Electric heating – Metal heating – By arc
Reexamination Certificate
2000-01-13
2001-05-15
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121570, C219S121550, C219S121440
Reexamination Certificate
active
06232574
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to plasma arc torches and, more particularly, to a method and plasma arc torch apparatus for stopping a cutting operation of the plasma arc torch such that the life of the torch consumable is improved.
BACKGROUND OF THE INVENTION
Plasma arc torches have a wide variety of applications for the working of metals, including cutting, welding, and annealing. Such torches usually include an electrode, also known as a consumable, for supporting an arc that extends from the electrode to a workpiece. A plasma gas such as an oxidizing gas is typically directed through a nozzle assembly to a plasma chamber between the electrode and the nozzle such that the plasma gas is operable to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. The electrode includes an emissive insert which emits electrons upon an electrical potential being applied between the electrode and the workpiece. When the cutting or welding process is complete, the torch may be turned off through a stop program which extinguishes the arc.
It is a common practice to flush oxidizing gas out of the torch following a work operation by flowing a non-oxidizing gas through the passages and nozzle of the torch. The oxidizing and non-oxidizing gases may be selectively introduced into the plasma chamber via a pair of actuatable valves. One or more gas feed lines direct the oxidizing or non-oxidizing gas from the solenoid valves to the plasma chamber at the tip of the torch. Thus, the actuatable valves may be opened and closed to exchange the oxidizing gas for the non-oxidizing gas at the end of the cutting process. A common method for shutting down the torch following a plasma arc cutting process typically includes the following simultaneous operations: switching off the power supply to the electrode; switching off the oxidizing gas, such as oxygen; and switching on a non-oxidizing gas, such as nitrogen, to flush the oxidizing gas from the torch. By simultaneously performing all three operations, however, the plasma arc extinguishes before the non-oxidizing gas completely purges the undesirable oxidizing gas from the plasma chamber of the torch, such that the arc extinguishes in a non-inert environment in the vicinity of the front emitting face of the electrode.
More specifically, the arc extinguishes before the non-oxidizing gas has purged the oxidizing gas from the plasma chamber due to the time delay or lag that is inherent in the purging of the oxidizing gas from the torch. This time delay or lag is due to the volume of gas contained within the tubing and passageways extending between the valves and the plasma chamber adjacent the electrode and nozzle assembly. All of the undesirable oxidizing gas to be purged must be ejected through the nozzle of the torch, which is a time consuming process dependent on the size of the nozzle orifice, the length and volume of the gas tubing, gas passageways and plasma chamber, the rate of flow of the non-oxidizing gas into the tubing, passageways and plasma chamber, and the rate of flow of the oxidizing gas through the nozzle. As a result of the arc extinguishing in a non-inert environment, oxides form on the outer surface of the electrode. Although the formed oxides are typically burned off when the arc is restarted, the repeated formation and burning off of oxides on the electrode contributes to accelerated consumable wear and poor starting performance.
Several methods have been developed to address various deleterious effects that can result when a plasma arc torch is shut down. One method for stopping the cutting operation of the plasma arc torch is described in U.S. Pat. No. 5,166,494, and includes ramping down the mass flow rate of the oxidizing gas as the torch approaches the end of a cutting path. When the electrical current to the torch is cut to extinguish the arc, a residual flow rate of the oxidizing gas remains in the plasma chamber to maintain the stability of the arc, which is said to be desirable to prevent damage to the electrode. As such, the arc extinguishes in a non-inert environment which causes the formation of oxides on the outer surface of the electrode.
Another method for stopping the cutting operation of the plasma arc torch, as exemplified in U.S. Pat. No. 5,070,227, includes venting the plasma chamber to atmosphere while reducing or cutting the flow of oxidizing gas to the plasma chamber, such that the oxidizing gas ejects both through the nozzle of the torch and through the vent to atmosphere to facilitate a more rapid change in the gas flow pattern in the plasma chamber. However, a residual flow rate of the oxidizing gas remains in the plasma chamber to maintain the stability of the arc in order to prevent damage to the electrode. Thus, the arc extinguishes in a non-inert environment which causes the formation of oxides as described above.
Thus, there is a need for a method and apparatus for stopping a cutting operation of a plasma arc torch in which formation of oxidation on the electrode is minimized.
SUMMARY OF THE INVENTION
The above and other objects and advantages of the present invention are achieved by a stopping process for plasma arc torches of the type having a metallic electrode and nozzle assembly positioned adjacent a discharge end of the electrode and defining a plasma chamber between the electrode and the nozzle, wherein the process minimizes the oxidation of the electrode and thereby extends the life of the electrode.
In one embodiment of the present invention, a flow of an oxidizing gas, such as oxygen, is first directed by a valve and passageway through the plasma chamber and nozzle of the plasma arc torch. Electrical current is supplied by a power source and directed through the electrode to form a plasma arc which extends between the electrode and a workpiece during a cutting operation. The electrical current is stepped down at a time F for extinguishing the plasma arc. The flow of oxidizing gas is switched off by a valve at a predetermined time interval before time F, and a flow of non-oxidizing gas, such as nitrogen, is switched on by a valve such that no later than at time F the non-oxidizing gas has substantially completely purged the oxidizing gas from the plasma chamber. The predetermined time interval is dependent upon several constants measurable for a given torch, such as the size of the nozzle, the volume of the gas passageways and plasma chamber, the rate of flow of the non-oxidizing gas into the passageways and plasma chamber, and the rate of flow of the oxidizing gas through the nozzle. By purging the oxidizing gas from the torch with the non-oxidizing gas before the plasma arc is extinguished, an inert environment is present when the arc is extinguished.
A plasma arc torch is also provided by the present invention. The torch includes a nozzle defining a gas flow path therethrough and an electrode positioned adjacent the nozzle. The electrode is capable of supporting an electrical arc which may be supplied by an electrical power supply operably connected to the electrode. The torch also includes pairs of valves and passageways for directing flows of oxidizing and non-oxidizing gas through the nozzle. A processor, such as a microprocessor, is operatively coupled to the valves and power supply. The processor is operable to close the valve controlling the oxidizing gas flow and to open the valve controlling the non-oxidizing gas flow at predetermined time intervals before the plasma arc is extinguished, such that at a time no later than when the arc is extinguished the non-oxidizing gas has substantially completely purged the oxidizing gas from the nozzle creating an inert environment adjacent the electrode. Thus, the arc extinguishes in an inert environment.
REFERENCES:
patent: 5801355 (1998-09-01), Saio et al.
patent: 6093905 (2000-07-01), Hardwick et al.
Alston & Bird LLP
Paschall Mark
The ESAB Group, Inc.
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