Abstract:
A method of supplying a plasma torch with a gas, mixed gas or gas mixture, in which the volume flow of the gas, mixed gas or gas mixture is controlled, characterised in that the volume flow control is effected in combination with a pressure control of the gas, mixed gas or gas mixture in such a way that the pressure control is used to adjust the level of the total volume flow through the nozzle of the plasma torch, and the volume flow control is used to adjust the volume flow portions producing the total volume flow, taking the desired gas composition into account, and an arrangement for carrying out said method.

Description:
BACKGROUND 
   The present invention relates to a method of supplying a plasma torch with a gas, mixed gas or gas mixture, in which the volume flow of the gas, mixed gas or gas mixture is controlled. In addition, the present invention relates to an arrangement for supplying a plasma torch with a gas or mixed gas or gas mixture, with an arrangement for delivering the gas or mixed gas or gas mixture to the plasma torch and a volume flow control for controlling the volume flow of the gas or mixed gas or gas mixture. 
   Various gases are used as plasma gases, such as monatomic argon and/or the diatomic gases hydrogen, nitrogen, oxygen or air. These gases are ionized and dissociated by the energy of the plasma arc. A plasma mixed gas is a plasma gas that has already been premixed by the supplier, whereas a plasma gas mixture is a plasma gas that is mixed on the spot. 
   As a rule, the plasma in a plasma torch is constricted by a water-cooled nozzle. In this way, energy densities of up to 2×10 6  W/cm 2  can be achieved. In the plasma arc of a plasma torch, temperatures of up to 30,000° C. arise, which, in combination with the high flow rate of the plasma gas, result in very high cutting speeds in all electrically conductive materials. 
   For a plasma cutting process, a pilot arc is first ignited between the nozzle and the cathode of the plasma torch with a high voltage. This low-energy pilot arc prepares the path between the plasma cutting torch and the work piece using partial ionisation. When the pilot arc touches the work piece, a cutting arc forms. 
   Plasma cutting is an established method of cutting electrically conductive materials. Depending on the cutting task, various gases and gas mixtures are used. Conventional gases and gas mixtures are, for example, air, oxygen, nitrogen and their gas mixtures, and also argon/hydrogen/nitrogen mixtures. 
   Unalloyed steels are normally cut with air or oxygen. Alloyed steels and non-ferrous metals are preferably cut with special argon/hydrogen, nitrogen/hydrogen or argon/hydrogen/nitrogen mixtures. In order to improve the quality of a cut, an additional secondary gas can also be employed, which also flows around the plasma jet. The function of the additional secondary gas is to protect the nozzle of the plasma torch against material from the work piece splashing back when piercing the work piece, thus protecting against damage. The additional secondary gas also influences the molten metal during cutting in such a way that the cut produced is free of dross and acts as a shield gas to protect the surface of the cut, which is still hot after it has been cut, against oxidation. 
   The plasma and secondary gases, mixed gases and gas mixtures are delivered to the plasma cutting torches via lines and solenoid valves. Gases are usually metered by setting or adjusting pressure. Pressure can be controlled either mechanically via pressure reducers, or electronically via pressure control valves. The use of electronic pressure regulators is common, especially in automated systems, in which a wide range of plasma cutting parameters, such as cutting current, cutting voltage, gas pressure, cutting rate, the thickness of the material and plasma cutting torch distance are stored in databases in order to achieve the greatest possible reproducibility of cutting results. 
   German document DE 195 36 150 C2, for example, describes a means and method for gas control in a plasma torch, in which gas flow is set by an arrangement consisting of a proportional valve, a pressure sensor and a shield in a plasma torch. 
   In European Patent Specification EP 0 697 935 B1, gas is metered by means of changeable needle valves. The cross-section of the needle valves, in combination with the pressure set, determines the amount of gas. Volume flow can be indicated in the process by means of variable-area flow meters. 
   Particular gas mixtures needed for processing alloyed steels and non-ferrous metals cannot however be reproducibly generated by means of pressure control. Attempts have therefore been made to reduce this disadvantage by using auxiliary devices. East German document DD 54437, for example, describes the use of a mixing chamber with pressure shields. However, this does not solve the problem since the resulting mixing ratio is very restricted. 
   It is particularly difficult to mix gases of different densities and in widely varying mixing ratios. For example, even the use of various known mixing devices, including T-fittings, injectors, labyrinth arrangements and arrangements of nozzles, as are described in East German Document DD 132247, cannot produce the varying optimum mixing ratios required. 
   It is possible to perform gas metering by means of pure volume flow control. This method can reproducibly create defined gas mixtures. 
   U.S. Pat. No. 6,248,972 B1 discloses a method and arrangement for reducing electrode and nozzle wear in oxygen plasma cutting by using a mixture of oxygen and nitrogen instead of pure oxygen. In the method, a constant volume flow of the individual plasma gases is produced by means of an arrangement consisting of needle valves and differential pressure gauges, such that the differential pressure upstream and downstream of the differential pressure gauges is kept constant by means of the needle valves upstream. Between the controlled member and the plasma torch there are pressure-reducing valves, which limit the maximum supply pressure to the plasma torch. 
   German utility model DE 201 21 641.8 U1 describes a method of supplying a plasma torch with a gas, mixed gas, or gas mixture, in which the volume flow of the gas, mixed gas, or gas mixture is controlled, using an arrangement for supplying a plasma torch with a gas or mixed gas or gas mixture with a means for delivering the gas or mixed gas or gas mixture to the plasma torch and a volume flow control means for controlling the volume flow of the gas or mixed gas or gas mixture. 
   However, even if a volume flow control is able, depending on the measuring method used, to generate a more or less constant volume flow of a gas or gases and can consistently and accurately reproduce such a gas mixture, the quality of a resulting cut is inadequate. This is especially true at the start of cutting a given material. Such inadequate quality cuts can take various forms. A lack of or delayed transfer of a pilot arc can lead to unreliable piercing of a work piece to be cut. In some cases, unreliable through cuts can occur and material can be left behind on the work piece. The formation of dross can occur and slag can form on the underside of the work piece. A major deviation in the desired angle can also lead to exceeding the tolerances of a desired right angle or slope of the work piece. 
   SUMMARY 
   A method of supplying a plasma torch with one or more plasma gases allows the volume flow of gas to be controlled. The method involves determining a desired composition of the one or more gases to be supplied to the plasma torch, and providing, from at least one gas source, a volume of the desired composition of the plasma gas or gases. Volume flow control of the plasma gas or gases to the plasma torch is effected by using pressure control to adjust the level of the total volume of the plasma gas or gases flowing through the plasma torch and by using volume flow control to adjust volume flow portions producing the total volume flow of the plasma gas or gases flowing through the plasma torch. This allows for the production of the desired composition of the plasma gas or gases. Secondary gases can also be supplied to the plasma torch. In some embodiments, volume flow can also be controlled in a manner analogous to the control of the supplied plasma gases. 
   An arrangement for supplying a plasma torch with a gas or gases allows the volume of the gas or gases to be controlled. The arrangement includes at least one gas source for providing the gas or gases to be supplied and at least one volume flow portion producing a total volume flow of the gas or gases. An effected volume flow control controls volume flow of the gas or gases, using pressure control to adjust the level of the total volume of the gas or gases as the gas or gases flow through the plasma torch. The volume flow control also adjusts the volume flow portions producing the total volume flow of the gas or gases flowing through the plasma torch, thereby producing the desired gas composition. 
   Some embodiments of the invention include a gas mixture delivery apparatus that includes one or more pressure measures for measuring pressure of the gas or gases, a logic controller responsive to the pressure measures, and one or more volume flow controllers responsive to the logic controller for controlling gas volume flow. Such embodiments can be configured to allow for pressure to measured between the one or more volume flow controllers and the plasma torch. Some embodiments may also include a secondary gas mixture delivery apparatus for analogously controlling volume flow of a secondary gas or gases provided to the torch. 
   Those skilled in the art will realize that this invention is capable of embodiments that are different from those shown and that details of the disclosed invention can be changed in various manners without departing from its intended scope. Accordingly, the following drawings and descriptions are to be regarded as including such equivalents as do not depart from the spirit and scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding and appreciation of this invention, and many of its advantages, reference will be made to the following detailed description taken in conjunction with the accompanying drawings. 
       FIG. 1  depicts a schematic arrangement for supplying a plasma torch, in the form of a plasma cutting torch, with a plasma gas mixture and a secondary gas mixture in accordance with one embodiment of the invention; 
       FIG. 2  depicts a schematic arrangement for supplying a plasma torch, in the form of a plasma cutting torch, with a plasma gas mixture and a secondary gas mixture in accordance with one embodiment of the invention; 
       FIG. 3  depicts a schematic arrangement for supplying a plasma torch, in the form of a plasma cutting torch, with a secondary gas mixture in accordance with one embodiment of the invention; and 
       FIG. 4  depicts detail of the schematic arrangement of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts an arrangement  10  for supplying a plasma torch in the form of a plasma cutting torch, the torch including an electrode  12 , a plasma torch nozzle  14  and a secondary gas nozzle  16 , and using an argon/hydrogen/nitrogen mixture for plasma cutting alloyed steels and non-ferrous metals. The arrangement  10  includes a plasma gas mixture delivery apparatus  18  having a single gas source (not shown) for each individual gas, namely argon (Ar), hydrogen (H 2 ) and nitrogen (N 2 ), of the plasma gas mixture (argon/hydrogen/nitrogen mixture). The plasma gas mixture delivery apparatus  18  receives plasma via respective source hoses  6   a ,  6   b , and  6   c  and delivers plasma gases to a plasma gas mixer  22  via respective supply hoses  7   a ,  7   b  and  7   c . Each of the source hoses  6   a ,  6   b , and  6   c  and respective supply hoses  7   a ,  7   b  and  7   c  are capable of accommodating a volume flow portion of the total volume flow of gas to the plasma torch. The plasma gas mixer  22  is connected to the plasma torch nozzle  14  via a plasma gas mixture hose  9   a.    
     FIG. 1  further depicts a secondary gas mixture delivery apparatus  20  for delivering a secondary gas mixture. In the arrangement  10  of  FIG. 1 , the secondary gases are nitrogen (N 2 ) and hydrogen (H 2 ), supplied to the secondary gas mixture delivery apparatus  20  from sources (not shown) for the individual gases via respective source hoses  6   d  and  6   e . The secondary gas mixture delivery apparatus  20  delivers the secondary gas mixture via a gas supply hose  7   d  and a secondary gas mixture hose  9   d  to the secondary gas nozzle  16 . 
   A separate pressure switch  2   a ,  2   b ,  2   c ,  2   d , and  2   e , volume flow controller  1   a ,  1   b ,  1   c ,  1   d , and  1   e , and supply solenoid valve  3   a ,  3   b ,  3   c ,  3   d , and  3   e , are arranged in respective series along each source hose  6   a ,  6   b ,  6   c ,  6   d , and  6   e . Within the plasma gas mixture delivery apparatus  18 , there is also a pressure measure  4   a ,  4   b , and  4   c  located downstream of each respective supply solenoid valve  3   a ,  3   b  and  3   c . The pressure measures  4   a ,  4   b  and  4   c  are each connected via signal lines to a common logic controller  5 , which in turn is connected to each of the volume flow controllers  1   a ,  1   b  and  1   c  via a respective control line. The volume flow controllers  1   a ,  1   b  and  1   c  are each responsive to the logic controller  5 . 
   Downstream of the pressure measures  4   a ,  4   b  and  4   c , delivery solenoid valves  8   a ,  8   b  and  8   c  are provided along the gas supply hoses  7   a ,  7   b  and  7   c . Downstream of the delivery solenoid valves  8   a ,  8   b  and  8   c , the gas supply hoses  7   a ,  7   b , and  7   c  connect to a plasma gas mixer  22 , which allows for mixing and supplying of the plasma gas mixture to the plasma gas mixture hose  9   a.    
   In the secondary gas mixture delivery apparatus  20 , the source hoses  6   d  and  6   e  are combined via a secondary gas mixer  26  into the supply hose  7   d  downstream of the supply solenoid valves  3   d  and  3   e . The supply hose  7   d  connects the secondary gas mixer  26  to a delivery solenoid valve  8   d . The secondary gas mixture hose  9   d  leads from the delivery solenoid valve  8   d  to the secondary gas nozzle  16  of the plasma torch. 
   In the operation of the arrangement  10  of  FIG. 1 , the pressure switches  2   a ,  2   b  and  2   c  monitor the presence of a minimum required gas pressure as the individual plasma gases, argon, nitrogen, and hydrogen, are delivered to the volume flow controllers  1   a ,  1   b  and  1   c  via the source hoses  6   a ,  6   b  and  6   c . Individual volume flow set values w 1 , w 2 , and w 3 , each corresponding to one individual plasma gas and a corresponding volume flow portion, are related by the logic controller  5  according to the selected parameters for the respective volume flow controllers  1   a ,  1   b  and  1   c.    
   Before the plasma cutting process begins, the supply solenoid valves  3   a ,  3   b  and  3   c  and, initially, the delivery solenoid valves  8   a ,  8   b  and  8   c  are opened to purge the source hoses  6   a ,  6   b  and  6   c . The delivery solenoid valves  8   a ,  8   b  and  8   c  are then closed and the source hoses  6   a ,  6   b  and  6   c  are filled with respective plasma gases, via the volume flow controllers  1   a ,  1   b  and  1   c , to the pressure determined by the logic controller  5 , the pressure being detected by and monitored with the pressure measures  4   a ,  4   b  and  4   c . The closed delivery solenoid valves  8   a ,  8   b  and  8   c  allow pressure to accumulate in the source hoses  6   a ,  6   b  and  6   c  and supply hoses  7   a ,  7   b  and  7   c . It is advantageous for the source hoses  6   a ,  6   b  and  6   c  to be filled to the same pressure, for example 4 bar, to prevent equalization processes from occurring between the individual gases at the beginning of the plasma cutting process. 
   As the plasma cutting process begins, the supply solenoid valves  3   a ,  3   b , and  3   c  and delivery solenoid valves  8   a ,  8   b  and  8   c  are all opened. This allows for the setting of the corresponding volume flows of the individual plasma gases and thus the total volume flow of the plasma gas mixture. This setting involves measuring pressure, with for example the pressure measure  4   a , the measurement being evaluated by the logic controller  5 . The pressure measurement is made while the supply solenoid valves  8   a ,  8   b  and  8   c  are all open so that each of the source hoses  6   a ,  6   b  and  6   c  are open to each other. 
   Some embodiments may also allow for measurement of individual gases by measuring individual mixed gases and forming a mean pressure determination. For example, it would also be possible for the logic controller  5  to evaluate all of the pressure measures  4   a ,  4   b  and  4   c  and then, for example, calculate a mean pressure based on the pressures measured. Alternatively, individual gases or individual mixed gases can be combined and the resulting pressure measured. This can allow for a reduction in the number of pressure measures needed and can serve to reduce the amount of equipment needed. 
   In the pre-flow period, which is immediately before the pilot arc is ignited, a defined plasma gas mixture is allowed to flow through the plasma torch at a preselected pressure, for example 4 bar. The resulting pressure is related to the logic controller  5  and processed to convert the selected volume flow set values w 1 , w 2  and w 3  into new volume flow set values w 1 *, w 2 * and w 3 *. This conversion allows for the setting of the desired pressure in the interior of the plasma torch at a constant gas mixture between the volume flow controllers  1   a ,  1   b  and  1   c  and the plasma torch. 
   After ignition of the pilot arc, pressure is increased to a level required for the plasma cutting process, for example 6 bar. This is best understood by comparing  FIG. 1  to  FIG. 4 , with  FIG. 4  depicting example logic operational details of the logic controller  5  with respect to the pressure measure  4   a  and volume flow controller  1   a  along the source hose  6   a  for argon gas (Ar), the source hose  6   a  being capable of accommodating one volume flow portion of the total volume flow of gas. The pressure increase is effected by raising the pressure set value p soll  within the logic controller  5 , whereby the increased pressure set value p soll  raises the volume flows of the individual plasma gases accordingly. This ensures that the desired pressure is always present in the interior of the plasma torch and further ensures that plasma gas mixing occurs in the desired ratio. This also allows for compensation in pressure fluctuations upstream of the plasma torch during certain operating conditions, for example when the current is reduced when cutting a corner or at the end of a cut. 
   The volume flows of the individual plasma gases, and thus the mixing ratio, are selected with the volume flow set values w 1 , w 2  and w 3 . The pressure upstream of the plasma torch determines the pressure in the interior of the plasma torch between the electrode  12  and the plasma torch nozzle  14 , and therefore also the volume flow that ultimately flows through the plasma torch nozzle  14 . For example, referring to  FIG. 4 , the pressure achieved by the set volume flow is measured with the pressure measure  4   a  as the pressure actual value p ist  and is related to the logic controller  5 . If this pressure actual value p ist  is not identical to the selected pressure set value p soll , wherein the volume flow through the volume flow controllers  1   a ,  1   b  and  1   c  are not sufficient to achieve the pressure set value p soll , the pressure difference Δp=p soll −p ist  is calculated. Δp is then multiplied by a constant factor k, and the resulting product is added to the set value for the volume flow w 1 , w 2  or w 3 , respectively, of the volume flow controllers  1   a ,  1   b  and  1   c . This relation between a volume flow set value w and a new volume flow set value w* can be expressed with the following equation:
 
 w*=w+k×Δp.  
 
   Accordingly, the values for the new volume flow set values are denoted w 1 *, w 2 * and w 3 *. Referring briefly to  FIG. 4 , if the pressure actual value p ist  is greater than the pressure set value p soll , Δp is negative. As a result, the volume flow set values for the volume flow controllers  1   a ,  1   b  and  1   c  are reduced. Desired allowable ranges for volume flows, gas mixture ratios, and pressures can be electronically preselected with appropriate electronics, control software, and/or with other appropriate automation or controls. 
   Although the invention has been shown and described in an arrangement in which the volume flow of a mixture of argon, hydrogen, and nitrogen are controlled, it will be appreciated that the invention can also be used to regulate individual gases, including oxidizing gases and mixtures such as air and oxygen, and non-oxidizing gasses such as argon, hydrogen, nitrogen or mixtures thereof. 
   It is further contemplated that in some embodiments, plasma torches of the invention can be used to cut unalloyed steel using air and oxygen. Alternatively, plasma torches of the invention can also be used to cut alloyed steels using an argon/hydrogen/nitrogen mixture. 
   In addition, it is contemplated that some embodiments of the invention allow for plasma cutting and plasma marking by modifying the arrangement  10  depicted in  FIGS. 1 and 4 . Modifying the arrangement  10  for plasma cutting structural steel, for example, involves the use of oxygen as an alternate plasma gas. Modifying the arrangement  10  for plasma marking involves the use of an argon/nitrogen mixture. When switching between alternate plasma gases, such switching should preferably occur as quickly as possible in order to maintain high productivity. However, such switching must also be performed in a manner that ensures that the replaced or removed plasma gas has been removed completely from the arrangement  10 . 
   Switching between plasma gases normally involves venting and purging the source hoses  6   a ,  6   b  and  6   c  completely prior to refilling with the new plasma gas mixture. Since plasma torch nozzles  14  frequently have a very small bore, often with a diameter on the order of 0.7 mm, this process can require a relatively long interval depending on the length of the hoses. Venting and purging intervals of 10 seconds or longer can be anticipated. To shorten this interval, an additional delivery solenoid valve  8   e  is provided to allow for the rapid venting of the supply hoses  7   a ,  7   b  and  7   c  when the delivery solenoid valves  8   a ,  8   b  or  8   c  are open. The inclusion of the additional delivery solenoid valve  8   e  can reduce venting and purging to an interval of less than 3 seconds. 
     FIG. 2  depicts an arrangement  10  of the invention that includes a combined pre-flow and post-flow gas delivery configuration, having a pre-delivery solenoid valve  3   f , a supply hose  7   f , and a delivery solenoid valve  8   f , for delivering pre-flow and post-flow gases to the plasma torch separately. A pressure controller  17  controls the pressure of the pre-flow and post-flow gas. A plasma gas mixer  24  allows plasma gases argon and nitrogen to be mixed within the plasma gas mixture delivery apparatus  18 . 
   The configuration of the arrangement  10  of  FIG. 2  allows pre-flow gas to flow through the plasma torch at a different pressure, for example at 4 bar, than gases that are present at the delivery solenoid valves  8   a  and  8   c  which are delivered at the pressure required for plasma cutting, for example 6 bar, before the plasma cutting begins. This eliminates a need to adjust the pressure from 4 bar to 6 bar when the pilot arc is ignited. 
   During the pre-flow period, the supply solenoid valves  3   a ,  3   b ,  3   c , pre-delivery solenoid valve  3   f , and delivery solenoid valve  8   f  are all open. The supply hoses  7   a  and  7   c  are filled by the volume flow controllers  1   a  and  1   c  up to the pressure determined by the logic controller  5 , which is detected by the pressure measures  4   a  and  4   c . The delivery solenoid valves  8   a  and  8   c  of the plasma torch are closed so that an adequate pressure, for example 6 bar, can accumulate. 
   After the pilot arc is ignited, the delivery solenoid valves  8   a  and  8   c  are opened while the pre-delivery solenoid valve  3   f  and delivery solenoid valve  8   f  are closed. The pressure values measured by the pressure measure  4   a  are processed in the logic controller  5 . The logic controller  5  then adjusts the volume flows of the individual gases to the desired pressure to ensure that the desired plasma gas mixing ratio is continuously supplied to the plasma torch. After the plasma cutting has finished, the delivery solenoid valves  8   a  and  8   c  are closed while the pre-delivery solenoid valve  3   f  and delivery solenoid valve  8   f  are opened. This allows for post-flow gas to then be supplied to the plasma torch. 
   In the arrangements  10  of both  FIGS. 1 and 2 , the secondary gas is controlled only by the volume flow controllers  1   d  and  1   e , which keep the secondary gas volume flow constant throughout the entire plasma cutting process while the supply solenoid valves  3   d  and  3   e  and the delivery solenoid valve  8   d  are open. This can usually be sufficient for operation if the plasma torch is constructed such that the bore of the secondary gas nozzle  16  is not significantly narrowed by a plasma jet, and normally applies to plasma torches in which the bore of the secondary gas nozzle  16  is at least twice as large as the plasma torch nozzle  14 . 
   When such diameter ratios are smaller, the secondary gas must be supplied using a method similar to the method used for supplying the plasma gas. For example,  FIG. 3  depicts a secondary gas mixture delivery apparatus  20  of the invention in which a combined volume flow and pressure control of the secondary gas is incorporated with the inclusion of an additional pressure measure  4   d  connected to the logic controller  5 , corresponding by analogy with the volume flow and pressure control employed in supplying plasma gas in accordance with  FIGS. 1 and 2 . 
   It will be appreciated that the invention can be incorporated into a variety of plasma technologies, such as plasma welding, plasma gouging and plasma marking, where plasma gas is supplied to a work piece. 
   It will be further appreciated that while the invention has been shown and described as having pressure in the interior of a plasma torch measured indirectly via pressure measures upstream of the plasma torch, such as with the pressure measures  4   a ,  4   b  and  4   c  of  FIG. 1 , some embodiments will allow for direct measuring of the pressure within the interior of the plasma torch without departing from the contemplated scope of the invention. 
   In some embodiments, it may be advantageous for at least one volume flow to be controlled on the basis of a calorimetric measurement of the volume flow, on the basis of the measurement of the volume flow from the differential pressure, or on the basis of a pulse measurement. 
   It can also be advantageous if, before being supplied with the gas, mixed gas or gas mixture, the plasma torch is separately supplied with a pre-flow gas at a controlled pressure. The torch can also be advantageously separately supplied with a post-flow gas at a controlled pressure. 
   The quality of a cut is ultimately dependent on the volume flow actually passing through the plasma torch nozzle  14 , and not on the gas volume flow flowing through the volume flow controllers  1   a ,  1   b , and  1   c . Due to the presence of gas hoses  7   a ,  7   b , and  7   c  connecting the plasma torch to volume flow controllers  1   a ,  1   b , and  1   c , volume flow through the volume flow controllers  1   a ,  1   b , and  1   c  may not be identical to the volume flow actually passing through the plasma torch nozzle  14 . The difference between the volume flow in the volume flow controllers  1   a ,  1   b , and  1   c  and the plasma torch nozzle  14  is due to the volume of the gas hoses therebetween and the compressibility of gases. 
   This is particularly noticeable in transitions between the different operating conditions that occur during plasma cutting. In the interior of the plasma torch, between the electrode  12  and the plasma torch nozzle  14 , depending on the operating condition, such as the start-up of the process, the pilot arc, the main arc and the end of the process, different internal pressures are required in order to achieve a particular volume flow. Such are generated by the changing arc currents, which produce different diameters of the plasma jet. In this way, the nozzle channel is narrowed. For example, the currents may be 10 to 25 A in the pilot arc, and 20 to 1000 A in the main arc. 
   The invention allows for quick reactions to rapidly changing pressure conditions in the interior of the plasma torch, especially during the transition processes, such as igniting the pilot arc, transferring the pilot arc to a work piece, and forming the main arc (cutting), without altering the mixing ratio of the gas mixture. This is done by superimposing the result of the pressure measurement on the set value of the volume flow control in such a way that a pressure independent of the operating condition of the plasma torch is created in the space between the volume flow controllers  1   a ,  1   b , and  1   c  and the plasma torch, or in the interior of the plasma torch. The mixing ratio of the gas mixture remains unchanged. Thus an ideal plasma gas mixture is available for the cutting process from the outset. 
   Both individual gases and the individual gases of gas mixtures can be controlled over wide ranges, and thus be adapted to a particular cutting task. This allows for a high level of reproducibility in cutting results. 
   Volume flow can also be set with the aid of devices such as proportional valves or motor operated valves. Pressure measurement, such as that performed by the pressure measures  4   a ,  4   b , and  4   c , can be performed by pressure transmitters which are well known in the art. 
   Volume flow control and pressure control can be analogue or digital and can be actuated accordingly. Measured volume flow can be visualised and monitored. 
   The method of the invention can be integrated into a quality control and documentation system. Evaluations together with other process parameters enable conclusions to be drawn with regard to cutting quality. 
   This invention has been described with reference to several preferred embodiments. Many modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such alterations and modifications in so far as they come within the scope of the appended claims or the equivalents of these claims.