Electric heating – Metal heating – By arc
Reexamination Certificate
2000-09-14
2001-08-14
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06274842
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods for cutting holes and other arcuate shapes in metal using a plasma arc torch.
BACKGROUND OF THE INVENTION
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treating, melting, and annealing. Such torches include an electrode which supports an electric arc that extends from the electrode to a workpiece. A plasma gas such as an oxidizing gas is typically directed to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. In some types of torches, a second shielding gas is used to surround the jet of plasma gas and the arc for controlling the work operation. In other types of torches, a swirling jet of water is used to surround the jet of plasma gas and the arc and impinge on the workpiece for controlling the work operation.
In a variety of circumstances, it is desired to cut metal workpieces along cutting paths that are at least partly arcuate in shape such that the torch has a nonzero angular velocity during at least portions of the cutting operation. The advance rate of the torch in surface feet per minute is generally a function primarily of the type and thickness of the material being cut and the current density of the torch expressed in amps of arc current per square inch of nozzle area. Thus, in existing plasma arc cutting methods, the advance rate of the torch typically is selected independently of shape or contour of the cutting path along which the torch is moved. Accordingly, when the torch is moving along an arcuate path, the angular rate of movement of the torch increases in an inversely proportional manner to the radius of curvature of the cutting path.
A phenomenon which has been noted in cutting small holes (e.g., hole diameters of about 1 inch or less) with a plasma arc torch is that the increased angular rate of the torch results in the arc not following the desired noncircular or circular path, but rather “whipping” around. Although not wishing to be bound by theory, it is thought that centrifugal effects become more and more significant as the angular velocity of the torch increases, such that the centrifugal effects are great enough to influence the movement of the arc, perhaps because the plasma gas flow does not follow the torch as accurately as it does at lower angular velocities. The result of this arc whipping is that the workpiece is cut along a path that does not conform to the desired cutting path. Problems of nonconformance are especially likely at the end of a hole cut where the finishing end of the cutting path joins the starting end of the cutting path. However, nonconformance caused by arc whipping can result whenever the torch is moved along a nonlinear path during a cutting operation.
SUMMARY OF THE INVENTION
The above needs are met and other advantages are achieved by the present invention, which provides improved methods for cutting with a plasma arc torch. In accordance with preferred embodiments of the invention, the current supplied to the torch is regulated as a function not only of the linear advance rate but also as a function of the angular rate of movement of the torch. It has been found that, for reasons that are not understood, increasing the current supplied to the torch tends to make the arc less susceptible to whipping around when the torch is moved along an arcuate path. Accordingly, in preferred embodiments of the invention, the current is increased when the angular rate of the torch increases.
In accordance with one preferred embodiment of the invention, the method comprises the steps of directing a plasma gas through a nozzle of a plasma arc torch and toward the workpiece, supplying current to the electrode to establish an electric arc from the electrode to the workpiece such that the arc cuts through the workpiece, moving the torch along a predetermined arcuate cutting path at a predetermined linear advance rate such that the torch has a determinable angular rate of movement, and regulating the current supplied to the electrode as a function of both the linear advance rate and a control parameter that is proportional to the angular rate of movement of the torch.
The invention provides improved methods for cutting circular holes in a workpiece, wherein the torch is moved along a circular cutting path having a predetermined diameter, and wherein the current is regulated as a function of the linear advance rate and the diameter of the cutting path which is inversely proportional to the angular rate of movement of the torch. Preferably, the current is regulated to a predetermined first value when the diameter of the cutting path is greater than a predetermined limit, and is increased to a predetermined second value when the diameter of the cutting path is equal to or less than the predetermined limit.
The method in one embodiment comprises increasing the arc density, expressed in amps per square inch of nozzle area, from a nominal arc density when the diameter of the cutting path is greater than the predetermined limit, to an arc density about 15 to 50 percent above its nominal level when the diameter is equal to or less than the predetermined limit. For example, the arc density advantageously can be about 75,000 amps per square inch for cutting holes greater than about 1 inch in diameter, and can be increased to an average of about 90,000 amps per square inch for cutting holes of about 1 inch or less in diameter. This is merely an illustrative example, and it will be appreciated that the arc density can vary depending on the material type and thickness of the workpiece as well as other factors.
In accordance with a preferred embodiment of the invention, the increased arc density is accomplished by pulsing the current, i.e., periodically increasing the current to a higher level for a short period of time and then reducing the current back to a lower level, such that the average current over time is greater than nominal by 15 to 50 percent. It is thought that pulsing the current may be less degrading to the electrode and other consumable components of the torch than a constant current at the higher level would be.
In accordance with yet another preferred embodiment of the invention, a method for cutting a workpiece of known material and thickness along a cutting path that includes one or more arcuate portions comprises the steps of moving the torch along the cutting path, determining a linear advance rate of the torch at a point along an arcuate portion of the cutting path, determining a control parameter that is a function of an angular rate of movement of the torch at said point, and regulating the current supplied to the electrode at said point as a function of the material and thickness of the workpiece, the linear advance rate, and the control parameter. If desired, the radius of curvature of the cutting path can be selected as the control parameter, and the current can be increased whenever the radius of curvature is less than a predetermined value. Such predetermined value can itself be a function of the linear advance rate.
The method of the invention is well-suited for, but not limited to, cutting operations performed by a numerically controlled plasma arc torch system. Accordingly, a preferred embodiment of the invention provides a method including the steps of moving the torch along a predetermined cutting path by operating first and second linear motion actuators that are operable respectively to move the torch in X- and Y-directions and to provide output signals indicative of X- and Y-coordinates of the torch, determining derivatives of the X- and Y-coordinates from the output signals of the actuators, and determining the control parameter based on said derivatives. The linear advance rate can also be determined from the derivatives. Advantageously, the control parameter can be selected to be the radius of curvature of the cutting path, which can be determined, for example, from a second derivative of the Y-coordinate with respect to the X-coordinate. Alternatively, the control parameter
Turner Barry Gaskins
Warren Joseph Valerious
Alston & Bird LLP
Paschall Mark
The ESAB Group, Inc.
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