Spot Welding
Questions and Answers
Why do so many projection welds continue to be made on AC machines when MFDC has so many advantages?
MFDC systems over the last twenty years have been the dominant new welding system. It has found usage in most new installations. That is not to say that AC systems do not have their place in the industry. They do and will continue to be used. Below is a comparison of the two systems pro & con:
An inquiry came in suggesting that two forces are responsible for creating a resistance weld. Let’s examine this question. To do so we will look at two previously published articles in this blog:
“WHAT IS AC RESISTANCE WELDING”
AND
“WHAT IS THE DEFINITION OF RESISTANCE WELDING”
Per the definition of the Resistance Welding Manufacturers Alliance:
RESISTANCE WELDING IS THE JOINING OF METALS BY APPLYING
PRESSURE
AND PASSING
CURRENT
FOR A LENGTH OF
TIME
THROUGH THE METAL AREA WHICH IS TO BE JOINED
When a machine has been running good parts and a brief stop is made one expects to start back up and make good parts. The stop could be for a break, electrode change, electrode dressing, lunch or shift change. These occur every day and should not create a problem. If something is occurring what is the variable?
Some have said that the electrode cools off during the break. Possibly but it cools to merely water tower temperature of 85 deg F or if refrigerated 65 deg F. Considering that the electrode will reach red hot on the face which is 1400 deg F in less than a second. A little cooling does not matter.
One must look elsewhere. It is troubleshooting time. Something else in the system has changed and needs to be investigated. It is time to get out the current meters and force gauges. It is highly likely that in the force or voltage current world something is not being delivered as desired. Every shop should have or access to a current meter and a force gauge.
Spot welding is a form of resistance welding. The heat is generated by two opposing electrodes passing current through two or more sheets of material. The resistance to that heat flow in the materials causes them to heat up and form a weld nugget. Which when solidified bonds the two sheet together. This heating process is governed by:
The heat is generated by the current squared times the resistance in the materials by the time it is heated.
Water flow measurement is critical to ensure the components of the resistance welder and especially the electrodes are receiving the proper amount of cooling water during operation.
AWS J1.2 Guide to Installation and Maintenance of Resistance Welding Machines offers information about the setup and maintenance of resistance welders.
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