Electric heating – Metal heating – By arc
Reexamination Certificate
1999-12-21
2001-07-10
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130310, C219S137610
Reexamination Certificate
active
06259059
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to an arc welder and a unique welding torch for use in electric arc welding.
BACKGROUND OF INVENTION
Gas metal arc welding (GMAW) involves applying electrical current to a contact tip in a welding torch through which a consumable electrode or welding wire is passed as it moves toward a workpiece. Electrical current connected to the welding wire by the contact tip creates the electric arc for the welding process in accordance with standard welding technology. A power source, normally a high speed switching inverter, creates the welding current and is normally regulated on the basis of arc voltage to maintain a stable arc length in a constant voltage welding or pulse welding. The electrode or welding wire advancing through the contact tip toward the workpiece is electrically connected to the workpiece by the electric arc having a length that should have a given length for a specific process. The spacing between the contact tip and the workpiece is the welding parameter known as the contact tip to work distance (CTWD). This parameter is distinguished from the electrode stickout (ESO) which is the length of the advancing welding wire measured from the contact tip to the arc and includes a summatio of ESO and arc length. In gas metal arc welding it is desirable to control CTWD to maintain a stable are length. CTWD varies due to process disturbances, such as weldment dimensional tolerances experienced during production, tooling/fixture positioning accuracy and part distortion during welding. To compensate for CTWD variations and, thus, maintain arc stability, it is normal procedure in GMAW processing to adjust the arc current to change the heat input and, thus, vary the burn-off rate of the wire. Changing arc current, while maintaining voltage constant, leads to substantial changes in the heat input per unit length of the weld bead and substantially affects the cooling rate of weld pool solidification. Using variations in the heat input by changing the current to adjust for variations in CTWD or arc length, has been found to cause corresponding changes in the weld surface profile, penetration profile, base metal dissolution, metallurgical properties and mechanical properties in the weld bead. Consequently, changing the current in a constant voltage welding process to compensate for changes in CTWD presents substantial process variations exhibited primarily in the heat affected zone. Further, distortion level of the weldment and quality of the weld structure is more difficult to maintain when using arc current to compensate for changes in CTWD and, thus, in changes of arc length. To avoid the necessity of increasing or decreasing arc current based upon variations in CTWD, especially on robotic applications, the arc current is measured and used to mechanically adjust the actual spacing between the contact tip and the workpiece. This procedure is often employed in seam tracking of robotic welding equipment wherein the torch weaves back and forth as it moves along a seam. The CTWD at each end of the weave is compared to determine if the torch is centered in the joint. Variations cause a change in the path of the torch to track the seam.
Because of the mechanical inertia and the dependency on a feedback loop process when adjusting CTWD as well as heat differences, a method using the arc current for arc stability is not used, especially in a high speed welding application. Another arrangement for physically adjusting the spacing of the tip with respect to the workpiece is a vision system that actually scans the position of the tip with respect to the workpiece during the welding operation. Such vision systems are not employed due to high cost and maintenance in hostile environments. In summary, the most accepted way of maintaining arc stability is to adjust the welding or arc current to compensate for variations in CTWD. This presents the problems heretofore explained in detail.
BRIEF DESCRIPTION OF INVENTION
The present invention relates to a method of controlling CTWD without changing heat input and without physically moving the contact tip mechanically with respect to the workpiece. Since the procedure for controlling CTWD does not involve a feedback loop controlling welding current and does not involve physically moving the contact tip or torch with respect to the workpiece, the disadvantages experienced with respect to prior efforts to maintain CTWD during normal welding or seam tracking are alleviated. The invention maintains stable arc length in a constant voltage welding operation or pulse welding operation without disadvantages associated with the prior control procedures.
In accordance with the present invention there is provided a modification of the contact tip used in the welding gun or torch assembly. The invention involves separating the contact tip into two electrically isolated sections comprising an upper contact tip section and a lower contact tip section. The two tip sections are separated by an air gap or other insulator so that the consumable welding wire or electrode passes through first the upper tip section and then through the lower tip section as it advances toward the workpiece. The power supply of the welder that drives the torch or gun has one lead which is controlled by two power electronic switches to split the output current of the power supply. The arc current flows to either top or upper contact tip section or the bottom or lower contact tip section, or both sections, according to shifting the switches selectively into the respective conductive states. The total welding current is maintained with current passing through either the upper contact tip, the lower contact tip or both contact tips. When the welding current flows only to the upper contact tip, CTWD is the distance determined by the spacing of the upper contact tip from the workpiece. When current is directed to the bottom tip, CTWD is the spacing of the lower tip from the workpiece. Consequently, as the current is directed to the upper tip a greater CTWD is created than when the current is directed to the lower tip. In accordance with the invention, the current is time shared between the upper or top contact tip and the lower or bottom contact tip. In accordance with the preferred embodiment, a pulse width modulator operated at a low frequency, such as less than about 1 kHz, includes a controlled duty cycle. The output logic on the pulse width modulator causes the welding current to be directed to the upper tip or to the lower tip. There is time sharing basis based upon the duty cycle of the pulse width modulator. Consequently, detected CTWD of the welding process is not the distance from the lower contact tip or the upper contact tip. The “effective” CTWD a distance is somewhere between the spacing of the two contact tip sections based upon the time during which the welding current is directed to the upper tip or the lower tip. By increasing the time during which the welding current is directed to the upper tip, compared to the time the current is directed to the lower tip, the effective CTWD is increased. In a like manner, an increase in the time the current is directed to the lower contact tip compared to the upper contact tip, the effective CTWD is reduced. By merely adjusting the duty cycle of the pulse width modulator, the effective CTWD is changed to maintain the desired arc length and, thus, the stability of the welding process. Arc length and arc voltage is controlled by adjusting the duty cycle or relative switch operating times of the arc current time sharing network. When the torch assembly or gun is physically too close to the weldment, more current is directed to the upper contact tip section to reverse the effective torch-to-work distance. In a like manner, when the torch or gun is pulled away from the weldment, more current is directed to the bottom contact tip to maintain an effective CTWD to maintain the arc length and arc voltage for stability of the welding process.
Since there is no mech
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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