Abstract:
A hydrostatic guidance system for the torch carriage used in an MCVD system has a plurality of air bearings mounted on the carriage and a pressurized fluid manifold device for routing pressurized fluid, such as air, to the several air bearings. The several bearings are located and oriented on the carriage adjacent the rails or ways of the lathe bed so that the carriage is made to float, contact free, over the ways for smooth, jerk free movement. The manifold house valve adjustments for controlling the amount of air routed to each air bearing, and the spacing of each bearing from the adjacent way is controlled by adjustable studs having the air bearings mounted on the ends thereof.

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. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an apparatus for supporting and 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 refraction 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 larger 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.—&#39;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 are in direct contact with the bed of the lathe or machine. 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 non-uniform velocity profile. 
     SUMMARY OF THE INVENTION 
     The present invention 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 machined 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, 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 without friction along the lathe bed, thereby insuring a substantially uniform velocity profile. 
     In a first embodiment of the invention, each side wall has three air passages drilled therethrough at each end, and three of the air hoses from the corresponding manifold are coupled through each group of three passages to the three pads at each end. In a second embodiment, the manifold outputs are coupled directly, by means of air hoses, to each of the pads. In both embodiments, 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 invention 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. 
     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 apparatus of the invention mounted on a lathe bed; 
     FIG. 2 is a perspective view of the carriage embodying a portion of the apparatus of the invention; 
     FIG. 3 is an exploded perspective view of the carriage of FIG. 2; 
     FIG. 4 is a perspective view of the manifold device for use with the carriage of FIGS. 2 and 3; 
     FIGS. 5 a ,  5   b , and  5   c  are several views of the manifold of FIG. 4; 
     FIGS. 6 a  and  6   b  are views of the threaded stud for use in the carriage of FIGS. 2 and 3; and 
     FIGS. 7 a  and  7   b  are views of the air bearing or pad used with the carriage of FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a perspective view of the carriage  11  of the present invention depicting the essential parts thereof as mounted on a lathe bed  12 . As noted hereinbefore, the invention will be described as used on a lathe bed  12  used in the MCVD process. However, the invention, as embodied in the carriage  11  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 or 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 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  of the present invention is designed and constructed to overcome these problems and to achieve a substantially uniform velocity profile. 
     In accordance with the invention, 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 lo 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 can be made to function as an output which, as best seen in FIG. 5 a  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 . It is, of course, possible to use a second air supply  33  to supply pressurized air directly to the second manifold  31 . The operation of the pads  28  and manifolds  31  will be discussed more fully hereinafter with reference to FIG.  4 . However, 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 . In a preferred embodiment of the invention, 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. In a second embodiment of the invention, not shown, the conduits  37  are 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 the remainder of the discussion, the first embodiment will be the focus, however, it should be appreciated that the second arrangement could just as readily be used, or a combination of the two arrangements for routing the conduits is feasible. 
     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 manifolds  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 . 
     As best seen in FIG. 4, each needle valve  43  has an adjusting slot  44  and an output coupling  46  to which the conduits  37  (not shown in FIG. 4) are connected. Also shown are two of the air or gas inlet holes  32  which are to be coupled by suitable means to the pressurized fluid supply  33  by means of one or more conduits  34 . It is usually the case that one inlet  32  is all that is necessary, in which case the remaining inlets are plugged by suitable means. However, there may be instances where more than one inlet is used for achieving an even pressurized fluid distribution within the manifold  31 . Adjustment of the needle values  43  fine tunes the air bearings (pads  28 ) to achieve a balance among the several bearings by varying the amounts of pressurized air emerging from the front faces and hence, to a degree, the spacing between the front face and the lathe bed ways  13  and  14 , thereby insuring a linear, non-cocking or canted separation of the carriage  11  from the ways. 
     FIGS. 5 a ,  5   b , and  5   c  are respectively, a plan view, a side elevation view, and an end elevation view of the manifold  31 , illustrating how the input ports or bores  32  distribute the pressurized fluid to the several output ports or bores  36  through a network of internal bores  47 , shown in dashed lines. Also shown are mounting bores  48  which match with the bores  41  in the sidewalls for insertion of bolts (not shown) therein. As an example of the manifold operation, fluid applied to input  32 ′ is emitted through outlet  36 ′, and through bore  47 ′ to outlet  36 ″ and also passes through bore  47 ″ to outlet  36 ″′ (in this example, all other inputs  32  are plugged so that no fluid may escape). The fluid also travels through bore  47 ″′ to outlet  36 ″″, beyond outlet  36 ″″ bore  47 ″′ to outlets  36 ″″′. At a fluid pressure of, for example, fifty-five pounds per square inch (55 psi) there is a substantially even distribution of fluid to the outlets  36 , which, through adjustment of the needle valves, impinges upon the ways  13  and  14  to insure rectilinear floating of the carriage. It can be seen that all of the ports are connected together, hence, any one of them may function, as an output to supply air to the second manifold  31 , as discussed hereinbefore. 
     FIGS. 6 a  and  6   b  are side elevation and rear elevation views of a threaded stud  29  which is threaded into the threaded bores  30 . Stud  29  has, at one end, an adjusting slot  51  for adjusting the distance the threaded body  52  is inserted into the bore and, at the other end has a substantially convex hemi-spherical or domed nose  53  of a slightly reduced diameter. As pointed out hereinbefore, each of the studs after adjustment, is locked in place by lock nuts  35 , for example. 
     FIGS. 7 a  and  7   b  are a side elevation view and a plan view of a pad  28 , a commercially available item commonly referred to as an air bearing. Pad  28  is preferably made of porous graphite and has a smooth, flat front face  54  through which pressurized fluid is emitted. The remainder of the exterior of the body of pad  28  is non-porous so that fluid introduced into the body thereof through an inlet coupling hole  56  can only be emitted through face  54 . On the rear face  57  of pad  28  is a centrally or axially located semi-spherical concave recess  58  which is dimensioned to receive domed nose  53  of stud  29 , thereby forming a quasi ball joint when the two are assembled. Pad  28  is free to wobble on nose  53 , but when pressurized fluid is applied and face  54  is adjacent a flat portion of one of the rails or ways  13  and  14 , face  54  is forced into a position that is parallel to the surface of way  13  and  14 . This effect is optimized by adjustment of stud  29 , by adjustment of the pressurized fluid source, and by adjustment of the needle valves  43 . As pointed out hereinbefore, such adjustments insure that carriage  11  floats free of any contact with the lathe throughout the length of its travel, assuming completely flat rails  13  and  14 . Although the pad  28  is shown in FIG. 7 b  as being circular, it may have other configurations such as, for example, rectangular or elliptical or other shapes. 
     From the foregoing, it can be seen that the carriage  11  of the invention does not require lubricant, and is substantially completely free of “stick-skip” and “jerk”, thereby insuring consistently high quality product and reduced down time for apparatus cleaning. 
     The carriage of the invention has been shown and described as having pads  28  positioned to cause floating of the carriage with rails or ways having upper, lower, and side guiding surfaces. It is possible that the carriage may be used on a lathe bed wherein the ways have only one or two guiding surfaces, in which case some of the pads  28  will not be necessary in achieving substantially frictionless movement of the carriage. The principles and features of the invention nonetheless apply to such arrangements. It is also possible that a combination of air bearings and contact bearings may be desired in which case only enough pads  28  necessary to achieve such a configuration need be used. 
     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.