Wire thermal spray gun and method

In an angular gas cap on a wire thermal spray gun, a forward channel extends from a rearward channel at an oblique angle thereto so as to have a lateral directional component. The rearward channel at the nozzle has a channel axis parallel to the central axis of the nozzle and is offset from the central axis in a direction opposite the lateral directional component. Immediately upon termination of spraying the wire is retracted into the nozzle. A wire positioner includes a hollow collet with the wire extending therethrough. A linear actuator retains the collet against a wall to hold the collet open from the wire during spraying. Upon termination of spraying the actuator retracts to release the collet from the wall so the collet is sprung to engage the wire during the retraction. Upon startup the wire is advanced into the gas cap faster than normal spraying speed.

This invention relates to thermal spraying and particularly to a thermal 
spray gun and method for spraying at an oblique angle. 
BACKGROUND OF THE INVENTION 
Thermal spraying, also known as flame spraying, involves the heat softening 
of a heat fusible material such as metal or ceramic, and propelling the 
softened material in particulate form against a surface which is to be 
coated. The heated particles strike the surface where they are quenched 
and bonded thereto. A thermal spray gun is used for the purpose of both 
heating and propelling the particles. 
In one type of such gun (e.g. U.S. Pat. No. 2,961,335, Shepard) the 
material is fed into a heating zone in the form of a heat fusible powder, 
generally in a size between about 5 and 150 microns. In another type a rod 
or wire is fed such as described in U.S. Pat. No. 3,148,818 (Charlop). The 
heating zone is formed by a flame of some type, such as a combustion flame 
where it is melted or at least heat-softened. A melted wire tip is 
atomized by an atomizing blast gas such as compressed air, and thence 
propelled in finely divided form onto the surface to be coated. The spray 
head includes a nozzle and a gas cap for providing an annular flame around 
an axially fed spray material. 
Ordinarily a thermal spray gun has a spray head including the nozzle and 
gas cap mounted directly on a gun body for spraying in a forward 
direction, for example for coating a flat or external cylindrical surface. 
However, some applications involve spraying into restricted areas such as 
the inside of bore holes, for example cylinder bores of pumps or 
combustion engines. In such cases it is necessary to use an extension for 
the spray head adapted to deflect or otherwise direct the spray stream 
transversely so as to coat a side wall. Examples of extensions for wire 
thermal spray guns are disclosed in U.S. Pat. Nos. 3,122,321 (Wilson), 
3,136,484 (Dittrich), 3,056,558 (Gilliland et al) and 3,085,750 (Kenshol). 
It may be seen that there are several basic types: one uses a blast gas 
for deflecting the spray stream, another has an angular gas cap to deflect 
the spray, and yet another combines these two. 
In some circumstances there is a tendency for spray material from the wire 
tip to build up inside of the gas cap and/or on the nozzle face. This can 
occur in an ordinary straight-spraying gun, but particularly may occur 
with an extension in which the spray stream is deflected by an angular gas 
cap, as there is more enclosure of the spray in the gas cap. Also, the 
typically constricted spray region in a bore hole raises the temperature 
of the spray head, encouraging adhesion, and causes back deflection of 
spray particles. 
A specific material with a buildup problem in the nozzle is molybdenum 
spray wire, with which oxidation has caused jamming in the nozzle, a 
condition to which U.S. Pat. No. 2,960,274 (Shepard) is directed by 
providing a wire guide insert in the nozzle. Buildup is also associated 
with starting and stopping of spraying, as in repetitive operations. A 
bulge or "mushroom" may develop on the wire tip under ordinary stopping 
conditions, which may jam or spit off and stick to the gas cap upon 
subsequent startup. 
As generally shown in the aforementioned patents, a spray wire is driven by 
an electric motor or air-driven turbine. Further details of mechanisms 
including drive rolls for gripping and feeding the wire are illustrated in 
the aforementioned U.S. Pat. No. 3,148,818. As also pointed out in U.S. 
Pat. Nos. 2,150,949 (Stevens) and 3,378,203 (Stanton), the conventional 
practice is to coordinate starting and stopping of wire feed with 
simultaneous changing of gas flows. 
SUMMARY OF THE INVENTION 
Objects of the present invention include the providing of a novel angular 
gas cap for coupling over a nozzle of a thermal spray gun, and an improved 
process for using an angular gas cap, particularly to reduce or eliminate 
buildup of spray material in the gas cap or on the nozzle face. Another 
object is to provide an improved thermal spray apparatus incorporating 
such a gas cap. Further objects are to provide a novel apparatus and 
process for retracting a thermal spray wire upon stopping of wire feeding 
so as to further minimize buildup particularly with an angular gas cap, 
and more particularly with the angular gas cap of the invention. 
The foregoing and other objects are achieved with an angular gas cap for 
coupling over a nozzle of a thermal spray gun, preferably a wire type of 
gun. The angular gas cap has a passage therethrough including a forward 
channel with an open end and a rearward channel adapted to extend from the 
nozzle. The forward channel extends from the rearward channel at an 
oblique angle thereto so as to have a lateral directional component. The 
rearward channel has a channel axis that is parallel to the central axis 
of the nozzle and is offset from the central axis in a direction opposite 
the lateral directional component. 
The objects are further achieved with a positioning means disposed on a 
thermal spray gun for transitorily retracting the wire rearwardly 
immediately upon termination of feeding the wire. The wire tip should be 
retracted into the nozzle sufficiently fast upon termination of feeding 
the wire to prevent significant mushrooming of the wire tip. The 
retracting means is advantageously utilized with an angular gas cap, and 
preferably with the angular gas cap of the invention. The positioning 
means also advantageously includes advancing means for momentarily 
advancing the wire forwardly from the nozzle into the gas cap at a rapid 
speed greater than normal wire speed, upon startup of spraying. 
In a preferred embodiment the positioning means comprises a guide means, a 
linear actuator and a chuck assembly. The guide means is connected to the 
gun in alignment therewith for guiding a spray wire into the gun. The 
guide means includes a rearwardly facing guide wall with an orifice 
therein for the wire. The linear actuator is connected to the gun and has 
an actuating motion substantially parallel to the center axis. 
The chuck assembly is attached to the linear actuator so as to be 
longitudinally positionable by the actuating motion. The assembly 
comprises a collet chuck, a collet disposed in the chuck so as to protrude 
from the chuck toward the guide wall, and a spring means for urging the 
collet forwardly in the chuck so as to normally engage the wire. The 
linear actuator is selectively controlled to a first position or a second 
position. The first position is such that the collet is urged against the 
guide wall so that the collet is disengaged from the wire, and the second 
position is such that the chuck assembly is retracted away from rear wall 
so that the spring means causes the collet to engage the wire. Thus, with 
the linear actuator in the first position the wire is free to feed through 
the gun, and during a transition to the second position the wire is 
engaged by the collet and retracted thereby. 
The objects are also achieved by a method for thermal spraying with a 
thermal spray gun, the gun including a gun body, a nozzle mounted on the 
gun body, and an angular gas cap extending forwardly from the nozzle. The 
gas cap has a passage therethrough defining a combination chamber. The 
passage includes a forward channel with an open end and a rearward channel 
to extending from the nozzle on a channel axis. The forward channel 
extends from the rearward channel at an oblique angle thereto so as to 
have a lateral directional component. The method comprises effecting an 
annular flame from the nozzle in the combustion chamber feeding a wire 
forwardly through the nozzle on a central axis parallel to the channel 
axis and offset therefrom in a direction coinciding with the lateral 
directional component such that the wire has a tip melted by the annular 
flame, and providing pressurized gas into the angular gas cap for 
atomizing the melted tip into a spray stream that is propelled generally 
at the oblique angle. 
The method preferably further comprises stopping the feeding of the wire 
and retracting the wire rearwardly into the nozzle immediately upon 
stopping feeding. The retracting should be effected sufficiently fast to 
prevent significant mushrooming of the wire tip. The method also includes 
momentarily advancing the wire forwardly from the nozzle into the gas cap 
at a rapid speed greater than normal wire speed, upon startup of spraying. 
Objects also achieved by a method for thermal spraying with a thermal spray 
gun including a gun body, a nozzle mounted on the gun body, and a gas cap 
mounted over the nozzle. The method comprises thermal spraying normally, 
and then subsequently terminating the thermal spraying by stopping feeding 
of the wire and retracting the wire rearwardly into the nozzle immediately 
upon stopping feeding. The method is advantageously effected with an 
angular gas cap, preferably of the present invention.

DETAILED DESCRIPTION OF THE INVENTION 
A basic thermal spray apparatus for certain aspects of the present 
invention is illustrated in FIG. 1. A thermal spray gun 10 has a gas head 
11 including a gas head body 12 with a gas cap 14 mounted with a retainer 
ring 15 thereon, and a channeling section 16 for fuel, oxygen and air. 
This section has a hose connection 18 for a fuel gas. Two other hose 
connections (not shown) for oxygen and air are spaced laterally from 
connector 18, above and below the plane for FIG. 1. The three connections 
are connected respectively via valves 19 and hoses to a fuel source 20, an 
oxygen source 22 and an air source 24. The valves control the flow of the 
respective gases from their connections into the gun. 
A cylindrical siphon plug 28 is fitted in a corresponding bore in the gas 
head, and a plurality of O-rings 30 thereon maintain gas-tight seals. The 
siphon plug is provided with a central passage 32, and with an annular 
groove 34 and a further annular groove 36 with a plurality of 
inter-connecting passages 38 (two shown). Oxygen is passed by means of a 
hose 40 through its connection (not shown) and into a passage 42 
(partially shown) from whence it flows into groove 34 and through passage 
38. 
A similar arrangement is provided to pass fuel gas from source 20 and a 
hose 46 through connection 18, and a passage 48 into groove 36, mix with 
the oxygen, and pass as a combustible mixture of the combustion gases 
(fuel and oxygen) through passages 50 aligned with passages 38 into an 
annular groove 53. Groove 53 is adjacent to the rear surface of a nozzle 
member 54 which is provided with an annular arrangement of orifices 55 
leading to the nozzle face 58 at the forward end of the nozzle, fed by an 
annular channel 56 from groove 53. Orifices 55 exit at a circular location 
on face 58 coaxial with gas cap 14. The combustible mixture from groove 53 
passes through channel 56 to produce an annular flow and is ignited at 
face 58 of nozzle 54. The annular arrangement of orifices 55 inject 
annular jets of the combustible mixture into the combustion chamber. 
A nozzle nut 62 holds nozzle 54 and siphon plug 28 on gas head body 12. 
Further O-rings are seated conventionally between nozzle 54 and siphon 
plug 28 for gas tight seals. Burner nozzle 54 extends into gas cap 14 
which extends forwardly from the nozzle. Nozzle member 54 is also provided 
with an axial bore 64 extending forwardly as a continuation of passage 32, 
for a spray wire 63 which is fed from the rear of gun 10. (As used herein 
and in the claims, "forward" or "forwardly" denotes toward the open or 
spraying end of the gun; "rear", "rearward" or "rearwardly" denotes the 
opposite.) 
Air or other non-combustible pressurized gas is passed from source 24 and 
hose 65 through its connection (not shown), cylinder valve 26, and a 
passage 66 (partially shown) to a space 68 in the interior of retainer 
ring 15. Lateral openings 70 in nozzle nut 62 communicate space 68 with a 
cylindrical combustion chamber 82 in gas cap 14 so that the air may flow 
as a forward sheath from space 68 through these lateral openings 70, 
thence through an annular slot 84 between the forward surface of nozzle 54 
and an inwardly facing cylindrical wall 86 defining combustion chamber 82, 
through chamber 82 as an annular forward flow, and out of the open end 88 
in gas cap 14. Chamber 82 is bounded at its opposite, rearward end by face 
58 of nozzle 54. 
A rear body 94 contains a drive mechanism for wire 63. Such mechanism 
includes an electric motor 93 (or air turbine), with conventional gearing 
(not shown) driving a pair of rollers 95 which have a geared connector 
mechanism 96 and engage the wire. The gearing should include a mechanism 
97 for disengaging the rollers from the wire, for example as disclosed in 
the aforementioned U.S. Pat. No. 3,148,818. 
An annular space 100 between wire 63 and the forward wall of central 
passage 32, which also extend through nozzle 54, provides for an annular 
rearward sheath flow of gas, preferably air, about the wire extending from 
the nozzle. This rearward sheath of air is a conventional method of 
preventing backflow of hot gas along the wire and normally contributes to 
reducing a tendency of buildup of spray material on wall 86 in the aircap. 
The sheath air is conveniently tapped from the air supplied to space 68, 
via a duct 102 in gas head 12 to an annular groove 104 in the rear portion 
of siphon plug 28, and at least one orifice 106 into annular space 100 
between wire 63 and siphon plug 28. 
FIG. 2 shows an extension 110 of a thermal spray gun incorporating an 
embodiment of the invention. Although such an extension is useful for 
powder thermal spraying, preferably the extension connects to a gun body 
of the type shown in FIG. 1, replacing the conventional nozzle/cap 
assembly. For some applications the extension may be rotated for spraying 
circumferentially in a bore hole. The siphon plug 28, nozzle 54 and some 
associated components are the same as for a conventional gun as described 
for FIG. 1. These are given the same numeral designations in FIG. 2, and 
the above descriptions are applicable. One change is a steel nozzle 
bushing 112 retained with a threaded member 113, replacing the nozzle 
unit, the bushing having the openings 70. 
An annular gas cap 114 is attached to a tubular housing 116 with a threaded 
retainer ring 118 which provides a gas-tight seal joint. The housing 
extends rearwardly over member 113 and a tubular gas head 120 which 
connects into the gun body. The gas cap and forward end of the housing are 
mounted over the gas head by a forward bearing 122 which allows rotation 
of the gas cap/housing assembly on the gas head if such is desired in 
utilizing the extension. The bearing is advantageously a bronze bushing 
press fitted on a rearward protrusion 124 of the gas cap, and slidingly 
fitted into the bushing 112 of hardened steel that also acts as the nozzle 
retainer. 
Rearwardly (FIG. 2b) the housing is threaded onto a rotatable tubular 
member 126 which effectively constitutes a rearward extension of the 
housing 116. A locking collar 128 is threaded on the tubular member 
abutting the housing to lock the housing in place on the member. An O-ring 
seal 130 is disposed between the housing and the member. 
A rear bearing 132 such as a needle bearing supports the tubular member 126 
and consequently the housing 116 rotatingly on the gas head 120, in 
accurate alignment with the main axis 134. The tubular member 126 extends 
back to the rear body of the gun where it is fitted into a hole in the 
body, for example with a double O-ring lubricated to effect a rotatably 
sliding seal. 
The tubular member 126 contains a central pipe 136 for wire and a pair of 
rigid pipes 138,140 for conveying the combustion fuel and oxygen 
respectively, the pipes fitting into corresponding channels 144,146,148 in 
the gas head 120. The remaining space 142 in the elongated member conveys 
the atomizing air. The corresponding channels and space communicate with 
appropriate passages in the siphon plug 28 (FIG. 2a). 
A conventional drive means (not shown) for rotating the housing on its axis 
may include gear teeth or a drive pulley on the perifery of the tubular 
member. An electrical motor mounted on the rear body is geared down with a 
similarly mounted gear box from which a drive shaft extends. A drive gear 
or pulley on the shaft engages the gear teeth or belt to rotate the 
assembly of the tubular member, housing and gas cap, for example at 200 
rpm. 
The angular gas cap 114 mounts over the nozzle 54. The angular cap 
comprises a cap body 150 and further comprises coupling means 152 
extending therefrom for coupling the cap body on the extension 110 of the 
thermal spray gun. Although not shown, the angular cap may be utilized 
without an extension and so may be mounted directly over the nozzle of 
FIG. 1, replacing the conventional gas cap, if an elongated extension is 
not needed. The cap body (FIG. 2) has a passage 154 therethrough formed of 
a forward channel 156 with an open end 158, and an rearward channel 160. 
The rearward channel is adapted to extend from the nozzle 54. The forward 
channel extends from the rearward channel at an oblique angle A thereto so 
as to have a lateral directional component 161. Preferably, the oblique 
angle is between about 30.degree. and 90.degree., for example 60.degree.. 
The high pressure atomizing gas atomizes the melted wire tip 162 in the 
passage into a spray stream and propels the spray stream (not shown) at 
about the oblique angle. 
The rearward channel has a channel axis 164 located so as to be parallel to 
the central axis 166 of the nozzle and, according to the invention, the 
channel axis is offset from the central axis in a direction 168 opposite 
the lateral directional component 161. The amount of offset O is 
preferably between about 1.5% and 20% of the exit diameter E at the open 
end of the gas cap; for example, for an exit diameter of 8.71 mm (0.343 
in), the offset is between about 0.13 mm (0.005 in.) and 1.57 mm (0.062 
in.). The coupling means 152 for the gas cap has a coupling axis 
coinciding with the central axis 166. Thus the channel axis is also offset 
from the coupling axis. 
The cap body 150 has a rearward end 170 opposite the forward channel 156. 
The coupling means includes the tubular protrusion 124 extending 
rearwardly from the rearward end coaxially with the coupling axis so as to 
encompass the nozzle 54, leaving an annular passage 172 for conveying the 
pressurized air along the nozzle into the gas cap body. Preferably the 
rearward channel 160 diverges slightly conically toward the forward 
channel, to the same degree as a conventional gas cap. 
The coupling means further includes a radial flange 174 extending outwardly 
from the rearward end, for engagement with the tubular housing by the 
retainer ring 118. 
The cap body is bounded at the open end by a planar surface 176 
perpendicular to the channel axis 178 of the forward channel 156, the 
channel axis being at the oblique angle A. Advantageously the forward 
channel is defined by a truncated cylindrical surface 180, preferably of 
uniform diameter equal to the exit diameter. The truncation is defined by 
the rearward channel wall 182 and a transition surface 184. The 
cylindrical surface 180 should have a shortest length 186 between the 
planar surface and the rearward channel between about 1.5% and 15% of the 
exit diameter E at the open end of the forward channel for example, for an 
exit diameter of 8.71 mm (0.343 in.), surface 180 is between about 0.13 mm 
(0.005 in) and 1.27 mm (0.05 in.). The transition surface should connect 
smoothly to the forward channel at the side opposite the lateral 
directional component. Conveniently the transition is effected by a ball 
milled spherical section, preferably with a radius equal to the forward 
channel diameter. The rearward channel should converge to a minimum 
diameter slightly less than the forward channel diameter. 
The axis 178 of the forward channel has an intersection point 188 with a 
plane 190 extended across the planar surface, and the gas cap should be 
mountable on the gun so that this intersection point is spaced from the 
nozzle face 58 by a distance D between about 0.75 and 2.5 times the exit 
diameter E. For example, for an exit diameter of 8.71 mm (0.343 in.), 
distance D is between about 6.35 mm (0.25 in.) and 19 mm (0.75 in.). 
According to a further aspect of the invention, to prevent mushrooming of 
the wire tip upon shutdown, and subsequent jamming or loading in the gas 
cap, the wire tip is retracted rapidly into a retracted position 
preferably within the nozzle upon shut down of the spraying operation. 
Such retraction should be useful under some conditions with a 
conventional, forward spraying aircap. Such conditions are where certain 
wire materials such as bronze are particularly susceptible to loading an 
air cap and/or the wire forms an objectionably large "mushroom" tip upon 
normal shut-down. However, retraction is particularly advantageous with an 
angular aircap, preferably an aircap of the type disclosed herein as in 
FIG. 2. The retracted tip is shown by broken lines at 298. 
A positioning means in the form of an assembly 200 for retracting the wire 
upon shut-down of an thermal spraying operation is shown in FIG. 3. A 
support member such as a bracket 202 is mounted with bolts (not shown) on 
the rear plate 204 of the thermal spray gun 10 (See also FIG. 1). The 
bracket comprises a forward section 206 and a rear section, 208 both 
connected by a base section 205. Other components in the assembly are 
mounted in the bracket, so as to be connected to the gun with tandem 
passages aligned with the central gun axis for leading a thermal spray 
wire 63 into the gun. 
A guide means 212 comprising a first threaded tube 214 extends rearwardly 
at the forward section 206. A retaining nut 209 is threaded onto the tube. 
A tubular member 210 is also threaded onto the tube, rearwardly of the 
nut, and is retained in a selected position by the nut tightened against 
it. The rear wall 211 of the guide means has an orifice 213 therein sized 
to loosely fit the wire and guide the wire into the gun. A main coil 
spring 216 may be fitted loosely over the tubular member 210 extending 
rearwardly therefrom. The forward end of the spring is positioned against 
the nut which either is larger than the member 210 or, as shown, has a 
flange 217 for positioning the spring. 
A second threaded tube 218 extends forwardly from the rear section 208. A 
cylinder body 220 is threaded onto the second tube so as to extend 
forwardly therefrom, and is held in place with a jam nut 221. A rearward 
circular opening 222 is provided in the body, and a removable face plate 
224 with a forward circular opening 226 is threaded into the forward end 
of the body. An elongated tube 228 is fitted slidingly through the 
openings with respective o-ring seals 230. The tube bore 232 is aligned 
with the gun so as to pass the spray wire through the guide means 212. A 
piston 234 is affixed to the tube and has an o-ring seal 236 slidingly 
engaging the cylinder wall, defining a rearward chamber 235 and a forward 
chamber 237 in the cylinder. The actuating motion 243 of the piston should 
be substantially parallel to the center axis 166 of the gun. 
A pair of gas connectors 239 extend through the cylinder wall, one at each 
end of the cylinder. Gas hoses 241 lead from the connectors through 
respective valving 240,242 to a source of compressed gas 244, conveniently 
air. The valving is controlled to provide the gas to either chamber in the 
cylinder, and release gas from the other chamber, to selectively force the 
tube toward or away from the gun. The valving may consist of valves that 
also release the gas pressure downstream upon closing, or each set of 
valving may consist of a pair of valves in which one is opened to release 
the pressure in the cylinder upon closing of the valve to the gas supply. 
The valving is operated by a controller 246. 
A chuck assembly 248, of the general type used with drills, includes a 
collet chuck 250 and a collet 252 mounted on the forward end of the 
elongated tube 228. The chuck is attached to the tube with an adaptor ring 
254, is fitted into the main spring 216 and has a chuck flange 218 to 
compress the spring. The collet in the chuck protrudes from the chuck 
toward the rear wall 212 of the tubular member 210, and is held in a 
normally forward and closed on the wire by a strong spring system 256 
compressed between the adaptor ring and the collet. Advantageously the 
spring system comprises a stack of Belleville springs. A thick elastomer 
(e.g. rubber) ring block 258 is fitted loosely on the elongated tube 
between the chuck assembly and the face plate of the cylinder body. 
During thermal spraying compressed gas (air) from source 244 is maintained 
in the rearward chamber 235 of the cylinder, thereby urging the assembly 
with its collet 252 in a first position against the rear wall 212 of the 
tubular member which acts as a stroke stop for the chuck assembly. In this 
position the collet is open so as to allow free wire travel through the 
retracting means and the gun, so that the motor can pull the wire through. 
Upon termination of the spraying process, the drive rollers are released 
conventionally from the wire, such as by the mechanism 97 (FIG. 1) of the 
aforementioned U.S. Pat. No. 3,148,818. Simultaneously with shut-off and 
release of the wire drive, the compressed air is reversed to release the 
pressure in the rearward chamber 235 and supply compressed air into the 
forward chamber 237. The main spring 216 and/or air cause the collet to be 
backed from the stroke stop, so that the Bellville springs urge the collet 
to engage the wire. The wire is then retracted rapidly for a short 
distance into a second position, preferably within the nozzle 54, as the 
piston, tube and chuck assembly are moved rearwardly. In operation the 
control means 248 regulates the valving so as to control the piston 234 
alternatively between the first position or the second position. 
With a sufficiently strong spring 216, the air supply and valving to the 
forward chamber 237 may be omitted with that chamber being open to air. In 
such case, when air pressure in the rear chamber 235 is released, the 
spring alone effects the retraction. Thus, for the first position, the 
control means 246 causes the linear actuator to urge the chuck assembly 
against the main spring into the first position and, for the second 
position, the control means releases the linear actuator such that the 
main spring urges the chuck assembly into the second position. 
The ring block 258 cushions the assembly 248 at the end of the rearward 
stroke. In the present example the cylinder body 220 and the tubular 
member 210 each may be prepositioned longitudinally on the respective 
threaded tubes 218,209 and affixed in place by the jam nut 221 and the 
retaining nut 209. Once suitable positions are established, similar but 
permanent attachments may be substituted without such threadings. For 
example, the guide member simply may be a part of the forward section with 
a suitable bore and shoulder for a main spring (if any). 
The cylinder, piston, tube and compressed air supply constitute a linear 
actuator for longitudingly positioning the chuck assembly. Such means may 
be provided by alternative methods such as a magnetic (e.g. solenoid) 
actuator or a linear stepper motor. 
In a further alternative embodiment, the linear actuator is mounted offset 
from the central axis but has an actuating motion substantially parallel 
to the axis. In this aspect there is no need for the actuator to have a 
wire passage therethrough. Instead, the actuator is located to one side 
of, e.g. above, the wire and has a side arm connecting the actuator to the 
chuck assembly. All other components and operation are essentially the 
same as described with respect to FIG. 3. 
As a further alternative for feeding and retracting, the motor 93 (FIG. 1) 
for driving the wire may simply be a quickly reacting reversible servo 
motor through drive rolls 95 maintained in permanent engagement with the 
wire (except for removing and replacing the wire). Such a servo motor, 
e.g. Model DXM-202 of Emerson Electric Motor Company is operable in a 
first mode to drive the wire forwardly and in a second mode to retract the 
wire. Advantageously the motor is controlled by computer program in the 
controller 246' which reverses the motor only for the transitory moment of 
retraction of the wire tip into the nozzle, and then stops the motor. 
In any event the wire tip should be retracted sufficiently fast to 
substantially prevent mushrooming of the wire tip upon termination of the 
spraying process. The retraction should be within 0.5 seconds of 
termination of forward wire feed, for example 0.2 seconds. 
In another aspect of the invention to further reduce buildup, particularly 
with the angular cap, it was discovered to be advantageous during startup 
of a spraying operation to momentarily advance the wire tip out of the 
nozzle at a very rapid speed greater than normal wire speed. Preferably 
the rapid speed is between 5% and 25% greater than normal. Normal gas 
flows (fuel, oxygen and pressurized gases) for the thermal spraying 
process are preset and flowing before this advance. These flows as well as 
normal wire speed are typically provided in instructions for the gun 
and/or material being sprayed. When the wire tip reaches its normal 
location in the gas cap passage, the wire feed speed is reduced to normal. 
The advance should occur at a speed of at least 5 cm/sec (2 in/sec), e.g. 
50 cm/sec (20 in/sec) for a normal wire speed of 2.8 cm/sec (1.1 in/sec). 
This sequence may be effected with a servo motor if such is also used for 
normal wire feed and the retraction. 
Alternatively the initial rapid advance may be accomplished with a 
positioning means such as the same assembly 200 used for retracting. Thus, 
at such time when it is desirable to restart the wire feeding, the 
compressed air to the cylinder 220 is reversed, i.e. by releasing the 
pressure in the front chamber 237 and supply compressed air into the 
rearward chamber 235. The collet 252, which has continued to grip the wire 
in its retracted position, advances and pulls the wire until the collet 
strikes the wall 212 to be urged into the chuck 250 so as to thereby 
release the wire. This advance with the wire is effected with sufficient 
air pressure to chamber 235 to provide the desired rapid speed. 
Simultaneously with the wire reaching the forward position, the wire is 
re-engaged by the feed mechanism 97 being signaled by the controller, and 
is fed by the motor at its normal speed. 
As an example of a thermal spray gun incorporating the present invention, a 
Metco Type 5K wire gun sold by The Perkin-Elmer Corporation, Westbury, 
N.Y. is modified as described herein. The gas cap is an angular cap or, 
for a simple embodiment with a retractor, an EC air cap, or alternatively 
a J air cap. 
As an example of a angular gas cap of the invention, the oblique deflection 
angle is 60.degree., exit diameter is between 8.13 and 9.27 mm, the offset 
F is 0.38 mm, and distance D is 9 mm. The normal wire speed should be 
adjusted so that wire tip 134 being melted is located proximate open end 
88. 
The wire or rod should have conventional sizes and accuracy tolerances for 
thermal spray wires and thus, for example may vary in size between 6.4 mm 
and 0.8 mm (20 gauge). The wire or rod may be formed conventionally as by 
drawing, or may be formed by sintering together a powder, or by bonding 
together the powder by means of an organic binder or other suitable binder 
which disintegrates in the heat of the heating zone, thereby releasing the 
powder to be sprayed in finely divided form. Any conventional or desired 
thermal spray wire of heat fusible material may be utilized, generally 
metal although ceramic rod may be utilized. 
While the invention has been described above in detail with reference to 
specific embodiments, various changes and modifications which fall within 
the spirit of the invention and scope of the appended claims will become 
apparent to those skilled in this art. The invention is therefore only 
intended to be limited by the appended claims or their equivalents.