Abstract:
An arc spray gun has a pair of wire guides that guide two metal wires to an arc zone near the wire tips where an electric current through the wires effect an arc, thereby melting the wire tips. The wire guides are of a two-part design. One part is easily removable without the need for tools. Generally, each removable part of the wire guide is temporarily held within a stationary wire guide by an O-ring. A retainer then holds the removable wire guide firmly in place. A gas cap keeps the retainer firmly against the removable wire guides while the gun is in use.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to thermal spray apparatus and particularly to a dual wire, arc type of thermal spray gun. 
     BACKGROUND OF THE INVENTION 
     Thermal spraying is a process of melting and propelling fine particles of molten material such as metal to form a coating. One type of thermal spray gun is a dual wire, arc thermal spray gun in which two wires are fed into electrical contact at the wire ends. The ends are melted by an electrical arc with current passed through the wires. A jet of compressed gas (usually air) is blown through the arc zone to atomize (i.e. nebulize) the molten metal and effect a spray stream of molten metal particles. Arc current can be of the order of hundreds of amperes. Typically the power is brought through cables connected to feed rollers and/or wire guides in the gun that electrically contact the wires and guide them to the point of arcing. 
     Wire guides are used to both guide the wire to the arc zone and to transfer electrical energy from the DC power source to the wire. Because they need to be efficient conductors of electricity, they are usually made of copper or copper-base alloys. Due to the friction of the feedstock wire, some of which can be extremely abrasive, wire guides need to be changed on a frequent basis. 
     Various configurations for jetting the atomizing air to the melting wire tips have been used in efforts to provide an effective spray stream, and for introducing auxiliary air to modify and improve the spray stream, for example as taught in U.S. Pat. No. 5,964,405 (Benary et al.) However, there has remained a need for improvement in the wire guides. In the existing art, the wire guides are screwed into a stationary wire guide that requires a tool in order to replace the guide. On production lines where the spray guns are used, it may take an average of five minutes to change the wire guides in one spray gun. This is true if there are no problems, such as binding between the threaded portion of the wire guide. In an effort to properly align the wire guides, a technician may use a tool to bend the wire guide after it has been screwed into place. This can cause difficulty when the time arrives for replacement of the wire guide. Some companies have several spray guns on line, and may need to shut down production for a significant amount of time between each guide change-over. Accordingly, a need exists for an improved spray gun having wire guides that can be changed relatively quickly. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved, dual wire, arc thermal spray gun that allows the user to change the wire guides without the need for any tools. The wire guides may be removed and replaced by hand without screwing them into place. An arc thermal spray apparatus of the present invention generally includes a spray gun body housing, a pair of wire guides that can receive wire, and a gas cap or nozzle attached to the body adjacent the wire guides. The wire guides are positioned so that their ends converge to guide the two metal wires to a point of contact at their spraying tips. A gas tube is positioned in the gun body on an axis located centrally with respect to the stationary wire guides, and is used to channel gas such as air to the spraying tips. A retainer is attached to the gun body coaxially with the central axis to hold the removable wire guides in place. The gas cap is attached to the gun body coaxially with the central axis to serve as a nozzle and to hold the retainer in position with respect to the removable wire guides. 
     The removable wire guides can be removed from the stationary wire guides without the use of a tool. The user can quickly change the removable wire guides by simply removing the gas cap and retainer, and pulling the removable wire guides from the stationary wire guides housed by the gun body. A fresh pair of removable wire guides are pushed into the stationary wire guides so that they are temporarily frictionally retained by the O-rings seated within the stationary wire guides, and the retainer slides over the guides and gas cap is reattached to the gun body. The relatively quick ability to change wire guides results in significant time savings. Further, the retainer ensures optimal alignment of the removable wire guides. 
    
    
     While the present invention is particularly useful in dual wire arc thermal spray guns, other applications are possible and references to use with dual wires should not be deemed to limit the application of the present invention. The present invention may be advantageously adapted for use where similar performance capabilities and characteristics are desired. These and other objects and advantages of the present invention will become apparent from the detailed description, claims, and accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of an arc spray apparatus of the present invention; 
     FIG. 2 is cross-sectional view of the apparatus shown in FIG. 1, taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is an exploded view of the apparatus shown in FIG. 1; 
     FIG. 4 is a front perspective view of a portion of the retainer as seen in FIGS. 2 and 3; 
     FIG. 5 is a rear perspective view of the retainer shown in FIG. 4; 
     FIG. 6 is a side cross-sectional view of the retainer shown in FIGS. 4 and 5, taken along line  6 — 6  in FIG. 4; 
     FIG. 7 is a side elevational view of an embodiment of the stationary wire guide of the present invention, prior to bending; 
     FIG. 8 is a top plan view of the stationary wire guide of FIG. 7; and 
     FIG. 9 is a front perspective view of the stationary wire guide of FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A dual wire, arc thermal spray gun  10  (FIGS. 1-6) incorporating the invention may be a conventional type except with respect to the wire guides described herein. In the present example, a gun body  12  has three general portions, namely a forward portion  14 , a middle portion  16  and a rear portion  18  (FIG.  1 ). As best seen in FIG. 2, the middle portion  16  contains the wire guides  20  and a gas tube  22 . From the forward portion  14  projects a nozzle or gas cap  23 , and from the rear portion  18  projects the rear portion  24  of the wire guides  20 . A retainer  30  is positioned on the wire guides  20  in the middle portion  16 . 
     (As used herein, the terms “forward” and “front” are with reference to the direction in which the wires are driven, and “rear” and “rearward” denote the opposite direction. The terms “inner” and “inward” mean facing or directed toward the axis.) 
     The rear portion  18  can be connected to a conventional wire drive mechanism (not shown). The type of wire drive is not important to this invention, and any suitable conventional or other desired mechanism may be used. A wire drive may be used to replace or supplement the push drive in the gun. 
     In the present example, electrical contact is made through the wire guides  20  to the wires (not shown). Electrical connections to the wire guides  20  can be made by a typical screw/nut connection to a pair of electrically conductive coaxial cables (also not shown) which extend from a power source to the rear of the gun at rear portion  24 . The type of electrical contact to the wires is not important to the present invention, and any other conventional or desired contact means such as compressive joints may be used. 
     Referring to FIGS. 2 and 3, the gas cap  23  is held to the forward gun body by a threaded portion  40  located on the exposed section of adapter  32 , at the front portion  14  of the gun body. The gas cap  23  is positioned coaxially with a central axis  42  located centrally between the wire guides  20 . The gas cap  23  in the present embodiment has a radius section  44 , rearward of a tubular section  46 . Radius section  44  can generally be described as a conicalshape with a decreasing interior radius. The radius section  44  narrows forwardly to a constricted aperture  48  to form a nozzle directing the atomizing air flow from the gas tube  22  to the wire tips (not shown). Atomizing air or other gas from a source of compressed gas is brought through hoses or the like and into the gas tube  22  at the rear portion  18  of body  12 . This gas cap  23  is known in the art, and can be replaced by other suitable gas caps. 
     The wire guides  20  converge in a forward direction so that the tips of the metal wires feeding therethrough will approach each other at a point forward of the guides, just inside the aperture  48 . With a conventional source of arc power (typically DC) applied through the wires, just before they contact each other an electric arc will be formed, thus melting the wire ends. From the internal radius section  44  of gas cap  23 , issues a jet of air axially through the molten wire tips to atomize and propel a spray stream of molten metal particles (designated schematically by an arrow  50 ) to a substrate for deposition. 
     Each wire guide  20  has a two-piece construction, generally, a stationary wire guide  60  that is slidably connectable to a removable wire guide  62 . Stationary wire guides  60  are fixed within the body  12 , whereas the removable wire guides  62  can be selectively attached to stationary wire guides  60 . 
     The specific construction of the stationary wire guide  60  is as follows. As seen in FIGS. 7 through 9, the guide  60  can be machined on a lathe from a round metal stock or square metal stock (as shown), preferably, of copper or copper alloy. The exterior surface of guide  60  has three primary features. At the rear portion  24  there is preferably an inner or outer thread for connection to a wire source. At the opposite end of the guide  60  is preferably a tapered nose  64 . The purpose of the tapered nose  64  is to aid in alignment as the guides  60  are set into the molded body  12 . This feature is optional, as there may be other ways to aid in such alignment. Somewhere in the middle section of guide  60  is a series of partial necks  66  between collars  68  that are formed on a lathe, or the like. Preferably, there are three necks  66  and two collars  68 . The purpose of the necks  66  is to relieve stress at the middle section so that it can be more easily bent and to prevent adverse deformation such as cracking in the tensile area  70  of the outer skin, or crumpling the compressive inner area  72  of the outer skin. 
     An inner bore  80  runs through the length of stationary guide  60 . The purpose of inner bore  80  is to guide the wire (not shown) from a feed mechanism (also not shown), and to accommodate the removable wire guide  62 . Therefore, the bore  80  preferably has at least two distinct diameters. At the tapered nose  64  the bore is preferably of a diameter that allows the removable wire guide  62  to maintain a steady position during gun operation, yet allow the user to remove the removable wire guide  62  from the guide  60  without the use of any tools. In at least the middle portion of the guide  60 , the bore is of a diameter that can accommodate wire as it slides through the guides. Near the rear portion, the guide  60  can be counter-bored to a larger diameter bore  81  to accommodate a friction-reducing coaxial cable-liner. It is preferable to offset the necked portion  66  with respect to the longitudinal axis of inner bore  80  to help prevent cracking of the outer skin when guide  60  is bent. 
     The specific construction of the removable wire guide  62  is as follows. As seen in FIGS. 2 and 3, guide  62  can be constructed from tubular metal stock, such as copper or copper alloy. As seen in FIG. 2, at a forward end  84 , removable guide  62  is preferably tapered so that the guides two  62  can converge more closely together when assembled into stationary guides  60 . Near a center portion of guide  62  is an outwardly extending collar or shoulder  86 , which has a forward side  88  and a rear side  90 . The forward side  88  of the shoulder provides a surface upon which a retainer  30  (described herein) bears, once gun  10  is assembled. When assembled, the rear side  90  of the shoulder bears against the forward end of stationary guide  60  so that there is adequate electrical contact between the guides  60  and  62 . There is an inner bore  92  of substantially uniform diameter through the length of guide  62 . A preferable exception to this uniformity is a tapered counter-bore or radius  94  located at the rear end of bore  92 . The purpose of radius  94  is to prevent wire from getting caught at the junction between the stationary and removable guides. Therefore, it is most preferable that the radius is smooth. 
     As seen in FIGS. 2 and 3, the adapter  32  is made of a generally tubular material, and preferably of aluminum or the like. The purpose of adapter  32  is to provide a means of attachment to the gun body  12 . As mentioned previously, the forward end of adapter  32  is preferably threaded on its exterior (or interior) surface so that the air cap can be threadedly attached thereto. In one embodiment, the rear portion of adapter  32 , two notches  102  are symmetrically positioned along the adapter  32  diameter to accommodate the stationary wire guides  60 . A collar  104  may be used to aid in the assembly of the gun  10 , which involves a molding process. The collar  104  is generally flush with respect to the front surface of gun body  12 . In addition, as seen in FIG. 3, there is preferably a shoulder  106  at the rear end of adapter  32  for the purpose of anchoring adapter  32  into the molded gun body  12 . Other means of attaching the gas cap  23  to gun body  12  may be used. For example, a snap-on attachment may be utilized. 
     The retainer  30  is preferably molded or machined from a non-conductive, heat-resistant material such as a high performance plastic. As seen in FIGS. 4 through 6, retainer  30  is generally a cylindrical-shaped member with a slot  110  extending therethrough. Slot  110  accommodates and properly positions removable guides  62  by being longer at the rear than at the front. The forward end of retainer  30  preferably has a substantially flat forward face with slot  110  having a relatively short opening therein. Also, preferably, the angle of the slot is somewhat less divergent than the outside taper of the properly aligned guides  62  so that the there is some clearance between the guides  62  and retainer  30  except at shoulders  86 . Specifically, the rear surface of retainer  30  is a “peaked surface” resulting from two converging planes  112  and  114 , wherein the peak  116  is laterally positioned with respect to the relatively large opening of slot  110 . Inside the slot  110  at the peaked surface is an inner rim  115 . The inner rim  115  bears against shoulder  86  on the removable guide  62 . Preferably, on each end of slot  110 , the surface  113  surrounding rim  115  fits against shoulder  86  with a clearance fit. The peaked surface and the inner rim are at substantially the same angle as removable wire guide shoulders  86  so the retainer can put a substantially uniform pressure thereon. As can be seen in FIG. 2, such pressure keeps the guides  62  in place and properly aligned during use of the gun  10 . 
     Optionally, retainer  30  may include one or more air channels  117  running from the peaked surface to the opposite flat face. Preferably, there are about six equally spaced air channels  117  as seen in FIGS. 4 and 5. The purpose of the air channel  117  is to provide a decrease in air pressure at the arc zone to prevent distortion of the electric arc. However, this may also be achieved by reducing the gas or air pressure fed through the gun  10 , or by using more or less air channels  117 . 
     Referring again to FIGS. 2 and 3, gas tube  22  is generally a tubular member that has a taper at the forward end  120  so as not to interfere with the converging ends of the stationary wire guides  60 , and has a necked portion  122  so it cannot be pulled out of the gun body  12 . Further, the tube  22  preferably has a hexagonal profile, or some other profile that can resist torque so that it does not become dislodged from gun body  12 . An inner bore  124  extends through the length of gas tube  22 . Preferably, a threaded chamfered counter-bore  126  extends inwardly from the rear portion  18  of the gun body  12  for attachment to an air or gas source (not shown). It will be appreciated that the body  12  could contain a bore rather than a gas tube  22 . 
     Preferably, the apparatus  10  is generally formed by setting the stationary components such as the adapter  32 , gas tube  22 , and stationary guides  60  in a non-conductive epoxy or resin material. Threaded sleeves  130  (as seen in FIG. 1) may be embedded in the side of the mold for the attachment of a handle or robotic arm. After the molding process is complete, O-rings  101  are seated into the grooves  99  within guides  60 . Removable guides  62  are placed into the stationary guides  60  so that shoulder  86  butts against the tapered nose of guide  60 . The O-rings  101  temporarily hold the guides  62  in position. Retainer  30  is then placed onto the removable guides  62  so that it is seated against the forward side  88  of shoulder  86 . Gas cap  23  is attached to adapter  32  to hold the retainer  30  firmly against guides  62 , to provide stability, alignment and electrical contact. 
     In operation, the changing of the removable wire guides  62  is conveniently simple and expeditious. First, the gas cap  23  and retainer  30  are removed. Next the removable wire guides  62  are replaced with a fresh pair. The gas cap  23  and retainer  30  are reattached. It is not necessary to use tools for this procedure. 
     Other styles for the atomizing gas cap  23  may be used. For example a nozzle orifice may be used in place of the tapering section of air cap. Alternatively, two or more gas jets may be utilized, preferably axisymetrically or concentrically, for example concentric passages. However, it is advantageous to incorporate the tapering section into the gas cap  23 , for simplicity and effective atomization. 
     Although the invention has been herein shown and described in what is perceived to be the most practical and preferred embodiments, it is to be understood that the invention is not intended to be limited to the specific embodiments set forth above. Accordingly, it is recognized that modifications may be made by one skilled in the art of the invention without departing from the spirit or intent of the invention and therefore, the invention is to be taken as including all reasonable equivalents to the subject matter of the appended claims.