown in that application is a hydrostatic guidance and support system for the movable carriage upon which the torch for the MCVD process is mounted. The carriage, as used on the MCVD lathe, is equipped with integral air bearing components which, in their geometry, match the lathe bed cross-section. Fluid, such as air, under pressure, is delivered to the bearings which, under pressure of the air or whatever fluid is used, in use, cause the carriage to float in spaced relationship to the lathe, thereby producing a nearly friction free support and guide for the carriage, which results in a smooth velocity profile, which, in turn, produces a drastic improvement in the quality (and quantity) of the MCVD product. The terms “fluid” and “air” will be used interchangeably hereinafter. 
     In greater detail, the carriage comprises a top plate to which the torch is mounted, first and second side walls depending from the top plate, and first and second inward facing guidance members in the form of flanges extending inwardly from the bottoms of the side walls. The top plate has four downwardly oriented threaded bores extending therethrough which are spaced to overlie the rails or ways of the lathe bed. Threaded studs are mounted in the bores, each stud having a partially spherical end face which fits into a hole having a spherically shaped bottom in a porous pad member thereby creating a ball joint to hold the member in place, especially while in motion. In like manner, each of the side walls has similar bores aligned with the sides of the lathe rails and in which similar studs are mounted which hold similar porous pads. Each of the flanges has a pair of bores therein for studs which also hold porous pads, beneath the ways or rails of the lathe. 
     On each of the side walls is mounted an air manifold having at least one air input, and six outputs having needle valves mounted therein. Thus, when pressurized air is supplied from a source to the manifold, each needle valve has a quantity of pressurized air emerging therefrom. The output of each needle valve is supplied by means of suitable tubing, to a porous pad, and each manifold supplies air to six of the pads of which there are twelve in all. Each pad, which preferably comprises porous graphite and which has a smooth porous face, has an input to which the pressurized air from the manifold is supplied. With all of the pads in place and with its pressurized air from the source being at an adjusted value of, for example, fifty-five (55) pounds per square inch, the needle valves and the threaded studs are used to fine tune the air pressure to the point where the carriage floats free of contact with the lathe bed, but properly centered on all axes. The carriage, which may be moved longitudinally by any of a number of drives, such as a worm drive, a rack and pinion drive, or a belt drive, for example, is then movable substantially without friction along the lathe bed, thereby insuring a substantially uniform velocity profile. 
     Inasmuch as there is no contact between the carriage and the lathe bed, lubrication and contamination of the MCVD process are eliminated. 
     The hydrostatic carriage arrangement of the application eliminates most of the maintenance associated with existing mechanical linear slide systems, the clogging of the lubricants in the elements, the contaminants to the process area, and velocity uniformities. 
     Also, because friction is substantially eliminated, the prime mover of the carriage, e.g., rack and pinion, having less of a load thereon, may be downsized in terms of the power requirements necessary to move the carriage. 
     Heretofore, in the prior art carriage arrangements wherein rolling or sliding elements on the carriage are in physical contact with the rails, for example, of the lathe bed, the movements of the carriage over time create wear on the moving surfaces. The wear is generally non-uniform and may progress to the point where gapping between the moving elements occurs. As the carriage traverses along the length of the bed, areas of binding or loosening may be encountered due to the wear. If a worn condition is present, the maintenance is usually directed to eliminating binding at the tightest point, which means that there will be portions of the carriage traverse that are loose. Some prior art arrangements make use of pre-loaded pivots or other spring loaded systems to maintain a uniform contact force between the moving elements. However, the number of components, which may include moving components, and their complexity impact the effectiveness of the system, and the velocity profile offer time of the carriage is directly depending upon the aforementioned factors. 
     The floating carriage arrangement of the aforementioned Mueller application overcomes, as pointed out, most of the problems of sticking and binding, provided the lathe bed has not been previously distorted through excess wear. Ideally, it would be a near perfect solution if the existing lathes were replaced with ones having no wear, rail bowing, or the like, but such a replacement would not be economically feasible. It would be preferable if the floating carriage arrangement could be modified to match existing rails and the like of existing lathe beds, thus making retrofit possible. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to imparting to the floating carriage of the Mueller application structural elements preferably integral therewith which act as bending beam elements. The properties of the beams which extend substantially parallel to the opposite sidewall, upon which air pads are mounted allow for a spring rate to be designed into the air bearing area which can be tuned for the necessary displacement or force functions to compensate for profile irregularities. By use of such structural pre-load, gapping and binding of the carriage to the bed can be avoided and a more uniform velocity profile obtained. 
     In more detail, the carriage, which has two sets of air bearing pads, each set having two upper, two lower, and two side pads which are supplied with pressurized fluid, which, preferably, in the MCVD configuration, is air has first and second beams in the side walls thereof having distal ends to which the side pads are mounted. The beams may be machined into the side walls of the carriage or may be mounted thereon, and the beam properties of the geometry allow for a spring rate to be designed into the bearing area to provide adequate compensation by movement of the air bearing pads for carriage contact surface or profile irregularities in the lathe bed. Thus gapping and binding of the carriage to the bed is avoided and a more uniform velocity profile obtained. The deflection and stiffness characteristics of the beams can be matched to the bed vector loads to achieve the desired result of a floating carriage, hence a more uniform operation of the MCVD process. 
     These and other features and advantages of the present invention will be readily apparent from the following detailed description, read in conjunction with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the floating carriage arrangement of Mueller application Ser. No. 09/500,154 
     FIG. 2 is a perspective view of the carriage of FIG. 1; 
     FIG. 3 is an exploded perspective view of elements of the carriage of FIG. 2; 
     FIG. 4 is a perspective view of the floating carriage of the present invention in place on the lathe bed; 
     FIG. 5 is a perspective view of the carriage of the invention; and 
     FIGS. 6 a  through  6   c  are a front elevation view, a side elevation view, and a plan view of the carriage of FIGS.  4  and  5 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a perspective view of the carriage  11  of the aforementioned Mueller application Ser. No. 09/500,154 depicting the essential parts thereof as mounted on a lathe bed  12 . As noted hereinbefore, the present invention will be described as used on a lathe bed  12  used in the MCVD process. However, the invention may be adaptable for other configurations where jerk-free, smooth movement of an element is desired in order, primarily, to produce a uniform velocity profile, as well as to reduce wear. As can be seen in FIG. 1, lathe bed  12  comprises first and second spaced rails or tracks  13  and  14  extending along the length of the bed onto which carriage  11  is movably mounted. Carriage  11  may be driven longitudinally by any suitable or conventional means  16 , which schematically represents a rack and pinion drive, but is also intended as a representation of a worm drive or a belt drive, for example. Thus, the carriage  11  is mounted on the rails  13  and  14  and, during operation, driven back and forth along the length thereof by means of the drive  16 . Mounted on the top plate  17  of the carriage  11  is a bracket and support member  18  upon which is mounted the torch or heater member  19  used in the MCVD process. As can be seen, torch  19  has a vertical adjustment  21  for fine tuning its vertical height above the bracket  18  and hence, the lathe bed  12 . Top plate  17  has depending therefrom spaced side walls  22  and  23  at the bottom  24  of each of which is an inwardly extending flange member  26 . As thus far described, carriage  11  is similar to carriages in present use, and may be milled from a single block of suitable metal, such as aluminum, or made from separate metallic parts  17 ,  22 ,  23 ,  26  bolted together as shown by bolts  27 , for example. In previous practice, carriage  11  has bearings or slides (not shown) which bear against the rails  13  and  14  and which, as discussed previously, are lubricated to reduce “stick-skip” and “jerk” during movement along lathe bed  12 . The carriage  11  is designed and constructed to overcome these problems and to achieve a substantially uniform velocity profile. 
     As shown in the Mueller application, the usual bearings or slides are replaced by a plurality of pads or air bearings  28  which are porous to the passage of air or other fluid therethrough, being made of, for example, a porous graphite material which has, as will be discussed more fully hereinafter, a smooth, flat, porous face adjacent the rails. Pads  28  are held in place by threaded studs  29  which are carried in threaded bores  30  and which provide adjustment of the pads  28  and thus separation from the surfaces of the rails or ways  13  and  14 . While the term “air” is used herein, it is to be understood that other fluids, preferably gaseous but in some cases, possibly liquid, may be used instead of air. An air manifold  31  is mounted on each of the side walls  22  and  23 . Each of the manifolds  31  has several air inputs  32 , at least one of which (not shown) is connected to a source  33  of pressurized air by an air conduit  34 . Where only a single air source  33  is used, one of the input ports  32  on the first manifold  31  can be made to function as an output which is directly connected to the input port  32  that is connected to air source  33 , to supply air through an air passage conduit  40  to an input port of the second manifold  31 , which is not shown in FIG. 1 but which is substantially identical to the one shown. Alternatively, a bore such as bore  45  in FIG. 3 which passes through carriage  11  can function as an air passage or as an internal passageway for a conduit  40 . The second manifold  31  is then connected to the air passage in the same manner as described hereinafter with respect to the air supply to pads  28  through conduits  42  at best seen in FIG.  2 . It is, of course, possible to use a second air supply  33  to supply pressurized air directly to the second manifold  31 . In FIG. 1 manifold  31  is shown with six air outlets  36 , one of which is shown connected through wall  23  to a pad  28  by means of a conduit  37 . Six conduits  37  are connected, each through a bore  38  in the side wall, to a pad  28  in the interior open volume defined by the carriage. The conduits  37  can, if desired, be routed around the ends of the carriage  11 . The first arrangement is preferred in that the conduits  37  are less likely to become snagged or otherwise interfered with by the lathe mechanisms. 
     In operation, when air or other fluid material under controlled pressure is applied to the manifold inlet  36 , with inlets not in use being plugged, the air is evenly divided among the six outlets  36  and passes through conduits  37  to the individual pads  28 , to emerge from their flat faces and force the pads  28  away from the surfaces of the lathe ways  13  and  14 . The studs  29  are adjusted to control the limiting spacing of the faces from the ways  13  and  14 , and, inasmuch as there are a total of twelve pads; two beneath each way; two adjacent the side of each way; and two above the top surface of each way; the carriage actually floats in contact-free relationship on each of the three axes relative to the lathe  12 . The studs  29  enable fine tuning of the structure to set the most desirable spacing of the face of the pads from the adjacent surface of the way. Once tuned, the studs are locked in place by suitable locking means, such as lock nuts  35 , one of which is shown in FIG.  3 . 
     FIG. 2 is a perspective view of the carriage  11  showing, in more detail, some of the elements referred to in the discussion of FIG.  1 . It can be seen that, adjacent one of the studs  29  in the sidewall, the bores  38  have couplings  39  mounted therein to which are to be attached the conduits  37  from manifold  31 . It is to be understood that all of the bores  38 , which total twelve, are to have couplings  39  affixed therein. Alternatively, bores  38  may be made large enough for conduits  37  to pass therethrough, to couple directly to pads  28 , or an interior coupler  39  to which conduits  42  are connected. Also shown are bores  41  in sidewall  23  for mounting manifold  31 . Although not shown, sidewall  22  has like bores  41  for mounting the second of the two manifold  31 . Also shown are two of the twelve pads  28 , one mounted on the interior of sidewall  22  facing inwardly and the other mounted on flange members  26  and facing upwardly. The pads  28  are connected via conduits  42  through the bores  38  and couplers  39  to the manifold  31 , not shown. The pads  28  are located such that the lower pads are beneath and closely adjacent to and face the smooth undersides of rails  12  and  14 ; the sidewall pads are closely adjacent to and face the smooth sides of rails  12  and  14 ; and the upper pads are closely adjacent to and face the smooth top surfaces of the rails  12  and  14 . Thus, when pressurized air or other fluid is applied to the porous pads  28 , a space is maintained between all of the pads and their corresponding rails and the carriage  11  floats without contacting the rails  12  and  14 . Further in order to insure stability of the carriage and prevent it from cocking relative to any of the three axes, the pads are placed relatively far apart so that they are closely adjacent the front and rear ends of the carriage. As will be seen more clearly hereinafter, the pads  28  are not fastened to their corresponding studs  29 , being free to “wobble” relative thereto. Thus, the pads  28  are, in effect, self leveling and free from any binding to the end of the stud. It can be seen that, with the arrangement just described, it is not necessary to use lubricants to insure smooth movement of the driven carriage inasmuch as there is virtually no friction between the carriage and its bearings (pads  28 ) and the lathe. 
     FIG. 3 is an exploded perspective view of the carriage  11  as formed in a single block, having been milled from a block of suitable metal, such as, for example, aluminum, and showing one of the manifolds  31  with needle valves  43  mounted in the outlet holes  36 . 
     FIG. 4 is a perspective view of the floating carriage  51  of the present invention, as formed from a single block and mounted on a lathe bed  12  having first and second rails or ways  13  and  14 . In order to avoid confusion, like paris or elements have been assigned the same reference numerals throughout the several views. As can be seen in FIG. 4, carriage  51  has a top plate  17  upon which is mounted the plate of support member  18 . On one side of plate  17  and depending therefrom is sidewall  22  on the bottom edge of which is an inwardly projecting flange member  26  (see FIG.  3 ). Air bearing pads  28  are positioned on the underside of plate  17 . 
     As thus far described, carriage  51  is substantially the same as carriage  11  of FIGS. 1,  2 , and  3 . In accordance with the present invention, plate  17  has a second sidewall  52  depending therefrom which comprises a central portion  53 , to which an air manifold  31  is mounted, and first and second longitudinally extending cantilevered beam members  54  and  56 , which are affixed to, preferably integrally with, central portion  53 . Beams  54  and  56  and have distal ends  57  and  58 , respectively, upon which are mounted air bearing pads  59  (only one of which is shown) and their respective mounted studs  29  in holes  60 . It will be noted that pads  59  are rectangular in shape, which illustrates the fact that any or all of the air bearing pads  28  and  59  may be shaped to produce the most desirable result. The beams  54  and  56  are preferably integral with center portion  53  and, as shown in FIG. 5, the entire carriage  51  may be milled from a single block of suitable metal, such as aluminum. Alternatively, the beams  54  and  56  may be mounted to the portion  53 . In either case, beams  54  and  56  are constructed to function as bending beam elements, their particular geometry allowing for a spring rate to be designed into the contact area of the air bearings  59  to cause bending from an increase in air pressure. The structure as thus described can be tuned for the necessary displacement of the air bearing pads to compensate for contact surface or profile irregularities. This structural preload compensates for such irregularities, and involves no moving parts (other than bending of the beams  54  and  56 ). Thus a more uniform motion profile of the carriage velocity is obtained. As the carriage  51  moves along the lathe bed, an irregularlity in the bed, such as bowing, will cause the beam to flex, due to the air pressure emanating from the air bearings  59 , rather than causing the carriage itself to move sideways, for example. Thus, the movement of carriage  51  remains smooth, without jerkiness, binding, or yawing. In the arrangement depicted in FIG. 4, only side wall  52  is shown with bending beams  54  and  56 , and the other air bearing locations and mountings are substantially the same as shown in the aforementioned Mueller patent application. It is possible, and may even be desirable in certain applications to use more than one set of bending beams. In general, it is desirable to have the bending beams, such as beams  54  and  56 , opposite a “hard” site of air bearings  28  mounted in depending wall  22 . The “hard” site functions as a reference, and follows any bends, for example, in the rail  13 . The bending beams  54  and  56  will, however, compensate for such bends and maintain the air bearings  59  at the proper gap relative to rail  14 , thereby preventing binding or contact between the rails and the carriage. In the arrangement of FIGS. 4 and 5, there are two air bearing pads  59  opposite two pads  28  in sidewall  22 , thus presenting two reference points and two flex points in a symmetrical “square” configuration. Such an arrangement works well in preventing wobbling or hunting of the carriage, and is a preferred configuration. It is possible, however, to use other configurations such as, for example, triangular. It is also possible to use bending beams in either the top or bottom of the carriage, or to use flex points opposite each other, such as, for example, in both sidewall  22  and sidewall  52 . This latter arrangement, unless the deviations in the lathe bed are known, so that the degree of flexure may be precisely set, will not necessarily function as well as the other arrangements tending to cause, among other things, hunting of the carriage as is moves along the track. 
     FIGS. 6 a,    6   b  and  6   c  illustrate the overall configuration of the carriage  51  in a front elevation view, a side elevation view, and a top plan view respectively. Carriage  51  as depicted in these figures has its top plate  17  milled out (or cast) to form reinforcing ribs  61  in order to lighten the overall carriage  51 . It can also be seen in these figures that the beam  54  and  56  are of a lesser thickness than sidewall  52 , or, more specifically, center portion  53 . Whether the carriage is cast, milled from a solid block, or pieced together, the thickness of the beams  54  and  56  are such that there is sufficient flexure to compensate for changes in spacing or gap between the air bearing pad and the lathe rails or ways. The beams can be “tuned” by varying their thickness, with the thinner beams having greater flexure. Thus, the velocity profile remains substantially uniform despite variations in the lathe ways which would otherwise cause variations in the velocity profile. Tuning of the beam essentially consists of designing the beam to have a spring rate which is matched to the lathe bed vector loads. 
     While the present invention has been shown and described in the context of the moving carriage in the MCVD process, it is readily adapted to other equipment or machines wherein a uniform velocity profile, or at least uniform air bearing action is required or desired, without the introduction of separate moving parts. 
     It is to be understood that the various features of the present invention might be incorporated into other types of apparatus and that other modifications or adaptations might occur to workers skilled in the art. All such variations and modifications are intended to be included herein as being within the scope of the invention as set forth in the claims. Further, in the claims hereinafter, the corresponding structures, materials, acts, and equivalents of all means or step-plus-function elements are intended to include any structure, material, or acts for performing the functions in combination with other elements as specifically claimed.