GB2192821A - Air plasma arc torch - Google Patents

Abstract

An air plasma arc torch capable of cutting thick metal sections and high performance steels is encapsulated in a thermosetting plastics material 42 formed around metal parts of the torch by high pressure transfer moulding. The torch is provided with means for regulating the plasma air so that the torch can be adjusted to different power requirements. The means may be a replaceable air regulator tube 16, the presence, or absence of a protrusion 62 and its shape determining what proportion of air, which has travelled down the tube and up into chamber 20, leaves via ports 22 to form the plasma and what leaves via ports 24 to form an air shield. The power circuit for the torch is completed by a ring 56 within a removable shield 52. The invention can be applied to water cooled as well as air cooled torches. <IMAGE>

Description

SPECIFICATION Air plasma arc torch This invention relates to air plasma arc torches and more particularly, but not exclusively, air cooled air plasma arc torches for cutting relatively thick metal sections.

Air plasma arc cutting has been known for many years because of the recognised advantage of higher arc temperatures than oxyfuel torches and plasma arc cutting using separated gases such as nitrogen, carbon dioxide and so on. In addition the presence around the air plasma arc of free oxygen from the air shield creates the conditions where an exothermic reaction can occur when cutting ferrous metals so that the quality of the cut surface is in most respects similar to that produced using oxyfuei.

Air plasma are torches for hand held operation have hitherto been confined to the relatively low power torches employed for cutting metal up to about 5 mm thickness. Recently, a requirement for high power air plasma arc torches has become apparent following the introduction of high performance steels into, for example, vehicle bodies. Torches for such materials have a power requirement similar to that needed for a torch for cutting metal up to about 50 mm thickness.

The traditional method of cooling high power torches is by water. Water cooling ensures that the torch was maintained at a sufficiently low temperature that permitted the use of low pressure low temperature encapsulation materials due to the complexity of the metal parts of the torch. However, the provision of water cooling requires additional equipment and limits the usefulness of a torch. Air cooling of a torch is clearly a far more simple and economic alternative. However, air cooling cannot maintain a high power torch at a sufficiently low temperature which will permit the use of the aforesaid materials for encapsulation.

It is often necessary for the user of a torch to remove the shield from the torch in order to gain access to parts such as the electrode, the cutting tip and so on. When the torch is being used at a distance from the power source the user may not take the trouble to check that the power is switched off before dis-assembling the torch service components with the consequent risk that the user may receive a severe electric shock from electrically live components.

Adjustment of the power supplied to a low power air plasma arc torch was initially effected without any adjustment of other parameters, such as the air supply. It has now been discovered that, for optimum operation, the air plasma supply is not the same for different power inputs. To deal with this problem it is possible to provide different torches for different power ranges but that is not particularly satisfactory.

As has been mentioned above torches require frequent servicing. It often happens that following such servicing a torch is not correctly re-assembled or even that components are not replaced at all. In that event when power is reapplied to the torch it may not be possible for the arc to strike at the intended site, that is to say in the plasma chamber. For example the arc would strike across a nonpreferred path. Repeated arcing over plastics causes the plastics to carbonise. Once that has occurred the low resistance path of the carbonised plastic will thereafter be the preferential site for the arc to strike even if the torch is subsequently correctly reassembled.

Once a torch has reached this condition it may not be possible to make it serviceable again.

The invention has been made from a consideration of the above disadvantages and problems and also having regard to other difficulties which will be explained hereinafter.

According to the invention there is provided an air plasma arc torch wherein the torch is encapsulated in a thermosetting plastics material formed around the metal parts of the torch by high pressure transfer moulding.

It has been discovered that encapsulation with the high temperature cross-linked thermosetting plastics materials permits operation of the torch at higher temperatures consequent upon increased power inputs, than was formerly possible with the aforementioned low pressure techniques.

According to another aspect of the invention the torch comprises a hollow shield removably mountable on the torch so as to be concentric with the torch electrode, and an eccentric ring within the shield whereby the shield can be locked on to the torch. In this embodiment of the invention the eccentric ring is preferably of electrically conductive material and is adapted to bridge across contacts in a power control circuit when the shield is locked on to the torch.

According to a further aspect of the invention the torch includes an electrode, an air channel for the supply of plasma air, an air regulator removably located in the channel, said air regulator being replaceable by a different air regulator to alter the supply of plasma air.

The torch also preferably comprises one or more further channels for leading air to form a plasma arc and/or an air cooling shield, the surface of said one or more further channels being chosen such that the power required for an arc to strike thereacross is greater than the power required to form an arc in the plasma chamber.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is an axial section through a torch head; Fig.2 is a section on the line Il-Il of Fig.1 with some parts omitted for clarity; Fig.3 is an exploded view of the general arrangement of the torch; Fig.4 is a section through a modification to the shield; and Fig.5 is a side elevation partly in section, of a torch for receiving the modified shield of Fig.4.

Referring to the drawings, the torch head comprises a body 10 having an axial bore 12.

The bore is internally threaded at 14 for receiving a hollow externally threaded open ended air regulator member 16. The air regulator member is locatable co-axially within the bore 12. The air regulator member projects beyond the bore 12 and into cathode 18 terminating just behind the rear face of the cathode electrode holder. The cathode electrode holder is removably secured to the exterior of the body 10.

The air regulator member is of smaller cross section than the bore in the body thereby defining a chamber 20. A shoulder 21 on the air regulator member closes-off the rear of the chamber and in graduated form restricts shield ports 24. A first series 22 of radial ports leads from chamber 20 for supplying plasma air. The second series 24 of radial shield ports also leads from chamber 20 for supplying air to form an air shield. The ports 24 lead to a recess 26 formed around the exterior of the body. A main insulator 28 preferably of high temperature, high dielectric material such as silicon nitride is formed over the body to cover the recess thereby defining a chamber. The insulator extends along the body 10 to a point just short of the ports 22. The surface 30 of the insulator adjacent ports 22 is not a simple radial plane but is profiled so as to increase the surface area.The purpose for this will be explained hereinafter.

An anode 32 having the general form of an open ended tube is concentrically secured to the insulator 28. The anode extends beyond surface 30 of the insulator so as to define a chamber 34 between it and the body 10 into which ports 22 lead. A tip holder 36 remova bly attached to the anode, concentrically sur rounds the cathode with a space between it and the cathode forming an annular passage 38 leading from chamber 34. A plasma arc constricting tip 40 is detachably mounted on the tip holder. The tip is concentric with both the tip holder and the cathode.

The torch head is encapsulated in high temperature, addition cross-linked plastics material 42, conferring non-corrosive properties at elevated temperatures.

The encapsulation is formed by high pres sure moulding. Conductors 44, 46 for a control circuit for the power supply to the torch are embedded in the encapsulation save for the-ends 48, 50 which are exposed and extend parallel to the torch axis. A tubular shield 52, preferably of ceramic material, is removably fitted to encapsulation so as to surround the anode, tip holder and tip and define an annular air gap 54. A guide clip 55 is releasably fitted to the shield 52 for spacing the torch from a workpiece 57. The shield has a conductive section 56 internally adjacent the end of the shield that is fitted over the encapsulation. As can be seen in Fig.2 the conductive section 56 is of eccentric form. The centre of the inner surface of conductive section 56 is eccentric with the shield axis.The shield is always held concentric with the other torch parts by virtue of its engagement over the encapsulation. However, rotation of the shield about its axis (from the position shown in Fig.2) will bring the eccentric conductive section 56 into contact with the ends 48 and 50 thereby completing the control circuit. In this position the shield is, in effect, locked onto the torch and can only be removed by counter-rotation to free the ends 48 and 50 from the conductive section 56 whereafter the shield can be withdrawn axially. This arrangement provides that the torch cannot be powered unless the shield is correctly positioned and locked onto the torch.

In operation air is supplied to the bore 12 in the body by a power and air supply tube 60.

The air travels through the regulator member 16 and strikes the rear of the cathode electrode holder 18, thereby assisting in cooling of the cathode. The air supply then travels in the opposite direction along chamber 20 around the outside of the regulator member 16. The sharp change of direction of the air stream causes entrained matter such as water, oil or solid contaminants which might impair the operation of the torch to be deposited on the rear surface of the cathode electrode holder.

The air stream flowing through chamber 20 is divided into a primary plasma stream which is led through ports 22 and a secondary stream for the air shield which is led through ports 24. The division of the air stream is effected by the profiling 62 that extends around the regulator member adjacent ports 22. As can be seen in Fig. 1 the profiling re sembles the upper surface of an aircraft wing and has the effect of increasing the velocity and reducing the pressure of the air stream just as it passes ports 22. The precise dimen sions and location of the profiling 62 com bined with the graduation of shoulder 21 determines the proportion of the air stream that will pass through ports 22 to form the plasma. Thus the torch can be readily adapted for use at different power levels when differ ent amounts of air for plasma are required by changing the air regulator member 16. The other permanent parts of the torch do not need to be changed.

The air from ports 22 flows through chamber 34 and to passage 38 around the cathode in order to form an arc. Air from ports 24 passes to chamber 26 and from there through passages 64 in the insulator and passages 66 in the anode to the air gap 54 to form an air shield. It will be noted that there are a number of changes of direction on the part of the air stream to form the shield. These changes also help to cause deposition of foreign matter from the air stream.

The interior of the shield 52 is preferably shaped so that air shield flowing therethrough will be focussed at the cutting point of the torch thereby providing more oxygen gas at that location for the exothermic reaction which takes place during cutting.

As previously pointed out the surface 30 is not a flat radial plane but is profiled to increase the surface tracking path. If it should happen that the torch is assembled without the consumable components, an attempt to operate the torch with the shield in position may cause an arc to strike from the cathode body to the anode. This arc travels across the surface 30 of the insulator and the increased length thereof means that a higher power input is required than would be necessary if the torch were properly assembled. It will also be noted that the passage 64 in the insulator is inclined in the radial plane to increase the length thereof. For an arc to strike through passage 64 between the cathode body and the anode will require yet even a higher power than for an arc to strike over surface 30.

These graduated power requirements are critical to the functional performance of the system.

Instead of an eccentric arrangement for fitting the shield onto the torch other arrangements can be provided which give the same safety control. For example the conductive interior section 56 of the shield may have an internally tapered surface 70 as illustrated in Fig.4. The conductive ring 56 is located concentrically within the shield and is adapted to be received on a correspondingly externally tapered part 72 of the torch proper (Fig.5). The ends 48 and 50 of the conductors which extend over the tapered part 72 are correspondingly tapered also. It is important that the coefficient of expansion of the ring 56 and the tapered part 72 of the nozzle should be substantially the same so that no problems arise from thermal expansion when the torch is in use.

The advantage of this kind of arrangement is that the shield can be secured by moving it axially onto the torch with a slight rotation in either direction. The embodiment of Figs. 1 and 2 can generally only be fitted onto the torch by rotation in one direction.

In the embodiment described with reference to Fig.1 the regulator member 16 is profiled externally at 62 so as to increase the velocity of the air stream as it passes ports 22. It is within the scope of the invention to provide different regulators thereby changing the proportion of plasma air and shield air. Regulators can be provided with a smaller profile 62 so that the velocity of air passing ports 22 is less and the proportion of plasma air increased. Yet a greater increase in plasma air is achieved when the profiled part 62 is omitted altogether. A further change in the proportions of plasma air and shield air can be attained by providing a shoulder or other profiling on the regulator which reduces the amount of air which can pass through ports 24 to form the shield air. The ability to regulate the air flow by means of different regulators provides a torch in which the plasma air volume can be varied by +10% from a neutral value, that is when a regulator with a plain surface is used.

The invention is not restricted to the above described embodiment. Many modifications can be made. Although the invention has been specifically described with reference to air cooled dtorches, it can be used with water cooled torches also.

Claims (11)

1. A plasma arc torch wherein the torch is encapsulated in a thermosetting plastics material formed around metal parts of the torch by high pressure transfer moulding.

2. A plasma arc torch as claimed in Claim 1, wherein the torch includes an electrode, an air channel for the supply of plasma-air, an air regulator removably located in the channel, said air regulator being replaceable by a different air regulator to alter the supply of plasma air.

3. A plasma arc torch as claimed in Claim 1, wherein further air channel means are provided for optionally supplying plasma air and an air shield.

4. A plasma arc torch as claimed in Claim 3, wherein the further air channel means is arranged such that the power requirement to strike an arc thereacross is greater than the power required to form the torch arc.

5. A plasma arc torch as claimed in Claim 3, wherein the passage means are formed with abrupt changes of direction to cause deposition of solids carried by air flowing through said passage means.

6. A plasma arc torch as claimed in Claim 1, and further comprising a hollow shield removably mounted on the torch so as to be concentric with the torch electrode.

7. A plasma arc torch as claimed in Claim 5, wherein the torch includes a power control circuit and wherein the electrically conductive material is located within the shield for bridging across spaced contacts in the power control circuit when the shield is fitted on to the torch, the power control circuit being broken when the conductive material is disengaged from the contacts.

8. A plasma arc torch as claimed in Claim 6 or Claim 7, wherein the electrically conductive material is in the form of an internally tapered ring adapted to engage on a correspondingly externally tapered part on the torch.

9. A plasma arc torch as claimed in Claim 8, wherein the coefficients of thermal expansion of the internally tapered ring and the externally tapered part of the torch are substantially the same.

10. A plasma arc torch as claimed in any of Claims 6 to 9, wherein the internal surface of the shield is profiled so as to focus air passing between the shield and the torch substantially at the cutting point of the torch.

11. A plasma arc cutting torch substantially as described herein with reference to Figs. 1 to 3 or Figs.1 to 3 as modified by Figs.5 and 6 of the accompanying drawings.