Page 36 - MetalForming-January-2019-issue

Welding Well
Wheel overshoots when rolling up onto front of part
Wheel bounces when it lands on top of part
Wheel starts rolling off of back of part
Wheels bounce against each other after rolling off of back of part
Velocity
Heat automatically reduces itself and shuts off as wheel rolls off of part
Displacement of wheel when rolling up onto front of part triggers current
Velocity
Each 5-msec.-duration weld-current pulse (below) is adjusted every millisecond to control weld based upon displacement, velocity and force
Data collected and current synthesized with WeldComputer Adaptive Control
Data collected and current synthesized with WeldComputer Adaptive Control
Fig. 3—An adaptive RSEW control automatically adjusts weld-heat input in relation to the position of the electrode wheels. This is shown on the left, the front edge of the part, and on the right, the back edge of the part.
ations varied over a wide enough range to produce welds that were either too hot or too cold.
The monitor also documented repeated occurrences of heat starting before the part reached the welding wheels, and occurrences where the wheels rolled up onto the part before current initiated. When heat started before the part contacted the welding wheels, the welds at the front edge of the part were too hot. Sparks resulted at the onset of con-
tact, and weld spatter contaminated the welding wheels. In addition, when the wheels rolled up onto the part before current started, undersized welds resulted on the front edge of the part. A similar phenomenon occurred on the back edge of the part. Excessive heating and expulsion of material occurred whenever the heat remained on as the wheels rolled off of the back end of the part, and inadequate welding occurred when the heat cut off before the wheels started rolling off of the back end of the part.
Furthermore, the monitor recorded instances of the heat starting too soon on one part and too late on the next part, without any adjustments made on the production line. This provided quantitative evidence that the system in place could not reliably coordinate the synchronization of heat versus time needed to apply proper heating to every part as it passed through the machine (Figs. 1 and 2).
Adaptive Welding Solution
To remedy the deficiencies of conventional seam-welding operations, fabricators can employ adaptive control to detect when the wheels start to roll up onto the front of the part, and then dynamically adjust the heat in relation to the profile pattern of the wheels rolling up onto the part. The same approach can ensure optimum heating on the back end of the part—profiling the heat in direct response to the wheels rolling off of the back end of the part. An adaptive control also can instantly terminate the flow of heat, within 1 msec., upon detecting that the wheels have finished rolling a specified distance off of the back of the part (Fig. 3). This limits the susceptibility of the process to sparking and expul- sion of material, caused by keeping current on too long. It also extends the amount of time that production can continue before cleaning the electrodes.
Part two of this article, to appear in the March 2019 issue of MetalForming, will discuss operational efficiency of the seam-welding process, and the use of inverter-based controls and MFDC transformers. MF
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34 MetalForming/January 2019
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