Spot Welding

Questions and Answers

In North America the most commonly used electrode materials for galvanized steel are RWMA Class 1, 2 and 20. By the quantity Class 1 and 2 are the most popular by far. Class 20 finds applications where the fabricator has weld schedules that make use of its unique properties – good conductivity and high heat resistance. Class 20 is sold at a premium for its higher heat properties.

 Resistance to Annealing RWMA Copper Alloys

In a previous article there is a complete discussion of this topic:

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Answers to the questions asked do not exist in the literaturer.  This question has entered an area of academia and testing to reach definitive answers to the questions presented. This is beyond the scope of this blog.

Reference: RWMA - Resistance Welding Manual 4th Edition

This question is asked often “How much power does my resistance welder use?” To answer this, we first need to calculate the Duty Cycle of the process. The formula for this is:

Duty Cycle rev

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Proper alignment of the resistance welding electrodes is very important to insure proper force application and current flow through the electrode face. If not set up properly the part can be distorted and/or the electrode face can have exceedingly high current densities. This can result in distorted parts, expulsion and excessive electrode face wear.

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This is the description of a typical cosmetic weld where one surface must be preserved as is. All heat and distortion must come from the other side, we will call it the back side. To do this a large “FLAT” faced RWMA “C” electrode is used on the good side. On the back side an RWMA “A” or pointed nose or in this specific case one with a face of 1.5 mm (0.060”).

The weld schedule should be designed to generate the heat from the back side and stop as the weld nugget just begins to grow into the top sheet. This will not be the best weld that could be produced. It will be a tack weld just good enough to hold the part together until the braze is made.

If discoloration is an issue water misting cooling, a gas shield or any other means could be used to prevent heat or oxidation on the one side.

                  Cosmetic Weld Nugget No electrodes


Reference: RWMA - Resistance Welding Manual 4th Edition

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