Improving Resistance Weld Quality with Monitoring
Putting Monitoring to Work for You
More than
Questions to Consider
What Monitors Cannot Do

Weld monitoring can help you spot setup or electrical problems. Often, bad welds are produced because an operator improperly sets the welding control. Sometimes, unauthorized adjustments have been made. With wear, electrodes may have "mushroomed" until current density is too low, requiring tip-dressing or replacement. Electrical connections may have loosened, or cables may have been frayed.

A weld monitor can also help you discover mechanical problems. The airline regulator may fail to deliver the required pressure. Weld current may start to flow before adequate electrode force is reached. Tooling may have worn or fixtures may be improperly assembled. A fastener, such as a weld nut, may have been positioned upside-down. Hold time may not be long enough to ensure proper and consistent setdown.

From a process optimization standpoint, you may find that welding time can be shortened. The need for extra safety welds can be reduced because the primary welds you have made are known to be of consistently high quality. Of course, you may find that you need to extend welding time to improve quality.

In all these cases, measuring as much that goes on during the weld cycle as possible - current, voltage, cycles, resistance, force, and displacement - will help you develop the best weld schedule for your application. This measurement will help ensure higher weld quality with less need for destructive testing. Your ability to describe in numeric terms what a good weld is and to recall its signature for comparison with subsequent welds will help prevent rejects. In short, the more you measure, the more you know.

Quality concerns are another driving factor behind the implementation of weld monitoring. Inadequate weld quality can involve both loss of productivity and safety issues. The definition of some automotive welds as MVSS (Motor Vehicle Safety Standard) means they must be monitored and known to be good. The failure of an MVSS weld may result in personal injury.

ISO 9000 requires that you know what your process should be and prove it with documentation. In many instances, the ability to evaluate welds according to a statistical process control (SPC) sampling plan is adequate. In other situations, the characteristics of each weld must be recorded. The existence of printed documentation of weld quality is a requirement for proof of process.

Remember that monitoring itself is not a solution to weld problems. You need to make monitoring work for you, and learn what monitoring trends are telling you.

For example, if welds are suddenly rejected, you might find that the welding control settings have been changed. By noting the monitor's time and date stamp on the weld record (if the monitor provides this), you can determine when the change was made. In another instance, you may find that current and voltage are decreasing over a large number of welds. This might point to problems with the secondary connections or cables. Next >>

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