Abstract:
Apparatus and methods for thermally processing a workpiece include directing a plasma arc to the workpiece and using a dielectric shield or dielectric coating to protect a forward portion (e.g., a torch head) of a plasma arc torch. The dielectric shield or dielectric coating covers a nozzle disposed within the torch head and protects the nozzle from the effects of slag and double arcing. The shield also improves operator visibility due to the spatial relationship between the dielectric shield and the nozzle.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 11/432,282 entitled “Generating Discrete Gas Jets in Plasma Arc Torch Applications,” filed on May 11, 2006. This application claims the benefit of U.S. Provisional Application Ser. No. 60/825,477, entitled “Dielectric Shield for a Plasma Arc Torch,” filed on Sep. 13, 2006. The entire disclosures of U.S. Ser. Nos. 60/825,477 and U.S. Ser. No. 11/432,282 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to use of a dielectric device with a plasma arc torch. Specifically, the invention relates to a dielectric device positioned relative to, or on a nozzle such that operator visibility of the plasma arc is increased and the risk of double arcing is decreased. 
     BACKGROUND 
     Plasma arc torches are widely used in the cutting, welding and heat treating of metallic materials. A plasma arc torch generally includes a cathode block with an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, a shield, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which includes a constricted ionized jet of a conductive plasma gas with high temperature and high momentum. The plasma gas, when energized by a DC source, forms a current path between the electrode and the nozzle (positive potential) creating the plasma arc pilot. Placing the nozzle near the workpiece causes the current path to flow between the workpiece and the electrode because the workpiece rests at a higher positive potential then the nozzle. Many of the torch components are consumable in that they deteriorate over time and require replacement. These “consumables” include the electrode, swirl ring, nozzle, retaining cap, and shield. 
     Frequently during torch operation, the operator is constrained by space or visibility, which may lead to inadvertent contact of the side of the nozzle to the workpiece resulting in “double arcing.” Double arcing is a condition where the plasma arc deviates from its intended electrode to workpiece path and instead goes from the electrode to the nozzle and then to the workpiece—causing electrical continuity between the nozzle and the workpiece. Double arcing causes premature wear to the nozzle and results in frequent nozzle replacement and additional expense. In addition, double arcing can cause nozzle stickiness, which inhibits accurate hand control of the torch. The use of a shield, which is electrically floating, around the nozzle helps to eliminate the risk of double arcing, but currently available shields have undesirable limitations. 
     Despite nozzle shields being pervasive in the commercial market, they are often bulky and inhibit visibility of the plasma arc by the operator. One design difficulty for conductive shields is establishing a sufficient dielectric gap. That is, a conductive shield must be positioned or spaced away from the nozzle to prevent the plasma arc from jumping from the nozzle to the shield. The desired gap or distance between the shield and nozzle is a function of the dielectric strength of the medium within the gap, gas dynamics, metal contamination within the gap, tolerance stack up, and the physical condition of the shield and/or nozzle. The arcing distance is the minimum distance required between a conductive shield and a nozzle to prevent the plasma arc from jumping the gap between the shield and the nozzle. In conventional torches, the conductive shield is positioned at least an arcing distance away from the nozzle causing the total covered volume surrounding the plasma arc to be large, thereby reducing operator visibility. 
     A ceramic shield can be used in place of a conductive shield, but problems associated with these consumables exist. One difficulty with ceramic shields in plasma arc torch systems, despite their ability to solve the spacing and electrical isolation problems, is that they cannot withstand the thermal and impact shocks that occur during normal industrial use. In addition, ceramic shields are generally bulky and therefore decrease operator visibility. Moreover, ceramic shields are often too brittle for most hand torch systems. 
     SUMMARY OF THE INVENTION 
     The subject matter of the invention generally relates to devices for protecting the nozzle in a plasma arc torch. In particular, the devices protect the nozzle by decreasing or eliminating double arcing events. In addition, the devices protect the nozzle by decreasing damaging interactions between the nozzle and the workpiece by increasing operator visibility. In one aspect, the invention relates to a dielectric shield for a plasma arc torch including a nozzle. At least a portion of the shield can include a non-ceramic substrate and a dielectric coating disposed on the non-ceramic substrate. The dielectric shield is sized to inhibit protrusion of the nozzle pass an end face of the dielectric shield. 
     Embodiments of this aspect of the invention can include one or more of the following features. The non-ceramic substrate can be a metal, such as, for example, copper, aluminum, steel, or an alloy. In certain embodiments, the non-ceramic substrate includes an electrically conductive material. In one embodiment, at least a portion of the dielectric shield includes a dielectric coating of an anodized material. The anodized material can be, for example, anodized aluminum or anodized copper. The dielectric coating can be formed of a ceramic layer, such as, for example a deposited layer of aluminum oxide. In some embodiments, the dielectric shield is made out of a composite material including a metallic inner substrate and an outer layer of ceramic. In another embodiment, the shield includes multiple coatings, which can be layered. The dielectric coating can be on an interior surface of the shield, on an exterior surface of the shield, over an entirety of the shield, and/or on an end face of the shield body. In another embodiment, the dielectric shield can have spring tangs for connecting or disconnecting the shield from the plasma arc torch. The shield can include multiple connecting portions, or multiple disconnecting portions, or both multiple connecting and disconnecting portions. The connecting and disconnecting portions allowing for portions of the dielectric shield to be replaced without having to replace the entire dielectric shield. 
     Another aspect of the invention relates to a torch head for a plasma arc torch for processing a metallic workpiece. The torch head includes a nozzle and an electrode and, in some embodiments, a shield. The nozzle of the torch head is mounted relative to an electrode in a torch body to define a plasma chamber in which a plasma arc is formed. The nozzle includes a conductive nozzle body portion and defines a nozzle exit orifice extending therethrough. The shield of the torch head is capable of being secured to the torch body such that at least a portion of a surface of the shield directly contacts the nozzle body portion. The shield is sized to inhibit protrusion of the nozzle pass an end face of the shield and at least partially defines a cooling passage for providing a cooling gas to the torch head. The shield includes a non-ceramic body and a dielectric coating disposed on at least a portion of the non-ceramic body. 
     Embodiments of this aspect of the invention can include one or more of the following features. The non-ceramic body of the shield can be form of an electrically conductive material, a metal, an alloy, or a conductive plastic. In certain embodiments, the non-ceramic body comprises a polymer, a plastic, a metal, or an alloy. In some embodiments, the non-ceramic body is conductive. In certain embodiments, the shield includes an anodized body. That is, the non-ceramic body portion of the shield is formed of a metallic material and the dielectric coating disposed on at least a portion of the non-ceramic body is an oxide layer formed from the anodization of the metallic material. In some embodiments, the shield is formed of an anodized aluminum body. In some embodiments, the dielectrically coated surface is an interior surface of the shield. The shield can electrically isolate the nozzle body portion, e.g., from double arcing. 
     Another aspect of the invention relates to a torch head for a plasma arc torch for processing a metallic workpiece. The torch head includes a nozzle mounted relative to an electrode in the torch body, thereby defining a plasma chamber in which a plasma arc can be formed. The nozzle includes a conductive nozzle body portion and defines a nozzle exit orifice extending therethrough. The shield of the torch head includes a non-ceramic portion, a dielectric portion, and an end face portion. The dielectric shield portion can inhibit the nozzle body portion from extending pass the end face and preventing arcing within the torch head when the shield is secured within an arcing distance of the nozzle. 
     Embodiments of this aspect of the invention can include one or more of the following features. In one embodiment, the non-ceramic portion of the shield is formed from an electrically conductive material, such as, for example, a metallic material or a conductive plastic material. In another embodiment, the non-ceramic portion of the shield is formed from a non-conductive material, such as, for example, a non-conductive polymer or plastic. The shield can include an anodized body, such as anodized aluminum body or a anodized copper body. In one embodiment, the shield is configured for cooling by a secondary or shield gas supplied from the plasma arc torch. 
     Yet another aspect of the invention relates to a nozzle for a plasma arc torch. The nozzle is adapted to be mounted relative to an electrode in a torch body, thereby defining a plasma chamber. The nozzle includes a hollow nozzle body portion and a nozzle head portion in contact with the nozzle body portion. The nozzle head portion defining a nozzle exit orifice extending therethrough. A surface of the nozzle head portion includes one or more dielectric coating(s) disposed thereon. 
     Embodiments of this aspect of the invention can include one or more of the following features. In one embodiment, the dielectric coating is applied to an exterior surface of the nozzle head portion. The nozzle can include multiple coatings disposed on the surface of the nozzle. In certain embodiments, all of the multiple coatings are dielectric coatings. In certain embodiments, the dielectric coating is applied to an exterior surface of the nozzle head portion and the nozzle body portion. The dielectric coating need not be applied to an interior surface of the nozzle head portion. The hollow nozzle body portion and/or the nozzle head portion can include copper. In one embodiment, the nozzle head portion can include at lest one of copper or aluminum. In certain embodiments, the nozzle body portion and the nozzle head portion are integrally formed. That is, the nozzle body portion and the nozzle head portion are formed as a single piece. 
     Another aspect of the invention relates to a method of protecting a plasma arc torch that includes an electrode and a nozzle disposed within a torch body. The method includes the steps of securing a shield including a non-ceramic substrate and a dielectric coating to the torch body between the workpiece and at least a portion of the nozzle. The method also includes the step of cooling the shield with a gas flowing through the torch body. In one embodiment, the shield includes a metallic, conductive substrate. In another embodiment, a surface of the shield contains anodized aluminum. 
     Another aspect of the invention relates to a method of protecting a plasma arc torch including an electrode. The method includes mounting a nozzle relative to the electrode in a torch body to define a plasma chamber, the nozzle including a nozzle body portion and a nozzle head portion in contact with the nozzle body portion. The nozzle defining a nozzle exit orifice extending through the nozzle head portion. An exterior surface of the nozzle head portion includes a dielectric coating disposed thereon. For example, the nozzle head portion can be formed of an anodized metal to provide a conductive nozzle head portion with a dielectric coating disposed thereon. The method further includes cooling the nozzle with a gas flowing over a portion of the exterior surface of the nozzle head portion. In one embodiment, the method also includes securing a shield to the nozzle. In an alternative embodiment, the method does not include securing a shield. That is, the plasma arc torch is used without a shield. 
     There are numerous advantages to the aspects of the invention described above. For example, the dielectrically coated shields and/or nozzles described above electrically insulate the nozzles from the workpieces. As a result, double arcing events are reduced and in some embodiments eliminated. In addition, the width of the torch head (i.e., the overall width of the combined electrode, nozzle, and shield) is reduced, thereby increasing operator visibility. Another advantage of using a dielectric device that includes a non-ceramic substrate and a dielectric coating is increased impact and thermal resistance. In conventional torches with non-conducting, ceramic shields, damage to the ceramic shields occurs often due to its brittle nature and inability to withstand thermal abuse. In the present invention, the dielectric devices provide comparable electrical isolation as ceramic shields, however, the dielectric devices in accordance with the invention can withstand greater impacts and thermal stresses due to the underlying non-ceramic substrate. In certain embodiments, convenience and efficiency are increased by include spring tangs and/or connecting and disconnecting portions of the shield. That is, a shield with spring tangs and/or connecting and disconnecting portions can be quickly and easily attached and removed from a torch body, thereby saving operational costs. In addition, shields including connecting and disconnecting portions can be piecemeal replaced. That is, as a portion of the shield wears away or becomes covered in slag, that portion can be removed and replaced without sacrificing the entire shield. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a vertical cross sectional view of an embodiment of a portion of a plasma arc torch with an electrode, a nozzle with a central exit orifice, a retaining cap, and a shield positioned relative to the nozzle. 
         FIG. 1B  is a perspective view of a nozzle with flutes that allow for a secondary gas passage when the dielectrically coated shield having a non-ceramic substrate is in contact with the nozzle. 
         FIG. 2A  is a perspective view of a dielectrically coated shield having spring tangs for easy connection and disconnection relative to the torch. 
         FIG. 2B  is a perspective view showing a dielectrically coated shield having a single dielectric coating disposed over the entirety of the shield. 
         FIG. 2C  is a perspective cross sectional view showing a dielectrically coated shield with multiple dielectric coatings and/or layers. 
         FIG. 3A  is a cross sectional view of a portion of a torch head including a nozzle surrounded by a conductive shield located at an arcing distance away from the nozzle. 
         FIG. 3B  is a cross sectional view of a portion of a torch head including a dielectric shield located a distance less than the arcing distance of  FIG. 3A  away from the nozzle. 
         FIG. 3C  is a cross sectional view of a portion of a torch head including a dielectric shield having a surface in contact with the nozzle. 
         FIG. 3D  is a cross sectional view of a portion of a torch head including a dielectric shield in direct contact with a nozzle. 
         FIG. 4  is a vertical cross sectional view of a torch head with a dielectrically coated nozzle. 
         FIG. 5  is a vertical cross sectional view of an embodiment of the plasma arc torch with an electrode, a nozzle with a central exit orifice, a retaining cap, and a shield having multiple portions. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention features a device for a plasma arc torch that minimizes the possibility of double arcing and maximizes cutting accuracy by improving operator visibility and edge starting (i.e., minimizing nozzle stickiness). 
       FIG. 1A  shows a vertical cross sectional view of one embodiment of a plasma arc torch  100 . The torch includes an electrode  140 , a nozzle  150  with a central exit orifice  160 , a retaining cap including an inner portion  120  and an outer portion  110 , and a dielectric shield  130 . The dielectric shield  130  can be positioned to contact the nozzle  150  without the threat of double arcing, due to the non-conductive nature of dielectric materials. That is, the dielectric shield  130  electrically insulates the conductive nozzle  150 . The dielectric shield  130  extends at least to the end face of the nozzle  170  and is sized so that the nozzle  150  does not protrude pass an end face  132  of the shield  130 . The plasma arc torch  100  produces a plasma arc, which is an energized conductive plasma gas that forms a current path between the electrode  140  and a workpiece. During torch start up, a current flows between the electrode  140  and the nozzle  150  facilitating the formation of a plasma arc pilot from gas flowing within a plasma chamber (i.e., a space between the nozzle  150  and the electrode  140 ). Positioning the nozzle  150  near the workpiece causes the arc to transfer, such that the torch current flows between the electrode  140  and the workpiece due to electrical potential of the workpiece. The dielectric shield  130  prevents double arcing caused by the formation of a second current path, protects the nozzle  150  and retaining cap  110  and  120  from slag, and protects the nozzle  150  and electrode  140  from the damaging effects of a torch head/workpiece collision. 
     In order to minimize the dielectric shield&#39;s  130  bulkiness and at the same time provide the shield with enough strength and rigidity to withstand use in the plasma arc torch, the dielectric shield is formed of multiple materials (i.e., is a composite material). For example, the body or substrate of the dielectric shield  130  can be formed of an electrically conductive, resilient material (e.g., a non-ceramic material, such as a metal, alloy, or conductive plastic) and a dielectric or insulative material (e.g., a ceramic coating) can be disposed over at least one surface (e.g., a surface adjacent to the nozzle  150 , the end face  132  of the shield) of the body of the shield  130 . The dielectric coating on the body of the shield  130  allows for positioning of the shield in direct contact with or proximate to the nozzle  150 , while still reducing or eliminating double arcing. 
     The dielectric shield  130  can be positioned relative to the nozzle  150  such that at least portion of an interior surface of the shield directly contacts the nozzle.  FIG. 1B  shows a nozzle  175  with flutes  177 . The flutes  177  form a secondary gas passage, which can allow for the flow of gas (e.g., plasma arc cooling gas or plasma arc shield gas) while the dielectric shield  130  directly contacts the nozzle  150 . The cooling gas is commonly used to cool the nozzle or impinge on the plasma arc. An example of a nozzle with flutes is shown in U.S. application Ser. No. 11/432,282. There are several advantages to having the dielectric shield  130  in contact with the nozzle  150  or  175 , such as higher operator visibility, lack of an otherwise required shield assembly, and longer nozzle and shield life. In addition, contact between the dielectric shield  130  and nozzle  150  can prevent slag from wedging in between the nozzle  150  and the dielectric shield  130 . Slag prevention reduces the risk of double arcing, thereby allowing the nozzle end face  170  to be exposed. 
       FIG. 2A  shows a perspective view of an embodiment of a dielectrically coated shield  200 . The dielectrically coated shield  200  has spring tangs  201  for quick removal and attachment to the plasma arc torch  100 . In addition, the dielectrically coated shield  200  includes a frustro-conically upper body portion  202  integrated with a cylindrically shaped lower body portion  203 . The upper and lower body portions  202  and  203  can be formed of the same non-ceramic material. Alternatively, in some embodiments, the upper and lower portions  202  and  203  are formed from different non-ceramic materials. For example the upper portion  202  can be made of a copper alloy, while the lower portion  203  can be formed of copper, aluminum, or steel. In the embodiment shown in  FIG. 2A , interior and exterior surfaces  205  and  206  of the shield  200  are coated with a dielectric coating  208 . 
     The dielectric coating can be applied to the different portions of the shield and cover various percentages of the surface of the shield. The thickness of the dielectric coating and percentage of shield surface area coated is such that only a portion of the surface of the shield large enough to electrically isolate the nozzle needs to be coated. For example, if only 30 percent of an interior surface of the shield surrounds the nozzle, then about 30 percent of that interior surface is dielectrically coated. In some embodiments, 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 99, 99.9 or more percent of a surface of the shield can be dielectrically coated. Alternatively, in some embodiments, it is desirable to coat the entire surface area of the shield (e.g., both interior and exterior surface area and the end face), such as by dielectric coating using an anodized bath. In the embodiment shown in  FIG. 2B , dielectrically coated shield  210  includes a dielectric material disposed over both interior surface  212  and exterior surface  213 , as well as end face  215 . In certain embodiments, the dielectric coating is even disposed within openings  218  configured for cooling or shielding gas flow. 
     The dielectric coating  211  can be formed of any type of dielectric material, such as, for example, porcelain, plasma sprayed ceramics, ceramic paint, titanium oxide, aluminum oxide, or any anodized material. Anodization of material occurs, for example, when a conductive substrate material, such as copper or aluminum, is submerged in an acidic charged bath, which causes an exterior surface of the material to oxidize and become non-conductive. An advantage of an anodized material, such as anodized aluminum, is that it can make an otherwise conductive durable material electrically insulative, therefore electrically insulating the shield while, e.g., absorbing torch head-to-workpiece impacts. 
     There are numerous combinations of non-ceramic substrates and dielectric coatings materials. Examples of some combinations include porcelain on a steel substrate, plasma spray ceramic on a copper substrate, ceramic paint on a steel substrate, titanium oxide on a titanium substrate, anodized aluminum on an aluminum substrate, anodized copper on a copper substrate, and ceramic on a plastic substrate. Other combinations are also possible. 
       FIG. 2C  shows another embodiment of a dielectric shield  220  having multiple dielectric coatings. For example, the bottom layer  222  can be an insulative ceramic coating and the top layer  221  can be a durable coating that is either insulative or conductive (e.g., a polymer layer or a chromate layer). By using multiple layers to form the coating, the material properties of the shield  220  can be enhanced. For example, by including a durable layer on top of a less durable or fragile layer, the durability of the coating is enhanced while its complementary property of electrical insulation is achieved by the bottom layer  222 . Another possible embodiment includes providing multiple dielectric layers, such that the body of the shield is dielectrically coated multiple times to increase material strength and resist torch head-to-workpiece impacts. There are many ways to dielectrically coat materials, for example, by chemical vapor deposition (see, e.g., U.S. Pat. No. 5,451,550), physical vapor deposition, vacuum deposit (see, e.g., U.S. Pat. No. 5,312,647), powder coating, spraying (see, e.g., U.S. Pat. No. 5,900,282), dipping, over-molding and/or brushing, each of which can be used with the invention. 
     As previously described, conventional conductive shields require a gap or spacing from the nozzle equal to or greater than the arcing distance d,  305 , in order to decrease or prevent the occurrence of double arcing.  FIG. 3A  illustrates the minimum distance d,  305  required in conventional torches. Due to the isolative properties of the dielectric coating, shields in accordance with the present technology, such as, for example shield  301 , can be positioned at a smaller distance s,  310 , away from the nozzle  303  (i.e., within the arcing distance  305 ) as shown in  FIG. 3B . By providing a small gap  310  between the nozzle and the shield cooling gasses can flow through the gap  310  and cool the exterior of the nozzle  303 , while at the same time increasing operator visibility over conventional torches that have the larger spacing of d,  305  or greater. In addition, as shown in  FIGS. 3C and 3D , at least a portion of the shield  301  can be in direct contact with the nozzle  303  while still preventing double arcing events. Positioning the dielectric shield  301  in contact with the nozzle  303  is advantageous because it reduces the total overall width of the torch head, thereby permitting better operator visibility of the workpiece and plasma arc. Direct contact between the nozzle and the shield can also reduce or eliminate slag wedged between the shield and nozzle. To cool the nozzle  303  in direct contact embodiments, the nozzle  303  and/or shield  301  includes flutes to form fluid passageways for flow of a cooling gas about the exterior of the nozzle. The gas used to cool the nozzle  303  and shield  301  escapes through openings disposed within the shield (e.g., openings  218  shown in  FIGS. 2B and 2C ). 
     While the above embodiments show a dielectrically coated shield device for protecting the nozzle from double arcing events, there are other devices that can also be used. For example, embodiments can feature a plasma arc torch having a nozzle with a dielectric coating disposed on an exterior surface. Referring to  FIG. 4 , a dielectric coating  401  can be disposed on an exterior surface of the nozzle head  402  of a nozzle  400  for a plasma arc torch. In cutting situations where a shield is not needed to protect the nozzle  400  from collision, one or more dielectric coating(s)  401  on the nozzle head  402 , (e.g., on an exterior surface of the nozzle) prevents arcing with the nozzle and increases operator visibility by reducing the total cross-sectional area and width of the torch head (e.g., the nozzle and electrode). The dielectric coating  401  need not be applied to an interior surface  403  of the nozzle head. One skilled in the art will recognize that the one or more dielectric coating(s) must be applied to a portion of a nozzle  400  that electrically insulates the electrode and maintains nozzle conductivity for the pilot arc between the electrode and the nozzle head portion during pilot arc operation of the torch. The dielectrically coated nozzle head portion  402  may be formed of copper or aluminum and is coated with an insulative material  401 . In certain embodiments, a nozzle hollow body portion  404  integrally connected to the nozzle head  402  is formed of the same material as the nozzle head portion  402 . In other embodiments, the nozzle body portion is formed from a different material than the nozzle head  402 . Examples of materials for use as the nozzle head portion  402  and/or the nozzle body portion  404  include, copper, aluminum, steel, gold, silver, titanium, and alloys thereof. The dielectric coating  401  material can be made of any dielectric, electrically insulating material, such as ceramics or an anodized metal layer. 
     Another embodiment of the invention features a dielectric shield that has connectable portions. For example,  FIG. 5  shows the shield with a bottom portion  510  connected to a top portion  570 . These two portions are mechanically connectable to form the dielectric shield. Other embodiments include a shield that has a bottom portion  510  that disconnects from a top portion  570 . Another example is a dielectrically coated shield that includes a bottom portion  510  that connects and disconnects to a top portion  570 . An advantage of connecting and disconnecting two shield portions is that the bottom portion can be made out of an expensive robust material, which easily protects the nozzle, without having to manufacture the entire shield of the expensive material. Slag created during torch operation is more likely to attach to the bottom part of the shield. Over time, the slag builds up or the bottom part of the shield wears away to a point that the shield needs replacement. By providing detachable top and bottom shield portions, replacement of only bottom portion  510  of the shield is necessary. 
     To protect an electrode and a nozzle from double arcing and damaging contact with a workpiece caused by poor operator visibility, an operator can remove an old or used shield surrounding the nozzle, and secure a shield including a non-ceramic substrate and a dielectric coating to the torch body. The shield should be secured such that at least a portion of the nozzle is covered by the shield. Thus, the shield with its dielectric coating electrically insulates the nozzle from the workpiece, thereby decreasing damage caused by double arcing. To further protect the nozzle and the electrode, cooling gas is flowed through the torch body between the nozzle and the shield. As a result, the consumable portions of the torch are cooled during use and wear at a slower rate than without the cooling. 
     A nozzle and electrode can also be protected against double arcing by mounting a nozzle including at least one dielectric coating on its exterior surface to the torch body. Specifically, by mounting a nozzle with a dielectric coating on its exterior, such as the nozzle illustrated in FIG.  4 , to a torch body, the electrode becomes insulated from double arcing events due to the dielectric coating on the exterior of the nozzle. In addition, the operator does not have to secure an additional shield over the nozzle. As a result, operator visibility of the plasma arc is maximized because the nozzle is no longer covered by or obstructed by the shield and optional shield assembly. The nozzle can be further protected by flowing cooling gas over a portion of the exterior surface of the nozzle during operation. There are many possible embodiments of a dielectrically coated nozzle ( 400 ,  401 ). For example, the dielectrically coated nozzle can include multiple coatings some which can be formed of dielectric materials. In certain embodiments, it is advantageous to apply multiple dielectric coatings. The dielectrically coated nozzle can also have various configurations. For example, the dielectrically coated nozzle can also include flutes  177  (see  FIG. 1B ) or other passageways through or around the nozzle head and/or body portions. 
     All patents cited here are incorporated by reference in their entirety. One skilled in the art will realize the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than by the foregoing description, and all changes, which come within the meaning and range of equivalency of the claims, are therefore intended to be embraced therein.