Patent Application
20100018954
The present disclosure relates generally to plasma arc torches and more particularly to methods for improving piercing operations.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode during piloting. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, often referred to as the plasma arc chamber, wherein the pilot arc heats and subsequently ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece with the aid of a switching circuit activated by the power supply. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
In one mode of operation, commonly referred to as “piercing,” the plasma arc torch is started at a location on the workpiece rather than on an edge of the workpiece to start a cut. Piercing becomes more difficult as the workpiece thickness increases, and in general, piercing workpieces that are thicker than about one inch is often challenging. Additionally, piercing thinner workpieces at lower current levels can prove to be difficult as well. With thinner workpieces, the pierce time is relatively short and the arc has a tendency to stretch as material is removed rather quickly. The stretched arc can cause damage to components of the plasma arc torch, such as the tip, and can also cause an over voltage condition such that the power supply cannot deliver the requisite amount of power. Moreover, during piercing operations, molten metal, or slag, has a tendency to splatter onto components of the plasma arc torch and reduce their effectiveness and overall useful life. Therefore, significant efforts are undertaken to design proper gas shielding to protect the plasma arc torch and its components from molten slag during piercing.
During piercing, the plasma arc creates a semi-ellipsoid shape in the workpiece, and molten metal travels away from the pierce location, taking on multiple trajectories and spanning radially and azimuthally. In order to deflect the molten metal away from the plasma arc torch and its components, and also to cool the molten metal such that it has less of a tendency to adhere to components of the plasma arc torch, shield gases are employed to exert a proper deflection force and for cooling. Compared to controlling current, the type and amount of shield gas is often difficult to control in order to effect proper deflection/cooling of the molten metal, and thus improved methods of piercing are continuously being pursued in the art of plasma arc cutting.
In general, the present disclosure provides an innovative plasma arc torch and methods to deflect metal spatter away from the plasma arc torch and its components during piercing operations. In general, the methods involve optimizing a current profile as a function of workpiece thickness in order to more efficiently deflect metal spatter away from the plasma arc torch and its components. Various forms of current profiles are employed, which are further a function of an operating current level in other forms of the present disclosure. The current profiling is used in combination with shield gases to exert a proper deflection force to the metal spatter, which is described in greater detail below. In general, an effective deflection will depend on the ratio of momentum of the shield gas available to that of the metal spatter.
In one form, the present disclosure provides a method of piercing a workpiece with a plasma arc torch of the type having a plasma gas flow path for directing a plasma gas through the torch and a secondary gas flow path for directing a secondary gas through the torch. The method comprises directing a flow of shield gas along a distal end portion of the plasma arc torch to deflect metal spatter generated from the piercing, ramping a current provided to the plasma arc torch along a profile during piercing and controlling current ramp parameters as a function of a thickness of the workpiece and an operating current level to reduce the impact of molten metal splatter during piercing, wherein the current ramp parameters comprise a length of time, a ramp rate, a shape factor, and a modulation.
In another form of the present disclosure, a method of piercing a workpiece with a plasma arc torch of the type having a plasma gas flow path for directing a plasma gas through the torch and a secondary gas flow path for directing a secondary gas through the torch is provided. The method comprises directing a flow of shield gas along a distal end portion of the plasma arc torch to deflect metal spatter generated from the piercing, ramping a current provided to the plasma arc torch along a profile during piercing, and modulating the current profile as a function of a thickness of the workpiece and an operating current level to decrease the impact of molten metal splatter during piercing.
In yet another form of the present disclosure, a method of piercing a workpiece with a plasma arc torch of the type having a plasma gas flow path for directing a plasma gas through the torch and a secondary gas flow path for directing a secondary gas through the torch is provided. The method comprises directing a flow of shield gas along a distal end portion of the plasma arc torch to deflect metal spatter generated from the piercing, ramping a current provided to the plasma arc torch along a profile during piercing, and decreasing and increasing a slope of the current profile as a function of a thickness of the workpiece to reduce the impact of molten metal splatter during piercing.
The present disclosure also includes a plasma arc torch of the type having a plasma gas flow path for directing a plasma gas through the torch and a secondary gas flow path for directing a secondary gas through the torch. The plasma arc torch comprises a piercing current that flows through a tip extending from a distal end portion of the torch. The piercing is controlled along a profile during piercing and is controlled by current ramp parameters as a function of a thickness of a workpiece and an operating current level to increase the effectiveness of a shield gas in deflecting metal splatter during piercing. In this form, the current ramp parameters comprise a length of time, a ramp rate, a shape factor, and a modulation.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawing, in which:
a-5i are exemplary shape factors for current ramp parameters in accordance with the principles of the present disclosure;
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.
Referring to
As used herein, a plasma arc torch, whether operated manually or automated, should be construed by those skilled in the art to be an apparatus that generates or uses plasma for cutting, welding, spraying, gouging, or marking operations, among others. Accordingly, the specific reference to plasma arc cutting torches, plasma arc torches, or automated plasma arc torches herein should not be construed as limiting the scope of the present disclosure. Furthermore, the specific reference to providing gas to a plasma arc torch should not be construed as limiting the scope of the present invention, such that other fluids, e.g. liquids, may also be provided to the plasma arc torch in accordance with the teachings of the present invention. Additionally, as used herein, the words “proximal direction” or “proximally” is the direction as depicted by arrow X, and the words “distal direction” or “distally” is the direction as depicted by arrow Y.
Referring now to
The consumable components also include a shield device 50 that is positioned distally from the tip 42 and which is isolated from the power supply. The shield device 50 functions to shield the tip 42 and other components of the plasma arc torch 20 from molten splatter during piercing and also from heat flux emanating from the workpiece, in addition to directing the flow of shield gas S that is used to deflect molten splatter and to stabilize and control the plasma stream. Additionally, the gas directed by the shield device 50 provides additional cooling for the consumable components of the plasma arc torch 20.
In general, the present disclosure sets forth methods by which the shield design and energy input to the pierce location are closely coupled in order to effect an improved piercing operation. More specifically, the present disclosure provides control of energy input to the pierce location through control of a current profile during piercing. Such control allows for the use of one particular pierce profile optimized for a current level and shield design across a range of material thicknesses and also optimization of current profile for a particular thickness. This in fact becomes particularly useful with automated plasma cutting systems.
With reference to
An increase in the capacity of pierce will depend on the effectiveness of pushing the molten metal at the bottom of the well. Referring to
1) Amplitude modulation—varying the magnitude of the current profile over time;
2) Frequency modulation—varying the frequency of the current waveform over time;
3) Phase modulation—delaying the natural flow of the current profile;
4) Pulse modulation—pulsing current level during profiling;
5) Phase Shift Keying—the phase of the current profile is varied to tailor the energy delivered during piercing; and
6) Multi-Modulation—combining two or more of the above current signals into the current profile.
More specifically, in accordance with the specific forms of the present disclosure, a sinusoidal wave superimposed with a linear ramp as shown in
Certain current ramp parameters are controlled in order to effect more efficient piercing in accordance with the principles of the present disclosure. These current ramp parameters include, by way of example:
1) Length of ramp up time;
2) Ramp rate;
3) Shape factor of the current ramp (described in greater detail below); and
3) Modulation of the current ramp.
With reference to
In general, the current ramp parameters are adjusted for current level and thickness of the workpiece 22. For example, in accordance with various testing and analysis, it has been shown that a sharp increase in current will deposit metal spatter 30 on the plasma arc torch 20 and damage the shield device 50. In a similar fashion, a decrease of the slope, especially on thicker workpieces 22, produces a more controlled pierce with controlled trajectories of the metal spatter 30. In accordance with one form of the present disclosure, the slope of the current profile is decreased as a function of an increase in pierce location of the workpiece 22. With the sinusoidal modulations as shown in
An exemplary method of piercing a workpiece 22 with a plasma arc torch 20 of the type having a plasma gas flow for directing a plasma gas through the torch and a secondary gas flow for directing a secondary gas through the torch is illustrated in
Referring now to
With reference to
As further shown in
In one form of the present disclosure, the workpiece thickness is about 1.50 inches (3.91 cm), and the length of time of the current ramp is between about 2 seconds and about 4 seconds. In another form, the workpiece thickness is between about 1.00 inches (2.54 cm) and about 1.25 inches (3.18 cm), the operating current level is about 250 amps, the length of time of the current ramp is between about 400 milliseconds and about 800 milliseconds, and the shape factor of the current profile is linear. In yet another form, the workpiece thickness is between about 1.00 inches (2.54 cm) and about 1.25 inches (3.18 cm), the operating current level is about 200 amps, the length of time of the current ramp is about 400 milliseconds, and the shape factor of the current profile is an S-curve.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the substance of the present disclosure are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure.
