Electric heating – Metal heating – By arc
Reexamination Certificate
2000-11-21
2003-12-23
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121500, C219S121670, C239S590000
Reexamination Certificate
active
06667459
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a material processing apparatus and, more specifically, to a nozzle used therein and apparatus and methods for regulating flow through such nozzle.
BACKGROUND OF THE INVENTION
Material processing apparatus, such as lasers and plasma arc torches, are widely used in the cutting, welding, and heat treating of metallic materials. A laser-based apparatus generally includes a nozzle through which a gas stream and laser beam pass to interact with a workpiece. The laser beam 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.
Similarly, 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 that exits through the nozzle orifice and impinges on the workpiece. Gases used in the torch can be non-reactive (e.g., argon or nitrogen), or reactive (e.g., oxygen or air).
It is generally desirable that the results of any material processing be of high quality. For example, the edges of the cut kerf produced by laser and plasma cutting should be straight and uniform. Edge irregularities caused by, for example, uneven heating of the workpiece by the laser, excessive chemical reactions between the assist gas and workpiece, or incomplete removal of cutting debris, should be minimized.
One way to improve process quality is by optimizing the flow of gas that impinges on the workpiece coincident with the energy beam. This gas, sometimes called an “assist gas” or a “cutting gas,” can be supplied through a single nozzle or a multitude of nozzles. In the case of a single nozzle, the assist gas flow can be optimized by the nozzle contour to achieve the desired flow characteristics (see, for example, U.S. Pat. No. 6,118,097 and Japanese Patent No. 8118063). In the case of a multiple nozzle design, the additional nozzles can be distributed around the central nozzle in a discrete or axisymmetric fashion (see, for example, U.S. Pat. No. 5,786,561).
In the case of oxygen assisted laser cutting, the assist gas flow must be adjusted to provide sufficient shear force to the liquefied material in order to ensure complete removal of the liquid (leaving no dross). Concurrently, the level of workpiece oxidization must be controlled to prevent excessive material removal. These two limitations oppose each other in most laser cutting applications because the oxygen velocity must be increased to remove the liquid metal more effectively. Nevertheless, since the nozzle diameter is larger than the laser beam and therefore the kerf width, the increased oxygen velocity increases the stagnation pressure near the kerf entrance region that promotes unwanted material burning and poor cutting quality.
Increasing gas pressure to improve system performance has drawbacks. First, it increases gas consumption. This degrades the operational efficiency of the cutting apparatus.
Second, increasing gas pressure can enlarge the width of the laser cut due to “overburning” when a reactive gas is used. This occurs because the increased pressure expands the physical scope of the reaction between the gas and the workpiece (e.g., by enhancing oxidation) beyond the dimensions of the laser beam. This is generally undesirable in the material processing industry, where narrow cuts are favored.
Finally, the increased gas pressure tends to damage the top area of the kerf, resulting in a “ragged” or “jagged” edge. One manifestation of this occurs when cut pieces fail to separate because the irregularities on their adjacent cut faces become seized.
Nozzles with multiple orifices may be used to modify the gas flow for impact on the workpiece. Such nozzles typically allow a laser beam and a gas to pass through an oversized center orifice. Several other orifices surrounding the center orifice, either concentrically or peripherally, also can deliver gas to the workpiece. An example of this is a “shower nozzle” where several smaller peripheral orifices encircle the large center orifice. In nozzles with multiple orifices, it is possible to isolate each orifice from the other orifices. This would allow, for example, each orifice to deliver independent gas flows at different pressures to different areas of the workpiece located under the nozzle. Through such tailoring of the gas flows, the quality of the cut can be improved. For example, regions of the cut that would benefit from a high pressure gas flow (e.g., the deep parts of the cut) would be subjected to such a flow that could be delivered through one or more orifices of the nozzle adjacent to that region. Conversely, workpiece regions better served by a low pressure gas flow (e.g., the top area of the kert) would receive a low pressure flow from one or more different orifices adjacent to that region. This delivery of high pressure and low pressure gas flows would occur concurrently and be provided through a single nozzle. It should be noted, though, that nozzles with multiple orifices can be complicated to manufacture. Furthermore, to achieve the positional accuracy between the specific gas flows and selected regions of the workpiece requires maintaining alignment between the laser beam and the centerline of the nozzle. This can be difficult to attain in the typical rugged field environment.
Also important in the operation of the material processing apparatus is the accuracy of the cut. For example, the location of the laser beam relative to the centerline of the nozzle orifice influences accuracy. Depending on the direction of the cut, any misalignment can result in the production of workpieces with improper dimensions and distorted edges. Asymmetric wear of the nozzle orifice also typically results when the energy source is a plasma jet, requiring premature replacement of the nozzle. Because nozzle replacement is ideally performed in the field, typically under harsh conditions, it is desirable that proper alignment of the energy beam and gas flow be achieved quickly and without difficulty.
From the foregoing, it will be apparent that there exists a need for a low cost, readily manufacturable and easily replaceable nozzle that can create the gas velocity profile necessary to produce high quality, accurate cuts. Such a nozzle should also promote efficient apparatus operation and be easy to align with the laser beam.
SUMMARY OF THE INVENTION
The present invention features a material processing apparatus that includes a nozzle having a baffle that creates a gas velocity profile that improves the quality and accuracy of, for example, cuts made in a workpiece. Additionally, the nozzle aligns itself with the axis of an energy beam that is a component of the material processing apparatus, further improving accuracy and increasing its operational lifetime. Calibration of the baffle and alignment of the nozzle occur with little or no user intervention, making the invention simple to operate, which is desirable under typically rugged field conditions.
In one embodiment, a material processing apparatus includes an energy source, such as a laser or plasma source, which provides an energy beam. The beam passes into a processing head assembly that includes a chamber or plenum to receive a fluid, such as an assist gas. The processing head assembly also includes a nozzle having a central exit orifice and a configurable baffle. The configurable baffle
Dean, Jr. Robert C.
Garg Sanjay
Hackett Charles M.
Woods Kenneth J.
Evans Geoffrey S.
Hypertherm, Inc.
Testa Hurwitz & Thibeault LLP
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