Electric heating – Metal heating – By arc
Reexamination Certificate
2002-05-21
2003-09-02
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06614001
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the design and manufacture of material processing apparatus and, more specifically, to consumables used in the apparatus and methods for aligning the consumables with an axis of the apparatus.
BACKGROUND OF THE INVENTION
Material processing apparatus, such as plasma arc torches and lasers, 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, 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 is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
Similarly, a laser-based apparatus generally includes a nozzle into which a gas stream and laser beam are introduced. A lens focuses the laser beam which then heats the workpiece. Both the beam and the gas stream exit the nozzle through an orifice and impinge on a target area of the workpiece. The resulting heating of the workpiece, combined with any chemical reaction between the gas and workpiece material, serves to heat, liquefy or vaporize the selected area of workpiece, depending on the focal point and energy level of the beam. This action allows the operator to cut or otherwise modify the workpiece.
Certain components of material processing apparatus deteriorate over time from use. These “consumable” components include, in the case of a plasma arc torch, the electrode, swirl ring, nozzle, and shield. Ideally, these components are easily replaceable in the field. Nevertheless, the alignment of these components within the torch is critical to ensure the reasonable consumable life, as well as accuracy and repeatability of plasma arc location, which is important in automated plasma arc cutting systems.
In a plasma arc torch, the location and angularity of the arc is determined by the relative location of the electrode and nozzle or, more specifically, the location of an insert disposed in a tip of the electrode relative to a centerline of the nozzle orifice. Since the plasma gas flowing through the orifice tends to center the arc in the orifice, it is desirable that the insert is concentrically aligned with the orifice, as any misalignment skews the arc relative to the centerline datum of the torch. As used herein, the term “axially concentric” and variants thereof mean that the centerlines of two or more components are substantially collinear. Depending on the direction of cut, any misalignment can result in the production of parts with improper dimensions and non-normal edges. Asymmetric wear of the nozzle orifice also typically results, requiring premature replacement of the nozzle.
Tolerances associated with conventional methods of mounting the electrode and nozzle render systems employing such torches incapable of producing highly uniform, close tolerance parts due to the errors inherent in positioning the electrode relative to the nozzle. One method of mounting the electrode and nozzle employs close tolerance sliding fits. For example, a cathode block having a bore for receiving a base of the electrode has a nominal diameter of 0.272 inches (0.691 cm) with a machining tolerance band of plus or minus 0.001 inches (0.003 cm). Accordingly, the bore can have a maximum diameter of 0.273 inches (0.693 cm) and a minimum diameter of 0.271 inches (0.688 cm). In order to ensure the electrode can be inserted reliably in the block without interference, the electrode base has a nominal diameter of 0.270 inches (0.689 cm) with a machining tolerance band of plus or minus 0.001 inches (0.003 cm). Accordingly, the electrode base can have a maximum diameter of 0.271 inches (0.688 cm) and a minimum diameter of 0.269 inches (0.683 cm). The diametral clearance between the base and bore can range between zero and 0.004 inches (0.010 cm) yielding a maximum radial displacement of the electrode relative to a centerline of the torch of 0.002 inches (0.005 cm). This maximum radial displacement is also called the worst case stacking error which results from employing a minimum allowable diameter electrode base with a maximum allowable diameter cathode block bore.
The worst case stack error of the nozzle is added to that of the electrode to determine the combined total maximum radial displacement for the nozzle and electrode in the torch. Calculation of nozzle location error is similar to that of the electrode. For example, a torch body having a bore for receiving a base of the nozzle has a nominal diameter of 0.751 inches (1.908 cm) with a machining tolerance band of plus or minus 0.001 inches (0.003 cm). Accordingly, the bore can have a maximum diameter of 0.752 inches (1.910 cm) and a minimum diameter of 0.750 inches (1.905 cm). In order to ensure the nozzle can be inserted reliably in the body without interference, the nozzle base has a nominal diameter of 0.747 inches (1.897 cm) with a machining tolerance band of plus or minus 0.002 inches (0.005 cm). The larger tolerance band is attributable to the increased difficulty of machining larger diameter parts to close tolerances reliably at reasonable cost. Accordingly, the nozzle base can have a maximum diameter of 0.749 inches (1.902 cm) and a minimum diameter of 0.745 inches (1.892 cm). The diametral clearance between the base and bore can range between 0.001 inches (0.003 cm) and 0.007 inches (0.018 cm) yielding a maximum radial displacement of the nozzle relative to a centerline of the torch of 0.0035 inches (0.0089 cm).
The combined total maximum radial displacement of the nozzle relative to the electrode is the sum of the individual maximum radial displacements or 0.0055 inches (0.0140 cm). For a torch having an axial distance between a tip of the electrode insert and an entrance to the nozzle orifice of 0.140 inches (0.3556 cm), the angularity of the arc relative to the torch centerline may be related to the angularity of the consumables relative to the torch centerline, the latter of which is calculated geometrically as about 2.25 degrees. Accordingly, if the axial distance from the tip of the insert to the workpiece surface is 0.274 inches (0.696 cm), the maximum dimensional error from the centerline of the torch projected on the workpiece to the actual entrance of a cut on the workpiece may be calculated geometrically as about 0.0108 inches (0.0274 cm). Depending on the direction of arc misalignment and the direction of the cut, the component cut from the workpiece may have cut edge angularity of 2.25 degrees and the dimensional error of the finished part may be up to twice the 0.0108 inches (0.0274 cm), or 0.0216 inches (0.0549 cm), in the case where opposite edges of the workpiece are both cut with the maximum skew. This magnitude of errors is unacceptable for reliably producing parts and features therein having total dimensional tolerance of between about plus or minus 0.005 inches (0.013 cm) and about plus or minus 0.010 inches (0.025 cm). Further, for a small nominal diameter nozzle orifice such as 0.018 inches (0.046 cm), the combined maximum radial displacement of 0.0055 inches (0.0140 cm) and angularity of 2.25 degrees result in asymmetric wear of the nozzle entailing premature replacement.
Diametral tolerances of plus or minus 0.001 inches (0.003 cm) for each of an electrode base, cathode block bore, and torch body bore and plus or minus 0.002 inches (0.005 cm) for a nozzle base are necessary to ensure the capability to replace readily the consumable components in the field. While tighter tolerances could be employed, such practices typically would entail higher manufacturing costs and likely complicate the field replacement of the consumables. Attempts to rely on O-rings for sealing the radial clearances as well as centering ar
Brandt Aaron D.
Currier Brian J.
Hackett Charles M.
Lu Zhipeng
Nakano Yutaka
Hypertherm, Inc.
Paschall Mark
Testa Hurwitz & Thibeault LLP
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