Page 35 - MetalForming-January-2019-issue

Wheel starts rolling up onto part
Current starting before wheel comes in contact with part, which overheats front edge of part
Displacement data (top) and current data (bottom) collected with WeldView monitor
Fig. 2—When applying current too early, the front edge of the part overheats.
velocity will produce hot welds.
Fabricators select from three modes of continuous RSEW: 1. Welds produced using the same wheel velocity—The
wheels clamp the parts and start rolling. Welding does not commence until after the wheels accelerate to the programmed welding velocity, and the wheels do not decelerate back to zero until the last weld in the seam has been completed.
This is the easiest arrangement for fabricators to manage with conventional welding equipment. As long as consistent parts are presented to the welding machine with consistent tooling, and the process maintains consistent control of electrode force, wheel velocity, heat and time, then managing the shunting phenomenon during the first few welds in the seam generally becomes the only remaining process-specific condition to address.
2. Welds produced with varying wheel velocity—The wheels clamp the part and begin rolling, and welding com- mences before the wheels finish accelerating to the pro- grammed welding velocity. Welding at the end of the seam continues, even as the wheels decelerate toward zero.
This arrangement requires special actions at the beginning and end of the seam to avoid forming excessively hot welds at the lower velocities. One such action: Employ upslope heat at the start of the seam and downslope heat at the end of the seam. Achieving consistent welding performance requires precise scaling and coordination of the heat upslope with the rising wheel velocity at the beginning of the seam, and precise scaling and coordination of the heat downslope with falling velocity at the end of the seam. This coordination can be difficult to achieve in practice. Also, as wheel speed increases, instantaneous velocity fluctuations increase from factors such as variable loading of the part presented to the machine, gear backlash, belt stretching and vibration, and machine mechanical resonances. These variations can lead to inconsistent weld-nugget size.
3. Welding occurs edge-to-edge across the entire part—
Fabricators use edge-to-edge seam welding to manufacture products such as water heaters, 55-gal. drums, pails and aerosol cans. As each weldment feeds through the RSEW machine, the wheels must roll up onto the front edge of the part, travel
along its entire length, and roll off of the back edge.
Most manufacturers of these types of parts attempt to control the process by employing upslope heat at the start of the seam and downslope heat at the end. A limit switch or proximity sensor detects the part approaching the seam wheels and triggers the start of the weld-schedule sequence. A sensor that detects the approach of the back end of the
part triggers the downslope at the end of the seam.
Often, this process produces high scrap rates from incon- sistent weld performance, and as production speed increases so do welding inconsistency and scrap rate. Typically, welds on the front edge of the part are either too cold (because welding heat started after the wheels already started to roll up onto the part), or too hot (because welding heat started before the wheels began to roll up onto the part). Regardless of how the fabricator may adjust the proximity sensors, the time uncertainty of the part front-end detection system, coupled with variability in the time from when the detection takes place until the part contacts the seam wheels, makes it nearly impossible to accurately synchronize the start of
heat with the front edge of the part entering the wheels. Synchronizing the downslope on the back end of the part, and turning off the heat at the right time, creates similar problems. If the heat shuts off before the wheels begin to roll off of the back edge, the welds will be too cold, and if the heat stays on too long, after the wheels roll off of the back edge, the welds will be too hot. If the last weld on the part remains in progress when the wheels have rolled too far off the back edge of the part, excessive sparking from expulsion and material loss will occur. In addition to affecting weld integrity, debris from the expulsion can stick to wheels
and accelerate wheel-surface degradation.
Case Study: Steel-Drum Welding
A New Jersey manufacturer of 55-gal. steel drums performs edge-to-edge seam welding at a rate of 50 ft./min. To improve weld consistency and reduce scrap, the manufacturer replaced its single-phase AC welding transformer and SCR-based weld control with a medium-frequency direct-current (MFDC) transformer and conventional inverter control. Instead of increasing production throughput and decreasing scrap, the equipment upgrades resulted in decreased throughput and increased scrap. An analysis of the welding operation revealed the causes of inconsistent welding performance, and resulted in recommendations to correct the problems.
To gather welding diagnostics, a portable WeldView monitor was connected to a machine on the production line. Examining the data, recorded over several hours of production, revealed multiple problems, including inconsistent heat-control deliv- ery during each welding impulse, and inconsistent synchro- nization of the start of heat with the front edge of the part and of the stop of heat with the back edge of the part.
The monitor documented multiple occurrences of greater than 10-percent current fluctuations and greater than 50- percent weld-impulse duration fluctuations. These fluctu-
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MetalForming/January 2019 33
Welding Well